Technical Guide to 

Managing Ground 

Water Resources 

United States

Department of

Agriculture

Forest Service

Minerals and 

Geology 

Management

Watershed, Fish,

Wildlife, Air, and

Rare Plants

Engineering

FS-881

May 2007
Acknowledgments

This technical guide was written by Steve Glasser, James 

Gauthier-Warinner, Joseph Gurrieri, and Joseph Keely of 

the U.S. Department of Agriculture Forest Service; Patrick 

Tucci of the U.S. Geological Survey; Paul Summers of the 

U.S. Bureau of Land Management; and Michael Wireman 

and Kevin McCormack of the U.S. Environmental Protection 

Agency. The document benefited greatly from our colleagues, 

too numerous to list, who reviewed the content and provided 

valuable comments. Editing was provided by Joe Gurrieri, 

Jim Gauthier-Warinner, and Christopher Carlson of the Forest 

Service. Special recognition is extended to Steve Glasser, 

without whose vision and perseverance this effort would never 

have come to fruition.

Cover photograph: Spouting Rock, a spring that feeds Hanging 

Lake, White River National Forest, east of Glenwood Springs, 

CO. (Photo by Patrick Tucci.)
Contents

List of Figures...................................................................................................v

List of Tables ................................................................................................. viii

Part 1. Introduction ..........................................................................................1

Purpose and Objectives...............................................................................2

Scope and Organization..............................................................................2

Importance of Ground Water Resources on NFS Lands.........................2

Part 2. Managing Ground Water Resources....................................................5

Federal Statutes ...........................................................................................5

Overview of the National Ground Water Policy.......................................5

Land Management Planning......................................................................6

Water Development...................................................................................6

Water Quality ............................................................................................7

Ground Water-dependent Ecosystems.......................................................7

Inventory and Monitoring .........................................................................8

Data Management .....................................................................................8

Partnerships...............................................................................................8

Ground Water Uses .....................................................................................8

Management of Drinking-water Supplies.................................................9

Source-water Protection .....................................................................11

Coordination with Public Water-supply Purveyors........................11

Protecting Public Water Supplies Through Forest Planning ..........11

Special-use Authorizations for Water Wells and Pipelines ................12

Initial Screening..............................................................................17

Second-level Screening ..................................................................17

Environmental Analysis .................................................................18

Exploratory Drilling Procedures.....................................................19

Construction and Production Permitting ........................................19

Monitoring and Mitigation .............................................................19

Case Study: Ground Water Development, 

Tonto National Forest, AZ..............................................................20

Carlota Copper Company Wellfield ...............................................20

Sunflower Well ...............................................................................23

Stock Watering on Public Land...............................................................24

Managing Ground Water Quantity Problems............................................25

Detection and Monitoring ..................................................................25

Limiting Withdrawals.........................................................................27

Increasing Recharge ...........................................................................27

Conjunctive Management of Surface Water and Ground Water........27

Case Study: Conjunctive Use of Ground Water and Surface Water, 

Tonto National Forest, AZ..............................................................27

Dowsing ..................................................................................................31

Ground Water Quality ..............................................................................33

Application of Water Quality Regulations to Ground Water ..................34

Ground Water Classification ..............................................................35

Ground Water Discharge Permits.......................................................36

Total Maximum Daily Loads .............................................................36

Water-quality Databases.....................................................................37
ii

Ground Water-dependent Ecosystems.....................................................37

Types of Ground Water-dependent Ecosystems........................................39

Terrestrial Vegetation and Fauna........................................................39

Ecosystems in Streams and Lakes Fed by Ground Water..................39

Hyporheic and Hypolentic Zones.......................................................41

Springs................................................................................................41

Aquifer, Karst, and Cave Ecosystems................................................42

Wetlands.............................................................................................42

Management Considerations and Protection Strategies..........................43

Case Study: Importance of Ground Water in Alpine-lake

Ecosystems, Cabinet Mountains Wilderness, MT.........................45

Activities That Affect Ground Water ......................................................47

Mineral Development..................................................................................47

Hardrock Mining................................................................................47

Ore Processing................................................................................47

Water Management.........................................................................48

Waste Management.........................................................................49

Mine Closure ..................................................................................50

Potential Impacts to Ground Water Resources...............................50

Case Study: Depletion of Streamflow by Underground Mining, 

Stillwater Mine, Custer National Forest, MT............................53

Coal Mining........................................................................................55

Potential Impacts to Ground Water Resources...............................55

Oil and Gas Exploration and Development............................................56

Potential Impacts to Ground Water Resources...............................57

Coal-bed Methane ..............................................................................58

Disposal of Production Water.........................................................59

Ground Water Pumping...........................................................................60

Declining Water Levels......................................................................60

Land Subsidence ................................................................................63

Impacts of Subsidence ....................................................................67

Ground Water and Slope Stability...........................................................67

Effects of Vegetation Management on Ground Water.............................73

Phreatophyte Management.................................................................73

Upland Forest Management ...............................................................74

Case Study: Effects of Acidic Precipitation on Ground Water, 

 Hubbard Brook Experimental Forest, NH......................................75

Impacts from Fires ..................................................................................76

Wildland/Urban Interface........................................................................77

Monitoring..................................................................................................77

Monitoring Program Objectives..............................................................77

Background or Baseline Monitoring..................................................79

Monitoring for Specific Land-use Impacts ........................................79

Case Study: Ground Water Monitoring in the Turkey Creek

Watershed, Jefferson County, CO..............................................79

Monitoring for Facility-based Compliance ...........................................81

Monitoring of Ground Water-dependent Ecosystems........................81

Case Study: Monitoring of Ground Water-dependent Ecosystems,

Arrowhead Tunnels, San Bernardino Mountains, CA...............83

Monitoring for Environmental Change .............................................87
iii

Staffing and Resource Needs....................................................................95

Expertise..................................................................................................95

Hardware/Software..................................................................................95

Field and Laboratory Requirements........................................................96

Training ...................................................................................................96

Part 3. Hydrogeologic Principles and Methods of Investigation ..................97

Basic Hydrogeologic Principles................................................................97

Hierarchical Classification of Aquifers...................................................97

Ground Water Regions of the United States......................................97

A Classification Framework for Ground Water..................................99

Geology and Ground Water...................................................................100

Ground Water Flow Systems.................................................................104

Recharge and Discharge...................................................................104

Movement of Ground Water.............................................................107

Regional and Local Flow Systems ..................................................107

Shallow, Intermediate, and Deep Aquifers...........................................112

Ground Water Development and Sustainability....................................114

Ground Water Quality ...........................................................................117

Ground Water/Surface Water Interactions.............................................120

Case Study: Contribution of Metal Loads to Daisy Creek 

from Ground Water, Custer National Forest, MT ......................121

Gaining and Losing Stream Reaches ...............................................124

Hyporheic Zone and Floodplain Mixing..........................................129

Influence of Wells on Streams..........................................................131

Springs..............................................................................................133

Types of Springs...........................................................................133

Physical Environment and Water Chemistry of Springs ..............136

Field Observations of Seeps and Springs.....................................137

Ground Water Exchange in Reservoirs, Lakes, and Ponds .............138

Sensitive Hydrogeological Settings ......................................................139

Karst Terrains...................................................................................139

Unconsolidated Deposits..................................................................140

Volcanic Terrains..............................................................................142

Fractured-rock Settings....................................................................142

Ground Water Investigation Methods...................................................145

Ground Water Resource Inventories and Evaluations...........................145

Aquifer Delineation and Assessment ...............................................146

Hydrogeological Mapping................................................................148

Remote Sensing ............................................................................151

Well and Borehole Logs...............................................................154

Design of a Ground Water Monitoring Network.....................................154

Use of Existing Wells.......................................................................155

Installation of New Monitoring Wells..............................................156

Well Placement.................................................................................157

Case Study: Landfill Evaluation, West Yellowstone, 

Gallatin National Forest, MT...................................................159

Water-level Monitoring ....................................................................161

Quality Assurance.........................................................................162

Water-level Monitoring Equipment..............................................162

Ground Water Quality Sampling and Analysis ................................164

Sources of Sampling Error ...........................................................167

Selection of Ground Water-quality Indicators..............................167

Well Purging and Sampling ..........................................................169

Quality Assurance and Quality Control Plans..............................172

Monitoring Network Cost Considerations .......................................174
iv

Aquifer Testing Techniques...................................................................175

Geophysical Techniques........................................................................177

Ground Water Tracing Techniques........................................................180

Natural Tracers.................................................................................181

Artificial Tracers ..............................................................................183

Field Methods...................................................................................184

Analysis of Hydrogeological Data ........................................................186

Analytical Methods..........................................................................186

Potentiometric Maps.....................................................................187

Calculating Ground Water Flow...................................................191

Flow Nets......................................................................................191

Analysis of Aquifer Test Data.......................................................195

Numerical Models............................................................................197

Synthesis and Interpretation..................................................................200

Ongoing Data Analysis Costs................................................................201

References .....................................................................................................202

Resources.......................................................................................................223

U.S. Geological Survey Ground Water Contacts..................................223

National Issues ......................................................................................223

Regional Issues......................................................................................223

State or Local Issues..............................................................................223

USGS Online Resources..........................................................................224

Other Online Resources ........................................................................224

Appendix I. 

Legal Framework for Ground Water Use 

 in the United States..............................................................................225

Appendix II. 

Common Ground Water Terms and Definitions..................................229

References.................................................................................................236

Appendix III. 

Contaminant Fate and Transport.........................................................237

References.................................................................................................242

Appendix IV. 

Ground Water Remediation...................................................................243

Appendix V. 

Ground Water Modeling .......................................................................249

References.................................................................................................257

Appendix VI. 

Water-quality Data: Statistics, Analysis, and Plotting.........................259

References.................................................................................................266

Appendix VII.

Geophysics...............................................................................................267

References.................................................................................................280
List of Figures

Figure 1. Decision tree for issuance of special-use permits for proposals to 

develop water supply wells on NFS land.

Figure 2. Ground water development project locations on the Tonto National 

Forest.

Figure 3. Pinto Creek.

Figure 4. Map of the Carlota Wellfield and associated monitoring locations.

Figure 5. Hydrographs showing the decrease in stream flow and decline of 

the alluvial water table during the pump test.

Figure 6. Map showing the Sunflower well and associated monitoring 

locations. 

Figure 7. Hydrograph showing the decrease in stream flow during the pump 

test.

Figure 8. Example of poor management of a spring on NFS lands.

Figure 9. Water well instrumented for water-level data collection, satellite 

transmission, and real-time reporting on the Internet.

Figure 10. Ground water development near a stream can reduce streamflow 

and harm riparian vegetation, requiring management decisions to 

restore the system.

Figure 11. Schematic of artificial recharge processes.

Figure 12. Highway corridor and the location of ground water sources 

investigated.

Figure 13. Locations of the production wells and associated monitoring 

locations. 

Figure 14. Mitigation and conjunctive use measures incorporated into the 

project.

Figure 15. Cliff Lake, Cabinet Mountains Wilderness, MT.

Figure 16. Water and solute budgets for Rock Lake and Cliff Lake in percent 

of ice-free season inflow.

Figure 17. Hydrographs of No Name Creek and flow rate of the East Adit, 

Stillwater Mine, MT.

Figure 18. Before mining, ground water discharged to the spring and 

maintained the flow of No Name Creek. During development of 

the East Adit, ground water that would have discharged to the 

spring was intercepted and diverted into the adit.

Figure 19. Pumping a single well in an idealized unconfined aquifer. 

Dewatering occurs in a cone of depression of unconfined aquifers 

during pumping by wells.

Figure 20. Comparison of drawdowns after 1 year at selected distances 

from single wells that are pumped at the same rate in idealized 

confined and unconfined aquifers.

Figure 21. A reduction in the total storage capacity of the aquifer system 

can occur if pumping of water causes an unrecoverable reduction 

in the pore volumes of compacted aquitards because of a collapse 

of the sediment structure.
vi

Figure 22. Land subsidence in the San Joaquin Valley, 1926-70.

Figure 23. Cross sections through the agricultural area of a portion of former 

Everglades area, central Florida, showing the decrease in landsurface elevation.

Figure 24. Cover-collapse sinkholes may develop abruptly and cause 

catastrophic damage.

Figure 25. Aerial view of the Mill Creek landslide blocking U.S. Highway 

50.

Figure 26. Some aspects of ground water flow in rock slopes: (1) possible 

large differences in fluid pressure in adjacent rock joints; (2) 

comparison of transient water-table fluctuations in porous 

soil slopes and low-porosity rock slopes; (3) fault as a lowpermeability ground water barrier, and as a high-permeability 

subsurface drain.

Figure 27. Ground water seeps are evident in a roadcut located 30 miles from 

Lohman, ID.

Figure 28. Ground water table in the Turkey Creek watershed, September 

2001.

Figure 29. Chloride concentrations in the Turkey Creek watershed, fall 1999.

Figure 30. Aerial photograph of showing the East Tunnel alignment and 

monitoring points on the San Bernadino National Forest.

Figure 31. Spring snail, Pyrgulopsis californiensis.

Figure 32. Western spadefoot toad, Spea hammondii.

Figure 33. Mitigation flow chart for the Arrowhead Tunnels Project.

Figure 34. Ground water regions of the United States (A), and alluvial valley 

aquifers (B).

Figure 35. Principal aquifers of the United States.

Figure 36. Types of openings in selected water-bearing rocks.

Figure 37. In the conterminous United States, 24 regions were delineated 

by the USGS where the interactions of ground water and surface 

water are considered to have similar characteristics.

Figure 38. Two-dimensional conceptual model of a ground water recharge 

system in a Basin and Range hydrogeological setting.

Figure 39. Using known altitudes of the water table at individual wells (A), 

contour maps of the water-table surface can be drawn (B), and 

directions of ground water flow (C) can be determined.

Figure 40. If the vertical distribution of hydraulic head in a vertical section is 

known from nested piezometers (wells completed at discrete 

intervals below land surface), vertical patterns of ground water 

flow can be determined.

Figure 41. A regional ground water flow system entails subsystems at 

different scales and a complex hydrogeological framework.

Figure 42. Diagrams illustrating water budgets for a ground water system for 

predevelopment and development conditions.

Figure 43. Iron oxyhydroxide and associated heavy metals from acidic 

inflows degrade the water quality of Daisy Creek, Park County, 

MT.
vii

Figure 44. Sources of dissolved copper to subreaches of Daisy Creek, 

including relative contributions of copper from surface-water and 

ground-water sources.

Figure 45. Interaction of streams and ground water.

Figure 46. Flow-duration curve for the Rio Camuy near Hatillo, PR.

Figure 47. Streamflow and baseflow hydrographs for the Homochitto River in 

Mississippi.

Figure 48. Effects of pumping from a hypothetical ground water system that 

discharges to a stream.

Figure 49. Comal Springs, near San Antonio, TX, discharges ground water 

from the highly productive Edwards aquifer.

Figure 50. Ground water from a large regional limestone aquifer discharges at 

Crystal Spring in Ash Meadows National Wildlife Refuge, NV.

Figure 51. Elements of a ground water inventory and assessment.

Figure 52. Vulnerability map of a portion of the Beaverhead-Deerlodge 

National Forest constructed using the DRASTIC method.

Figure 53. Interpretation of drainage patterns from aerial photos and 

corresponding rock types and structure.

Figure 54. Common environmental factors that influence the choice of 

frequency of water-level measurements in observation wells.

Figure 55. Measuring water levels in an observation well with a steel tape.

Figure 56. Downloading data from an automatic water-level recorder.

Figure 57. Cross-sectional diagram showing the water level as measured by 

piezometers located at various depths.

Figure 58. Common errors in contouring water table maps: (a) topographic 

depression occupied by lakes and (b) fault zones.

Figure 59. Error in mapping potentiometric surface because of mixing of two 

confined aquifers with different pressures.

Figure 60. Example of a flow net for a simple flow system.

Figure 61. Effect of fracture anisotropy on the orientation of the zone of 

contribution to a pumping well.

Figure 62. Plan view of flow net and cross-section through losing stream 

segment.

Figure 63. Plan view and cross-section of flow net for gaining stream.

Figure 64. Example of a Theis type curve and a curve-matching plot for 

analysis of aquifer-test data.
viii

List of Tables

Table 1. A common ground water classification system based on TDS 

concentration (mg/L).

Table 2. Hydrogeoindicators that may be suitable for monitoring to assess 

environmental responses to changes in ground water systems on 

NFS lands. 

Table 3. Recommended indicators in the ground water environment.

Table 4. General attributes of local-scale assessments of ground water 

systems.

Table 5. Attributes of a regional-scale assessment of a hydrogeologic unit 

or group of units.

Table 6. Possible sources of water entering and leaving a shallow ground

water system under natural conditions. 

Table 7. Major, minor, and trace dissolved inorganic constituents in ground 

 water.

Table 8. Hardness classification based on equivalent concentration of 

CaCO3 (mg/L).

Table 9. Types of studies that evaluate water-flow and water-quality 

interactions between ground water and surface water.

Table 10. DRASTIC computation matrix showing methods for computing 

index values for various hydrogeological settings in the Pioneer 

Mountains, Beaverhead-Deerlodge National Forest, MT.

Table 11. Generalized diagram of the steps involved in sampling and the 

principal sources of error.

Table 12. Summary of aquifer test methods.

Table 13. Relative advantages and disadvantages of analytical and numerical 

models.
Part 1. Introduction 

Ground water is the Nation’s principal reserve of fresh water and represents 

much of its potential future water supply. Ground water on National Forest 

System (NFS) lands is a major contributor to flow in many streams and rivers 

and has a strong influence on the health and diversity of plant and animal 

species in forests, grasslands, riparian areas, lakes, wetlands, and cave systems. 

It also provides drinking water to hundreds of communities. Demands for safe 

drinking water and requirements to maintain healthy ecosystems are increasing, 

and complex social and scientific questions have arisen about how to assess 

and manage the water resources on NFS lands. This technical guide was 

developed to help address these issues. It describes the national ground water 

policy and provides management guidelines for the NFS.

Today, many of the concerns about ground water resources on or adjacent 

to public land involve questions about depletion of ground water storage, 

reductions in streamflow, potential loss of ground water-dependent ecosystems, 

land subsidence, saltwater intrusion, and changes in ground water quality. The 

effects of many human activities on ground water resources and on the broader 

environment need to be clearly understood in order to properly manage these 

systems. Throughout this technical guide, we emphasize that development, 

disruption, or contamination of ground water resources has consequences for 

hydrological systems and related environmental systems.

Ground water and surface water are interconnected and interdependent in 

almost all ecosystems. Ground water plays significant roles in sustaining 

the flow, chemistry, and temperature of streams, lakes, springs, wetlands, 

and cave systems in many settings, while surface waters provide recharge to 

ground water in other settings. Ground water has a major influence on rock 

weathering, streambank erosion, and the headward progression of stream 

channels. In steep terrain, it governs slope stability; in flat terrain, it limits soil 

compaction and land subsidence. Pumping of ground water can reduce river 

flows, lower lake levels, and reduce or eliminate discharges to wetlands and 

springs. It also can influence the sustainability of drinking-water supplies and 

maintenance of critical ground water-dependent habitats. 

Increasingly, attention is being placed on how to manage ground water (and 

surface-water) resources on public lands in a sustainable manner. The potential 

for ground water resources to become contaminated from anthropogenic as 

well as natural sources is being scientifically assessed. Each ground water 

system and development situation is unique and requires a specific analysis to 

draw appropriate conclusions. 

This technical guide begins by reviewing the legislative and policy framework, 

and the issues related to ground water inventory, monitoring, contamination, 

and development. Individual sections then focus on key concepts, principles 
Purpose and 

Objectives

Scope and 

Organization

Importance 

of Ground 

Water 

Resources on 

NFS Lands

and methods for managing ground water resources. Relevant special topics, 

case studies, and field examples are highlighted throughout the text. Additional 

information on some topics can be found in the appendixes.

This technical guide provides guidance for implementing the U.S. Department 

of Agriculture (USDA) Forest Service national ground water policy. It 

describes hydrological, geological, and ecological concepts, as well as the 

managerial responsibilities that must be considered to ensure the wise and sustainable use of ground water resources on NFS lands. 

This document is one part of a four-part information system on ground water 

management on the national forests and grasslands. The other three parts are 

(1) Forest Service policy on ground water (Forest Service Manuals [FSM] 

2543 and 2880); (2) a Forest Service sourcebook on State ground water laws, 

regulations, and case law for all 43 States with NFS land; and (3) a ground 

water inventory and monitoring technical guide. When complete, the four parts 

will provide line officers and technical specialists at all field levels with the 

science, policy, and legal framework for Forest Service ground water-resource 

management. Users of this document are strongly encouraged to refer to all of 

these documents when dealing with a ground water-resource issue.

This technical guide is intended for Forest Service line officers and managers 

and their technical-support staffs. Managers will be interested in Parts 1 and 2, 

in which information is presented on management considerations and on the 

importance of ground water issues. Part 3 and the appendixes provide more 

detailed information on basic hydrogeological principles and ground water 

investigation methods that may be most appropriate for technical support staffs. 

Ground water is a valuable commodity and its use is growing nationwide. The 

NFS contains substantial ground water resources, for which stewardship and 

protection are mandated by congressional acts. Many other natural resources 

on NFS lands rely, directly or indirectly, on ground water and would be 

damaged or destroyed if that water were depleted or contaminated. Careful 

inventory of the quantity and quality of ground water on the NFS is needed to 

provide sufficient information to appraise the value and provide appropriate 

stewardship of these ground water resources. The following are the objectives 

of ground water inventory and monitoring:

•	 To ensure timely availability of hydrogeological resource information 

needed for the periodic assessment required by the Forest and 

Rangeland Renewable Resources Planning Act, as amended, and for 

land and resource management planning.

•	 To provide regionwide status and change data and to enhance the 

potential for combining data sets across geographic areas to address 

national trends. 
•	 To classify aquifer types, establish baseline ground water quality, map 

flow systems and ground water-dependent ecosystems, and assess 

aquifer vulnerability based on a consistent standard throughout the 

NFS. 

Ensuring sustainability of natural resources has become a fundamental 

requirement for Federal land management. In preparing to manage ground 

water resources within this framework, the following interdependent questions 

must be addressed:

•	 How much ground water is there, where is it, and what is its 

quality?

•	 What are the existing uses of ground water?

•	 What is the nature of the interconnections between the ground 

water and surface water systems?

•	 To what extent do other natural resources depend on ground 

water?

•	 How vulnerable are the aquifers to contamination or depletion?

To answer these questions, ground water resources need to be inventoried and 

assessed. 

Overuse of ground water may impact streams, wetlands, riparian areas, forest 

stands, meadows, grasslands, seeps, springs, cave systems, and livestock and 

wildlife watering holes. It may lower lake and reservoir levels, and promote 

land subsidence, sinkhole formation, and cave collapse. Reduced water-table 

levels can impact biota that depend on ground water, particularly in riparian 

and wetland ecosystems. 

When water is removed from saturated soils and deeper sediments, the soil, 

sediment, or rock structure that remains may partially collapse and result 

in visible slumping of soils, widespread subsidence of the land surface, or 

the formation of sinkholes. These changes in the land surface may damage 

highways, bridges, building foundations, and other structures. They also may 

damage natural resources. In addition, excessive well withdrawals can affect 

water quality in the aquifer. Saltwater may intrude into the aquifer, poorquality or contaminated water may migrate from adjoining areas or surface 

water bodies, or chemical components of the desaturated aquifer may be 

mobilized. Ground water levels or pressures may drop, causing shallow wells 

to go dry and requiring deepening or replacement. Increased drawdown can 

impact ecological resources by depleting ground water that supports riparian 

vegetation, wetlands, or sensitive flora and fauna.

The list of elements and chemical compounds that may be accidentally or 

purposely released into the environment, and transported by ground water, is 

seemingly endless. The NFS contains thousands of public and private drinkingwater supply systems located at campgrounds, rest areas, permittee sites, 
private in-holdings, and in-forest communities. The NFS also contains the 

headwaters of many streams that flow off-system lands and the recharge areas 

for many aquifers from which water is drawn for human use. The protection 

of all sources of public drinking water from contamination is a nationwide 

imperative, heralded by the Safe Drinking Water Act (SDWA) of 1974. 

Many activities have the potential to contribute contamination to soils and 

ground water simply through the presence and use of fuels, oils, solvents, 

paints and detergents, and by the generation of solid or liquid wastes. Typical 

contamination sources on NFS lands include mines, oil and gas wells, landfills, 

and septic systems. Contamination of soils and ground water can be difficult, 

time-consuming, and expensive to address. 

Although numerous Federal and State programs regulate activities that may 

release contaminants to soils and ground water, the implementation of these 

programs in rural areas generally lags behind that in urban areas. Because the 

release of even small amounts of stored chemicals or fuels may substantially 

damage soil and ground water resources, efforts must be made to ensure that all 

Forest Service activities and facilities comply with regulations for preventing 

soil and ground water contamination. Similarly, efforts must be made to 

collaborate or partner with States, permittees, owners of in-holdings, and 

forest-bounded communities to institute appropriate ground water protection 

measures.
Federal 

Statutes

Overview of 

the National 

Ground 

Water Policy

Part 2. Managing Ground Water Resources

This section reviews the types of ground water issues that are important for 

all USDA Forest Service units, line officers, and staff to consider. Legal 

requirements and ground water-management strategies are discussed. 

In addition to the Federal land management statutes cited in Forest Service 

Manual (FSM) 2501, the following Federal statutes provide pertinent direction 

to the Forest Service for its management of ground water resources in the 

National Forest System.

Safe Drinking Water Act of 1974, as amended. (42 U.S.C. §300f et seq). 

The intent of the SDWA is to ensure the safety of drinking-water supplies. 

Its authority is used to establish drinking-water standards and to protect 

surface- and ground water supplies from contamination.

Resource Conservation and Recovery Act of 1976, as amended. (42 U.S.C. 

§6901 et seq) The Resource Conservation and Recovery Act (RCRA) 

regulates the generation, transportation, treatment, storage and disposal of 

waste materials. It has very specific requirements for the protection and 

monitoring of ground water and surface water at operating facilities that 

may generate solid wastes or hazardous wastes. 

Comprehensive Environmental Response, Compensation, and Liability 

Act of 1980, as amended. (42 U.S.C. §6901 et seq). Also known as 

“Superfund”, the Comprehensive Environmental Response, Compensation, 

and Liability Act (CERCLA) regulates cleanup of existing environmental 

contamination at non-operating and abandoned sites (see also FSM 2160).

In addition, judicial doctrine and water-rights case law provide the legal 

interpretations of Federal and State statutes about usage and management of 

ground water (see FSM 2541.01 and Forest Service Handbook [FSH] 2509.16 

for procedures to be followed for complying with Federal policy and State 

water-rights laws). 

The national ground water policy sets out the framework in which ground 

water resources are to be managed on NFS lands. The policy is designed to 

be located in two parts of the Forest Service Manual, FSM 2880, Geologic 

Resources, Hazards, and Services, and FSM 2543, Ground Water Resource 

Management. As of the publication date of this technical guide, FSM 2543 

is in draft form and may change due to agency and public comment prior to 

finalization. Regional Foresters and Forest Supervisors are directed by the 

national ground water policy to perform the duties detailed below.


Water 

Development

Land 

Management 

Planning

•	 Protection and sustainable development of ground water resources are 

appropriate components of land and resource management planning for NFS 

lands. Ground water inventories and monitoring data shall be integrated into 

the land and resource management process. 

•	 When evaluating project alternatives or revising national forest plans, use 

the best available science, technology, models, information, and expertise 

to determine the location, extent, depths, amounts, flow paths, quality, 

and recharge and discharge areas of ground water resources and their 

hydrological connections with surface water.

•	 Conduct appropriate National Environmental Policy Act (NEPA) analyses 

when evaluating applications for water wells or other activities that propose 

to test, study, monitor, modify, remediate, withdraw, or inject into ground 

water on NFS lands (see also FSH 2509).

•	 Always assume that hydrological connections exist between ground water 

and surface water in each watershed, unless it can be reasonably shown 

none exist in a local situation.

•	 Ensure that ground water that is needed to meet Forest Service and 

authorized purposes is used efficiently and, in water-scarce areas or time 

periods, frugally. Carefully evaluate alternative water sources, recognizing 

that the suitable and available ground water is often better than surface 

water for human consumption at administrative and public recreational 

sites. 

•	 Prevent, if possible, or minimize the adverse impacts to streams, lakes, 

ponds, reservoirs, and other surface waters on NFS lands from ground 

water withdrawal.

•	 As applicable under State water-rights laws and adjudications, file wateruse-permit applications and water-rights claims for beneficial uses of 

ground water by the Forest Service. Consult with the Office of General 

Counsel prior to filing (see also FSM 2541). 

•	 Comply with wellhead protection (U.S. Environmental Protection Agency 

[EPA] 1994), sole-source aquifer, and underground injection control (UIC) 

requirements of Federal (40 Code of Federal Regulations [CFR] 144), 

State, and local agencies. Ensure that all public water systems (PWSs) on 

NFS lands that use ground water comply with EPA’s ground water rules. 

•	 Require all drinking-water systems that withdraw water from aquifers on 

NFS lands, and that are classified as community water systems (those that 

serve 25 year-round residents or have 15 or more service connections), to 

have flow meters installed and operating. Require wells on NFS lands that 

provide ground water that is later sold to consumers or used for industrial 

or commercial purposes to have flow meters installed and operating. Wells 

equipped with hand pumps are not required to have flow meters. Require 

injection wells with discharge pipes that are 4 inches inside diameter or 

larger to be metered.
Ground Waterdependent 

Ecosystems

Water Quality •	 Identify the needs and opportunities for improving watersheds and 

improving ground water quality and quantity. Take appropriate steps to 

address the needs and take advantage of the opportunities. 

•	 In areas where ground water on NFS land has become contaminated from 

human sources, evaluate the potential receptors, technical feasibility, costs, 

and likelihood of finding potentially responsible parties (PRPs), the risks 

 of exacerbating the problem, and other relevant factors before making a 

decision to try to cleanup the ground water. 

•	 Complete removal and/or remedial actions for ground water contamination 

at CERCLA/Superfund sites on NFS lands. Identify the PRPs and seek 

to have them perform the cleanup work, where possible, to minimize the 

cost of the cleanup to the Forest Service. At sites where the Forest Service 

is a PRP, the cleanup work should be aggressively performed in a timely 

manner to fulfill the agency’s trustee responsibilities. Inform owners of 

non-federal property abutting NFS lands that overlie contaminated ground 

water of the existence of the contamination, the types of contaminants 

present, and the Forest Service plan for managing the contaminated ground 

water. 

•	 Ecological processes and biodiversity of ground water-dependent 

ecosystems must be protected. Plan and implement appropriately to 

minimize adverse impacts on ground water-dependent ecosystems by (1) 

maintaining natural patterns of recharge and discharge, and minimizing 

disruption to ground water levels that are critical for ecosystems; (2) not 

polluting or causing significant changes in ground water quality; and (3) 

rehabilitating degraded ground water systems where possible.

•	 Manage ground water-dependent ecosystems to satisfy various legal 

mandates, including, but not limited to, those associated with floodplains, 

wetlands, water quality and quantity, dredge and fill material, endangered 

species, and cultural resources.

•	 Manage ground water-dependent ecosystems under the principles of 

multiple use and sustained yield, while emphasizing protection and 

improvement of soil, water, and vegetation, particularly because of effects 

upon aquatic and wildlife resources. Give preferential consideration to 

ground water-dependent resources when conflicts among land-use activities 

occur. 

•	 Delineate and evaluate both ground water itself and ground waterdependent ecosystems before implementing any project activity with 

the potential to adversely affect those resources. Determine geographic 

boundaries of ground water-dependent ecosystems based on site-specific 

characteristics of water, geology, flora, and fauna.

•	 Establish maximum limits to which water levels can be drawn down at a 

specified distance from a ground water-dependent ecosystem in order to 

protect the character and function of that ecosystem. 

•	 Establish a minimum distance from a connected river, stream, wetland, 

or other ground water-dependent ecosystem from which a ground water 

withdrawal may be sited.
Data 

Management

Partnerships

Ground 

Water Uses

Inventory and 

Monitoring

•	 Design inventory and monitoring programs to (1) gather enough 

information to develop management alternatives that will protect 

ground water resources, and (2) evaluate management concerns and 

issues expressed by the general public. Assign high priorities for survey, 

inventory, analysis, and monitoring to municipal water-supply aquifers, 

sensitive aquifers, unique ground water-dependent ecosystems, and highvalue or intensively managed watersheds. 

•	 Develop estimates of the usable quantity of ground water in aquifers while 

protecting important NFS resources and monitor to detect excessive water 

withdrawal.

•	 Define the present situation and detect spatial or temporal changes or trends 

in ground water quality or quantity and health of ground water-dependent 

ecosystems; detect impacts or changes over time and space, and quantify 

likely effects from human activities. 

•	 Establish guidelines and standards for the acquisition and reporting of 

ground water information to meet the specific needs of Forest Service 

programs. The storage of ground water data must conform to Forest Service 

Natural Resource Applications (FSNRA) standards and servicewide 

Geographic Information System (GIS) data standards. Storage will be in 

FSNRA databases upon availability.

•	 Close collaboration and partnership with other Federal Agencies and States/

Tribes, regional and local governments and other organizations is essential 

in gathering and analyzing information about ground water resources for 

which the Forest Service has stewardship.

 

Some 83.8 billion gallons per day of fresh ground water were pumped in 

the United States in 2000 (Hutson and others 2004). This total was about 8 

percent of the estimated daily natural recharge to the Nation’s ground water. 

Much of this water was being withdrawn in excess of the recharge capabilities 

of local aquifers (“overpumping”). Withdrawals significantly in excess of 

natural recharge are located predominantly in coastal areas of California, 

Texas, Louisiana, Florida, and New York, in the Southwest, and in the Central 

Plains. In the United States, management of ground water is primarily the 

responsibility of State and local governments. The authority and responsibility 

for overseeing the allocation and development of water resources typically 

resides with the State’s department of natural resources or water resources 

or the State engineer’s office. The authority and responsibility to prevent 

undue contamination of ground water typically resides with the State’s 

health department or department of environmental quality or environmental 

management and with local government (e.g., health department, county 

commissioners, city council). In addition on most Federal lands some 

overlapping responsibilities for both ground water and quantity resides with the 

management agency. 
Management of 

Drinking-water 

Supplies

Management of water resources includes the management of land-use 

activities that include potential sources of contamination. As population 

density increases, an ever-increasing demand on water resources and an everincreasing complexity of management issues are created. This complexity 

results from the uncertainties related to (1) how to manage water resources 

in a manner that achieves a sustainable annual supply, (2) how to prevent 

unplanned contamination of ground water, and (3) how to balance competing 

uses of interconnected water resources. 

Pumping of ground water results in changes to the ground water system and, 

potentially, to the ecosystem of the region being developed. These changes 

may take many years to be observed because of the commonly slow movement 

of ground water. Some changes, such as the loss of aquifer storage capacity 

from land subsidence, may be irreversible. Some changes may not be readily 

observable because they are incrementally small with time, occur underground, 

or slowly affect the chemistry of the ground water or surface water. The 

consequences of pumping should be assessed for each level of development, 

and safe yield should be the maximum withdrawal for which the consequences 

are considered acceptable. 

Ground water is one of the Nation’s most important natural resources, 

providing about 40 percent of the Nation’s public water supply (Alley and 

others 1999). In addition, more than 40 million people, including most of the 

rural population, supply their own drinking water from domestic wells. As a 

result, ground water is an important source of drinking water in every State 

and is also the source of much of the water used for agricultural irrigation. 

Therefore, protection of those water resources is an important goal of land-use 

planning and management nationwide. A valuable reference to use in assessing 

how much risk of contamination is associated with different land-use practices 

commonly occurring on NFS lands is titled “Drinking Water from Forests and 

Grasslands: A Synthesis of the Scientific Literature” published in 2000 by the 

Forest Service Southern Research Station as General Technical Report SRS039 (http://www.srs.fs.fed.us/pubs/gtr/gtr_srs039).

The SDWA Amendments of 1996 (P.L. 104-182, 42 U.S.C. 300f et. seq.) 

revised the original 1974 Act by adopting a multiple barrier approach to 

the protection of drinking water from its source to the tap, creating a State 

revolving fund for financing water treatment improvements, and establishing 

reporting on the quality of water served to all water consumers by the water 

provider. All PWSs have to be assessed for vulnerability to current and 

potential sources of contamination and the source of water must be delineated. 

By definition, PWSs provide drinking water to at least 25 people or 15 service 

connections for at least 60 days a year. About 170,000 PWSs in the United 

States provide water to more than 250 million people. Of these, at least 3,500 

communities and 60 million people get water directly from NFS lands. The 

EPA defines two main types of PWSs:
10

1. Community water systems that provide drinking water to the same 

people yearlong. All Federal drinking-water regulations apply to 

these systems.

2. Noncommunity water systems that serve customers on less than a 

yearlong basis. Such systems are considered to be transient if they 

serve people who are passing through the area and nontransient if 

they serve at least 25 of the same people more than 6 months in 

a year but not yearlong. Most federal drinking-water regulations 

apply to systems in the latter category, while only regulations 

concerning contaminants posing immediate health risks apply to 

systems in the transient category.

The Forest Service owns and operates about 6,000 water systems, most of 

which fall into the noncommunity, transient category. District Rangers and 

Forest Supervisors should make sure that these water systems are meeting 

all requirements of the law, are being tested in compliance with the law 

and regulations, and, if found to fail the bacteriological or any of the other 

standards, that the system will be immediately shut off and not reopened until 

all tests are in compliance. Exposure of the public or Forest Service employees 

to unsafe or SDWA non-compliant drinking water must not take place at Forest 

Service managed facilities. In addition, civil penalties can be imposed by States 

and EPA for violations of the SDWA.

Water systems owned and operated by the Forest Service should be maintained 

properly to ensure that the water provided to the public and employees is safe 

and meets all applicable standards. Systems nearing the end of their service 

life may need major overhaul or replacement. Line officers are responsible 

for requesting sufficient funding to maintain water systems or close down 

obsolete systems and switching those facilities to other water supplies, such as 

municipal water if available. Additional guidance can be found in FSM 7420 or 

through consultation with the regional environmental engineer.

Effective management of water resources in fractured-rock hydrogeological 

settings must be based on a sound conceptual understanding of the ground 

water flow system(s) that occur in the area to be managed. Because of the 

heterogeneous and anisotropic nature of fractured-rock settings, it has proven 

difficult to manage water resources in these settings. In fractured-rock settings, 

sustainable development is greatly complicated by uncertainties about actual 

watershed dimensions and annual water budgets in associated aquifer systems. 

The relationship between “deep” ground water in fractured rock and surface 

water is still not well understood at the watershed scale. At the watershed level, 

significant uncertainties also exist about whether water from non-consumptive 

uses returns to the deep ground water system or moves as interflow directly to 

a nearby stream.
11

Source-water 

Protection

Coordination with

Public Water-supply

Purveyors

Protecting Public

Water Supplies

Through Forest

Planning

Because, in part, of the relative ease of delineating and recognizing watershed 

boundaries in mountainous areas, the concept and practice of watershedbased resource management has evolved more rapidly in those regions of the 

country. Watershed-based management is a holistic approach that requires an 

understanding of ground water flow and the relationship between ground water 

and surface water. In fractured-rock settings such understandings, however, 

may be very difficult to achieve. 

Another key provision of the 1996 Amendments to the SDWA was an increased 

focus on the prevention of contamination of drinking water at its source 

within a surface watershed or within a defined area surrounding a ground 

water extraction site, such as a well. States are required to do Source Water 

Assessments (U.S EPA 1997) for all public water supplies. For ground water, 

States commonly use one of two methods to define a well-head protection 

area: (1) “fixed radius,” which is the area defined by a radius of set distance 

from a well, such as 1,000 feet or 1 mile; or (2) “time of travel,” which is the 

area from which ground water flows horizontally to a well in a set time period, 

such as 1 year. Each method has its advantages and problems, and neither 

can provide 100 percent assurance that the ground water supply is really safe 

from contamination if appropriate land-use restrictions are applied within the 

deliineated area. 

PWS utility operators near or within the NFS may request the Forest Service to 

add water-quality protective measures, including additional “best management 

practices” (BMPs), for many land uses and activities on NFS lands within 

delineated source watersheds and well-head protection areas. The Forest 

Service should work with water supply utilities and others to evaluate the 

likely effectiveness of such additional practices and the means for paying for 

their installation and maintenance. The Forest Service should also determine 

whether any reimbursement for revenues forgone to the U.S. Treasury expected 

from any contracts, leases or permits that are being ended or prevented should 

be required of the utility, municipality, or other entity (see also FSM 2542).

During the first round of forest planning, a provision in the 1982 NFMA 

regulations required that municipal water-supply watersheds be identified as 

separate management areas. Many forests identified these watersheds and 

developed separate standards for them, but some did not. The new NFMA 

planning process does away with the requirement to delineate municipal 

watersheds, but it continues the emphasis on collaboration with stakeholders 

in the management of NFS lands, and the need to identify and quantify the 

amount and quality of water needed for multiple uses on and off these lands. 

As forest plans are revised, Forest Service units should invite participation 

from local water-utility managers and their staffs to help the agency make sure 

that forest plans recognize the importance of drinking-water sources on and 

under NFS lands and of developing and implementing sound water-quality and 

quantity protection strategies and measures.
12

Special-use 

Authorizations 

for Water Wells 

and Pipelines

This section addresses the authorization of water extraction or injection wells 

and water pipelines through special-use permits. Guidance in FSM 2729, 

FSM 2543, and FSH 2509, specifically addresses the authorization of water 

developments on NFS lands, and a decision tree summarizing the process is 

shown in figure 1. The basic laws authorizing water wells on NFS lands are 

the Organic Administration Act and the Federal Land Policy and Management 

Act (FLPMA). The permitting process for wells and pipelines is a discretionary 

activity; a permit for a well or pipeline may be denied if the agency’s analysis 

indicates that NFS resources, including water, will not be adequately protected. 

Except when authorized by either U.S. Department of the Interior or USDA 

regulations for the management of mineral or energy exploration, development, 

or production, a special-use permit is required for all entities other than the 

Forest Service to drill water wells or construct water pipelines on NFS lands. 

Where a State-based water right or State approval is needed for a water 

development, the process for securing State approvals should follow after 

or run concurrently with the Forest Service process for authorizing a water 

development. In all cases, State law must be observed when a State-based 

water right is involved. When a project proponent proposes to drill a well 

on NFS lands and/or to transport ground water across NFS lands through a 

pipeline, an analysis of the potential impacts of water removal from the aquifer 

along with the impacts of well drilling and/or pipeline construction is required 

(40 CFR 1508.25 Scope and 1508.7 Cumulative Impacts). For development of 

a water-injection well, the impact on ground water quality from the addition 

of non-native water and the impact that added volume would have on aquifer 

structure and function must be analyzed.

Laws and regulations governing wells include both State requirements 

for notification, drilling permits, well logs, well completion or 

abandonment procedures and documentation, and Federal requirements 

and recommendations for construction, sampling, and abandonment of 

monitoring wells. Under 36 CFR 251.51, the Forest Service has the authority 

to grant or deny a request for special-use authorization for a water diversion, 

extraction, or conveyance facility. No legal obligation exists to grant a specialuse authorization for a water facility, even if the applicant controls a valid 

State-issued water right. When considering whether to grant a special-use 

authorization for a water diversion, the law requires the inclusion of terms and 

conditions necessary to protect national-forest resources as part of any decision 

granting a right-of-way across NFS lands (see Section 1765 of the FLPMA).

A solid administrative record must be developed to support decisions on 

special-use authorizations. Include the impacts to national-forest resources, 

details on the basis for mitigation measures required to protect those resources, 

and the reasons why the extraction or conveyance of ground water is consistent 

or inconsistent with the applicable land management plan. 
13

Is proposal consistent with 

applicable laws, regulations, 

policies, rules executive orders,

treaties, decrees, and NFS land 

and resource management 

plans (FSM 2702 and 2703)?

Return proposal to 

proponent verbally 

or in writing 

(36 CFR 251.54[e][2]).

Second Level Screening.

Figure 1. Decision tree for issuance of special-use permits for proposals to develop water supply wells on NFS land.

Is proposal consistent 

with national policy not

to encumber NFS lands 

just because it affords 

proponents a lower 

cost when compared 

with alternatives located 

on non-NFS lands 

(FSM 2703.2)?

Yes

Yes

No

No

Initial Screening.

Proposal received verbally or in writing (CFR 251.54[b]).
14

Second Level Screening.

Return proposal to proponent 

with written reason for 

rejection (36 CFR 251.54[g][1]). 

NEPA analysis is not 

necessary (36 CFR 251.54[e][6]). 

Begin appropriate NEPA analysis 

(36 CFR 251.54[g][2][ii]).

Has proponent identified purpose 

and quantity of water needed?

Does proponent include 

appropriate water 

conservation measures 

(FSM 2541.21L)?

Return proposal to 

proponent verbally or 

in writing 

(36 CFR 251.54[e][2]).

Would drilling activities 

negatively affect NFS 

resources?

Is there a reasonable 

likelihood of successfully 

completing a well?

Notify proponent to submit 

written formal application 

(36 CFR 251.54[g][1]).

Would proposed well 

location(s) be likely to affect 

key NFS resources or 

neighboring water supplies?

Can impacts be avoided

or mitigated?

No

No

No

No

No

Can resource damage be 

adequately mitigated?

Yes

Yes

Yes

Yes

Yes

Yes

Yes
15

Has applicant 

obtained all 

necessary State 

authorizations 

for Phase 1 

(FSM 2541)?

Deny temporary SUP for exploration & impact evaluation

(36 CFR 251.54[g][4]).

Issue temporary SUP for exploration & impact evaluation

(36 CFR 251.54[g][4]).

Does exploration drilling result 

in sufficient water to meet 

applicant’s needs?

No

Conduct Phase 1 NEPA for exploration and impact

evaluation.

Approve proposed use 

with modifications or deny 

SUP (36 CFR 251.54[g][4]).

Has applicant obtained 

all necessary State 

authorizations (FSM 2541)?

Issue SUP with appropriate 

monitoring, mitigation and 

fees. (36 CFR 251.54[g][4]).

No

Does proposal adequately 

protect NFS resources 

and neighboring water 

supplies?

No

Complete NEPA analysis.

Begin appropriate NEPA anlaysis 

(36 CFR 251.54[g][2][ii]).

Does proposal include

substantial ground water 

production rates?

Conduct NEPA analysis in 

two phases.

Yes

Yes

Yes

Yes

No
16

Have applicant prepare for 

Forest Service review and 

approval detailed monitoring 

plan for aquifer testing of the 

well(s) and assessing impacts 

to NFS resources and 

neighboring water supplies.

Conducting aquifer testing and 

impact assessment.

Conduct Phase 2 NEPA for

construction and production.

Has applicant obtained all 

necessary State authorizations 

for Phase 2 (FSM 2541)?

Can wells be used for NFS purposes?

Deny SUP (36 CFR 251.54[g][4]). 

Retain in Federal own ership and 

acquire necessary water rights.

Deny SUP (36 CFR 251.54[g][4]). 

Retain in Federal ownership and 

Have applicant abandon wells 

according to State and Federal 

regulations.

Can impacts be avoided or mitigated?

Approve proposed use with 

modifications and appropriate 

moniitoring, mitigation and fees or 

deny SUP (36 CFR 251.54[g][4]).

Yes

Does testing and monitoring 

indicate that NFS resources 

and neighboring water 

supplies are adequately 

protected?

Does exploration drilling result 

in sufficient water to meet 

applicant’s needs?

No

No

Yes

Yes

Yes

No

Issue SUP with appropriate 

monitoring, mitigation and 

fees (36 CFR 251.54[g][4]). 

Incorporate conditions that 

could result in modification or 

termination of SUP under 

appropriate circumstances 

(36 CFR 251.60[a][2][D]).

No

Yes

No
17

Initial Screening

Second-level

Screening

As provided for in 36 CFR 251.54(b), initial proposals for ground water 

developments may be presented to the Forest Service either orally or in 

writing. Water developments related to a CERCLA response action are not 

subject to this initial NEPA screening, but are subject to CERCLA analysis in 

the engineering evaluation/cost analysis for removals or remedial investigation/

feasibility study for remedial actions. To pass the initial information screening 

requirements, proposals to construct wells on NFS lands and/or pipelines 

across NFS lands must meet the following conditions:

1. The proposal to pump, inject, or transport water must be consistent 

with applicable laws, regulations, policies, rules, executive 

orders, treaties, compacts, and Forest Service land and resource 

management plans (FSM 2702 and 2703). Proposals are evaluated 

as specified in 36 CFR 251.54(e). 

2. The proposal must be consistent with national policy not to 

encumber NFS lands just because it affords a proponent a lower 

cost when compared with alternatives located on non-NFS 

lands. If the intent of the proposal is to use ground water derived 

from NFS lands for a non-NFS purpose, the proponent must 

demonstrate that alternative water sources do not exist (FSM 

2703.2).

Proposals that do not meet the minimal requirements of the initial information 

screening process are returned to the proponent as insufficient. The authorizing 

officer shall reply in writing if the proposal was presented in writing, or may 

reply orally if the proposal was presented orally (36 CFR 251.54[e][2]).

Additional information is required for proposals that pass initial information 

screening. In second-level screening, the proposal is evaluated as described in 

36 CFR 251.54(e)(5) and as follows:

1. The quantity of water the proponent is seeking to pump from 

beneath NFS lands and the purpose of use of such water must be 

identified. If the proponent anticipates increased water needs in 

the future, those needs must be quantified. If the proponent seeks 

to inject water into the ground, the quantity, source(s), and quality 

of the injection water and the likely effects of this action must be 

identified.

2. Proposals to use ground water underlying NFS lands must include 

appropriate water conservation measures (FSM 2541.21h) and 

all community water system wells must be equipped with a flow 

metering device in good working order.

3. Drilling activities themselves can negatively impact NFS 

resources. In instances in which considerable disturbance may 

result from the drilling process, the proponent must demonstrate 

that a reasonable likelihood exists of successfully completing any 

water wells and adequately mitigating any resource damage. 
18

Environmental

Analysis

4. Identify all anticipated facilities, such as roads, power lines, 

pipelines, water storage tanks, and pumps that could ultimately 

be needed to produce or inject, and convey water across NFS 

land. Proposals that involve construction and/or use of roads shall 

conform to the requirements of FLPMA, specifically Sections 502 

and 505. 

5. Identify key resources and existing water supplies to assist in 

evaluating the potential for the proposal to affect NFS resources 

and neighboring water supplies. 

6. Return proposals that fail to pass second-level screening to 

the proponent with a written reason for rejection (36 CFR 

251.54[g][1]). NEPA analysis is not required to make this 

determination (36 CFR 251.54[e][6]).

Where proposals pass second-level screening, notify the proponent that the 

Forest Service is prepared to accept a formal written application for a specialuse authorization. Previously submitted information may be included in the 

application by reference. The Forest Service should begin the appropriate 

NEPA analysis on receipt of the formal application (36 CFR 251.54[g][2][ii]) 

and notify Federal, Tribal, State, and local entities involved in the management 

of water resources as early in the process as possible (FSM 1909.15, Conduct 

Scoping). Advise the proponent that any information provided will become 

public information once the formal application is received and the NEPA 

process initiated. Once the formal application is received, the proponent is 

referred to as the applicant. 

If information screening indicates that the proposal includes higher-thanaverage ground water production rates and/or potentially high-impact well(s) 

or transmission facilities, substantial additional analysis may be necessary. 

An application may be approved in two phases: (1) exploration, and (2) 

construction of water-production facilities. Using that approach, each phase 

requires separate NEPA analysis and documentation (refer to FSH 1909.15, 

chapters 20, 30, and 40). When the application uses existing wells, many of the 

evaluation procedures described here may still apply. The project applicants 

should be advised that obtaining approval for exploratory drilling and/or 

evaluation does not guarantee that construction of production phase facilities 

will be authorized. They should also be advised that there may be substantial 

mitigation measures required by the terms of a production authorization and 

that the scope of those measures may not be identified until the conclusion of 

the appropriate environmental analysis. 

Where water supplies in sufficient quantities to meet the applicant’s needs are 

located in existing wells or found through exploration, require a detailed plan 

to determine impacts. This plan must be site-specific and designed to identify 

potential impacts to NFS resources and neighboring water supplies, and must 

be approved before testing for impacts. In the absence of sufficient information 

to model impacts, an aquifer test with long-term pumping of existing and/or 
19

Exploratory Drilling

Procedures

Construction

and Production

Permitting

Monitoring and

Mitigation

exploratory wells and monitoring of observation wells and surface water 

may be required. The purpose of the test is to evaluate the potential impacts 

of removing water at production levels from the well(s) under consideration. 

Where the proposal involves the transport of ground water pumped from 

nearby non-NFS lands across NFS lands, the above testing may still be 

required to evaluate impacts of the ground water withdrawal on NFS resources 

and neighboring water supplies (40 CFR 1508.25, Scope). When an injection 

well is proposed, a site-specific analysis of the impacts from the introduced 

water is required to determine potential impacts to NFS resources and 

neighboring water supplies. The results of testing, monitoring, and/or modeling 

should be shared with the appropriate State and local agencies. 

NEPA documentation must be completed, appropriate to the scale of 

operations, when screening indicates a reasonable likelihood of producing 

ground water or of injecting water without negative impacts to NFS resources 

or neighboring water supplies and all applicable State authorizations have been 

obtained. If the responsible official decides to allow exploration on NFS lands, 

a temporary permit may be issued for the exploration and impact-evaluation 

phase of the proposal. This temporary permit shall contain any conditions 

necessary to minimize impacts to NFS resources.

The construction and production phase includes the construction of all 

infrastructure needed to pump, store, and convey water from its source to the 

location of use. Once a NEPA decision and all applicable State authorizations 

are in place, a special-use authorization is needed to occupy and use NFS lands 

for the purposes of constructing and operating facilities designed to produce, 

inject, and/or convey ground water (36 CFR 251.54 [g][5]). Refer to FSM 

2711 for guidance on the type of permit or easement to issue. Refer to 36 CFR 

251.56 for terms and conditions for permit issuance. Construction may be 

permitted separately from production. Once a permit is issued, the applicant is 

referred to as the holder. The Forest Service may amend the permit at any time, 

regardless of the length of time for which a permit is issued (FSM 2711.2). 

Continued monitoring of water developments is necessary to verify that their 

operation remains in the public interest.

Monitoring and/or mitigation measures necessary to ensure protection of 

NFS resources during the construction of water pumping, injection, storage, 

or transport facilities are included in annual plans of operation. Mitigation 

measures can include the cessation of pumping during critical times of the 

year or replacing water to streams and springs. If long-term monitoring 

detects additional or unforeseen adverse impacts to forest resources, or if 

mitigation measures do not adequately protect forest resources, the permit can 

be suspended or revoked (36 CFR 251.60[a][2][D]). To reverse or prevent a 

suspension, the holder shall undertake such efforts as are necessary to eliminate 

adverse impacts. 
20

Case Study: 

Ground Water 

Development, 

Tonto National 

Forest, AZ

Figure 2. Ground water development project locations on the Tonto National Forest.

The Carlota Copper Mine site is 6 miles west of Miami, AZ (fig. 2), at an 

elevation of approximately 3,700 feet above mean sea level in a rugged, 

mountainous, semiarid region. The Carlota Copper Company proposed to 

mine 100 million tons of ore from open pits over a 20-year period to produce 

900 million pounds of copper. The ore would be leached with a sulfuric acid 

solution in a heap leach process. Predicted water requirements for the mine 

averaged 590 gallons per minute (gpm) with peak water requirements of 850 

gpm during dry months.

The mine was proposed to be located in the Pinto Creek watershed (fig. 3), 

which drains into Roosevelt Lake, a major water supply reservoir for the 

Phoenix metropolitan area. Pinto Creek, which becomes perennial below the 

project area, is a valuable resource on the forest. The creek is a rare perennial 

stream in the Sonoran Desert and has been designated as an Aquatic Resource 

of National Importance by the EPA, studied for eligibility for inclusion in the 

National Wild and Scenic River System, nominated for unique waters status, 

named as one of the 10 most endangered rivers in the nation by American 

Rivers, and called a “jewel in the desert” by the late Senator Barry Goldwater. 

To protect the stream, the Tonto National Forest applied for and received an 

instream flow water right from the State that seeks to maintain existing median 

monthly flows along a 9-mile reach of the stream located approximately 4 

miles below the Carlota project area. These flows range from 1 to 2.7 cubic 

feet per second (cfs). 

Carlota Copper

Company Wellfield

The Tonto National Forest’s ground water policy evolved from experiences 

with ground water development projects on or adjacent to the forest. The 

discussion that follows briefly describes the Carlota Copper Company and 

Sunflower projects (fig. 2).
21

The mine conducted an extensive search for water and ultimately elected to 

use ground water from a wellfield approximately two miles downstream of 

the main project area in an area adjacent to the confluence of Pinto Creek and 

Haunted Canyon (fig. 4).

Three test wells ranging in depth from 755 feet to 1,220 feet were drilled at the 

site. All three wells experienced artesian flows with artesian discharge from 

the middle well (TW-2) flowing at 250 gpm. These wells were test pumped to 

evaluate the long-term yield potential of the aquifer, and the impact of pumping 

on surface-water resources and on water table elevations in alluvium. Well 

TW-2 was pumped for 25 days at a rate of 600 gpm. The monitoring network 

consisted of three shallow alluvial monitoring wells, four bedrock monitoring 

wells, weirs at two springs, and a weir or Parshall flume at two locations in 

Haunted Canyon and Pinto Creek.

During the 25-day pump test of TW-2, streamflow at a weir in Haunted 

Canyon approximately 2,300 feet south of the TW-2 well declined from 

approximately 45 gpm at the start of the test to 5 gpm at the end of the test 

(fig. 5). Flow increased progressively to approximately 27 gpm within a few 

days of shutting off the pump. The water level in an alluvial monitoring well 

in Haunted Canyon, located approximately 1,550 feet south of TW-2, declined 

approximately 1 foot during the 25-day test and recovered slowly following the 

test. 

Based on these test results the Tonto National Forest sent a letter to the Arizona 

Department of Water Resources (ADWR) requesting an appropriability 

determination. In Arizona, water pumped from a well is considered to be 

appropriable if withdrawing that water tends to directly and appreciably 

reduce flow in a surface water source. ADWR concluded that the well was 

withdrawing appropriable water and would need a water right if it was to be 

Figure 3. Pinto Creek.
22

Figure 4. Map of the Carlota Wellfield and associated monitoring locations.

used. The Carlota Copper Company subsequently submitted a water rights 

application. The Forest protested the application based on its instream flow 

water right downstream on Pinto Creek. The Forest negotiated a wellfield 

mitigation program with the mine that seeks to maintain median monthly flows 

in Haunted Canyon and Pinto Creek in exchange for the Forest’s withdrawal of 

its protest.

Figure 5. Hydrographs showing the decrease in stream flow and decline of the alluvial water 

table during the pump test.
23

The second ground water development project influencing the development 

of the Forest’s ground water policy was the Sunflower Well. This well was 

proposed as a water supply source for upgrading a portion of State Highway 

87 that carries heavy traffic from the Phoenix metropolitan area to summer 

recreation areas in the high country along the Mogollon Rim in north-central 

Arizona. Water requirements for highway construction were estimated to be 

about 200 gpm for compaction of fills and for dust control. 

The Sunflower well was to be located on private land near Sycamore Creek 

(fig. 6), which has stream reaches with both intermittent and perennial flow 

near the well. Sycamore Creek, like Pinto Creek, is a stream with reaches of 

perennial flow in the Sonoran Desert. It supports valuable riparian vegetation, 

provides habitat for native fish, and is a popular recreation area. The Record 

of Decision for the Environmental Impact Statement (ROD) prepared for 

the highway upgrading project stated that construction water would not be 

withdrawn from Sycamore Creek. To evaluate the effects of the well on 

Sycamore Creek an aquifer test with observation wells and a streamflow 

monitoring flume was conducted.

The proposed production well (fig. 6) was completed to a depth of 240 feet 

in fractured basalt. Water rose under artesian pressure to a depth of about 20 

feet below ground surface. The monitoring network consisted of four shallow 

observation wells in the alluvium bordering the creek, two deep observation 

wells in bedrock, and a Parshall flume in a perennial reach of Sycamore Creek 

just downstream of the well. The aquifer test was originally scheduled for 3 

days with the production well pumping at an average rate of 250 gpm. 

Sunflower Well

Figure 6. Map showing the Sunflower well and associated monitoring locations. The private 

lands are outlined.
24

Figure 7. Hydrograph showing the decrease in stream flow during the pump test.

Stock Watering 

on Public Land

Water levels in the shallow monitoring wells declined before, during, and 

after the test. Water levels declined at a slightly greater rate during the test. 

The majority of the decline is believed to be attributable to natural conditions. 

The impact of pumping on streamflow through the flume was dramatically 

different than the impact to the shallow observation wells. Prior to beginning 

the test, the flow rate through the flume was about 90 gpm (fig. 7). About 6 

minutes after the pump in the production well was turned on, flow through the 

flume started to decline. Approximately 6 hours into the test, flow in Sycamore 

Creek declined to the point where there was no longer flow through the flume. 

One hour and twenty minutes after the pump was turned off, Sycamore Creek 

started flowing through the flume again. Two hours after the pump was turned 

off, flow through the flume was 37 gpm; 10 hours after turning the pump off, 

flow through the flume was 61 gpm.

Based on the results of this test, the contractor was not allowed to use the well 

for the highway upgrade project under the criteria of the ROD. 

Springs for stock watering have been developed with little regard for the 

effects on ground water-dependent ecosystems that depend on springs (fig. 8). 

Spring development generally consists of excavation and conveying all water 

to a single discharge point. This type of spring development deprives the flora 

and fauna that depend on the spring water. A portion of the water from a spring 

should be allocated to protect the viability of the dependent ecosystem and the 

area should be fenced to eliminate trampling.
25

Figure 8. Example of poor management of a spring on NFS lands.

Managing 

Ground Water 

Quantity 

Problems

Detection and 

Monitoring

According to Galloway and others (2003), ground water management includes 

the engineering, economic, and political factors that affect the locations, rates, 

and timing of hydrological stresses to the ground water system (ground water 

withdrawals, artificial recharge, and so forth). These imposed stresses then 

affect the responses of the ground water system (ground water levels, discharge 

rates, and water quality), which in turn may affect streamflow rates, aquatic 

habitats, and other environmental conditions.

In managing withdrawals from a ground water system, it is important to 

understand the status of the system and the impacts of any withdrawals. To 

understand the status of a ground water system, basic information is needed 

on the geologic framework, boundary conditions, hydraulic-head distribution, 

water-transmitting and water-storage properties, and chemical distribution. 

Any quantitative analysis depends on the availability of data, the development 

of a conceptual model based on these data, and an understanding of the factors 

affecting the movement of ground water (Galloway and others 2003). 

To monitor and evaluate changes in the ground water system, baseline 

conditions for the system must first be established. An inventory of existing 

wells or other sources of data is a first step in establishing baseline conditions. 

Such information may be obtained, often online, from the U.S. Geological 

Survey (USGS) water science center that covers the study area, or from the 

State engineer’s or State geologist’s offices. Once the status of existing data is 

established, areas where additional data are needed can be identified and new 

data can be obtained. Examples of needs may include new wells and water 
26

Figure 9. Water well instrumented for 

water-level data collection, satellite 

transmission, and real-time reporting 

on the Internet. (Photo by William 

Cunningham, USGS Circular 1217, 35.)

levels, new stream gages and stream flows, water-quality data, and water-use 

data. After baseline conditions are established, new data are collected from 

the monitoring network at a frequency appropriate for the problem. For many 

problems involving development of new wells or well fields, system response 

usually occurs quickly at first, particularly close to the new wells, then more 

slowly with time. Daily, or even hourly, observations may be needed close to 

the new wells at first. Weekly or monthly observations may be sufficient as 

transient effects of pumping begin to decrease.

Ground water systems are dynamic. They respond to short- and long-term 

changes in climate, ground water withdrawal, and land use (Taylor and Alley 

2001). Monitoring of ground water conditions in response to these changes 

requires a monitoring network of observation wells, stream and spring 

gages, and meteorological and water-quality stations. Long-term, systematic 

measurements from such a network provide essential data needed to evaluate 

changes in the ground water system over time, to develop flow models and 

forecast trends, and to design, implement, and monitor the effectiveness of 

ground water management and protection programs (Taylor and Alley 2001). 

Water-level measurements from 

observation wells (fig. 9) are the principal 

source of information about the hydrologic 

stresses acting on aquifers and how these 

stresses affect ground water recharge, 

storage, and discharge (Taylor and Alley 

2001). The ideal observation network 

consists of wells drilled specifically for 

that network, as well as instrumentation 

to collect ancillary hydrological data such 

as rainfall and streamflow. Budgetary 

constraints may require the use of existing 

wells for all or part of the network, but 

care must be taken in the selection of 

existing wells for use in the network to 

enable correct interpretation of the data.

Particularly during low-flow conditions, 

measurement of stream and spring flow 

may also provide insights about the 

response of the ground water system to changing conditions. For example, 

decreasing flow in streams or from springs, despite average or above-average 

precipitation, may indicate adverse ground water response to pumping. 

Changes in ground- and surface-water quality may also be indicative of 

ground water responses to both natural and manmade system changes.

Monitoring of ground water use can be a critical component of a monitoring 

network. Ideally, pumping wells in the area of interest are equipped with meters 

to record the amount pumped, and these values are collected and documented. 
27

Limiting 

Withdrawals

Increasing 

Recharge

Conjunctive 

Management of 

Surface Water 

and Ground 

Water

Case Study: 

Conjunctive Use 

of Ground Water 

and Surface 

Water, Tonto 

National Forest, 

AZ

Where metering is lacking, electric-power-consumption records or rated 

capacity of the well can be used as surrogates for actual pumpage data. Wellcompletion information is also crucial to understanding the impacts of ground 

water withdrawals. If undesirable effects because of new stresses on the 

ground water system are detected, informed management decisions can then 

be made. Some of the management options are described in the next sections. 

These options may be best evaluated through the use of numerical models, 

particularly in areas of complex hydrogeology.

If groundwater withdrawals or springflow diversions are negatively affecting 

the ground water system, one management option is to limit the withdrawals to 

an established safe yield. Another is to specify the location of the new wells to 

minimize negative impacts (fig. 10). Although water levels near the pumping 

wells may recover relatively quickly, water-level declines (drawdown) may 

still occur at larger distances from the wells until new equilibrium conditions 

become established. 

Increasing recharge to the ground water system through the use of infiltration 

ponds, streamflow diversions, or injection wells can help to offset the effects 

of additional pumping by establishing new equilibrium conditions (fig. 11) 

(Galloway and others 2003). These methods, however, often require a high 

degree of maintenance to the recharge system facilities and equipment to keep 

it operating efficiently and may not result in a net positive effect on the targeted 

resources. 

Conjunctive use is the combined use of surface and ground water to optimize 

resource use and minimize adverse effects. Conjunctive use is often a costeffective way to mitigate the negative impacts of excessive use of either 

resource (Galloway and others 2003). Moreover, the likelihood of more 

frequent surface water shortages, as urban and environmental demands on 

existing supplies increase, accentuates the differences in reliability between 

surface water and local ground water supplies. Conjunctive use can increase 

the yield of a water system by using existing resources more efficiently. By 

coordinating the use of surface- and ground water supplies at different times, 

in response to varying conditions, the overall use of water supplies can be 

improved in the short term and better sustained in the long term. Conjunctive 

use also can address ground water depletion problems, and help ensure the 

adequacy of ground water resources for periods of drought and surface water 

shortages.

The Arizona Department of Transportation (ADOT) proposed to upgrade a 

52-mile stretch of Highway 260 from Payson to Heber, AZ. Portions of the 

highway were to be realigned and the entire stretch was to be upgraded from 

two to four lanes. Construction would be completed in segments and plans 

called for an 8-year construction period.

Water was required for embankment compaction, dust control, and paving. 

Peak water requirements were estimated at approximately 180 gpm. ADOT 

investigated several water supply sources, including both ground- and surfacewater supplies (fig. 12). 
28

Figure 10. Ground-water development near a stream can reduce streamflow and harm riparian 

vegetation, requiring management decisions to restore the system (last panel) (Galloway and 

others 2003).
29

Figure 11. Schematic of artificial recharge processes (Galloway and others 2003).

Figure 12. Highway corridor and the location of ground-water sources investigated. 
30

A recreational vehicle (RV) site on NFS land was selected for further study 

after investigations of the other sites suggested insufficient water supplies 

were available or the likelihood of adverse environmental impacts was too 

great. This site was located within a half mile of a stream with reaches of both 

perennial and intermittent flow (Little Green Valley Creek), within a mile 

of two private land subdivisions that relied on shallow wells for their water 

supply, and within a mile of three springs, two on NFS land and one on private 

land (fig. 13). 

Figure 13. Locations of the production wells and associated monitoring locations. The private 

lands are outlined.

Before permitting ADOT to use this well field, the Forest Service required a 

long-term pump test (38 days) to assess impacts to the springs, Little Green 

Valley Creek, and wells on private land. Several wells were drilled at the site 

and selected wells were completed as potential production wells. Monitoring 

wells were installed at strategic locations around the production wells to 

monitor changes in water levels during the extended aquifer test. In addition, 

weirs were installed at springs on NFS land and in the channel of Little Green 

Valley Creek to monitor changes in flow during the test. 

Flow in one of the springs stopped during the test, and flow in the other spring 

and in Little Green Valley Creek appeared to be declining, but impacts were 

unclear because of storm events during the pump test. Water table elevations 

declined in most production and monitoring wells during the test and recovered 

to varying degrees following the test. Aquifer test data indicated that a fractureflow model should be used to analyze the data. A site-specific fracture-flow 

model was subsequently developed for the well field and calibrated against 

the pump test data. The model was then run to simulate well field operations 
31

Dowsing

for the life of the highway construction project (8 years). Model simulations 

suggested well field operations had the potential to lower water table elevations 

in the wells on private land.

To mitigate impacts to NFS resources and to wells on private land, the Forest 

Service required ADOT to discharge 10 gpm into Little Green Valley Creek 

and to install two monitoring wells next to the private land. When water table 

elevations in these monitoring wells drop more than 10 feet, ADOT is required 

to cease pumping.

ADOT was concerned that the aquifer at the RV site would not be able to 

supply the water needed for highway construction without dropping below the 

10-foot mitigation threshold. After consultations with the Forest Service, the 

Arizona Game and Fish Department, the U.S. Fish and Wildlife Service, the 

ADWR, and the Salt River Project, a program to divert and use surface water 

during winter months and to inject surface waters into the RV site aquifer 

during the same time period was developed to prevent adverse water-table 

drawdowns in wells on private land. The well-field injection program consists 

of three wells with the capacity to inject from 225 to 450 gpm during periods 

when surface water is available for injection. The intent is to use the aquifer to 

store surface waters when surface waters are available, to restore water-table 

elevations in the aquifer, and then to withdraw stored water during periods 

when surface water is not available. Note that water-quality issues apparently 

were not a significant consideration in this case; careful consideration should 

be given to water-quality impacts on the aquifer system(s) used for storage of 

surface water prior to approval of any ground water-storage proposal.

ADOT is allowed to divert surface water from Tonto Creek, a tributary to 

Roosevelt Lake, from December to April, when riparian vegetation along Tonto 

Creek is dormant. Flows are allowed to be diverted from Tonto Creek when 

streamflows exceed threshold rates at two gages on Tonto Creek. No more than 

10 percent of the flow in Tonto Creek up to a maximum of 1 cubic foot per 

second of stream flow can be diverted. Figure 14 displays the mitigation and 

conjunctive use measures incorporated into this project. 

ADOT must repay the Salt River Project for the water diverted from Tonto 

Creek with water from the Central Arizona Project Canal. In Arizona, the 

ADWR reviews exchange agreements such as these to ensure that other water 

rights holders will not be injured. 

In many parts of the world, particularly in rural areas, water-well locations may 

be determined by using the services of a “dowser” or “water witch.” Dowsing 

is the action of a person who uses a rod, stick, or other device (“dowsing rod” 

or “divining rod”) to locate ground water, metallic ores, oil, or other objects 

that may be hidden from sight. Dowsers may practice their art either in the field 

or over a map of the area of interest. The most common divining rods consist 

of either a forked stick or a pair of metallic rods. When the dowser crosses the 
32

target (either in the field or over the location on a map) the forked stick bends 

downward or the pair of metal rods crosses. Many dowsers believe that water 

occurs in underground streams or rivers. Although such features are known to 

occur in Karst areas, they are relatively rare.

Dowsing has been practiced for hundreds, if not thousands, of years. Dowsers 

claim a high success rate, and many anecdotes of successful dowsing can be 

found; however, when subjected to scientifically controlled tests, the success 

rates of dowsers are no better than random chance (Carroll 2001). The natural 

explanation of the dowser’s success is that in many areas water would be hard 

to miss. In a region of adequate rainfall and favorable geology, it is difficult 

not to drill and find water (U.S. Geological Survey 1993). In fact, some water 

exists under the Earth’s surface almost everywhere. To accurately estimate 

the depth, quantity, and quality of ground water, a number of techniques must 

be used. Hydrological, geological, and geophysical knowledge is needed to 

determine the depths and extents of the different water-bearing strata and the 

quantity and quality of water found in each.

In response to many inquiries about dowsing, the USGS published a report 

on the subject in 1917, which was reprinted several times because of its 

popularity. They advised people “not to expend any money for the services of a 

‘water witch’ or for the use or purchase of any machine or instrument devised 

for locating underground water or other minerals.” Subsequent reprints (Ellis 

1938), however, distinguished geophysical methods and equipment, which 

Figure 14. Mitigation and conjunctive use measures incorporated into the project. The private 

lands are outlined.
33

Ground 

Water Quality

are commonly used to assist hydrologists in their search for ground water and 

minerals, from these types of “water finders.” Federal employees should 

never expend public funds on the services of a dowser.

Protection and management of ground water resources includes the 

establishment and implementation of water-quality standards that are designed 

to (1) protect public health, (2) maintain legally established designated uses, 

and (3) minimize impacts to ground water-dependent ecosystems. 

One definition of water quality consists of the biological, chemical, and 

physical conditions of a water. Contamination can be defined as the 

introduction of substances into the hydrological environment that can adversely 

affect water quality as a result of human activities. Pollution then occurs when 

contaminant concentrations attain objectionable levels (in excess of applicable 

standards, health advisories, action limits, and so forth). 

Certain land uses are known to cause ground water contamination. Specific 

types of contaminants are associated with specific types of land uses and 

industries. The Office of Technology Assessment of the US Congress (1984) 

identified the following six categories of major sources of ground water 

contamination:

1. Sources designed to discharge substances—septic tanks, injection wells, 

land application.

2. Sources designed to store, treat, or dispose of substances—landfills, 

surface impoundments, mine waste, storage tanks.

3. Sources designed to retain substances during transport—pipelines, 

material transport and transfer.

4. Sources discharging substances as a consequence of other planned 

activities—irrigation, pesticide and fertilizer application, road salt, 

urban runoff, mine drainage.

5. Sources providing a conduit for contaminated water to enter aquifers—

wells, construction excavation.

6. Naturally occurring sources whose discharges are created or enhanced 

by human activity—ground water/surface-water interaction, natural 

leaching, saltwater intrusion.

Ground water quality is protected by Federal, State, local and tribal 

governments through rules and regulations aimed at managing these categories 

of contaminant sources. During the 1990s the EPA and State, local and tribal 

governments developed ground water protection strategies aimed at preventing 

ground water contamination. These strategies focus on proactive measures, 

including education, source-water protection, and utilization of public health 

authorities to prevent ground water contamination. Also, during the past 10 

years the watershed management approach has proven to be effective as a way 

to manage water resources, including ground water. 
34

Application of 

Water Quality 

Regulations to 

Ground Water

Designated uses of water that are protected against water-quality degradation 

include domestic, municipal, agricultural, and industrial supply; power 

generation; recreation; aesthetic enjoyment; navigation; and preservation and 

enhancement of fish, wildlife, and other aquatic resources or preserves. 

Water-quality standards that are applied to ground water have been established 

by the EPA and State and tribal governments as authorized under the Safe 

Drinking Water Act (SDWA), the Clean Water Act (CWA), and State and 

tribal laws and regulations. Standards are derived for constituents that may be 

harmful to human health or the environment and for constituents that affect 

other designated uses. The promulgated values established for individual 

constituents are often based on toxicological studies. Applicable standards 

for a particular aquifer or ground water system are determined based on State 

requirements that may include ground water classification systems, ground 

water cleanup goals, or ground water discharge permit requirements. Most 

States also have primacy for enforcing the SDWA water-quality standards 

for drinking water systems. The EPA has direct implementation authority for 

Indian reservations and other selected lands.

Water-quality standards promulgated under the SDWA apply to public-water 

systems as defined in the SDWA. A PWS is a system for the provision to the 

public of water for human consumption through pipes or other constructed 

conveyances, if such system has at least 15 service connections or regularly 

serves at least 25 individuals. Numeric standards include the Maximum 

Contaminant Levels (“MCLs”) and Secondary Maximum Contaminant Levels 

for public drinking-water supplies as established by the SDWA regulations. 

In some States, some numeric standards are set based on human-health 

risk assessment levels. In other States, the standards for some potential 

toxic pollutants, primarily pesticides, are set at laboratory detection limits 

(nondetectable levels).

Degradation has been defined as a change in water quality that lowers the 

quality of high-quality waters for a particular parameter. States determine 

whether a proposed activity may cause water-quality degradation based on 

information submitted by an applicant. Contaminants other than carcinogens 

are generally regulated under an anti-degradation policy for most natural 

waters. That policy allows for an increase in concentration of a contaminant 

in ground water, but does not allow for a standard to be exceeded. In at least 

one State (Montana), non-degradation rules apply to any activity resulting in a 

new or increased source that may cause water-quality degradation because of 

carcinogens. 

Some States have applied some existing surface water-quality standards to 

ground water through statutes or rules administered under State ground water 

protection programs. Some States have established preventive action limits 

as an early warning of the presence of pollution before beneficial uses are 

adversely affected. The purpose is to achieve more stringent protection for 
35

Ground Water 

Classification

higher quality ground water. For example, in Utah preventive levels are set in 

ground water discharge permits. The levels are set at 10 to 50 percent of the 

standard; if pollutant concentrations are detected that exceed the protection 

levels, then the source of the problem must be evaluated for potential 

correction.

In many States, ground water classification schemes are used to help 

determine which standards may be applicable to selected aquifers or ground 

water beneath certain areas. Schemes are typically based on the current 

and/or potential future beneficial uses of the resource and existing water 

quality. Examples are drinking-water use, agricultural use, and industrial use. 

Boundary criteria for the classified areas may be physically based or otherwise 

determined, such as an aquifer or aquifer zone, a watershed, or a permitted 

discharge facility. Total dissolved solids (TDS) concentrations and specific 

conductance are commonly used to define the various classes of ground 

water (table 1). The classifications are used to establish in situ water-quality 

standards for implementing ground water protection programs, permitting 

discharges to ground water, and setting cleanup goals at contaminated sites.

Table 1. A common ground water classification system based on TDS concentration (mg/L).

TDS* Classification

Less than 1,000 Fresh

1,000 to 3,000 Slightly brackish

3,000 to 10,000 Brackish

10,000 to 50,000 Saline

More than 50,000 Brine

*As a point of reference, the TDS concentration in seawater is approximately 35,000 mg/L.

Uncontaminated fresh ground water is generally suitable for human 

consumption, for livestock and other agricultural uses, and for most industrial 

uses. Slightly brackish water may not be suitable for those uses, depending on 

the relative amounts of the various major ions and trace elements. Brackish, 

saline, and brine waters are never suitable for human consumption. In some 

cases, brackish water can be used for livestock, but saline and brine waters 

never can (National Research Council 1981).

Most State ground water classification schemes are based on TDS. For 

example, in North Dakota and South Dakota ground water is classified as 

“potentially suitable” for drinking-water use if the TDS level is less than 

10,000 parts per million (ppm), and suitable for no specific beneficial uses if 

the TDS level exceeds 10,000 ppm. In North Dakota, a second classification 

system based on aquifer sensitivity is also used to prioritize ground water 

monitoring to track the occurrence of agricultural chemicals and to help 

determine State activities in the Underground Injection Control (UIC) Class V 

Program. 
36

Ground Water 

Discharge 

Permits

Total Maximum 

Daily Loads

In Colorado, a public hearing process in front of the State Water Quality 

Control Commission is required to classify specific ground water to set the 

applicable ground water-quality standards for protection and regulatory 

purposes. The classification scheme includes (1) quality for domestic use, (2) 

quality for agricultural use, (3) surface water-quality protection, (4) potentially 

usable quality, and (5) limited use and quality.

In many States, facilities that discharge waste or pollutants directly or 

indirectly into ground water (other than those regulated under the UIC or 

National Pollution Discharge Elimination System [NPDES]) may be required 

to apply for a ground water discharge permit. The goal of this program is 

to allow economic development while maintaining ground water quality; 

in most cases, a limited zone of pollution (mixing zone) is permitted and 

quarterly compliance monitoring is instituted by the permittee. Ground waterquality standards and/or protection levels are used to determine the discharge 

requirements.

Facilities required to apply for ground water discharge permits are identified 

in the regulations. For example, Colorado requires all facilities under certain 

standard industrial classifications to apply for permits and some of these 

facilities are covered under a general permit for the UIC program. In Utah, 

facilities that pose little or no threat to ground water quality or that are 

permitted by other State ground water protection programs (such as septic 

tanks and discharges from permitted RCRA units) receive a permit by rule.

Generally, a facility needing a permit submits information to the State 

that describes the extent and quality of the ground water, the volume and 

composition of the discharge, how the discharge will be controlled or treated 

to meet standards and/or protection levels, and proposed inspection and 

monitoring plans to ensure compliance with the terms of the permit. In some 

States, the permitting process requires a contingency plan to bring the facility 

into compliance in the event of a significant release of contaminants to ground 

water from the facility. In South Dakota, a discharge plan includes three 

permits: (1) a ground water-quality variance, (2) a facility construction permit, 

and (3) a discharge permit from the Ground Water Quality Program.

Regulations issued by EPA in 1985 and 1992, pursuant to the CWA, require 

the quantification of specific pollutants that impair the quality of surface-water 

bodies. The regulations require that total maximum daily loads (TMDLs) be 

established for selected streams or stream reaches that exceed water-quality 

standards because of contaminant loading. States typically have primacy for 

the TMDL program under their water-quality programs. The EPA is required 

to determine TMDLs if a State does not do so. While the TMDL programs 

typically focus on point-source loads to surface water, loading from ground 

water should be considered. It is not a requirement, however.
37

Ground 

Waterdependent 

Ecosystems

Water-quality 

Databases

The USGS, EPA, and many States maintain a number of water-related 

databases that contain water-quality information. Some of these systems are 

available on the Internet; however, access to some of them may necessitate a 

direct request to the right agency. 

The USGS National Water Information System stores data on surface water 

stages and flows, ground water elevations, and water quality (http://waterdata.

usgs.gov/nwis). The USGS National Water Quality Assessment Program is 

a commonly used source of information on ground water-quality available 

in the United States today. Under this program, USGS collects water-quality 

data in 60 special study regions of the country, conducts retrospective analyses 

of existing data (such as State data), and prepares national-scale syntheses of 

the results. Information from this program is also available separately on the 

Internet (http://water.usgs.gov//nawqa/data.html).

The EPA maintains the nationwide STORET database for water information, 

including water quality (http://www.epa.gov/storet/dbtop.html). The EPA is 

also developing a National Contaminant Occurrence Database (NCOD) to track 

contaminants in ground and surface sources of drinking water. The NCOD 

will aid in the identification and selection of contaminants for future drinkingwater regulations, support regulation development or other appropriate actions, 

and assist in the review of existing regulations for possible modification. The 

NCOD will incorporate data of documented quality from existing Federal 

databases on regulated and unregulated physical, chemical, microbial, and 

radiological contaminants, as well as other contaminants that are known or are 

likely to occur in the source and finished waters of PWSs of the United States 

and its territories.

Ground water-dependent ecosystems are communities of plants, animals and 

other organisms whose extent and life processes depend on ground water. The 

following are examples of some ecosystems that may depend on ground water:

•	 Wetlands in areas of ground water discharge or shallow water 

table.

•	 Terrestrial vegetation and fauna, in areas with a shallow water 

table or in riparian zones.

•	 Aquatic ecosystems in ground water-fed streams and lakes.

•	 Cave and karst systems. 

•	 Aquifer systems.

•	 Springs and seeps.

Ecological resources include sensitive fish, wildlife, plants, and habitats 

that are at risk from exposure to ground water contaminants or ground water 

depletion. Some examples are breeding, spawning, and nesting areas; early 

life-stage concentration and nursery areas; wintering or migratory areas; rare, 

threatened, and endangered species locations; and other types of concentrated-
38

population or sensitive areas. These areas contain ecological resources that 

potentially are highly susceptible to permanent or long-term environmental 

damage from contaminated or depleted ground water.

Ground water-dependent ecosystems vary dramatically in how extensively 

they depend on ground water, from being entirely dependent to having 

occasional dependence. Unique ecosystems that depend on ground water, fens 

for example, can be entirely dependent on ground water, which makes them 

very vulnerable to local changes in ground water conditions. Ground water 

extraction by humans modifies the pre-existing hydrologic cycle. It can lower 

ground water levels and alter the natural variability of these levels. The result 

can be alteration of the timing, availability, and volume of ground water flow 

to dependent ecosystems. 

Ground water-dependent ecosystems can be threatened by contamination and 

extraction. Particular threats include urban development, contamination from 

industry, intensive irrigation, clearing of vegetation, mining, and filling or 

draining of wetlands. In some caves and peatlands, scientific research into past 

environments relies upon the fossil record, and fluctuating water levels and 

changes in water quality can destroy this record.

The role ground water plays in controlling ecosystems on public land is poorly 

understood. Little information exists in the literature on this topic. Hatton 

and Evans (1998) provide an excellent discussion of ground water-dependent 

ecosystems in Australia, and the U.S. Bureau of Land Management (U.S. 

BLM 2001) discusses the occurrence, ecological values, and management 

of springs in the Western United States. Unseen and sometimes poorly 

understood, ground water nonetheless fundamentally controls the health of 

many ecosystems. 

Ground water-dependent ecosystems have many values, including the 

following: 

•	 Water-quality benefits. Microfauna in ground water help cleanup 

contaminants and may play an important, but not yet fully understood, 

role in maintaining the health of surface waters.

•	 Biodiversity value. Many species depend on habitats maintained by 

ground water discharge. They add to the ecological diversity of a region 

and can be indicators of the overall biological health of a system. Some 

plants and animals that depend on ground water are rare, unique, or 

threatened. The ecosystems in aquifers and caves may be among the 

oldest surviving on earth. They can be connected to other non-ground 

water-dependent ecosystems and thus integrated into many broader 

regional ecosystems. 
39

Types of Ground 

Water-dependent 

Ecosystems

Terrestrial 

Vegetation 

and Fauna

Ecosystems in 

Streams and 

Lakes Fed by 

Ground Water

•	 Archeological and social value. Some sites, such as springs, may have 

cultural significance, especially for Native Americans, and can have 

csocial, esthetic, and economic values.

Shallow ground water can support terrestrial vegetation, such as forests 

and woodlands, either permanently or seasonally (Baird and Wilby 1999). 

Examples occur in riparian areas along streams (Hayashi and Rosenberry 2002) 

and in upland areas that support forested wetland environments. Phreatophytes 

are plants whose roots generally extend downward to the water table and are 

common in these high-water-table areas. Some fauna depend on this vegetation 

and therefore indirectly depend on ground water. Terrestrial vegetation may 

depend to varying degrees on the diffuse discharge of shallow ground water, 

either to sustain transpiration and growth through a dry season or for the 

maintenance of perennially lush ecosystems in otherwise arid environments. 

Ground water-dependent terrestrial plant communities provide habitat for a 

variety of terrestrial, aquatic, and marine animals (U.S. BLM 2001), which by 

extension must also be considered ground water dependent. Some species are 

quite restricted to these habitats. For example, in Montana northern leopard 

frogs occur in fewer than six ponds or sloughs in the Flathead Lake watershed, 

and the northern bog lemming is known only from one fen complex in the 

Stillwater River watershed (Greenlee 1998). 

An additional group of ground water-dependent fauna (including humans) rely 

on ground water as a source of drinking water. Ground water, as river baseflow 

or discharge to springs, is an important source of water across much of the 

country, particularly in the Southwestern United States and other areas with 

semiarid climate. Its significance is greater for larger mammals and birds, as 

many smaller animals can obtain most of their water requirements from other 

sources.

Ranchers in the West have made extensive use of ground water to supply 

drinking water to grazing stock. In addition to watering stock, ground water is 

also used by native fauna. Provision of water has allowed larger populations 

of both wildlife and pest animals to be sustained than would otherwise be 

the case. Ground water-dependent vegetation and wetlands may be used 

by terrestrial fauna as drought refuges. Access to ground water allows the 

vegetation to maintain its condition and normal phenology (for example, nectar 

production, new foliage initiation, seeding). Populations of some birds and 

mammals retreat to these areas during drought and then recolonize drier parts 

of the landscape following recovery. The long-term survival of such animal 

populations relies on maintaining the vegetation communities and ensuring that 

their water requirements are met. 

This category of ecosystem includes many ecosystems that are dependent on 

ground water-derived baseflow in streams and rivers (Gilbert and others 1998). 

Baseflow is that part of streamflow derived from ground water discharge and 

bank storage. River flow is often maintained largely by ground water, which 
40

provides baseflow long after rainfall or snow melt runoff ceases. On average, 

up to 40 percent of the flow of many rivers is estimated to be made up of 

ground water-fed baseflow. The baseflow typically emerges as springs or as 

diffuse flow from sediments underlying the river and banks. This water may 

be crucial for in-river and near-river ecosystems (Stanford and Gonser 1998). 

Localized areas of ground water discharge have a largely stable temperature 

and provide thermal refuges for fish in both winter and summer (Hayashi 

and Rosenberry 2002). Ground water also influences the spawning behavior 

of some fish. Reducing the baseflow to ground water-fed rivers could reduce 

upwelling or dry out riffles and reduce spawning success.

The ambient ground water flux is likely to be the key attribute influencing a 

surface-water ecosystem’s dependency on ground water. The ground water 

level in riverine aquifers is important for maintaining a hydraulic gradient 

towards the stream that supports the necessary discharge flux. Sufficient 

discharge of ground water is needed to maintain the level of flow required 

by the various ecosystem components. Contamination of riverine aquifers by 

nutrients, pesticides, or other contaminants may adversely affect dependent 

ecosystems in baseflow-dominated streams. 

Lakes, both natural and human made, can have complex ground water flow 

systems (Fetter 2000). Lakes interact with ground water in one of three basic 

ways: (1) some receive ground water inflow throughout their entire bed; (2) 

some have seepage loss to ground water throughout their entire bed; and (3) 

others, perhaps most, receive ground water inflow through part of their bed 

and have seepage loss to ground water through other parts (Winter and others 

1998). Changes in flow patterns to lakes as a result of pumping may alter 

the natural fluxes to lakes of key constituents, such as nutrients. As a result, 

the distribution and composition of lake biota may be altered. Conversely, 

lakes perched well above local ground water year around may be immune to 

depletion of the underlying ground water system. 

The chemistry of ground water and the direction and magnitude of exchange 

with surface water significantly affect the input of dissolved chemicals to lakes 

(Hayashi and Rosenberry 2002). In fact, ground water can be the principal 

source of dissolved chemicals to a lake, even in cases where ground water 

discharge is a small component of a lake’s water budget. The importance of 

ground water is accentuated for dilute lakes (low TDS concentration), such as 

those in mountainous regions that rely on ground water as their primary source 

of dissolved solids and nutrients. In addition, a considerable proportion of 

the buffering capacity in many lakes is because of acid neutralizing capacity 

(ANC) contributed by influent ground water. ANC is particularly important for 

soft water lakes because of their extreme sensitivity to the adverse effects of 

acidic atmospheric deposition.

The transport of nutrients by ground water can be a significant source of 

water-quality degradation in lakes. Major sources of nutrient enrichment are 

inadequately designed and maintained household septic systems and nonpoint 
41

Hyporheic and 

Hypolentic 

Zones

Springs

pollution sources, such as construction-project and agricultural runoff. The 

Lake Tahoe Basin Framework Study Groundwater Evaluation (U.S. Army 

Corps of Engineers 2003) was designed to enhance the understanding of the 

role ground water plays in eutrophication processes that reduce lake clarity. 

The study revealed that ground water contributed 12 percent of the nitrogen 

loading and 15 percent of the phosphorous loading to Lake Tahoe. While best 

management practices in the Lake Tahoe Basin represent an important step 

toward improving lake clarity, BMPs do not always take into account effects on 

ground water of either the original practice or the BMP itself. 

The interface between saturated ground water and surface water in streams 

and rivers is a zone of active mixing and interchange between the two and is 

known as the hyporheic zone (Jones and Mulholland 2000, Stanford and Ward 

1988, 1993). In mountain streams with typical pool-and-riffle organization, 

ground water enters streams most readily at the upstream end of deep pools, 

and conversely, surface water moves into the subsurface beneath and to the 

sides of riffles (Harvey and Bencala 1993). The hyporheic zone is generally 

confined to the near stream area; however, in large alluvial or glacial outwash 

valleys (for example, Flathead River, MT) this zone may extend hundreds of 

feet away from the river channel. Hyporheic zones can be important for aquatic 

life (Gilbert and others 1998, Stanford and Ward 1993). In both gaining and 

losing streams, water and dissolved chemicals can move repeatedly over short 

distances between the stream and the shallow subsurface below the streambed. 

The spawning success of fish may be greater where flow from the stream 

brings oxygen into contact with eggs that were deposited within the coarse 

bottom sediment or where stream temperatures are controlled by ground water 

inflow. Upwelling of ground water provides stream organisms with nutrients, 

while downwelling stream water provides dissolved oxygen and organic matter 

to microbes and invertebrates in the hyporheic zone. This exchange zone is 

an important habitat for many invertebrates, and a refuge for some vertebrates 

during droughts and floods. 

A similar mixing zone, called the hypolentic zone, occurs at the interface 

between saturated ground water and surface water in lakes and wetlands. 

In many lakes, the most active portion of the hypolentic zone is located in 

the littoral zone in close proximity to the shoreline (Hunt and others 2003, 

McBride and Pfannkuch 1975). Distinct vegetation and aquatic communities 

are likely to be associated with focused and diffuse discharge of ground water 

(Rosenberry and others 2000).

Springs typically are present where the water table intersects the land surface. 

In fractured-rock terrain, springs are fed through faults or fractures. Springs 

are important sources of water to streams and other surface-water features. 

They also may be important cultural and aesthetic features. The constant 

source of water at springs leads to the abundant growth of plants, and many 

times to unique habitats for endemic species like spring snails (U.S. BLM 

2001). Ground water development can reduce spring flow, change springs from 
42

Aquifer, Karst, 

and Cave 

Ecosystems

Wetlands

perennial to intermittent, or eliminate springs altogether. Springs typically 

represent points on the landscape where ground water flow paths from different 

sources converge. Ground water development may affect the amount of flow 

from these different sources to varying extents, thus affecting the chemical 

composition of the spring water.

This category comprises the aquatic ecosystems that may be found in free 

water in cave and karst systems (Fetter 2000) and within aquifers themselves 

(Gilbert and others 1998). Aquifer ecosystems represent the most extended 

array of freshwater ecosystems across the entire planet (Gilbert 1996). Their 

fauna largely consists of invertebrates and microfauna. Very little is known 

about aquifer ecosystems under NFS lands, their importance for biodiversity, or 

their importance to the systems into which they discharge. 

Some ecosystems, such as floodplains, exist along a continuum between 

fully aquatic communities and fully aquifer communities (Danielopol 1989). 

Aquifer ecosystems are not confined to near-surface environments. Stygofauna 

(animals occupying cave or aquifer habitats) have been identified at depths 

of up to 600 meters (Longley 1992). Aquifer ecosystems are characterized 

by darkness, consistency, persistence of habitat, and low energy and oxygen 

availability. The organisms that inhabit these environments are often 

specialized morphologically and physiologically. Their stable and confined 

environment results in high levels of endemism and high proportions of 

relict species compared with surface environments. Some cave fauna may 

have changed very little over the last hundreds of millions of years. Recent 

work in northwestern Australia has identified entire major lineages (orders or 

classes) of stygofauna that are thought not to have been represented in surface 

ecosystems since the Mesozoic Era. 

Ground water level, flux, and quality are the three attributes of greatest 

significance to cave-karst and aquifer ecosystems. Ground water level 

and flux determine the amount of ground water available to support these 

ecosystems. Where the composition of aquifer ecosystems changes with depth, 

reductions in ground water levels may result in the loss of particular species 

or communities of aquatic organisms. Such aquifer ecosystems are highly 

specialized and may be lost entirely with changes in ground water level of only 

1 to 2 meters (Humphreys 1999).

Many aquifer ecosystems have developed in very stable environments. Subtle 

changes in ground water quality because of contamination by agricultural 

chemicals, sediment, or septic tank effluent may alter ecosystem function. The 

potential sensitivity of aquifer ecosystems to changes in ground water quality 

can make them useful as bioindicators (Gilbert 1996). 

Wetlands occur in widely diverse settings from coastal margins to floodplains 

to mountain valleys. Similar to streams and lakes, wetlands can receive inflow 

from ground water, recharge ground water, or do both. The persistence, size, 
43

Management 

Considerations 

and Protection 

Strategies

and function of wetlands are controlled by hydrologic processes active at each 

site (Carter 1996). For example, the persistence of wetness for many wetlands 

depends on a relatively stable influx of ground water throughout seasonal and 

annual climatic cycles. Characterizing ground water discharge to wetlands and 

its relation to environmental factors such as moisture content and chemistry 

in the root zone of wetland plants is a critical but highly challenging aspect of 

wetlands hydrology (Hunt and others 1999).

Wetlands can be quite sensitive to the effects of ground water pumping. This 

pumping can affect wetlands not only by lowering the water table, but also by 

increasing seasonal changes in the elevation of the water table and exposing 

accumulated organic and inorganic material to oxidation. Some peat-forming 

wetlands are highly stable environments that may contain fossil material that 

provides insights into past environments. Overextraction of water, like the 

draining of wetlands for agriculture and other development, can destroy this 

valuable source of scientific data. 

Fens are peat-forming wetlands that receive recharge and nutrients almost 

exclusively from ground water. The water table is at or just below the ground 

surface. Water moves into fens from upslope mineral soils, and flows through 

the fen at a low gradient. Fens differ from other peatlands because they are 

less acidic and have higher nutrient levels; therefore, they are able to support 

a much more diverse plant and animal community. Grasses, sedges, rushes, 

and wildflowers often cover these systems. Over time, peat may build up 

and separate the fen from its ground water supply. When this happens, the 

fen receives fewer nutrients and may become a bog. Patterned fens are 

characterized by a distribution of narrow, shrub-dominated ridges separated by 

wet depressions. 

In North America, fens are common in the northeastern United States, 

the Great Lakes region, the Rocky Mountains, the Cascade and Siskiyou 

Mountains, and much of Canada. They are generally associated with low 

temperatures and short growing seasons. Slow decomposition of organic matter 

allows peat to accumulate. Fens provide important benefits in a watershed, 

including preventing or reducing the risk of floods, improving water quality, 

and providing habitat for unique plant and animal communities. Like most 

peatlands, fens have experienced a decline in acreage, mostly from mining and 

draining for cropland, fuel, and fertilizer. Because of the large historical loss of 

this ecosystem type, remaining fens are rare, and it is crucial to protect them. 

While mining and draining these ecosystems provide resources for people, up 

to 10,000 years are required to form a fen naturally.

The Forest Service ground water policy is specifically designed to protect 

ground water-dependent ecosystems so that, wherever possible, the ecological 

processes and biodiversity of their dependent ecosystems are maintained, 

or restored, for the benefit of present and future generations. The general 

level of understanding of the role of ground water in maintaining ecosystems 
44

throughout the public lands is very low. Ground water resource managers 

and investigators tend to underestimate ecosystem vulnerability to ground 

water development, pollution, and land-use change. Planners must recognize 

ecosystem dependence on ground water and related processes. Perhaps such 

recognition can be best achieved by incorporating ground water resource 

inventory, monitoring, and protection into management plans. 

The initial step in protecting ground water-dependent ecosystems is developing 

an inventory of those systems on NFS lands. Identify and describe their 

locations, ecological values, and degrees of dependence on ground water. 

Land management plans should then be reviewed and revised as necessary to 

incorporate ground water-level, ground water extraction-rate, ground water 

recharge-rate targets or other management rules that minimize localized 

impacts on dependent ecosystems. The degree of protection will vary according 

to the characteristics and dynamics of each ground water system and the 

significance of the ground water-dependent ecosystems. Protection may range 

from minimal where the aquifer is deep and has little connection to the surface, 

to significant where the connection is strong and the conservation value of 

dependent ecosystems is high. More localized measures for protecting ground 

water-dependent ecosystems may include the following steps:

•	 Establishing buffer zones around dependent ecosystems, within 

which ground water extraction is excluded or limited.

•	 Establishing maximum limits to which water levels can be drawn 

down at a specified distance from a dependent ecosystem. 

•	 Establishing a minimum distance from a connected river, creek or 

other dependent ecosystem from which a well could be sited.

•	 Protecting ground water quality in areas that provide recharge to 

dependent ecosystems by limiting the types of activities that can 

take place there.

The social and economic costs of the recommended management prescriptions 

and protections, as well as the costs related to impacts from use, also need to be 

considered. Ground water extractions should be managed within the sustainable 

yield of aquifer systems so that the ecological processes and biodiversity 

of their dependent ecosystems are maintained or restored. In this process, 

threshold levels that are critical for ecosystem health should be estimated and 

considered. Planning, approval, and management of developments and land 

uses should aim to minimize adverse impacts on ground water systems by 

maintaining natural patterns of recharge and discharge, and by minimizing 

disruption to ground water levels that are critical for ecosystems.

Ground water-dependent ecosystems can have important values for ground 

water users, ecosystem managers, scientists, and the wider community. These 

values, and how threats to them may be avoided, should be identified in land 

management plans, and actions should be taken to ensure that the ecosystems 

are protected. 
45

Case Study: 

Importance of 

Ground Water 

in Alpine-lake 

Ecosystems, 

Cabinet 

Mountains 

Wilderness, MT

An investigation in the Cabinet Mountains Wilderness, Kootenai National 

Forest, in northwestern Montana was conducted to assess the potential for 

adverse impacts from ground water withdrawal from a proposed underground 

mine. The study was prompted by concerns that mining under the wilderness 

could modify ground water hydraulics in the fractured bedrock aquifer and 

adversely impact the water balance, chemistry, and ecology of the overlying 

wilderness lakes.

Ground water plays an important role in the chemical composition of lakes, and 

the aquatic ecology of lakes is defined, in large part, by their hydrochemistry. 

The importance of ground water is accentuated for dilute lakes, like those in 

the Cabinet Mountains, which rely on ground water inputs as their primary 

source of dissolved solids and nutrients. Even though the volume of ground 

water inflow to these lakes is a small fraction of the annual hydrological 

budget, during the short ice-free period when peak biological activity takes 

place, ground water inflow can contribute considerable amounts of water and 

solutes. 

Hydrological and chemical budget evaluations of Cliff Lake (fig. 15) and Rock 

Lake, two of the lakes overlying the Rock Creek ore body, were performed 

to help predict potential impacts from proposed mining. Nonsteady-state 

mass balances using naturally occurring tracers (solutes and stable isotopes) 

provided a means for estimating the quantity of ground water inflow into the 

lakes and evaluating the water balance over the short summer season. 

Over the summer, the chemical composition of the lakes shifts toward that 

of local ground water, indicating a direct hydraulic connection to the ground 

water system. Compared with solute mass fluxes from precipitation or surface 

water, ground water is the principle source of dissolved solute load (fig. 16). 

For Rock Lake, ground water supplies about 59 percent of the ice-free season 

inflow but contributes 71 percent of the solute load. Similarly, for Cliff Lake, 

ground water supplies about 83 percent of the inflow and 96 percent of the 

solute load. In addition, a considerable proportion of the buffering capacity is 

a result of the ANC contributed by ground water. ANC is important for dilute 

lakes, such as these, because of their extreme sensitivity to the adverse effects 

of acid deposition.

Unless a surface-water body is directly connected to the underlying ground 

water system being affected by such mining, it will not experience significant 

disruptions in water or chemical budgets. This study established that Rock 

Lake and Cliff Lake are directly connected to the ground water system. 

Depletion of ground water inflow by mining-induced changes in hydraulic 

gradients and ground water flow paths could cause a shift in the hydrological, 

chemical, and consequently, the biological composition of these lakes. 

For more information, see Montana Department of Environmental Quality and 

USDA Forest Service (2001) and Gurrieri and Furniss (2004). 
46

Figure 15. Cliff Lake, Cabinet Mountains Wilderness, MT.

Figure 16. Water and solute budgets for Rock Lake and Cliff Lake in percent of ice-free season inflow.
47

Activities 

That Affect 

Ground Water 

Mineral 

Development

Hardrock Mining

Ore Processing

This section describes some of the activities that commonly cause ground 

water problems on NFS lands. See appendix IV for a discussion of possible 

techniques for remediating existing ground water contamination.

Prospecting and developing mineral resources on NFS lands, including such 

materials as base and precious metals, oil and gas, coal, phosphate and gypsum, 

and aggregate and building stone, involve activities and land uses with the 

potential to significantly affect both the quantity and quality of the ground 

water resource associated with those lands. The primary issues associated with 

the major types of mineral development are presented below.

Hardrock mining is defined as the extraction of precious and industrial metals 

and nonfuel minerals by surface and underground mining methods (Lyon and 

others 1993). In the United States, extensive hardrock mining started in the 

1880s and, for the next 70 to 80 years, it was a major industry in many States. 

In 1992, more than 500 operating hardrock mines were located in the United 

States, of which more than 200 were gold mines. In 1997, approximately 60 

mine sites in 26 States were on the Federal Superfund National Priorities List.

Many ore bodies and mines (both old and operating) are on public land 

administered by the Federal land management agencies. They are frequently 

in areas with relatively little other development. During the first half of the 

20th century, environmental controls were very limited or nonexistent. As a 

result, numerous abandoned mines are currently causing serious environmental 

damage. Many thousand abandoned and inactive mines are on public land. 

The USDA Office of the Inspector General estimates that more than 38,000 

abandoned and inactive hardrock mines are located on land administered by the 

Forest Service.

The two primary methods used to mine metals and minerals include surface, 

or open-pit, mining and underground mining. Surface mining methods are 

typically used for shallow ore bodies or ore bodies that have a low metal or 

mineral value per unit volume of rock, while underground mining methods are 

typically used when the ore body is deep or occurs in veins. Hardrock mining 

is a large-scale activity that typically disturbs large areas of land. The siting of 

a mine is largely dictated by the location of the ore body. Because of the high 

waste-to-product ratios associated with mining most ore bodies, large volumes 

of mining-related waste are generated. Mine waste includes all of the leftover 

material generated as a result of mining and processing the ore. 

Ore processing, or milling, refers to the altering of ore rock to (1) create a 

desired size of product, (2) remove unwanted constituents, and (3) concentrate 

or otherwise improve the quality of the product. Applicable milling processes 

are determined based on the physical and chemical properties of the target 

metal or mineral, the ore grade, and environmental considerations. Each 

method creates its own set of potential contaminants.
48

Water Management

Amalgamation. In this process, metallic mercury is added to gold ore to 

separate the gold from the ore rock. When liquid mercury comes in contact 

with gold, it bonds with the surface of the gold particles (amalgamation). The 

mercury-coated gold particles coalesce or collect into a gray plastic mass. 

When this mass is heated, the mercury is driven off and the metallic gold 

remains. 

Flotation. The physical and chemical properties of many minerals allow for 

separation and concentration by flotation. Finely crushed ore rock is added 

to water containing selected reagents. These reagents create a froth that 

selectively floats some minerals while others sink. Common reagents include 

copper, zinc, chromium, cyanide, nitrate and phenolic compounds, and sulfuric 

acid and lime for pH adjustment. The waste (tailings) and the wastewater are 

typically disposed of in large, constructed impoundments.

Leaching. Leaching typically involves spraying, pouring, or injecting an acid 

or cyanide solution over crushed and uncrushed ore to dissolve metals for later 

extraction. The main types include dump, heap, vat, and in situ leaching. For 

each type, a nearby holding area (typically a pond) is used to store the pregnant 

solution prior to recovery of the desired metal using chemical or electrical 

processes. Once the desired metal is recovered, the solution is reused in the 

leaching process. 

In recent years, the most common and problematic technology has been 

cyanide heap leaching. In this process, the ore is usually crushed and is placed 

on a pad constructed of synthetic materials or clay. A leaching solution is 

sprayed or dripped over the top of the pile. Leaching can recover economic 

quantities of the desired mineral for months, years, or decades. When 

leaching no longer produces economical quantities of metals, the spent ore is 

typically rinsed to dilute or otherwise detoxify the reagent solution to meet 

environmental standards. If standards are met, the rinsing may be discontinued 

and the leached material may be allowed to drain. The spent ore is then 

typically left in place.

Management of water at large mine sites is a critical element of mine 

operation. At large mine sites that include a mill and a tailings impoundment, 

water management can be difficult and complex. The many management 

requirements include (1) the dewatering of open pits and/or underground 

mine workings, (2) the routing of surface runoff across mine sites, (3) the 

use and containment of water used for ore processing, and (4) the need to 

meet applicable water-quality standards for all discharges from the mine 

site. Historically, the management of water has not focused on prevention of 

environmental impacts. Nationwide, there have been numerous incidents in 

which contaminated water from a mine site has been improperly discharged to 

surface water and/or ground water. 
49

Waste Management

Both surface and underground mines typically extend below the local 

or regional water table. The ground water that flows into the mine pit or 

underground workings must be removed to maintain acceptable working 

conditions. In open-pit mines, this water is typically pumped out and 

discharged to nearby surface waters or ephemeral drainages. In underground 

mines, the water can be pumped out and similarly discharged or, under certain 

conditions, drainage adits can be constructed at or below the lowest mine level 

to allow for free drainage of the water entering the workings. Many precious 

metal ore bodies occur in mountainous terrains or regions of continental shield 

where the host rock is commonly comprised of igneous and/or metamorphic 

rocks. In these types of rocks, ground water occurrence and flow are controlled 

by the distribution and orientation of geologic structures, such as fractures, 

joints, and faults. In these types of hydrogeological settings, ground water 

inflow into mine workings largely occurs only where the mine workings 

intersect water-bearing structures.

Hardrock mining typically produces large volumes of solid waste, 

including overburden (spoil), development rock, waste rock, spent ore, and 

tailings. Waste rock, and in some cases development rock and spoil, can 

contain significant concentrations of metals, and therefore may present an 

environmental problem. In both surface and underground mining, extraction 

of ore waste materials requires the use of heavy equipment and explosives. 

The most commonly used explosive is ANFO, ammonium nitrate and fuel oil. 

Residual nitrogen in the waste rock and development rock can be leached out 

by precipitation and cause contamination of water resources.

Tailings are the waste solids remaining after ore processing. Commonly, 

tailings leave the mill as slurry consisting of 40 to 70 percent liquid and 

30 to 60 percent fine-grained solids. Tailings and the associated carriage 

water (usually mill process water) can contain significant concentrations of 

heavy metals and other contaminants. Most tailings are disposed in onsite 

impoundments. Historically, tailings impoundments were not lined and were 

located without consideration of potential environmental impacts on streams 

and floodplains. Modern tailings impoundment design often includes lowpermeability clay or synthetic liners designed to minimize seepage from 

the tailings, engineered caps designed to minimize infiltration of water into 

the tailings, and collection systems to capture leachate that collects within 

the impoundment. Some seepage from tailings impoundments is often 

unavoidable, and leachate may infiltrate to underlying ground water.

Spent ore is a waste material that is generated at mines that utilize a 

leaching process. The volume of spent ore can be very large and can contain 

environmentally significant residual amounts of leaching reagent and dissolved 

metals. Both spent ore and tailings need to be actively managed for years after 

mine closure to ensure that leachate does not contaminate underlying ground 

water.
50

Potential Impacts

to Ground Water

Resources

Mine Closure Closure of a mining operation occurs during a temporary shutdown of 

operations or when the facilities are permanently decommissioned. Depending 

on the type of mine, the size and nature of the area of disturbance, and the 

type of ore processing used, active management of the mine site and water 

management may be necessary for years or even decades after closure. Until 

recently, reclamation was limited to grading and revegetating waste materials 

and pits to minimize erosion and improve the visual landscape. Permanent 

closure now routinely includes some or all of the following: removal/disposal 

of stored fuels and chemicals; structure demolition; removal of unnecessary 

roadways and ditches; shaft and adit plugging; waste detoxification; capping 

of tailings and waste rock; backfilling pits; and active water management, 

including assuring that all applicable water-quality standards are met. In 

numerous cases, this has meant operating and maintaining a water-treatment 

facility. At sites where acid drainage is a problem, post-closure water treatment 

may be necessary for decades. 

Information on potential environmental impacts related to hardrock mining 

has increased greatly in recent years. Numerous investigations and published 

reports have documented the release of toxic metals to ground water and 

surface water resulting from mobilization and transport of metals from mines 

and mine-related facilities.

In hardrock mines, adits and shafts, underground workings, open pits, 

overburden, development rock and waste rock dumps, tailings impoundments, 

leach pads, mills, and process water ponds are recognized as potential sources 

of acidity, metals, sulfate, cyanide, and nitrate. If released in environmentally 

harmful concentrations, these contaminants can significantly reduce the quality 

and usability of both ground and surface waters. Dissolved metals in ground 

waters can make it unsuitable for consumption. If contaminated ground water 

provides baseflow to a stream, the aquatic health of the stream and riparian 

ecosystems can be impacted. The impacts can be long term and large scale. 

They differ with the physical and geological setting of the ore body, type of ore 

extracted, the mining method, the method of ore processing, the effectiveness 

of water management, and the nature of mine closure.

A variety of complex geochemical and hydrogeological processes control 

the transport, attenuation, and ultimate distribution of metals and other minerelated contaminants in ground water (Drever 1997). Dissolved contaminants 

are transported to aquifers through complex overland and subsurface pathways. 

This complexity, combined with the large scale of many mining activities 

and the numerous mine-related sources of contaminants, makes water-quality 

assessments and restoration and remediation of mine sites very difficult.

Precious and heavy metal ore bodies are typically found in fractured-rock 

hydrogeologic settings. The extraction and processing of ore over the past 100 

years has resulted in the release of heavy metals into the aquatic environment 

in mining districts across North America. During the past 10 years, research 
51

has shown that ground water flow can deliver significant metal loads to 

streams in mountainous areas. Adequate control of metal mobility at active 

and abandoned hardrock mine sites requires a good understanding of the local 

fractured-flow system and its geochemical conditions. The two major types 

of potential long-term quality impacts to ground water, acid drainage and 

dissolution and transport of contaminants, are discussed below.

Acid Drainage. A major problem at some hardrock mine sites is the formation 

of acid drainage, also known as acid rock drainage (ARD) or acid mine 

drainage (AMD), and the associated mobilization of toxic metals, iron, 

sulfate, and TDS. ARD results from the exposure of sulfide minerals (such 

as, pyrite, pyrrhotite, galena, sphalerite, and chalcopyrite) to air and water. 

Sulfide minerals are commonly associated with coal deposits and precious 

and heavy metal ore bodies. Pyrite (FeS2), the most common sulfide mineral, 

reacts with water and oxygen to produce ferrous iron (Fe+2), sulfate (SO4), and 

acid (H+). In oxygenated water with a pH greater than 3.5, ferrous iron will 

oxidize to ferric iron (Fe+3), much of it will then precipitate as iron hydroxide 

(Fe[OH]3), and additional acidity will be released. Some ferric iron remains 

in solution and continues to chemically accelerate the further oxidation of 

pyrite and subsequent generation of acidity. As the pH continues to decrease, 

the oxidation of ferrous iron and the precipitation of iron hydroxide decreases. 

The result is a greater dissolved concentration of ferric iron and therefore a 

greater rate of sulfide (pyrite) oxidation. The oxidation of sulfide minerals 

can be catalyzed by bacteria; Thiobacillus ferrooxidans is one example. 

This bacterium, which is common in the subsurface, can increase the rate of 

sulfide oxidation by 5 or 6 orders of magnitude. When low pH water comes 

in contact with metal-bearing rocks and minerals, a number of toxic metals 

go into solution and are transported by the water. Different metals dissolve 

over different ranges of pH. The most common metals associated with sulfide 

minerals include lead, zinc, copper, cadmium, and arsenic.

Water, oxygen, and sulfide minerals are necessary ingredients to generate acid 

drainage. Water serves as both a reactant and as a medium for the oxidation 

process. Water also transports the oxidation reaction products and the 

associated dissolved metals. Atmospheric oxygen is a very strong oxidizing 

agent and is important for bacterially catalyzed oxidation at pH values below 

3.5. Surface water and shallow ground water typically have relatively high 

concentrations of dissolved oxygen.

Acid drainage can be discharged from underground mine workings, open-pit 

walls and floors, tailings impoundments, waste rock piles, and spent ore from 

leaching operations. It can also be released naturally from mineralized rock 

located at or near the surface; though, anthropogenic activity in such areas can 

enhance its release. It occurs at both active and abandoned mines. No easy 

or inexpensive solutions to acid drainage are currently available. The best 

approach is to avoid development of a problem through appropriate upfront 

planning and analysis. An appropriate management approach to possible acid 
52

generation is to isolate or otherwise segregate and specially handle wastes 

with acid generation potential. Oxygen contact and water contact with the 

isolated material should be minimized. Another approach is to ensure that an 

adequate amount of natural or introduced material is available to neutralize 

any acid produced. The neutralization approach, however, may not adequately 

address all of the solutes that could be released into solution during the 

oxidation process. Techniques used to isolate acid-generating materials include 

subaqueous disposal, barrier covers, waste blending, hydrologic controls, and 

bacterial control.

Transport of Dissolved Contaminants. Dissolved contaminants (primarily 

metals, sulfate, and nitrate) can migrate from mining operations to underlying 

ground water and surface water. Process water, mine water, and runoff and 

seepage from mine waste piles or impoundments can transport dissolved 

contaminants to ground water. The likelihood of contaminants dissolving 

and migrating from mine waste materials or mine workings to ground water 

depends on the nature and management of the waste materials and liquids, the 

local hydrogeological setting, and the geochemical conditions in the underlying 

vadose zone and aquifer. 

Distinguishing between “natural” or background metal loadings and those 

resulting from mining is an issue that often arises at hardrock mine sites. 

A number of studies have attempted to separate “natural” loading from 

anthropogenic loading (Nimick and von Guerard 1998). Researchers have 

used water-quality data, including isotopes and tracers, to try to “fingerprint” 

water in an attempt to identify loading caused by leaching of unmined ore 

bodies from leaching of metals that is enhanced by mining activities. To date, 

however, no reliable technique has been developed to clearly separate these 

two general sources of loading.

At some locations, naturally occurring substances other than the target minerals 

can be a significant source of contaminants. The rocks that comprise ore 

bodies contain varying concentrations of nontarget minerals, often including 

radioactive minerals. Other minerals may be present at concentrations that 

can be toxic and can be mobilized by the same geochemical and hydrological 

processes that control transport of contaminants from mine sites. Nontarget 

substances that can pose a risk to ground water include aluminum, arsenic, 

asbestos, cadmium, chromium, copper, iron, lead, manganese, mercury, nickel, 

silver, selenium, sulfate, thallium, and zinc.

The impacts from mining can last for many years. As a result, environmental 

monitoring (including early warning, facility specific, and compliance 

monitoring), contingency planning and financial assurance have to be in place 

for many decades. Geochemical conditions within the ore body, waste rock, 

and tailings can change over time and must be tracked. Flexibility therefore is 

needed to make necessary changes in water control and water treatment after 

mine closure.
53

Case Study: 

Depletion of Stream- flow by Underground

Mining, Stillwater

Mine, Custer

National Forest, MT

Dewatering of shallow aquifers that are directly connected to surface water 

bodies can have a significant effect on the movement of water between these 

two water bodies. In mountainous terrain, the fracture-dominated ground water 

flow system adds complexity to predicting or monitoring the effects from 

mine dewatering. The disappearance of No Name Creek at the Stillwater Mine 

illustrates what can happen to surface water resources when mining disrupts 

the underlying ground water flow system.

The Stillwater Mine is an underground platinum and palladium mine located 

on the Custer National Forest in south-central Montana. The ore body is part of 

the Stillwater Complex, a vertically dipping, Precambrian-aged igneous rock 

unit. The mine began operations in 1986 and in July 1987 began developing the 

East Adit. After driving the adit about 4,000 feet, a large inflow of water was 

encountered that peaked at 884 gpm on May 25, 1988 (fig. 17). By July 1988, 

the inflow had decreased to its present steady-state rate of 200 gpm. 

Overlying the adit is a 60-acre watershed that contained a perennial stream 

called No Name Creek. Baseflow of the stream was supported by a bedrock 

fracture spring located 830 feet vertically above the adit (fig. 18). At the same 

time the large adit inflow was encountered, No Name Creek began to dry up. 

By July 1988, the stream and another spring near the portal ceased to flow and 

have remained dry ever since. 

Under predevelopment conditions, the ground water system was in a state of 

dynamic equilibrium and ground water discharging at the spring maintained 

the baseflow in No Name Creek. A new state of dynamic equilibrium was 

achieved after development of the adit and ground water that previously 

discharged to the spring was intercepted by the adit and now discharges 

out the portal (fig. 18). Tunneling activities induced a downward hydraulic 

gradient in the overlying fractured bedrock aquifer, and subsequent lowering 

of the potentiometric surface in the aquifer caused the spring to stop flowing. 

The enhanced vertical permeability along preexisting fractures created by the 

vertically dipping rocks likely contributed to the strong hydraulic connection 

between the adit and the overlying spring. 

Interestingly, the flow of Nye Creek located adjacent to No Name Creek 

and also overlying the adit was not affected by the initial tunneling. In 1994, 

however, three springs in the upper Nye Creek basin were rendered dry by 

continued underground development of the ore body. Potential mitigation 

measures have been discussed. The most promising is grouting off the 

inflows in the underground adit. This effort could reestablish the spring as 

well as the baseflow of No Name Creek. This case illustrates consequences 

of ground water depletion and the difficulty of predicting the spatial as well 

as the temporal impacts from human activities on ground water/surface water 

interactions in a fractured bedrock aquifer. 
54

Figure 17. Hydrographs of No Name Creek and flow rate of the East Adit, Stillwater Mine, MT. 

Figure 18. Before mining, ground water discharged to the spring and maintained the flow of No Name Creek. During 

development of the East Adit, ground water that would have discharged to the spring was intercepted and diverted into the adit.
55

Coal Mining

Potential Impacts

to Ground Water

Resources

Coal accounts for one-third of the total energy usage and more than one-half 

of the electricity generated in the United States (USGS 1996). In 1998, total 

domestic production was 1.18 billion tons (National Mining Association 1999). 

Coal production in the West has almost doubled since 1991. Wyoming leads 

the nation in coal production; West Virginia and Kentucky are second and third, 

respectively. About 60 percent of domestic production is from surface mines 

and 40 percent from underground mines. On NFS lands, active coal mining 

occurs in Utah, Wyoming, Colorado, and West Virginia.

Strip mining is the most common method of producing coal from surface 

mines. Strip mining commonly includes the removal and storage of topsoil, the 

removal of any overburden material (spoil), and the subsequent excavation of 

the coal seam. As an individual “strip” advances across the land surface, only 

a relatively small area of the coal seam is actively mined. With this method, 

the spoil is removed from the advanced side of the active mine face and 

concurrently placed on the retreat side where the coal has been mined out. 

Two methods of underground mining are commonly used: (1) room and pillar, 

and (2) longwall. In room and pillar mining, “entries” or adits are driven into 

the coal seam and crosscuts are driven at right angles to the adits at spacings 

dictated by the individual mine plan. The result is a checkerboard pattern 

of interconnected tunnels or “rooms” and unmined supports or “pillars.” In 

longwall mining, numerous crosscuts are developed around a large block of 

coal. Once the crosscuts are fully developed the large block is completely 

excavated. Longwall mining results in fairly predictable subsidence of the 

overlying ground surface. 

In surface coal mines, dewatering may be required to lower the water table 

so that mining can proceed. Depending on the stratigraphic occurrence of 

the coal beds and the aerial extent of the economic coal seams, dewatering 

can result in a cone of depression that can extend for miles in the upgradient 

direction. Water levels can be lowered in ground water wells that are in the 

same hydrostratigraphic unit as the coal. Coal beds are often characterized 

by high hydraulic conductivity, and the associated high transmissivity 

often makes them attractive for accessing ground water for domestic use, 

livestock, and irrigation. It is not uncommon for coal-mining companies to 

enter into agreements with well owners to provide alternative water supplies 

if domestic, stock, or irrigation wells are impacted. Dewatering can also 

reduce ground water discharge to wetlands and springs, particularly if the coal 

beds to be mined occur in a confined hydrostratigraphic unit. In this type of 

hydrogeological setting, a small lowering of the potentiometric surface can 

cause a significant reduction in ground water discharge to surface waters.

Waste materials are generated from coal mining and coal preparation. Spoil 

materials removed for surface mining are often used to backfill the excavated 

area. Waste material from underground mining is disposed of in mined-out 

workings to the extent possible, but it often is placed in a designated waste 

rock disposal area on the surface. The waste material from the coal preparation 
56

Oil and Gas 

Exploration and 

Development

process (both coarse material and fine-grained slurry) is typically disposed 

in disturbed portions of the mine site. The fine-slurry waste is commonly 

disposed of in an impoundment, where the slurry solid settles and the water can 

be reclaimed.

As with precious metal mining, coal mining can expose sulfide minerals to 

oxygen, water, and bacteria. Pyrite and, less commonly, marcasite (FeS2) and 

greigite (Fe3S4) are the primary sulfide minerals found in coal and adjacent 

rock. Oxidation of these minerals can generate acidic water and mobilize 

and transport heavy metals to ground water and surface water. Mine waste 

and coal preparation waste can contain significant amounts of pyrite and 

metals, including cadmium, chromium, mercury, nickel, lead, and zinc. These 

metals and sulfur can be concentrated in waste materials by factors of 3 to 

10 compared to raw coal (National Research Council 1979). Therefore, acid 

drainage and associated mobilization of metals and sulfate are potentially 

significant threats to ground water resources from coal mining. 

Underground coal mining using the longwall extraction method directly leads 

the overlying strata to break and fracture as subsidence occurs. Room and pillar 

extraction can also lead to fracturing and subsidence. This fracturing of the 

overlying strata changes the intrinsic permeability of the strata, and can alter 

ground water flow paths, create areas of increased permeability, and cause 

fluctuations in the water table. Any changes to the ground water can take years 

to establish a new equilibrium. Where the overlying rock strata are thin (less 

than about 600 feet) between the mined coal seam and the land surface, rock 

fracturing associated with longwall mine subsidence can also directly affect 

surface water. With respect to ground water, shallow aquifers could drain 

into subsidence fractures, or surface waters and recharge could be diverted 

into fractures. Sometimes, underground mining can encounter faults in the 

subsurface. The faults can sometimes contain ground water that discharges 

into the underground mine. The effects discussed above, however, do not occur 

everywhere, and the local geology, occurrence of ground water and surface 

water, and mining scenario must be evaluated carefully to ensure an adequate 

understanding of a particular site. 

Although geophysical and geological investigations are useful for oil and 

gas exploration, only exploratory drilling can confirm the presence of 

commercially valuable oil and gas reserves. Tens of thousands of exploration 

holes are completed every year. The majority of these wells are “dry” holes, 

meaning that no commercially significant quantities of oil and gas are 

encountered. Oil and gas companies are required to properly plug and abandon 

“dry” holes as well as exploration and production wells and injection and 

disposal wells that are no longer in use. Plugging and abandonment must 

be completed in accordance with State law. By 1993, more than 3.3 million 

wells had been drilled in the United States by the petroleum industry, and 

approximately 1.2 million wells had been plugged and 1 million had been 

abandoned or were inactive (American Petroleum Institute 1993). 
57

Potential Impacts

to Ground Water

Resources

Oil and gas contained in geologic formations is often not under sufficient 

hydraulic pressure to flow freely to a production well. The formation may 

have low permeability or the area immediately surrounding the well may 

become packed with cuttings. A number of techniques are used to increase or 

enhance the flow. They include hydraulic fracturing and acid introduction to 

dissolve formation matrix and create larger void space. The use of these flowenhancement techniques and secondary recovery methods result in physical 

changes to the geologic formation that will affect the hydraulic properties 

of the formation. Typically, the effects of these techniques and methods are 

localized to the area immediately surrounding the individual well, are limited 

to the specific oil and gas reservoir, and do not impact adjacent aquifers.

The Forest Service plays only a partial role in the regulation of oil and gas 

production activities on NFS lands under lease for oil and gas. The BLM 

oversees oil and gas drilling on NFS lands and is the formal leasing agency. 

The Forest Service only has responsibility for surface activities and surface 

impact evaluation. The EPA and State agencies regulate the disposal of wastes 

generated by the development and production of oil and gas. Underground 

waste disposal is regulated under the UIC program, which was authorized 

under the SDWA. RCRA conditionally exempted wastes associated with 

exploration, development, and production of oil and gas from regulation as 

a hazardous waste. Exempted wastes include well completion, treatment and 

stimulation fluids, workover wastes, packing fluids, and constituents removed 

from produced water before disposal.

Exploration, development, and production of traditional oil and gas resources 

typically do not significantly deplete ground water. Oil and gas resources are 

often developed from geological reservoirs that do not contain significant 

amounts of freshwater; however, the development and production of oil and 

gas can affect adjacent or nearby aquifers. Potential impacts result from the 

creation of artificial pathways between oil and gas reservoirs and adjacent 

aquifers. Modification of ground water flow paths may cause fresh ground 

water to come in contact with oil or gas. In addition, improper disposal of 

waste waters (brine, storm runoff), drilling fluids, and other wastes can impact 

the quality of underlying ground water (U.S EPA 1987).

A high risk of fluid migration exists along the vertical pathways created 

by inadequately constructed wells and unplugged inactive wells. Brine or 

hydrocarbons can migrate to overlying or underlying aquifers in such wells. 

This problem is well known in the oil fields around Midland, TX. Since 

the 1930s, most States have required that multiple barriers be included in 

well construction and abandonment to prevent migration of injected water, 

formation fluids, and produced fluids. These barriers include (1) setting surface 

casing below all known aquifers and cementing the casing to the surface, and 

(2) extending the casing from the surface to the production or injection interval 

and cementing the interval. Barriers that can be used to prevent fluid migration 

in abandoned wells include cement or mechanical plugs. They should be 

installed (1) at points where the casing has been cut, (2) at the base of the 
58

Coal-bed 

Methane

lowermost aquifer, (3) across the surface casing shoe, and (4) at the surface. 

Individual States and the BLM have casing programs for oil and gas wells to 

limit cross contamination of aquifers.

Coal-bed methane is natural methane gas that is produced during the 

transformation of plant and other organic material to coal (a process called 

coalification) and subsequently trapped in coal beds (DeBruin and others 

2001). As the coalification process proceeds and lignite, sub-bituminous, and 

bituminous coal are formed, various gases, including methane, carbon dioxide, 

and nitrogen are released. These gases can then be trapped in the coal beds by 

ground water pressure. Two types of methane gas can be created during the 

coalification process: (1) biogenic methane, which is produced by bacterial 

activity; and (2) thermogenic methane, which is produced by heating, usually 

during burial. Coal-bed methane can be stored in four different ways within 

coal beds: (1) as a free gas within micropores, (2) as dissolved gas in ground 

water that occurs within the coal beds, (3) as adsorbed gas, and (4) as absorbed 

gas. 

Economically viable coal-bed methane resources can occur in coal fields 

that include shallow, thick, laterally continuous coal beds. Historically, coalbed methane production focused on high-rank, high-gas-content coal beds. 

Recently a new production technique has been developed that makes it more 

economical to produce methane from shallow, low-gas-content coal beds. 

Using this technique, coal-bed methane well casings are set to the top of the 

target coal bed, and the underlying target zone is reamed. A submersible pump 

is then used to pump water up the tubing, and the methane gas separates from 

the water and flows up the annulus. The flow of methane gas up the annulus 

is facilitated by a decrease in hydraulic head because of dewatering. At the 

wellhead, gas is piped to a compressor and the “produced” water is discarded. 

Coal-bed methane wells go through three stages of production: (1) dewatering 

stage—water production exceeds gas production, (2) stable production stage—

maximum methane production and stable water production, and (3) declining 

stage—methane production declines until it becomes uneconomic. 

In some locales, the production of coal-bed methane requires that large 

volumes of ground water be pumped out of the coal seams to recover the 

gas. These amounts can vary widely depending on the local hydrogeological 

regime. The depletion and disposal of the “produced” ground water is a 

significant water-management issue. Because the annual amount of ground 

water produced from a coal-bed aquifer can easily exceed the annual recharge, 

removing large volumes of ground water can lower local and even regional 

aquifer water levels. The result can be reduced yields and increased pumping 

costs for wells developed in these aquifers. It is fairly common for companies 

that produce coal-bed methane to enter into agreements to provide water to 

owners of impacted wells. In most coal-bed methane production areas great 

uncertainty exists as to how long it will take to recharge the depleted aquifers 
59

Disposal of Production Water

after methane production has ceased. Depending on the disposal method, the 

use of ground water resources in coal-bed methane production areas may be 

severely limited for years or decades into the future.

The Western Governors’ Association has published the handbook Coal Bed 

Methane Best Management Practices (Western Governors’ Association 2004). 

The reader is advised to refer to it. 

The quality of coal-bed methane “produced” water can vary significantly. 

The quality of some ground water contained in coal beds is very good and is 

sometimes used for domestic consumption. Ground water that occurs in coal 

bed aquifers can contain significant concentrations of cations such as sodium, 

calcium, and magnesium. Many cations are readily sorbed to clay particles 

and can be easily exchanged for other cations. Excess sodium sorbed to clay 

soil will cause the soil to swell and reduce the soil permeability. The sodium 

adsorption ratio (SAR) is a measure of the ratio of sodium to calcium plus 

magnesium and is used to provide an indication of the degree to which free 

sodium ions could occupy exchange sites on clay particles. High SAR values 

can indicate that the use of water for irrigation purposes should be limited. This 

is an important issue where “produced” water is discharged to streams above 

locations where stream water is diverted for irrigation of crops.

Ground water from coal-bed methane wells is most often disposed of by 

direct discharge to ephemeral or intermittent streams. Other disposal methods 

include direct discharge to perennial streams, disposal through shallow or 

deep injection wells, and recharge to the subsurface through infiltration from 

recharge basins (Wireman 2002). It is important to adequately evaluate the 

technical and environmental issues associated with the disposal of ground 

water produced as a result of coal-bed methane production. Disposal of 

“produced” water through injection wells or via infiltration from recharge 

basins or spray-irrigation areas facilitates ground water recharge and can result 

in lower net loss of the resource. Disposal to perennial streams is more legally 

complicated and may require a National Pollution Discharge Elimination 

System (NPDES) permit or the State equivalent. 

Whether recharge to the subsurface via infiltration or injection is a viable 

disposal option for “produced” water will depend on a number of legal, 

engineering, and hydrogeological factors. Legal factors that need to be 

considered include permitting requirements and the potential for infiltrated or 

injected water to resurface in nearby drainage channels. Engineering factors 

include cost, geotechnical considerations, and operation and maintenance 

requirements. Hydrogeological factors that need to be considered include (1) 

the volume, rate and quality of water to be disposed, and (2) the hydraulic and 

chemical characteristics of the soils/rock to receive the water. In addition, for 

recharge basins or irrigation areas the thickness and hydraulic properties of the 

unsaturated zone beneath the recharge basin are important. The construction 

and use of recharge basins or injection wells may need to be permitted. The 
60

Ground Water 

Pumping

Declining Water 

Levels

need for a permit and the permit conditions will depend, in part, on whether or 

not the infiltrated water will discharge back to the land surface at some distance 

from the recharge basin or injection well or discharge directly to a nearby 

stream or lake. 

It is important to site recharge basins in locations where hydrogeological 

conditions will prevent or minimize the local discharge of infiltrated water. 

The rate of infiltration depends on the infiltration capacity of the soil or 

sediment underlying the recharge basin. The rate of infiltration will decline 

from an initial faster rate to an approximately constant rate for water with 

low suspended solids and low to moderate dissolved solids. For any given 

soil or sediment type a limiting curve defines the maximum possible rates of 

infiltration versus time (Horton 1933). The final constant rate is lower for clay 

soils with fine pores than for open-textured sandy soils or sediments. The final 

constant infiltration rate is numerically equivalent to the saturated hydraulic 

conductivity of the soil or sediment (Rubin and Steinhardt 1963, 1964). The 

latter can be determined from saturated hydraulic conductivity data, which are 

more readily available. 

As surface water resources become fully developed and appropriated, ground 

water commonly offers the only available source for new development. In 

many areas of the United States, however, pumping of ground water has 

resulted in significant depletion of ground water storage (Alley and others 

1999). These ground water depletions can result in lowered water levels in 

wells, hydraulic interference between pumping wells, reduced surface water 

discharge, land subsidence, and adverse changes in water quality.

It is useful to consider three terms that have long been associated with ground 

water sustainability: (1) safe yield, (2) ground water mining, and (3) overdraft. 

The term “safe yield” commonly is used in efforts to quantify sustainable 

ground water development. The term should be used with respect to specific 

effects of pumping, such as water-level declines, reduced streamflow, and 

degradation of water quality. The consequences of pumping should be assessed 

for each level of development, and safe yield should be taken as the maximum 

pumpage for which the consequences are considered acceptable. The term 

“ground water mining” typically refers to a prolonged, progressive, and, in 

many cases, permanent decrease in the amount of water stored in a ground 

water system. This phenomenon may occur, for example, in heavily pumped 

aquifers in arid and semiarid regions. Ground water mining is a hydrologic 

term without connotations about water-management practices (U.S. Water 

Resources Council 1980). The term “overdraft” refers to withdrawals of ground 

water from an aquifer at rates considered to be excessive and therefore carries 

the value judgment of overdevelopment. Thus, overdraft may refer to ground 

water mining that is considered excessive as well as to other undesirable effects 

of ground water withdrawals.
61

Figure 19. Pumping a single well in an idealized unconfined aquifer. Dewatering occurs in a 

cone of depression of unconfined aquifers during pumping by wells (Alley and others 1999).

Pumping ground water from a well always causes (1) a decline in ground water 

levels at and near the well; and (2) a diversion of ground water to the pumping 

well that was moving slowly to its natural, possibly distant, area of discharge 

(fig. 19). Pumping of a single low-capacity well typically has a local effect on 

the ground water flow system. Pumping of high-capacity wells or many wells 

(sometimes hundreds or thousands of wells) in large areas can have regionally 

significant effects on ground water systems. Where a new pumping well is 

installed near an existing pumping well and both are tapping the same aquifer, 

overlapping cones of depression (well interference) can result (Fetter 2000). 

The effect on the existing well from pumping the new well is lowered water 

levels, an increased rate of decline, and reduced yield. In addition, changes in 

water chemistry at the existing well can result. The new well likewise has a 

lower yield than if it had been placed farther from the existing pumping well. 

Ground water heads respond to pumping to markedly different degrees in 

unconfined and confined aquifers. Pumping the same quantity of water from 

wells in confined and in unconfined aquifers initially results in much larger 

declines in heads over much larger areas for the confined aquifers. This is 

because less water is available from confined aquifers for a given loss of head 

compared to similar unconfined aquifers.
62

Figure 20. Comparison of drawdowns after 1 year at selected distances from single wells that 

are pumped at the same rate in idealized confined and unconfined aquifers (Alley and others 

1999).

As might be expected, declines in heads and associated reductions in storage 

in response to pumping can be large compared to changes in unstressed 

ground water systems. For example, declines in heads as a result of intense 

pumping can reach several hundred feet in some hydrogeological settings. 

Drawdown is typically larger in confined aquifers (fig. 20). Widespread 

pumping that is sufficient to cause regional declines in aquifer heads can 

result in several unwanted effects: (1) substantially decreased aquifer storage, 

particularly in unconfined aquifers; (2) dried up wells in places because the 

lowered heads are below the screened or open intervals of these wells; (3) 

decreased pumping efficiency and increased pumping costs because the vertical 

distance that ground water must be lifted to the land surface increases; (4) 

changed rates of movement of low quality or contaminated ground water and 

increased likelihood that the low quality or contaminated ground water will be 

intercepted by a pumping well; and (5) land subsidence or intrusion of saline 

ground water may result in some hydrogeologic settings.
63

Perennially flowing springs can be adversely affected by too much water 

well pumping. Flows may diminish or cease if too much pumping occurs in 

an aquifer where a hydrologic connection exists between a spring and a well. 

Many examples of this phenomenon can be found in all parts of the United 

States. The same holds true for surface streamflows, especially during baseflow 

periods and in times of drought when all of the streamflow comes from ground 

water discharge. 

Depletion of ground water also can lower water levels in lakes, ponds, 

wetlands, and riparian areas. Water temperatures can rise from solar heating 

of smaller volumes of water and depletion of cooler ground water inflows. 

In turn, geochemical reaction rates may increase and affect the organisms in 

those waters, possibly to their detriment. Algae blooms are more likely in these 

lakes, ponds, and reservoirs, and when the algae die, fall to the bottom, and 

decompose, dissolved oxygen is consumed in the water body, causing stress to 

or killing fish and other aquatic species.

Where the depletion of ground water causes a decline in surface water or even 

total stream dewatering, terrestrial species may be adversely affected similarly 

to aquatic species. If any species so affected are listed under the Endangered 

Species Act of 1973, the Forest Service line officer has a duty to consult with 

the appropriate agency responsible for administering that act (U.S. Fish and 

Wildlife Service or National Marine Fisheries Service) and implement its 

recommendations for species protection or recovery. Recommendations can 

include modifying or canceling an authorization for water extraction from NFS 

land. 

Land subsidence is a gradual settling or sudden sinking of the Earth’s surface 

because of subsurface movement of earth materials. More than 80 percent 

of the identified subsidence in the United States is a consequence of human 

impact on subsurface water. This effect is an often-overlooked environmental 

consequence of our water-use practices. Impacts from land subsidence include 

damage to manmade structures, such as buildings and roads, as well as 

irrecoverable damage to aquifers. Subsidence is a global problem. In the United 

States, more than 17,000 square miles in 45 States have been directly affected 

by subsidence. In the late 1980s, the estimated annual costs in the United States 

from flooding and structural damage caused by land subsidence exceeded $125 

million (National Research Council 1991). This section provides an overview 

of land subsidence principles and impacts. Detailed information on subsidence 

is provided by Galloway and others (2003). 

In some areas, excessive pumping can cause the collapse of the framework 

of aquifer materials. The result is aquifer compaction and subsidence at the 

land surface (fig. 21). This compaction results in the permanent loss of aquifer 

storage, even if the water table should later recover when pumping stops. 

Although the water table may recover to prepumping levels, resumption of 

pumping will result in rapid drawdown because of the loss of aquifer storage 

capacity. In some parts of Florida, the lowering of the water table from 

Land Subsidence
64

pumping has resulted in sinkhole development. Subsidence resulting from 

drainage of organic soils is a problem in wetland areas, and such changes can 

adversely affect wetland ecosystems. Subsidence also can severely damage 

building foundations, roads, and buried pipelines, and can increase the 

frequency of flooding in low-lying areas.

A time lag often occurs between the dewatering of an aquifer and subsidence 

because much of the compaction results from the slow draining of water 

from confining units adjacent to the aquifer (Galloway and others 1999). This 

phenomenon is called “hydrodynamic consolidation.” It is also responsible for 

residual compaction, which may continue long after water levels are initially 

lowered or even after pumping stops.

Three distinct processes account for most water-related subsidence: (1) 

compaction of aquifer systems, (2) drainage and subsequent oxidation of 

organic soils, and (3) dissolution and collapse of susceptible rocks. Other 

Figure 21. A reduction in the total storage capacity of the aquifer system can occur if pumping of 

water causes an unrecoverable reduction in the pore volumes of compacted aquitards because 

of a collapse of the sediment structure (Galloway and others 1999). 
65

causes of subsidence include underground mining (particularly coal mining), 

removal of oil and gas reserves from the subsurface, thawing of permafrost, 

consolidation of sedimentary deposits over geologic time, and tectonism.

Examples of subsidence caused by overdraft of ground water and aquifer 

compaction include the San Joaquin Valley in California (fig. 22), agricultural 

areas in south-central Arizona, the Houston-Galveston area of Texas, and 

Las Vegas, NV. Subsidence because of drainage and subsequent oxidation 

of organic soils is a major problem in the Florida Everglades. The causes are 

conversion of marshland to urban areas and farmland, periodic droughts, and 

associated wildfires. Subsidence exceeds 5 feet in the agricultural areas (fig. 

23). This amount of subsidence is especially significant to this near-sea-level 

wetlands system in which flow is driven by less than 20 feet of relief.

Figure 22. Land subsidence in the San Joaquin Valley, 1926-70 (modified from Poland and others 1975). The approximate 

location of maximum subsidence (28 feet) in the United States is Mendota, San Joaquin Valley, CA. Signs on the pole show 

approximate altitude of the land surface in 1925, 1955, and 1977 (Galloway and others 1999).
66

Figure 23. Cross sections through the agricultural area of a portion of former Everglades area, central Florida, showing the 

decrease in land-surface elevation (Galloway and others 1999).

Discrete collapse features (sinkholes and cavities) tend to be associated with 

specific rock types, such as evaporites (salt, gypsum, and anhydrite) and 

carbonates (limestone and dolomite). These rocks are susceptible to dissolution 

in water and the formation of cavities. This process can occur naturally where 

these rocks are present near the surface. Evaporite and carbonate rocks underlie 

about 35 to 40 percent of the United States, but they are buried at great depths 

in many areas. Collapse of the land surface above cavities can be triggered by 

ground water-level declines caused by pumping and by enhanced percolation of 

ground water (fig. 24). It often results in some of the most visually spectacular 

examples of subsidence. Large-scale pumping can induce sinkholes by abruptly 

changing ground water levels and disturbing the equilibrium between a buried 

cavity and the overlying earth materials (Newton 1986). Rapid water-level 

declines can cause a loss of fluid-pressure support, bringing more weight to 

bear on the soils and rocks that span the buried voids. As stresses on these 

supporting materials increase, the cavity roof may fail and the ground suddenly 

collapse. Although the collapses tend to be highly localized, their effects 

can extend beyond the collapse zone through the potential facilitation of 

contaminant movement into and through the ground water system.
67

Localized surface impacts of subsidence include earth fissures and sinkholes. 

Earth fissures occur as a result of ground failure in areas of uneven or 

differential compaction. Most fissures occur near the margins of alluvial basins 

or near exposed or shallow buried bedrock in regions where differential land 

subsidence has occurred. They tend to be concentrated where the thickness of 

alluvium changes markedly. When they first open, fissures are usually narrow 

vertical cracks, less than an inch wide and up to hundreds of feet long. They 

can subsequently lengthen to many thousands of feet and widen to more than 

10 feet as a result of erosion and collapse. Vertical offset along the fissure 

is usually no more than a few inches, but a fissure in central Arizona has a 

vertical offset of more than 2 feet. Apparent depths of fissures range from a few 

feet to more than 30 feet.

The large-scale and differential settling of the ground surface that accompanies 

subsidence can have a profound impact on manmade structures. The cost of 

damage caused by subsidence is estimated to be millions of dollars each year 

(National Research Council 1991). Types of potential damage to manmade 

structures caused by subsidence include the following:

•	 Damaged roads. 

•	 Broken foundations.

•	 Severed utilities and pipelines.

•	 Damaged underground and above-ground storage tanks.

•	 Damaged storage reservoirs and treatment lagoons.

•	 Cracked canals and aqueducts.

•	 Broken well casings and damaged pumps.

•	 Damaged railroad tracks, bridges and tunnels.

•	 Flood damage in low-lying areas and damage to flood-control dikes.

•	 Damage to irrigated fields.

•	 Loss of property because of catastrophic sinkhole collapse.

Ground water can play an important role in slope movements because its 

presence in soil pores reduces slope stability. Slope movements often occur 

during the wet season, or following major rainfall or snowmelt events 

(Terzaghi 1950). They are quite common in the forests of the Western United 

Figure 24. Cover-collapse sinkholes may develop abruptly and cause catastrophic damage. They occur where the covering 

sediments contain a significant amount of clay (Galloway and others 1999).

Impacts of

Subsidence

Ground Water 

and Slope 

Stability
68

States. Intense rainfall associated with hurricanes and large frontal systems 

also can produce landslides in forests of the Southeast and on the Caribbean 

National Forest in Puerto Rico (Neary and Swift 1987). Heavy rainfall on 

January 24, 1997, triggered the Mill Creek landslide on the Eldorado National 

Forest in the Sierra Nevada of California. This landslide damaged or destroyed 

three cabins and dammed the South Fork of the American River for 5 hours; 4 

weeks and $4.5 million were required to remove the slide from U.S. Highway 

50 (fig. 25) (Reid and LaHusen 1998). In addition to the potential loss of life 

and property associated with landslides, they result in other environmental 

impacts such as soil erosion and increased sediment concentrations in streams.

Figure 25. Aerial view of the Mill Creek landslide blocking U.S. Highway 50 (Photo courtesy of 

Lynn Harrison, CalTrans).
69

Slopes move when gravitational forces exceed the strength of earth material 

making up the slopes. These movements, landslides, involve both rock and 

soil (Cruden and Varnes 1996). Movement of rock or soil masses on a slope 

is resisted along contacts between rock block surfaces or between individual 

soil particles, and any fluids present in the voids (spaces) between them tend 

to decrease such resistance (Kenney 1984). Rock blocks and soil particles 

derive their resisting strength mainly from friction at the points of contact 

with surrounding blocks or particles. Increased pore pressure within the voids 

reduces this resisting strength. As the pore pressure increases, the potential for 

slope movement increases along planes where gravitational force and resisting 

force becomes nearly equal (Keppeler and Brown 1998). 

In crystalline rock masses, pore pressure changes are rapid and they can lead to 

slope movement in highly fractured masses (fig. 26). Horizontally bedded rock 

masses, in which the principal direction of possible movement is horizontal, do 

not develop pore pressures as great as in rock masses where bedding and the 

principal direction of movement are parallel to the slope face (fig. 26) (Freeze 

and Cherry 1979). 

Figure 26. Some aspects of ground water flow in rock slopes: (1) possible large differences 

in fluid pressure in adjacent rock joints; (2) comparison of transient water-table fluctuations in 

porous soil slopes and low-porosity rock slopes; (3) fault as a low-permeability ground water 

barrier, and as a high-permeability subsurface drain (after Patton and Deere 1971). 
70

Similarly, granular soils experience a decrease in strength as pore pressure in 

the intergranular spaces reduces the contact between adjacent particles. In soil 

with a significant proportion of clay-size particles, the effect of pore pressure 

is somewhat more complex. Such a soil has resistant strength because of 

frictional contact between particles and interparticle attraction, called cohesion, 

between the finer sized particles. Pore pressure increases, however, lead to 

strength decreases in these soils, too. A detailed presentation of ground water 

and slope stability can be found in both “Slope Stability Guide for the National 

Forests in the United States” by Prellwitz and others (1994) and “Landslides—

Investigation and Mitigation” edited by Turner and Schuster (1996).

It has long been recognized in the fields of soil mechanics, agronomy, 

geological engineering, and environmental geology that soil erosion on a slope 

depends greatly on the amount of water the slope contains. If ground water 

recharge is sufficient in a given location to bring the ground water level near 

to the land surface, the erosion potential of the surface soil in response to 

runoff events will be much higher than with lower ground water levels. Little 

infiltration capacity is available when ground water levels are high, so that 

virtually all of the rainfall or snowmelt that occurs becomes runoff. In addition, 

the surface soil grains may be nearly buoyant and easily dislodged by runoff 

water. On even modest hillslopes, bare soils may be eroded rapidly under such 

circumstances. Gully formation occurs most rapidly under such circumstances, 

with saturated soils yielding high runoff and offering little resistance to erosion. 

High ground water levels adjacent to streams lead to unstable, easily erodible 

streambanks and streambeds. Just as dry garden soils may be so hard that it 

is difficult to push in a shovel blade, but when saturated with water are easily 

worked with a shovel, so too are streambanks much softer and more erodible 

when saturated than when dry. Streambeds produce sediment much more easily 

when high ground water levels are providing a buoyant effect (by pushing up 

through the streambed to discharge into the stream) than when low ground 

water levels allow water to escape the stream by seeping through the streambed 

to recharge the ground water that lies below. In addition to increasing particle 

buoyancy and reducing particle friction, ground water may play a significant 

role in weathering of streambank materials into smaller, more transportable 

particles. Pore pressures within soils exposed on the free face of the 

streambank can detach particles and lead to differences in erosion on the bank 

face. This seepage pressure often leads to part of the bank being undermined 

either because of a difference in permeability between the layers of soil 

exposed or because of the height of the saturated zone. Sufficient undermining 

then results in a part of the streambank moving as a small landslide or slump. 

As long as the basic conditions persist, this action continues to modify the 

streambank. 

Where ground water is found at shallow depths, it may dominate and accelerate 

the headward progression of stream channels and the formation and shape 

of tributaries. Several field observations, laboratory flume experiments, and 
71

computer modeling efforts have provided evidence that the presence of ground 

water may dominate the initiation and rate of headcut progression. Laboratory 

flume experiments of headcut migration under hydrogeological conditions 

similar to those on the Colorado Plateau yielded patterns of stream networks 

(long valleys, short tributaries, and amphitheater heads) that compared well 

with field descriptions. Various computer models indicated that headcuts 

formed spontaneously with the introduction of ground water seepage and that 

headward erosion rates increased by as much as 60 times. 

The presence of water in the subsurface offers several mechanisms to 

influence geomorphic processes and rock weathering. These mechanisms 

include freezing and thawing cycles, wetting and drying cycles, chemical 

dissolution, and particle transport and piping (Higgins and Coates 1990). 

Often, geomorphic development of gullies, streambank erosion, and sediment 

production may involve more than one of the mechanisms described below. 

Freeze-Thaw Cycles. Water that infiltrates the voids between soil particles 

(grains) expands as it freezes and exerts sufficient force to disrupt the existing 

order of the soil particles. At the ground surface, this is often evident as frost 

heaving, which is an upward swelling of the soil surface during freezing. 

Similarly, water that percolates into the fractures of otherwise impermeable 

rock expands as it freezes, widening, deepening, and lengthening the fractures, 

and initiating other fractures. As many fractures propagate and many others are 

initiated, this weathering process cleaves pieces of rock from the parent body. 

Water also enters fine fractures in the cleaved pieces of rock. When it freezes, 

it leads to additional cleaving and the creation of smaller pieces of rock.

Wet-Dry Cycles. As in the freeze-thaw cycle, water in this process infiltrates 

into the voids between particles in soils that have a significant proportion of 

clay particles (Dunne 1990). These cohesive soils tend to crack as they dry. 

These shrinkage cracks significantly influence the development of gullies. 

The effect of seasonal wetting and drying can be accentuated when the clays 

present in the soil have great shrinking and swelling potential. In massive 

bedrock, this wetting and drying promotes chemical weathering of the rock 

minerals exposed along cracks penetrated by the ground water. Over time, the 

zone of weathered minerals associated with soil development on either side of 

the crack becomes wider. 

Chemical Dissolution. Some kinds of rock, such as limestone and gypsum, 

are somewhat soluble in water. Given sufficient contact time, appreciable 

masses of these rocks may be dissolved away by the presence of even small 

volumes of water. Given large volumes of water, the rate of dissolution may be 

dramatic and the ultimate impacts catastrophic. Karstic limestone, for example, 

is so riddled with solution cavities and widened fractures that extensive cave 

systems may form and evolve quickly; the collapse of cavern ceilings and the 

formation of sinkholes are major adverse repercussions.
72

Seepage Erosion and Piping. In subsurface strata that consist of uncemented 

and unconsolidated sediments such as boulders, cobbles, gravels, sands, silts, 

and clays, it is possible for very fine particles to be transported by water 

through the voids between larger particles. This process, along with animal 

burrowing and decaying of plant roots, creates major conduits for water flow. 

The phenomenon, called “piping,” can be a major concern (fig. 27). Some 

research has focused on the effects of logging on piping (Ziemer 1992).

Civil engineers have spent a great deal of time studying the piping of small 

particles from the sediments at the bases of dams; leakage through the base 

of a dam must not be allowed to become large enough to exert buoyant forces 

at the toe or immediately downstream of the dam. Otherwise, a disaster like 

the Teton Dam collapse may result. In karst terrain, the solution cavities and 

passageways open to flow may be large enough to permit very large sediment 

sizes, such as gravels and cobbles, to be transported. This phenomenon can be 

a major concern (fig. 27). 

Figure 27. Ground water seeps are evident in a roadcut located 30 miles from Lohman, ID. It 

is clear that ground water seepage has been eroding the rocky face of the roadcut. Note the 

headward progression of erosion, which may continue until the gully becomes a permanent 

stream channel fed by ground water.
73

Midslope and valley-bottom roads in mountainous terrain often intercept and 

redistribute shallow ground water flow. Effects on ground water-dependent 

ecosystems and streamflow timing and duration can be significant. Roads also 

may aid contaminant and hazardous waste migration.

Manipulation of forest vegetation, including both trees and shrubs, can directly 

and indirectly affect ground water. Vegetation influences the water budget 

through its effects on water inputs to the basin and more directly through plant 

water use. By intercepting rain and snow, the vegetation canopy can facilitate 

water loss to sublimation and evaporation. This interception loss may affect the 

amount of water available for ground water recharge. By shading ground and 

water surfaces, vegetation can also influence the rate and timing of snowmelt 

and evaporation from those surfaces. Plants with access to ground water 

(phreatophytes) also influence ground water quantity. They take up ground 

water directly for transpiration. Management activities that intentionally or 

unintentionally influence the density, structure, and species composition of 

vegetation may have measurable effects on the quantity and quality of ground 

water.

Plants growing in valley bottoms and along river margins generally have 

better access to water than plants growing in upland areas. Although most 

phreatophytic plants utilize soil water when available, phreatophytes primarily 

use ground water (Smith and others 1998). This use may cause quite dramatic 

diurnal fluctuations in shallow alluvial aquifers in areas near streams. Because 

of higher water availability in areas adjacent to stream channels and on 

floodplains, plants growing in these areas generally transpire at higher rates 

than vegetation in uplands where water is limiting. As a consequence of 

these high rates of water use by plants with access to ground water, attempts 

have been made to estimate potential water salvage through the removal of 

phreatophytes. Although the volumes of salvaged water proposed in these 

studies are often quite impressive (U.S. Bureau of Reclamation 1963), very 

few studies have actually demonstrated that removal of even extensive areas 

of vegetation have resulted in measurable increases in streamflow (Muckel 

1966). Most studies have indicated that clearing of phreatophytes results 

in no measurable change in streamflow (Culler and others 1982, Welder 

1988). Removal of phreatophytes, however, does often result in increases in 

water table elevations in shallow aquifers (Welder 1988) and destabilization 

of streambanks. Water salvage from removing such vegetation is often 

significantly less than expected and sometimes results in higher water loss from 

an area than before removal (Welder 1988). Depending on the depth from the 

soil surface to the water table, an elevated water table may result in increased 

evaporative losses from the site if the capillary fringe comes into contact with 

the atmosphere. Furthermore, water is used by the vegetation that replaces the 

phreatophytes.

Effects of 

Vegetation 

Management on 

Ground Water

Phreatophyte 

Management
74

Evapotranspiration in stands dominated by phreatophytes has been estimated 

to be from 1.1 to 9 acre-feet of water per acre per year in arid areas of the 

Southwestern United States (Anderson and others 1976). Robinson (1967) 

reported that annual savings in areas of dense vegetation may amount to 2 

to 3 feet of water, depending on depth to the water table. Years of effort and 

tens of millions of Federal dollars were spent in the 1970s to eliminate or 

thin phreatophytes in New Mexico, Arizona, Texas, and elsewhere, when 

phreatophytes were viewed only as “water thieves.” The benefits of riparian 

vegetation to fish, wildlife, and humans are now recognized and far fewer 

projects to eliminate them are being undertaken (Campbell 1970). The recent 

drought throughout the Western United States, however, has stimulated a new 

push for control of nonnative phreatophytes (mainly Tamarix spp.[tamarisk]) 

as well as native species such as mesquite (Prosopis spp.) and willow (Salix 

spp.). Two recently (2004) signed bills will commit $100 million to removal 

of tamarisk from western rivers over several years for the purpose of water 

salvage.

The presence, density, and composition of phreatophytes can affect the quality 

of ground water through uptake of nutrients and pollutants. Phreatophytic 

vegetation has been used for bioremediation of soil and ground water toxicity 

caused by mining and solid waste disposal. Certain species can take up 

and store particular ions, heavy metals, and other pollutants. Phreatophytic 

vegetation may be very effective in removing nitrate from ground water as well 

as phosphorous and other nutrients (Griffiths and others 1997, Dosskey 2001). 

Removal of the forest canopy affects the amount of interception of snow 

and rain by the canopy, as well as the infiltration rate of the precipitation 

that reaches the forest floor. Both of these processes can affect ground water 

recharge and the rate of ground water movement at a local scale. Anderson 

and others (1976) summarized interception in rain-dominated areas as ranging 

from about 8 percent of annual precipitation in hardwoods to about 20 percent 

for conifers. In snow-dominated regions, interception losses ranged from about 

10 to 30 percent for conifers. Intercepted water is not available for ground 

water recharge; however, if the forest canopy is reduced or removed, this water 

can become available as long as the forest floor has not been compacted by 

heavy machinery such as log skidders or removed by erosion. Under certain 

conditions, forest fires can form impermeable layers (hydrophobicity), which 

hinder or even prevent infiltration of water on the forest floor, limiting water 

on the ground surface from recharging shallow aquifers. Slow drainage of 

soil moisture in the range of field capacity is the source of a large proportion 

of the baseflow of forested headwaters streams, where organic matter content 

of forest soils tends to be high. Depth of forest soils throughout the country 

varies widely but generally ranges from 2 to 8 feet before parent material or 

impermeable layers are found. Some areas like the Midsouth and the Pacific 

Northwest have deeper forest soils and, hence, deeper rooting zones with 

probable larger effects on ground water if the tree roots are killed by logging, 

fire, or other means.

Upland Forest 

Management
75

Studies have shown that management of upland forests can increase total 

annual water yield in a basin, particularly if total annual precipitation in the 

watershed exceeds 450 millimeters (118 inches) and deep rooted plants can 

be replaced by shallow-rooted species (Woods 1966, Hibbert 1983). Increases 

in water yield can be accomplished through mechanical thinning and removal 

of existing trees and deep rooted shrubs through use of herbicides. The use of 

herbicides and pesticides, as well as fertilizers, to treat forest stands or selected 

understory species can affect the quality of ground water and surface water. 

The fate and transport of these chemicals is reported on elsewhere in this 

technical guide. The human health aspect of these chemicals in the forested 

environment is covered extensively in Dissmeyer (2000). 

Interdisciplinary studies of the northern hardwoods ecosystem at Hubbard 

Brook Experimental Forest in the White Mountains of New Hampshire began 

in 1955 by the Northeastern Research Station of the Forest Service. Much 

of the early focus was on the effects on small watersheds of clear-cutting 

and herbicide spraying to prevent regrowth of vegetation with respect to 

streamflow quantity and quality. Over time, the studies expanded to include 

the effects of acid rain on soil chemistry, nutrient cycling, and ground water 

chemistry.

Nilsson and others (1982) found that acid deposition could include mobile 

anions that fall directly on stream surfaces or on soils where they are routed 

quickly to streams and ground water. They concluded that, over time, acid 

deposition can be expected to lead to stream-water acidification. Where a 

hydrologic connection exists between surface water and ground water, it is 

likely that the ground water also will become increasingly acidic. Surface 

streams in that part of New Hampshire typically have pH values between 

4.0 and 6.0, while the long-term average pH of the precipitation falling at 

Hubbard Brook is 4.4, which is typical for much of New England, according to 

Hornbeck and Leak (1992).

Studies at Mirror Lake in the Hubbard Brook Experimental Forest by the 

U.S. Geological Survey and others found that the extent of ground water flow 

systems tributary to surface-water bodies were much larger than the surface 

watershed divides would indicate. Also, multiple vertical layers of ground 

ground water flows with deeper layers coming from increasingly larger 

recharge areas. Therefore, the deeper layers would be expected to be fed by 

larger quantities of acidic precipitation, which was contaminated primarily by 

sulfate in that area. This conclusion illustrates the need to do a thorough study 

before significant land- or water-use decisions are made in areas known to 

experience high levels of acidic deposition.

For more information about effects of acidic deposition on surface and ground 

water and northern hardwood ecosystems, visit http://www.lternet.edu/ or 

http://www.hubbardbrook.org The USGS Circular 1139, “Ground Water and 

Surface Water: A Single Resource” by T.C. Winter and others (1998), also 

contains information on the effects of acidic deposition on ground water (http://

pubs.usgs.gov/circ/circ1139/).

Case Study: 

Effects of Acidic 

Precipitation on 

Ground Water, 

Hubbard Brook 

Experimental 

Forest, NH
76

Young and others (2003) present the latest research on the effects of fire on 

aquatic ecosystems. The effects of fire on ground water have not been well 

established or researched. The ultimate impacts of fire on ground water are 

generally manifested as slope failures and increased baseflow in streams 

and springs. Burned areas typically yield more runoff to streams and more 

infiltration to ground water, compared to preburn conditions. Although the 

soil surface is typically rendered slightly to highly hydrophobic by fires, with 

more intense burns and higher loads of vegetation yielding more hydrophobic 

character, the hydrophobic soil surface is easily disturbed by differential 

solar heating and frost heaving. Rapid infiltration of precipitation may then 

occur through discontinuities in the hydrophobic soil surface. Normally, the 

forest canopy and ground cover afforded by living vegetation intercept a fair 

amount of precipitation and much of that is returned to the atmosphere by 

evaporation. When fire destroys the above-ground tree and plant structures, 

far less interception of precipitation and its subsequent evaporation occur. The 

increased infiltration that occurs may result in slope failures, as the moisture 

contents of the soil and subsoil increase. 

Since transpiration and interception of precipitation decrease after a fire 

(at least until substantial new vegetation growth takes place), ground water 

discharges to streams and springs often increase after a fire. Year-round 

baseflow and the severity of floods also increase. Gaining streams exhibit 

higher flows and losing streams may become gaining streams, depending 

on the increased magnitude of ground water recharge versus the increases 

overland flow and runoff from the burned areas. Fire-induced vegetation 

changes can alter the water-holding capacity of soils, the rate of snow melt, 

and local water tables, and these factors can lead to changes in the timing of 

peak and low-water events and the formation of small forest pools (Pilliod 

and others 2003). Small pools often form in areas of gentle slope after loss of 

vegetation from logging or fire, because decreased evapotranspiration results in 

elevated water tables and increased soil saturation. 

Ground water is a factor in the ecology of the forest. Its decline in dense 

forests may be a factor in the decline of species diversity, and its increase in 

burned areas may cause shifts away from naturally occurring forest species to 

those that are more competitive in habitats with wetter soils and ponded water. 

Ground water discharges to wetlands and riparian areas may significantly 

increase after fires and result in shifts in amphibian populations. Small isolated 

wetlands are particularly important amphibian habitats, and their formation in 

burned forests may benefit some amphibians. 

Increasing ground water levels may significantly increase the potential for 

slope failures and landslides. The loss of vegetation during a fire causes soil 

moisture contents to increase and water tables to rise. When plants die, their 

roots decay, creating passages through which infiltrating precipitation may 

move rapidly into the subsurface. The loss of root structures through decay 

Impacts from 

Fires
77

can be an important factor in destabilizing slopes. The increased water content 

in the affected soils and subsurface sediments may destabilize steep slopes, 

generating slope failures and landslides. 

Residential and commercial development has been rapid adjacent to national 

forest boundaries and on in-holdings. As water supplies become stressed, land 

managers will be pressured to permit additional municipal drinking-water 

wells on NFS land. In the future, ground water management is likely to evolve 

toward total aquifer management. Protection measures such as limitations on 

activities in recharge areas, reservation of some areas for production of highquality water, and protection of unique ground water-dependent ecosystems 

will be incorporated into land management plans. It will no longer be sufficient 

to manage for operators and users. Managers must recognize that ground water 

serves diverse functions, some of which are ecological.

In unincorporated areas, residential growth is characterized by the use of 

individual domestic wells and individual sewage disposal systems (ISDS; 

also known as septic systems). In the fractured-rock settings typical of much 

NFS land, proper siting and design of an ISDS is problematic. The traditional 

ISDS; design is appropriate for installation in areas underlain by sufficient 

soil thickness and porous media aquifers. The use of these types of ISDS in 

fractured-rock settings often results in contamination of nearby domestic wells 

or surface waters. Primary causes include insufficient filtering and treatment 

by the typically thin soils that overlie fractured bedrock and the difficulty in 

determining the nature and orientation of ground water flow paths. When 

properly designed, installed, and operated, some advanced ISDS systems, such 

as those based on the “mound” concept, have been effective in many areas. 

These advanced systems, however, can cost substantially more to install and 

operate. 

This section outlines issues that relate to the design and implementation of 

ground water monitoring programs. These issues were addressed in the report 

of the Intergovernmental Task Force on Monitoring Water Quality (Franke 

1997), and much of this section is taken from that document. Specific details 

for designing and implementing a ground water monitoring program can also 

be found in Sanders and others (2000), proper sampling protocols are described 

in U.S. Geological Survey (1997 to present), and statistical methods for 

analyzing water-quality data are found in Helsel and Hirsch (1992).

Ground water monitoring is critical for appropriate water-resource 

management. The hydrological connections between ground water and surface 

water mandate that monitoring programs for all water resources be closely 

linked. By acknowledging this close hydrological connection, ground water 

monitoring can provide critical support to surface water and ground water 

management programs. 

Wildland/Urban 

Interface

Monitoring

Monitoring 

Program 

Objectives
78

Monitoring of ground water quality is defined as an integrated activity 

for obtaining and evaluating information on the physical, chemical, and 

biological characteristics of ground water in relation to human health, aquifer 

conditions, and designated ground water and surface water uses. With accurate 

information, the current state of ground water resources on NFS lands can 

be better assessed, water-resource protection programs can be run more 

effectively, and long-term trends in ground water quality and the success of 

land management programs can be evaluated. 

Many Forest Service units do not have the capability or sufficient resources 

to undertake a water-monitoring program in a short timeframe for all aquifers 

within their jurisdictions. Therefore, it is recommended that the agency 

combine resources and talents with others to begin a systematic process 

of sampling aquifers that are the highest priority, such as those that have 

the largest human water use. Depending on the availability of resources, 

this approach may extend the amount of time needed to assess all aquifers 

in a unit’s jurisdiction, but the most important ones from a human health 

perspective will be addressed first. Monitoring ground water in a systematic 

manner will gradually result in the development of high-quality, comparable 

data sets that will increase knowledge of the occurrence and distribution 

of constituents in ground water and environmental settings where different 

indicators should be included in monitoring programs. 

When designing and implementing monitoring programs, it is vital to consider 

the differences in the spatial and temporal characteristics of ground and surface 

waters. Ground water has a three-dimensional distribution within a geological 

framework and is characterized by contrasting aquifer and geological features. 

In addition, of course, access is limited because ground water must be sampled 

through a well or spring. Therefore, the design and implementation of a ground 

water quality monitoring program must be based on a thorough understanding 

of the unique hydrogeological characteristics of the ground water flow 

system under investigation and the locations of particular land uses and other 

contaminant sources likely to affect ground water quality. 

An important aspect of any program for monitoring ground water quality is 

the sharing and using of data from various sources. One such area of exchange 

is among programs designed to gather background or ambient-monitoring 

data. Another is among programs designed to gather data about regulatory 

compliance. 

Monitoring programs have the following general objectives: 

•	 Assess background ground water-quality and quantity conditions. 

•	 Comply with statutory and regulatory mandates. 

•	 Determine changes (or lack of change) in ground water quality and 

quantity over time to define existing and emerging problems, to guide 

monitoring and management priorities, and to evaluate effectiveness of 

land and water management practices and programs. 
79

•	 Improve understanding of the natural and human-induced factors 

affecting ground water quality and quantity. 

Several types of ground water monitoring are conducted by Federal, State, 

local, and private organizations to accomplish one or more of the objectives 

stated above. 

Background or baseline monitoring of water resources often is needed when 

sampling an area for the first time or in advance of the initiation of a new 

activity that could affect ground water quality. A wide variety of chemical, 

physical, and biological contaminants may affect ground water resources 

(Fetter 1999). As a result, background monitoring programs are designed to 

establish baseline water-quality characteristics and to investigate long-term 

trends in resource conditions. Parameters are selected to provide data on 

general ground water conditions or on conditions relevant to a new activity. 

Baseline concentrations of elements, species, or chemical substances in ground 

water present are those that occur naturally from geological, biological or 

atmospheric sources, or from existing human activities. Established waterquality limits may be exceeded by natural or anthropogenic processes for 

various elements. 

These monitoring programs typically focus on assessing the impact from 

stressses or contaminant sources that are related to specific land uses. For 

these monitoring efforts, parameters are identified on the basis of a thorough 

understanding of the resource to be evaluated and the sources of stress or 

contamination.

The Turkey Creek watershed in Jefferson County, southwest of Denver, 

CO, is an area of rapidly developing communities in the foothills of the 

Colorado Rocky Mountains. About 5,000 households in the watershed 

depend on domestic wells for their water needs, and individual septicdisposal systems are used for wastewater. County government agencies 

are concerned about the impacts of the development on water quality 

and water quantity.

To understand the hydrological conditions in the Turkey Creek watershed, the 

USGS and Jefferson County undertook a cooperative study to evaluate the 

water resources of the watershed from 1998–2001. A critical component of 

this study was the establishment of monitoring networks for both ground water 

levels and ground water quality. These networks provided baseline hydrologic 

information as well as data necessary for the construction and calibration of a 

precipitation-runoff model of the study area.

Background 

or Baseline 

Monitoring

Monitoring for 

Specific Landuse Impacts

Case Study: 

Ground Water

Monitoring in the

Turkey Creek

Watershed, Jefferson

County, CO
80

Ground water levels were monitored monthly at 15 monitoring wells, 

beginning in 1999. These wells are no longer used by homeowners, and are 

considered reliable indicators of static water levels. Three of the wells are 

shallow, hand-dug wells, and the remaining wells are completed at depths 

ranging from 70 to 505 feet. Water levels also were measured in 131 domestic 

wells from September 24 to October 4, 2001 to complete a water-table map for 

that time period. The resulting map (fig. 28) can be used to indicate areas of 

ground water recharge and discharge and directions of ground water flow. The 

water table generally mimics the topography, with ground water flowing from 

higher recharge areas to lower discharge areas near streams.

Water-quality data were obtained quarterly from 22 surface water sites and 110 

wells and springs from October 1998 to September 1999. A few miscellaneous 

samples were obtained during 2000 and 2001 to fill data gaps. Samples 

were analyzed for temperature, specific conductance, major ions, nutrients, 

bacteria, and minor elements from 1998 to 1999 (fig. 29). During the 2000–01 

Parmalee Gulch

S. Turkey Creek

N. Turkey Cr

Giant Gulch

Turkey Creek Iowa Gulch

Lone Pine

Estates

Wild Rose Grange

Marshdale

Tiny Town

Indian

Hills

Fenders

Phillipsburg

Homewood Park

Oehlmann

Park

Conifer

8

To Morrison

C470

Shadow Mountain Road

N. Turkey Creek Rd

Parma lee Gulch

Road

285

285

Marshdale

�

�

D

A

B

E

D

C

F

L

M

H

P

G

I

J

K

N

O

39°32'24"

39°30'36"

39°34'12"

39°36'

39°37'48"

105°23'24" 105°21'36" 105°19'48" 105°18' 105°16'12" 105°14'24" 105°12'36" 105°10'48"

EXPLANATION

Study area

Subbasins

Drainage

Roads

Ground-water

 elevation contour, 

 in feet

Subbasin code

Population center

Well and ground-water

 elevation, in feet

0

0 0.5 1 1.5 2 2.5 3 KILOMETERS

0.5 1 1.5 2 2.5 3 MILES

7711

7742

7974

7974

7866

7749

7763

8240 8341

7625

7585

7582

7780

8002

7828 7685

7649

7716 7738

7661 7794

8002 8017

8042

8080

7343

7236

7231

7550

7029

6925

6547

6499

6805

7232

7020

7228

7156

7249

7324

7377

7473

7430

7041

7137 7174

8216

8280 8581 8635

8582

9139

9381

9311

9550

8949 8929 8842

8442

8626

8459

8484

8419

8712 8732 8700

9100

9000

7900

7900

7400

7500

7300 7200 7100

7000

6900

6800

6700

6600

6500

8300

8300

8400

8100

8200

8000

7900

7800

7900

7400

76007700

8000

9200

8800

8700

8900

9500

9400

8100

8000

8200

8300

8500

8600

8200

8400

8100

8724

9727

9677

9307

7406

7434 7326 7371

7070

74217188 7129

7845

7872

7570

7813 7380

8081

7789

7739

7884

7847

7774

7644

7787 7938

8134

8079

7684

7610

7309

8209

8208

8145

8019 7920

7884

8246

8188

8046

8049

8135

8143

8435

8257

8154 8250

8145 7844

7873

8091

8070

8712

7880

7943

7976

7981

7783

7896

7906

8036

8016 7884

Base modified from U.S. Geological Survey

digital data for 1:24,000 maps

Figure 28. Ground water table in the Turkey Creek watershed, September 2001.
81

sampling, water was analyzed for bromide, selected inorganic ions, wastewater 

compounds indicative of septic-tank effluent, and tritium, which is indicative 

of modern recharge. The water-quality data indicate that some wastewater 

compounds are present in the ground water and surface water, and that most of 

the ground water has been recharged within the past 50 years.

For additional information see Bossong and others (2003). Also see 

http://water.usgs.gov/pubs/wri/wri03-4034/.

Monitoring is often needed to be certain that specific facilities are complying 

with specific regulatory requirements or permit conditions. These efforts may 

be designed to comply with various laws such as RCRA, or to support remedial 

activities such as those under CERCLA (Superfund).

Monitoring is required to inform management and to help develop an 

understanding of ecological processes in ground water-dependent ecosystems. 

It should address the environmental condition of ground water-dependent 

ecosystems at particular points in time and the trend in condition over time. 

Subject to resource availability, such monitoring could address key ecological 

Parmalee Gulch

S. Turkey Creek

N. Turkey Cr

Giant Gulch

Turkey Creek Iowa Gulch

Lone Pine

Estates

Wild Rose Grange

Marshdale

Tiny Town

Indian

Hills

Fenders

Phillipsburg

Homewood Park

Oehlmann

Park

Conifer

8

To Morrison

C470

Shadow Mountain Road

N. Turkey Creek Rd

Parma lee Gulch

Road

285

285

Marshdale

�

�

D A

B

E

D C

F

L M

H

P

G

I

J

K

N

O

39°32'24"

39°30'36"

39°34'12"

39°36'

39°37'48"

105°23'24" 105°21'36" 105°19'48" 105°18' 105°16'12" 105°14'24" 105°12'36" 105°10'48"

EXPLANATION

Study area

Subbasins

Drainage

Roads

Subbasin code

Population centers

Chloride in ground

 water, fall 1999, in

 mg/L

 Less than or equal to 5

 Greater than 5 and less

 than or equal to 35

 Greater than 35

Chloride in surface

 water, fall 1999, in

 mg/L

 Less than or equal to 5

 Greater than 5 and less

 than or equal to 35

 Greater than 35

0

0 0.5 1 1.5 2 2.5 3 KILOMETERS

0.5 1 1.5 2 2.5 3 MILES

Base modified from U.S. Geological Survey

digital data for 1:24,000 maps

Figure 29. Chloride concentrations in the Turkey Creek watershed, fall 1999.

Monitoring for 

Facility-based 

Compliance

Monitoring of 

Ground Waterdependent 

Ecosystems
82

processes and any changes in vulnerability to processes or events that threaten 

the integrity of the ecosystem. Monitoring of important processes such as water 

regime and allocation, water quality, and usage of ground water will enable 

detection of changes detrimental to the health of the ecosystem.

In addition to contributing flow to surface waters, ground water directly 

sustains wetlands, riparian zones, meadows, marshes, some forest tree stands 

and some grasslands, as well as aquatic species in lakes, streams, cave systems, 

and springs. The loss of ground water flow to these ground water-dependent 

ecosystems can have adverse impacts on the flora and fauna of the NFS. The 

dependency of ecosystems on ground water is based on at least one of three 

basic ground water attributes: quality, flux, and level (Sinclair Knight Merz Pty 

Ltd. 2001).

Quality. Ground water quality is typically measured in terms of electrical 

conductivity (or salinity), nutrient content, concentrations of major ions, 

and/or concentrations of contaminants such as metals and organic chemicals. 

Ecosystems and their component species typically function adequately over 

certain ranges in water quality. Outside these ranges, the composition and 

health of the ecosystem is likely to decline. A ground water attribute can 

become important to an ecosystem when a sustained change in quality or trend 

away from the natural water-quality state occurs. Salinity is typically key 

inorganic indicator of ground water quality for such ecosystems. Terrestrial 

ecosystems may also be sensitive to ground water contamination by nutrients, 

pesticides, or metals, but little is known about most ecosystem responses. 

Phytotoxicity of metals, however, has been established for some plant species 

(Kabata-Pendias and Pendias 2000).

Flux. Ground water flux or flow is the rate of surface or subsurface discharge 

of an aquifer. It is relevant to the provision of an adequate quantity of water 

to sustain an ecosystem or of a sufficient quantity to dilute more saline water 

(in wetland systems) to allow an ecosystem to function. Quantity of water 

is critical to ecosystems that occupy discharged ground water, such as cave 

systems, aquatic ecosystems in baseflow-dependent streams and many ground 

water-fed wetlands, or ecosystems whose sole or principal source of water 

is ground water. For terrestrial vegetation, the ground water flux needs to be 

sufficient to sustain a level of uptake by vegetation that at least partly satisfies 

evaporative demand.

Level. Ground water level is the depth of the water table. It is relevant to a 

broad range of ecosystems, including wetlands fed by unconfined aquifers, 

many coastal lacustrine and estuarine ecosystems, some cave and aquifer 

ecosystems, and baseflow-dependent ecosystems. The ecosystem’s location 

or usage of ground water depends on the level of the water table remaining 

within a certain range. Aquifer pressure has a similar role in ecosystems fed 

by confined aquifers to that of level in systems fed by unconfined aquifers. It 

determines discharge rates from springs and from fractured bedrock aquifers.
83

The response of ecosystems to change in these attributes is variable. There 

may be a threshold response in some cases, whereby an ecosystem collapses 

completely if a certain required attribute value is not met. Examples might be 

springs or fens supported by ground water discharge. These would cease to 

exist if pressures in the supporting aquifer fell to the point where no further 

surface discharge occurred. In other cases, a more gradual change in the health, 

composition, or ecological function of communities is expected. For example, 

an ecosystem may change slowly in response to gradually increasing ground 

water salinity. 

An assessment of ecosystem dependency on ground water can be performed 

by identifying ecosystem traits that imply such dependency (Sinclair Knight 

Merz Pty Ltd. 2001). A high level of dependency on ground water makes an 

ecosystem vulnerable to change in water regime. Many of such ecosystems 

have relatively high levels of endemism. The following checklist can be used 

to help determine ground water dependency: 

•	 Is the ecosystem identical or similar to another that is known to be 

ground water dependent?

•	 Is the distribution of the ecosystem associated with surface water 

bodies that are or are likely to be ground water dependent? Examples 

are permanent wetlands and streams with consistent or increasing flow 

along the flow path during extended dry periods.

•	 Is the distribution of the ecosystem consistently associated with known 

areas of ground water discharge from springs or seeps?

•	 Is the distribution of the ecosystem typically confined to locations 

where ground water is known or expected to be shallow, such as 

topographically low areas and major breaks of topographic slope?

•	 Does the ecosystem withstand prolonged dry conditions without 

obvious signs of water stress?

•	 Is the vegetation community known to function as a refuge for mobile 

fauna during times of drought?

•	 Does the vegetation in a particular community support a greater leaf 

area index and more diverse structure than those in nearby areas in 

somewhat different positions in the landscape?

•	 Does expert opinion indicate that the ecosystem is ground water 

dependent?

Underground tunnel construction can disrupt ground water flow systems and 

cause dewatering of overlying springs and riparian areas that provide valuable 

habitat for flora and fauna. The surface resources monitoring and mitigation 

plan for the Arrowhead East and West Tunnels Project is an example in which 

the potential for ground water disruption was recognized and addressed. The 

Forest Service objective is to maintain ecosystem health at each of the surface 

water features being monitored. 

Case Study: 

Monitoring of Ground

Water-dependent

Ecosystems, Arrow- head Tunnels, San Bernardino

Mountains, CA
84

The Arrowhead East and West tunnels are part of the Metropolitan Water 

District (MWD) of Southern California’s Inland Feeder System, which 

connects the California and Colorado Aqueducts in the southern part of 

California. The project consists of two 16-foot diameter tunnel segments that 

pass under the San Bernardino National Forest (fig. 30). The combined length 

of the tunnels will be more than 8 miles and depths will reach 2,040 feet under 

the San Bernardino Mountains. The tunnels cross active splays of the San 

Andreas Fault in three locations. Ground water levels are as high as 1,100 feet 

above the tunnels, and significant ground water inflows have been encountered 

during the tunneling operations.

A key requirement of the Arrowhead Tunnels project is to protect the water 

resources in the San Bernardino National Forest. Limits have been placed on 

ground water inflows into the tunnels by the Forest Service under a specialuse permit issued to the MWD. In 1993, the MWD and the Forest Service 

recognized the potential for construction of the Arrowhead Tunnels to affect 

local surface water resources in the San Bernardino Mountains, and they 

adopted a water-resources monitoring and mitigation plan that will provide 

data that can be used to identify construction-related effects. The plan targets 

selected ground water-dependent surface water and biological resources in the 

vicinity of the tunnel segments. 

A total of 126 spring, stream, rain gage, and well sites were identified for 

monitoring during the preconstruction, construction, and postconstruction 

periods (fig. 30). The monitoring effort has provided an unprecedented amount 

of data about the hydrological characteristics of the ground water regime in 

the project area. These data have proven to be quite valuable for assessing 

the hydrological trends across the mountains and for identifying the variables 

that most significantly influence baseflow at the spring and stream monitoring 

points. 

In addition, biological monitoring and several focused biological surveys have 

been completed for mollusks (fig. 31), amphibians (fig. 32), birds, and riparian 

vegetation. These have provided detailed information about the plant and 

animal species in the project area that depend on ground water discharge and 

their responses to normal fluctuations in rainfall, temperature, and wildfires. 

Spring snails (fig. 31) in the Transverse Range of Southern California, are 

particularly good indicators of spring ecosystem health because they occupy 

only springs that are minimally disturbed and have persisted for thousands 

of years. They do not occupy habitats that are scoured by floods, or that 

periodically dry, and they are susceptible to establishment of nonnative species 

and cultural activities affecting the quality of spring-fed aquatic habitats.
85

Figure 30. Aerial photograph of showing the East Tunnel alignment and monitoring points on the San Bernadino National 

Forest.

Regression models have been developed to help estimate what the seasonally 

adjusted baseflow should be at each surface-monitoring site within 2,500 feet 

of the tunnels. In addition to surface-related variables, ground water levels 

in wells, tunnel-heading inflow, probe-hole flow, and portal discharge are 

regularly measured. These data will provide early warning of the potential for 

a surface-related impact from tunnel construction. They also will be used to 

corroborate the occurrence and to assess the magnitude and extent of tunnelrelated surface impacts.

The flow chart in figure 33 shows the mitigation triggers that would occur if 

a tunnel-related hydrologic effect were suspected. During the supplemental 

biological monitoring phase, plant-water potential, general observations 

of plant health, soil moisture, and animal condition and habitat will be 

evaluated and compared with reference sites. Indicators that provide important 

information for evaluations include (1) willow or sycamore trees that begin 

dropping leaves early in the season, (2) the herbaceous understory that begins 

to desiccate early in the season, (3) soil moisture readings that indicate unusual 

drying of soils, and (4) water potential measurements at predawn that are 

increasingly elevated. For more information on spring snails see Sada (2002).
86

Figure 31. Spring 

snail,Pyrgulopsis 

californiensis.

Figure 32. Western 

spadefoot toad, Spea 

hammondii.
87

Geoindicators have been developed to assist in assessments of natural 

environments and ecosystems. They provide an approach for identifying rapid 

changes in the natural environment (Berger and Iams 1996). An international 

working group of the International Union of Geological Sciences developed 

geoindicators to assess common geological processes occurring at or near the 

Earth’s surface that may undergo significant change in magnitude, frequency, 

trend, or rate over periods of 100 years or less. Geoindicators measure both 

catastrophic events and those that are more gradual but evident within a human 

lifespan. Geoindicators that focus on environmental changes in ground water 

systems are described here. For the purpose of this technical guide, they are 

called hydrogeoindicators. As descriptors of hydrogeological processes that 

operate in many settings, hydrogeoindicators can be used by the Forest Service 

to monitor natural as well as human-induced changes in ground water systems 

and in the ecosystems they sustain. 

The most effective use of hydrogeoindicators is in environmental monitoring 

programs. They are designed for use on local or national scales. They can help 

to answer four basic questions:

1. What is happening in the environment (conditions and trends)?

2. Why is it happening (causes, links between human influences and 

natural processes)?

3. Why is it significant (ecological, economic, and health effects)?

4. What are we doing about it (implications for planning and policy)?

The use of hydrogeoindicators presents several specific challenges. One is 

to define more closely the thresholds or critical levels involved, so that it is 

possible to specifically express the relative stability of a particular environment 

to management. For each indicator, target, trend, or threshold values will need 

to be set. If a threshold is reached, action of some type should be required. 

Eleven important hydrogeoindicators are presented in table 2. Appropriate 

indicators can be selected from this list depending on the terrain and the 

environmental issues under consideration. 

Edmunds (1996) proposed a monitoring scheme for ground water designed 

to detect changing conditions using a set of parameters that have global or 

regional significance and undergo changes over a time scale of 50 to 100 years. 

The primary and secondary indicators shown in table 3 monitor both natural 

changes in ground water chemistry and effects from human influences. 

These indicators have been developed from standard approaches used in 

geology, geochemistry, geophysics, geomorphology, hydrology, and other 

earth sciences. For the most part, the expertise and technology already exist to 

monitor and analyze the resulting data and most indicators are relatively simple 

and inexpensive to apply.

Monitoring for 

Environmental 

Change
88

Figure 33. Mitigation flow chart for the Arrowhead Tunnels Project.
89

Ecosystem management, reporting, and planning generally focus on biological 

issues such as biodiversity, threatened and endangered species, exotic species, 

and biological and chemical parameters that describe air and water quality. 

Much less attention is paid to the physical processes that shape the landscape—

the natural, changing foundation on which humans and all other organisms live 

and function. 

Hydrogeoindicators can help answer Forest Service resource management 

questions about what is happening to the hydrological environment, why 

it is happening, and whether it is significant. They can establish baseline 

conditions and trends, so that human-induced changes can be identified. 

Applying this approach will provide science-based information to support 

resource management decisions and planning. Hydrogeoindicators help nongeoscientists focus on key geological issues. They can help forests managers 

to anticipate changes that might occur in the future, and to identify potential 

management concerns from a hydrogeological perspective. 

Hydrogeological processes are integral to forest management and planning. 

When measures of natural change are omitted from monitoring and planning, 

the assumption that natural systems are stable, fixed, and in equilibrium is 

perpetuated. Natural systems are dynamic, and some may be chaotic; change 

is the rule, not the exception. Using hydrogeoindicators shifts management 

actions from response (crisis mode) to long-range planning, so issues can be 

recognized before they become serious concerns.
90

Hydrogeoindicator Significance Types of monitoring sites Method of 

measurement

Frequency of 

measurement

Thresholds

Ground water quality

The chemical composition of ground water can be used as a measure of its suitability for human and animal consumption, irrigation, and for industrial and other purposes. It also influences ecosystem health and function, so it is important to detect change and early warnings of change, both natural and resulting from human activity.

Wells, springs, wetlands, adit discharges, lakes, stream baseflow. Focus on major aquifers providing water supplies or substantial discharge to important surface waters. Monitor downgradient of potential problem areas such as mines, urban areas, waste disposal sites, burned areas, and timbered areas. Relate individual pollutants to their sources; include sampling of potential sources when they can be identified and accessed. Wherever possible, integrate monitoring with other national, State, or local water-quality networks.

Standard sampling and laboratory techniques and equipment. Some monitoring can be conducted remotely using dataloggers and sensors placed in wells or at points of ground water discharge. In many cases, it will be important to collect data over a time period sufficient to be able identify normal seasonal variability. Statistical analysis of temporal and synoptic data may be appropriate. [Specialized sampling and analysis for isotopes]

Usually on a seasonal or annual basis; maximum frequency of 4 times a year is suggested to detect changes in shallow ground water sources, but annual measurements are often sufficient for deeper sources.

Water-quality standards for applicable beneficial uses, established trends, statistically significant changes in concentration.

Ground water level (elevation)

The availability of water is of fundamental importance to the sustainability of life. Ground water levels are essential to be able to identify the extent of the resource and to determine the recharge. Regularly measure water levels in wells and boreholes and/or ground water-fed surface waters. Results are the simplest indicator of changes in ground water resources. The level of ground- water is an essential parameter to understand the ground water system.

Boreholes, wells, and ground water discharge areas associated with springs, streams, lakes, and wetlands representative of the particular aquifer.

Depth to the water table can be measured manually, with automatic water-level recorders, or with pressure transducers. Standard hydrogeological methods are used to calculate a water balance for the ground water system of interest.

At least monthly to reflect seasonal as well as annual changes. The state of fossil aquifers should be assessed at about 5-year intervals. Water levels can be measured both seasonally and annually over decades to determine overall trends.

A threshold is crossed when the rate of extraction exceeds the rate of recharge, and a sustainable, renewable resource becomes a non-renewable, mined one. Drying up of wells, springs, wetlands, and so on.

Table 2. Hydrogeoindicators that may be suitable for monitoring to assess environmental responses to changes in ground water systems on NFS lands. Appropriate indicators are selected depending on the terrain and the environmental issues under consideration.
91

Hydrogeoindicator Significance Types of monitoring sites Method of 

measurement

Frequency of 

measurement

Thresholds

Vadose zone chemistry

Changes in recharge rates have a direct relationship to water resource availability. The unsaturated zone may store and transmit contaminants, the release of which may have a gradual or sudden adverse impact on ground water quality.

Unconsolidated sediments or consolidated porous media (sand, till, sandstone, chalk, calcarenite, volcanic ash) on relatively level terrain that has negligible surface runoff. The best records are obtained where the unsaturated zone is 10–30m thick, and where sediments and flow are relatively homogeneous.

Sampling from either material samples from borings or from lysimeters. Samples can be collected from borings/wells placed by hollow stem auger, percussion, air-flush rotary, or dual tube drilling. Pore water is extracted from sediments by high-speed centrifuge (drainage or immiscible liquid displacement) or, for nonreactive components such as Cl and NO3, by elution with de-ionized water. For isotopic samples (3H, δ18O, δ2H), vacuum distillation may be used. Lysimeters are monitoring devices designed to collect soil water, either as drainage (basins) or from within the matrix (suction).

For collected soil samples, 5- to 10-year intervals to confirm movement of solutes toward the water table.For soil water, sampling quarterly or more frequently, to ensure proper operation of the equipment, until conditions are identified. May be repeated with new equipment installations on a similar 5- to 10-year basis.

Vadose zone begins to measurably affect saturated zone water quality.

Stream and spring baseflow

Ground water discharge to streams and springs is vital for the regulation and maintenance of aquatic health and biodiversity. Humaninduced depletion of baseflow has major implications for the health of riparian ecosystems.

Stream channels and springs. Standard techniques for 

measuring streamflow, hydrograph separation. Synoptic sampling—tracer injection studies, streambank piezometers.

Continuous to periodic. Established critical flow 

for sustaining healthy aquatic ecosystems.

Surface water quality

The quality of surface water in rivers, streams, lakes, ponds, and wetlands is influenced by interactions with ground water. The bulk of the solutes in surface water are often derived from ground water baseflow where the influences of waterrock interactions are important.

Stream channels, lakes, wetlands, and springs.

Standard sampling and laboratory techniques and equipment. Statistical analysis of data. [Specialized sampling and analysis is needed for isotopes and trace metals.] Bioindicators for inferring past lake- water chemistry.

4–6 times yearly for major ions, twice yearly for radionuclides and organic chemicals.Continuous, real-time monitoring systems provide the most complete information for field parameters.

Water-quality standards and trends. Human and aquatic health criteria.

Table 2—Cont.
92

Hydrogeoindicator Significance Types of monitoring sites Method of 

measurement

Frequency of 

measurement

Thresholds

Lake levels Lakes are sensitive to local 

climate and to land-use changes in the surrounding landscape Lakes can also be valuable indicators of near-surface ground water conditions. Where not directly affected by human activities, lake level fluctuations are excellent indicators of drought conditions. Lake level fluctuations vary with the water balance of the lake and its catchment, and may reflect changes in shallow ground water resources.

Lakes with ground water exchange. Shoreline gauges. Areal 

extent from successive aerial photos, satellite images, and geomorphology. 

Lake level monthly to annually. Areal extent every 5 years.

Critical inflow-outflowstorage water balance perturbation.

Subsurface temperature

The thermal regime of soils and bedrock strongly influences the soil ecosystem, near-surface chemical reactions involving ground water, and the ability of these materials to sequester or release greenhouse gases. It may affect the type, productivity, and decay of plants, the availability and retention of water, the rate of nutrient cycling, and the activities of soil microfauna.

Sites remote sites from human disturbances, bodies of surface water, or areas of high geothermal flow where the ground cover is left undisturbed. The best results are obtained from measurements in relatively impermeable bedrock or where there has been minimal ground water movement.

Data loggers, thermocouples, thermistors.

Once every 5 years for deep boreholes, more frequently (as often as twice daily) for nearsurface temperatures.

Operationally defined change in temperature.

Wetlands Wetlands have high biological 

productivity and diversity. They are important for wildlife habitat, water storage, and human recreation. Wetlands can affect local hydrology by acting as filters, sequestering and storing heavy metals and other pollutants, and serving as flood buffers.

Ground water supported fens and marshes.

Areal extent and distribution. Permanent transects and plots can be set up for ease of data comparison and establishing temporal trends in vegetation distribution, surface morphology, accumulation rates, hydroperiods, water levels, and hydrochemistry. Piezometers, wells, and weirs can be used. Variations in the chemistry of water inflows and outflows, changes in water levels and in seasonality of flow patterns.

Every 5–10 years for distribution, extent, and structure; for water levels, and hydrochemistry, initial measurements should be weekly to monthly (more frequently in times of rapid change such as spring thaw) until important times and parameters have been identified, then less frequently.

Change in areal extent, vegetation community type, water-flow regime.

Table 2—Cont.
93

Table 2—Cont.

Hydrogeoindicator Significance Types of monitoring sites Method of 

measurement

Frequency of 

measurement

Thresholds

Karst features Karst landscapes occupy 

up to 10% of the Earth’s land surface, and as much as a quarter of the world’s population is supplied by karst water. The karst system is sensitive to many environmental factors. Instability of karst surfaces leads annually to millions of dollars of damage to roads, buildings, and other structures. Radon levels in karst ground water tend to be high in some regions.

Caves allow direct observation and mapping of underground features and their relation to the surface and to ground water flow. Wells, borings, and quarries may be less useful as monitoring sites because they may provide only discontinuous points of information.

Pumping tests on wells, dye tracing. Hydrological and geochemical measurements of springs, sinking streams, drip waters into caves, and cave streams provide records of shortterm changes in water quality and chemical processes. In builtup areas, locate buried cavities and monitor their potential for collapse, using a combination of geophysical surveys, exploratory drilling, and repeated leveling.

Continuous measurements are needed to interpret the karst system. Surface features, ground water chemistry, and contamination in karst terrains are notoriously unstable and can change rapidly.

Operationally defined change between dissolution and precipitation of calcite, critical change in water flow or water quality.

Slope failure Slope failure is one of the most widespread causes of land 

disturbance, so the initiation and development of landslides should be closely monitored. Ground water is a major factor in causing landslides.

The highest part (crown) of landslides and other potential slope failures is generally the most important place for monitoring cracks, subsidence, and sagging. Upheaval or buckling generally begins in the toe area. As failure progresses and the slide or flow develops, cracks and ground subsidence may form at any point, including the toe.

Satellite images, air photos, repeated conventional surveying, installation of various instruments to measure movements directly, and inclinometers to record changes in slope inclination near cracks and areas of greatest vertical movement. Subsurface methods include installation of inclinometers and rock noise instruments, and geophysical techniques for locating shear surfaces.

Dictated by changes in rate of crack propagation and ground deformation and by the degree of potential damage if the slope fails. If little activity has taken place in a particular area, re-assessment can be delayed for several years or more. Critical periods for monitoring are during and immediately after intense rains and rapid snowmelt.

Slope instability affects or threatens structures such as roads or buildings.

Subsidence/ Surface displacement

Subsidence from extraction of ground water or oil and gas, and mining activities can damage buildings, foundations, and other structures. Displacements of the ground surface can be used to assess and warn of environmental problems, especially in areas liable to subsidence from bedrock solution, mining and fluid extraction.

Areas extracting ground water, oil or gas, underground mines, active fault zones, and reservoirs.

Repeated precise leveling and ground surveys. Standard geodetic techniques, especially using Global Positioning System and laser range finders.

Depends on the nature of the movement taking place.

A threshold is crossed when the rate of movement crosses an operationally defined value or range, affects or threatens structures.
94

Table 3. Recommended indicators in the ground water environment (after Edmunds 1996).

Priority Issue Primary indicators Secondary indicators Frequency of 

measurement (yrs)

Water 

level

HCO3 pH DOC2 Cl SO4

*

Changing water table

x

Spring discharge

0.25

*

Total ground water reserves x

Water-quality indices; storage changes

5

*

Acid neutralization

x x

Al, Ca

0.5

Salinity

x

Mg/Cl, Br, δ18O, δ2H, total dissolved solids, 

SpC3 0.5

Agricultural impact

x x

K, Na, PO4, pesticides

0.5

Urban industrial impact

x x x

B, PO4, solvents, metals

0.5

Radioactive contamination

3H, 36Cl, 85Kr

2

Aquifer redox status

x x

Eh, Fe2+, HS

0.5

*

Land-use/forestry change

x x 2

Depletion of paleowater

x s

δ18O, δ2H, 14C, CFC’s

2

*

Changing recharge and climatic influence x

δ18O, δ2H, 

2

*

Mining impact

x x

Metals

0.5

1DO = dissolved oxygen2DOC = dissolved organic carbon3SpC = specific conductance (electrical conductivity at 25°C)
95

Staffing and 

Resource 

Needs

Expertise

Hardware/

Software

In the United States, management of the development and use of ground water 

resources is primarily the responsibility of State and local governments. No 

Federal law or regulation applies across the entire country. Increasingly, the 

Forest Service is involved in ground water issues to ensure that ground water 

users or ecological resources on NFS lands will not be impacted and that 

development will not impair ground water quality. Restoration or remediation 

of contaminated ground water is typically achieved under the authorities 

established in Federal laws; for example, CERCLA, RCRA, SDWA, and 

CWA. Remediation projects are typically directed and overseen by appropriate 

State or Federal agencies. The following staff and resource requirements are 

considered essential for effective management of ground water resources on 

NFS lands.

A staff with the pertinent expertise is critical for any ground water 

management organizational unit or program. The study of ground water is 

interdisciplinary. It requires knowledge of many of the basic principles of 

geology, physics, chemistry, and mathematics. The most appropriate disciplines 

are hydrogeology, hydrology, and geochemistry. Ground water occurs, flows, 

and obtains its chemical signature in the geological environment; therefore, it 

is critical to be able to characterize and understand the geological environment 

and its control on the movement and chemistry of ground water. Knowledge 

in the fields of biology, geophysics, soil science, and statistics, and geographic 

information systems (GIS) provides the hydrogeologist with additional tools 

needed to describe ground water flow systems and to assess human impact on 

these systems.

Many universities offer specific degrees in these areas. It is not appropriate 

to staff a ground water program with people who have training and university 

degrees that do not include these areas. Unfortunately, it has been very 

common to staff ground water programs in other organizations with people 

who have some experience in water-related issues but no formal training as 

ground water scientists.

Ground water scientists rely on data analysis, mapping and analytical and 

numerical models to help develop and evolve conceptual understandings 

related to ground water flow, chemistry, and interaction with surface water. 

Sound conceptual understandings are essential for wise management of ground 

water resources in a given aquifer or area.

Computer models. Many sophisticated models have been developed for 

simulating ground water flow and contaminant transport. Purchasing and using 

these models can be expensive and time consuming. Public domain ground 

water models, such as MT3D and the USGS code MODFLOW, have been 

extensively used and improved and are available at nominal cost. The same 

is true for public domain geochemical speciation and mixing models such 

as EPA’s MINTEQA2 or USGS’ PHREEQC. Other proprietary software for 

modeling or developing model inputs is also available. 
96

Field and 

Laboratory 

Requirements

Training

GIS. In the past 10 years the development and use of GIS software has 

increased dramatically. GIS software can be used to perform spatial analyses 

and develop spatially-based input files for other programs. GIS technology is 

essential for performing ground water inventories.

Other applicable software. Geological software packages such as 

ROCKWORKS include a number of analytical and semi-analytical programs, 

data plotting programs, and cross-section programs. Statistical analysis of 

water-quality data can be performed using WQStat Plus. Geochemical analysis 

of water chemistry data can be evaluated with Aquachem.

A ground water program requires field staff and access to facilities for waterquality analysis. It is common for many State and Federal water-quality 

programs to require responsible parties to bear the analytical costs, but this 

requirement may not be realistic when a ground water inventory is needed. 

There are hundreds of EPA-certified water-quality laboratories across 

the United States. EPA certification ensures the use of consistent sample 

management protocols and analytical and reporting methods. All analytical 

water-quality testing in support of Forest Service or special-use activities on 

NFS lands should be conducted by EPA- or State- certified laboratories. 

Some standard water-quality measurements should be conducted in the field at 

the time of sampling. Standard field equipment for the hydrogeologist includes 

a pH meter, specific conductance meter, temperature meter, water-level 

indicator, bailer, sampling pump, and water-sampling equipment.

It is imperative that any ground water management staff have access to 

continuing education. The fields of hydrogeology, hydrology, and geochemistry 

are dynamic. New things are learned and new tools and techniques are being 

developed regularly. Budgets for ground water programs should include 

adequate funds for training. A number of national and international professional 

organizations specialize in ground water and provide training opportunities. 

These include the National Ground Water Association, the International 

Association of Hydrogeologists, the Geological Society of America, the 

American Geophysical Union, and the American Institute of Hydrology. Many 

States have active ground water or water-resources associations. In addition, 

certifications for ground water professionals can be acquired through the 

National Ground Water Association, the American Institute of Hydrology, and 

many States.
97

Basic 

Hydrogeologic 

Principles

Ground Water 

Regions of the 

United States

Hierarchical 

Classification 

of Aquifers

Part 3. Hydrogeologic Principles and Methods of 

Investigation

The following discussion is abstracted from Heath (1984), to which the 

reader is referred for additional details of the ground water characteristics of 

individual regions. Additionally, an updated (2001) “Ground Water Atlas of the 

United States” is available on line from the USGS at http://capp.water.usgs.

gov/gwa/gwa.html. The atlas consists of 13 chapters that describe the ground 

water resources of regions that collectively cover the 50 States, Puerto Rico, 

and the U.S. Virgin Islands. Definitions of common hydrogeological terms and 

concepts are presented in appendix II of this technical guide.

To divide the country into ground water regions, a classification was developed 

that identifies features of ground water systems that affect the occurrence 

and availability of ground water. The five features of this classification are as 

follows:

(1) the components of the system and their arrangement (confined and 

unconfined aquifers, confining units), 

(2) the nature of the water-bearing openings of the dominant aquifers 

(primary vs. secondary porosity), 

(3) the mineral composition of the rock matrix of the dominant aquifers 

(soluble vs. insoluble), 

(4) the water storage and transmission characteristics of the dominant 

aquifers, and 

(5) the nature and location of recharge and discharge areas.

The first two features are primary criteria used in all delineations of ground 

water regions. The remaining three are secondary criteria that are useful in 

subdividing regions into more homogeneous areas. On the basis of the these 

criteria, the United States, Puerto Rico, and the U.S. Virgin Islands are divided 

into 15 ground water regions plus alluvial valley aquifers (fig. 34).

The nature and extent of the dominant aquifers (fig. 35) and their relation 

to other units of the ground water system are the primary criteria used in 

delineating the regions. Consequently, the boundaries of the regions generally 

coincide with major geological boundaries, rather than with drainage divides.
98

A

B

Figure 34. (A) Ground water regions of the United States, and (B) alluvial valley aquifers (Heath 

1984).
99

A Classification 

Framework for 

Ground Water

Figure 35. Principal aquifers of the United States (Miller 1998).

The first step in an inventory is to identify and map the areal extent of aquifers. 

The classification framework for surface water employs a hierarchy of units 

for resource characterization and management purposes (watersheds, basins, 

hydrologic units, and so on). A similar framework for ground water can be 

useful for understanding, classifying and mapping ground water resources 

(Maxwell and others 1995). The hierarchical classification presented here is 

based on mappable features that control ground water occurrence, flow and 

quality. In order of descending scale, the following is the hierarchy of units:

Ground water regions

 Hydrogeological settings

 Aquifers

 Aquifer zones

 Aquifer sites

Ground water regions are geographic areas where the composition, 

arrangement, and structure of rock units that affect the occurrence and 

availability of ground water are similar. Heath (1984, 1988) built on the work 

of Meinzer (1923) and Thomas (1952) to map 15 ground water regions in the 

United States (fig. 15). Ground water regions coincide closely with 
100

physiographic provinces (Fenneman 1938) and their boundaries reflect the 

nature and extent of dominant aquifers and their relations to other units of 

the ground water system. Ground Water regions can underlie large areas. 

For example, the High Plains Ground Water Region underlies eight river 

basins in Nebraska, Oklahoma, and the Texas Panhandle. Most ground water 

regions underlie tens of thousands to hundreds of thousands of square miles. 

Exceptions are segments of the Alluvial Valleys Ground Water Region, which 

are so narrow that they typically underlie tens to hundreds of square miles.

Ground water regions have been subdivided into hydrogeological settings 

(Aller and others 1987). A hydrogeological setting is defined as a composite 

description of all the major geological and hydrological factors that affect 

and control ground water movement into, through, and out of an area. It 

is a mappable unit with common hydrogeological characteristics, and as a 

consequence, has common vulnerability to contamination by introduced 

pollutants (Aller and others 1987). Although not yet mapped for most of the 

United States, a suite of hydrogeological settings has been described for each 

ground water region. Hydrogeological settings range in size from tens to 

hundreds of square miles. A typical map scale is 1:250,000.

An aquifer is a water-bearing geological formation, group of formations, or 

part of a formation that contains sufficient saturated permeable material to 

yield usable quantities of water to a well or spring (Lohman 1972). Within 

each aquifer, ground water moves from areas of recharge to areas of discharge. 

Flow direction, velocity, and discharge rates are controlled by aquifer porosity, 

hydraulic conductivity, and hydraulic gradient. Aquifers range in area from a 

few to hundreds of square miles. The recommended mapping scale is in a range 

from 1:24,000 to 1:63,000.

Aquifer zones are subdivisions of aquifers with differing hydrological 

conditions. Aquifer zones include recharge and discharge areas as well as 

confined and unconfined areas. Locally important hydraulic connection to 

surface-water systems that may be obscured at coarser hierarchical levels are 

identified at this level. Recharge may occur through direct precipitation, losing 

streams and lakes, or leakage from other aquifers. Discharge may occur to 

springs, seeps, gaining streams, lakes, and wetlands, by evapotranspiration, or 

by seepage into adjacent aquifers. Recharge zones are usually greater in area 

than discharge zones. Regionally significant recharge and discharge zones 

can occur in discrete localized areas. Recharge can be through fault zones or 

sinkholes; discharge can be through springs, and so on. Any one stream, lake, 

or wetland may have both gaining and losing portions, but in certain locations, 

either recharge or discharge may dominate. 

It is not uncommon for a single aquifer to include areas where the ground water 

is confined as well as areas where it is not confined. This condition occurs 

because many aquifers outcrop or subcrop along part of their areal extent and 

are buried beneath other geologic units along other portions of their areal 

extent. These areas should be considered distinct aquifer zones. 
101

Geology and 

Ground Water

Recharge and discharge can also occur at aquifer sites, which are specific 

features such as sinks and springs. Sinks and springs may be single points, 

clusters of points, or linear features along streams. They are most common 

in karst areas. A spring is ground water that naturally discharges from a 

geologic unit or aquifer onto the land surface or into surface waters. Sinks are 

commonly formed by the dissolution of soluble bedrock or semiconsolidated 

sediments (e.g., calcite-, dolomite-, and gypsum-bearing materials). 

Ground water occurs in openings in the rocks that form the Earth’s crust. The 

volume of the openings and the other water-bearing characteristics of the rocks 

depend on the mineral composition, age, and structure of the rocks. Therefore, 

to understand the occurrence of ground water in an area, it is necessary to have 

an understanding of the geology of that area. Below is a summary of a detailed 

discussion of geology and ground water by Heath (1984).

The United States is underlain by many different rock types. The nature of the 

water-bearing openings (porosity) in these rocks depends to a large extent on 

the geological age of the rocks as well as the processes that formed and may 

have subsequently modified the rocks. The youngest rocks are unconsolidated 

sedimentary deposits such as sand, gravel, clay, and glacial till, as well as 

volcanic rocks. The openings in sedimentary rocks generally are pores between 

the mineral grains (fig. 36). The openings in volcanic rocks include cooling 

fractures, pores in ash deposits, and lava tubes. Both of these geologically 

young rocks tend to be able to store and transmit more water than do older 

rocks of the same type.

At the time of their formation, crystalline rocks, such as granite, do not 

contain any appreciable porosity. Over the course of geological time, various 

tectonic forces and release of confining pressure cause the rocks to break along 

horizontal and vertical sets of fractures, which can then serve as water-bearing 

openings (fig. 36). Similar fractures can also form in sedimentary rocks that 

have been deeply buried and then are exposed by erosion of overlying rocks.

Carbonate rocks (limestone and dolomite) are soluble in weak acidic solutions, 

such as rainwater that percolates through the soil. As the rocks dissolve, often 

along existing fractures or bedding planes, these openings can enlarge to form 

large passages, sinkholes, and caverns (fig. 36). Areas in which these processes 

have occurred are called karst areas. Such enlarged solution openings can 

contain and transmit huge quantities of ground water, and these areas can 

have true “underground rivers.” Karst areas in which sinkholes are common 

are particularly vulnerable to contamination from the surface because the 

contaminants can travel rapidly to the water table.

Although nearly all rock types can contain ground water, the earth materials 

that are most important as sources of ground water include sand and gravel, 

limestone/dolostone, sandstone, and extrusive volcanics, such as basalt and 

rhyolite. Earth materials with limited fractures and few or extremely small 
102

Figure 36. Types of openings in selected water-bearing rocks. The size of the blocks can range from a few 

millimeters (A) to tens of meters (C and D) (Heath 1983).

intergranular openings generally do not readily yield water to wells and act 

to impede ground water flow. Earth materials that primarily act as barriers 

to ground water flow include silt, clay, shale, glacial till, and unfractured 

crystalline rocks. Clay deposits are composed of microscopic, flat particles 

that form an irregular (but very open) structure laced with very small pores. 

The pores are so small that most of these openings are occupied by water that 

is bound to the surface of the clay particles. Only minute amounts of water 

within these deposits are free to move. Although typically composed of up to 

50 percent water, saturated clays may release less than 1 percent of that water 

when allowed to drain freely by gravity. 

Of primary interest in hydrogeology is the capability of the various rock units 

to store and transmit water. Aquifers are identified on that basis. Geological 

units can be categorized as potential aquifers by describing the rock unit, 
103

interpreting the environment in which the rock unit was deposited, and 

interpreting the post-depositional conditions experienced by the unit. For 

purposes of assessing ground water potential, rock units generally have the 

following characteristics:

Massive Shale/Clay/Silt/Glacial Till. Thick-bedded shale, claystone, siltstone, 

glacial till, or clay typically yields only small quantities of water from 

fractures. Wells drilled into these units are often dry. These units often serve as 

confining layers in sedimentary sequences, producing artesian aquifers where 

an aquifer exists below the unit and that aquifer is connected to a recharge 

area. These units are distributed throughout the country, and are often found in 

conjunction with water-bearing units resulting in complex ground water flow 

systems.

Unweathered Metamorphic/Intrusive Igneous Rock. Consolidated bedrock 

of metamorphic or igneous origin contains very little or no primary porosity 

and yields water only from fractures or joints within the rock (secondary 

porosity). Typically, well yields are very low; dry holes very often occur, 

or wells go dry after producing only for a short time. Very low yields are 

sometimes obtained from fractures. In general, however, these units neither 

store nor transmit much water and are of only minor importance as aquifers. 

Weathered Metamorphic/Igneous Rock. Unconsolidated material, commonly 

termed regolith or saprolite, is derived by weathering of the underlying 

consolidated bedrock, and contains only primary porosity. Water generally 

moves readily in this rock, but well yields are commonly low because the 

available thickness often is insufficient to adequately supply a well.

Bedded Sandstone/Limestone/Dolostone. Typically, thin-bedded sequences 

of consolidated sedimentary rock contain substantial porosity. The primary 

porosity in sandstones is generally substantial and minor in limestones and 

dolostones, while the secondary porosity in limestones and dolostones may be 

considerable.

Bedded Shale/Clay/Silt. Thin-bedded shale typically contains some secondary 

porosity as fractures and minor primary porosity along bedding planes. Thinbedded clay and silt contains substantial primary porosity, but generally does 

not yield adequate water to supply a well.

Massive Sandstone. Consolidated sandstone bedrock contains both primary 

and secondary porosity and is typified by thicker deposits than the bedded 

sandstone deposits, which are distinguished by several identifiable beds over 

a distance of a few tens of feet. Sandstone is most important as a source of 

ground water where the cementing minerals have been deposited only around 

the points of contact of the sand particles, resulting in appreciable intergranular 

porosity. Bedding plane openings and other fractures in sandstone may also
104

Ground Water 

Flow Systems

Recharge and 

Discharge

yield substantial amounts of water to wells. Sandstone is an important source of 

ground water in the north-central part of the country, in Texas, and in a narrow 

zone west of the Appalachian Mountains.

Massive Limestone/Dolostone. Consolidated limestones and dolostones 

are generally characterized by substantial secondary porosity, usually from 

fractures, but they can also have significant porosity developed from solution 

cavities that form along fractures. Limestones and dolostones are the sources 

of some of the largest well and spring yields in the United States. Yields of 

thousands of gallons per minute are common from springs and wells that 

are developed in carbonate rocks. These rocks underlie large areas in the 

Southeastern and Central United States.

Sand and Gravel. Unconsolidated mixtures of sand- to gravel-sized particles 

contain varying amounts of fine materials. The fine materials can be clays and 

silts and limit the interconnectivity of the porosity, or they can be sands and 

not substantially limit that connectivity. In the latter case, these materials are 

capable of being very productive aquifers if a sufficient thickness of material is 

present. Sands and gravels that contain only small amounts of fine materials are 

termed “clean.” Their ability to move ground water can be high, and wells in 

them can be highly productive. Sand and gravel deposits from glacial activity, 

stream deposits, or mass movements, such as landslides and debris flows, are 

the sources of much of the ground water pumped from wells in the United 

States. These deposits occur throughout most of the country. The importance 

of sand and gravel deposits as a source of ground water is a result of both their 

widespread distribution and their capacity to yield water to wells at large rates.

Volcanic Rock. Consolidated extrusive igneous rock generally contains 

secondary porosity along fractures, interflow zones, and in vesicles. When well 

fractured, it often has high well yields. Basalts, rhyolites and other volcanic 

rocks are also among the most productive water-bearing formations. Basalt, 

which may be composed of thick layers that represent individual lava flows, is 

common in the northwestern United States. Large amounts of ground water can 

be pumped from both fractures within the flow units and from coarse-grained 

sediments that may be present between the individual lava flows. Lava tubes, 

common in Hawaii’s volcanic rocks, act as channels for ground water flow.

Karst/Fractured Limestone/Dolostone. Consolidated limestone/dolostone 

that has been dissolved to the point in which large, open interconnected cavities 

are present is known as karst. Both karstic and fractured limestone/dolostone 

are capable of very large well yields, but water quality may be more like 

surface water than most ground water. 

The addition of water to an aquifer is called recharge. It often occurs through 

infiltration of rainwater or snowmelt through the surface soil, followed by 

downward percolation through the unsaturated zone. The portion of infiltrating

water that percolates to the water table is termed recharge. The amount of 

recharge by precipitation depends on factors such as the amount of rainfall, 
105

soil type, subsurface geology, slope, aspect, depth to the water table, and 

vegetation cover. Rates of recharge can range from less than an inch per year in 

the desert areas of the Southwest to more than 30 inches per year in karst areas 

of the Southeast. Other mechanisms of naturally occurring recharge include 

infiltration from streams and lakes and ground water flow from adjacent 

aquifers. Recharge can also be artificially created through establishment of 

infiltration ponds and galleries and by injection of water through wells.

Ground water leaves an aquifer (known as discharge) by several mechanisms. 

In areas where the water table is relatively shallow, transpiration by plants or 

direct evaporation from the water table is a common discharge mechanism. 

A large percentage of the baseflow to streams can be made up of discharged 

ground water (fig. 37). Ground water also discharges to ponds and lakes, 

wetlands, and the sea, as well as to adjacent aquifers. In addition, water 

withdrawn from wells accounts for the discharge of millions of gallons of 

ground water each day.

Under natural conditions, the ground water system develops a quasi 

equilibrium (or “steady state”) with its recharge and discharge. That is, 

averaged over some period of time, the amount of water entering the system is 

about equal to the amount of water leaving the system. Because the system is 

in equilibrium, the amount of water stored in the system is constant or varies 

about some average condition in response to annual or climatic variations. 

As humans develop the ground water resources of an area, the natural system 

equilibrium begins to change (becomes “transient”), and the result can be an 

increase in recharge, a decrease in discharge, removal of ground water from 

storage in the aquifer, or a combination of all three.

An example of a recharge/discharge system in the Basin and Range 

hydrogeologic setting is shown in figure 38. Most recharge to basin aquifers 

occurs from precipitation falling on bedrock highlands. This water makes its 

way to the ground water reservoir along the basin margins, and from losing 

reaches of the larger intrabasin streams. 

At present, our ability to quantify recharge and discharge is limited, and no 

uniformly acceptable methods exist for measuring recharge and discharge 

fluxes (National Research Council 2004, De Vries and Simmers 2002, Halford 

and Meyer 2000). Methods that have been used successfully in specific 

situations include measurements in surface water using channel water budget, 

baseflow discharge, seepage meters, heat tracers, isotopic tracers, solute massbalance, and watershed modeling. Measurements in ground water using age 

dating, environmental tracers (CFCs, 3H/3He, 14C), Darcy’s Law, and numerical 

modeling also have been successful. In general, the interconnected nature of 

the hydrologic system necessitates that some combination of information from 

both surface and ground waters be used to in order to develop a comprehensive 

view of an aquifer’s water budget.
106

Figure 37. In the conterminous United States, 24 regions were delineated by the USGS where the interactions of ground water 

and surface water are considered to have similar characteristics. The estimated ground water contribution to streamflow is 

shown for specific streams in 10 of the regions (Winter and others 1998). 
107

Movement of 

Ground Water

Regional and 

Local Flow 

Systems 

Ground water moves from areas of high hydraulic head (usually upland areas) 

to areas of low hydraulic head (such as lowland areas, marshes, springs, and 

rivers). This allows a hydrogeologist to make use of water-level data obtained 

from wells, springs, and surface water features to determine the direction of 

ground water movement, both horizontally and vertically, as well as to estimate 

the quantity of ground water flow.

A potentiometric surface is an imaginary surface that represents the total 

head in an aquifer. It represents the height above a datum plane at which 

the water level stands in tightly cased wells that penetrate the aquifer in 

multiple locations. The water table is a special type of potentiometric surface. 

Potentiometric-surface maps can be constructed from water-level data by 

plotting these data on a map and contouring the interpreted surface based on 

these data. The potentiometric contours are also called equipotential lines. 

In most cases, the direction of ground water flow is perpendicular to the 

potentiometric contour. Figure 39 shows an example of a potentiometricsurface map and the inferred directions of ground water flow. Figure 40 

shows an example of a cross section used to infer vertical ground water flow 

directions.

The areal extent of ground water flow systems varies from a few square miles 

or less to tens of thousands of square miles. The length of ground water flow 

paths ranges from a few feet to tens, and sometimes hundreds, of miles. A deep 

ground water flow system with long flow paths between areas of recharge 

and discharge may be overlain by, and in hydraulic connection with, several 

shallow, more local, flow systems (fig. 41). Thus, the definition of a ground 

water flow system is to some extent subjective and depends in part on the scale 

of interest in a given study.

Figure 38. Two-dimensonal conceptual model of a ground water recharge system in a Basin 

and Range hydrogeological setting (Mifflin 1988).
108

Figure 39. Using known altitudes of the water table at individual wells (A), contour maps of 

the water-table surface can be drawn (B), and directions of ground water flow (C) can be 

determined (Winter and others 1998).
109

Figure 40. If the vertical distribution of hydraulic head in a vertical section is known from nested 

piezometers (wells completed at discrete intervals below land surface), vertical patterns of 

ground water flow can be determined (Winter and others 1998).

Figure 41. A regional ground water flow system entails substyems of at different scales and a complex hydrogeological 

framework (after Sun 1986).
110

Significant features of the flow system depicted in figure 22 include (1) local 

ground water subsystems in the upper water-table aquifer that discharge to the 

nearest surface water bodies (lakes or streams) and are separated by ground 

water divides beneath topographically high areas; (2) a subregional ground 

water subsystem in the water-table aquifer in which flow paths originating at 

the water table do not discharge into the nearest surface water body but into 

a more distant one; and (3) a deep, regional ground water flow subsystem 

that lies beneath the water-table subsystems and is hydraulically connected to 

them. The hydrogeologic framework of the flow system exhibits a complicated 

spatial arrangement of high hydraulic-conductivity aquifer units and low 

hydraulic-conductivity confining units. The horizontal scale of the figure could 

range from tens to hundreds of miles.

Ground water study areas can range from less than 1 square mile to several 

square miles for local-scale studies, to hundreds of square miles for regional 

studies. Examples of local studies include those associated with problems 

involving drainage from individual mines, leaking underground storage tanks, 

accidental spills of hazardous materials, and parts of hydrogeological units 

that are near heavily pumped public-water-supply wells in which contaminated 

ground water is present. Regional studies include those covering an entire basin 

or region (Brahana and Mesko 1988). Basin-wide studies can include proposed 

mine or well-field development, ground water/surface water interaction 

problems, or mine reclamation. Regional-scale studies are generally associated 

with resource inventories that cover more than one drainage basin. The general 

types of information needed for studies of each of these scales are similar, but 

the amount of detail needed for each can be very different.

Attributes of local-scale studies of ground water are listed in table 4. Because 

of the range in possible project objectives, these attributes are quite general. 

Many local-scale studies, particularly on surficial hydrogeological units, are 

based on newly constructed project wells, the locations of which are guided 

by patterns of flow in the local ground water system. Examples of localscale studies include (1) local-scale aquifer assessments, (2) early warning 

monitoring studies, (3) monitoring of point sources of contamination, (4) 

flow-path water-quality studies, and (5) local-scale studies of the interactions 

between ground water and surface water.

Local-scale assessments typically are used for areas and volumes of 

hydrogeological units in which potentially high concentrations of contaminants 

or locally high variability in water quality or quantity are expected. Early 

warning monitoring studies are conducted in areas where important ground 

water bodies are vulnerable to gradual inflow of contaminated ground water 

or to changes in water budget. Frequent sampling of monitoring wells is 

characteristic of local-scale studies.
111

Table 4. General attributes of local-scale assessments of ground water systems.

Attribute Explanation

General objectives Objectives of local-scale ground water assessments and research studies range 

from a survey of local-scale occurrence and distribution of water-quality and 

aquifer characteristics, to research studies on the transport and degradation of 

selected analytes, particularly in surficial hydrogeological units. The focus of 

many local-scale water-quality studies is to relate water quality explicitly to the 

ground water flow system.

Volume of earth material targeted for 

sampling

Most frequently, a small part of a hydrogeological unit.

Existing wells or new wells Likely new wells, possibly supplemented by existing wells.

Number of wells to be sampled Variable, depending on objectives and project design.

Well-selection strategy Depending on study objectives, locations for new wells may be selected randomly 

or nonrandomly. Nonrandom locations may, for example, be in relation to the 

local ground water flow system and additional physical and cultural features, 

such as surface-water bodies, potential sources of contamination, and discharge 

locations, including water wells.

Temporal sampling strategy Depends on study objectives; the objectives of many types of local-scale studies 

would require multiple samples from at least some of the wells.

Selection of target analytes Analytes are targeted to meet study objectives.

For local-scale problems, information from a just a few wells may be 

sufficient, but the information needed for each of the wells may be extensive. 

Aquifer characteristics and water-quality samples might need to be defined for 

several intervals within the well, and the lithology may need to be described 

at intervals of less than a foot vertically within the well. Characterization of 

the physical and hydrologic properties of many individual fractures within the 

bedrock may also be required. Stream discharge may also need to be defined 

throughout many small segments of the stream for a local study. Topographic 

information, typically available from USGS quadrangle-scale (1:24,000) 

topographic maps, may not be of sufficient detail for a local study. 

Regional-scale assessments or occurrence and distribution surveys of 

hydrogeological units are characterized by a wide spatial coverage and a 

broad array of analytes. The principal purposes of these broad surveys are 

(1) to provide evidence for naturally occurring constituents, including natural 

and anthropogenic contaminants that are present in water samples derived 

from a hydrogeological unit; (2) to provide an indication of contaminant 

concentrations by geographic location; and (3) to define general aquifer 

characteristics. General attributes of regional-scale studies of ground water are 

listed in table 5. 
112

Table 5. Attributes of a regional-scale assessment of a hydrogeologic unit or group of units (occurrence and distribution survey).

Attribute Explanation

General objective To supplement existing data by providing a broad overview of ground water in 

a targeted hydrogeological unit or group of units—an occurrence survey and the 

beginning of a study of spatial distribution of water-quality constituents and aquifer 

characteristics in the hydrogeological unit(s).

Volume of earth material targeted 

for sampling

Generally, an entire hydrogeological unit or group of units; in thick 

hydrogeological units in which significant changes in water quality with depth are 

known or anticipated, dividing the hydrogeological unit into two or more parts 

based on lithology, depth, or both may be advisable. Sampling of these parts would 

then be structured separately.

Existing wells or new wells Generally, existing wells are used exclusively.

Number of wells to be sampled The number depends, in part, on the quality and breadth of existing water-quality 

data and on the known or anticipated spatial variability in water quality and aquifer 

heterogeneity; for example, in some surficial hydrogeological units, a considerably 

larger number of wells may be needed for a reasonable occurrence survey 

compared to some deeper confined hydrogeological units.

Well-selection strategy A random component in well selection is usually highly desirable; sampling as 

few different types of wells as possible is advisable as long as the desired spatial 

coverage is achieved.

Temporal sampling strategy Most wells are sampled once or twice unless (1) the entire assessment survey is 

repeated at some later time (generally 10 years or more).or (2) a well is selected to 

be part of a long-term monitoring (trend) network that is sampled at a fixed time 

interval.

Selection of analytes Broad array of analytes, encompassing all project and monitoring-program 

objectives.

For a regional-scale study, the information needs generally are the same as 

those in a local study, but the level of detail required is usually less. Well 

information is still needed, but data may have to be collected from many wells 

over the entire study area. At each well, aquifer characteristics and waterquality samples that are representative of the area surrounding each well may 

suffice, and lithology may be described in very general terms. Characterization 

of only major fracture sets may suffice, as will streamflow information for only 

major stream reaches and tributaries. Topographic information from regionalscale (1:100,000 or 1:250,000) maps may suffice.

Shallow aquifers generally are the focus of local-scale inventories, but the term 

“shallow” is relative. In parts of New England or southern Florida, shallow 

aquifers generally occur within the first 30 feet or so of the surface. In parts of 

the arid Southwest, where the depth to the water table is much greater, shallow 

aquifers may be at depths of hundreds of feet. Shallow aquifers tend to be 

recharged relatively quickly through infiltration of precipitation, and ground 

Shallow, 

Intermediate, 

and Deep 

Aquifers
113

water is generally young (less than 50 years old). These aquifers also tend to be 

drained by small streams, or, in agricultural areas, by drainage tiles or ditches. 

They may respond rapidly to local stresses. Understanding of ground water/

surface water relationships is particularly critical in assessments of shallow 

aquifers, and these aquifers can be very susceptible to contamination from 

surface sources. 

The amount of ground water in shallow aquifers can be highly variable, 

depending on the season or climatic conditions. During winter and spring, 

when evapotranspiration is low and precipitation is high, water levels may 

rise significantly, adding a large volume of water into storage. During summer 

and fall, when evapotranspiration is high and precipitation may be low, water 

levels tend to fall, draining ground water from storage. Ground water flow 

rates and directions, therefore, may be variable during the year or over a period 

of several years. Because of this temporal variability, data must be collected 

over relatively short intervals—from hourly or daily to weekly or monthly, 

depending on the objectives of the study. Shallow aquifers are most susceptible 

to periods of drought, and monitoring of ground water conditions is critical 

during those times. 

Deep aquifers are often the focus of basin-wide or regional-scale inventories. 

As with shallow aquifers, the term “deep” is relative, depending on 

hydrogeologic and climatic conditions. Deep aquifers in the Eastern United 

States may be at depths of hundreds of feet, but in the western part of the 

country, deep aquifers may be several thousand feet deep. Deep aquifers 

generally receive less recharge than shallow aquifers, and recharge mechanisms 

are variable. A deep aquifer may receive recharge from precipitation at outcrop 

areas that can be hundreds of miles away from the area of study, or recharge 

may occur by leakage from overlying or underlying aquifers. Ground water 

ages of deep aquifers generally are much greater than those of shallow aquifers. 

The ground water age of a deep aquifer can be on the order of thousands of 

years. Discharge of water from a deep aquifer tends to occur only to large, 

regional rivers or to fracture- or fault-controlled springs that are connected to 

the aquifer. Deep aquifers are often confined and hydraulically isolated from 

overlying shallow aquifers, and ground water flow direction can differ from 

that of the overlying aquifers. The chemical quality of deep-aquifer water is 

often different from the quality of water in shallow aquifers. Deep aquifers may 

have high dissolved-solids concentrations because of dissolution of minerals 

along the long flow paths in the aquifer. Because of their hydraulic isolation, 

deep aquifers tend to be less susceptible to anthropogenic contamination 

than shallow aquifers. Deep aquifers tend to be less affected by short-term 

drought conditions, and respond very slowly to changing climatic conditions. 

Data collection frequency, therefore, generally usually can be less (quarterly 

or annually) than that needed for shallow aquifers. Intermediate aquifers are 

transitional between shallow and deep aquifers, and have characteristics of 

both types of aquifers.
114

A ground water system consists of a mass of water flowing through the 

pores or cracks below the Earth’s surface. This mass of water is in motion. 

Water is constantly added to the system by recharge from precipitation, and 

water is constantly leaving the system as discharge to surface water and as 

evapotranspiration. Each ground water system is unique in that the source and 

amount of water flowing through the system is dependent on external factors 

such as rate of precipitation, location of streams and other surface water bodies, 

and rate of evapotranspiration. The one common factor for all ground water 

systems, however, is that the total amount of water entering, leaving, and being 

stored in the system must be conserved. An accounting of all the inflows, 

outflows, and changes in storage is called a water budget (Alley and others 

1999).

Human activities, such as ground water withdrawals and irrigation, change 

the natural flow patterns, and these changes must be accounted for in the 

calculation of the water budget. Because any water that is used must come 

from somewhere, human activities affect the amount and rate of movement of 

water in the system, entering the system, and leaving the system.

Some hydrologists believe that a predevelopment water budget for a ground 

water system (that is, a water budget for the natural conditions before humans 

used the water) can be used to calculate the amount of water available for 

consumption (or the safe yield). In this approach, the development of a ground 

water system is considered to be “safe” if the rate of ground water withdrawal 

does not exceed the rate of natural recharge. This concept has been referred to 

as the “Water-Budget Myth” (Bredehoeft and others 1982). It is a myth because 

it is an oversimplification of the information that is needed to understand the 

effects of developing a ground water system. As human activities change the 

system, the components of the water budget (inflows, outflows, and changes 

in storage) also will change and must be accounted for in any management 

decision. Understanding water budgets and how they change in response to 

human activities is an important aspect of ground water hydrology; however, 

a predevelopment water budget by itself is of limited value in determining the 

amount of ground water that can be withdrawn on a sustained basis.

Under predevelopment conditions, the ground water system is generally in 

long-term equilibrium. That is, averaged over some period of time, the amount 

of water entering or recharging the system is approximately equal to the 

amount of water leaving or discharging from the system. Because the system is 

in equilibrium, the quantity of water stored in the system is constant or varies 

about some average condition in response to annual or longer term climatic 

variations. This predevelopment water budget is shown schematically in figure 

42(A).

We also can write an equation that describes the water budget of the 

predevelopment system as:

Recharge (water entering) = Discharge (water leaving).

Ground Water 

Development

and 

Sustainability
115

Table 6. Possible sources of water entering and leaving a shallow ground water system under 

natural conditions.

Inflow (recharge) Outflow (discharge)

1. Areal recharge from precipitation that 

percolates through the unsaturated 

zone to the water table.

1. Discharge to streams, lakes, wetlands, 

saltwater bodies (bays, estuaries, or 

oceans), and springs. 

2. Recharge from losing streams, lakes, 

and wetlands. 

2. Ground water evapotranspiration. 

3. Flow from an adjacent aquifer. 3. Flow to an adjacent aquifer.

Figure 42. Diagrams illustrating water budgets for a ground water system for predevelopment 

and development conditions. (A) Predevelopment water-budget diagram illustrating that inflow 

equals outflow. (B) Water-budget diagram showing changes in flow for a ground water system 

being pumped. The sources of water for the pumpage are changes in recharge, discharge, and 

the amount of water stored. The initial predevelopment values do not directly enter the budget 

calculation (Alley and others 1999).

The water leaving often is discharged to streams and rivers and is called 

baseflow. The possible inflows (recharge) and outflows (discharge) of a 

shallow ground water system under natural (equilibrium) conditions are listed 

in table 6.

Humans change the natural or predevelopment flow system by withdrawing 

(pumping) water for use, changing recharge patterns by irrigation and urban 

development, changing the type of vegetation, and other activities. Focusing 

our attention on the effects of withdrawing ground water, we can conclude that 
116

Pumpage = Increased recharge + Water removed from storage + Decreased discharge.

the source of water for pumpage must be supplied by (1) more water entering 

the ground water system (increased recharge), (2) less water leaving the system 

through other discharge mechanisms (decreased discharge), (3) removal of 

water that was stored in the system, or (4) some combination of these three. 

This statement, illustrated in figure 42B, can be written in terms of rates (or 

volumes over a specified period of time) as:

It is the changes in the system that allow water to be withdrawn. That is, 

the water pumped must come from some change of flows and storage in the 

predevelopment system (Lohman 1972). The predevelopment water budget 

does not provide information on where the water will come from to supply 

the amount withdrawn. Furthermore, the predevelopment water budget only 

indirectly provides information on the amount of water perennially available, 

in that it can only indicate the magnitude of the original discharge that can be 

decreased (captured) under possible, usually extreme, development alternatives 

at possible significant expense to the environment.

Regardless of the amount of water withdrawn, the system will undergo some 

drawdown in water levels in pumping wells to induce the flow of water to 

these wells, which means that some water initially is removed from storage. 

Thus, the ground water system serves as both a water reservoir and a waterdistribution system. For most ground water systems, the change in storage 

in response to pumping is a transient phenomenon that occurs as the system 

readjusts to the pumping stress. The relative contributions of changes in 

storage, changes in recharge, and changes in discharge evolve with time. The 

initial response to withdrawal of water is changes in storage. If the system can 

come to a new equilibrium, the changes in storage will stop and inflows will 

again balance outflows and can be written as follows:

Pumpage = Increased recharge + Decreased discharge.

Thus, the long-term source of water to discharging wells is typically a change 

in the amount of water entering and/or leaving the system. How much ground 

water is available for use depends on how these changes in inflow and outflow 

affect the surrounding environment and what the public defines as undesirable 

effects on the environment.

In determining the effects of pumping and the amount of water available for 

use, it is critical to recognize that not all the water pumped is necessarily 

consumed. For example, not all the water pumped for irrigation is consumed 

by evapotranspiration. Some of the water returns to the shallow ground water 

system as infiltration (irrigation return flow). Most other uses of ground water 

are similar in that some of the water pumped is not consumed but is returned to 

the system. However, depending on the source of the water pumped and the
117

Ground Water 

Quality

nature of the ground water system, the portion not consumed may be lost from 

the part of the system from which it was withdrawn. Thus, it is important to 

differentiate between the amount of water pumped and the amount of water 

consumed and to understand the source of the water pumped and the recharge 

location of the water not consumed when estimating water availability and 

developing sustainable management strategies.

The possibilities of severe, long-term droughts and climate change also should 

be considered. Long-term droughts, which virtually always result in reduced 

ground water recharge, may be viewed as a natural stress on a ground water 

system that in many ways has effects similar to ground water withdrawals 

through reductions in ground water storage and accompanying reductions in 

ground water discharge to streams and other surface water bodies. Because a 

climate stress on the hydrologic system is added to the existing or projected 

human-derived stress, droughts represent extreme hydrologic conditions that 

should be evaluated in any long-term management plan.

Nearly all active ground water originates as rain or snow that infiltrates 

through the vadose zone to the water table or saturated zone. Because most 

water vapor that becomes precipitation occurs as a result of evaporation, 

it typically contains low concentrations of dissolved solids. Consequently, 

the chemical composition of natural ground water is primarily a result of 

physical, chemical, and biological processes that occur as water interacts with 

geologic materials as it moves downward through the vadose zone (in recharge 

areas) and flows (as ground water) to areas of discharge. Some of the more 

important processes include weathering of rock and soil, mineral dissolution 

and precipitation reactions (including for example, oxidation and reduction, 

ion exchange, and adsorption), and interactions between water and air. The 

types and concentrations of dissolved constituents in ground water are net 

effects of chemical reactions that have dissolved material from solid phases, 

altered previously dissolved constituents, or removed dissolved constituents by 

precipitation or other processes (Hem 1989). Biological activity and numerous 

physical processes influence these chemical processes. 

Commonly, precipitation that infiltrates to the subsurface moves vertically 

through a thickness of unsaturated (vadose) zone before reaching the water 

table. In the vadose zone, carbonic acid (H2CO3) is generated as water 

interacts with soils and oxygen. Carbonic acid typically lowers the pH of the 

water slightly. As ground water flows from recharge areas to discharge areas, 

residence time increases and continuing rock-water interaction results in an 

increase in total dissolved solids in the downgradient direction. As a result, 

ground water contains a wide variety of dissolved inorganic constituents in 

various concentrations. The concentration of TDS in ground water varies from 

less than 500 to more than 100,000 mg/L. The TDS of seawater is 35,000 mg/

L. The EPA secondary drinking-water standard is 500 mg//L.
118

Table 7. Major, minor, and trace dissolved inorganic constituents in ground water.

Major dissolved constituents

(> 5 mg/L)

Minor dissolved constituents 

(> 0.01–10mg/L)

Trace dissolved constituents

(< 0.1 mg/L)

Carbonic acid (H2CO3) Boron (B) Arsenic (As) Chromium (Cr)

Chloride1 (Cl) Carbonate1 (CO3) Cadmium (Cd) Phosphate (PO4)

Sulfate1 (SO4) Fluoridel (F) Zinc (Zn) Copper (Cu)

Bicarbonate1 (HCO3) Iron2 (Fe) Lead (Pb) Silver (Ag)

Calcium2 (Ca) Nitrate1 (NO3) Manganese (Mn) Selenium (Se)

Magnesium 2 (Mg) Potassium2 (K) Aluminum (Al) Radium3 (Ra)

Sodium2 (Na) Strontium2 (Sr) Antimony (Sb) Uranium3 (U)

Silicon2 (Si) Barium (Ba) Thorium3 (Th)

1

 anions

2

 cations

3

 radioactive

Most of the chemical constituents that are dissolved in ground water occur in 

ionic form. Ions that have negative charge (excess electrons) are referred to as 

anions. Ions that have a positive charge (excess of protons) are referred to as 

cations. Common major, minor, and trace dissolved inorganic constituents are 

listed in table 7. In intermediate and regional ground water flow systems the 

dominant anion often changes from bicarbonate to sulfate to chloride. Types 

and concentrations of dominant cations vary depending on the mineralogy 

and chemical composition of the rock or sediment and the dominant chemical 

reactions. 

Dissolved organic constituents and dissolved gases also occur in ground water 

but concentrations are usually low. Dissolved organic matter is ubiquitous in 

natural ground water and is thought to be primarily fulvic and humic acids. The 

most abundant dissolved gases in ground water are nitrogen (N2), oxygen (O2), 

carbon dioxide (CO2), methane (CH4), and hydrogen sulfide (H2S). The first 

three are atmospheric gases and the last two are products of biogeochemical 

processes that occur in anaerobic subsurface zones. Other minor dissolved 

gases include radon (radioactive), argon, helium, and neon.

Hardness is a water-quality property that has had widespread interest for 

centuries. Hardness refers to the effects observed in the use of soap with some 

types of water or to the encrustations left by heating some types of water. 

Hardness is further defined as the content of metallic ions that react with 

sodium soaps to form a scummy residue. Because hardness results primarily 

from the presence of calcium and magnesium, it is typically reported as the 

total concentration of Ca2+ and Mg2+ expressed in terms of an equivalent 

concentration of CaCO3. The designation of “soft” and “hard” water is 

somewhat arbitrary. Table 8 presents a commonly used classification developed 

by Durfor and Becker (1964). 
119

Description Hardness

Soft 0–60

Moderately hard 61–120

Hard 121–180

Very hard > 180

Table 8. Hardness classification based on equivalent concentration of CaCO3 (mg/L).

Alkalinity and acidity are other important properties of natural ground water. 

These properties refer to the capacity of ground water to neutralize an acid 

or base. Chemical reactions related to rock-water interaction result in low 

dissolved concentrations of hydrogen (H+) and hydroxyl (OH-) ions, which 

contribute significantly to acidity and alkalinity, respectively. Alkalinity of 

ground water is defined as the capacity for solutes it contains to react with 

and neutralize acid. Acidity is defined as the quantitative capacity of aqueous 

media to react with and neutralize a base. In most natural waters, the alkalinity 

is largely produced by dissolved carbon dioxide species (CO2), bicarbonate 

(HCO3

–

), and carbonate (CO3

2–). Hydroxide, silicate, and borate are important 

noncarbonate contributors to alkalinity. Alkalinity is most often reported as 

an equivalent amount of CaCO3 (mg/L). The pH of water is a measure of the 

concentration (activity) of H+ ions. Sources of acidity in natural ground water 

include low pH rain and snow, dissolved CO2, solution of volcanic gases or 

gaseous discharges in geothermal areas, and the oxidation of sulfide minerals 

and ferrous iron. Acidity is reported as meq/L or mg/L of H+.

As discussed previously, the chemistry of natural ground water is generally 

influenced greatly by the geological materials through which it flows. As 

a result, ground water in similar geologic materials tends to exhibit similar 

chemistry. In saturated sedimentary rock sequences, characterized by active 

ground water flushing through well-leached rocks, the ground water tends 

to be low in TDS with bicarbonate as the dominant anion. In rock sequences 

characterized by intermediate and regional-scale ground water flow systems, 

ground water circulation is relatively slow, and residence times are relatively 

long. In these hydrogeological settings, TDS concentrations in ground water 

tend to be higher with sulfate and chloride as the dominant anions. Ground 

water in carbonate rock aquifers tends to be higher in calcium, magnesium, 

and bicarbonate and lower in sodium, potassium, chloride, and sulfate. 

Ground water contained in crystalline igneous and metamorphic rocks is 

commonly soft and slightly acidic, with low concentrations of TDS and high 

concentrations of dissolved silica. It is important to note that ground water 

in karst aquifers can often be an exception to these general tendencies, since 

water flow may be quite rapid.

Ground water quality in glacial deposits is quite variable because of the large 

variability in mineralogy of glacial deposits. Glacial deposits that overlie the 

North American Pre-Cambrian Shield commonly contain soft, slightly acidic 

water with TDS concentrations less than 100 mg/L. Sodium, calcium, and
120

Ground Water/ 

Surface Water 

Interactions

magnesium are the common cations, and bicarbonate is the dominant anion. 

Ground water in glacial deposits overlying the interior plains of the United 

States commonly contains high concentrations of TDS. Sodium, magnesium, 

calcium, and sulfate occur in major concentrations. Ground water in shallow 

fluvial deposits is generally low in TDS and slightly acidic if derived from 

infiltration of precipitation. 

Ground water and surface water both originate as precipitation. From the 

moment that water from precipitation reaches the soil surface, its chemistry 

begins to change. Water that infiltrates surface soils and is underground 

for long periods tends to develop considerably different water-quality 

characteristics (chemical composition, temperature, and microbiological 

quality) than water that flows overland. Ground water and surface water 

interact (join and mix) at many locations in most watersheds; consequently, 

their flow rates, chemistries, temperatures, and microbiological qualities are 

often neither uniform nor estimable by simple extrapolation downstream or 

downslope. Ground water that originated as infiltrating precipitation today may 

be discharged months or years later to a stream or lake that also contains water 

recently contributed by precipitation. Conversely, surface water that originated 

as runoff from recent precipitation may be lost by seepage downward through 

the streambed to mix with ground water of much greater age.

Ground water and surface water interact on many physical scales and over a 

wide range of time periods. Some of these interactions may be observed and 

measured directly, while others may be detected and evaluated only by indirect 

methods or surrogate measures. The interactions of significant interest include 

(1) ground water supply to the baseflow of perennial streams and full flow of 

some ephemeral streams; (2) ground water supply of flow to springs, seeps, and 

cave systems; (3) streamflow supply of recharge to the ground water system; 

(4) ground water flow into, and ground water recharge from, reservoirs, lakes, 

ponds and lagoons; and (5) ground water controls on landforms and stream 

morphology. Observations and measurements of these interactions may be 

used to provide key inputs and constraints for watershed models and ecological 

assessments, thereby greatly improving their reliability and usefulness.

Ground water and streams may interact in a variety of ways. Ground water 

may flow directly into a stream through seeps or springs in the streambanks 

or streambed. Surface water may be lost by seepage from a stream channel 

to underlying ground water. Ground water and surface water may exchange 

repeatedly along the length of a stream or cyclically over time in a given 

stream reach in response to changing water-table and/or runoff conditions. 

Streams may disappear into the ground, and reappear elsewhere, especially in 

karst (limestone) terrain and in fractured rock settings.

The water regime of a stream is defined in terms of the presence of running 

water in the channel. Perennial streams flow year-round and are generally 

supported by abundant ground water discharge during dry periods. Some 

drainages are intermittent, containing perennial water only in certain segments 
121

fed by springs or ground water and dry for long distances during dry periods. 

Many drainages in semiarid areas and most desert drainages are ephemeral, 

containing running water only seasonally, usually in response to rainfall, and 

not necessarily every year. 

Ground water contributes to streamflow under at least some conditions in most 

physiographic and climatic settings. The proportion of stream water that is 

derived from ground water varies across physiographic, climatic, and seasonal 

settings. Knowledge of the amount of ground water recharge and discharge 

to streams and other surface-water bodies is important in quantifying the 

total ground water available in an area. In areas where streams primarily lose 

water to ground water, such as the arid Southwest, discharge of ground water 

may supply the drainage at its head, while in downstream areas infiltration of 

streamflow may be the major source of recharge to the ground water system. 

Ground water discharge to streams can be estimated by measuring streamflow 

during “baseflow” periods, when streamflow is almost entirely supported by 

ground water inflow to the stream channel. The baseflow of a stream is that 

portion of streamflow in the channel that has been contributed by ground water 

inflow to the stream. Baseflow may constitute a small portion or a majority 

of the streamflow. The average proportion of baseflow to the total streamflow 

ranges from a few percent annually to almost all of the streamflow in the 

channel annually.

Streamflow can be measured by several methods. The most common method 

currently used by the USGS involves obtaining point velocity measurements 

at predetermined locations along a cross-section of stream channel and 

multiplying these velocities by the area represented by each velocity 

measurement to get volumetric discharge. The velocity measurements may be 

made by either wading in small streams or by taking measurements in boats or 

from bridges and cableways. This method provides a measurement of stream 

discharge at one point along a stream at a single point in time. Continuous 

measurements are obtained at gaging stations by continuously measuring 

stream stage along a section of the stream, and applying a uniquely determined 

stage-discharge relation for that site. Streamflow obtained at these sites are 

often presented graphically as a “hydrograph.” Real-time stream-discharge data 

are available on the Internet for many streams at the USGS Web site at 

http://water.usgs.gov. Historic stream-discharge data can be obtained from 

individual water science centers of the USGS. Historic data can be used to 

develop flow-duration curves for gaged reaches, which can be used to help 

develop bounding estimates of baseflow in perennial streams.

Water quality and aquatic habitat in Daisy Creek on the Custer National 

Forest has been adversely affected by drainage from the McLaren Mine, as 

well as by natural weathering of pyrite-rich mineralized rock. Specific surface 

and subsurface sources of metals to the creek were identified by a synoptic 

sampling and tracer injection study. Knowledge of the main sources and 

pathways of metals and acid to Daisy Creek has aided resource managers in 

planning and conducting cost-efficient remediation activities.

Case Study:

Contribution of 

Metal Loads to 

Daisy Creek from 

Ground Water, 

Custer National 

Forest, MT 
122

Acid drainage from the McLaren Mine affects the water quality of Daisy 

Creek, an alpine headwater tributary of the Stillwater River. Water quality and 

aquatic habitat have been severely affected by drainage from mining as well 

as by natural weathering of pyrite-rich mineralized rock (fig. 43). Effective 

planning for remediation requires detailed knowledge of the sources of metals 

and how the metals from these sources enter the stream. Metal-loading studies 

have been useful in characterizing water quality in historical mining areas and 

identifying surface as well as subsurface metal sources and pathways. The 

USGS in cooperation with the Forest Service conducted a constant-rate tracer 

injection synoptic sampling study to quantify the principle sources of metal 

loads to Daisy Creek. 

In August 1999, a sodium chloride tracer was added to the stream for 29.5 

hours to provide a hydrologic context for synoptic sampling of metal chemistry 

in the stream and its inflows. Detailed profiles of metal loads along Daisy 

Creek were developed from streamflow data (obtained by tracer injection) 

and metal-concentration data (obtained by synoptic water-quality sampling) 

collected at many closely spaced sites. These profiles helped to identify reaches 

of Daisy Creek where most of the metal loading occurs. 

Inflows to the stream can be divided between visible surface inflows, which 

were sampled, and subsurface inflows, which were not sampled, but the effects 

of both types of inflows on the stream were quantified. Substantial loads were 

attributed to both sources (fig. 44). About 54 percent of the total copper load 

was contributed by surface inflows. Copper loading from ground water inflows 

was also substantial, contributing 46 percent of the total dissolved copper load 

to Daisy Creek. 

The upper 270 feet of Daisy Creek are relatively unaffected by historical 

mining activity and resulting water-quality impacts (fig. 44). Once the 

tributaries draining the McLaren Mine and related ground water inflows are 

encountered, however, significant impacts of acidity and elevated metals 

are encountered in Daisy Creek. The principal observable impacts from the 

McLaren Mine occur in the subreach from 270 feet to 611 feet downstream. 

The subreaches from 611 feet to 5,475 feet receive little surface water inflow, 

but ground water inflow into Daisy Creek continues to provide copper loading

. 

Flow through the shallow subsurface appears to be a major copper transport 

pathway from the McLaren Mine and surrounding mineralized bedrock 

to Daisy Creek during baseflow conditions. These results indicate that 

remediation of large visible inflows could still leave ground water-derived 

metal concentrations in Daisy Creek at levels that may adversely affect aquatic 

life.

For additional information see Nimick and Cleasby (2001).
123

Figure 43. Iron oxyhydroxide and associated heavy metals from acidic inflows degrade the water 

quality of Daisy Creek, Park County, MT.

Figure 44. Sources of dissolved copper to subreaches of Daisy Creek, including relative 

contributions of copper from surface water and ground water sources. Copper loading occurs 

primarily from surface inflow in the upper reaches, while ground water contributes substantial 

loads in the lower reaches. 
124

Gaining and 

Losing Stream 

Reaches

Some shallow ground water has a water level that lies above the elevation of 

the water surface in an adjacent stream channel. In such cases, ground water 

seeps through the stream bank and bed to discharge into the stream, which is 

referred to as a “gaining stream or reach” (fig. 45A). Where shallow ground 

water has a water level that lies below the elevation of the water surface in an 

adjacent stream, water may seep out of the channel through the stream bank 

and bed to locally recharge the ground water. In such a situation, the channel is 

referred to as a “losing stream or reach” (figs. 45B and C). Many streams have 

reaches of both types, gaining in some and losing in others. Most mountain 

streams have gaining reaches from their headwaters on downstream to a midvalley location, where they may have losing reaches. Farther downstream, near 

the mouths of the streams, additional gaining reaches are frequently found; 

these are typically the discharge zones of shallow ground water flow systems. 

The flow directions between ground water and surface water can change 

seasonally as the elevation of the ground water table changes with respect to 

the stream-surface elevation. They can change over shorter timeframes when 

rises in stream surfaces during storms cause recharge to the streambank. Under 

natural conditions, ground water makes some contribution to streamflow in 

most physiographic and climatic settings. Thus, even in settings where streams 

are primarily losing water to ground water, certain reaches may receive ground 

water inflow seasonally or under particular hydrologic conditions.

Losing streams can be connected to the ground water system by a continuous 

saturated zone (fig. 45B), or they can be disconnected from the ground water 

system by an unsaturated zone (fig. 45C). An important feature of streams that 

are disconnected from ground water is that pumping of ground water near the 

stream does not substantially affect the flow of the stream near the pumped 

well. A more thorough discussion of the interaction of ground water and 

surface water is presented, in a generally nontechnical format, by Winter and 

others (1998).

Many graphical techniques exist to analyze streamflow data, and several 

are applicable to ground water problems. Techniques and procedures for 

quantifying baseflow in streams are described in Fetter (2001) and McCuen 

(1998). The techniques can be used to estimate hydraulic properties of an 

aquifer and to estimate ground water recharge and discharge in a basin. These 

techniques have the advantage of providing information over a wide area 

(as compared to an aquifer-test analysis), integrating the effects of climate, 

topography, and geology in a basin; however, they have the disadvantages of 

being somewhat subjective and nonunique.

Flow-duration curves are cumulative frequency curves that show the 

percentage of time during which specified discharges of streams were equaled 

or exceeded in a given period of time (Searcy 1959). Comparison of flowduration curves can provide valuable insights into the drainage characteristics 

of different streams or of different reaches of the same stream. Steep curves
125

Figure 45. Interaction of 

streams and ground water 

(after Winter and others 

1998).
126

Figure 46. Flow-duration curve for the Rio Camuy near Hatillo, PR (Tucci and Martinez 1995).

indicate a high degree of runoff; flat curves indicate a high degree of surface 

or subsurface storage in the basin. Because the distribution of low flows is 

controlled chiefly by the geology of the basin, the lower end of the curve is 

a valuable means for studying the effects of geology on the ground water 

discharge to a stream (Searcy 1959). Many studies have used a flow-duration 

curve value as a substitute for direct estimates of mean baseflow. Some 

researchers select the 90 percent flow-duration value (the flow that is equaled 

or exceeded by 90 percent of the flow on record) as a conservative estimator of 

ground water discharge (Rutledge and Mesko 1996), but individual basins are 

highly variable. Flow duration values of as low as 40 percent have been found 

to represent a reasonable estimate of mean ground water discharge. Figure 46 

shows an example of a flow-duration curve for Rio Camuy, near Hatillo, PR. 

An independent evaluation of ground water discharge for Rio Camuy at this 

site indicated a baseflow of about 72 cfs, which approximately corresponds to 

the 60 percent flow-duration value on this curve (Tucci and Martinez 1995).
127

Figure 47. Streamflow and baseflow hydrographs for the Homochitto River in Mississippi (Winter and others 1998).

Streamflow recession methods, also referred to as hydrograph-separation 

methods, can provide information not only on baseflow to streams but also on 

transmissivity and storage values for a basin. These methods characterize the 

portions of the hydrograph following a recharge event, which are represented 

by a sharp increase in streamflow followed by a decline (or “recession”) (fig. 

47). The method was described in detail by Rorabaugh (1964), and computer 

programs to apply the method are available (Rutledge 1998, 2000). Several 

techniques for assessing the quantity and quality of ground water discharging 

to streams are presented in table 9.
128

Table 9. Types of studies that evaluate water-flow and water-quality interactions between ground 

water and surface water.

Type of study Explanation

Ground Water Contributions to Stream Flow and Quality

(1) Hydrograph 

separation

The objective is to divide the total streamflow hydrograph into two parts: (1) Storm runoff 

or quick-response flow that is related to storms and (2) ground water, which may augment 

storm runoff during storms, but occurs mainly as normal ground water discharge to streams 

during periods of streamflow recession when there is no precipitation. Hydrograph separation 

provides a useful index for the long-term proportion of streamflow that is derived from ground 

water, particularly when this index is used for comparative purposes—for example, between 

long-term averages for different seasons or periods of years for the same streamflow record or 

between records from different streamflow-gaging stations. Well-documented computer software 

packages that automatically perform hydrograph separation using daily flow records include the 

package by Rutledge (1993). 

(2) Synoptic sampling 

– tracer injection 

studies

The objective is to define the downstream changes in metal or other constituent loads in the 

stream and attribute them to sources along the stream as well as to instream geochemical 

reactions. Part of the cumulative total load can be attributed to visible surface inflows, and 

another calculation gives a maximum load due to diffuse ground water inflows. Comparisons 

of these different load profiles provide important chemical characteristics of streams useful for 

remediation planning. An approach that has worked well for mountain watersheds combines 

discharge measurements, using dye or salt, with synoptic sampling to provide spatially detailed 

concentration data (Kimball 1997). Detailed profiles of load along a stream are developed from 

streamflow data and constituent concentration data obtained by synoptic sampling at many 

closely spaced sites. 

(3) Sampling baseflow 

of streams

The objective of these studies is to quantify the contribution of ground water to the quality 

of total streamflow during different times of the year. The approach is to sample streamflow 

when the streamflow hydrograph indicates that all or most of the streamflow can reasonably 

be assumed to derive from ground water. To relate water quality sampling to the streamflow 

hydrograph, sampling generally is done at or near a gaging station, or a stream-discharge 

measurement is made as part of the sampling process.

(4) Determining 

integrated ground 

water inflow along 

stream reaches

The objective is to quantify the volume and quality of ground water inflow along a particular 

stream reach. The approach is to select two measuring points on a stream at which flow and 

water quality are measured. The ground water contribution to flow and water quality along 

the stream reach are determined by difference. These studies are more locally focused than 

assessments of the baseflow of streams.

(5) Evaluating 

discharging ground 

water

The objective is to determine the quality of shallow ground water that soon will discharge into a 

stream. The approach is to sample using streambed and streambank piezometers and to compare 

the ground water quality with stream water quality. Another possibility is direct sampling 

of ground water discharge to streams by means of seepage meters. Sampling from shallow 

streambed piezometers can be used in reconnaissance surveys to characterize ground water 

quality. The design of these surveys is guided by knowledge of flow patterns in the shallow 

ground water flow system and land use near the streams.
129

Table 9.— Cont.

Hyporheic Zone 

and Floodplain 

Mixing

Type of study Explanation

(6) Assessing spring or 

seep water

Springs are points of concentrated ground water discharge. They represent an opportunity to 

sample ground water discharge directly. Although often difficult to determine, the contributing 

area of a sampling site is a useful concept for springs. The sampled water quality from a spring 

may vary for some constituents, depending on where and how the spring is sampled—for 

example, as ground water from a piezometer immediately upgradient from the orifice or as 

surface water after discharge. A seep is an area where ground water oozes from the earth in small 

quantities. Therefore, seeps can be viewed as low-discharge end members of springs.

(7) Measuring surface 

water capture from 

shallow pumping wells 

located near a surface 

water body

Generally, the objective is to determine the proportion of the water pumped from a well field 

that is derived from surface water at different pumping rates. An additional objective may be to 

determine if pumping-induced movement of surface water through the shallow ground water 

system to the well results in changes in the original quality of the surface water; for example, 

one can determine whether the concentrations of key constituents from the surface water are 

decreased or eliminated before the stream water reaches the pumping well. Tools of analysis 

for this type of study and bank storage/overbank-flooding studies include water-mixing models, 

analysis of isotope data, and local-scale simulation of the ground water flow system.

Determining Interactions Related to Increases in Stream Stage

(1) Bank storage The objectives of bank-storage studies include determining (1) the movement of stream water 

into the ground water system during periods of rising stream stage and (2) the volume, time of 

release, and quality of former surface water, possibly mixed with original ground water, that 

returns to the stream during periods of falling stream stage. These studies rely on determining 

the quality of the surface water and shallow ground water near the stream.

(2) Overbank flooding The objective of overbank-flooding studies (similar to the objectives for bank storage studies) is 

to determine the volume and quality of surface water that recharges shallow ground water from 

the flooded land surface, as well as surface water that enters the shallow ground water system 

through the stream banks and bed. Given sufficient overbank flooding, parts of the underlying 

surficial hydrogeological unit may become completely saturated to the land surface. 

In certain circumstances, surface water and ground water may mix and 

remix rapidly over short distances. Hyporheic water is stream water that 

flows through shallow unconfined aquifers and is returned to the stream over 

relatively short time periods. It ranges from 100 percent for recent streamsource water in downwelling locations, to some mixture of longer residence 

time “ground water” and recent stream-source water. A popular convention 

is to limit the spatial extent of the hyporheic zone to areas where the water 

in the unconfined aquifer is composed of 10 percent or more recent streamsource water (Triska and others 1989). The following are among the hydrologic 

features that lead to hyporheic exchange flows:
130

•	 Any change in the longitudinal profile of the stream (pool-step or poolriffle sequences).

•	 Changes in aquifer thickness or width.

•	 Changes in saturated hydraulic conductivity.

•	 Presence of multiple channels (channel splits around islands, secondary 

channels, floodplain spring brooks).

•	 Buried relic channels that create longitudinally continuous preferential 

flow paths. 

•	 Channel meander bends (channel sinuosity).

•	 Interactions between streamflow and channel bed forms.

•	 Bank storage or overbank flooding and infiltration of flood water.

•	 Entrainment of stream water into mobilized bed sediments during 

floods.

Ground water and surface water may exchange over short distances along a 

stream because of streambed slope changes in high-gradient step-pool streams, 

and between meanders in lowland valley streams. In step-pool streams, ground 

water may flow into the stream channel at the upstream ends of the pools and 

return as stream water recharging the ground water at the downstream ends 

of the pools. This sequence of flows creates a unique ecological environment 

within the streambed sediments and their immediate surroundings. The water 

that washes back and forth through these sediments is much richer in oxygen 

and nutrients than that found deeper in the subsurface. This especially active 

envelope of sediments around and including the streambed can support unique 

biota that have evolved in and inhabit the hyporheic zone.

The meandering of streams offers opportunities for stream water and ground 

water to mix. Often, the direction of shallow ground water flow is roughly 

coincident with the predominant course of the stream. This assumption is 

reasonable for small undeveloped valleys, but less so for large valleys and 

where development has led to significant use of ground water. If flows are 

coincident with high-stage conditions in the stream channel, water moves from 

the stream to the ground water. For relatively short duration events, only bank 

storage of stream water occurs during the high stage; the water stored in the 

streambanks is released back into the stream as the high stage subsides. For 

relatively long duration events, the water that is moving continuously from 

the stream into the streambanks is pushed through the streambank and farther 

into the adjacent sediments, where it blends with the local ground water and is 

carried downgradient as part of the ground water flow system. Consequently, 

only some of the water lost by the stream may return to the channel as ground 

water inflow after the high stream stage subsides.

Overbank floods provide additional opportunities for the mixing of stream 

water and ground water. Stream water spreads out over the floodplain during 

overbank flooding and infiltrates the floodplain sediments to mix with ground 

water. Because overbank flooding may occur infrequently and be of very 

limited duration, the influx of oxygen and nutrients may be too short-lived 
131

Influence 

of Wells on 

Streams

to evolve and sustain unique biota. Certain riparian plants, however, require 

occasional inundation by overbank flooding for proper establishment and 

growth. Although the specific role of the influx of nutrients in floodwaters may 

not be known as yet for most riparian plant species, ecological principles

suggest that the role is not incidental.

Ground water pumping can substantially affect the quantity of surface waters, 

including not only downstream water supply for human consumption but 

also the maintenance of instream-flow requirements for fish habitat and other 

environmental needs. Long-term reductions in streamflow can affect vegetation 

along streams (riparian areas) that serve critical roles in maintaining wildlife 

habitat and in enhancing the quality of surface water. Pumping-induced 

changes in the flow direction to and from streams may affect temperature, 

oxygen levels, and nutrient concentrations in the stream, which may in turn 

affect aquatic life in the stream.

Figure 48 illustrates the following discussion on the source of water to wells 

(from Alley and others 1999). Under natural conditions (A), recharge at the 

water table is equal to ground water discharge to the stream. Assume a well 

is installed and is pumped continuously at a rate, Q1, as in (B). After a new 

state of dynamic equilibrium is achieved, inflow to the ground water system 

from recharge will equal outflow to the stream plus the withdrawal from 

the well. In this new equilibrium, some of the ground water that would have 

discharged to the stream is intercepted by the well, and a ground water divide 

(a line separating directions of flow) is established locally between the well and 

the stream. If the well is pumped at a higher rate, Q2, a different equilibrium 

is reached, as shown in (C). Under this condition, the ground water divide 

between the well and the stream is no longer present, and withdrawals from the 

well induce movement of water from the stream into the aquifer. Thus, 

pumping reverses the hydrologic condition of the stream in this reach from 

ground water discharge to ground water recharge. Note that in the hydrologic 

system depicted in (A) and (B), the quality of the stream water generally will 

have little effect on the quality of ground water; however, the loss of ground 

water to the stream could have an effect on water quality in the stream. In the 

case of the well pumping at the higher rate in (C), however, the quality of the 

stream water can affect the quality of ground water between the well and the 

stream, as well as the quality of the water withdrawn from the well. Although a 

stream is used in this example, the general concepts apply to all surface water 

bodies including lakes, reservoirs, wetlands, and estuaries.

The factors that influence the location of areas contributing water to wells can 

be categorized as dependent either on the ground water system or the well 

(Franke and others 1998, Reilly and Pollock 1993). The ground water factors 

that affect the paths of water movement in ground water systems are (1) the 

hydrogeological framework, (2) system boundary conditions, (3) aquifer 

properties, and (4) other transient effects, such as rainfall. The well factors are 

the location of the well, the depth of the screened or open-hole section of the 

well, and pumping rates. 
132

Figure 48. Effects of pumping from a hypothetical ground water system that discharges to a stream (after Heath 1983).
133

Springs

Types of Springs

The adjustments to pumping of an actual hydrological system may take place 

over many years, depending on the physical characteristics of the aquifer, 

the degree of hydraulic connection between the stream and aquifer, and the 

locations and pumping histories of wells. Reductions of streamflow as a result 

of ground water pumping are likely to be of greatest concern during periods 

of low flow, particularly when the reliability of surface water supplies is 

threatened during droughts.

Characterizing a ground water flow system involves definition of the aquifers 

and confining units that comprise the system, as well as quantification of the 

amount of ground water present in the system at any one time and the amount 

of ground water that is entering and leaving the system at that time. Definition 

of the system is scale dependent. For example the ground water system of 

interest in a problem involving a leaking underground storage tank can be very 

different from the system of interest when trying to quantify the amount of 

ground water that flows through a national forest. 

Springs are important sources of hydrogeological information. They occur 

because the hydraulic head in the aquifer intersects the land surface. The 

distribution, flow characteristics, and water quality of springs can provide as 

much, or more, information about an aquifer system as a well. Springs are 

relatively small riparian ecosystems that are maintained by water flowing from 

the ground (Hynes 1970). The classic definition is from Meinzer (1923, 48): “A 

spring is a place where, without the agency of man, water flows from a rock or 

soil upon the land or into a body of surface water.” Spring ecosystems include 

aquatic and riparian habitats that are similar to those associated with rivers, 

streams, lakes, and ponds. They are distinctive habitats because they provide

relatively constant water temperature, depend on subterranean flow through 

aquifers, and on occasion provide refuge habitats that support species that 

occur only in springs (Hynes 1970, Erman and Erman 1995, O’Brien and Blinn 

1999). 

Springs are replenished by precipitation that percolates into aquifers by seeping 

into the soil and entering fractures, joints, bedding planes, or interstitial 

pore space. Springs occur where water flowing through aquifers discharges 

at the ground surface through fault zones or fractures, or by flow along an 

impermeable layer (fig. 49). They can also occur where water flows from large 

orifices that occur when water creates a passage by enlarging fractures or joints 

by dissolving carbonate rock. Characteristics of regional and local geology 

influence spring occurrence and flow rates. 

Fetter (2001) identifies five types of springs: (1) depression springs, (2) 

contact springs, (3) fault springs, (4) sinkhole springs, and (5) fracture springs. 

Depression springs form in low topographic spots where the water table 

reaches the surface. Where permeable rocks overlie rocks of much lower 

permeability, a contact spring may result. Such a lithologic contact between 

rock of contrasting permeability is often marked by a line of springs. Faulting 
134

Figure 49. Comal Springs, near San Antonio, TX, discharges ground water from the highly 

productive Edwards aquifer. (Photo by Robert Morris, USGS.)

can form a boundary to ground water flow and force water in the aquifer to 

discharge as a fault spring. Sinkhole springs are formed where water dissolves 

the limestone beneath the surface and creates a sinkhole. If the artesian 

pressure in the subterranean solution cavities is high enough to reach the 

surface, a sinkhole spring is formed. Fracture springs form where ground water 

flowing along a fracture or joint intersects the land surface. 

Springs can also be classified as gravity springs and artesian springs, with 

thermal springs classed as a type of artesian spring. Water that moves along an 

elevation gradient emerging at the surface creates gravity springs. Depression

springs, contact springs, and fracture springs are different types of gravity 

springs. They are the result of ground water discharging from a permeable rock 

unit in contact with impermeable rocks or rocks having lower permeability. 

Fracture springs, for example, are often the result of fractured basalt or 

limestone overlying an impermeable rock stratum, and water flows along the 

outcrop of the two units. The temperature of the water will approximate the 

mean annual atmospheric temperature of the location. If movement of water 

occurs through passages that are open to the circulation of air, cooling to as 

much as several degrees below mean annual temperature will occur. If water 

is not in contact with circulating air and the depth to the water table is several 

hundred feet, the water will be a few degrees warmer than the mean annual 

temperature (generally about 1 degree for each 100 feet in depth). 

Artesian springs occur where the potentiometric level of the ground water 

flow system is above land surface and water flows at the land surface under 

artesian pressure, or where water is forced to the surface from deep sources 

by thermal and pressure gradients. They usually occur at lower elevations in 
135

mountainous areas, especially along mountain fronts. Aquifer outcrop springs 

and fault springs are the two main types of artesian springs. Thermal springs 

are usually a variation of artesian spring that connect to deep-seated thermal 

sources, and they are classed as volcanic springs or fissure springs (Milligan 

and others 1966). Temperatures of thermal springs can be greater than 100o 

C. Fault-related springs can also be thermal if they are from a deep source of 

water. This type of spring is common in the Great Basin (UT, NV and adjacent 

States), where mountain blocks are faulted along the margins, allowing water 

from deep sources to rise along the fault. Devil’s Hole, NV, is an example. 

Karst springs can be classified by mode of ground water recharge. The three 

major recharge modes are (1) diffuse through permeable material producing 

network conduit/cave patterns; (2) authigenic through many discrete sources 

such as sink holes, producing dendritic conduit/cave patterns; and (3) allogenic 

through a few major inflow points such as sinking streams, producing braided 

conduit/cave patterns. Springs are natural ground water discharge points, while 

sinks can be ground water recharge or discharge points.

Springs can be regional (long flow paths that may connect more than one 

surface water basin) or local discharge points (short flow paths). Local 

springs are comparatively small, can be low flow and low temperature, and 

are typically from shallow aquifers. The discharge from these springs often 

fluctuates either seasonally or in greater cycles, sometimes in response 

to local precipitation. Local aquifers are quickly recharged and water 

movement through them is comparatively rapid, resulting in water that is 

low in mineralization. Springs supported by local aquifers are more likely to 

periodically stop flowing than springs supported by regional aquifers. Springs 

at higher elevations generally display greater fluctuations in flow rates, and dry 

more frequently than regional springs or springs at lower elevations; however, 

they are generally less susceptible to impacts from dewatering at mining 

operations or from pumping wells.

Springs fed from regional aquifers typically have large discharge, and are 

discharge points for aquifers covering hundreds of square miles (fig. 50). 

In the Great Basin, the majority of springs with high discharge rates occur 

in intermontane basins of the carbonate rock province and are often closely 

associated with limestone outcrops (Mifflin 1988). Regional springs are 

typically of nearly constant discharge, and can be more mineralized than local 

springs because of their long flow paths. Their temperatures can be cold or 

warm depending on the depth of circulation. Seasonal and annual variations in 

discharge from regional springs are usually limited, and they are comparatively 

stable aquatic environments. Regional springs rarely stop flowing, even during 

long droughts, but they can be affected by pumping from the regional aquifer 

(Dudley and Larson 1976). 
136

Physical Environment

and Water Chemistry

of Springs

Figure 50. Ground water from a large regional limestone aquifer discharges at Crystal Spring in 

Ash Meadows National Wildlife Refuge, NV. (Photo by Pat Tucci, USGS.) 

Springs occur in many sizes, types of discharge points, and location with 

respect to topography. They occur at the highest elevations of mountainous 

areas and they occur in valley floors. Many springs on public land are small, 

provide limited aquatic habitats, and are intermittent in flow. They sometimes 

support limited amounts of riparian vegetation. Some small springs, however, 

provide aquatic habitat, are permanent, and support high species diversity over 

large riparian areas. Springs can be categorized by the morphology of their 

discharge area. Limnocrenes are springs where water flows from large deep 

pools, helocrenes are marshy bogs, and rheocrenes flow from a confined

channel (Hynes 1970). It is often difficult, however, to categorize springs 

because morphology can involve a combination of features from more than one 

of these categories.

Springs may occur singly or in groups that can include dozens of habitats in 

various sizes and morphologies. Many springs are tributaries to rivers, lakes, 

or streams. A few are even the major source for a river or lake. Many springs 

are isolated from other surface waters and frequently flow a short distance on 

the surface before drying. Springs in dry regions may stop flowing seasonally 

or during droughts. Some groups of springs can support wetland areas with 

unique habitat and species; examples are Ruby Marsh in northeastern Nevada, 

Ash Meadows in southern Nevada (fig. 50), Fish Springs in northwestern Utah, 

and San Bernardino Ranch in southern Arizona (Hendrickson and Minckley 

1984, Dudley and Larson 1976). Some springs support fens that are at middle 

to low elevations in the watershed, usually in large open areas or parks, such as 

in South Park in south-central Colorado. Some springs are the source for loworder streams high in watersheds.
137

Field Observations

of Seeps and Springs

Riparian vegetation may be narrowly restricted to immediate boundaries 

of the aquatic habitat, or it may extend outward for substantial distances. 

Narrow riparian areas are typically dominated by sedges, grasses, and woody 

phreatophytes such as willows and mesquite. Wider riparian ecosystems are 

generally associated with spring provinces where water seeps outward from 

aquatic habitats, which saturate and create hydric soils. In these provinces, 

riparian ecosystems are characterized by marsh vegetation or expansive mesic 

alkali meadows. 

Physical and chemical conditions of springs vary (Hynes 1970). They can be 

cold (near or below mean-annual air temperature), thermal (5o to 10o C above 

mean annual air temperature) (van Everdingen 1991), or hot (more than 10o 

C above mean annual air temperature) (Peterken 1957). The temperature of 

spring water is also an indicator of the flow path of water discharging to the 

spring and its recharge area. Shallow circulating ground water has temperatures 

generally within a few degrees of the mean annual ambient air temperature 

(Mifflin 1988). Higher temperatures are usually indicative of deeper, regional 

circulation, although some cool regional springs exist. Thermal springs 

may gain their heat when water comes in contact with or in close proximity 

to recently emplaced igneous masses, such as at Steamboat Springs, NV; 

Yellowstone National Park; and Geyser, CA (Wood and Fernandez 1988), or 

through the higher temperatures encountered at large depths caused by the 

natural geothermal gradient. 

Springs may be highly mineralized, especially thermal springs and regional 

springs that have a very long flow path. Dissolved oxygen (DO) concentrations 

are primarily a function of temperature and pressure; as temperature increases, 

the DO concentration decreases (Hem 1989). As a result, DO concentrations 

are frequently very low (less than 2 parts per million) in hot springs and high 

(greater than 5 ppm) in cold springs. However, DO can also be substantially 

affected by the nature of the geologic materials along the flow path. For 

example, a flow path that involves materials with high organic content will 

generally have low DO concentrations. Electrical conductance may range from 

very low (near 0 micromhos per centimeter) to very high (greater than 10,000 

micromhos per centimeter). Local low-flowing springs may freeze during 

winter while the larger and warmer regional springs generally do not. 

Springs and seeps provide a means of assessing ground water quality and 

of helping to determine ground water flow patterns. It is essential that flow 

from a spring is identified as to the geologic formation from which the water 

discharges. Springs may be caused by bodies of perched ground water, water 

under artesian pressure, or outcrops of the main water table. Gains or losses 

in baseflow of streams mark reaches affected by ground-water discharge or 

recharge. The following are among the basic data collection requirements for 

springs: 

1. Elevation of the spring. 

2. Uses of the spring water (stock watering, domestic, unused).
138

Ground Water 

Exchange in 

Reservoirs, 

Lakes, and 

Ponds 

3. Permanence of flow (perennial or seasonal).

4. Discharge of the spring, including date and time of measurement.

5. Chemical characteristics of the water.

6. Type of spring (perched, contact, fracture, and so on).

7. Source aquifer.

Examination of water quality in the field is an important part of 

hydrogeological studies, especially when investigating spring sources. Certain 

properties of natural water, especially pH and DO, are so closely related to 

the environment of the water that they are likely to be altered by sampling 

and storage, and a meaningful value can be obtained only in the field. Other 

properties that should be sampled while in the field are specific conductance, 

redox potential, and temperature. 

When conducting spring investigations, any geological outcrops at the spring 

need to be evaluated to determine the hydrogeological setting for the spring. 

Assess how water is being recharged into the ground water system, how it 

moves, and what mechanism forces the water to the surface at that particular 

point. Features such as fractures, faults, sand-and-gravel layers overlying 

impermeable bedrock, and silt or shale layers that impede downward flow of 

water should be noted. Before development of a spring is attempted, the type 

of spring must be determined to properly design a collection system that will 

result in a reliable water source without damaging the natural condition or 

ecological values of the spring. A classification of the spring can often be made 

from geological and topographical observations. 

The evaporation of water from reservoirs, lakes, and ponds often maintains 

a water level that is somewhat lower than the local ground water table. In 

such circumstances, the primary productivity of the surface water body may 

be greatly enhanced by the nutrients carried by the ground water flowing 

into it (Kenoyer and Anderson 1989). Another potential influence on primary 

productivity is that the temperature of ground water is usually fairly constant, 

so that it is colder in the summer and warmer in winter than the water in 

the receiving surface-water body. The amount of ground water flowing to a 

water body may be estimated by mapping the slope of the ground water table, 

performing water-budget analysis (Winter 1995), conducting seepage studies 

(Carr and Winter 1980, Paulsen and others 2001), applying chemical massbalance methods (Krabbenhoft and others 1990a, Lerman and others 1995, 

Sacks 2002, Stauffer 1985,), or performing numerical modeling (Krabbenhoft 

and others 1990b). In some instances, the inflows from surface streams and 

runoff to a surface-water body, usually one with no surface outlet, may be 

sufficient to maintain its level at an elevation that is higher than the local 

ground water table. In those cases, seepage from the surface-water body is a 

recharge source for local ground water (Winter 1981). 
139

Sensitive 

Hydrogeological 

Settings

Karst Terrains Many professionals consider the most sensitive aquifers to be those that 

are composed of karst limestone or dolomite. Dissolution of portions of a 

soluble rock body by water flowing through the pores and fractures generates 

preferential pathways of flow, which can vary in size from lengthy but small 

diameter solution cavities to cave systems and large caverns. Because the rock 

itself is often highly porous, karst aquifers offer the possibility of enormous 

withdrawals of ground water; however, the consequences of excessive 

withdrawals from karst aquifers can be highly destructive. A sinkhole is one 

possible result. The ready movement of large amounts of ground water within 

karst aquifers, coupled with the presence of preferential pathways of flow, can 

make ground water contamination spread quickly and often in unpredictable 

patterns.

Karst is a general term for a wide range of landscape settings in which the 

underlying rocks have been modified by solutional processes. Ground water 

interactions with stream water in karst settings include springs, sinkholes, 

swallows, and resurgences. The flow of ground water through karstic limestone 

formations occurs through the pores of the bulk rock, through fractures in the 

bulk rock, and through solution cavities and channels, including cave streams, 

pools, and waterfalls. Ground water emerges as springs, seeps, and wetlands of

various kinds, including fends and marshes. Streams that flow over karst terrain 

may swell or shrink in size sporadically, in response to passing over springs 

and sinks. Streams may disappear completely into sinkholes or swallows (the 

land surface entry points of solution channels or cavities), only to reappear by 

resurgence farther downslope. Tracer studies have shown that the flow paths 

within karst limestone can be circuitous and multibranched, so that it is not 

uncommon for swallowed streamflow to reappear miles away, on the opposite 

side of a ridge, or at several widely separated locations. Similarly, tracer studies

have indicated that the water in some streams in karst terrain contained ground 

water from various springs that originated in widely separated areas.

Focused recharge and discharge can be readily identified in many karst systems 

from remote sensing and mapping of geomorphic features such as sinkholes, 

stream networks, and vegetation patterns defining fracture traces. The National 

Research Council (2004) suggests the following field components for assessing 

the hydrogeology of a karst area:

1. Develop a long-term water balance for the watershed.

2. Measure discharge and geochemical parameters at all major springs.

3. Measure travel times and residence times using environmental isotopes, 

geochemistry, temperature, and tracer tests.

4. Install monitoring wells and meteorological stations and monitor 

continuous water levels and geochemistry.
140

Unconsolidated 

Deposits

5. Map in detail the topography, soils, karst features, and vegetation that 

correlates with discharge zones and seeps.

6. Estimate stream-hydrograph separation using both physical and 

chemical parameters to discern baseflow, stormflow, and old and new 

water components (Kendall and McDonnell 1998).

Additional information on characterization of karst and fractured rock 

hydrogeologic systems can be found in American Society for Testing and 

Materials (ASTM) standard D5717-95e1 (ASTM 1996).

Unconsolidated deposits comprise the most common and most accessible 

aquifers in the United States. The unconsolidated-rock aquifers occur as 

alluvium, colluvium, and glacial drift deposits. These aquifers are typically 

composed of sand or sand and gravel, often intermixed with finer-grained 

sediments. They are usually unconfined aquifers, but may also occur as 

partially confined or confined aquifers. Because unconsolidated aquifers are 

generally shallow and well connected to surface water, knowledge of ground 

water/surface water interactions is critical to understanding these aquifers. 

Those same characteristics also make unconsolidated aquifers often highly 

susceptible to contamination.

Alluvial aquifers generally occur along rivers and streams and were deposited 

as coarse-grained sediments by streams. Their extent in area may be restricted 

to a zone on either side approximately parallel to the stream or it may be quite 

extensive, especially along major rivers. An example of an extensive aquifer 

is the Mississippi Aquifer in western Tennessee and eastern Arkansas. Alluvial 

aquifers generally are tens to hundreds of feet thick. The distribution of coarsegrained sediments, which are the most productive parts of the aquifers, is 

controlled somewhat by the type of stream that deposited the sediments. In 

streams that are somewhat confined in area and have a relatively steep gradient, 

coarse-grained sediments may be distributed throughout the aquifer. In larger, 

meandering streams, the coarse-grained sediments tend to be associated with 

sand and gravel bars distributed between finer grained sediments. Alluvial 

aquifers are usually well connected to the nearby streams, which can provide a 

source of recharge to the aquifer. Because of their shallow, unconfined nature, 

these aquifers are susceptible to contamination from human activities at the 

surface.

The High Plains Aquifer is major alluvial aquifer that underlies some National 

Grasslands. It is one of the largest aquifers in the United States. It was 

formed from sediments eroded from the Rocky Mountains to the west. It is a 

highly productive, thick, generally unconfined aquifer, which has undergone 

significant water-level declines because of irrigation pumpage.
141

Some alluvial aquifers are buried beneath more recent stream or glacial 

sediments. These aquifers most often occur within bedrock channels carved by 

ancient rivers in Northeastern and Midwestern States. An example is the Teays 

Aquifer of Ohio, Indiana, and Illinois (Sharp 1988). 

Valley-fill aquifers, sometimes termed basin-fill aquifers, occur in the Western 

United States and are the most important aquifers in the Basin and Range 

physiographic province. These aquifers, which often are adjacent to NFS 

lands, were deposited as a combination of alluvium and colluvium as the 

basins subsided relative to the surrounding mountain ranges. They tend to be 

coarser grained and most productive along the basin margins and near modern 

stream channels. They tend to be finer grained and less productive near the 

basin centers, but this general pattern can be altered somewhat by the structural 

history of the basin (Anderson and others 1992). In some basins, evaporite 

deposits occur within the valley-fill aquifer, and these deposits can degrade the 

quality of the ground water. Ground water generally occurs under unconfined 

conditions, but confined conditions can occur where extensive fine-grained 

sediments are present. Valley-fill aquifers can be very productive, and some 

wells can yield more than 1,000 gallons/minute. Because of the typically arid 

environments in which these aquifers occur, recharge to them is small. Much 

of the water pumped from them is removed from storage, causing large waterlevel declines. In some basins, large ground water withdrawals have resulted in 

land subsidence.

Glacial aquifers are generally derived from coarse-grained sediments 

associated with glacial outwash and ice-contact stratified deposits associated 

with fast-moving glacial meltwaters. Some sandy tills and loess (aeolian silt) 

deposits can provide adequate water to domestic wells. In mountainous areas, 

the productive glacial aquifers are confined to valley bottoms and sides. In 

areas subjected to continental glaciation, the productive aquifers may occur 

along the surface or may be buried by sediments deposited by subsequent 

glaciation or other processes. Hydraulic properties and thickness of glacial 

aquifers are highly variable, depending on the type of glacial deposits and 

subsequent modification to those deposits (Stephenson and others 1988). 

Glacial aquifers are generally shallow and unconfined, but ground water can 

occur under confined conditions where the aquifers occur beneath glacial lake 

deposits.

Characterizing unconsolidated aquifers is generally straightforward because 

much of the theoretical basis for quantification of ground water flow was 

developed from studies of these types of deposits. Aquifer tests and computer 

modeling are well-suited to analysis of unconsolidated aquifers because 

porous-media flow is commonly a reasonable assumption. Heterogeneity of 

unconsolidated deposits, however, greatly complicates the characterization 

of these aquifers, particularly for small-scale problems such as ground water 

contamination; for example, braided-stream or glacial-outwash deposits 

can vary greatly within short distances both horizontally and vertically. 
142

Volcanic Terrains

Fractured-rock 

Settings

These variations can make correlation of units within these deposits almost 

impossible. In addition, many of the fine-grained tills and lacustrine 

deposits associated with glacial aquifers developed fractures as a result of 

unloading following ice retreat or periglacial freeze-thaw action. Due to 

the high variability in these systems, large numbers of boreholes and wells, 

completed at different depths, may be required for unconsolidated aquifer 

characterization. 

Geophysical methods may help in characterization for large-scale problems; 

for example, gravity and seismic surveys have been used successfully to 

map the extent of buried alluvial aquifers. Electrical and electromagnetic 

geophysical methods can be used to map the extent of fine-grained materials 

within the unconsolidated aquifers. Basic knowledge of the geomorphic and 

sedimentologic characteristics of unconsolidated deposits can be used to map 

the location of the various depositional facies and estimate the productive 

portions of these aquifers.

Volcanic rocks retain porosity associated with lava-flow features and 

pyroclastic deposition. Hydraulic conductivity can be quite high, but ash 

beds, intrusive dikes, and sills may be barriers to ground water flow. Flow 

features such as vertical contraction joints and stream gravels buried between 

successive flows contribute to overall permeability and produce some of the 

most productive aquifers. Flow distribution, timing, volumes, and rates can be 

affected by stratigraphic differences in texture, jointing and fracture patterns 

and spacing, contact relationships between lithologic units, and the presence 

or absence of lava tubes. Ground water geochemistry can be directly affected 

by venting volcanic gases and ground water circulation driven by geothermal 

systems. The Columbia Lava Plateau is the largest sequence of basalt flows 

and interbedded sediments in the United States. Ground water is replenished 

by precipitation, runoff from adjacent mountains, and excess irrigation water 

applied to the surface. 

Much of the NFS lands are located in mountain-dominated terrain. The 

landscape is rugged and composed of exposed igneous, metamorphic, and 

sedimentary bedrock. A weathered zone of soil a few meters to tens of meters 

thick may exist. Sources of recharge largely involve diffuse infiltration of 

precipitation, including melting snowpack. Discharge occurs locally as focused 

spring flows and seeps, as diffuse inflow to streams, and as transpiration 

in riparian areas. Ground water flows through pores and fractures. In rock 

formations with large numbers of fractures that are highly interconnected, 

ground water flow can be very similar to that through porous sediments. A 

predominant flow direction exists, and responses to pumping or intersection 

with a lake or stream are predictable. In rock formations with few or poorly 

connected fractures, ground water flow occurs in a far less predictable manner. 

Discrete fracture flows may be independent of ground water flow through the 

bulk rock. Consequently, the seepage of ground water from, and the seepage 
143

of streamflow into, fractured rock can result in the mixing of chemically 

dissimilar waters at flow rates that are very difficult to predict. Some suggested 

methods for studying the recharge and discharge characteristics in mountainous 

hydrogeological settings are described in National Research Council (2004).

The majority of NFS land is underlain by fractured-rock aquifers, and demands 

are increasing on ground water on and around the NFS. These types of aquifers 

occur in the igneous, metamorphic, and sedimentary rocks that form the 

mountain uplifts and in sedimentary rocks that flank uplifts. The occurrence 

and flow of ground water in these types of aquifers is controlled by the 

spacing, aperture size, orientation, and connectivity of permeable “preferential” 

pathways that occur within discontinuities created by structural processes 

related to uplift and mountain building (Caine and others 1996). Types of 

discontinuities that facilitate ground water flow include joints and fractures, 

foliation, faults, shear zones, geological contacts, and bedding planes. It should 

be noted that some structures, such as dikes and faults, may also function as 

barriers to ground water flow. The quantitative aspects of ground water flow 

in fractured rocks are not well understood, particularly at the fracture scale; 

however, in many fractured-rock settings, the watershed or surface drainage 

basin can be an appropriate, natural unit within which to characterize and 

manage surface-water and ground water resources. 

Heath (1988) divided North America into 28 ground water regions based on, 

among other things, the nature of the water-bearing openings of the dominant 

aquifer or aquifers. Eleven of these regions are underlain by mountainous 

areas dominated by fractured-rock hydrogeologic settings. Six of the 

regions (Western Mountain Ranges, Columbia Lava Plateau, Sierra Madre 

Occidental, Sierra Madre Oriental, Sierra Madre Del Sur, and Faja Volcanica 

Tansmexicano) occur along the western edge of North America. Three regions 

(Northeastern Appalachians, Appalachian Plateaus, and Piedmont and Blue 

Ridge) occur along the eastern edge of North America. The other two are the 

West Indies and the Hawaiian Islands.

Hydrogeological settings in the 11 ground water regions in mountainous 

areas are typically characterized by steep slopes on the sides of ridges and 

mountains, and thin soil or regolith overlying moderately to highly fractured 

and/or folded bedrock. Exceptions are the Northeastern Appalachian and 

Appalachian Plateau ground water regions, where the regolith is thick. In some 

bedrock types, the upper 10 to 100 feet are commonly highly weathered and, 

when saturated, comprise a significant water-bearing unit separate from the 

bedrock. In areas where snowmelt is a significant seasonal event, interflow is 

often a dominant process in this zone during the snowmelt flux. Below this 

zone, ground water flow occurs predominantly in individual fractures, fracture 

zones, faults, fault zones, and other structural discontinuities (Gerhart 1984). 

The rock matrix in igneous and metamorphic bedrock typically plays a minor 

role in ground water flow and usually has low porosity and permeability. As a 
144

result, ground water flow in these settings is highly preferential and controlled 

primarily by the spacing, orientation, hydraulic properties, and connectivity of 

the permeable discontinuities (Forster and Smith 1988b). 

Because of the steep topography in mountainous and upland areas, hydraulic 

gradients along preferential pathways can be very high, causing relatively high 

ground water flow velocities. Ground water flow along preferential pathways is 

generally toward valley bottoms, where ground water discharges at seeps and 

springs, which are very common in fractured-rock settings, or to an intermittent 

or perennial stream in the valley bottom. Strong upward gradients are common 

in bedrock underlying mountain valley bottoms. In mountainous watersheds, 

the topographic drainage basins are not always coincident with ground water 

flow divides. Because of low porosity and storage, seasonal ground water 

levels in fractured-rock aquifers commonly vary from 10s to 100s of feet. 

As a result, the location of ground water divides can shift seasonally because 

more and different flow paths are available to ground water when water 

levels are high than when water levels are low. Hydrogeological conditions in 

mountain watersheds often result in distinct ground water flow systems that are 

temporally and spatially dynamic. 

Inadequate collection, interpretation, and use of fracture-scale hydraulic 

data continue to be general deficiencies in fractured-rock hydrogeological 

investigations. In such settings, the relationship between seasonal interflow 

in a surficial water-bearing zone and recharge to an underlying fracture flow 

system is poorly understood. A number of factors control ground water flow 

through fractures or other discontinuities, including fracture aperture and 

length and the degree of roughness and nonparallelism of the fracture walls. 

The hydraulic conductivity through a fracture is directly proportional to the 

aperture width and inversely related to normal stress and depth. Fracture 

permeability is affected by rock temperature, cementation, in-filling, and 

chemical and physical weathering. In fractured-rock settings it is important 

to distinguish between the hydraulic conductivity of a fracture, the hydraulic 

conductivity of the rock matrix and the hydraulic conductivity of a rock mass. 

As the development of ground water increases, a better understanding of the 

quantitative aspects of ground water flow at the fracture scale will be essential 

to adequately manage the spatial and temporal withdrawal of ground water for

human use. This will also be essential for characterizing contaminant transport 

in fractured-rock settings.

As more attention is being focused on the hydrogeology of fractured-rock 

settings, a wide variety of tools are being used to characterize (1) ground water 

storage and flow and development; (2) ground water/surface water interaction; 

(3) chemical, isotopic, and biological quality of ground water and surface 

water; and (4) contaminant transport in these settings. Given the complexity, 

it is advisable to use a number of different tools and data sets (multiple lines 

of evidence) for evaluation. Some of the more appropriate characterizations 

tools include aquifer tests, evaluation of drill core and drillholes, isotopes, and 

geophysics.
145

As the population density has increased in these fractured-rock settings, 

the development of ground water resources for domestic, municipal, and 

commercial uses has increased significantly. Concurrent with this growth has 

been an increase in anthropogenic contamination of ground water. The issues of 

population pressures and contamination are becoming increasingly important 

in and around the NFS. As a result of the increasing stress on water resources, 

ground water scientists and water-resource managers have recognized the 

need to develop a more appropriate approach to characterizing the occurrence, 

movement, and chemistry of ground water in fractured rock. A concurrent need 

exists to develop more appropriate ways to characterize contaminant transport 

in fractured rocks and to select, design, and operate remedial technologies 

at ground water contamination sites. As the overall understanding of ground 

water occurrence and flow in fractured rock has improved, a new conceptual 

model has evolved that is more appropriate for characterizing water resources 

in these types of hydrogeological settings. Ground water scientists have been 

forced to move beyond conventional approaches that have traditionally been 

used for unconsolidated deposits and consolidated rock aquifers where porous 

media flow is dominant. Other, more thorough, discussions of these topics are 

provided by the National Research Council (1996), the National Ground Water 

Association (2002), and ASTM standard D5717-95e1 (ASTM 1996).

Ground water inventories can help provide the basis for selecting suitable areas 

for major land uses, identifying areas that need more intensive investigation, 

evaluating various land-management alternatives, and predicting the effects 

of a given activity on resource health or condition. The resultant maps, data, 

descriptions, and management interpretations provide basic ground water 

resource information necessary for ecological assessments, project planning, 

watershed analysis, forest plan revisions, and implementation and monitoring 

of forest plans. The information provided can be used for activities such as 

assessing resource conditions, conducting environmental analyses, defining 

and establishing desired conditions, and managing and monitoring natural 

resources. Ground water inventories and monitoring programs will necessarily 

involve various levels of detail, focus, and spatial extent depending on the 

geographic location of a national forest and the specific resource issues that 

that national forest is dealing with. The basic elements in a ground water 

inventory are shown in figure 51.

The discussion that follows explains how various strategies, field methods, 

and data analyses are useful for accomplishing ground water inventories and 

evaluations. Although each ground water inventory will be more or less unique, 

general guidelines for a successful inventory can be followed. 

Ground 

Water 

Investigation 

Methods

Ground Water 

Resource 

Inventories 

and Evaluations
146

Aquifer assessments can be used by land-management agencies to define the 

overall usefulness of an aquifer and/or its susceptibility to contamination or 

hydraulic disruption. Ground water assessments of various kinds are needed 

in many Federal, State, and local water-management programs. An assessment 

should include the identification and location of sustainable sources of 

drinking water, State pesticide management plans, underground injection of 

waste, and confined animal feeding operations. A National Research Council 

(1993) publication summarizes the broad array of definitions and approaches 

that are used by government as well as private and academic organizations in 

assessing the vulnerability of ground water to contamination. The National 

Research Council (1993) defines vulnerability as “the tendency or likelihood 

for contaminants to reach a specified position in the ground water system after 

introduction at some location above the uppermost aquifer.”

Depending on specific objectives and available resources, assessments can be 

designed to include individual wells or entire aquifer systems. They can target 

one contaminant, or contamination in general. They can focus on hydraulic 

disruption. The effectiveness of individual assessments will be linked to the 

degree to which the important physical/chemical processes have been identified 

and accounted for, the manner in which uncertainty is addressed, and the extent 

to which the original science and management objectives are met.

 

The vulnerability of ground water to contamination depends on intrinsic 

susceptibility as well as the locations and types of potential sources of 

contamination, the relative locations of wells, and the fate and transport 

Figure 51. Elements of a ground water inventory and assessment.

Aquifer 

Delineation and 

Assessment
147

of potential contaminant(s). The intrinsic susceptibility of a ground water 

system depends on its geologic setting, the aquifer properties including 

hydraulic conductivity, porosity, and hydraulic gradients, and on the associated 

sources of water and stresses for the system. Key elements are recharge, 

interactions with surface water, travel through the unsaturated zone, and well 

discharge. Intrinsic susceptibility assessments do not target specific natural or 

anthropogenic sources of contamination but instead consider only the physical 

factors affecting the flow of water to, and through, the ground water resource. 

Karst aquifers typically have a high intrinsic vulnerability because of the ease 

and speed with which contaminants can enter and move within the system 

(Zwahlen 2003). Some volcanic aquifers are similarly vulnerable.

Assessments of the vulnerability of ground water to contamination range in 

scope and complexity from simple, qualitative, and relatively inexpensive 

approaches to rigorous, quantitative, and costly ones. Tradeoffs must 

be carefully considered among the competing influences of the cost of 

an assessment, the scientific defensibility, and the amount of acceptable 

uncertainty in meeting the objectives of the water-resource decision maker. 

Subjective rating methods focus on policy or management objectives. 

Relative degrees of ground water vulnerability are usually delineated as low, 

medium, and high. These classes are common endpoints for all subjective 

rating methods. These broad classes are appropriate for the “index” methods 

described below, but not for the more costly and involved statistical and 

process-based methods. 

Index methods and closely associated “overlay methods” assign numerical 

scores or ratings directly to various physical attributes to develop a range of 

vulnerability categories. The index method is one of the earliest and most 

commonly used categorical rating methods (National Research Council 1993). 

The most widely used index method is DRASTIC, which is an acronym for the 

seven factors considered in the method: Depth to water, net Recharge, Aquifer 

media, Soil media, Topography, Impact of vadose zone media, and hydraulic 

Conductivity of the aquifer (Aller and others 1985, 1987). The point rating 

system for DRASTIC was determined by the best professional judgment of the 

original method developers. The DRASTIC method has been used to produce 

maps in many parts of the United States (Durnford and others 1990). The maps 

have a variety of scales, including national (Kellogg and others 1997, Lynch 

and others 1994), statewide (Hamerlinck and Ameson 1998, Seelig 1994), and 

individual counties and townships (Regional Groundwater Center 1995, Shukla 

and others 2000). The index method is popular for ground water vulnerability 

assessments because it is relatively inexpensive and straightforward, uses data 

that are commonly available or estimated, and produces an end product that is 

easily interpreted and incorporated into decision-making processes. 

Figure 52 shows how the DRASTIC method can be applied to NFS land. This 

coverage was constructed using GIS layers for topography, slope aspect, and 

geology. Depth to water was estimated from water well information stored in a 

Statewide well database. Hydraulic conductivity, recharge, and soil thickness 
148

were estimated by consulting a hydrogeologist and soil scientest familiar with 

the area. Index value compuations for the various hydrogeological settings 

are presented in table 10. Geologic units were combined into hydrogeologic 

settings based on similar hydrogeological properties, as suggested by Aller and 

others (1985, 1987).

Hydrogeological 

Mapping

Figure 52. Vulnerability map of a portion of the Beaverhead-Deerlodge National Forest constructed using the DRASTIC 

method. Aquifers are rated from high to low vulnerability based on hydrogeological factors.

Geological mapping for hydrogeological purposes involves standard geological 

mapping procedures. Aerial photographs (1:24,000 scale or less) are used to 

delineate geological contacts. Photo data are combined with field checks to 

correlate map units, characterize rock units, measure stratigraphic sections, 

and measure strike and dip of formations. Of primary interest in hydrogeology 

is the ability of the various rock units to store and transmit water and act as 

aquifers.

Understanding geological conditions is the cornerstone of any ground water 

evaluation. Geology forms the physical framework for the flow of ground 

water. Primary and secondary porosity, storage properties, and transmitting 

properties are largely a function of the geological materials present. 

Stratigraphy affects local and regional ground water flow. Structural features, 
149

Table 10. DRASTIC computation matrix showing methods for computing index values for various hydrogeological settings in the Pioneer Mountains, Beaverhead-Deerlodge National Forest, MT.

Map units DRASTIC

 Index

Aquifer media Rating Depth 

to water 

(ft)

Rating Recharge 

(in)

Rating Soil Rating Topography 

(% Slope)

Rating Vadose zone Rating K (gpd/

ft2)

Rating

Qo 180 glacial 

outwash

8 5-15 9 4-7 6 gravel 10 2-6 9 sand & gravel 

(s & g)

8 700-1000 6

Qm 124 glacial till 5 15-30 7 4-7 6 sandy 

loam

6 6-12 5 s & g with silt 

and clay

6 1-100 1

Ql 130 landslide 8 50-75 3 7-10 8 silty 

loam

4 2-6 9 s & g with silt 

and clay

6 300-700 4

Qf 160 alluvial fan 8 30-50 5 7-10 8 sandy 

loam

6 2-6 9 s & g 8 700-1000 6

Qtg, Tbz 134 alluvial 

gravels, Bozeman group

8 30-50 5 7-10 8 silty 

loam

4 2-6 9 s & g with silt 

and clay

6 100-300 2

Ym, Ymm,Kbgg,Kgtd,Tkg

65 metamorphic

igneous (east slopes)

3 100+ 1 0-2 1 thin or 

absent

10 18+ 1 metamorphic 

igneous

4 100-300 2

70 metamorphic

igneous (west slopes)

3 75-100 2 0-2 1 thin or 

absent

10 18+ 1 metamorphic, 

igneous

4 100-300 2

Cu,Kk,Ks,Pmu,IPmu,Pp,Tc,Tvu,Tru

83 bedded ss, ls, 

sh sequences (east slopes)

6 75-100 2 0-2 1 sandy 

loam

6 12-18 3 sandstone, 

limestone, 

shale

6 100-300 2

106 bedded ss, ls, 

sh sequences (west slopes)

6 30-50 5 2-4 3 sandy 

loam

6 12-18 3 sandstone, 

limestone, 

shale

6 100-300 2

Mmm 142 limestone 9 100+ 1 4-7 6 silty 

loam

4 12-18 3 limestone 9 1000-

2000

10

*Map Units: Qo - Quaternary glacial outwash; Qm - Quaternary glacial moraine; Ql - Quaternary landslide deposits; Qf - Quaternary alluvial fan; Qtg - Quaternary alluvial gravels; Tbz - Tertiary Bozeman group; Ym - Precambrian Missoula group; Ymm - Precambrian Missoula group; Kbgg - Cretacious biotite grandiorite and granite; Kgtd - Cretacious granite and diabase; Tkg - Tertiary-Cretacious granite; Cu - Precambrian Cherry Creek series; Kk - Cretacious Kootenai Fm; Ks - Cretacious sediments, undifferentiated; Pmu - Permian, undifferentiated; IPmu - Triassic Permian, undifferentiated; Pp - Permian Phosphoria Fm; Tc - Tertiary colluvium; Tvu - Tertiary volcanics, undivided; Tru - volcanics, undifferentiated; Mmm - Mississippian Madison Mission Canyon Fm.
150

such as the folding and fracturing of rock by tectonic processes, may alter 

directions of ground water flow compared to horizontal sediments by changing 

the inclination of permeable sediments and confining units. Displacement 

of sediments by faulting may either provide zones of increased permeability 

through fracturing or create aquifer boundaries when impermeable strata block 

the flow of water through permeable strata. Secondary fracture porosity results 

primarily from tectonic stresses.

Geological maps are the basis for interpreting the movement of ground water. 

Distinctions between unconsolidated and consolidated, and permeable and 

impermeable rock units are made on a qualitative basis, using rock type, 

structure, and knowledge about depositional environments. Interpretations 

of hydraulic conductivity can be made using such information, and estimates 

of potential ground water movement through the rock unit. Geological maps 

showing rock type and genesis are more useful for hydrogeological purposes 

than maps that classify rock units only as to their stratigraphic age. In addition 

to bedrock maps, those showing surficial geology are also very useful. In 

fractured-rock settings, maps that show geological structures such as faults, 

folds, joint orientation, strike and dip of beds, and cross-sections are useful 

for hydrogeological purposes because geologic discontinuities frequently are 

preferential ground water flow paths. 

Hydrogeological mapping requires the systematic and integrated appraisal 

of soils, geomorphology, geology, hydrology (including meteorology), 

geochemistry, and water chemistry as they affect the occurrence, flow, and 

quality of ground water. It is also important to understand the hydrogeological 

setting as whole, including (1) surface water hydrology, (2) other nonfracturedrock aquifers that occur within the setting, and (3) data on meteorological and 

other water-budget elements in the watershed.

The character and distribution of soils and landforms are major considerations 

in hydrogeological mapping in humid areas where unconfined aquifers 

develop in unconsolidated materials and lie relatively near the land surface. 

In such settings, the water table generally follows the land surface, but 

with more subdued relief. Recharge areas are generally located in upland 

areas, and ground water divides tend to coincide with surface watershed 

boundaries. Valley bottoms and floodplains with perennial streams represent 

discharge areas. For all areas, soils and topography are the primary features 

that determine how much precipitation infiltrates into the ground to recharge 

ground water, and how much runs off to surface streams. In general, highly 

permeable soils and flat topography favor infiltration; less permeable soils 

and steep slopes promote surface runoff. However, steep, forested slopes 

with near-surface exposure of bedrock can serve as focused recharge areas for 

associated aquifers (Potter and others 1995).

Although the focus of hydrogeological mapping is the saturated ground water 

system, the occurrence and flow of ground water must be understood in the 

context of the larger hydrological cycle, which includes atmospheric water, 
151

water in the vadose zone (unsaturated ground water), and surface water. Such 

understanding is especially important for unconfined aquifers, which are 

intimately connected to the hydrological cycle. Complete characterization of 

unconfined aquifers requires consideration of infiltration of precipitation, the

effects of evapotranspiration, and the relationship between the ground water 

and surface water systems. Potentiometric surface mapping is one of the most 

important aspects of hydrogeological characterization. Confined aquifers 

that are distant from areas of surface recharge can be considered effectively 

isolated from the hydrological cycle, provided that they are highly confined. 

Such an assumption greatly simplifies analysis of a ground water flow system. 

A potentiometric-surface map is one of the most basic and useful tools 

available for characterization of ground water flow systems. A water-table map 

depicts the elevation of saturated ground water in an unconfined aquifer; a 

piezometric (pressure) surface map depicts the pressure potentials of confined 

aquifers. Either type of map is called a potentiometric-surface map. In practice, 

the terms water table, potentiometric, and piezometric are all often used 

interchangeably. 

Interpretation of aerial photographs for hydrogeological purposes generally 

has two purposes: (1) location of potential sites for drilling water supply wells, 

and (2) analysis of regional or local ground water flow systems. Methods 

employed in such investigations include (1) analysis of soil patterns that may 

reflect on infiltration potential, drainage characteristics that suggest rock 

type, and soil/rock permeability (permeable soils will have good drainage, 

reflected in drainage patterns that are course textured or even absent); (2) 

lineament analysis; (3) mapping and interpretation of joints and fractures; (4) 

land form analysis, which gives suggestive evidence of the kind of geologic 

material making up the landform; and (5) observation of vegetation patterns 

or types that provide inferences about the presence or preferential movement 

of water or its chemical quality. Other related uses of aerial photographs in the 

assessment of hydrogeology include the interpretation of the geological history 

of an area, using landforms, channel geometry, and the identification of fluvial 

or lacustrine sediments and bedrock contacts.

Aerial photography is an essential element of many geological or 

hydrogeological studies. Considerable information on ground water conditions 

can be obtained from stereo pairs of low-level black and white (panchromatic) 

or color air photos. The pairs provide a three-dimensional image of the 

topography when they are viewed through a stereoscope. Patterns of 

vegetation, variations of gray or color tones in soil and rock, drainage patterns, 

joint patterns, and linear features (landforms, fault traces) allow preliminary 

interpretations of geology, soils, and hydrogeology. Historic aerial photography 

can also be useful in documenting preexisting physical conditions and 

monitoring the progress of cleanup operations at hazardous waste sites.

Aerial photos of areas with near-surface bedrock often reveal linear features 

called fracture traces, which indicate zones of relatively high permeability 

in the subsurface. Fracture-trace analysis on aerial photographs can provide 

Remote Sensing
152

preliminary information on possible preferential movement of ground water 

or contaminants. Fetter (2001) provides a useful introduction to fracture-trace 

analysis. Sonderegger (1970) describes use of panchromatic, color, and infrared 

photography to locate fracture traces as an aid to the interpretation of 

the occurrence and movement of ground water in limestone terrain. Parizek 

(1976) thoroughly reviews the North American literature on fracture-trace and 

lineament analysis. If possible, aerial-photo analysis of fracture traces should 

be supplemented with surface analysis of bedrock fracture orientations. Tracing

and analysis of drainage patterns on aerial photos using overlays can suggest 

various rock types and geological structures, based on characteristic drainage 

patterns and densities (fig. 53).

Color infrared photography is particularly useful for identifying ground water 

discharge areas or areas where contamination changes vegetation; for example, 

it can help identify a failed septic tank absorption system (Farrell 1985), areas 

where fertilizer has been applied, or areas of oil pollution. Thermal infrared 

scanning can detect ground water discharge into surface waters by sensing

temperature differences in the ground water and surface water. Ellyett and 

Pratt (1975) considered this type of photography to be potentially the most 

useful remote sensing tool in the study of hydrogeological indicators. The use 

of thermal infrared imagery to measure soil moisture (Jackson 1986, Jackson 

Figure 53. Interpretation of drainage patterns from aerial photos and corresponding rock types and structure.
153

and others 1982, Price 1980, U.S. Geological Survey 1982) and evaporation 

(Price 1980, U.S. Geological Survey 1982) is reasonably well established. The 

National Research Council (2004) evaluated remote sensing technology as a 

means of detecting shallow aquifers and concluded that it is not practical for 

measuring ground water depth directly.

Airborne geophysical methods such as side-looking radar (SLAR), airborne 

electromagnetic imaging, and aeromagnetics have not been widely used in 

ground water studies, but the potential exists for their use in regional waterquality studies. A special feature of SLAR is its ability to distinguish grain size 

in alluvium. This technique requires unvegetated surfaces, a condition that is 

more likely to occur in arid areas (Ellyett and Pratt 1975).

Black and white photographs are available from various State and Federal 

agencies for almost any location in the United States and are the least 

expensive type of photo to obtain. Other types of photography are available at 

greater expense and should be used for special applications or to expand the 

scope of the photographic interpretation of the study area. These include the 

following photo types:

•	 True color photos that record all colors in the visible spectrum.

•	 Color infrared film that records yellows and reds as green and the near infrared (not 

visible to the eye) as red. Since vegetation reflects near-infrared radiation, this image is 

especially useful for observing vegetation patterns. Other types of images that record or 

display colors differentially (false color) can be created.

•	 Ultraviolet (UV) photography uses special film and filters to record UV energy. Oil and 

carbonate minerals are fluorescent in UV bands when they are stimulated by sunlight. 

A disadvantage of UV wavelengths is that they are scattered in the atmosphere and 

result in a low-contrast image, especially when dust or haze is present.

•	 Multiband or multispectral images use multiple lenses and filters to record simultaneous 

exposures of different portions of the visible and near-infrared spectra of the same 

area on the ground. Images can also be recorded electronically using a multispectral 

scanning system.

Because aerial photography is basic to preparation of geological maps, its use 

is crucial for hydrogeological studies if published geological maps are not 

available. Contracts for aerial photography should be awarded only for areas 

that are not mapped and only when the project has high priority. Existing 

aerial photography can often be purchased, realizing significant cost savings. 

A hydrogeologist can make basic interpretations about hydrogeological 

conditions of an area using aerial photography along with field verification. A 

general geological map can be prepared for many study areas using the aerial 

photography. The lengthy time requirements and great expense required for 

preparation of geological maps restrict the use of map preparation to small 

project areas. Detailed geological mapping for large areas (for example, er 
154

1:24,000) can often take years, and makes such efforts impractical for most 

hydrogeological studies. Satellite images can also provide useful geological 

and hydrological information (Salama and others 1994), but at a scale that is 

useful primarily for large regional assessments. Imagery can be purchased from 

several commercial vendors.

Inventory data about hydrogeological conditions are available from well logs 

filed with State regulatory agencies. Well logs are drillers’ descriptions of 

lithological units penetrated by drilling. Unfortunately, these logs are often 

only minimally descriptive and sometimes describe rock types inaccurately; 

however, well logs are very descriptive if a geologist was present during 

drilling and supervises the logging, or the driller knew various rock types and 

could identify subtle changes in drilling that translate into changes in rock type. 

These logs also include specific capacity test data that can be used to infer 

relative transmissivity of the formation (Theis and others 1963, Bradbury and 

Rothschild 1985). Regardless of the quality of the logs, well logs can provide 

important information about the subsurface and, where available, should be 

incorporated into any ground water investigation.

Hydrogeological studies may require installation of monitoring wells. Often, 

such installations involve the detailed logging of the associated borehole by 

a geologist. In addition, there may be other boring logs or detailed soil pit 

information generated by other ground water investigations conducted in the 

area. These detailed logs can provide much needed information during the 

early stages of an investigation.

Ground water monitoring networks are should be designed for the specific 

purpose for which they are established. For example, a network may be used to 

(1) monitor long-term effects of climatic changes on ground water systems, (2) 

monitor the effects of a new well field adjacent to a national forest on ground 

water levels within the national forest boundaries, or (3) monitor contaminant 

movement within national forest boundaries from a landfill or other pollution 

source. Each of these networks has different design considerations.

Often overlooked in ground water investigations is the need for an observation 

network to collect other types of hydrological data, in addition to ground water 

levels and water-quality data (Taylor and Alley 2001). Because meteorological 

data aid in the interpretation of water-level changes in observation wells, 

rain gages should be included as part of a network. Where observation wells 

are located in aquifers that have a strong hydraulic connections to streams 

or lakes, hydrological data, such as stream discharge and stage or lake stage, 

are important to examine the interaction between ground water and surface 

water (ASTM 1996). In addition, water-use data, such as rates and volumes 

of extracted ground water, can greatly enhance the interpretation of trends 

observed in water levels.

Design of a 

Ground Water 

Monitoring 

Network

Well and Borehole

Logs
155

Contaminant detection is generally the most important aspect of a waterquality program, and must be considered in network design. False negative 

contaminant readings because of the loss of chemical constituents or

the introduction of interfering substances that mask the presence of the 

contaminants in water samples can be very serious. Such errors may delay 

needed remedial action and expose either the public or the environment to an 

unreasonably high risk. False positive observations of contaminants may call 

for costly remedial actions or more intensive study, which are not warranted 

by the actual situation. Thus, reliable sample collection and data interpretation 

procedures are central to an optimized network design.

The ideal observation network consists of monitoring wells constructed 

specifically for that network, as well as instrumentation to collect ancillary 

hydrological data such as rainfall and streamflow. Budgetary constraints, 

however, may require the use of existing observation, domestic, or other wells 

for all or part of the network. 

Extreme care must be taken in the selection of existing wells for use in the 

networks; for example, water-supply wells are drilled for maximum capacity, 

and may be completed so that they tap more than one aquifer or water-bearing 

unit. Such wells may provide data of minimal value in a study of contaminant 

movement from a pollution source. If existing wells must be used, all available 

well-construction information should be obtained so that the usefulness of the 

well for the network can be evaluated. If a pumping well is to be used for the 

network, both the pumping level and the static water level in the well should be 

obtained.

Identification of existing wells that are suitable for sampling may be divided 

into four steps: (1) identifying all the wells that exist in the area of interest; 

(2) identify those wells that are screened only in the hydrogeological unit 

targeted for sampling; (3) applying a screening process to the wells identified 

in step 2 to determine which wells meet the explicitly defined suitability 

criteria for sampling; and (4) evaluating the spatial distribution of wells that 

are suitable for sampling, not only in map view but also relative to the depths 

of the screened intervals of these wells within the hydrogeological unit. This 

evaluation is accomplished most efficiently by plotting available wells on a 

map, showing depth of screened interval below the water table and/or depth of 

screened interval relative to total thickness of the hydrogeological unit.

Criteria for wells that are suitable for sampling may vary for different projects; 

therefore, the first step in defining suitable wells is to list explicitly (and 

document subsequently in the monitoring project database) a set of criteria 

that must be present, and information about the well that must be available 

to meet the minimum-acceptable criteria for sampling. These same criteria 

are a starting point in developing specifications for newly constructed project 

wells. The criterion for a monitoring well that must be met is that it must yield 

water from, and only from, the particular zone (hydrostratigraphic unit) that is 

targeted for sampling. 

Use of Existing 

Wells
156

A second criterion, the well type (primary purpose for which the well was 

constructed), relates to existing wells and is a key consideration in judging 

their suitability for sampling to meet project objectives. Wells may be divided 

into two major size categories: (1) high-capacity wells and (2) low-capacity 

wells. Sometimes the type of well to be sampled is an explicit part of the 

project objectives; for example, interest may be only in low-capacity domestic 

wells or wells constructed to a certain depth or screened in a particular zone. It 

is essential that well construction information be available for any well that is 

to be sampled. Key information includes screened interval, total depth, casing 

material, filter pack, and surface seal design. 

A third criterion involves construction of the well. Key considerations include 

(1) length of intake interval (well screen or open hole)— project objectives 

may not be served by very long well screens or long open-hole intervals in 

bedrock wells because these create uncertainties in the actual water source; 

(2) the type of casing and screen material—results of sampling for metals 

may be compromised by metal casing, and sampling for some volatile organic 

compounds (VOCs) may be compromised by polyvinyl chloride (PVC) casing, 

particularly if casing joints are glued; and (3) methods and materials used to 

drill, complete, and develop the well—contaminants could be introduced into 

the strata or change the chemical environment in the vicinity of the well bore 

(Brobst 1984).

In addition to these criteria for selecting existing wells suitable for sampling 

is the availability of detailed information about these wells. The process of 

evaluating the suitability of existing wells for sampling begins as part of the 

assembling and evaluating of existing geologic and water-quality information 

in the different hydrogeological units of interest. An important prerequisite 

for screening existing water-quality data is the existence (preferably in an 

electronic database) of basic information on well location, well-construction 

details, and at least one water level when the well was not being pumped. Large 

numbers of otherwise suitable wells may have to be eliminated as candidates 

for sampling because essential information about the wells is not available.

Following the determination of the number and spatial distribution of existing 

wells that are suitable for sampling, as well as the number of wells to be 

sampled, the next question is whether a subset of the existing suitable wells 

can be selected that will meet project objectives. If not, new project wells are 

needed either for all samples in the study, or to merely “fill in the gaps” where 

wells do not exist, or suitable wells are not available. The obvious advantages 

of drilling new project wells include (1) selection of the well location and 

access to the well for sampling, (2) designation of the screened interval of the 

well within the hydrogeological unit, (3) control over specific construction 

features of the well, (4) possible assurance of long-term availability of the 

well for sampling; and (5) control over the collection of detailed geologic and 

hydrologic information during well construction. The principal disadvantages 

of drilling new project wells are the potentially large additional drilling costs 

Installation of 

New Monitoring 

Wells
157

for the project, and the increased time and cost associated with difficulties in 

obtaining permission and legal easements to drill wells in desired locations. 

Additional time is also required for completing all required environmental 

assessments.

A type of monitoring well that often justifies special design and construction is 

one that will be used for long-term monitoring and trend analysis. Because of 

the large costs to collect and analyze the samples over a period of many years, 

reliability of the data is imperative, and a well specifically constructed for this 

purpose would likely be cost-effective. 

Effective design and construction of a monitoring well require considerable 

care and at least some understanding of the hydrogeology and subsurface 

geochemistry of the site. Preliminary borings, well drilling experience, and 

the details of the operational history of a site can be very helpful. Common 

monitoring well design criteria include depth, screen size, gravel-pack 

specifications, and yield potential. These considerations differ substantially 

from those applied to production wells. The simplest, small-diameter well 

completions that will permit development, accommodate the sampling gear, 

and minimize the need to purge large volumes of potentially contaminated 

water are preferred for effective routine monitoring activities. Helpful 

references include Barcelona and others (1983), Scalf and others (1981), 

Wehrmann (1983), Aller and others (1991), Lapham and others (1997), and 

Driscoll (1986).

The placement and number of wells in a network, as well as the placement 

and number of rain gages or stream and lake gaging stations, depends on the 

purpose of the network and the hydrogeologic complexity of the study area. 

Ideally, the wells chosen for an observation network provide data representative 

of the topographic, geologic, climatic, and land-use environments present in 

the area of interest. The more varied these environments are within that area, 

the more wells will be needed. Subsurface geophysical techniques can be very 

helpful in determining the optimum placement of monitoring wells under 

appropriate conditions and when sufficient hydrogeological information is 

available (Evans and Schweitzer 1984). The placement and number of wells 

will also depend on the degree of spatial and temporal detail needed to meet the 

objectives of the program. Both the directions and approximate rates of ground 

water movement must be known to satisfactorily interpret the chemical data. 

An understanding of the variability or distribution of hydraulic conductivity, 

in both the vertical and horizontal dimension, allows one to isolate the 

major zones of water transmission and, therefore, to select the proper depths 

of wells and the position and length of well screens. The same is true for 

offsite, upgradient, and downgradient monitoring wells. With this knowledge, 

it also may be possible to estimate the nature and location of pollutant 

sources (Gorelick and others 1983). Well placement should be viewed as an 

evolutionary activity that may expand or contract as the needs of the program 

dictate.

Well Placement
158

For contaminant monitoring, wells should be placed near the area of suspected 

contamination pathway, as well as upgradient of the site. Initial investigations 

need to be conducted to determine the flow system before monitoring wells 

can be effectively installed. If several aquifers are present and of interest, 

observation wells will be required that monitor each individual aquifer. 

Individual sampling can be accomplished by using multiple wells, each 

completed in an individual aquifer and isolated from the others at a site, or by 

using multiple screened intervals isolated from other aquifers with packers or 

some other isolating medium, such as bentonite or cement. Wells completed 

at multiple depths may also be needed where there are vertical head gradients 

within a single aquifer (such as near a lake or stream that receives ground 

water discharge), or where contaminant migration may be along preferential 

flow paths (such as fractures or sand lenses). Where multiple-completion wells 

are required, care should be taken to physically isolate each zone of interest. 

Lapham and others (1997) provide detailed discussions of well design, well 

completion, and well development that provide optimal information for waterquality studies. Generally, the placement of nested piezometers in closely 

spaced, separate boreholes of different depths generally is the preferred method 

to determine vertical head differences and the potential for vertical movement 

of contaminants, while monitoring wells with appropriately located screens 

are used to determining the lateral movement of contaminants in the saturated 

zone. One must also consider whether vadose-zone monitoring is required. 

Nested lysimeters can be used to detect contaminants in the vadose zone, but 

great care must be taken to ensure that the collected samples are representative 

and not affected by preferential flow paths and sorption and volatilization of 

the contaminant.

The length and position of well screens also must be predicated on the nature 

of the contaminant; for example, if the contaminants are miscible with the 

liquid phase, it may be possible to use only one well per sampling point. It 

also may be possible to use only one well if the transmissive zone is very thin. 

If the contaminants are immiscible with the liquid phase (sinkers or floaters), 

the well screen must be located accordingly. Selection of a length of well 

screen depends on the vertical scale of investigation, and on the thickness and 

properties of the hydrogeological unit of interest. The longer the screened (or 

open) interval relative to aquifer thick­ness, the less likely dif­ferences in water 

quality at specific depth intervals will be able to be distinguished. Mixing of 

waters within the screened interval can lead to constituent concentrations that 

do not necessarily represent the maximum or minimum concentrations of those 

constituents at any point. For this reason, rela­tively short screens are generally 

used if the objective is to inves­tigate water quality at discrete intervals and to 

define chemical stratification within the aquifer. If determin­ing the vertical 

distribution of water quality in an aqui­fer is a data-collection objective, 

installing wells at different depths, each with a relatively short screen length, is 

often the most effective design.
159

Screen lengths for monitoring wells typically range from 2 to 20 feet. As a 

general rule, screen lengths of 20 feet or less generally are appropriate for most 

resource assessment studies, while screen lengths of 5 feet or less generally 

are better suited for stud­ies to determine fate, transport, and geochemistry 

of ground water constituents. A screen length of 5 feet might be too long if 

information suggests that marked vertical differences in the distribution of 

hydraulic head or water quality occur on the order of a few feet or less.

The length of the open interval also depends on the scale of the investigation. 

For example, a 20-foot-long screen is too long for an investigation of a 5-footthick contaminant plume; however, such a screen might be considered too short 

in an investigation of the general water quality of an aqui­fer that is several 

hundred feet thick. The following are additional factors to consider when 

deciding on screen length:

•	 A short screen generally provides measurements of hydraulic 

head and ground water quality that more closely represent point 

measurements in the aquifer than measurements provided by a 

long screen.

•	 Samples taken from wells with long screened inter­vals could 

exhibit smaller concentrations or a higher frequency of samples 

with undetectable concentrations (leading to a “false negative” 

assess­ment) in comparison with samples taken from wells with 

short screened intervals.

A long well screen also can induce mixing of waters of different chemistry in 

comparison to a short well screen because of vertical flow along the screened 

interval resulting from differences in head (well-bore flow). Well-bore flow can 

occur even in homogeneous aquifers with very small vertical head differences. 

Well-bore flow might contribute to aqui­fer contamination by providing a 

pathway for contaminant movement from contaminated to uncontaminated 

zones along the screened inter­val(s).

The selection of a particular drilling technique for observation-well 

construction depends on the geology of the site, the expected depths of the 

well, the requirements for subsurface lithologic samples, and the suitability of 

drilling equipment for the contaminants of interest. Available drilling methods 

include auger; rotary, using water-based fluids or air; cable-tool, jet-wash 

and jet-percussion; coring; direct push; and vibration. The advantages and 

disadvantages of each method are described in detail by Lapham and others 

(1997). Regardless of the technique used, every effort should be made to 

minimize subsurface disturbance. For environmental applications, the drilling 

rig and tools should be steam cleaned to minimize the potential for crosscontamination between formations or successive borings. 

Leaching of contaminants from unlined landfills is a common problem. This 

case study documents a hydrogeological investigation of a landfill on Forest 

Service land used by the city of West Yellowstone, MT. The investigation

Case Study: Landfill

Evaluation, West

Yellowstone, Gallatin

National Forest, MT
160

detected a plume of heavy metals and VOCs migrating from the landfill toward 

the nearby Madison River, resulting in a decision by the city to close the 

landfill and truck all of its trash to a distant certified landfill.

In 1971, the city of West Yellowstone obtained a special-use permit from 

the Gallatin National Forest to locate and operate a Class II (household 

wastes) landfill on national forest land 4 miles north of the city and east of 

U.S. Highway 191. It replaced an older dump 1 mile away. The site is on a 

flat alluvial terrace formed by the Madison River. The geological setting is 

medium- to coarse-grained, highly permeable, obsidian sand that is about 

600 feet thick and derived from volcanic activity originating in Yellowstone 

National Park. Annual precipitation in the area averages 40 inches, mostly as 

snowfall from October through May. Elevation is about 6,700 feet.

By most accounts, the landfill was well-operated and maintained by the private 

contractor to whom the city leased the site. In 1976, the Forest Service zone 

sanitary engineer asked the Soil Conservation Service (SCS) to investigate 

and evaluate the landfill. The SCS drilled two monitoring wells and described 

the landfill’s trenches as being about 25 feet deep with about 25–35 feet of 

obsidian sand separating the garbage from the water table. The ground water 

flow was estimated to be southwesterly toward the Madison River about 3,700 

feet away. The potential for leachate from the landfill to ultimately reach the 

river and Hebgen Lake was considered to be high.

Monitoring results from the two wells in 1976 revealed high levels of carbon 

dioxide, iron, manganese, lead, mercury, cadmium, biological oxygen demand, 

chemical oxygen demand, and a trace of 2,4-dichlorophenoxyacetic acid (2,4-

D), indicating leachate contamination. A decision was made to drill seven more 

4-inch diameter, shallow wells, 44–62 inches deep, between the landfill and 

the river. A background well was located inside Yellowstone National Park 

upgradient of the landfill. The state office of the SCS performed the drilling 

in 1976–77. The water table depth ranged from 42 to 60 feet below the land 

surface, and was shown to fluctuate 6 to 8 feet annually.

Water-quality samples were collected at each well in 1977–78, 1980–81, 

and 1985. Results revealed a leachate plume in ground water, moving 

steadily toward the Madison River. The Solid Waste District sampled for 10 

parameters in 1989, 1990, 1992, and added VOCs in 1995 and 1996. Lowlevel concentrations of several VOCs were detected, but none exceeded 

human health standards. Dissolved metals (iron, manganese, zinc) and specific 

conductance values indicated that the leachate plume was continuing to move 

toward the river.

In 1982, dumping of household wastes at the landfill ceased, but Class III 

waste (trees, cars, demolition debris) dumping continued until 1988 when a 

soil cap was constructed. A solid waste transfer facility was built in 1983–84 
161

Water-level 

Monitoring

Figure 54. Common environmental factors that influence the choice of frequency of water-level measurements in observation 

wells (Taylor and Alley 2001).

and continues in operation. A 1994 special-use permit required testing of VOCs 

twice at the closed landfill site. These tests were done, and no additional testing 

is planned.

Old, unlined landfills can leach heavy metals, VOCs, and other constituents 

into ground water and eventually into surface water bodies hydrologically 

connected to the ground water. Hydrogeological investigations are necessary 

to quantify the magnitude and extent of ground water contamination and to 

determine the direction and rate of ground water movement. Cooperation of 

local, State, and Federal agencies produces better results than going it alone.

Water-level measurements from observation wells are the principal source of 

information about the hydrologic stresses acting on aquifers and how these 

stresses affect ground water recharge, storage, and discharge (Taylor and Alley 

2001). The frequency of water-level measurements from the network is an 

important design consideration. Water levels may be measured continuously 

using floats and paper strip charts, at frequent intervals ranging from seconds 

to hours using transducers and data loggers, or periodically (daily, weekly, 

monthly) by obtaining manual measurements on site. Again, the purpose of the 

network determines the frequency of measurements that is required. Although 

influenced by budgetary considerations, the frequency of measurements should 

be determined to the extent possible with regard to the anticipated variability 

of water-level fluctuations in the observation wells and the amount of detail 

needed to fully characterize the hydrological behavior of the aquifer or the rate 

of contaminant movement (Fig. 54).
162

Quality Assurance

Water-level

Monitoring

Equipment

Synoptic water-level measurements are a special type of periodic measurement 

in which water levels in wells and nearby surface waters are measured 

within a relatively short period and under specified hydrologic conditions. 

Synoptic measurements provide a “snapshot” of aquifer water levels. These 

measurements commonly are taken when data are needed for mapping the 

altitude of the water table or potentiometric surface, for determining hydraulic 

gradients, or for defining the physical boundaries of an aquifer. Regional 

synoptic measurements made on an annual or multiyear basis can be used as 

part of long-term monitoring to complement more frequent measurements 

made from a smaller number of wells.

Good quality assurance helps to maintain the accuracy and precision of waterlevel measurements, ensure that observation wells reflect conditions in the 

aquifer being monitored, and provide data that can be relied on for many uses 

(Taylor and Alley 2001). Therefore, field and office practices that will provide 

the needed levels of quality assurance for water-level data should be carefully 

considered and consistently employed. Some important field practices that 

will ensure the quality of ground water-level data include the establishment 

of a permanent datum (a reference point for water-level measurements) for 

observation wells, periodic inspection of well structures, and periodic hydraulic 

testing of the well to ensure its communication with the aquifer. The locations 

and elevations of the wells should be accurately surveyed initially and checked 

periodically (annually or every other year). Existing wells selected for use in 

the network should be carefully inspected, with a downhole video camera if 

necessary, to ensure that no construction defects are present that might affect 

the accuracy of the water-level measurements. Water levels are typically 

measured to within 0.01 foot or 1 millimeter.

To help ensure quality, both paper and electronic files should be established 

containing information for each observation well. The files should include 

a physical description of well construction, location (both horizontal and 

vertical) in an appropriate coordinate system, coordinate system datum, 

and results of hydraulic tests. Recent water-level measurements should be 

compared with previous measurements made under similar hydrological 

conditions on a regular basis to identify potential anomalies in water-level 

fluctuations that may indicate measurement equipment malfunction or a defect 

in well construction. Ongoing data evaluations can help identify issues early to 

allow correction or equipment repair prior to the end of the study.

The type of water-level monitoring equipment needed depends on several 

factors, such as study objectives, depth to the water table, required accuracy, 

type of well to be monitored (pumping or observation well) and frequency of 

measurements. Manual-measurement equipment tends to be used only when 

periodic (monthly, quarterly, or annual) measurements are needed. Continuousmeasurement equipment is most useful when short-term (minutes, hours, days) 

measurements are needed.
163

Manual-measurement equipment includes steel or electric tapes. Steel tapes 

with attached “poppers” (Fig. 55) are most useful at shallow depths, generally 

less than 200 feet, and can be accurate within 0.01 foot. Electric tapes are used 

at shallow to intermediate depths, generally less than 1,000 feet, but can be 

less accurate at greater depths or in wells with cascading water. Correction 

factors for both types of tapes (particularly electric tapes) may be required 

for measurements greater than 1,000 feet, because of stretching of the tape at 

those depths (Garber and Koopman 1968).

Common continuous-measurement equipment includes floats with strip 

charts and pressure transducers with data-logger systems. Strip charts and 

floats monitor water levels truly continuously (barring equipment failures, 

such as hung-up floats), but transducer and data-logger systems can be 

programmed to monitor water levels at any desired interval. In order to save 

battery power or data storage space, data loggers can even be programmed to 

measure only when water levels change by a specified value, over a specified 

time interval, or when also connected to a rain gage, during or immediately 

following storm events. The data are recorded and stored electronically, and 

can be transmitted by radio, phone, or satellite to an office miles away. Such 

capability is especially useful for monitoring in remote locations, and for early 

identification of potential equipment problems prior to significant loss of data. 

Figure 55. Measuring water 

levels in an observation well 

with a steel tape. (Photo by 

Heather S. Eppler, USGS.)
164

Ground Water 

Quality Sampling 

and Analysis

Several manufacturers produce downhole pressure transducer-data logger 

packages to measure water level changes in wells (fig. 56). These units 

generally have small diameters (5/8 inch) and short lengths (1–2 feet), which 

permit them to be installed inside the well casing and left in place for long-term 

monitoring. They have no exposed wires or other indications that anything 

is inside the well, which minimizes vandalism. The units can be set to record 

water levels every few seconds if necessary (during pump tests for example), 

or once or twice per day, or even monthly for long-term monitoring. Data is 

downloaded into either a laptop computer or a handheld device. Data are then 

transferred to a database through data management software or analyzed with 

graphic analysis software. 

Water-quality sampling and field analysis for Forest Service ground water 

studies, inventories, and investigations should conform to techniques and 

protocols established in the National Field Manual for the Collection of 

Water-Quality Data (National Field Manual) by the USGS (1997 to present). 

The National Field Manual describes protocols and provides guidelines for 

personnel who collect data used to assess the quality of the Nation’s surface 

water and ground water resources. A chapter of the National Field Manual

addresses field-trip preparations, including selection of sample-collection sites 

for studies of surface water quality. It also covers site reconnaissance and well 

selection for studies of ground water quality, and the establishment of field files 

for a sampling site. Each chapter of the National Field Manual is published 

separately and revised periodically. Newly published and revised chapters 

are announced at http://water.usgs.gov/ under “New Publications of the U.S. 

Geological Survey.” 

Figure 56. Downloading data 

from an automatic water-level 

recorder. (Photo by Michael D. 

Unthank, USGS.)
165

The National Field Manual is targeted specifically for field personnel to (1) 

establish and communicate scientifically sound methods and procedures; (2) 

provide methods that minimize data bias and, when properly applied, result in 

data that are reproducible within acceptable limits of variability; (3) encourage 

consistent use of field methods for the purpose of producing nationally 

comparable data; and (4) provide citable documentation for USGS waterquality data-collection protocols. Formal training and field experience are 

needed to correctly implement the protocols described in this manual.

Sampling protocols addressed in the USGS National Field Manual for the 

Collection of Water-quality Data include the following:

•	 Preparations for water sampling. 

•	 Selection of equipment for water sampling.

•	 Cleaning of equipment for water sampling.

•	 Collection of water samples. 

•	 Processing of water samples. 

•	 Field measurements. 

−	 Temperature. 

−	 Dissolved oxygen. 

−	 Specific electrical conductance. 

−	 pH. 

−	 Oxidation-reduction (redox) potential by the electrode 

method (also known as ORP or Eh). 

−	 Alkalinity and acid neutralizing capacity. 

−	 Turbidity.

•	 Biological indicators. 

−	 Five-day biochemical oxygen demand (also known as 

BOD5). 

−	 Fecal indicator bacteria. 

−	 Fecal indicator viruses. 

−	 Protozoan pathogens.

•	 Bottom-material samples. 

•	 Safety in field activities.

Whether the goal of the monitoring effort is inventory or detection of specific 

contamination, the information gathered in sampling must be of known 

quality and must be well documented. High-quality chemical data collection 

is essential in ground water monitoring programs. Each monitoring program, 

however, has unique needs and goals that are fundamentally different from 

surface-water investigative activities. The reliable detection and assessment 

of subsurface contamination require minimal disturbance of geochemical and 

hydrogeological conditions during sampling. The technical difficulties involved 

in “representative” samplings are well documented (Wood 1976). 

Gillham and others (1983) published a very useful reference on the principal 

sources of bias and imprecision in ground water monitoring. Their treatment is 

extensive and stresses the minimization of random error, which can enter
166

into well construction, sample collection, and sample handling. They further 

stress the importance of collecting precise data over time to maximize the 

effectiveness of trend analysis, particularly for regulatory purposes. Accuracy 

also is very important, because the ultimate reliability of statistical comparisons 

of results from different wells may depend on differences between mean 

values for selected constituents from relatively small numbers of replicates; 

therefore, systematic error must be controlled by selecting proven methods for 

establishing sampling points and collecting samples to ensure known levels 

of accuracy. Collecting representative samples that are free of artifacts and 

errors is a function of the degree of detail needed to characterize subsurface 

hydrological and geochemical conditions and the care taken to minimize 

disturbance of these conditions (EPA 1993a). Each well or boring represents 

a potential conduit for short-circuited contaminant migration or ground water 

flow, which must be considered a potential liability to investigative activities.

Filtration of samples being analyzed for contaminants has received considerable 

attention in the literature in the past several years because contaminants can be 

sorbed onto colloidal particles moving through an aquifer. Analysis of filtered 

ground water samples might underestimate the amount of a contaminant that is 

actively moving through an aquifer if colloidal transport is occurring. Analysis 

of unfiltered samples, however, may overestimate the amount of contaminant 

in the aquifer as a result of changed physical and chemical conditions in and 

immediately surrounding the well. Given the changing status of regulatory

thinking on this issue, consultation with the appropriate regulatory agency or 

water-quality expert is recommended to determine whether samples should be 

filtered. Filtering samples for total organic carbon, total organic halogens, or other 

organics is inadvisable because the increased handling required may result in the 

loss of the chemical constituents of interest (EPA 1991). Filtering of ground water 

samples that are to be analyzed for metals, however, may be appropriate. To 

minimize the problems associated with sample filtration, any filtration should be 

conducted in the field as soon after sample collection as possible.

Preparations for sampling involve extensive planning and logistical support to 

ensure that the sampling effort is conducted properly and will provide legally 

and scientifically defensible data. Planning may involve months of personnel 

time. Activities include (1) selection of wells to be sampled, if existing wells 

are to be used, or selection of drilling locations for placing monitoring wells 

for new monitoring locations; (2) ordering of suitable equipment to obtain the 

desired sample parameters and associated supplies, including health and safety 

gear; (3) training of field personnel in sampling protocols; (4) implementation 

of the quality assurance/quality control (QA/QC) program in connection 

with the training of field personnel, including analysis of equipment blanks 

and possibly other types of QA/QC samples; (5) selection of the lab and 

establishing procedures for receipt of samples and turnaround times for lab 

analysis; (6) establishing methods of shipping and ensuring that holding times 

are not exceeded, that proper sample preservation methods are employed, 

and that proper chain of custody control is maintained; (7) visits to all wells 
167

Sources of

Sampling Error

Selection of

Ground Waterquality Indicators

to confirm permission to sample, obtain access to wells (physical access and 

access into the well bore with sampling equipment), establish exact location 

of wells, and anticipate any sampling problems that might arise with new 

personnel on the site. In some cases, some of the time and expense associate 

with proper sampling may be addressed through the use of well-trained, 

experienced contractors.

To collect sensitive, high-quality concentration data, investigators must identify 

the types and magnitudes of errors that may arise in ground water sampling. 

Table 11 presents a generalized diagram of the steps involved in sampling and 

the principal sources of error.

Table 11. Generalized diagram of the steps involved in sampling and the principal sources of 

error.

Sampling activity Sources of error

Establishment of sampling points Improper well construction or placement.

Field measurements Instrument malfunction; operator error; 

poor field conditions.

Sample collection Improper protocols – cross 

contamination, sample exposure, 

degassing, oxygenation.

Preservation/storage Improper protocols – handling, labeling 

errors, wrong preservative; matrix 

interference.

Transportation Delay beyond holding times; sample 

loss.

Field and trip blanks, standards Contamination; operator error; matrix 

interferences.

Other factors controlling ground water sampling errors are the contamination 

of the subsurface by drilling fluids, grouts, or sealing materials; the sorptive 

or leaching effects on waters samples from well casing; pump or sampling 

tubing materials; and the effects on the solution chemistry from oxygenation, 

depressurization, or gas exchange caused by the sampling mechanism. Two 

of the most critical elements of a monitoring program are establishing both 

reliable sampling points and simple, efficient sampling protocols that will yield 

data of known quality.

Whether a program is focused on background/existing conditions, land-use 

impacts, or compliance monitoring, a key element is the selection of the 

properties, elements, and compounds (indicators) to be measured. Selection 
168

of indicators for monitoring programs should be based on their relevance to 

important water-quality issues, such as human or aquatic health protection, 

the need for gaining an understanding of important geochemical processes, 

as well as the existence of appropriate analytical methodologies. Because of 

differences in the importance of water-quality issues in various regions of the 

country and because of the potential for significant differences in the objectives 

of monitoring programs, no one set of indicators is suitable or appropriate for 

all Forest Service monitoring programs. 

Indicators appropriate for ground water-quality monitoring should meet 

two general criteria. First, a parameter should be a candidate for monitoring 

because it fulfills any of or all the following criteria: 

•	 The parameter is potentially toxic to human health and the environment, 

livestock, and plants; for example, pesticides, VOCs, trace elements, 

and nitrate. 

•	 The parameter impairs the suitability of the water for general use; for 

example, hardness, iron, manganese, taste, odor, and color. 

•	 The parameter is of interest in surface water and may be transported 

from ground water to surface water; for example, ammonia, nitrite, and 

nitrate. 

•	 The parameter is an important “support variable” for interpreting 

the results of physical and chemical measurements; for example, 

temperature, specific conductance, pH, major ion balance, depth to the 

water table, and selected isotopes. 

Second, analysis of the candidate indicator should be made using wellestablished analytical methods at appropriate minimum-detection and reporting 

levels necessary to achieve the objectives of study. In general, only published 

analysis protocols established or recognized by EPA, USGS, ASTM, or States 

should be used on Forest Service projects. An additional important source of 

portentially applicable analysis prototcols is Standard Methods 

http://www.standardmethods.org. Based on these criteria, the following 

general groups of indicators should be considered for ground water-monitoring 

programs: 

•	 Field measurements (temperature, specific conductance, pH, Eh (redox 

potential), dissolved oxygen, alkalinity, depth to water).

•	 Major inorganic ions and dissolved nutrients (total dissolved solids 

(TDS), Cl, NO3, SO4, PO4, SiO2, Na, K, Ca, Mg, NH4).

•	 Total organic carbon (also known as TOC) [Barcelona 1984].

•	 Pesticides.

•	 VOCs.

•	 Metals and trace elements (Fe, Mn, Zn, Cd, Cu, Pb, Cr, Ni, Ag, Hg, As, 

Sb, Se, Be, B).

•	 Bacteria.

•	 Radionuclides.

•	 Environmental isotopes (H, O, N, C, S).
169

The steps for selecting specific indicators for ground water monitoring are 

discussed below. 

1. Analyze existing information. The first step in the process is to use 

existing information to determine whether a recently documented 

occurrence of the indicator(s) exists. In many areas, large amounts 

of water-quality data have been collected by many organizations 

to address a wide range of objectives. Much of these data can be 

obtained from the EPA STOrage and RETrieval (STORET) database 

(http://www.epa.gov/storet/), the USGS National Water Information 

System (NWIS) database (http://waterdata.usgs.gov/nwis), and specific 

State ground water and/or water-quality databases.

2. Determine whether the constituent or contaminant is likely to occur in 

the ground water system. This step assesses the likelihood that specific 

indicators, which have no documented occurrence and have not been 

found in samples collected from the aquifer system, will be present. 

This assessment should take into account what is known about the 

potential sources of the contaminant(s) of interest, the physical and 

chemical properties of the contaminants, and knowledge of the local 

hydrogeology and the susceptibility of the aquifer to contamination. 

Franke (1997) provides detailed lists of indicators that could be 

considered for monitoring in areas with different types of land use and 

sources of contaminants.

3. Test and validate constituent occurrence. A screening sampling could 

be conducted to determine if the constituent of interest is present in 

selected wells in the aquifer system to be sampled. The number of 

wells to be assessed in such a screening survey should be determined 

on the basis of the size of the study region and the complexity 

of the hydrogeological setting. Using the results of this survey, 

the investigator should refine the list of constituents to include in 

subsequent sampling of the system. As knowledge of the occurrence of 

different constituents in different environmental settings improves, the 

uncertainty associated with the understanding of indicator occurrence, 

as well as the need for extensive verification, should decrease. 

It is generally accepted that water present within the well casing is not likely 

to be representative of the formation water and needs to be purged prior 

to collection of ground water samples. The water in the screened interval, 

however, may indeed be representative of the formation, depending on well 

construction and site hydrogeology. Wells are purged for the following reasons: 

(1) the presence of the air interface at the top of the water column results in an 

increased oxygen concentration within the well and surrounding materials, (2) 

the loss of volatiles up the water column, (3) the leaching from or sorption to 

the casing, filter pack, seal or fill, of constituents of interest, (4) the changes 

to the aquifer flow field from the physical presence of the well, and (5) the 

presence of stagnant water within the well casing. Low-flow purging has 

been found to minimize the amount of purging needed while obtaining a 

representative aquifer sample.

Well Purging and

Sampling
170

Low flow refers to the velocity with which water enters the pump intake and 

is transmitted to the formation pore water in the immediate vicinity of the well 

screen. It does not necessarily refer to the flow rate of water discharged at the 

surface, which can be affected by flow regulators or restrictions. Water-level 

drawdown provides the best indicator of the stress imparted by a given flow 

rate for a given hydrological situation. The objective is to pump in a manner 

that minimizes stress (drawdown) to the system to the extent practicable, 

taking into account established site sampling objectives. Typically, flow rates 

on the order of 0.1—0.5 liters/minute are used; however, rates depend on sitespecific hydrogeology. Some extremely coarse-textured formations have been 

successfully sampled in this manner at flow rates up to 1 liter/minute. The 

effectiveness of using low-flow purging is closely linked with proper screen 

location, screen length, and well construction and development techniques. 

The reestablishment of natural flow paths in both the vertical and horizontal 

directions is important for correct interpretation of the data. For high-resolution 

sampling needs, screens 1 meter in length or less should be used. 

Most of the need for purging has been found to be caused by passing the 

sampling device through the overlying casing water, which causes mixing of 

this stagnant water with the dynamic water in the screened interval. In addition, 

this action causes disturbance to sediment collected in the bottom of the casing 

and the displacement of water out into the formation immediately adjacent 

to the well screen. These disturbances and impacts can be avoided by using 

dedicated sampling equipment, which precludes the need to insert the sampling 

device prior to purging and sampling. 

Low-flow purging using portable or dedicated systems should be implemented 

with a pump intake located in the middle or slightly above the middle of the 

screened interval. Placement of the pump too close to the bottom of the well 

will cause increased entrainment of solids that have collected in the well 

over time. These particles are present as a result of well development, prior 

purging and sampling, natural colloidal transport and deposition, and changes 

to aquifer redox conditions within the well. Placement of the pump at the top 

of the water column for sampling is only recommended in unconfined aquifers 

that are screened across the water table, where the top of the water column is 

the desired sampling point. Low-flow purging has the advantage of minimizing 

mixing between the overlying stagnant casing water and water within the 

screened interval.

The water in the interval can be isolated from the overlying stagnant casing 

water by using low-flow minimal drawdown techniques. If the pump intake is 

located within the screened interval, most of the water pumped will be drawn 

directly from the formation into the well with little mixing of casing water 

or disturbance to the sampling zone; however, if the well is not constructed 

and developed properly, zones other than those intended may be sampled. At 

some sites where geologic heterogeneities are sufficiently different within the 
171

screened interval, the higher conductivity zones may be preferentially sampled. 

This is another reason to use short screened intervals, especially where high 

spatial resolution of the aquifer is a sampling objective.

Water-quality indicator parameters should be used to determine purging needs 

before sample collection in each well. Stabilization of parameters such as 

pH, specific conductance, dissolved oxygen, redox potential (ORP or Eh), 

temperature, and turbidity should be used to determine when stagnant water in 

the well is purged and sampling can begin. In general, the order of stabilization 

is pH, temperature, and specific conductance, followed by dissolved oxygen 

and turbidity. Performance criteria for determination of stabilization should be 

based on water-level drawdown, pumping rate, and equipment specifications 

for measuring indicator parameters. Special devices such as inline flow cells 

are available that continuously measure the above parameters.

Sampling aquifers that have low hydraulic conductivity presents unique 

difficulties. The traditional approach of purging the well of several well casing 

volumes may be ineffective because the drawdown is so rapid that perhaps 

only one casing volume can be obtained in a reasonable time. The approach 

used for low-yielding wells has been to pump the well until the water column 

is evacuated, wait for the well to recover, then repeat, if possible, or directly 

sample the water. This approach poses several concerns:

•	 The time required for sufficient recovery of the well may be excessive 

(perhaps days), affecting sample chemistry through prolonged exposure 

to the atmosphere. 

•	 Purging below the top of the screen (or open section) may cause 

“jetting” or cascading in the well screen (or open section) as the well 

recovers, resulting in a change in dissolved gases and redox state and 

ultimately affecting the concentration of the analytes of interest through 

the oxidation of dissolved metals and possible loss of VOCs if they are 

present.

•	 Draining water from the filter pack around the well screen can trap air 

in the pore spaces, causing lingering effects on dissolved gas levels and 

redox state.

•	 Increased sample turbidity can result from the stress on the formation 

and stirring up of any settled solids in the bottom of the well.

Low-flow sampling as described above is an alternative for sampling aquifers 

having low hydraulic conductivity. Use of these methods will minimize the 

pitfalls in traditional well purging and permit sampling of low-yield formations 

where traditional methods would not be effective. In addition, use of smalldiameter tubing and the smallest pump chamber (or bladder) volume minimizes 

the sampling system volume and the water displaced by the equipment. 
172

Quality Assurance

and Quality Control

Plans

A plan for QA/QC requires the establishment of a sampling protocol that is 

designed to minimize sources of error in each stage of the sampling process, 

from sample collection to analysis to reporting of analytical data. Key elements 

include (1) development of a statistically sound sampling plan for spatial and 

temporal characterization of ground water (EPA 1989); (2) installation of a 

vertical and horizontal sampling network that allows for the collection of 

samples that are representative of the subsurface; (3) use of sampling devices 

that minimize disturbance of the chemistry of the formation water; (4) use of 

decontamination procedures for all sampling equipment to minimize crosscontamination between sampling points (ASTM 1990); (5) collection of QA/

QC samples (trip blanks, field blanks, and duplicates); and (6) processing, 

preserving, and transporting samples to maximize the integrity of the 

samples. Additional QA/QC procedures must be followed in the laboratory. 

As requirements for precision and accuracy increase, the type and number of 

appropriate QA/QC samples will increase. 

QA/QC measures are activities undertaken to demonstrate the accuracy (how 

close to the real result) and precision (how reproducible the results are) of 

monitoring. QA generally refers to a broad plan for maintaining quality in all 

aspects of a program. This plan should describe how the monitoring effort 

would be undertaken. It should include proper documentation of all procedures, 

training of participants, study design, data management and analysis, and 

specific QC measures. QC consists of the steps to be taken to determine the 

validity of specific sampling and analytical procedures. The final quality 

assessment is the estimation of the overall precision and accuracy of the data 

after the analyses have been run. 

The following are among the internal checks that should be performed by 

project field and laboratory staffs:

•	 Trip blanks. A trip blank is a sample bottle filled with deionized 

water under laboratory conditions that is exposed to all conditions 

experienced by the sample bottles and samples throughout the sample 

event. It is processed like any of the other samples. It is used to 

identify sample contamination that may have occurred through ambient 

exposure associated with the sampling event, shipping, and storage.

•	 Field blanks. A field blank is a sample bottle filled with deionized 

water under field conditions using the same or similar equipment 

used to collect the rest of the samples. It is processed like any of the 

other samples. It is used to identify errors or contamination in sample 

collection and analysis. 

•	 Negative and positive plates (for bacteria). A negative plate results 

when the buffered rinse water (the water used to rinse down the sides 

of the filter funnel during filtration) has been filtered the same way as 

a sample. This material is different from a field blank in that it contains 

reagents used in the rinse water. There should be no bacteria growth on 

the filter after incubation. The purpose is to detect laboratory bacteria 

contamination of the sample. Positive plates result when water known 
173

to contain bacteria (such as waste-water treatment plant influent) is 

filtered the same way as a sample. There should be plenty of bacteria 

growth on the filter after incubation. It is used to detect procedural 

errors or the presence of contaminants in the laboratory analysis that 

might inhibit bacteria growth. 

•	 Field duplicates. A field duplicate is a duplicate sample collected by 

the same team or by another sampler or team at the same place, at the 

same time as a sample. It is used to estimate the precision of sampling 

and laboratory analysis. 

•	 Lab replicates. A lab replicate is a sample that is split into subsamples 

at the lab. Each subsample is then analyzed and the results compared. 

They are used to test the precision of the laboratory measurements. For 

bacteria, they are used to obtain an optimal number of bacteria colonies 

on filters for counting purposes. 

•	 Spike samples. A known concentration of the indicator being measured 

is added to the sample. This step should increase the concentration in 

the sample by a predictable amount. It is used to test the accuracy of the 

method. 

•	 Calibration blank. A calibration blank is deionized water processed 

like any of the samples and used to “zero” the instrument. It is the first 

“sample” analyzed and used to set the meter to zero. It is different from 

the field blank in that it is “sampled” in the lab. It is used to check the 

measuring instrument periodically for “drift” (the instrument should 

always read “0” when this blank is measured). It can also be compared 

to the field blank to pinpoint where contamination might have occurred. 

•	 Calibration standards. Calibration standards are used to calibrate a 

meter. They consist of one or more “standard concentrations” (made up 

in the lab to specified concentrations) of the indicator being measured, 

one of which is the calibration blank. Calibration standards can be used 

to calibrate the meter before running the test, or they can be used to 

convert the units read on the meter to the reporting units (for example, 

absorbance to milligrams per liter). 

The following external checks may be performed by field staff and a second 

laboratory. The results are compared with those obtained by the project lab. 

•	 Split samples. A split sample is a sample that is divided into two 

subsamples in the field or at the lab. One subsample is analyzed at the 

project lab and the other is analyzed at an independent lab. The results 

are compared. 

•	 Outside lab analysis of duplicate samples. Either internal or external 

field duplicates can be analyzed at an independent lab. The results 

should be comparable with those obtained by the project lab. 

•	 Knowns. The quality-control lab sends samples for selected indicators, 

labeled with the concentrations, to the project lab for analysis prior 

to the first sample run. These samples are analyzed and the results 

compared with the known concentrations. Problems are reported to the 

quality-control lab. 
174

•	 Unknowns. The quality-control lab sends samples to the project lab 

for analysis for selected indicators, prior to the first sample run. The 

concentrations of these samples are unknown to the project lab. These 

samples are analyzed and the results reported to the quality-control 

lab. Discrepancies are reported to the project lab, and a problemidentification and problem-solving process follows. In general, many 

of the lab-specific portions of the QA/QC program can be satisfied by 

using a laboratory certified by formal EPA or State environmental lab 

certification programs. It is recommended that only certified labs be 

used for most water quality analyses.

The total cost of an observation network includes the costs to plan, design, and 

construct the physical network, and the costs to operate and properly maintain 

the network for a specified period of time. Observation networks that include 

sites in remote areas can greatly add to the costs because of access restrictions. 

Managers must balance the needs and scope of the study with the budgetary 

constraints of the agency.

Construction costs are usually the largest costs encountered in observation 

networks. Drilling of observation wells can cost tens to hundreds of thousands 

of dollars for deep wells with multiple completions. Obtaining a continuous 

core sample provides a great deal of detailed geological information, but also 

can greatly increase the cost of a well. Simple, shallow wells or piezometers, 

however, can be relatively inexpensive, particularly if they are installed 

by hand in unconsolidated sediments. Well completion and development, 

borehole-geophysical logging, monitoring equipment, and construction of 

shelters for that equipment, however, add to the cost of monitoring wells. Use 

of existing wells may save on construction costs, but may not provide the 

information needed to correctly interpret the information obtained from those 

wells. The reader should consult a local contractor to ascertain the costs for the 

particular geographic area.

The cost of operation and maintenance of an observation network depends 

on a number of factors, such as measurement frequency, number of wells to 

be monitored, physical distance to and between wells and well access, and 

the frequency of well maintenance and testing. Manual measurements that 

are obtained infrequently (quarterly or annually) mainly involve relatively 

inexpensive equipment and personnel time, but may result in information that 

is difficult to interpret. If the network consists of many wells over a large 

distance that must be measured frequently, then self-operating automated 

systems (such as a transducer and datalogger) may actually be more costeffective, despite the higher equipment costs. A regionally distributed network 

in which wells are in remote locations can be expensive to operate and 

maintain because of travel time and costs, as well as vehicle costs. Maintenance 

costs for self-operating systems can also be considerable. Periodic site visits 

to automated wells are required to check on equipment operation, download 

Monitoring 

Network Cost 

Considerations
175

data, and upgrade or repair equipment. Monitoring equipment shelters often 

are favorite targets for vandalism. A local contractor should be contacted to 

ascertain the costs for a particular geographic area.

Knowledge of the hydraulic properties of the subsur­face systems being studied 

often is necessary for valid interpretation of ground water-quality data. A largescale aquifer test can provide an overall estimate of the hydraulic conductivity 

and storage of water-bearing units within several hundred feet or more of a 

pumping well. A large-scale test usually involves measur­ing the response of an 

aquifer system to pumping by measuring changes in water levels in observation 

wells in the vicinity of the pumping well. Analysis of the response to other 

hydraulic stresses, such as injection of water into the system, also is possible. 

Typically, wells for an aquifer test consist of one large-diameter (4 inches 

or greater) pumping well that is associ­ated with observation wells that can be 

of smaller diameter in which drawdown is measured as pumping proceeds. 

The larger diameter normally is required for the pumping well to ensure that 

pumping can be fast enough to cause measurable drawdown in the outlying

observa­tion wells. A diameter as small as 2 inches or less can be suitable 

for measuring water levels and can be used in an array of wells from which 

drawdown is to be determined. 

The hydraulic properties that can be determined from an aquifer test depend 

on the onsite test conditions and installations (see table 12). The most 

commonly determined hydraulic parameters are the transmissivity (T), 

hydraulic conductivity (K), and storage coefficient or storativity (S). 

Transmissivity is a measure of the ease with which the full thickness of the 

aquifer transmits water. Hydraulic conductivity is a measure of the ease 

with which a unit thickness of the aquifer transmits water (see appendix II 

for a chart of representative hydraulic conductivity values for different 

geological materials). Hydraulic conductivity measurements provide a basis 

for judging the hydraulic connection of the monitoring well and adjacent 

screened formation to the hydrogeological setting. These measurements 

also allow an experienced hydrogeologist to estimate an optimal sampling 

frequency for the monitoring program (Barcelona and others 1985). Hydraulic 

conductivity is most effectively determined under field conditions by 

aquifer testing methods, such as pump testing or slug testing. The waterlevel drawdown can be measured during water withdrawal. Alternatively, 

water levels can be measured after the static water level is depressed by 

application of gas pressure or elevated by the introduction of a slug of water. 

These procedures are rather straightforward for wells that have been properly 

developed (EPA 1991). 

Traditionally, hydraulic conductivity has been estimated by collecting 

drill samples, which were then taken to the laboratory for testing. Several 

techniques involving laboratory permeameters are routinely used. Falling-head 

or constant-head permeameter tests on recompacted samples in fixed wall or 

Aquifer Testing 

Techniques
176

triaxial test cells are among the most common. The relative applicability of 

these techniques depends on both operator skill and methodology because 

calibration standards are not available. The major problem with laboratory 

test procedures is that the determined values are based on remolded samples 

rather than on undisturbed materials. Work done to date with laboratory tests 

on “undisturbed” samples suggests that laboratory-determined values of 

hydraulic conductivity are three to six orders of magnitude smaller than values 

determined by in situ aquifer testing for unconsolidated, fine-grained material 

(Melby 1989). Therefore, considerable care must be exercised when evaluating 

laboratory-derived hydraulic conductivity information.

Storage coefficient (storativity), specific yield (Sy), effective porosity (ne), and 

drainable porosity are all terms that express information about the storage 

capacity of an aquifer. Storage capacity is a measure of the interconnected void 

space of an aquifer medium. . The storage coefficient of an unconfined aquifer 

is approximately equal to the effective porosity, and typically has values 

of 0.05 to 0.30, or 5–30%. The storage coefficient of a confined aquifer is 

typically much smaller than that of an unconfined aquifer, typically ranging 

from 10-5 to 10-3. Storage coefficients are low in confined aquifers because 

they are not drained during pumping, and any water released from storage is 

the result of a combination of compression of the aquifer and expansion of the 

water that is being pumped. As a result, a small amount of water is released 

per unit change in head. Pressure is reduced in the aquifer, but the aquifer is 

not dewatered. Therefore, for equal changes in head in an unconfined aquifer 

vs. a confined aquifer, the unconfined aquifer will produce a greater volume of 

water.

Aquifer tests do not provide a direct analysis of hydraulic conductivity or 

effective porosity; however, hydraulic conductivity can be determined from 

an aquifer test where the saturated thickness of the aquifer is known. The 

effective porosity can be estimated as the storage coefficient from tests of 

an unconfined aquifer. The determination of storage coefficient with some 

confidence from an aquifer test requires analysis of the drawdown response in 

observation wells rather than in the pumping well. Draw­down response in the 

pumping well alone can be used to estimate transmis­sivity, but it is unreliable 

for determining the storage coefficient because the effective radius of the 

pumping well is not known.

Each aquifer-test method is commonly assumed to be limited to a relatively 

simple set of aquifer characteristics and boundary conditions as opposed to the 

complexity of actual sites. A method should be selected on the basis of (1) the 

hydrogeology of the test site, and (2) the field-test conditions. An additional 

set of criteria that affects the method(s) selected often involves the available 

budget, the project timeline, and the consequence of the results on future work. 

The hydrogeology of the test site—such as a nonleaky confined aquifer, a 

leaky confined aquifer, or an unconfined aquifer, and other natural conditions 

of the site—determines the applicable set of aquifer-test methods. The number 

and location of observation wells, if any, the instrumentation for measuring 
177

water levels, and the screened interval of the well and the capacity of the pump 

determine which aquifer-test methods can be applied to the data. These and 

other factors determine the physical constraints on stressing the aquifer and on 

determining the aquifer response, and may further limit the applicable aquifertest methods. 

One relatively simple method of determining aquifer characteristics is the 

slug test (table 12). A slug test at an observation well can provide an estimate 

of hydraulic conductivity of the aquifer in the immediate vicinity of the well 

screen. It is not a good method to characterize an aquifer at great distance from 

the well used for the slug test. A slug test is conducted in a single observation 

or moni­toring well that usually is small in diameter (less than about 4 inches). 

Slug tests involve the instantaneous addi­tion or removal of water from 

the well, commonly done by lowering a solid cylinder into the well, and 

withdrawing it, causing the water level to first rise and then drop in the well 

casing. Measurements of the recovery of the water level in the well from both 

falling head and rising head are used to determine hydraulic conductivity of the 

screened inter­val. Techniques for conducting slug tests can be found in many 

publications by the USGS. 

Aquifer-test methods are numerous. The body of literature on these methods 

is extensive, and covers the selection, planning, design, and implementation 

of a test, and the analysis of results. Only a few of the many publications on 

aquifer-test methods are referenced below. A review of field procedures for 

conducting an aquifer test and a summary of the principal aquifer-test methods 

are provided in Bedinger and Reed (1988). Bedinger and Reed (1988) provide 

a glossary of terms and a sylla­bus of aquifer-test methods, classified by aquifer 

condition, control-well characteristics, recharge and discharge function, and 

boundary conditions. Description of the basic principles of well hydraulics and 

principal aquifer-test methods with examples of their application are described 

in Lohman (1972). Practical information related to aquifer-test plan­ning and 

interpretation of aquifer-test data is given in Kruseman and deRidder (1990), 

and Driscoll (1986).

Geophysical methods applicable to ground water investigations are generally 

described in two broad categories: surface methods and borehole methods 

(EPA 1993b) [See appendix VII for more detailed information on geophysics 

in hydrogeological studies]. Borehole geophysical methods have the 

greatest utility in ground water studies, but their use is limited after wells 

are completed. Surface geophysical methods are used to interpret geological 

conditions and their possible controls on ground water. In addition, surface 

methods can be used to map contamination under some conditions. Recently, 

considerable technology and methodology have been developed for use in 

fractured-rock settings. Weight and Sondregger (2001) summarize geophysical 

techniques commonly used in hydrogeology.

Geophysical 

Techniques
178

Surface geophysical methods provide for the areal reconnaissance of geology 

and shallow ground water conditions (Zohdy and others 1974). Four techniques 

are widely applicable to a variety of geological settings, and can be useful 

in hydrogeological studies: (1) electrical resistivity, (2) electromagnetic 

conductivity, and (3) seismic refraction. These methods are generally employed 

in hydrogeological applications for four broad objectives: (1) evaluating 

ground water quality, (2) determining the depth to the water table, (3) 

determining the depth to the bedrock surface, and (4) evaluating subsurface 

lithology and physical properties. Surface geophysical methods can be useful 

in determining the surface location and orientation of potential water-bearing 

fractures. Electrical methods, including square-array and azimuthal resistivity 

surveys and electromagnetic surveys, are particularly useful for locating 

fractures (Lane and others 1995, Taylor and Fleming 1988, Slater and others 

1998). 

Borehole geophysics is the science of recording and analyzing continuous or 

point measurements of physical properties made in wells or test holes (Keys 

1990). The terms borehole and downhole are used interchangeably to refer to 

such measurements. Most specific borehole geophysical techniques have long 

been in use by the petroleum industry, in which holes being logged are usually 

deep and filled with drilling muds or saline water. Many of these techniques 

are not suitable, or must be adapted, for use in freshwater aquifers, which are 

the focus of most near-surface hydrogeological investigations. Nevertheless, 

suitable borehole geophysical methods can greatly enhance the geological and 

hydrogeological information obtained from water supply or monitoring wells. 

The development of logging tools specifically designed for use in freshwater 

wells, such as the EM39 borehole conductivity meter (McNeill 1986), and 

high-precision thermal and electromagnetic borehole flowmeters (Paillet 1994, 

2000) should contribute to greater use of downhole methods in the future.

Borehole and core logging can provide data on the geology of the borehole, 

individual fractures, and the fluid in the hole. Commonly used borehole 

logging methods include caliper, fluid, resistivity, and gamma logs. Optical and 

acoustic imaging methods and heat pulse flow meters are particularly useful 

for detecting and evaluating individual fractures. Newer technologies that 

are not yet in common use include digital borehole imaging, borehole radar, 

and seismic and resistivity tomography. It is important to keep in mind that 

many geophysical methods yield non-unique results that are best interpreted in 

combination with other lines of evidence, especially physical and geological 

sampling. 
179

Table 12. Summary of aquifer test methods (National Academy of Sciences 1981). 
180

The characteristics of the borehole may constrain the type of borehole logging

method that can be used, and therefore may be primary considerations when 

identifying borehole logging methods of potential value in a specific situation. 

These characteristics include the following:

•	 Whether a casing is present (electrical methods, for example, require 

uncased holes).

•	 If cased, the type of casing (borehole radar, for example, can be used 

with a PVC casing, but not with a steel casing).

•	 Borehole diameter must be large enough for the instrument of interest 

(some logs, such as dielectric and nuclear magnetic resonance logs, 

require borehole diameters that are considerably larger than are 

typically drilled for monitoring wells). 

•	 Whether borehole fluid is present (electric logs, sonic logs, and any 

fluid characterization log require ground water or drilling fluid in the 

borehole).

•	 The radius of measurement of the specific method (radii can range 

from near the borehole surface for spontaneous potential and singlepoint resistance logs to more than 100 meters for borehole radar in 

highly resistive rock).

•	 Calibration (many logging methods require calibration for corrections 

of such factors as temperature, borehole diameter, and fluid resistivity). 

The most commonly used borehole logging methods in hydrogeological and 

contaminated-site investigations include spontaneous potential, single-point 

resistance, fluid conductivity, natural gamma, gamma-gamma, neutron, sonic, 

caliper, temperature, and flow meter.

In recent years ground water tracing techniques have been used in a variety 

of hydrogeological settings to help characterize ground water flow systems. 

Tracing techniques have proven to be especially useful in fractured rock 

and karst settings and have been helpful for identifying and characterizing 

contaminant transport pathways and transport velocities. Ground water tracing 

techniques often require fewer assumptions about hydrogeological conditions 

than do hypothetical or numerical simulations; therefore, they can be more 

reliable. Tracer tests can be used to obtain empirical data related to ground 

water recharge, flow direction, flow rates, flow destinations, and flow-system 

boundaries. Tracer recovery data, when combined with ground water discharge 

data, can also provide quantitative data that can be useful for assessing the 

fate of contaminants in the subsurface. Several tracers can be used together, 

allowing several potential pathways to be evaluated simultaneously.

In general, tracing can be divided into two categories: label tracing and pulse 

tracing. Using tracers as labels allows for identification of specific waters or 

plumes. Pulse tracing involves sending an identifiable signal through part of a 

ground water flow system at concentrations significantly above background. 

Ground water tracers can be divided into two types: natural and artificial. In 

general, natural tracers are more applicable for label tracing, while artificial 

Ground Water 

Tracing 

Techniques
181

tracers are more suitable for pulse tracing. Important natural tracers include 

stable and radioactive isotopes, selected ions, selected field parameters 

(specific conductance and temperature) and selected microorganisms. 

Commonly used artificial tracers include organic fluorescent dyes, 

chlorofluorocarbons, gases, and salts (like chloride and bromide).

Tracing methods that include the deliberate or incidental introduction of a 

tracer into a stream or ground water flow system have been increasingly 

used in hydrogeological investigations in fractured-rock settings. Tracing 

techniques do not require the assumptions of a porous-media approach and can 

be used to delineate ground water flow paths, determine ground water flow 

velocities along the delineated flow paths, and help estimate mass loading to 

a stream along a preferential ground water flow path. Tracer recovery data, 

when combined with ground water flow data, can provide quantitative data for 

evaluating contaminant behavior and fate in the subsurface. An ideal artificial 

tracer should be (1) quantitatively detectable in very small concentrations; (2) 

found in low concentrations in the water to be traced; (3) not readily attenuated 

by the aquifer material, geochemical reactions, or biological degradation; 

and (4) nontoxic to humans and the ecosystem (Todd 1980). The application 

of surface-water tracing in combination with ground water tracing provides 

detailed information on ground water inflow zones to streams. Stream-tracing 

techniques, which include the continuous injection of a constant concentration 

of tracer, also provide very accurate stream discharge measurements based 

on dilution of the tracer (Kimball 1997). Discharge calculated in this manner 

includes that flow which is in the hyporheic zone, which is typically a 

significant hydrologic zone in many streams, especially those with high 

gradients. It should be noted that no way exists to quantify how the discharge 

estimate is influenced by hyporheic exchange flows. The only way to ensure 

that all hyporheic exchange flows are included in the discharge estimate is to 

locate the downstream sampling site on exposed, impermeable bedrock where 

all flow is forced into the surface stream channel.

Naturally occurring isotopes are the most common natural tracers used in 

ground water investigations. These include isotopes of common elements, 

such as carbon, oxygen, and hydrogen, in addition to isotopes of radioactive 

elements, such as tritium. Analyzing water samples for stable and radioactive 

isotopes can provide data for characterizing sources of ground water recharge. 

This step can be very helpful in delineating and characterizing preferential flow 

paths. The information can also be used to establish relative, or sometimes 

absolute, ages of ground water withdrawn from various depths and locations in 

an aquifer, which can greatly assist in the identification and differentiation of 

local, intermediate, and regional flow systems. It can also greatly assist in the 

estimation of exchange rates and flow directions at aquifer-stream interfaces 

and aquifer-lake interfaces. For detailed information on isotope geochemistry, 

processes affecting isotopic compositions, and isotopes in ground water 

hydrology, see Kendall and McDonnell (1998) and Clark and Fritz (1997). 

Natural Tracers
182

Stable Isotopes. Stable isotopes have proven to be the most versatile natural 

tracers. For a given element, isotopic composition can vary because of 

partitioning or fractionation related to differences in reaction rates among the 

isotopes. Fractionation is typically proportional to the differences in isotopic 

mass for a given element of low atomic number. This property allows the ratios 

of isotopes of an element to become fingerprints of climatic and hydrologic 

conditions or serve as markers for different sources of that element. Variations 

in annual rainfall and snowmelt strongly affect the isotopic composition of 

waters. Data for ratios of stable isotopes of oxygen, hydrogen, sulfur, nitrogen 

and carbon can be especially useful for (1) characterizing ground water flow 

paths from areas of recharge to areas of discharge, (2) identifying mechanisms 

responsible for streamflow generation, and (3) testing flow path and waterbudget models developed using hydrologic data. 

Stable isotopes of oxygen and hydrogen are ideal tracers of water sources and 

movement because they are the two constituents of the water molecule and 

the ratios of each element tend to stay constant as long as the water has not 

experienced freezing or evaporation. Oxygen isotopes include 16O, 17O, and 18O 

and hydrogen isotopes include protium (1H), deuterium (2H), and tritium (3H). 

Stable isotopes of sulfur, nitrogen, and carbon are also important in 

environmental studies. These isotopes are constituents that are dissolved in 

water or carried in the gas phase. Stable isotopes of oxygen and hydrogen 

behave conservatively because interactions with organic and geologic material 

along the flow path will have a negligible effect on the ratios of isotopes in 

the water molecule. The ratios of stable isotopes of dissolved sulfur, nitrogen, 

and carbon can be significantly altered by reactions with organic and geologic 

material. Thus, these solute isotopes may have limited use for tracing water 

sources and flow paths. Solute isotope data, however, can provide information 

on the reactions that are responsible for their presence in the water and the flow 

paths implied by their presence (Kendall and McDonnell 1998). In addition, 

they may be able to be used to identify the source of a contaminant plume 

should each potential source have a different isotopic composition.

Radioactive Isotopes. Tritium, a radioactive isotope of hydrogen (half-life = 

12.43 years), is naturally produced in the upper atmosphere by bombardment 

of nitrogen by neutrons; however, large amounts of tritium were released to the 

atmosphere during the period of above-ground thermonuclear testing, which 

was at a maximum during the 1950s and was discontinued in 1963. Pre-bombtesting concentrations of tritium in water have been determined to be close to 

the detection limit of 1 tritium unit. The presence of tritium above background 

levels in ground water is an indication that recharge occurred during or after 

the bomb testing period. In addition to radioactive decay, tritium in ground 

water is subject to significant attenuation through mixing with waters with less 

tritium; consequently, tritium concentrations usually cannot be used to obtain 

an “absolute” age of ground water. 
183

Artificial tracers are those introduced into the ground water flow system either 

purposely as part of a designed tracer test or inadvertently as a spill or other 

anthropogenic activity. To serve as a suitable tracer, a substance must be (1) 

nontoxic to humans and the ecosystem; (2) either absent from the ground water 

system or present at very low, near-constant levels; (3) soluble in water with 

the resultant solution having nearly the same density as water; (4) nonreactive; 

(5) easy to introduce into the flow system; and (6) unambiguously detectable in 

very low concentrations. Many organic dye and salt tracers have been approved 

for use in aquifers and streams that are used to obtain drinking water; however, 

some States, such as Wisconsin, have restricted the use of all artificial tracers 

in ground water.

Salts. Chloride, bromide, and lithium solutions are commonly used for both 

ground water and surface water tracing. These salts are very soluble, relatively 

inexpensive, conservative, and nontoxic at concentrations typically used for 

tracing. They are also easily detectable at low concentrations. Some ecological 

considerations for the use of salts in tracing studies are covered by Wood and 

Dykes (2002). Chloride occurs naturally in some ground water, often in the 

tens to hundreds of parts per million. Natural concentrations of bromide are 

usually much lower. Chloride has commonly been used to trace contaminant 

plumes that originate at landfills or other industrial facilities. Stream-tracing 

techniques, which include the continuous injection of a constant concentration 

of a salt, provide very accurate stream discharge measurements based on 

dilution of the tracer. 

Organic Dyes. Fluorescent dyes are some of the most analytically sensitive, 

versatile, and inexpensive artificial water tracers available. Many references 

document the use of these dyes in stream and ground water tracing studies 

and their human and environmental toxicity (Field and others 1995, Smart 

and Laidlaw 1977). Fluorescent dyes commonly used in ground water 

investigations include uranine, fluorescein, rhodamine, eosin and phloxine, and 

sulpho-rhodamine B. Most fluorescent dyes work well in water with a nearly 

neutral pH. In acidic conditions, the fluorescence of some dyes is minimized; 

however, these dyes will fluoresce again if the pH of the sample is adjusted to 

more alkaline conditions. In addition, in ground water systems with substantial 

quantities of organic material, adsorption of the fluorescent dyes to the organic 

materials may limit their usefulness. Commercial grade, organic, fluorescent 

dyes can be purchased as liquid compounds or as powders. Uranine, eosin, and 

phloxine are FDA approved. 

Chlorofluorocarbons. The chlorofluorocarbon (CFC) gases CFC-11, CFC-12 

and CFC-13 were developed during the 1930s. These gases were chemically 

stable and safe and therefore found wide application, commonly as refrigerants. 

Unfortunately, waste CFCs accumulate in the atmosphere, where they are 

now thought to pose a serious hazard to stratospheric ozone. This problem 

has led to a very successful international action (Montreal Protocol) to reduce 

global CFC production. The known growth rates of atmospheric CFCs, their 

Artificial Tracers
184

rapid mixing worldwide, their solubility in water, and their good chemical 

stability have enabled CFCs to become a useful tool for hydrologists to trace 

water movement in the oceans, in surface water, and in ground water. In the 

case of ground water, the method rests on the assumption that ground water 

at the water table will be in equilibrium with atmospheric air concentrations, 

including its CFC component. Once water moves into the saturated zone below 

the water table, it will not be able to acquire or lose any additional CFC gas 

to the atmosphere. The CFC concentration in the water will be characteristic 

of the atmospheric CFC level prevailing during its last contact with the 

atmosphere. This characteristic forms the basis of CFC dating of ground water 

on a time scale of 0 to 50 years. The steep increase in atmospheric CFC levels 

over time ensures that fairly precise dates can be obtained. In contrast, the 

input curves for tritium and radiocarbon are rather flat. The development of 

a reliable sampling and analytical procedure has ensured wide application of 

this technique. For information on dating of ground water using CFCs, see 

Plummer and others (1993) and Plummer and Friedman (1999).

The usefulness and appropriateness of a ground water tracer test depend on 

the questions to be answered by a particular hydrogeological investigation. 

Tracer tests are appropriate when ground water flow velocities are such that 

results will be obtained within a reasonable period of time, usually less than 

a year. The usefulness of tracer test results are highly dependent on proper 

test design (particularly determination of sampling locations) and execution, 

the nature of the tracer, the ability to detect the tracer at low concentrations, 

and correct interpretation of recovery data. Before conducting a tracer test, 

it is very important to use other geological and hydrological information to 

develop a fundamental understanding of the hydrogeological setting and the 

ground water flow system to be traced. This understanding can then be used to 

(1) determine the appropriate type of tracer, (2) determine the tracer injection 

location and method, (3) determine appropriate sample collection locations, 

and (4) determine which tracers should be included in the test design. It is 

always advisable to sample more locations rather than fewer locations. For 

artificial tracers, it is important to know precisely how much tracer mass is 

injected. This knowledge will allow for a determination of the percent of tracer 

mass recovered at a given sampling location. This quantitative aspect of tracing 

can be important in helping to evaluate the significance of any given ground 

water flow path.

Isotopes. Chapter 10 of Clark and Fritz (1997) includes an excellent discussion 

and comparison of sampling and analytical protocols and procedures 

for collecting water samples for isotopic analysis. Sample size, filtering, 

preservation, container type, holding times, and method of analysis vary quite 

a bit between different stable and radioactive isotopes. In general, isotopes of 

water (oxygen 18, deuterium, tritium) have simpler sampling protocols than 

isotopes of carbon (carbon 13 and carbon 14), sulfur (sulfur 34), dissolved 

gases (helium, argon 39, krypton 85), and uranium (uranium 234 and uranium 

Field Methods
185

238). Water samples for isotopic analyses must be collected and stored in 

well-sealed bottles. Proper sealing and handling is necessary to prevent any 

additional fractionation.

Organic Dyes. The use of organic dyes as hydrogeological tracers requires 

specific field sampling and analysis procedures. Careful thought should be 

given to the selection of the proper dye and the method for introducing it into 

the ground water. Dyes should be selected based on their chemical and toxicity 

characteristics. Also, fluorescence is reduced in some dyes when dissolved in 

low pH waters or exposed to sunlight, and some dyes fluoresce better in cooler 

waters. It is also very important to carefully consider the best way to introduce 

the dye tracer into the ground water system. The following are four common 

methods:

•	 Inject into a well, making sure the well will take the desire quantities of 

water before introducing a dye tracer. The ability to take water can be 

ascertained by conducting a simple aquifer test.

•	 Inject into a stream, making sure the stream gradient is low and the 

reach of stream below the injection point is a losing reach.

•	 Inject into a constructed excavation, making sure the excavation will 

take the desire quantities of water in an appropriate time period.

•	 Inject into a sinkhole in karst terrain. 

Protocols for collecting water samples that may contain the dye tracer are 

relatively simple. Sample containers and storage should minimize all exposure 

to light to prevent photo degradation of organic dyes. Samples do not require 

filtering or preservation, but water samples that may contain dye should be 

kept cool until analysis is complete. They should be analyzed within 2 weeks to 

minimize bacterial degradation of organic dyes.

Water samples can be collected by grab sampling or with an auto sampler, 

which is useful if many samples must be taken in a short time and from 

locations with difficult access. Once the water samples are collected, samples 

containing organic dyes should be analyzed on a spectrofluorometer to confirm 

the nature of the fluorescence and then samples containing all tracers should 

be analyzed with wet chemistry methods for dye concentration. It is important 

to conduct both types of analyses to confirm the presence of the dye that was 

injected. Sophisticated sampling of organic tracers can be achieved by using 

flow-through flourometers, which measure the fluorescence of the dye in water 

on a real-time basis. This type of sampling requires a power source and data 

loggers, but is indispensable in surface-water tracer studies.

Small bags of activated charcoal can also be used to detect organic dyes. 

Organic dyes will sorb onto charcoal if water that contains dye comes into contact with the charcoal. Charcoal bags are placed in water at sampling locations 

and then retrieved for analysis at selected time intervals. It is important to note 

that determining the travel time from an injection location to a given charcoal 

bag is constrained by the time interval between retrieval of the bags.
186

The most important rule of thumb for sampling is to collect samples often at 

many places. Collecting samples for organic dye analysis is relatively easy and 

inexpensive. Because it is not always possible to predict all locations where 

dye may be recovered, it is best to have more rather than fewer sampling 

locations. By collecting samples frequently, at important locations, the data 

can be used to construct breakthrough curves of recovery versus time. Detailed 

breakthrough curves can be used in rigorous analyses of the recovery data.

This section presents techniques for assessing ground water flow conditions 

and hydraulic properties of aquifers. An equally important component for 

any hydrogeological investigation is analysis of water-quality data. Some 

techniques for analysis and plotting of water-quality data are detailed in 

appendix VI.

Analytical methods use exact closed-form solutions of the appropriate 

differential equations for particular sets of conditions and involve manually 

solving equations, such as Darcy’s Law or the Theis equation, or generating 

solutions using curve-matching techniques. These approaches may be used 

either independently or in concert to develop solutions to complex problems. In 

contrast, numerical models apply approximate solu­tions to the same equations. 

Semianalytical models use numerical techniques to approximate complex 

analytical solutions, allowing a discrete solution in either time or space. 

Analytical methods are most useful in the analysis of aquifer test data, simplified 

aquifer system evaluation, and to assist in the design of numerical models. 

Analytical models provide exact solutions, but employ many simplifying 

assumptions about the ground water system, its geometry, and external stresses 

to produce tractable solutions (Walton 1984). This approach places a burden on 

the user to test and justify the underlying assumptions and simplifications 

against the actual physical system (EPA 1991). For example, analytical models 

generally assume isotropic conditions and an infinite aquifer. These conditions 

may not exist in the problem at hand, and results may be inaccurate because of 

these constraints. The following are examples of the use of analytical models:

•	 Determining drawdown effects of pumping alluvial aquifers with 

relatively impermeable boundaries, as with mountain blocks 

bounding an alluvial valley floor. The use of image well theory 

provides for analysis of such a situation, and results in greater 

drawdown impacts than an infinite aquifer (Walton 1970). 

•	 Determining drawdown effects at a well field with several wells 

pumping simultaneously.

Analysis of 

Hydrogeological 

Data

Analytical 

Methods
187

•	 Determining the ground water flow rate to a finite line sink, as 

in the study of ditches, canals, strip mines or ground water flow 

to finite sections of rivers or streams. In this case, the head or 

drawdown at the line sink is known and the flow rate is unknown. 

•	 Mounding of ground water beneath a water body such as a tailings 

pond.

Semi-analytical models can provide streamline and travel time information 

through the use of numerical or analytical expressions in space or time. This 

information is especially useful for delineation of wellhead protection areas 

(EPA 1994). Analytical element models are a relatively recent development

in semi-analytical model­ing of regional ground water flow. They use approximate analytical solutions by superposing various exact or approximate analytical 

functions, each representing a par­ticular feature of the aquifer (Haijtema 1985, 

Strack 1989). A major advantage of these models compared to analytic models is 

greater flexibility in incorporating varying hydrogeology and stresses without a 

signifi­cantly increased need for data (van der Heijde and others 1988).

Developing a potentiometric map is not as straightforward as preparing a 

topographic map. An accurate potentiometric map requires enough well 

observations to develop contours of equal head that do not miss important 

features of the flow system. Considerable interpretation and judgment may 

be required in developing contours when well data points do not seem to fit 

into a coherent pattern; for example, if water-level data from wells are drawn 

from multiple sources, measurements in nearby wells may have been taken at 

different times of the year and may not be directly comparable. On the other 

hand, if all the data have been collected so as to minimize effects of shortterm or seasonal fluctuations, examination of individual well characteristics 

may yield explanations for anomalous data points; for example, a single well 

data point that is far out of line with nearby wells may be tapping a different 

aquifer. If an anomalous well data point cannot be readily explained as being 

unrepresentative for any reason, then further field investigation may be 

required to determine whether any localized hydrogeological conditions are 

causing the anomaly.

The starting point for a potentiometric map is a base map. The base map 

identifies well locations, water-level elevations in the wells, and other surface 

hydrologic features, such as streams, rivers, and water bodies. Drawing 

equipotential contours requires some skill and judgment. Errors in contouring 

fall into two general categories: (1) failure to exclude data points that are 

not representative and (2) failure to take into account subsurface features 

that change the distribution of potentiometric head as a result of aquifer 

heterogeneity or boundary conditions. Following are six situations in which 

contouring errors might occur:

Potentiometric Maps
188

1. For water-table maps, failure to exclude measurements from wells 

cased below the water-table surface in recharge and discharge 

areas; for example, only well c in figure 57 gives an accurate 

reading of the water table surface.

2. For water-table maps, failure to adjust contour lines in areas of 

topographic depressions occupied by lakes. Figure 58a illustrates 

the incorrect and correct interpretations in this situation. 

3. Failure to recognize locally steep gradients caused by fault zones. 

Figure 58b illustrates how conventional contouring methods 

erroneously portray the ground water flow systems on the two 

sides of a fault.

4. Failure to consider localized mounding or depression of 

the potentiometric surface from anthropogenic recharge or 

withdrawal. Pumping wells create a cone of depression around the 

well, with steepened hydraulic gradients. Agricultural irrigation, 

artificial recharge using municipally treated wastewater, and 

artificial ponds and lagoons usually cause a mounding of the water 

table. When the source of recharge is confined to a relatively 

small area, a localized mound develops with elevations increasing 

toward the center, rather than decreasing as in a pumped well. 

Area wide recharge will reduce hydraulic gradients compared to 

natural aquifer conditions. These features are especially significant 

when they are located near a ground water divide because small 

shifts in the location of a divide may have a major impact on the 

direction in which contaminants flow.

5. Failure to consider seasonal and other short-term fluctuations in 

well levels. If an aquifer experiences seasonal high and low water 

tables, well measurements are not comparable unless they are 

taken at the same time of year. Other factors, such as dramatic 

changes in atmospheric pressure and precipitation events, might 

reduce the comparability of well measurements even if the 

measurements are taken at about the same time of year.

6. Use of measurements from wells tapping multiple aquifers. Wells 

in which the screened interval includes multiple aquifers generally 

yield inaccurate water level or piezometric measurements because 

the measured head reflects the interaction between heads of 

the intersected aquifers. Figure 59 illustrates how the failure to 

differentiate measurements from wells completed in two aquifers, 

combined with a well that connects the two, results in an apparent 

depression in the potentiometric surface.
189

Figure 57. Cross-sectional diagram showing the water level as measured by piezometers located at various depths. The water 

level in piezometer c is the same as well b since it lies along the same equipotential line (after Mills and others 1985).
190

Figure 58. Common errors in contouring water table maps: (a) topographic depression 

occupied by lakes and (b) fault zones (Davis and DeWiest 1966).

Figure 59. Error in mapping potentiometric surface because of mixing of two confined aquifers with different pressures (Davis 

and DeWiest 1966).
191

The quantity of ground water moving through a volume of rock can be 

estimated using Darcy’s Law (Darcy 1856), 

Q = KIA, 

where Q is the quantity of ground water flow, K is the hydraulic conductivity, 

I is the hydraulic gradient, and A is the cross-sectional area of the aquifer of 

interest (saturated interval).

Note that the quantity of flow is directly proportional to the hydraulic gradient. 

This equation provides a rapid way to estimate the flow through an alluvial 

channel, for example.

A set of intersecting equipotential lines and flow lines, constructed according 

to a strict set of rules, is called a flow net. It can be a powerful analytical tool 

for the analysis of ground water flow (Freeze and Cherry 1979). A discussion 

of the rules governing the construction of flow nets is beyond the scope of this 

section, and the reader is referred to chapter 5 of Freeze and Cherry (1979) 

for a detailed description of flow net construction. Once a flow net is properly 

constructed, the amount of ground water flow through the area represented by 

the flow net, under steady-state conditions, can be calculated if the hydraulic 

conductivity of the aquifer is known. Figure 60 shows an example flow net for 

a simple system (modified from Freeze and Cherry 1979), in which ground 

water is flowing from the left side of the figure to the right side. 

Darcy’s Law allows the amount of ground water flow through the area 

represented in figure 60 to be calculated using a flow net and the following 

equation: 

Q = (mKH)/n,

where Q is the ground water flow rate, K is the hydraulic conductivity, H is 

the total change in hydraulic head across the flow net, m is the total number of 

flow tubes (the area between the flow lines), and n is the number of divisions 

of head in the flow net. 

Figure 60. Example of 

a flow net for a simple 

flow system. m = 3, 

n = 6, H = 60 feet, 

K = 10-3 feet/day, 

so that Q = 3.0 x 

10-2 ft3/d per square 

meter of section 

perpendicular to the 

flow net.

Calculating Ground

Water Flow

Flow Nets
192

Figure 61. Effect of fracture anisotropy on the orientation of the zone of contribution to a pumping well (Bradbury and 

others 1991).

A standard flow net assumes that the aquifer is isotropic. When an aquifer is 

anisotropic (commonly the case in unconsolidated and sedimentary aquifers), 

the actual direction of ground water flow will not be perpendicular to the 

equipotential contours. Instead, the direction of flow will deviate from the 

perpendicular at an angle that depends on the ratio of the horizontal to the 

vertical hydraulic conductivity. Figure 61 illustrates how anisotropy in a 

fractured rock aquifer alters the direction of ground water flow compared to 

that expected in an isotropic aquifer.

A potentiometric surface map can be developed into a flow net by constructing 

flow lines that intersect the equipotential lines or contour lines at right angles. 

Flow lines are imaginary paths that trace the flow of water particles through the 

aquifer. Although the number of both equipotential and flow lines is infinite, 

the former are constructed with uniform differences in elevation between 

them, while the latter are constructed so that they form, in combination 

with equipotential lines, a series of squares. A flow net carefully prepared 

in conjunction with Darcy’s Law allows estimation of the quantity of water 

flowing through an area, and of the variability of transmissivity and hydraulic 

conductivity. Plan and cross-section views of flow nets drawn for a losing 

stream are shown in figure 62 and a gaining stream in figure 63. Plan view 

flow nets are valuable for delineating the zone of contribution to a well, or for 

boundary conditions for pumping wells.
193

Figure 62. Plan view and cross section of flow net through losing stream segment 

(Heath 1983).
194

Figure 63. Plan view and cross section of flow net for gaining stream (Heath 1983).
195

Analysis of Aquifer

Test Data

Many different methods have been developed to analyze aquifer-test data, for 

both single-well and multiple-well tests. The correct analysis method to be 

used depends on the hydrogeological conditions at the test site, the type of data 

collected for the test, and how well the hydrogeological conditions match the 

assumptions inherent to each approach. For the test to be successful, it must be 

planned and conducted in a manner consistent with the site hydrogeology and the 

analysis method(s) to be used.

One of the simplest, and often the most cost-effective, aquifer test procedures 

is the specific-capacity test. This test, which is often conducted after well 

development by a driller, calculates the well yield per unit of drawdown in 

the well after a specified time (commonly 24 hours). The well is pumped at a 

constant, predetermined rate for the specified time, and the drawdown in the well 

is measured at the end of that time. The discharge divided by the drawdown is 

the specific capacity, usually reported in units of gallons per minute per foot of 

drawdown. The specific capacity value can change with the length of time that 

the well is pumped; for example, a short-duration test (1 hour or less) can result in 

a large value for specific capacity because of well-bore storage effects. For longer 

tests in unconsolidated aquifers, the specific capacity can decrease with time 

because of dewatering of the aquifer. Aquifer transmissivity (in gallons per day 

per foot) can be approximated from a specific-capacity value using the following 

equations (Driscoll 1986):

T = Specific capacity x 2000, for a confined aquifer.

T = Specific capacity x 1500, for an unconfined aquifer.

A more rigorous method for estimating transmissivity and hydraulic conductivity 

from specific capacity tests is described by Theis and others (1963). Bradbury 

and Rothschild (1985) describe a computer program to estimate hydraulic 

conductivity from specific-capacity tests.

Most aquifer-test data are analyzed using graphical procedures (many of which 

are now performed with the use of computer programs). One procedure involves 

analysis of the shape of a time-drawdown or distance-drawdown graph. Another 

involves curve-matching methods (Fig. 64). Detailed descriptions of each of these 

methods is beyond the scope of this section, and the reader is referred to one 

of the many textbooks or reports that can provide that level of detail, including 

Dawson and Istok (1991), Lohman (1972), Kruseman and deRidder (1991), or 

Driscoll (1986). Ground water flow models, which are discussed in a subsequent 

section, are also used to analyze aquifer-test data. Interpretation of aquifer-test 

data is often nonunique, however; for example, the time-drawdown responses are 

similar for leaky confined, unconfined, and bounded aquifer systems. Because a 

theoretical response curve can be matched to aquifer-test data does not prove that 

the aquifer fits the assumptions on which the curve is based (Freeze and Cherry 

1979). Therefore, the experience and judgment of the analyst is critical to the 

proper interpretation of aquifer test data.
196

Figure 64. Example of a Theis type curve and a curve-matching plot for analysis of aquifer-test data (Heath 1983).
197

Numerical 

Models

The use of aquifer tests to obtain hydraulic data in fractured-rock aquifers 

requires careful thought about the purpose and design of the test, the type of 

data to be collected, and the analyses of the data. Conventional slug tests and 

constant discharge/variable discharge pumping tests were designed for porous 

media flow and are difficult to apply to fractured rocks unless the fractures 

are highly connected. Identification and testing of water-bearing fractures are 

critical for the success of aquifer tests in fractured rocks. For single fractures or 

fracture zones, in situ measurements of average hydraulic conductivity can be 

made with a standard Lugen packer test (Singhal and Gupta 1999). Directional 

hydraulic conductivity can be measured with a modified Lugen packer test 

and/or a tracer injection test, and three-dimensional values of hydraulic 

conductivity can be measured with cross-hole hydraulic tests. For fracturedrock aquifers that have significant matrix porosity (with low matrix hydraulic 

conductivity) and regularly spaced fractures (high hydraulic conductivity), 

pumping test data can be used to estimate hydraulic characteristics of the 

fractures and the matrix blocks. Dual porosity models assume that porous 

media flow occurs within the matrix block and within the fractures.

All of the analysis methods described so far contain assumptions or limitations 

that make them unsuitable for large-scale problems in complex hydrogeological 

settings. Numerical methods implemented through computer programs 

(computer models), however, can be well suited to these types of problems. 

See appendix V for more detailed information on numerical modeling. 

Numerical models can be much less burdened by the sim­plifying assumptions 

used in analytical models; therefore, they are inherently capable of addressing more 

compli­cated problems. They require significantly more input, however, and 

their solutions are inexact (numerical ap­proximations); for example, in many 

models the assumptions of homo­geneity and isotropy are unnecessary because 

the model can assign point (nodal) values of transmissivity and storage to hundreds 

or thousands of nodes. Likewise, the capacity to incorporate com­plex boundary 

conditions provides greater flexibility, and computer models can be used for 

both small-scale, site-specific problems and for large-scale (basin or multiple 

basins), complex problems. The user faces difficult choices, however, about 

model selection, boundary conditions, grid discretization, time steps, and ways 

to avoid truncation errors and numerical oscillations (Remson and others 1971, 

Javendel and others 1984). Improper choices may result in errors, such as 

mass imbalances, incorrect velocity distributions, and grid-orienting effects that 

are unlikely to occur with analytical approaches. Reilly and Harbaugh (2004) 

provide some guidelines and discussion of how to evaluate complex ground 

water flow models used in the investigation of ground water systems. Listed 

in table 13 are the relative advantages and disadvan­tages of analytical and 

numerical models.

A fundamental requirement of most numerical approaches is the creation of a 

discretized grid or mesh that represents the flow system being simulated. This 

discretization usually consists of rectangular- or triangular-shaped cells covering 

the lateral dimensions of the area of interest for which ground water parameters 

must be specified. The grid also extends vertically to represent one or more 
198

Table 13. Relative advantages and disadvantages of analytical and numerical models.

Analytical Models

Advantages Disadvantages

Efficient when data on the system are sparse or uncertain Limited to certain idealized conditions with simple geometry

Economical May not be applicable to field problems with complex 

boundary conditions

Good for initial estimation of magnitude of contamination or drawdown

May not be able to readily handle spatial or temporal variations 

in system

Rough estimates of input data often possible from existing 

data sources

Input data for computer codes usually simple

Numerical Models

Advantages Disadvantages

Easily handle spatial and temporal variations of 

hydrogeologic system

Achieving familiarity with complex numerical programs can be 

time consuming and expensive

Easily handle complex boundary conditions Errors because of numerical dispersion (artifacts of the 

computational process) may be substantial for transport models

Three-dimensional transient problems can be treated 

without much difficulty

May not be able to readily handle spatial or temporal variations 

in system

Rough estimates of input data often possible from existing 

data sources

More data input required, and can be time consuming

Input data for computer codes usually simple

aquifers and/or confining units. The grid or mesh forms the basis for a matrix 

of equations to be solved. A new grid or mesh must be designed for each area to 

be modeled, based on the data col­lected during site characterization and on the 

conceptual model developed for the physical system. The size of the grid cells 

(or mesh elements) can vary from project to project, with smaller spacing (cell or 

element size) usually used in an area of the model where more detail is required 

(such as near well fields or sources of contaminants); however, this fine grid (or 

mesh) resolution also increases the requirements for data and the computational 

time necessary to reach a solution. Grid (or mesh) design is one of the most 

critical elements in the accuracy of computational results (EPA 1991).

Finite-difference and finite-element methods are the most frequently used 

numerical solution techniques. The finite-difference method approximates the 

solution of partial differential equations by using finite-difference equivalents. 

The finite-element method approxi­mates differential equations by an integral 

approach. Perhaps the most frequently used finite-difference ground water model 

is MODFLOW. This model was originally developed by the USGS (McDonald 
199

and Harbaugh 1988). The computer code has been modified though the years, 

adding modules and refining the code for various situations. A recent version, 

MODFLOW-2000, includes options for parameter estimation and statistical 

evaluation of model results (Harbaugh and others 2000). Hill and others 2000) 

(see appendix V). Graphical interfaces have been written by several companies to 

ease the process of data input and to visualize model input and output. 

Conceptualization, model design, and data input can take several hundred hours, 

but graphical interfaces and use of GIS techniques can substantially reduce that 

time. The time required to run the model is usually minimal, except for very 

large flow models with several hundred thousand nodes or contaminant transport 

models. The model is calibrated by adjusting model-input data until an acceptable 

match between simulated heads (concentrations for transport) and water-budget 

components and measured and estimated values are obtained. This process 

can take many months of effort. Model calibration can often result in a revised 

conceptualization of the ground water system and an identification of gaps in 

knowledge of the system and additional data needed to fill those gaps.

Additional numerical modeling tools may be necessary for particular 

investigations. If the purpose of the study is to predict the fate of a contaminant 

from a spill of hazardous chemicals, a solute-transport code may be required. 

Perhaps the most frequently used finite-difference solute-transport model is 

MT3D, which links easily with MODFLOW (Zheng 1990, Zheng and Wang 

1998). See appendix III for a discussion of contaminant fate and transport 

mechanisms in ground water. Geochemical models, in which ground water 

quality is altered by water-rock interactions in an aquifer, are also valuable 

in conceptualizing and evaluating flow systems. Examples of geochemical 

models include MINTEQA2 (Allison and others 1991), PHREEQC (Parkhurst 

and Appelo 1999) and NETPATH (Plummer and others 1994). Model codes 

developed by the USGS and documentation of these codes can be obtained at no 

charge at http://water.usgs.gov/software/.

It is important to distinguish between the software, or computer code, used in a 

model and the model itself. The software is simply the analytical equation(s) to 

be solved and the algorithms for reading input data and for outputting simulation 

results. MODFLOW is an example of a simulation code. The model is the set 

of input data, simulation software, and output from the software. The code is 

generic. A model, however, includes a set of boundary and initial conditions as 

well as a site-specific grid, parameter values, and hydrologic stresses.

According to Anderson and Woessner (1992), the following are two prevalent and 

opposing opinions about models:

1. “Models are worthless because they require too many data; therefore, 

they are too expensive to assemble and run. Furthermore, they can 

never be proved to be correct and suffer from lack of scientific 

certainty.”

2. “Models are essential in performing complex analyses and in making 

informed predictions.”
200

Synthesis and 

Interpretation

Models do require extensive field, and sometimes laboratory, information for 

input data and calibration, and model solutions may be nonunique so that results 

may be uncertain; however, good modeling practices and an adequate amount 

of good-quality data will increase confidence in modeling results (Hill 1998). 

A ground water model is often the best way to make an informed analysis or 

prediction about consequences of a proposed action on a ground water flow 

system. Anderson and Woessner (1992) also state, “Models provide a framework 

for synthesizing field information and for testing ideas about how the system 

works. They can alert the modeler to phenomena not previously considered. 

They may identify areas where more field information is required.” Much of the 

following discussion is taken directly from Anderson and Woessner (1992), and 

the reader is referred to that text for more detailed information.

Modeling is an excellent way to help organize and synthesize field data, but it is 

important to recognize that modeling is only one component of a hydrogeologic 

assessment and not an end in itself. In fact, the process of assembling and 

understanding the field data required for model input may provide the modeler 

with the answer to the problem before ever running the model. Conversely, a 

model that is based on inadequate field data can produce erroneous results that 

may not be obvious in the colorful graphical output from modern modeling 

software. The modeler must have some basic understanding of the geology and 

hydrology of the area being modeled, or should work in close collaboration 

with others who do have that understanding. In this way, model results that are 

hydrogeologically unreasonable, or that are based on unrealistic or erroneous 

data, can be recognized and addressed.

The adaptation of numerical ground water flow models to fractured-rock 

hydrogeological settings has progressed somewhat, but is still constrained in 

settings that exhibit significant anisotropy and heterogeneity (Forster and Smith 

1988a). Ground water flow in these settings is often simulated as flow through 

porous media using MODFLOW or similar programs. This simplification is 

often adequate for large-scale flow systems, but may not be appropriate for 

small-scale (well-field) systems or contaminant transport problems. Watershedscale models that distribute and attempt to balance elements of the water 

budget can be used to evaluate ground water and surface water development. 

Fracture network models, which utilize outcrop data on fracture geometry (for 

example, FRACMAN, Golder Associates), can be used to evaluate flow in 

discrete fracture networks (discrete volumes of rock), but they are constrained 

by the difficulties of obtaining sufficient data and by a poor correlation with 

depth.

A conceptual framework for a hydrological system is the final result of a 

hydrogeological study and pulls together all information gathered on the 

geological setting, the surface water and ground water system, and dependent 

ecosystems to provide a coherent, unified picture of the system and the 

important processes active within that system. Stone (1999) provides an 

excellent discussion on developing a conceptual framework (also known as a 
201

Ongoing Data 

Analysis Costs

“conceptual model,” though a much more refined version than the conceptual 

model step in the development of a numerical ground water model). Such a 

framework is the starting place for additional studies of water supply, waste 

disposal, inventory, and remediation. A conceptual framework varies with the 

scale of the study area. The study area can be conceptualized on a regional, 

aquifer, or project scale, or alternatively, in terms of flow systems. An ideal 

conceptual framework will include four components: geology, surface water, 

and saturated and unsaturated ground water. Formulating a conceptual 

framework involves describing the geological setting from a hydrological 

point of view and the interactions of surface, soil, and ground water within this 

setting. 

An important subcomponent of a hydrological conceptual framework is the 

hydrochemistry. The framework should address the relationship between the 

hydrogeologic setting and its hydrochemistry including the concentrations 

of chemical constituents, contamination, geochemical transformations taking 

place along a flow path, trends in water quality, and comparisons to waterquality standards. Appendix VI contains a discussion of analysis and statistical 

methods for evaluating water-quality data. 

Costs of ongoing data analysis for ground water studies include those 

associated with periodic evaluations and report writing on the status or changes 

in the hydrogeologic system. Water-quality studies have long-term costs 

associated with data interpretation during the study to detect trends and provide 

a means to modify the study if the data indicate a need to change the strategy. 

The data retrieved from a ground water monitoring program also must be 

managed. A database must be developed, data collection and input forms must 

be prepared for field personnel, and data must be entered into the database and 

evaluated on a frequent and routine basis.
202

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Resources

Office of Ground Water

USGS National Center, MS-411

12201 Sunrise Valley Drive

Reston, VA 20192

Phone: 703–648–5035

Northeast Region Ground water Specialist

USGS National Center, MS-433

12201 Sunrise Valley Drive

Reston, VA 20192

Phone: 703–648–5814

Southeast Region Ground water Specialist

USGS

Spalding Woods Office Park, Suite 160

3850 Holcomb Bridge Road

Norcross, GA 30092

Phone: 770–409–7716

Central Region Ground water Specialist

USGS, MS-406

Box 25046

Denver Federal Center, Bldg. 53, Room F-1200

Lakewood, CO 80225

Phone: 303–236–5950, ext. 213

Western Region Ground water Specialist

USGS

7801 Folsom Blvd., Suite 325

Sacramento, CA 95826

Phone: 916–379–3737

Contact the State ground water specialist, through the USGS State representative on the Internet at http://interactive2.usgs.gov/contact_us/index.asp.

National Issues

Regional Issues

State or Local 

Issues

U.S. 

Geological 

Survey 

Ground 

Water 

Contacts
224

The USGS home page, with links to many earth-science-related topics and 

information on USGS water programs, technical resources (such as computer 

programs), publications, and water data, can be obtained on the Internet at 

http://water.usgs.gov/.

Many USGS reports are now available online. These reports can be accessed at 

http://water.usgs.gov/pubs/.

You can search for a publication by report series (for example, Water-supply 

Paper, Open-file Report, Techniques of Water-resources Investigations) andS 

number, or by keyword. A listing of the most recently published USGS reports 

can be obtained at http://pubs.usgs.gov/publications/index.shtml.

The U.S. Geological Survey series of print publications The Ground Water 

Atlas of the United States describes the location, the extent, and the geologic 

and hydrologic characteristics of the important aquifers of the Nation. This 

series can be accessed online at http://capp.water.usgs.gov/gwa/gwa.html.

Current drought information can be obtained at http://water.usgs.gov/.

Information on various ground water issues being addressed by USGS can be 

obtained at http://water.usgs.gov/ogw/issues.html.

The home page for the USGS Office of Ground Water Branch of Geophysics, 

which specializes in the application of geophysical methods to ground water 

investigations, is at http://water.usgs.gov/ogw/bgas/.

The National Park Service National Cave and Karst Research Institute’s home 

page is http://www2.nature.nps.gov/nckri/.

The EPA ground water research lab in Ada, OK (USEPA GWERD Library) 

has published numerous ground water issue papers focused on contaminant 

hydrology. They can be found at http://www.epa.gov/ada/publications.html

The Association of American State Geologists (AASG) is an organization of 

the chief executives of the State geological surveys in 50 States and Puerto 

Rico. The responsibilities of the various State surveys differ from State to 

State, depending on the enabling legislation and the traditions under which the 

particular survey evolved. Some have regulatory responsibilities for water, oil 

and gas, land reclamation, and so on. http://stategeologists.org

USGS Online 

Resources

Other Online 

Resources
225

Appendix I.

Legal Framework for Ground Water Use in the 

United States 

This document provides an overview of doctrine governing ground water in the 

43 States in which National Forest System land is located. Information here 

is not a substitute for legal advice from the USDA Office of the General 

Counsel.

Rights to use ground water are regulated by States through application of 

common law, State statutes and regulations, and/or judicial precedent. The 

ownership and allocation rules applicable to ground water are usually different 

from those applying to surface water. A brief overview of ground water law in 

the United States is given below. While ground water schemes can be divided 

into a few general categories, there are variations in every State. The USDA 

Office of the General Counsel should be consulted as specific questions about 

ground water laws arise. States generally follow one of four basic systems of 

ground water allocation: (1) the “English” rule of absolute ownership, (2) the 

“American” rule of reasonable use, (3) the prior appropriation rule, and (4) the 

correlative rights rule.1

While the central focus of this document is an overview of state laws and 

regulations regarding ground water, Federal law may have limited application 

when managing ground water resources. This doctrine is known as reserved 

rights, and it applies to land reserved from the public domain. The U.S. 

Supreme Court has decided that when the Federal Government reserves land 

from the public domain, it also implicitly, or sometimes explicitly, reserves the 

water needed to fulfill the reservation’s primary legislative purposes.2 As part 

of the creation of national forests, water rights were reserved for the purposes 

of securing favorable conditions of water flows and to furnish a continuous 

supply of timber.3 The U.S. Supreme Court rejected the United States’ claim 

of reserved water rights for maintenance of in-stream flows, recreation, stock 

watering, and wildlife within the Gila National Forest.4 

The amount of water reserved is “only that amount of water necessary to 

fulfill the purpose of the reservation, no more.”5 However, the reservation 

encompasses an amount of water “sufficient for the future requirements of the 

area reserved.”6 The date of the reservation establishes the priority right and 

the water right applies only to previously unappropriated waters.7 In Cappaert, 

1

 Malone, Linda A., The Necessary Interrelationship between Land Use and Preservation of Ground 

water Resources, 9 UCLA J. Environmental Law & Policy 1, 5 (1990).

2

 Winters v. United States, 207 U.S. 564 (1908). 3

 16 U.S.C. § 475; United States v. New Mexico, 438 U.S. 696, 707-08, 718 (1978) 4

 Id. at 708, 716-17 5

 Cappaert v. United States, 426 U.S. 128, 141 (1976). 6

 Arizona v. California, 373 U.S. 546, 601 (1963). 7

 Cappaert v. United States, 426 U.S. at 139

The Reserved 

Rights Doctrine 
226

Absolute 

Ownership

Reasonable Use 

the Supreme Court held that the reservation of land withdrawn under the 

American Antiquities Preservation Act, reserved subterranean water necessary 

for the maintenance of the pupfish at Devil’s Hole National Monument, and 

the United States did not have to perfect its water rights according to State law. 

However, in doing so the Supreme Court did not define the subsurface waters 

where the pupfish lived as “ground water.”8 The Supreme Court and Circuit 

Courts of Appeal have never made a determination as to whether the reserved 

rights doctrine applies to water lying beneath federal lands.

The Federal Courts have left the question of whether reserved rights in ground 

water exist for a later day. Wyoming and Arizona have addressed whether 

there are federally reserved rights in ground water. Arizona came to the 

conclusion that the Federal Government did have reserved rights in stationary 

ground water and that those reserved rights entitle the federal government to 

greater protection than permittees with only State law rights. For additional 

discussion see section on Arizona Water Law.9

Should Federal Courts establish that the Federal Government has reserved 

rights in ground water, Federal Agencies will likely face similar difficulties 

to those encountered in the New Mexico decision; namely that the use of the 

ground water would be confined to the statutory purposes of the reservation of 

the land.

The absolute ownership doctrine is based on the English precedent of a 

landowner owning the airspace above and the soil beneath one’s property.10 

Under this doctrine, the landowner overlying an aquifer has an absolute right 

to extract all ground water from the aquifer beneath the landowner’s property. 

The overlying landowner can pump as much water as needed without regard 

to the needs or effect on other overlying landowners. The doctrine worked 

well in areas where abundant water was available; however, the drawbacks of 

the doctrine became apparent in the arid environment of the Western States.11 

Most of the States that initially followed this rule abandoned it during the 

late nineteenth and early twentieth century in favor of the reasonable use or 

“American” rule.12 States still following the absolute ownership rule include 

Connecticut, Georgia, Indiana, Louisiana, Maine, Massachusetts, Mississippi, 

Rhode Island, and Texas.13

The reasonable-use rule is a modified absolute ownership rule wherein ground 

water use by an overlying landowner must be “reasonable” and must be used 

for a beneficial purpose on the overlying land.14 Use of ground water on 

8

 Cf. Cappaert v. United States, 508 F.2d 313,317 (9th Cir., 1974) (the Ninth Circuit characterized the 

waters of Devil’s Hole were ground water and found a reserved right).

9

 In re General Adjudication of All Rights to use the Gila River System and Source III, 195 Ariz. 411 

(1999).

10 Acton v. Blundell, 152 Eng. Rep. 1223 (Exch. 1843). 11 Ashley, Jeffrey S. and Smith, Zachary A., Ground water Management in the West, University of 

Nebraska Press, 1999.

12 A. Tarlock, Law of Water Rights and Resources, §4.04, Clark Boardman Callaghan, 1997. 13 Malone at 5, fn. 25

14 Malone at 6.
227

nonoverlying land is considered unreasonable. Reasonableness is based on 

such factors as well location, amount of water, and the proposed use and 

placement of the water.15 Waste of water is not a reasonable use if it interferes 

with the right of adjacent landowners to use the water for the beneficial use 

of their overlying lands.16 If the requirements of the rule are met, a landowner 

may withdraw ground water even if doing so deprives another landowner of 

the reasonable use of the ground water.17 States applying the reasonable use 

rule include Alabama, Florida, Illinois, Kentucky, Maryland, New York, Ohio, 

North Carolina, and Tennessee.18

The prior appropriation doctrine gives priority to ground water users who put 

ground water to beneficial uses that are first in time. During water shortages, 

first in time appropriators have priority over later appropriators.19 Many States 

have statutory systems requiring permits to establish priority use. Idaho, 

Kansas, Montana, Nevada, New Mexico, North Dakota, Oregon, South Dakota, 

Utah, and Wyoming apply the doctrine of prior appropriation to ground water.20 

California applies it where surplus water exists above the needs of overlying 

owners. Arizona, once an absolute ownership State, now has a statutory 

scheme that creates Active Ground Water Management Areas, grandfathers pre1980 water rights in these areas, and sets up a permit administration system.21 

The States of Colorado, Kansas, Montana, Nebraska, Nevada, New Mexico, 

Washington, and Oregon have combined prior appropriation with critical 

area legislation to designate areas where new pumping may be prohibited and 

existing pumping may be restricted to preserve ground water.22 Courts in Idaho 

have upheld laws limiting water extraction to the annual recharge rate and 

have issued injunctions against junior wells that exceed reasonably anticipated 

future rate of recharge.23 Arizona, Colorado and New Mexico further limit 

ground water mining and extraction to a rate that will restore the aquifer 

to the level necessary for economically feasible extraction.24 Some States 

exempt ground water that is a by-product of secondary oil and gas recovery 

(Wyoming), geothermal resources (California), or water from mine dewatering 

(New Mexico).25

16 Id.16 Ashley at 9. 17 Tarlock at §4.05(1).

18 Malone at 6.

19 Malone at 8.

20 Turlock at §6.03(1).

21 Patrick, Kevin L and Archer, Kelly E., A Comparison of State Ground water Laws, 30 Tulsa L.J. 123, 

132-33.

22 L. Malone at 9-10.

23 Malone at 10, fn. 48.

24 Malone at 10.

25 Tarlock at 6.03(3).

Prior 

Appropriation
228

The correlative rights doctrine gives each overlying property owner a common 

right to the reasonable, beneficial use of the basin supply on the overlying land. 

This is similar to the doctrine of riparian rights to surface water. All overlaying 

landowners have equal rights to percolating ground water and all must share 

in any water shortages;26 however, overlying landowners do not have a right 

to maintenance of the natural water table.27 The States that have adopted the 

correlative rights doctrine include Arkansas, California, Delaware, Minnesota, 

Missouri, Nebraska, and New Jersey.28

Subject to future requirements on overlying lands, ground water that is surplus 

to the needs of overlying owners is available for appropriation for uses on 

non-overlying land. The burden of proof is on the appropriator to prove that a 

surplus exists beyond prior vested-right uses of overlying landowners. In the 

event of a shortage, overlying landowners have first priority.29

Some uses of ground water on land overlying a basin have been held to 

constitute appropriative uses. For example, the public use of ground water 

is typically not an overlying use. Municipalities or public water agencies 

generally have appropriative rights, not overlying rights, to the water pumped 

from a ground water basin to supply their customers. They do not exercise the 

overlying rights of their inhabitants.30

Most States have a permit system for ground water extraction. Permit 

requirements differ in each state. Some States require a permit for all 

extractions. Others require permits where water is proposed to be withdrawn 

from certain designated areas. Some States have a common permit system for 

surface and ground water.31 

The definition of “beneficial use” is a critical issue in analyzing ground water 

law in any State. Some uses are universally considered to be beneficial. They 

include the use of water for domestic, irrigation, manufacturing or stockwatering purposes;32 however, the States differ on whether protection of fish, 

recreation, aesthetic, or scenic uses are beneficial uses of water.33 

26 Tehachapi-Cummings County Water District v. Armstrong, 49 Cal. App. 3d 992, 1001 (1975). 27 Katz v.Walkinshaw 141 Cal. 116 (1903) [74 P. 766]. 28 A. Tarlock at §4.06(2).

29 Montecito Valley Water Co. v. Santa Barbara, 144 Cal. 578, 584-85 (1904).

30 Hutchins, The California Law of Water Rights, 1956, p. 458; San Bernardino v. Riverside, 186 Cal. 7, 

25 (1921) [198 P. 784].

31 Malone at 12.

32 Ashley at 10.

33 Id.

Correlative 
229

Figure 1. The relation 

between hydraulic head 

and water level in two 

observation wells. Well 1 is 

screened in an unconfined 

aquifer, and Well 2 is 

screened in a confined 

aquifer (Taylor and Alley 

2001).

Appendix II. 

Common Ground Water Terms and Definitions

The following section defines, in relatively simple terms, terminology 

and properties commonly associated with ground water. More thorough 

discussions of each term or concept can be found in ground water hydrology 

textbooks (for example, Fetter 2001, Freeze and Cherry 1979). A list of 

definitions of common hydrological terms is provided online by the U.S. 

Geological Survey at http://capp.water.usgs.gov/GIP/h2o_gloss/

Hydraulic head (often simply referred to as “head”) can be considered simply 

as the elevation of the water surface in a well, although the actual definition of 

hydraulic head is more complex. A water-level measurement made under static 

conditions is a measurement of the hydraulic head in the aquifer at the depth 

of the screened or open interval of a well (fig. 1). Because hydraulic head 

represents the energy of water, ground water flows from locations of higher 

head to locations of lower head. The change in hydraulic head over a specified 

distance in a given direction is called the “hydraulic gradient.” 

Hydraulic Head 

and Hydraulic 

Gradient
230

Saturated and 

Unsaturated 

Zones

Figure 2. The relationships among 

the unsaturated zone, capillary fringe, 

saturated zone, and water table.

When rain falls or snow melts, some of the water evaporates, some is 

transpired by plants, some flows overland and collects in streams, and some 

infiltrates into the pores or cracks of the soil and rocks. The first water that 

enters the soil replaces water that has been evaporated or used by plants during 

a preceding dry period. Between the land surface and the aquifer water is the 

unsaturated zone. In the unsaturated zone, there usually is at least a little water, 

mostly in smaller openings of the soil and rock; the larger openings usually 

contain air instead of water. After a significant rain, the zone may become 

almost saturated; after a long dry spell, it may become almost dry. However, 

some water is always held in the unsaturated zone by molecular attraction. 

After the water requirements for plant and soil are satisfied, any excess 

water will infiltrate to the water table —the top of the zone below which the 

openings in rocks are fully saturated (the saturated zone). The water table is 

often considered the boundary between the saturated and unsaturated zones, 

but in reality a capillary fringe often exists between the two zones (fig. 2). 

At the water table the fluid pressure within pore spaces is exactly equal to 

atmospheric pressure, but within the capillary fringe the fluid pressure is less 

than atmospheric. 

Figure 3. An example of a perched 

water body.

Complex geological environments can lead to more complex saturatedunsaturated conditions than those previously discussed. The presence of 

a low permeability layer, such as a clay layer, within a highly permeable 

formation can result in the formation of a discontinuous saturated lens in which 

unsaturated conditions exist both above and below the lens. Such a lens is 

called a “perched water body” (fig. 3). 
231

Aquifers and 

Confining Units

Figure 4. A typical ground water flow system showing the relation between an 

unconfined and a confined aquifer, a water table, and other hydrologic elements (Taylor 

and Alley 2001).

An aquifer is a geological formation, group of formations, or part of a 

formation that contains sufficient saturated, permeable material to yield 

significant quantities of water to wells and springs (Taylor and Alley 2001). 

Examples of aquifer materials include sand and gravel, cavernous or fractured 

limestone, sandstone, and fractured crystalline rock. 

Two general classes of aquifers — unconfined and confined — are recognized 

(fig. 4). In unconfined aquifers (sometimes referred to as “water-table 

aquifers”), hydraulic heads fluctuate freely in response to changes in recharge, 

discharge, and barometer pressure. Water levels measured in the upper part of 

an unconfined aquifer help define the elevation of the water table. In confined 

aquifers, water in the aquifer is confined by an overlying geologic formation 

that is much less permeable than the aquifer. Water levels in tightly cased 

wells completed in confined aquifers may rise above the elevation of the top 

of the aquifer (fig. 4), and may even flow at land surface. These aquifers are 

considered to be “artesian”. These water levels define an imaginary surface, 

referred to as the potentiometric surface, which represents the potential height 

to which water will rise in wells completed in the confined aquifer. Many 

aquifers are intermediate between being completely confined or unconfined, 

and in some cases an aquifer can be both confined and unconfined at different 

locations (fig. 5).
232

Figure 5. This aquifer is unconfined, in the area beneath the recharge area on the left, and 

confined on the right side of this illustration.

Hydraulic 

Conductivity

The geological unit that isolates a confined aquifer and restricts the movement 

of water between aquifers is called a confining unit (sometimes referred to 

as an “aquitard” or “aquiclude”). A confining unit is composed of geological 

materials that are significantly less permeable than the adjacent aquifer(s). 

Examples of confining unit materials include clay, shale, glacial till, and 

unfractured crystalline rock.

Hydraulic conductivity (K) is a measure of the capacity of an aquifer to 

transmit water, and it is expressed in units of velocity, such as feet per day 

or centimeters per second. In general, the greater the hydraulic conductivity 

of an aquifer, the greater is its ability to provide water to a well. Hydraulic 

conductivity is sometimes used interchangeably with the term “permeability,” 

but these terms are technically somewhat different. Permeability is an intrinsic 

property of the aquifer material, whereas hydraulic conductivity captures not 

only the size and interconnectedness of the water-filled openings in the aquifer 

but also the physical properties of the water. Hydraulic conductivity of earth 

materials is highly variable, and can range over 12 orders of magnitude 

(fig. 6). For example, sand and gravel, cavernous limestone, and highly 

fractured crystalline rocks have relatively large hydraulic conductivity values, 

but clay, shale, and unfractured crystalline rocks have relatively small hydraulic 

conductivity values.

Because earth materials are usually not uniform in their physical properties, 

hydraulic conductivity may be highly variable within a single geologic 

formation. Such variability is termed “heterogeneity.” Hydraulic conductivity 

may also vary with direction within a single formation, and this variability 

is termed “anisotropy.” In bedded sedimentary rocks, for example, hydraulic 

conductivity is usually greater in the direction parallel to the bed than in 

the direction perpendicular (for flat-lying units, this is known as “vertical 

anisotropy”). In fractured rocks, horizontal hydraulic conductivity is often 

greater in the direction parallel to the fracture planes (termed “horizontal 

anisotropy).
233

Transmissivity

Porosity and 

Effective 

Porosity

Figure 6. Range in hydraulic conductivity for selected earth materials (Heath 1983).

Transmissivity is another measure of an aquifer’s ability to transmit water. 

It is the product of the aquifer’s hydraulic conductivity (K) and the saturated 

thickness of the aquifer (b), such that:

T = Kb.

Transmissivity, commonly expressed in ft2/day or cm2/s, is usually the aquifer 

property that is solved for when analyzing an aquifer (pumping) test.

The ratio of openings (voids) to the total volume of a soil or rock is referred 

to as porosity, which is unitless and usually expressed as a percentage or a 

decimal fraction. Porosity depends on the range in grain size (sorting) and on 

the shape of the void spaces, but not necessarily on the size of the grains. For 

example a gravel deposit may be less porous than a clay deposit, because the 

clay is composed of a more uniform grain size and has a very open internal 

structure (often described as a “house of cards” structure). The individual pore 

spaces in the clay are smaller than those of the gravel, but the overall volume 

of pores in a clay will be tend to be greater than those of an equal volume of 

gravel.
234

Figure 7. A core sample, approximately four inches 

in length, of highly porous Miami oolite (limestone) 

obtained near Miami, FL. (USGS 2002)

Porosity also varies widely for different earth materials. Unfractured 

crystalline rocks can have almost no porosity, but clays and some modern 

carbonate rocks (fig. 7) may have porosities of 40 percent or more.

Effective porosity is that portion of the porosity that is interconnected and 

able to transmit fluids. The effective porosity is the ratio of the volume of 

interconnected voids to the total volume and is also unitless. Because it leaves 

out the dead-end void spaces and those void spaces that are too small to admit 

water molecules, it is typically less than the total porosity.

Specific Yield 

and Storage 

Coefficient

Porosity is important in 

ground water hydrology 

because it tells us the 

maximum amount of water 

that a rock or soil can contain 

when it is saturated (Heath 

1983). It is equally important, 

however, to know that only a 

part of this water is available 

to supply a well. Water in 

storage in the ground (total 

saturated porosity) is divided 

into the part that will drain 

under the influence of gravity 

(specific yield) and the part 

that is retained as a film on 

rock surfaces and in very 

small openings because of 

capillary forces (specific 

retention). Specific yield is 

the measure of how much 

water is available for use, 

and specific retention tells 

us how much water will 

remain in the rock after it is 

drained by gravity. Specific 

yield generally ranges 

between 10 and 30 percent 

in unconsolidated deposits, 

and is generally 10 percent 

or less in consolidated rocks. 

In confined aquifers, the 

term “storage coefficient” is 

usually used in place of specific yield. Storage coefficient values tend to be 

much smaller than values of specific yield, commonly on the order of less than 

1 percent.
235

Water Budget Budgets or balances of the amounts of precipitation, consumption, 

transpiration, evaporation, runoff, streamflow, and ground water flow within 

a basin may be performed to infer how much ground water is discharged to 

streams and becomes baseflow. A water budget is simply a statement of mass 

balance for hydrology (fig. 8). The following is the governing equation: 

Inflow – Outflow = Change in Storage

Watershed modeling and ground water modeling alike rely on the water 

balance approach. Although straightforward in concept, water budgets are 

difficult to determine in practice. The primary obstacle is obtaining the 

requisite data in sufficient detail, spatially and temporally. Many of the 

difficulties in making projections with watershed models attend attempts to 

perform complete ground water and surface water balances on basins and subbasins. Despite the difficulties, water budgets are useful and desirable. 

Figure 8. Water budget components for a typical watershed.
236

References 

Fetter, C.W. 2001. Applied hydrogeology. Englewood Cliffs, NJ: Prentice-Hall, 

598 p. 

Freeze, R.A.; Cherry, J.A. 1979. Groundwater. Englewood Cliffs, NJ: 

Prentice-Hall, 604 p.

Heath, R.C. 1983. Basic ground water hydrology. Water-Supply Paper 2220. 

Washington, DC: U.S. Geological Survey, 84 p.

Taylor, C.J.; Alley, W.M. 2001. Ground-water-level monitoring and the 

importance of long-term water-level data. Circular 1217. Washington, DC: U.S. 

Geological Survey. 68 p. [http://water.usgs.gov/pubs/circ/circ1217/]

U.S. Geological Survey (USGS). 2002. Report to Congress. Concepts for 

national assessment of water availability and use. Circular 1223. Reston, VA.: 

U.S. Geological Survey. 34 p. [http://water.usgs.gov/pubs/circ/circ1223/]
237

Appendix III. 

Contaminant Fate and Transport 

Several mechanisms influence the spread of a contaminant in a ground water 

flow field. Dispersion and differences in density and viscosity may accelerate 

contaminant movement, while various retardation processes slow the rate of 

movement compared to that predicted by simple advective transport. Fetter 

(1999) presents a comprehensive discussion of contaminant hydrogeology. 

The major mechanisms of contaminant fate and transport in the subsurface are 

summarized below. 

In its natural state, ground water moves very slowly, but continuously. 

Advection is the process by which dissolved solutes are carried along with 

the flowing groundwater. Advecting solutes are traveling at the same rate as 

the average linear velocity of the ground water if the solutes are not subject 

to any sort of reactions with the porous media. These movement patterns are 

generally governed by the space occupied by the mass of the liquid flowing 

through the media and the rate(s) of flow encountered within these spaces. 

The hydraulic conductivity of a geological formation depends on a variety of 

physical factors within the formation, such as effective porosity; particle size, 

arrangement, distribution, and shape; and secondary features, such as fracturing 

and dissolution. Generally, hydraulic conductivity values for unconsolidated 

porous materials vary with particle size. Fine-grained clayey materials exhibit 

lower values than those of coarse-grained, sandy materials.

Effective porosity is basically an estimated parameter, because the actual 

measurement of the volume of interconnected pore spaces in most porous 

media is not known. Therefore, effective porosity is usually estimated as 

being somewhat less than total porosity, which is calculated from the ratios 

of saturated and dry porous materials. In coarse-grained materials that drain 

freely, effective porosity is essentially equal to total porosity and is generally 

defined as the ratio of the volume of water that drains by gravity to the total 

volume of saturated porous material.

Diffusion is the process by which a solute moves from areas of higher chemical 

potential (high concentration) to areas of lower chemical potentials (low 

concentration). This process is also known as molecular diffusion. Diffusion 

occurs in the absence of any bulk hydraulic movement of the solution; that is, 

solutes diffuse (spread) regardless of whether the bulk mass of liquid is static 

or moving through the hydrogeological medium. 

Ground water molecules move at different rates depending on position 

within the aquifer and within the interconnected pores in the aquifer; some 

are faster than the average linear velocity while some are slower (Mills and 

others 1985). There are three causes for this phenomenon: friction on pore 

Ground Water 

Advection

Diffusion

Hydrodynamic 

Dispersion

Effective 

Porosity
238

walls, variations in pore sizes, and variations in path length. As ground water 

moves through the pores, it will move faster at the center of the pore than 

along the walls because of friction. In cases where different size pores exist, 

ground water will move through larger pores faster. Ground water molecules 

have tortuous flow paths and some will travel longer pathways than others. 

Because the invading solute-containing water is not all moving at the same 

rate, mixing occurs along the flow path. This mixing is termed mechanical 

dispersion. The mixing that occurs along the direction of fluid flow is 

termed longitudinal dispersion, whereas the mixing that occurs normal to the 

direction of fluid flow is termed transverse dispersion. Because molecular 

diffusion cannot be readily separated from mechanical dispersion in flowing 

ground water, the two are combined into a parameter called hydrodynamic 

dispersion. Because of hydrodynamic dispersion, the concentration of a 

solute will decrease over distance along the flow path. Generally speaking, 

the solute will spread more in the direction of ground water flow than in the 

direction normal to the ground water flow because longitudinal dispersivity is 

typically substantially higher than transverse dispersivity. Because dispersion 

anisotropy is often difficult to measure, a default value of a factor of 10 higher 

for longitudinal relative to transverse is often used. In fact, most solute plumes 

are long and thin.

Quantifying dispersion may be important in fate assessment, because 

contaminants can move more rapidly through an aquifer by this process than 

by simple plug flow (uniform movement of water through an aquifer with a 

vertical front). In other words, physical conditions, such as the presence of 

more permeable zones where water can move more quickly, and chemical 

processes, such as movement by molecular diffusion of dissolved species at 

greater velocities than the water, result in more rapid contaminant movement 

than would be predicted by ground water equations for physical flow, which 

assume average values for hydraulic conductivity. 

There are many types of chemical reactions that can be important in ground 

water systems. These include oxidation-reduction, acid-base, dissolutionprecipitation, sorption, complexation, and ion exchange. A detailed discussion 

of the chemistry of natural waters is beyond the scope of this document. Some 

additional information is provided below on two of the most common reactions 

in contaminant transport, ion exchange and sorption. More information on 

aquatic chemistry is available in Chapelle (2000), Drever (1997), Langmuir 

(1997), Stumm and Morgan (1996), and Morel and Hering (1993).

Ion exchange processes exert an important influence on retarding the 

movement of chemical constituents in ground water. In ground water systems, 

ion exchange occurs when ions in solution displace ions associated with 

geological materials. This process removes constituents from the ground 

water and releases others to the flow system. One major consideration in 

ion exchange is that the exchange capacity of a given geological material 

is limited. A measure of this capacity is quantified in a term called “ion 

exchange capacity” and is defined as the amount of exchangeable ions in 

Chemical 

Reactions

Ion Exchange
239

milliequivalents per 100 grams of solids at pH 7. Typically, clay materials such 

as montmorillonite exhibit greater cation (negatively charged ions) exchange 

capacities than other minerals such as quartz, which is the primary component 

of sand. This difference is attributable to the often much greater surface area of 

clays than other minerals.

Anionic (positively charged ions) exchange in aquifer systems is not as 

well understood as cationic exchange. Anions such as sulfate, chloride, and 

nitrate would not be expected to be retarded significantly by anion exchange 

because most mineral surfaces in natural water systems are negatively charged. 

Chloride ions may be regarded as conservative or noninteracting ions, which 

move largely unretarded with the advective velocity of the ground water mass.

It is important to recognize that the ion-exchange capacity of a geological 

material may retard contaminant movement from a waste or other source for 

years or even decades. However, if the source continues to supply a strongly 

ionic leachate, it is possible to exceed the exchange capacity of the geological 

material, eventually allowing unretarded transport of the contaminant. 

Changes in environmental conditions or ground water solution composition 

can also cause the release of constituents formerly bound to the geological 

materials.

Sorption involves the surface interaction of a dissolved constituent with a solid 

material. More specifically, the term encompasses both adsorption-desorption 

reactions and absorption. The former refers to a buildup or a release of a 

constituent on the surface of a solid as a result of molecular-level interactions, 

while the latter implies a more or less uniform penetration of the solid by 

a contaminant. In many environmental settings, this distinction may serve 

little purpose as there is seldom information about the specific nature of the 

interaction. A number of factors control the interaction of a contaminant and 

the surfaces of soil or aquifer materials. These include chemical and physical 

characteristics of the constituent, composition of the surface of the solid, and 

the fluid media encompassing both. By gaining an understanding of these 

factors, logical conclusions can often be drawn about the impact of sorption

on the movement and distribution of constituents in the subsurface. The 

failure to take sorption into account can result in a significant underestimation 

of the amount of a contaminant at a site, the time required for it to move 

from one point to another, and the cost and time involved for remediation. 

The properties of a contaminant that have a profound effect on its sorptive

behavior include water solubility, polar/ionic character, octanol/water partition 

coefficient, acid/base chemistry, and oxidation/reduction chemistry.

The transformation of both organic and inorganic chemicals by microorganisms 

readily occurs in many subsurface environments, including landfills and septic 

systems. Microbial processes may be a major factor in the transformation 

of both natural and anthropogenic organic materials present in ground water. 

These transformations usually result in the formation of CO2, CH4, H2, H2S, 

N2, NH3, and NO gases, among other compounds. Under the appropriate 

Sorption

Biotransformation and 

Biodegradation
240

circumstances, pollutants can be completely degraded to harmless products; 

whereas, under other circumstances, they can be transformed to new substances 

that are more mobile or more toxic than the original contaminant. Quantitative 

predictions of the fate of biologically reactive substances are primitive in 

comparison with predictions for other processes that affect pollutant transport 

and fate. 

Biotransformations in ground water were previously thought to mimic 

those known to occur in surface water bodies, but detailed fieldwork has 

demonstrated the fallacy of this assumption. With the relatively long residence 

times and stable environments in ground water systems, water-table aquifers 

are now known to harbor appreciable numbers of metabolically active 

microorganisms distinctly different from those in surface waters. These ground 

water organisms frequently can effectively degrade organic contaminants in 

the subsurface that would not be effectively degraded on the surface. Thus, it 

is necessary to consider biotransformation as a process that affects pollutant 

transport and fate. 

Contaminant residence time in ground water is usually long, at least measured 

in weeks or months, and frequently in years or even decades. Further, 

contaminant concentrations that are high enough to be of environmental 

concern are often high enough to elicit adaptation of the microbial community. 

For example, the U.S. Environmental Protection Agency maximum 

contaminant limit (MCL) for benzene is 5 ug/L. This is very close to the 

concentration of alkylbenzenes required to elicit adaptation to this class 

of organic compounds in soils. As a result, the biotransformation rate of a 

contaminant in the subsurface environment is not a constant, but increases after 

exposure to the contaminant in an unpredictable way. Careful fieldwork has 

shown that the transformation rate in aquifers of typical organic contaminants 

such as alkylbenzenes can vary as much as two orders of magnitude over a 

meter vertically and a few meters horizontally. This surprising variability 

in transformation rate is not related in any simple way to system geology or 

hydrology. Biological activity may promote or catalyze chemical reactions 

as well, and stimulation of the native microbial population and the addition 

of contaminant-specific “seed” microorganisms for the restoration of 

contaminated aquifers by in situ biological treatment has been explored 

vigorously.

Most ground water sources have very low levels of radioactive contaminants 

(“radionuclides”). The most natural radionuclides in ground water are referred 

to as primordial radionuclides and have exceptionally long half-lives. These 

very low levels are not considered to be a public health concern. Of the small 

percentage of drinking water systems with radioactive contaminant levels 

high enough to be of concern, most of the radioactivity is naturally occurring. 

Certain rock types have naturally occurring trace amounts of “mildly 

radioactive” elements (radioactive elements with very long half-lives) that 

serve as the “parent” of other radioactive contaminants (“daughter products”). 

These radioactive contaminants, depending on their chemical properties, 

Radionuclides 

in Ground 

Water
241

may accumulate in drinking water sources at levels of concern. The “parent 

radionuclide” often behaves very differently from the “daughter radionuclide” 

in the environment. Because of this, parent and daughter radionuclides may 

have very different drinking water occurrence patterns. For example, ground 

water with high radium levels tends to have low uranium levels and vice versa, 

even though uranium-238 is the parent of radium-226. 

Most parts of the United States have very low “average radionuclide 

occurrence” in ground water sources; however, some parts of the country have, 

on average, elevated levels of particular radionuclides compared to the national 

average. For example, some parts of the Midwest have significantly higher 

average combined radium-226/radium-228 levels. On the other hand, some 

Western States have elevated average uranium levels compared to the national 

average. In general, however, average uranium levels are very low compared 

to the EPA Maximum Contaminant Level for drinking water throughout the 

United States. While there are other radionuclides that have been known to 

occur in a small number of drinking water supplies, their occurrence is thought 

to be rare compared to radium-226, radium-228, and uranium. Uranium is 

present in ground water in amounts ranging from 0.05 parts per billion (ppb) to 

10 ppb (the median is about 1.5 ppb).

Radon-222, a naturally occurring radionuclide of concern in ground water, 

has a half-life of 3.8 days and is produced continuously in aquifers by the 

disintegration of the parent nuclide radium-226. Radioactivity in ground 

water is normally measured in the units of microcuries per milliliter (μCi/ml). 

Normal ground water contains from less than 1 x 10-7 μCi/ml to about 3 x 10-5

μCi/ml radon; the median is about 2 x 10-6 μCi/ml.

Ground water has been contaminated with radionuclides beyond background 

levels through the mining, refinement, and processing of uranium ore; initial 

production of nuclear fuels and explosives; reprocessing used reactor elements; 

discharge of cooling water that has been exposed to nuclear activation; escape 

of volatile material from evaporation and burning; dispersion of products 

of nuclear explosions; and the release of radionuclides used in science and 

medicine. The safe disposal of wastes from reactor operations and fuel 

reprocessing is one of the major problems in the widespread utilization of 

nuclear power. Disposal practices depend on the radioactivity of the waste, the 

general chemical character of the waste, the design of protective containment, 

and the physical environment of the disposal area. The radioactivity of liquid 

waste is broadly referred to as low level if it has fractions of a microcurie per 

gallon, intermediate level if it has a less than a few curies per gallon but greater 

than a microcurie per gallon, and high level if it has more than a few curies 

per gallon. Low-level wastes have been disposed into the subsurface at the 

National Reactor Testing Station in Idaho, at the Savanna River Plant, SC, and 

at Hanford, WA. Sorption on soil particles plus decay of the radionuclides 

with short half-lives has for the most part limited undesirable movement of 

contaminants into the ground water at these sites.
242

References 

Chapelle, F. H. 2000. Ground water microbiology and geochemistry. New 

York: John Wiley and Sons, 496 p.

Drever, J. I. 1997. Geochemistry of natural waters, the surface and groundwater 

environments. Upper Saddle River, NJ: Prentice Hall, 436 p.

Fetter, C.W. 1999. Contaminant hydrogeology. Englewood Cliffs, NJ: 

Prentice Hall. 500 p.

Langmuir, D. 1997. Aqueous environmental geochemistry. Upper Saddle River, 

NJ: Prentice Hall, 600 p.

Mills, W.B.; Borcella, B.B.; Ungs, M.J.; Gherini, S.A.; Summers, K.V.; 

Lingsung, M.; Rupp, G.L.; Bowie, G.L.; and Haith, D.A. 1985. Water Quality 

Assessment: A screening procedure for toxic and conventional pollutants in 

surface and ground water, Parts 1 and 2. Report EPA 600/6-85/002a,b. Athens, 

GA: Environmental Research Laboratory, U.S. Environmental Protection 

Agency. 

Morel, F. M. M.; Hering, J. G. 1993. Principles and applications of aquatic 

chemistry. New York: Wiley-Interscience, 608 p.

Stumm, W.; Morgan, J.J. 1996. Aquatic Chemistry, Chemical Equilibria and 

Rates in Natural Waters, 3rd ed. New York: John Wiley & Sons, Inc., 1022 p. 
243

Appendix IV. 

Ground Water Remediation

Once ground water is contaminated, it is difficult and typically very expensive 

to restore to natural or pre-contamination conditions. The broad range of 

chemical, physical, and biological characteristics of the thousands of potential 

ground water contaminants coupled with the complex heterogeneities of 

subsurface flow and contaminant transport make it very difficult to determine 

the exact nature and extent of ground water contamination in a given area or 

aquifer. If the value of the ground water that has been contaminated is great 

enough, it is very important to conduct an appropriate remedial investigation 

that is aimed at determining the nature and extent of the contaminant or 

contaminants. This remedial investigation is then used to scope and conduct a 

feasibility study that will focus on evaluating potential remedial options. 

Strategies and technologies that are typically used to remediate contaminated 

ground water include the following general categories:

(1) Those aimed at removing or controlling the source of contamination.

(2) Those aimed at hydraulically controlling the contaminant plume(s) to 

isolate the contaminated ground water.

(3) Those that include treatment of the contaminated ground water, either 

in situ or by collecting, treating and returning the ground water to the 

aquifer. 

The decision regarding which remedial option is appropriate for a given 

situation depends largely on the following factors:

(1) The compatibility of the remedial option to the hydrogeologic setting.

(2) The ability to achieve the remedial goals.

(3) The cost and time required to implement the remedy.

The objective of source removal is to reduce or eliminate the volume of 

waste (solid or liquid) or non-waste that is the source of the ground water 

contaminant(s). Removal should stop or minimize ongoing contamination; 

however, it is important to not transfer the problem from one location to 

another. To determine if source removal is a viable option, it is necessary to 

consider the following items: (1) problems associated with excavation and 

transport of the source material; (2) accessibility, distance, and road conditions 

between the origin and disposal sites; (3) cost; and (4) political, social, and 

legal factors.

Over the last couple of decades, a multitude of measures have been developed 

to control contaminant sources; some of those have been successful under 

certain conditions, while others have not demonstrated much success. Two 

Source Removal

Source Control
244

commonly utilized source control measures, surface runoff control and 

ground water barriers, are discussed below. Both of these options are aimed at 

preventing water from moving into and through the contaminant source, thus 

minimizing or stopping the leaching and subsequent transport of contaminants. 

A complete discussion of potential source control measures is beyond the scope 

of this document. 

Surface-runoff controls. Surface-runoff control measures are used to 

minimize or prevent infiltration of precipitation and overland flow. Overland 

flow over an area of concern can be prevented by contouring the land using 

dikes, berms, ditches, terraces, benches, levees, and sedimentation basins. 

These features can be used to divert or collect the overland flow to prevent 

infiltration. If feasible, a contaminated site or buried waste (landfill, mine 

waste) can be capped to control or prevent infiltration of precipitation into the 

underlying waste. In areas with low annual precipitation, a water balance cap 

may be appropriate. A water balance cap is constructed in a way that allows 

for infiltration of precipitation at a rate close to the uptake rate of grasses, 

shrubs or trees that are planted on top of the cap. This infiltration allows for 

a revegetated cap and minimizes water management considerations. In areas 

where a water balance cap is not appropriate, a low-permeability cap can be 

installed to divert most precipitation off the cap to a collection or diversion 

system. The goal of this type of cap is to limit infiltration into the underlying 

waste. Caps can be constructed of native soils, clays, synthetic membranes,

or a combination of these materials. Revegetation can also be a cost-effective 

method for helping to control overland flow and infiltration, especially if 

combined with contouring and/or capping. Vegetation reduces the impact of 

rainfall, decreases overland flow velocity, and strengthens soil structure.

Ground water barriers. Ground water barriers are designed to stop ground 

water flow into, through, or from a certain location, thus limiting the mixing 

of uncontaminated ground water with contaminated ground water or source 

materials. Ground water barriers are commonly used in combination with other 

treatment strategies and technologies, such as pump and treat. Common types 

of barriers include (1) slurry trench walls, (2) grout curtains and seals, and (3) 

cutoff walls.

Slurry trench walls are suitable for placing upgradient of a contaminated 

site to limit ground water flow through the site, downgradient to limit offsite 

flow of contaminated ground water, or completely around a site to contain 

contaminated ground water. Slurry walls can be constructed so that they 

extend well below the water table, if desirable. A slurry wall is constructed 

by excavating a trench to the desired depth (up to 100 feet, under appropriate 

conditions) and backfilling the trench with a slurry mixture that forms the 

final wall. The mixture can be composed of soil and bentonite, cement and 

bentonite, or concrete. Generally, a soil slurry should contain 5 to 7 percent by 

weight suspension of bentonite in water. The slurry will provide for trench wall 

stability and forms a low-permeability filter cake on the walls. Slurry walls are 
245

reported to have long service lives. Two separate slurry walls were constructed 

along the boundary of the Rocky Mountain Arsenal near Denver, Colorado in 

the early 1980s and are reported to still be in operation.

Grouting involves the pressure-injection of stabilizing materials into the 

subsurface to fill and seal voids, cracks, and fissures. Subsurface grouting 

has been used for decades in geotechnical applications related to subsurface 

and dam construction. Grouts are usually composed of mixtures of materials 

involving bentonite and cement with small amounts of additives to promote 

penetration and manage the set time. For application to ground water 

contamination problems, grout curtains are typically formed by injecting grout 

through tubes. Grout curtains can create a fairly effective barrier to ground 

water movement, depending on the degree of completeness of the curtain. The 

amount of grout needed is a function of the volume of void space, the density 

of the grout, and pressures required for injecting the grout. Grout can also 

be used to seal the bottom of an excavation or waste impoundment. Grout is 

injected through drill holes to form a curved or horizontal barrier to prevent 

downward migration of a leachate. 

A properly located array of recharge and discharge wells can be used to 

prevent a ground water contamination plume from (1) moving into the zone 

of influence of a water supply well or well field, (2) moving into another 

aquifer or aquifer zone, or (3) connecting to surface water. By controlling the 

rate of ground water discharge and recharge at selected locations and vertical 

intervals, the magnitude and direction of hydraulic gradient of the water table 

or potentiometric surface can be controlled. It can allow the location, direction 

of movement and velocity of a plume to be controlled. Discharge wells can be 

used to create cones of depression with known diameter and depths. Recharge 

wells can be used to develop a hydraulic pressure ridge that will function as a 

hydraulic barrier. 

The system design of these gradient-control techniques is very sensitive to the 

local subsurface geology. Optimizing system operations with regard to well 

construction and maintenance, pumping schedules, costs and time frames is 

very site specific. It is wise to utilize computer simulations to evaluate system 

design elements. It is important to recognize that establishing hydrodynamic 

control in a given situation can involve the management of large amounts of 

potentially contaminated water. 

If it is legally required or desirable to treat contaminated ground water, and 

it cannot be achieved by in situ treatment, it will be necessary to collect the 

contaminated water, treat it by methods appropriate for the contaminant(s), and 

return it to the subsurface. The pump-and-treat system is the most common and 

the most successful collection and treatment technique. Depending on the site 

hydrogeology, the nature of the ground water contaminant(s), and the extent 

of the plume, an array of extraction wells is installed to remove contaminated 

ground water for treatment. Once treatment has been completed, the “clean” 

ground water is returned to the subsurface through infiltration ponds/trenches, 

Hydrodynamic 

Controls

Ground Water 

Collection and 

Treatment
246

spreading basins, or injection wells or is discharge to the surface. The 

location and operation of extraction wells is highly dependent on subsurface 

hydrogeological conditions. It is important to recognize that the operation of 

an array of extraction wells will result in the formation of a stagnant zone—an 

area downgradient of an extraction well where ground water flow is not 

affected by pumping. Contaminated ground water within these stagnation 

zones will not be collected for treatment; thus, it is important to construct and 

locate a sufficient number of wells to mitigate this effect.

Another significant constraint associated with pump-and-treat systems is the 

asymptotic decrease in concentrations of low-solubility contaminants over 

time as ground water flows along a geologic pathway. This slow decrease 

in concentrations is caused by (1) the slow release of contaminants from 

small pore spaces into the larger pores that comprise the primary flow paths 

and (2) the desorption of sorbed contaminants as the concentration in the 

pore waters is reduced. This phenomenon can significantly increase the time 

required to achieve water-quality goals. It is also important to recognize that 

pump-and-treat systems are effective primarily for dissolved contaminants. 

Contaminants that readily sorb to organic and mineral particles in the aquifer 

will not be readily collected by pumping. “Pulsed” pumping can be used to aid 

in the effectiveness of removal of some contaminants. This method involves 

intermittent pumping, which allows time for contaminant concentrations to 

come into equilibrium with regard to diffusion and partitioning. Alternating the 

pattern of pumping within an array of extraction wells can also modify active 

flow paths.

Subsurface drains are an alternative collection system that may be more 

appropriate in some situations than an array of extraction wells. A subsurface 

or “French drain” functions as an infinite line of extraction wells and creates 

a zone of influence in which ground water passively flows towards the drain. 

French drains usually have a perforated polyvinyl chloride (PVC) pipe installed 

within a bed of high-permeability gravel. For some applications, it can be 

necessary to line the bottom of the trench with a low-permeability liner.

 

Ground water pump-and-treat systems are commonly used in combination 

with other remedial methods. Examples include the use of a barrier wall or 

a subsurface funnel and gate to control the location of a plume and optimize 

collection by extraction wells and the use of surface ponds or enhanced 

irrigation to flush contaminants from the unsaturated zone prior to collection 

by extraction wells.

In general, in situ remedial techniques have not been as effective for restoring 

large volumes of contaminated ground water as ground water collection and 

treatment systems. This is attributable to a number of factors, including: 

(1) Inability to deliver the in situ treatment to all parts of a contaminated 

aquifer.

In Situ 

Remediation
247

(2) Difficulty in maintaining the correct biological and chemical 

conditions for treatment optimization.

(3) Constraints presented by the considerable subsurface heterogeneity 

with respect to hydrogeological conditions.

In situ treatment techniques can be grouped into two categories: (1) physical/

chemical treatment processes and (2) biological treatment processes. In situ

treatment processes include the injection of a treatment medium into an 

aquifer. As contaminated ground water comes in contact with this medium, 

specific chemical/biological reactions are catalyzed, causing the contaminant to 

participate in reaction(s) that reduce its concentration and/or toxicity or break it 

down into nontoxic constituents. 

Oxidation-reduction reactions can be effectively used to remediate selected 

metals (chrome, copper, zinc, manganese). These reactions are catalyzed by 

microorganisms and oxygen and can cause metals to change from toxic to 

less toxic or nontoxic species. For example, calcium polysulfide has proven 

to be effective in reducing Cr6+ to Cr3+ in a high-permeability valley-fill 

aquifer along the South Platte River in Denver. Cr6+ concentrations have been 

reduced by two to three orders of magnitude after 2 to 3 years of injecting a 

calcium polysulfide slurry into the aquifer. Contaminants that are held within 

the aquifer by sorption can be mobilized by the introduction of a solvent 

or surfactant which can enhance the solubility of the sorbed contaminant. 

Examples of physical processes that can immobilize or reduce dissolved 

concentration include precipitation, volatilization, and polymerization. 

Biological treatment below ground involves the injection of nutrients 

and oxygen into the contaminated aquifer to enhance the activity of 

microorganisms that utilize the contaminant in their metabolic processes. 

This treatment facilitates the breakdown of toxic organic compounds into 

nontoxic constituents and results in the destruction of the contaminant. To 

effectively utilize subsurface biological treatment, it is necessary to (1) control 

the anaerobic/aerobic conditions, (2) provide the correct amount and timing 

of nutrients and (typically) oxygen to the consortium of microorganisms, (3) 

understand what the likely degradation products will be, and (4) maintain 

optimal conditions for the microorganisms for the period of time required 

to meet the water-quality goals. Experience at experimental sites as well as 

regulated ground water contamination sites has shown that it is difficult to 

maintain ideal chemical and biological conditions for the microorganisms to be 

effective over long periods of time. Delivery of the treatment medium to all of 

the contaminated parts of the aquifer has also proven to be difficult.

Performance 

Monitoring

A performance monitoring program must be developed and implemented at 

ground water contamination sites that are being, or have been, remediated. 

The performance monitoring plan should be designed to provide data that can 

be used to determine whether the remedies that were utilized have achieved 

the established water-quality goals. Ground water-quality goals for any 
248

particular ground water contamination site are established based on legal and 

political requirements, use requirements, and/or the constraints of remediation 

technology. When developing a performance monitoring program, the 

following factors should be considered: 

(1) The extent of the ground water contamination. 

(2) The potential receptors of the contaminated ground water, such as a 

stream or water-supply well.

(3) The applicable regulatory requirements.

(4) The hydrogeological setting.

(5) The sampling frequency and methodology.

(6) An appropriate parameter list. 

(7) Sample collection, transport, and analysis. 

(8) Sound quality assurance and quality control procedures.

In general, performance monitoring programs should include the following 

four features:

(1) Clearly established compliance locations. 

(2) Clearly established compliance limits and schedules. 

(3) Early warning and trigger-level limits and locations. 

(4) Appropriate contingency measures to be implemented in the event 

compliance cannot be achieved. 

When developing plans for managing a ground water contamination site, it is 

important to allocate an appropriate budget, staff time, field time, and lab time. 
249

Appendix V. 

Ground Water Modeling

A generalized process for developing a ground water model includes the 

following components: model conceptualization, code selection, model design, 

model calibration, sensitivity analysis, and prediction (fig. 1). Few modeling 

studies will incorporate all of the steps in the process shown in figure 1; however, 

all should include the steps through calibration and sensitivity analysis, and the 

model should be completely documented in a written report.

The Modeling 

Process

Figure 1. A generic process for modeling (Anderson and Woessner 1992).
250

The ideal modeling process would include the following steps (Anderson and 

Woessner 1992):

1. Establish the purpose of the model. The purpose will determine what 

type of model is needed and which codes should be considered.

2. Develop a conceptual model of the hydrologic system of interest. 

Hydrostratigraphic units and system boundaries are identified. Field 

data are assembled, including information on the water balance and 

data needed to assign values to aquifer parameters and hydrological 

stresses. A visit to the field site is essential during this step, to provide 

the modeler with information that cannot be adequately conveyed on a 

map or in a report. At this stage, a hydrogeological framework for the 

study area is developed by means of hydrogeologic maps and sections.

3. Select the appropriate computer code or codes for simulation. There 

are many codes commercially available for aquifer simulation, and 

the code(s) selected should be able to adequately simulate the field 

conditions of importance for the study. For example, if the study area 

contains multiple aquifers critical to the study that are separated by 

confining units, then code(s) that can simulate a fully three-dimensional 

system should be used. Whichever code is selected, it should be one 

that is verified (either by the developer or someone else) and fully 

documented. 

4. Design the model. The conceptual model is put into a form suitable for 

modeling. This step includes design of the model grid, setting boundary 

and initial conditions, and selection of values for aquifer parameters and 

hydrologic stresses. Figure 2 shows an example of the relation between 

geologic units, hydrogeologic units, and model layers that is typical of 

the conversion of the conceptual model to a computer model.

5. Calibrate the model. The purpose of calibration is to establish that 

the model can reproduce observed heads and flows, such as spring 

flow or measured ground water contribution to streams. Values for 

aquifer parameters and stresses are systematically adjusted through a 

reasonable range of values until the differences between simulated and 

observed heads and flows are minimized. Calibration can be done by a 

trial-and-error process or automatically by using a parameter-estimation 

code such as UCODE (Poeter and Hill 1998) or PEST (Doherty 

1994). This step is sometimes called solving the inverse problem. 

The calibration process may result in identification of areas of data 

deficiencies that need to be filled before the model can be adequately 

calibrated, or the process may result in a redefinition of the conceptual 

model. In the latter cases, the modeler may have to go back to Step 2, 

collect new data, and possibly redefine the conceptual model before 

proceeding to Step 6. Hill (1998) provides detailed guidance on model 

calibration.
251

6. Perform a sensitivity analysis for the calibrated model. The calibrated 

model can contain a large degree of uncertainty and nonuniqueness, 

because of the inability to exactly define the spatial and temporal 

distribution of aquifer parameters and hydrological stresses. During a 

sensitivity analysis, the calibrated parameters and stresses are varied 

over the range of uncertainty or over a range of hydrogeologically 

reasonable values to establish how calibrated results may vary because 

of this uncertainty.

7. Conduct a model verification. During model verification the calibrated 

model is used to reproduce a second set of observed heads and flows. 

For example, the model may have been calibrated to hydrological 

conditions for the period 1960–70, and the model is then used to 

simulate conditions observed for 1970–90 as a verification. If these 

later conditions are simulated within accepted criteria, then greater 

confidence can be placed in the model as a representation of the real 

world.

8. Use the model to predict the response to a new set of anticipated or 

proposed hydrological stresses. The model is run with calibrated values 

for parameters and stresses, except for those stresses that are expected 

to change in the future. 

9. Perform a sensitivity analysis for the predictive model. Uncertainty in 

the predictive simulation results is the result of uncertainties inherent 

in the calibrated model and the inability to accurately predict future 

hydrological conditions and stresses. The sensitivity analysis helps 

to bracket the range of possible predicted responses to the simulated 

stresses and reduce the uncertainty.

10. Document the model design, process, and results to effectively 

communicate the modeling effort for other potential users of the model 

or model results. It is important that this documentation include a 

discussion of model uncertainty and any limitations on future use of 

the model. For example, if the model was calibrated only to steadystate conditions, it may not be appropriate to use it to simulate transient 

conditions without additional calibration.

11. Conduct a “postaudit” of the model predictions. A postaudit is 

conducted several years after the modeling study is completed. New 

field data are collected to determine if the model predictions were 

accurate. Postaudits often show that model predictions were not 

accurate, primarily because simulated stresses did not accurately 

duplicate those that actually occurred. For example, a new well field 

may have come on line or a new surface-water source was developed 

that reduced the amount of pumping from an existing well field, and 

these conditions were not included in the predictive simulation.
252

12. Redesign and recalibrate the model, either as a result of the postaudit 

or because new data have been collected in areas where data were 

previously lacking. The redesigned model should also be documented.

The degree to which the modeling study incorporates all or some of these steps 

is dependent on the objectives of the study and the amount of resources (time, 

personnel, and funding) available for the study. Additional details for each of 

these steps are provided in Anderson and Woessner (1992).

Figure 2. The relationship between geologic units, hydrogeologic units, and model layers for the 

northern Mississippi Embayment area (Brahana and Mesko 1988).
253

The purpose of the study will determine the type of model and the computer 

code that are needed to achieve the study objectives. If the purpose of the study 

is simply to predict the effect of a new well field on the ground water system, 

a flow-model code such as MODFLOW (McDonald and Harbaugh 1988) or 

GFLOW (Haitjema Consulting, Inc.) may be all that is required. If the purpose 

of the study is to predict the fate of a contaminant from a spill of hazardous 

chemicals, then a solute-transport code such as MOC3D (Konikow and others 

1996) will also be required. If processes in the unsaturated zone are important 

to the study, then a code such as TOUGH2 (Pruess 1991) may be required. 

Geochemical models that account for changes in ground water quality as a result 

of water-rock interactions in an aquifer are also valuable in conceptualizing and 

evaluating flow systems. Examples of geochemical models include MINTEQA2 

(Allison and others 1991), PHREEQC (Parkhurst and Appelo 1999), and 

NETPATH (Plummer and others 1994). Each code has its own capabilities and 

limitations, and the modeler must carefully define what is required of the model 

when selecting the code to use.

There are two basic numerical methods used to incorporate the ground water 

flow equations into a model code: finite-difference methods and finite-element 

methods. Each of these methods requires discretizing the real world into distinct 

blocks for which the ground water flow equations are approximated. A detailed 

discussion of these methods is beyond the scope of this section. It is sufficient 

here to say that finite-difference model codes discretize the flow system into 

rectangular blocks and the flow equations are approximated for a point at either 

the center of the block or at the corners of the block. Figure 3 shows an example 

of a finite-difference grid. Finite-element codes discretize the flow system into 

triangular or other polygonal-shaped blocks (that can also be rectangular), and 

approximate the flow equations at the corners of the blocks. Finite-element codes 

allow the modeler to more closely approximate the shape of highly irregularly 

shaped aquifer systems and better represent seepage faces than a finite-difference 

code. Figure 4 shows an example of a finite-element grid. 

Several numerical codes exist for parameter-estimation-based calibration of a 

model (see modeling procedure Step 5). Codes such as PEST (Doherty 1994) 

and UCODE (Poeter and Hill 1998) are independent of the flow model used. 

MODFLOW2000 (Hill and others 2000) includes a parameter-estimation 

process that runs inside of the MODFLOW code. In parameter estimation, 

the hydrological parameters to be estimated such as hydraulic conductivity 

and recharge are automatically adjusted within a preset range to minimize 

the difference between simulated and observed heads and fluxes. Benefits 

of parameter estimation include the quantification of (1) the quality of the 

calibration, (2) data needs, and (3) confidence in estimates and predictions 

(Poeter and Hill 1997).

A more recent type of model code uses the “analytic-element” method, which 

does not require the use of a model grid or specification of boundary conditions 

at the grid perimeter (Strack 1989; Haitjema 1995). An analytic-element 

Types of 

Computer 

Models
254

Figure 3. Example of a finite-difference model grid and boundary conditions (Tucci 1994).
255

Figure 4. Example of a finite-element model grid (Wu and others 1999).
256

model uses superposition of closed-form analytical solutions to the differential 

equation to approximate both local and regional flow. These models allow for 

representation of large domains that include many hydrologic features outside 

the immediate area of interest, and easy modification of the regional flow field 

by adding analytic elements representing regional hydrologic features (Hunt and 

others 1998). 

Data needed for ground water flow models can be considered to fall into three 

general categories:

1. Data needed to define the physical and hydrogeological framework. 

Topographic maps, geological maps, cross sections, well and boring logs 

(driller’s, geological, geophysical), well-construction information, maps 

showing the areal extent and thickness of aquifers and confining units, 

hydraulic properties of aquifers and confining units, and streambed and 

lakebed characteristics.

2. Data needed to define the water budget. Precipitation, evapotranspiration, 

streamflow, springflow, and pumping.

3. Data needed to define the flow system. Ground water levels, 

potentiometric-surface maps, stream stages, lake stages, and spring 

discharge elevations. Information about how these data vary with time is 

also needed.

If simulation of solute transport is needed, additional data are needed on waterquality characteristics of the aquifer and/or contaminant plumes over time. Of 

course, if the model is being developed solely to test hydrological concepts that 

are generic in nature, no real data are required. The types and amount of data 

required, therefore, are directly related to the objective(s) of the modeling study.

According to Anderson and Woessner (1992, 6), “Modeling is an excellent way 

to help organize and synthesize field data, but it is important to recognize that 

modeling is only one component of a hydrogeologic assessment and not an end 

in itself.” In fact, the process of assembling and understanding the field data 

required for model input may provide the modeler with the answer to the problem 

before ever running the model. Conversely, a model that is based on inadequate 

field data can produce erroneous results that may not be obvious in the colorful 

graphical output from modern modeling software. The modeler must have some 

basic understanding of the geology and hydrology of the area being modeled, or 

should work in close collaboration with others who do have that understanding. 

In this way, model results that are hydrogeologically unreasonable, or that are 

based on unrealistic or erroneous data, can be recognized and addressed.

Private consultants, university researchers or other government agencies (e.g., 

USGS, state geological surveys) are often contracted to develop ground water 

flow models. Careful review and evaluation of such models on the part of the 

user is important to insure that the modeling was done correctly and fulfills 

the contract obligations. Reilly and Harbaugh (2004) provides guidelines for 

evaluating models, and is also useful in planning a modeling study. 

Data Needed 

for Models

A Word of 

Caution
257

References 

Allison, J. D.; Brown, D. S.; Novo-Gradac, K. J. 1991. MINTEQA2/PRODEFA2, 

A geochemical assessment model for environmental systems: version 3.0 user’s 

manual. Report EPA/600/3-91/021. Athens, GA: Environmental Research 

Laboratory, U.S. Environmental Protection Agency, 107 p.[http://www.epa.gov/

ceampubl/mmedia/minteq/index.htm].

Anderson, M.P.; Woessner, W.W. 1992. Applied groundwater modeling, 

simulation of flow and advective transport. San Diego, CA: Academic Press, Inc., 

381 p.

Brahana, J.V.; Mesko T.O. 1988. Hydrogeology and preliminary assessment 

of regional flow in the upper Cretaceous and adjacent aquifers in the northern 

Mississippi Embayment. Water-resources Investigations Report 87-4000. 

Washington, DC: U.S. Geological Survey. 65 p.

Doherty, J. 1994. Manual for PEST package – Fifth Edition. Watermark 

Numerical Computing, Brisbane, Australia, 2002. 

Haitjema, H. M. 1995. Analytic element modeling of groundwater flow. San 

Diego: Academic Press, 394 p.

Hill, M.C. 1998. Methods and guidelines for effective model calibration. Waterresources Investigations Report 98-4005. Washington, DC: U.S. Geological 

Survey. 90 p. [http://water.usgs.gov/nrp/gwsoftware/modflow2000/WRIR98-

4005.pdf].

Hill, M.C.; Banta, E.R.; Harbaugh, A.W.; Anderman, E.R. 2000. MODFLOW2000, the U.S. Geological Survey modular ground water model: User guide 

to the observation, sensitivity, and parameter-estimation processes and three 

post-processing programs. Open-file Report 00-184. Washington, DC: U. S. 

Geological Survey, 209 p. [http://water.usgs.gov/nrp/gwsoftware/modflow2000/

ofr00-184.pdf].

Hunt, R.J.; Anderson, M.P.; Kelson, V.A. 1998. Improving a complex finitedifference ground water flow model through the use of an analytical element 

screening model. Ground Water. 36 (6): 1011-1017.

Konikow, L.F.; Goode, D.J.; Hornberger, G.Z. 1996. A three-dimensional 

method-of-characteristics solute-transport model (MOC3D). Water-resources 

Investigations Report 96-4267. Washington, DC: U.S. Geological Survey, 87 p.

McDonald, M.G.; Harbaugh, A.W. 1988. A modular three-dimensional finitedifference ground water flow model. In Techniques of Water-resources 

Investigations. Book 6, Chapter A1. Washington, DC: U.S. Geological Survey, 

586 p. [http://water.usgs.gov/pubs/twri/twri6a1].
258

Parkhurst, D.L.; Appelo, C.A.J. 1999. User’s guide to PHREEQC (Version 2): 

A computer program for speciation, batch-reaction, one-dimensional transport, 

and inverse geochemical calculations. Water-resources Investigations Report 

99-4259. Washington, DC: U.S. Geological Survey. 312 p.

Plummer, L.N.; Prestemon, E.C.; Parkhurst, D.L. 1994. An interactive code 

(NETPATH) for modeling NET geochemical reactions along a flow PATH--

Version 2.0. Water-resources Investigations Report 94-4169. Washington, DC: 

U.S. Geological Survey. 130 p.[http://water.usgs.gov/software/netpath.html].

Poeter, E.P.; Hill, M.C. 1997. Inverse models: A necessary next step in ground 

water modeling. Ground Water. 35 (2): 250-260.

Poeter, E.P.; Hill, M.C. 1998. Documentation of UCODE, a computer code for 

universal inverse modeling. Water-resources Investigations Report 98-4080. 

Washington, DC: U.S. Geological Survey. 116 p. [http://water.usgs.gov/software/

ucode.html].

Pruess, K. 1991. TOUGH2: A general-purpose numerical simulator for 

multiphase flow and heat flow. Berkeley, CA: Lawrence Berkeley Laboratory 

Report LBL-29400, 102 p. 

Reilly, T.E.; Harbaugh, A.W. 2004. Guidelines for evaluating ground water flow 

models. Scientific Investigations Report 2004-5038. Washington, DC: U.S. 

Geological Survey. 30 p. [http://water.usgs.gov/pubs/sir/2004/5038].

Strack, O.D.L. 1989. Groundwater mechanics. Engelwood Cliffs, NJ: Prentice 

Hall. 732 p.

Tucci, P. 1994. Simulated effects of horizontal anisotropy on ground- water 

flow paths and discharge to streams. In Marston, R.A., and Hasfurther, V.R. 

(eds.) Effects of human- induced changes on hydrologic systems. New York, NY: 

American Water Resources Association Technical Publications Series TPS-94-3, 

pp. 851-860.

Wu, Y.S.; Ritcey, A.C.; Bodvarsson, G.S. 1999. A modeling study of perched 

water phenomena in the unsaturated zone at Yucca Mountain. Journal of 

Contaminant Hydrology. 38: 157-184.
259

Appendix VI. 

Water-quality Data: Statistics, Analysis, 

and Plotting

This section describes some statistical procedures recommended for analysis of 

water-quality data. Most of these statistical procedures are explained in Gilbert 

(1987), U.S. Environmental Protection Agency (1992), or Helsel and Hirsch 

(1992), Sanders and others (2000).

Water-quality data possess unique characteristics that may require specialized 

approaches to statistical analysis. Data sets generally have a base limit of zero 

because only positive values are possible for most parameters, and can contain 

censored (less than) values, outliers, multiple detection limits, missing values, 

and serial correlation. These characteristics commonly present problems 

in the use of conventional parametric statistics based on an assumption of 

normally distributed data sets. The presence of censored data, non-negative 

values, and outliers may lead to an asymmetric or non-normal distribution 

instead of a normal, symmetric, or bell-shaped (Gaussian) distribution, which 

is common for many data sets. These skewed data sets may require use of 

specific nonparametric statistical procedures for their analysis. The use of 

nonparametric statistical procedures is also preferred when determining 

trends of many constituents at multiple stations. Additionally, nonparametric 

statistical tests are more powerful when applied to non-normally distributed 

data, and almost as powerful (under certain conditions) as parametric tests 

when applied to normally distributed data (Helsel and Hirsch 1992).

A major cause of variation in water-quality data is the effect of seasonality, 

which needs to be compensated for to discern specific anthropogenic or 

natural processes that affect water quality over time. Seasonal variation may 

be the result of a variety of conditions, including specific land-use practices, 

biological activity, or changes in sources and volumes of water. As an example, 

precipitation-induced stream discharge may predominate during specific 

months of the year, whereas baseflow (driven by ground water seepage) 

may be dominant at other times of the year. Another example is the increase 

in biological activity that occurs in surface waters during summer because 

of warmer temperatures. The result may be seasonal variation in nutrient 

concentrations. 

A trend in water quality is defined as a monotonic change in a particular 

constituent with time. Investigators must employ parametric and 

nonparametric tests that are designed to deal with characteristics unique 

to water-quality data. Because all data may not have been collected at the 

same frequency for the duration of a project or monitoring program, specific 

seasonal definitions are needed to prevent bias in the trend results. 

Statistics for 

Water-quality 

Data

Seasonal 

Variation

Trend Analysis
260

The statistical approach that is used to compensate for seasonal variability in 

water-quality data is the distribution-free, nonparametric seasonal Kendall 

trend test. This test, modified from the Mann-Kendall test (Helsel and Hirsch 

1992), compares relative ranks of data values from the same season. For 

example, January values are compared to January values, February values 

are compared to February values, and so forth. No comparisons are made 

across seasonal boundaries. A plus value is recorded if the subsequent value 

in time is higher, and a minus is recorded if the subsequent value in time is 

lower. If pluses predominate, a positive trend exists; if minuses predominate, a 

negative trend results. No trend is the result of pluses and minuses being equal. 

The null hypothesis is that the concentration of the water-quality constituent 

is independent of time (Smith and others 1982). The test assumes that the 

data are independent and from the same statistical distribution. The seasonal 

Kendall test statistic is the summation of the Mann-Kendall test results from 

all the seasons. The attained significance level (or p-value) is the probability 

of incorrectly rejecting the null hypothesis of no trend when actually there is 

a trend. The seasonal Kendall slope estimator is computed according to the 

method of Sen (1968); it is the median slope of all the pairwise comparisons 

from all of the seasons expressed as rate of change per year in original units 

(usually in milligrams per liter depending on the constituent) and in percent per 

year.

Recommended 

Statistical 

Procedures

Summary 

Statistics

Summary statistics are simple procedures that allow an investigator to quickly 

analyze a data set.

Time-Series Plot. Displays the variability in concentration levels over time 

for constituents and can be used to examine possible outliers. More than one 

station can be compared on the same plot to look for differences between 

stations. They can be used to examine the data for trends.

Histogram. Displays the frequency distribution for constituents. More than 

one station can be compared on the same plot to look for differences between 

stations. 

Box-and-Whiskers Plot. An efficient way to visually display the distribution 

of data for constituents at a given station. This plot can also be utilized by year 

or by season. The plot locates the median and the 25th and 75th quartiles and 

the whiskers extend to the minimum and maximum values of the data set.

Wilcox Diagram. This plot can be used to quickly determine the viability of 

water for irrigation purposes (also known as the USDA diagram).
261

Kruskal-Wallis Test. Tests for seasonality. If seasonality is found to exist in a 

time series of concentrations, then data can be deseasonalized prior to running 

further statistical analyses. 

Rank Von Neumann. This procedure tests for serial correlation at a station 

and also reflects the presence of trends or cycles such as seasonality.

Statistical Outlier Test (Dixon’s Test). The outlier test identifies data 

points that do not appear to fit the distribution of the rest of the data set and 

determines if they differ significantly from the rest of the data set.

Shapiro-Wilk Test, Shapiro-Francia Test, Chi-Squared-Goodness-Fit Test. 

These tests, called normality statistics, evaluate the distribution of the data.

Detection Limit Substitution. All censored data are usually corrected to half 

of the detection limit or to the detection limit of the least sensitive analytical 

procedure prior to running statistical analyses. This procedure provides a closer 

comparison between samples and time periods, but results in a large loss of 

information. This procedure should not be used if the percentage of censored 

data exceeds 50 percent of the total number of records.

Cohen’s Adjustment, Aitchison’s Adjustment. As alternatives to replacement 

of the detection limit with arbitrary constant values, these procedures calculate 

a corrected sample mean that accounts for data below the detection limit. 

The methods use probability theory to estimate the shape of the tail of the 

population probability density function that was censored, thus preserving the 

sample variability and mean that would have been estimated had the detection 

limit been zero and had no values been censored.

Analysis of Variance (ANOVA) (Interstation or Intrastation; Parametric 

or Nonparametric). Compares the means or median values of different groups 

of observations to determine if a statistical difference exists among groups.

Mann-Whitney Test (Interstation or Intrastation). Tests whether the 

measurements from one population are significantly higher or lower than those 

of another population.

Sen’s Slope, Mann-Kendall Trend Tests. Used to detect a general increase 

or decrease in observed values over time and determine the significance and 

magnitude of the trend.

Seasonal Kendall Test. This test is an extension of the Mann-Kendall test that 

removes seasonal cycles and tests for trend.

Shewhart-CUSUM Control Chart. These charts monitor the statistical 

variation of data collected at a station and flag anomalous results. If a result 

falls outside the predetermined control limits, then the process is considered 

“out of control.”

Censored Data 

Substitution 

Functions

Mean/Median 

Analysis

Trend Analysis
262

Proportion Estimate. This test computes the proportion of observations in the 

record which exceed a stated excursion limit and computes a confidence limit.

Tolerance Limit (Interstation or Intrastation). Tolerance limits define an 

interval that contains a specified fraction of the population with specified 

probability. They are used to compare concentrations from compliance stations 

to the upper limit of the tolerance interval.

Prediction Limit (Interstation or Intrastation). Used to determine whether a 

single observation is statistically representative of a group of observations.

Confidence Interval. A confidence interval is constructed from sample data 

and is designed to contain the mean concentration of a station analyte, with a 

designated level of confidence.

The following graphical methods are used for displaying water-quality data on 

maps or for analyzing different water types and compositions.

Bar Chart. The most widely used graphical procedure for displaying ion 

concentrations is the vertical bar system developed by Collins (1923). 

This method uses a vertical bar whose weight is proportional to the total 

concentration of anions or cations in milliequivalents per liter (fig. 1). 

Horizontal lines are used to separate the concentrations of various ions. 

Usually, six divisions are used, but more can easily be added if required. 

Graphical 

Display of 

Chemical 

Data

Figure 1. Example of a 

Collins bar chart for depicting 

water quality. The numbers 

above the bars refer to 

specific analysis taken from 

a data table. In this way, 

specific sampling locations 

can be identified in the table 

and correlated to the bar 

chart (Hem 1989).

Excursion 

Analysis
263

Stiff Diagram. This is a pattern plot and can be used to evaluate the change in 

water quality at a location over time, or as the water passes through different 

geologic formations or subsurface conditions (Stiff 1951). This method uses 

four parallel horizontal axes extending from each side of a vertical zero axis 

(fig. 2). Concentrations of four cations may be plotted, one on each axis to 

the left of zero, and four anion concentrations in milliequivalents per liter may 

be plotted on each axis to the right of zero. This method gives a distinctive 

pattern, and is very useful in depicting water composition differences or 

similarities. The pattern for a particular water source tends to maintain its 

shape, even with concentration or dilution of the constituent. Thus, a study 

of water-quality patterns can often be utilized to identify different producing 

strata, and correlate water sources with strata over an area.

Figure 2. Examples of 

a Stiff diagrams for four 

samples. Cations are 

plotted as concentrations 

(in milliequivalents/liter) to 

the left of the axis, and 

anions are plotted to the 

right of the axis. The 

anions should always 

be plotted in the same 

sequence. Connecting 

the resulting points reveals 

an irregular pattern, as 

shown. Water-quality types 

can be readily identified by 

the shape of the pattern 

(Hem 1989).
264

Piper Diagram. This plot is useful for showing multiple samples and trends 

in major ions (Hem 1989). The central diamond-shaped field is used to show 

the general character of the water, and ground-water types can be quickly 

discriminated by position within the field. These diagrams are usually poor 

graphical representations to plot on maps showing water quality over a 

large area, because they take up a large amount of space and render the map 

ineffective. But they aid in interpreting the mixing of waters from different 

aquifers, especially when used as support with other kinds of interpretations. 

The circles plotted in the central field have areas proportional to dissolvedsolids concentrations and are located by extending the points for the sample 

in the lower two triangles to the point of intersection in the diamond-shaped 

field (fig. 3). In the example below, the samples designated 15-1 in the lower 

triangles are plotted on the diamond-shaped field, extending rays parallel to the 

triangle axes, to the point of intersection. Distinct ground water classifications 

can be quickly discriminated by their position on the diamond-shaped field, as 

indicated in figure 4. 

Figure 3. Example of the Piper trilinear diagram for four samples (Hem 1989).
265

Pie Diagram. This is perhaps the most flexible method to show quality 

of water (fig 5). The radius of the circle is proportional to the total 

milliequivalents per liter. Pie charts can be conveniently plotted on base maps 

to show the ground-water quality for the point source; however, they are time 

consuming to construct, unless computer plotting software is used. 

 Figure 4. Subdivisions 

of the diamond-shaped 

field of the Piper diagram. 

Interpretations are made by 

the following descriptions 

of areas: area 1, alkaline 

earths exceed alkalies; 

area 2, alkalies exceed 

alkaline earths; area 

3, weak acids exceed 

strong acids; area 4, 

strong acids exceed weak 

acids; area 5, carbonate 

hardness (“secondary 

alkalinity”) exceeds 50 

percent (that is, chemical 

properties of the ground 

water are dominated 

by alkaline earths and 

weak acids); area 6, 

noncarbonated hardness 

(“secondary salinity”) 

exceeds 50 percent; 

area 7, noncarbonated 

alkali (primary salinity) 

exceeds 50 percent (that 

is, chemical properties 

are dominated by alkalies 

and strong acids; ocean 

water and many brines 

plot in this area, near its 

right-hand vertex); area 8, 

carbonate alkali (primary 

alkalinity) exceeds 50 

percent (ground waters 

that are inordinately soft in 

proportion to their content 

of dissolved solids plot 

here); area 9, no single 

cation-anion pair exceeds 

50 percent (Piper 1953).

Figure 5. Examples of pie diagrams to depict water quality. The radii length on the scale 

indicates the concentration (in milliequivalents/liter), and the area of the circle indicates relative 

total ionic concentrations compared with other samples. The subdivisions of the circles 

represent proportions of the various ions. The numbers above the circles indicate the particular 

sample, taken from a table of water-quality data from the study area. By selecting samples to 

plot in this manner, an overall characterization of the water quality can be shown (Hem 1989).
266

References 

Collins, W.D. 1923. Graphic representation of analysis. Industrial and 

Engineering Chemistry. 15: 394.

Gilbert, R.O. 1987. Statistical methods for environmental pollution 

monitoring. New York: John Wiley and Sons, 336 p.

Helsel, D.R.; Hirsch, R.M. 1992. Statistical methods in water resources. 

Amsterdam, the Netherlands: Elsevier, 529 p.

Hem, J. D. 1989. Study and interpretation of the chemical characteristics of 

natural water. Water-supply Paper 2254 (third edition). Washington, DC: U.S. 

Geological Survey. 263 p.

Piper, A.M. 1953. A graphic procedure in the geochemical interpretation of 

water analysis. Ground Water Note 12. Washington, DC: U.S. Geological 

Survey. 14p.

Sanders, T. G.; Ward, R.C.; Loftis, J.C.; Steele, T.D.; Adrian, D.D.; Yevjevich, 

V. 2000. Design of networks for monitoring water quality. Highlands Ranch, 

CO: Water Resources Publications, 336 p.

Sen, P.K. 1968. Estimates of the regression coefficient based on Kendall’s 

Tau. Journal of the American Statistical Association. 63: 1379-1389.

Smith, R.A.; Hirsch, R.M.; Slack, J.R. 1982. A study of trends in total 

phosphorus measurements at NASQAN stations. Washington, DC: U.S. 

Geological Survey Water-supply Paper 2190, 34 pp.

Stiff, J.A., Jr. 1951. The interpretation of chemical water analysis by means of 

patterns. Journal of Petroleum Technology. 3 (10): 15-17.

U.S. Environmental Protection Agency. 1992. Statistical analysis of ground 

water monitoring data at RCRA facilities: Addendum to interim final guidance. 

Washington, DC: U.S. Environmental Protection Agency, http://www.epa.gov/

epaoswer/hazwaste/ca/resource/guidance/sitechar/gwstats/gwstats.htm.
267

Appendix VII. 

Geophysics

Use of geophysics can substantially reduce costs and improve the success 

of ground water investigations. For example, use of surface and borehole 

geophysical methods can provide a first estimate of the extent of a contaminant 

plume, thereby reducing the need for large numbers of wells to define the 

plume and allowing the needed wells to be optimally placed.

Surface geophysical methods can be part of the geological mapping phase 

of the project. They can assist in the delineation of areal geology and the 

identification of shallow ground water conditions. They provide an indirect 

means of assessing a variety of hydrogeological conditions, including (1) 

physical properties of bedrock and unconsolidated materials, (2) delineation 

of subsurface lithology, (3) depth to the water table, and (4) quality of 

ground water. Surface geophysical methods are used to indirectly assess 

hydrogeological conditions and their possible controls on ground water. Three 

surface geophysical techniques are widely applicable to a variety of geologic 

settings, and are particularly useful in hydrogeologic studies. These techniques 

are electrical resistivity, electromagnetic conductivity, and seismic refraction 

and reflection. In addition, gravity and magnetic techniques are often useful 

in defining the geometry of geological structures in deep aquifers (Zohdy and 

others 1974, Bartolino and Cole 2002), and ground-penetrating radar has been 

used successfully to locate buried drums and to delineate detailed shallow 

subsurface stratigraphy and voids.

Electrical resistivity has been used effectively for near-surface geophysical 

studies for more than 50 years. The technique involves inducing an electric 

current into the ground through two electrodes and measuring the potential 

differences between two points on the ground with two or more additional 

electrodes. In essence, it is a measurement of the electrical resistance of the 

surficial material. This property is of great interest because the electrical 

resistance of the ground is related to the composition of the near-surface 

material, its porosity, the pore fluid conductivity, and the degree of saturation. 

It is used to determine lithologic changes or pore-fluid conductivity changes. 

For example, sands and gravels and fresh water typically have high resistivity 

values, but clays and contaminated water typically have low resistivity values. 

Success of this method depends primarily on the resistivity contrast between 

various geologic materials or the contrast between varying water qualities. 

Usually a resistivity contrast of twofold or threefold is needed to make a 

change in lithology, water quality, or hydrologic character detectable.

 

There are a variety of electrode configurations, but the most widely used are 

the Wenner and the Schlumberger configurations (fig. 1). The Wenner array 

uses four equally-spaced electrodes. The array consists of two current input 

Surface 

Methods
268

electrodes, C1 and C2, and two potential measurement electrodes, P1 and P2. 

The spacing between electrodes is usually referred to as the “a” spacing. The 

Schlumberger array differs from the Wenner array in the electrode spacings, in 

that the distance between a current electrode and the nearest potential electrode 

(a) is not equal to the distance between the two potential electrodes (b). Larger 

depths are penetrated by expanding the electrode array outward from the 

center. A more recently developed array configuration, the square array, is 

particularly useful for detecting fractures (Lane and others 1993).

The results must be used with caution because a resistivity sounding can 

have more than one interpretation. More than one combination of layers, 

layer thicknesses, and layer resistivities can produce the same geophysical 

response. The number of possible combinations that will fit the data decreases 

as independent geological data obtained from sources such as boreholes or 

outcrops reduces the number of options available for the geophysical model. 

For this reason, resistivity surveys must be interpreted by someone experienced 

in correlating geological conditions with the resistivity measurements. 

An emerging technology for resistivity applications is continuous-resistivity 

profiling (Lane 2004). Continuous-resistivity profiling is a water-borne 

electrical geophysical method that is used to measure the apparent resistivity 

distribution of a surface water sub-bottom. This method is especially suited for 

delineating regions of focused ground water discharge in the sub-bottom or in 

the near-shore environment. It locates the freshwater/saltwater interface in the 

sub-bottom and images electrical properties for hydrogeological mapping of 

the sub-bottom. Data are collected by towing an electrode streamer behind a 

boat. Data collection is fast and easy, but data processing and interpretation can 

be time consuming.

Electromagnetic (EM) techniques were originally developed for the exploration 

of base metals. The electromagnetic conductivity technique provides results 

that are similar to resistivity methods. In recent years, the technique has been 

applied to waste-site monitoring, particularly tracing conductive leachate 

Figure 1. Common 

electrode arrays for surface 

geophysical surveys 

(Rehm and others 1985).

Electromagnetic 

Methods
269

plumes. The EM survey provides results that are similar to resistivity 

measurements. The approach involves the definition of areal anomalies in 

electrical conductivity that can be related to known or assumed conditions in 

the area. The anomalies can be the result of changes in geology, hydrology, or 

ground water quality.

 

The EM conductivity method has some distinct advantages over resistivity 

techniques. The equipment need not make contact with the ground, and it is 

very portable, making measurement taking faster. Hence, this method is very 

good for reconnaissance profiling. The EM transmitter and receiver coils are 

either held above the ground at approximately waist level or placed on the 

ground (fig. 2) and measurements are made as the investigator traverses across 

the site. The method is generally only useful at relatively shallow depths. The 

Geonics EM-3134, currently the most portable instrument, can penetrate to 

depths of 10 to 20 feet; the Geonics EM-34 can penetrate to depths of 100 to 

200 feet, depending on whether the coil is oriented vertically or horizontally 

(Rehm and others 1985, McNeill 1980). The basic operating principle of the 

EM technique is illustrated in figure 3.

Electromagnetic survey methods have been in used in hydrogeological 

applications since the early 1980’s. The success of the technique has varied, 

depending on the type of application, but the method has identified leachate 

plumes at landfills, plumes of contaminated ground water at mine tailings 

sites, and water table locations at mine spoils sites (Rehm and others 1985). 

It should be noted, however, that the limited number of layers of contracting 

conductivity that can be resolved constrain the method to simple geological 

settings.

 

34 Mention of trade names is solely to identify equipment used and does not imply endorsement 

by the U.S. Department of Agriculture.

Figure 2. Conducting an EM survey, with the transmitter and receiver coils in the vertical 

orientation. (Photo by Patrick Tucci, USGS.)
270

Although seismic refraction methods generally have less resolution than 

seismic reflection methods, they have been preferred for use in shallow 

hydrogeological investigations for the following reasons (Zohdy and others 

1974): 

1. Refraction methods generally yield superior results in areas of thick 

alluvial or glacial fill and where large velocity contrasts exist, such as 

buried bedrock valleys.

2. Personnel and equipment requirements are generally simpler and less 

expensive for refraction surveys than for reflection surveys.

Seismic refraction techniques are designed to obtain data near the surface 

(typically to a depth of about 30 meters, but depths in excess of 200 meters can 

be achieved with more powerful seismic sources). Such techniques provide 

data on the refraction of seismic waves at the interface between subsurface 

layers and on their travel times within the layers. Properly interpreted, 

the refraction data make it possible to estimate the thickness and depth of 

geological layers (including the water table) and to assess their properties. 

Also, changes in the lateral facies of aquifer material can sometimes be mapped 

with this method (Sandlein and Yazicigil 1981)

The seismic refraction method relies upon measuring the transit time from 

energy source to receiver of induced vibrational energy refracted along some 

geological boundary (fig. 4) and assumes increasing seismic velocity with 

depth. Through analysis of the measured transit times as a function of sourcereceiver separation, it is possible, given certain assumptions, to determine 

the thickness and seismic velocity of all units beneath a selected sourceFigure 3. Operating 

principle of the EM 

technique. The transmitter 

coil (1) generates a 

magnetic field (2). This 

primary field induces a 

current (3) in a mass of 

conductive earth. The 

induced current in turn 

generates a secondary 

magnetic field (4). The 

receiver coil (5) senses 

both the primary and 

secondary fields. The 

conductivity of the earth 

is proportional to the ratio 

of the intensity of the 

secondary field to the 

intensity of the primary field 

(Rehm and others 1985).

Seismic 

Refraction 

Methods
271

receiver geometry. Based on the calculated velocities, it is then possible to 

infer lithology and physical characteristics of the area selected for analysis. 

It is possible that measured seismic velocities, when combined with known 

lithology, could be converted to porosity through a method similar to that of 

Wyllie and others (1958); however, it would probably fail for fine-grained 

sediments and, in any event, would require establishing empirical curves 

appropriate to the particular area.

The primary application of seismic surveys is to determine the depth to the 

bedrock surface or to map the elevation of the bedrock surface, because of the 

large velocity contrast between bedrock and unconsolidated overburden. In 

selected cases, refraction surveys could be employed to determine the watertable depth. Using seismic refraction methods to determine water-table depths 

is geophysically equivalent to the determination of bedrock depth. From a 

practical standpoint, the water table in a course-grained, unconfined aquifer 

could easily be detected; however, in a fine-grained, unconfined aquifer, the 

transition from saturated to unsaturated conditions is too poorly defined to 

be detected by refraction measurements. Haeni (1988) lists hydrogeological 

settings in which seismic-refraction surveys (1) can be used successfully; (2) 

may work, but with difficulty; and (3) will not work. 

Figure 4. Typical refraction 

paths in seismic refraction 

geophysics (Rehm and 

others 1985).
272

Note that the method assumes increasing velocity with increasing depth. 

The presence of a decrease in velocity at depth can lead to significant 

interpretational errors. The problem is compounded by the fact that there is 

no way to establish from the field data whether low-velocity units at depth are 

affecting the results. Hence, the presence of low-velocity units at depth must 

be determined from logged boreholes in the area. As in electrical methods, 

analysis and interpretation of seismic survey data should only be done by 

experienced personnel with knowledge of the local geology of the area being 

surveyed.

Seismic reflection methods are similar to refraction methods. An acoustic 

signal is generated near the earth’s surface, and the travel times of acoustic 

pulses reflected at contacts between various earth materials are measured.

 

Reflection surveys are generally employed to identify geological contacts 

at depths greater than 61 meters (200 feet). The resolution at these depths is 

approximately 3 meters (10 feet). The accuracy of depth determinations is 

limited by the uncertainty in the seismic velocities of the subsurface materials. 

Special equipment is available for surveys as shallow as 30 to 60 meters 

(100–200 feet), but refraction surveys are generally better suited than reflection 

surveys for use at shallower depths.

An exception to this rule is in the application of marine seismic-reflection 

techniques to ground water problems near surface water bodies (Haeni 1986). 

Detailed stratigraphic and structural information (fig. 5) can be obtained using 

seismic-reflection methods below lakes, rivers, and canals, where standard 

exploration methods cannot easily be used. The sound source and receivers 

(hydrophones) are towed by a boat, so that a great deal of data can be collected 

in a short time. 

Seismic 

Reflection 

Methods

Figure 5. Detailed stratigraphic 

data obtained below 

Annabessacook Lake, 

Winthrop, ME, using seismicreflection profiling.
273

The measurement of physical earth properties using equipment lowered into 

drilled holes is known as geophysical well logging, borehole logging, wireline 

logging, or downhole logging. This type of logging requires a single hole, 

and thus, differs from cross-hole logging (tomography) which requires a 

minimum of two holes. It also differs from mud logging, core logging, or the 

driller’s log in that no physical sample from the hole is required. Borehole 

logging involves an instrumentation package, known as the probe, sonde or 

tool, which is attached to a cable and lowered into the borehole. Normally, 

the probe measures or “logs” selected physical properties of the material in 

or near the borehole as the probe rises from the bottom of the hole. The log 

output typically consists of a plot of geophysical responses as a function of 

depth. Usually several geophysical logs are plotted simultaneously (fig. 6). The 

resulting downhole measurements are related to geological and hydrological 

conditions near the borehole. 

Borehole 

Methods

Figure 6. A typical display 

of several geophysical logs 

simultaneously displayed 

as output: Gamma log, 

spontaneous potential 

(SP) log, deep and shallow 

resistivity, bulk density, and 

porosity (Rehm and others 

1985).
274

Borehole geophysical methods have great utility in ground water studies. 

The objective of the borehole logging effort is to provide greater detail about 

subsurface conditions than would be available from surface geophysical 

methods, drilling cuttings, or discontinuous representative or undisturbed 

samples. These methods are generally employed in hydrogeological 

applications to help meet five broad objectives: 

1. To evaluate ground water quality.

2. To determine the depth to the water table.

3. To determine the depth to the bedrock surface.

4. To evaluate subsurface lithology.

5. To locate water-producing fractures.

Paillet and Crowder (1996) describe an approach for the interpretation of 

borehole logs in ground water studies.

Generally, the logging applications can be classified according to the 

parameters evaluated: measuring water quality, determining lithology, locating 

permeable zones, locating bedding planes and fractures, determining fluid 

velocity, or determining porosity. In addition to these applications, material 

resistivity and seismic velocity can be directly obtained to enhance the value of 

surface electrical or seismic measurements.

Electric logging methods form the largest single group of borehole logging 

techniques. Logging methods that determine electrical conductivity or 

resistivity in or near the borehole are the most widely used methods normally 

considered under the heading of electrical methods. Self-potential and induced 

polarization methods have more restricted applicability to hydrological 

investigations. Electrical conductivity or resistivity logs are conveniently 

divided into two classes: (1) those methods that employ electrodes in contact 

with borehole fluid and (2) those that rely on electromagnetic induction and 

require no contact with borehole fluid. 

 

Borehole resistivity methods are generally classified according to the number 

of electrodes required to make a measurement. Resistance logs, also known 

as single-point and single-electrode logs, involve a single downhole electrode 

(fig. 7a). This method provides a measure of the electrical resistance associated 

with current flow from a point in the borehole to a point on the surface near 

the borehole. The log is used primarily to define contacts between materials 

of differing electrical properties. The primary advantage of the log is the very 

simple instrumentation requirements and therefore low equipment cost (Rehm 

and others 1985). 

Normal logs, also known as two-electrode logs, use an electrode configuration 

like the one shown in figure 7b. The tool is usually described by the distance 

between electrodes A and M. The petroleum industry has standardized twoprobe configurations: the short-normal at 0.41 meter (16 inches) and the 

Electrical 

Methods
275

long-normal at 1.63 meters (64 inches). For shallow hydrogeological studies, 

the distances do not appear to be standardized, but are generally within ± 50 

percent of the above. The radius of investigation is approximately twice the 

electrode separation, or 1 meter (3 feet) for the short-normal and 3 meters (10 

feet) for the long-normal (Rehm and others 1985).

The third type of resistivity log is the lateral log (fig. 7c). This log was 

introduced to petroleum logging to obtain the resistivity of the formation 

beyond the zone affected by drilling fluid. The tool is normally described by 

the spacing between the A and N electrodes, which in the petroleum industry 

is standardized at 5.69 meters (18.67 feet). In hydrological logging, there is 

apparently no standard, but approximately 1.8 meters (6 feet) is common. 

The radius of investigation for this log is approximately equal to the spacing 

between electrodes A and N (Rehm and others 1985).

Another type of electrical log, the micro or sidewall log, commonly employs 

both small-lateral, AN = 0.38 meter (1.5 inches), and small-normal, AM = 

0.051 meter (2 inches), electrode spacings. The electrodes are carried in a pad, 

which must be held in contact with the sidewall. These logs are measuring 

properties within a few centimeters of the borehole, and thus are only used 

where detailed hydrologic information is needed (Rehm and others 1985).

Induction logs rely on electromagnetic (EM) radiation from the tool to induce 

secondary currents in the formation near the tool. The magnitude of these 

currents is then detected by a receiver within the tool. The induction methods 

will, therefore, operate in oil- or air-filled boreholes and in the presence of 

Figure 7. Electrode arrays for borehole logs: (a) resistivity log, (b) normal log, and (c) lateral log 

(Rehm and others 1985).
276

polyvinyl chloride (PVC) well casing. Highly conductive borehole fluids and 

steel casing, however, prevent the use of induction methods. The tools respond 

directly to the inverse of formation resistivity or the formation conductivity and 

are, therefore, known as conductivity logs (Rehm and others 1985).

From a hydrological standpoint, the primary advantage of an EM induction 

tool is its ability to log through PVC casing or in air-filled holes. EM induction 

logs, in combination with other logs, are particularly useful in the delineation 

of contaminant plumes that may be constrained to discrete intervals in the 

subsurface (fig. 8) (Williams and others 1993). EM induction probes are 

readily available and are commonly replacing normal-resistivity logs in ground 

water investigations.

SP is the potential associated with natural current flow within and near the 

borehole. The SP log is a measurement of these potentials over the length of 

the borehole. The SP voltages result primarily from conductivity differences 

between the drilling fluid and formation waters or from actual flow of drilling 

fluid into the formation. The former are known as electrochemical potentials 

and the latter as electrokinetic, or streaming, potentials (Rehm and others 

1985).

The SP log is used primarily for determining the contacts between materials 

with different electrical properties, differentiating between permeable and 

non-permeable materials (such as sand vs. clay) and determining formationwater resistivity. In hydrological investigations, the formation waters and 

drilling fluid will probably display little difference in resistivity. Under these 

Figure 8. Electromagneticinduction log delineates 

a leachate plume in a 

sand-and-gravel aquifer 

downgradient of a 

municipal landfill. The 

most highly contaminated 

part of the plume is at a 

depth of 41 to 45 meters 

(Williams and others 

1993).
277

conditions, the electrochemical potential will be small or zero and, hence, of 

limited value. Streaming potentials may be generated as ground water flows 

from aquifers into the borehole or as borehole fluids flow into permeable 

materials (Rehm and others 1985). 

Nuclear logs are the second largest subset of logging technology. While many 

special purpose nuclear logs exist, the most common and widely applied 

nuclear logs are the natural gamma, the gamma-gamma, and the neutron 

log. These are all discussed in detail by Pirson (1963), Kelly (1969), Hilchie 

(1982), and Keys and MacCary (1976). The specific application of nuclear 

logging methods to hydrological problems is reviewed by the International 

Atomic Energy Agency (1968).

Natural-gamma logs measure the naturally occurring gamma radiation along 

the length of the borehole. The log can be obtained in cased or uncased holes, 

and in air- or fluid-filled holes. These logs may be taken in a broad energy 

window (natural-gamma logging) or in several narrow energy windows 

(spectrometric-gamma logging). Most radioactive elements are associated 

with clay minerals. The natural-gamma log is, therefore, primarily a claylithology log. The spectrometric-gamma log is intended primarily as a uranium 

exploration tool. The primary purpose of a natural-gamma log in the context 

of hydrogeological investigations is to identify clay layers penetrated by 

boreholes. 

Gamma-gamma logs, also known as density logs, measure the effect of 

material near the borehole on gamma radiation emanating from a source 

within the logging tool. While the effect can be measured in various ways, the 

measurements all relate to the electron density of material near the borehole. 

Because electron density is related to bulk density, the log is basically a bulkdensity log. If the densities of the rock matrix and pore fluid are known, the 

bulk density may be converted to porosity. In either case, the radius to which 

the measurement extends is approximately 0.3 meter (1 foot) in open holes and 

0.15 meter (0.5 feet) in cased holes. 

Neutron logs measure the response of material near the borehole to neutrons 

emitted from a source within the tool. The response may be measured in 

various ways, but all common measurements are related to the presence of 

hydrogen in the formation. Excluding potential problems with bound water, 

the response of the instrument is determined entirely by the amount to water in 

the porous medium. Hence, the tool is primarily a porosity log. The radius of 

influence for this tool is approximately 0.3 meter (1 foot) in open holes and a 

few centimeters in cased holes. 

Both gamma-gamma and neutron geophysical probes contain radioactive 

sources, whose use is regulated by the U.S. Nuclear Regulatory Commission 

as well as various State agencies. Special permits are needed to store and 

use these sources, and extensive training in their safe use is required. If these 

Nuclear Methods
278

types of logs are needed, it is best to contract with a commercial well-logging 

company. Natural-gamma logs do not require nuclear sources, and their use is 

not regulated.

Flow logs measure fluid movement within the borehole as a function of depth. 

Flow logs fall into two classes: (1) those that directly measure fluid motion 

with a mechanical impeller and (2) those that measure fluid motion indirectly 

by measuring heat flow away from a thermal source or by electromagnetic 

methods. More recent designs for borehole flowmeters include acousticdoppler and optical methods. The direct measuring devices are primarily 

designed for use in well production and well completion problems, and are 

useful for relatively large flow rates. The indirect measuring devices are 

better suited to lower flow rates that cannot be measured by direct measuring 

devices (< 2 m/min), but they require flow rates greater than 0.03 m/min (U.S. 

Geological Survey 1998). 

Flow measurements can be useful in determining vertical direction of ground 

water flow (up or down), flow from a particular zone of interest (such as a 

fracture), or the interaction between vertically connected aquifers. They also 

can measure the change in vertical flow as a function of depth. Methods have 

recently been developed to use borehole flowmeter data to estimate hydraulic 

conductivity in fractured-rock aquifers (Paillet 1998)

Several other types of borehole logs can also provide important information for 

ground water investigations.

•	 Caliper logs provide information on the diameter of the borehole, 

and can be used to indicate large open fractures within the borehole 

or screened intervals in cased wells. They can also be used to locate 

constrictions in the borehole that could prevent use of large-diameter 

probes or “washouts” in the borehole that could influence interpretation 

of other logs. Caliper logging is generally fairly inexpensive, but can 

only be used in open boreholes in bedrock.

•	 Acoustical logs provide information on the velocity of sound waves in 

the formation. Such information is useful in interpreting surface seismic 

surveys, and can provide indirect information on formation density. 

Special acoustical tools, called televiewers, provide images that indicate 

the size and orientation of fractures in the borehole (fig. 9). Recent 

advances in televiewer logs allow processing of the data to produce 

“virtual cores” from the data.

•	 Fluid-conductance and temperature logs are useful in obtaining 

information on zones where water enters or leaves the borehole, or for 

locating zones of high electrical conductivity ground water.

•	 Borehole radar is a relatively new, but expensive, technique that is 

particularly useful in detecting subsurface fractures. The technique can 

often see well beyond the immediate borehole, and can be used in a 

cross-hole technique to determine interconnected fractures. 

Flow Logs

Other Borehole 

Logs
279

•	 Borehole television cameras (optical televiewers) or other optical 

imaging devices are readily available, and are useful in detecting 

fractures (fig. 9) and conditions in the well or borehole prior to sending 

down more expensive probes.

Figure 9. Acoustic and 

optical televiewer images 

of a transmissive zone in 

a borehole (after Williams 

and others 2002).

An integrated surface- and borehole-geophysical approach has been termed 

the “toolbox” approach (Haeni and others 2001). It is particularly useful 

in fractured-rock hydrological settings commonly found in the national 

forests. Surface geophysical methods provide site reconnaissance suitable 

for the development of initial conceptual models of ground water flow in the 

formation and location of test holes. Conventional borehole-geophysical logs, 

borehole imaging, and advanced single- and cross-hole geophysical methods 

can be interpreted to identify the location and physical characteristics of 

fractures, and, potentially, their hydraulic properties. Integration of surfaceand borehole-geophysical data with geological, hydrological, and geochemical 

data provides a means for developing a comprehensive interpretation of 

the hydrogeological conditions at a site and a conceptual understanding of 

ground water flow (Haeni and others 2001). It is important to recognize that 

the implementation of a comprehensive integrated approach will be time 

consuming and resource intensive, making its application appropriate only in 

select circumstances.

Integrated 

Methods
280

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