U.S. Department of the Interior

U.S. Geological Survey

Circular 1293

Research Opportunities in Interdisciplinary 

Ground-Water Science in the U.S. Geological Survey
Cover.  Four images of different sections of the San Pedro River basin from a thermal infrared multispectral scanner (TIMS) used to calculate 

evapotranspiration and recharge at land surface. (The colors represent reflected infrared (thermal) wavelengths.) (Images provided by National 

Aeronautics and Space Administration/Advanced Spaceborne Thermal Emission and Reflection Radiometer Science Team)
Research Opportunities in Interdisciplinary 

Ground-Water Science in the U.S. Geological Survey

By W.E. Sanford, J.S. Caine, D.A. Wilcox, H.C. McWreath, and J.R. Nicholas

Circular 1293

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2006

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Suggested citation:

Sanford, W.E., Caine, J.S., Wilcox, D.A., McWreath, H.C., and Nicholas, J.R., 2006, Research opportunities in interdisciplinary ground-water science in the U.S. Geological Survey: U.S. Geological Survey Circular 1293, 21 p.

Library of Congress Cataloging-in-Publication Data

Research opportunities in interdisciplinary ground-water science in the U.S.

Geological Survey / by W.E. Sanford . . . [et al.].

 p. cm. — (Circular ; 1293)

 Includes bibliographical references.

 ISBN 1-4113-0958-8 (alk. paper)

 1. Groundwater—Research—United States. 2. Hydrogeology—Research—United

States. 3. Geological Survey (U.S.) I. Sanford, Ward E. II. U.S. Geological

Survey circular ; 1293.

 GB1002.5R47 2006

 551.49072'073—dc22

2006004647
iii

Preface

This report is written for the scientifically literate reader but is not limited to those who are 

involved in ground-water science. The report is intended to encourage U.S. Geological Survey 

scientists to develop a sense of excitement about ground-water science in the agency, to inform 

scientists about existing and potential ground-water science opportunities, and to engage 

scientists and managers in interdisciplinary discussions and collaboration. The report is intended 

for use by U.S. Geological Survey and Department of the Interior management to formulate 

long-term ground-water science programs and to continue sustained support of ground-water 

monitoring and research, some of which may not have an immediate impact. Finally, the report 

can be used to communicate the U.S. Geological Survey’s vision of ground-water science to 

Congress, partners, other agencies, and the research community at large with the goals of 

enhancing collaborative opportunities, sharing information, and maintaining dialogue regarding 

the directions of U.S. Geological Survey ground-water science.
iv

Acknowledgments

The authors express appreciation to the following for their guidance and review of this report: 

Bill Van Liew and Louise Liew of the National Park Service; and William M. Alley, Mary Jo 

Baedecker, Paul M. Barlow, William C. Burton, Kevin F. Dennehy, Norman G. Grannemann, 

Tien Grauch, Mary C. Hill, Randall J. Hunt, Kenneth J. Lanfear, Randall C. Orndorff, David L. 

Parkhurst, Herbert A. Pierce, Allen M. Shapiro, Edwin P. Weeks, Thomas C. Winter, and Chester 

Zenone of the U.S. Geological Survey.
v

Contents

Preface ...........................................................................................................................................................iii

Acknowledgments........................................................................................................................................iv

Introduction.....................................................................................................................................................1

Ground-Water Research Opportunities .....................................................................................................1 

Mapping in Three Dimensions—Geology and Geophysics ..........................................................2

Issues ...........................................................................................................................................2

Current State ...............................................................................................................................3

Research Opportunities .............................................................................................................4

Characterizing Aquifer Heterogeneity .............................................................................................4

Issues ...........................................................................................................................................4

Current State ...............................................................................................................................5

Research Opportunities .............................................................................................................6

Measuring Recharge to Ground-Water Flow Systems .................................................................8

Issues ...........................................................................................................................................8

Current State ...............................................................................................................................8

Research Opportunities .............................................................................................................8

Quantifying Ground-Water and Surface-Water Interaction ......................................................10

Issues .........................................................................................................................................10

Current State .............................................................................................................................11

Research Opportunities ...........................................................................................................11

Understanding the Relation of Ground-Water Flow Systems to Aquatic Ecosystems ..........13

Issues .........................................................................................................................................13

Current State .............................................................................................................................14

Research Opportunities ...........................................................................................................14

Developing More Effective Computer Models ..............................................................................16

Issues .........................................................................................................................................16

Current State .............................................................................................................................16

Research Opportunities ...........................................................................................................17

Summary .......................................................................................................................................................19

References Cited .........................................................................................................................................21

Figures

1. Three-dimensional (3–D) geologic framework model of Yucca Mountain, Nevada .........3

2. Three-dimensional (3–D) images from seismic surveys of the speed of shock waves 

through sediment ..........................................................................................................................5

3. Photograph of an alluvial deposit near Sacramento, California, that exhibits 

heterogeneity and vertical anisotropy where a 1- to 2-meter-thick, silty-clay 

layer at the ground surface is underlain by sand, sandy gravel, 

and gravelly sand layers ..............................................................................................................6

4. Simulated values of average annual net ground-water recharge in Southeastern 

Florida .............................................................................................................................................9

5. Photograph of USGS scientists installing instrumention in an ephemeral stream 

at Abo Arroyo, New Mexico, to monitor streamflow and calculate recharge using 

temperature as a tracer .............................................................................................................11
vi

6. Remote sensing image showing cold (blue) temperatures that reveal 

ground-water discharge to the Umatilla River in Oregon, August 10, 1998 ......................12

7. Four images of different sections of the San Pedro River basin from a thermal 

infrared multispectral scanner (TIMS) used to calculate evapotranspiration and 

recharge at land surface............................................................................................................13

8. Photograph of the Texas Blind Salamander, Typhlomolge rathbuni, which lives 

only in the caves of the Edwards aquifer, an important source of water, and is 

listed as endangered by both the State of Texas and the Federal government ...............15

9. Image showing rankings based on sensitivities of water-level observations and 

future forecasts of radionuclide transport paths to parameters of a regional 

model around Yucca Mountain, Nevada ................................................................................17

Conversion Factors for Units of Measure in this Report

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square kilometer (km2) 0.3861 square mile (mi2

)

Inch/Pound to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Flow

gallon per day (gal/d) 0.003785 cubic meter per day (m3/d) 
Introduction

The U.S. Geological Survey (USGS) has a longstanding reputation for providing unbiased scien­tific 

leadership and excellence in the field of ground-water 

hydrology and geological research. This report provides 

a framework for continuing this scientific leadership 

by describing six interdisciplinary topics for research 

opportunities in ground-water science in the USGS. These 

topics build on recommendations of the National Research 

Council (2000) contained in the report, “Investigating 

Groundwater Systems on Regional and National Scales,” 

and emphasize research topics that would benefit from 

the integrated capabili­ties of all parts of the USGS. 

Understanding the relations between ground water and the 

geological characteristics of aquifers within which ground 

water resides, and the relation of ground water to surfacewater resources and terrestrial and aquatic biota is increasingly important and presents a considerable opportunity 

to draw on expertise throughout the USGS, including the 

science disciplines of biology, geography, geology, and 

hydrology. The National Research Council (2000) also 

emphasizes that USGS regional and national assessments 

of ground-water resources should focus on aspects that 

foster the sustainability of the resource. The need for a 

comprehensive program address­ing the sustainability of 

ground-water resources can be stated very concisely—we 

need enough ground water of good quality to sustain our 

lives, our economy, and our aquatic ecosystems. 

Although societal needs for high-quality, objective 

ground-water science are increasing, current funding for 

USGS regional ground-water programs is about 40 percent 

of the funding available 20–25 years ago. Given the 

current challenges of budgetary con­straints, however, this 

report provides a flexible set of interrelated research topics 

that enhance the ability of the USGS to focus limited fiscal 

resources on developing ground-water science tools and 

methods that provide high-quality, objective scientific 

informa­tion. 

Ground-Water Research Opportunities 

The questions that traditional earth scientists generally 

ask are different from those typically asked by the society we 

serve. Scientists ask 

“What is the current state of the resource?” 

“What processes have created this state?” 

“How have these processes operated in the past?” 

Scientists tend to focus on the present, the past, and the 

processes. 

Society generally wants to know about the future—the 

present, the past, and the processes are of interest primarily as 

they relate to the future. Nearly every question about ground 

water from a private citizen, a municipal planning agency, or a 

state regulatory agency begins with “What will happen if...?” 

The “if” leads to a stressor, such as increased water withdrawals, the relation of multiyear droughts to long-term climate 

change, effects of urbanization, or land-surface contamination. 

Society’s focus requires that the USGS emphasize sciencebased forecasting. The USGS must rely on its knowledge of 

the present, the past, and the processes to provide sciencebased forecasts based on the effects of stressors yester­day, 

today, and tomorrow. 

Ground-water resources in many parts of the United 

States are severely stressed by human activity. For example, in 

the southwestern part of the Nation, large withdrawals result 

in land subsidence and water shortages. In coastal areas and 

some inland areas, withdrawals cause saline water to intrude 

aquifers that formerly contained freshwater. Throughout the 

Nation, even in water-rich areas, ground water is subject to 

competing uses where water is insufficient to meet the needs 

of everyone. Septic tanks, land­fills, applications of agricultural 

chemicals, disposal of chemical and radioactive wastes, and 

other land uses and activities intro­duce chemicals to ground 

water, potentially restrict the use of ground water for drinking, 

and contaminate aquatic ecosystems in streams, lakes, and 

wetlands. Furthermore, responses to natural stresses, such as 

droughts when surface-water supplies often are supplemented 

with ground water, reduce the availability of ground water for 

future uses.

•

•

•

Research Opportunities in Interdisciplinary 

Ground-Water Science in the U.S. Geological Survey

By W.E. Sanford, J.S. Caine, D.A. Wilcox, H.C. McWreath, and J.R. Nicholas
Advances in the six interdisciplinary topics for research 

opportunities in ground-water science described in this report 

could greatly improve our ability to forecast ground-water 

availability to sustain human use and support aquatic ecosystems. Briefly, these topics are as follows:

Improved understanding of the geologic framework 

of ground-water systems through the development of 

three-dimensional (3–D) mapping and visualization 

tools and new applications of geophysical methods;

Improved characterization of aquifer heterogeneity in 

unconsolidated aquifers, fractured aquifers, and karst 

aquifers;

Improved estimations of recharge, with emphasis on 

estimates that bridge local and regional scales;

Improved abilities to measure ground-water and 

surface-water interactions, which include both inflows 

to and outflows from ground-water systems;

Improved understanding of the relation between ground 

water and aquatic ecosystems; and

Improved computer mod­els of ground-water flow and 

transport, with emphasis on developing and applying 

new inverse methods and uncertainty analysis.

The first two topics build on the integra­tion of waterresources, mapping, geological, and geophysical characterization research. The next three topics emphasize the importance 

of quantifying the interactions of ground water with the 

biosphere and integrat­ing biological and water-resources 

research. The sixth topic provides a vital link between processbased understandings developed through the other topics and 

the goal of improved forecasting. 

The six ground-water research topics discussed in this 

report are not intended to be comprehensive, nor do they cover 

all aspects of ground-water science that will be emphasized 

by the USGS. These topics focus instead on the need for 

sub­stantial long-term, interdisciplinary commitment to make 

the scientific advances that will provide the information 

needed to support sustainable use of the Nation’s ground-water 

resources.

The USGS has a central role in ground-water research 

among Federal agencies and is the sole Federal agency with 

the mission to con­duct nationwide assessments of the status 

and trends of the surface and subsurface components of the 

hydrologic cycle. Federal involve­ment in the assessment of 

ground-water resources for the Nation is needed to (1) study 

complete aquifer systems that extend beyond state and international boundaries, because water behaves independently of 

political jurisdictions; (2) ensure nation­ally consistent methods 

of data collection; (3) use and improve the exist­ing monitoring 

infrastructure; and (4) provide basic research and understanding of the Nation’s valuable ground-water resources. The 

Federal role is to provide scientific information for states and 

•

•

•

•

•

•

local agencies to use in manag­ing water resources. Likewise, 

a Federal role is needed to support the management of large 

portions of land by the Bureau of Land Management, U.S. 

Fish and Wildlife Service, National Park Service, U.S. Forest 

Service, and Department of Defense, and of the land con­tained 

within Indian Reservations administered by the Department of 

the Interior. Several of the topics identified in this report (for 

example, those addressing karst resources and the effects of 

ground water on biological resources) are of special concern 

to other Department of the Interior agencies. The research 

opportunities emphasized in this report are of interest to 

university scientists, other agencies, and private organizations. 

Thus, we see many potential partners in advancing knowledge 

in these topics. 

This publication is intended primarily for persons with 

a scientific or technical back­ground in the natural sciences, 

although we attempted to make the report of interest and 

accessible to a wide audience. The research opportunities 

contained in this report build on an enormous amount of 

previous work by sci­entists within and outside the USGS. For 

the sake of brevity, we did not attempt to review the literature 

in the fields discussed, and litera­ture citations are used only to 

support particular statements. 

Mapping in Three Dimensions—Geology and Geophysics

Issues

The distributions of lithology and geologic structures in 

three dimensions are major con­trols on ground-water flow and 

solute trans­port. These physical features, however, usually are 

complex, difficult to characterize, and even more difficult to 

visualize. These features also are linked closely to the overall 

aquifer heterogeneity, the second research topic pre­sented 

in this report. Consequently, to understand a ground-water 

flow system fully, the geologic frame­work must be well 

understood and incorpo­rated into hydrogeologic analyses and 

ground-water flow models. As the demand increases for more 

accurate forecasts of the quantity and quality of ground-water 

resources, it will be necessary to more accurately characterize 

the geologic framework. 

Geologic mapping constitutes the basic foun­dation for 

modern 3–D geologic models. These models are excellent 

tech­nological tools for representing the natural sys­tem by 

assembling and integrating diverse geologic, geophysical, 

geochemical, and hydrogeologic information. A critical 

feature of modern 3–D geologic models developed for process 

modeling is the representation of uncertainties associated with 

the geometries and physical properties throughout the model 

domain and the ability to gener­ate multiple scenarios or realizations. Geo­logic mapping and field-based characterization; 

permeability of host-rock and fault-related materials; in-situ 

borehole and regional geophysical data acquisition; map­ping 

of mineralized and hydrothermally altered rock; cross-section 

construction; and fracture-network data collection, analyses, 

 Research Opportunities in Interdisciplinary Ground-Water Science in the U.S. Geological Survey
and modeling are all used to develop 3–D geologic framework 

models. The effects of lithologic and structural features on 

aquifers and the uncertainties involved are investigated in a 

variety of geologic settings and at a variety of scales ranging from local concerns about a community ground-water 

supply to regional concerns, such as understanding nat­ural 

as opposed to mining-related acid and metal loads to surface 

water, ground water, and aquatic ecosystems. Geochemical, 

environmental tracer, and ground-water age data also are used 

in combination with the above information to develop the 

numerical and predictive models and modeling strategies for 

ground-water studies. Much of these data also are integrated 

effectively and explored in sophisticated geographic information systems (GIS) that allow a wide range of users to query 

multidisciplinary data and gen­erate new and theme-specific 

earth-science maps.

Geophysical methods are critical for improv­ing 

knowledge of subsurface geologic struc­ture and lithologic 

heterogeneities at both local and regional scales. The methods 

provide information in both horizontal and ver­tical dimensions and information directly related to 

hydrogeology and water quality, which 

can be used to improve knowledge about 

con­ditions between wells, extrapolate 

information to areas away from wells, or 

provide a mapped view of a large region, 

such as a watershed or ground-water 

basin. Developing new applications of 

ground-based and airborne methods and 

a better understanding of the relations 

between physical and hydraulic properties 

are keys to extrapolating knowledge of 

the third dimen­sion over wide areas and 

directly importing this information into 

ground-water models.

Current State

Developing a useful 3–D geologic 

model depends on the use of powerful 

computer hardware, sophisticated software, and the geo­science tools required 

to integrate diverse data into a coherent 

3–D spatial framework. Mod­ern computer 

systems handle many complex modeling 

applications and are evolving rapidly in 

both speed and storage capacity. Sophisticated software programs are available for 

assembling, manipulating, and visualizing 

3–D geologic maps and models. Driven 

largely by the needs of petroleum and 

GIS industries, these programs continue 

to evolve and stay ahead of the needs of 

water-resource investigations. Methods 

and tools for developing 3–D geologic 

models currently are in the early stages of 

development, despite the useful knowledge gained from the petroleum industry, 

but developers of 3–D hydrogeologic 

models face somewhat different challenges 

and much additional work.

One of the challenges of 3–D 

modeling involves improving the scope 

and utility of geophysical methods that 

can be applied to hydrogeologic investigations. A variety of geophysical methods 

Figure 1. Three-dimensional (3–D) geologic framework model of Yucca Mountain, 

Nevada (from Potter and others, 2004).

Data used for 3–D framework modeling include geologic maps and cross 

sections, regional structural and lithostratigraphic facies analyses, geophysical 

investigations, and lithologic data from boreholes. This block diagram illustrates 

the geometry of principal structural elements of Yucca Mountain, Nevada. 

Cross sections, fence diagrams, and block diagrams are traditional tools that 

geologists use to portray 3–D relations of geologic units and develop a conceptual 

understanding of the system to be modeled.

116¡ 27' 116¡ 29' 116¡ 23' 116¡ 25'W

36¡ 52'

36¡ 50'N

36¡ 54'

Ground-Water Research Opportunities    
are available that can be designed for specific applica­tions, 

particular scales of investigation, and wide cost ranges. The 

types of information these geophysical methods provide 

include characterizing lithology (such as rock type and grain 

size), determining the boundaries between hydrogeologic 

units, locating faults that partition units or control groundwater flow, estimating fracture den­sity, mapping hydrothermal 

alteration or ther­mal waters, and detecting variations in 

salinity. Some geophysical methods, such as certain borehole 

and shallow-looking surface and marine geophysical methods, 

have become well accepted in ground-water applications. 

Other methods, such as airborne geophysics, are new to 

ground-water applications, and their full potential has not 

yet been developed. Whereas borehole and shallow-looking 

surface geophysical methods are best for detailed stud­ies, the 

airborne methods present new opportunities for efficiently 

and economically characterizing large regions and obtaining 

information over areas with poor accessibility. This type of 

information is essential for developing regional 3–D geologic 

and ground-water flow models.

Research Opportunities

Research and development are needed to bring 3–D 

capabilities for geologic modeling to a level that can be integrated routinely with hydrologic models. Methods are needed 

to (1) quantitatively and probabilisti­cally extrapolate surficial 

geologic and hydro­geologic information into the subsurface; 

(2) develop 3–D geologic models of alluvial basins from data 

typically available in most areas (water wells, drillers’ logs, 

surficial geol­ogy maps, gravity surveys, aeromagnetic sur­veys, 

and geotechnical well data and sampling); and (3) define 

and represent uncertainties in the geometries and properties 

continuously throughout the rock matrix. New geologic 

mapping strategies and methods also are needed during the 

course of routine geologic mapping studies to acquire data 

important to hydrologic models. Ideally, these strategies and 

methods will directly use hydrologic data and information as 

constraints in 3–D geologic mapping.

The abil­ity to determine the relation of geophysical 

responses to hydraulic properties (hydraulic conductivity, storage, or hydrologic changes) and the means to quantitatively 

incorporate this geophysical information and interpretation 

into hydrologic models are needed. Research opportunities in 

the application of geophysics include the continuing improvement and evolution of geophysical data acqui­sition and 

interpretation methods specific to ground-water issues. 

Improvements in geophysical methods that result from 

advancements in mathematical algorithms, visualization, 

and instrumentation are needed for ground-water studies. 

The investigation of geophys­ical responses to variations 

in hydrogeologic properties entails determining relations 

between physical properties and hydrogeol­ogy, observing the 

geophysical responses to physical property variations related 

to hydrogeology, and the difficulty of extending local-scale 

observations to a regional scale. Finding a relation between 

geophysical responses and hydraulic conductivity involves 

understanding which variables affect both and developing 

means to determine the variables that affect only one. By 

observing changes in geophysical responses over time, for 

example, one might be able to isolate the effects of changing 

water storage or track water-quality changes over time, such as 

incursions of saline water into coastal aquifers.

Important advancements in modeling capabili­ties, 

such as coupling 3–D seismic and other geophysi­cal data 

and interpretation with hydraulic parameterization routines, 

will be needed for future ground-water models. A set of 

techniques that has proved valuable for modeling sedimentary 

facies architec­ture in hydrocarbon reservoirs is 3–D seismic 

and stochastic modeling. During the last decade, petroleum 

companies have used 3–D seismic techniques to effectively 

characterize the 3–D geometries of sedimentary facies at the 

reservoir scale. This requires a great deal of data collection, 

and for typical petroleum-reservoir scales, the costs easily can 

be in the millions of dollars for a single survey. Hydrogeologists are just beginning to demonstrate that these techniques 

have potential for addressing environmental and ground-water 

issues. Successful test cases have been run in glaciated and 

alluvial fan settings where contrasts in sediment type are great. 

The techniques also have been shown to work with seismic 

cross-hole tomography. Techniques applied to shallower, more 

local systems than oil reservoirs are being developed along 

with an improved understanding of how to interpret the signals 

and results of model simulations. Hydrogeologists are increasingly more aware that to successfully model an aquifer system, 

equal attention must be paid to using scale-appropriate values 

of permeability and to correctly defining the geometry of 

permeability zonation, such as exists in sedimentary formations. Three-dimensional seismic techniques are being used 

to help define sedimentary zonation on the basis of acoustical 

properties, which are related in part to hydraulic properties. 

Characterizing Aquifer Heterogeneity

Issues

 Despite the importance of knowing the hetero­geneity of 

an aquifer, adequately characterizing it still remains elusive. 

The nature of aquifer heterogeneity varies with rock type, and 

different methods will be necessary to improve our abilities 

to characterize rock types. About half of all the ground water 

used in the United States is extracted from unconsolidated to 

semiconsoli­dated deposits of sand and gravel. Some areas of 

the Nation rely solely on ground water from fractured-rock 

aquifers. In addition, many communities that currently rely on 

surface water and ground water from shallow, uncon­solidated 

aquifers for water supply are evaluating the potential for 

bedrock aquifers to augment exist­ing supplies. 

A key requirement for assessing the sustain­ability of 

ground-water systems and their vulnerability to contamination 

is the spatial distribution of water-transmissive proper­ties. 

 Research Opportunities in Interdisciplinary Ground-Water Science in the U.S. Geological Survey
These properties commonly are highly variable spatially 

as a result of aquifer hetero­geneity. Aquifer heterogeneity 

arises from the complex geologic history of the host rock 

or sediment. Karst terrains are characterized by extreme 

heterogeneity and susceptibility to rapid contamination from 

the land surface. Characterizing the geology as a means of 

characterizing heterogeneity was discussed in the first topic of 

this report. The extremely wide range (more than 10 orders of 

magnitude) of hydraulic conductivity that commonly occurs 

in ground-water systems leads to the distribu­tion of hydraulic 

conductivity being the controlling factor in the move­ment 

of ground water and contaminants. The representation of 

hydraulic characteristics of aquifers in models is often sufficient to predict water-level changes in response to stresses, 

but the same type of representation rarely, if ever, is adequate 

to determine the flow paths and travel times necessary for 

contaminant transport studies.

Current State

Our current ability to characterize the hydrau­lic properties of unconsolidated sedimentary deposits and the distribution of these proper­ties is generally adequate for making 

predictions for water-supply purposes. To predict contaminant 

migration, however, a much more detailed description of 

the 3–D pattern of sediment properties is needed. Field 

studies have shown that, although the extent and movement 

of contaminant plumes in relatively homogeneous deposits 

can be delineated and are predictable to a reasonable degree, 

plumes in heterogeneous depos­its are more unpredictable. 

The heterogeneity of clastic sedimentary aqui­fers is determined primarily by depositional and burial-related diagenetic 

processes. Under­standing the depositional environments and 

processes is an important fac­tor in better characterizing these 

aquifers. Aquifer heterogeneity usually can be charac­terized 

by zones, or facies, that are determined to a large degree by the 

Figure 2. Three-dimensional (3–D) images from seismic surveys of the speed of shock waves through 

sediment (from Hyndman and others, 2000).

The speed of waves is controlled partly by the compressibility of the sediment, which is related to the hydraulic 

conductivity. Therefore, it may be possible to use seismic images to better map heterogeneity in unconsolidated 

aquifers.

Ground-Water Research Opportunities 
environment at the time of sediment deposition. These facies 

distributions can vary widely between, for example, riverine, 

coastal, or gla­cial environments. Some attempts have been 

made to use this type of geologic information in creating zones 

for hydraulic properties in ground-water models. Geostatistical methods have been used to create parameter fields that 

have statistical properties similar to the 

sedimentary units of interest. 

The heterogeneity of fractured-rock 

aquifers is determined by the distribution, 

aperture, and connectivity of fractures. 

These factors are determined, to some 

degree, by the tectonic history of the 

rock and the present-day stress field. 

Furthermore, the distribution of hydraulic 

con­ductivity and storage properties 

of fractured-rock aquifers can be 

substantially more vari­able than those of 

most unconsolidated aqui­fers. Borehole 

investigations (for example, geophysical 

logging, hydraulic and tracer tests) have 

been one effective means of characterizing 

fractured-rock aquifers in the vicinity of 

the bore­hole. 

Finally, heterogeneity in karst 

terrains is determined by the distribution 

of solution channeling along fracture 

networks that eventually develop into 

cavernous openings. Thus, it is important 

to know the locations of transmissive 

fractures, individual caves, and other solution openings in order to delineate flow 

paths. To this end, field studies in karst 

terrains have shifted from quantitative 

modeling to field-based, tracer techniques. 

Quantita­tive characterization of this rock 

type has been difficult, not only because 

of the extreme heterogeneity but because 

of the variability in types and degrees of 

solution channels involved. A combination 

of geophysical methods, tracer tests, and 

special modeling approaches has been 

giving the best results.

Research Opportunities

A better understanding of depositional environ­ments and processes is 

needed to improve characterization of 

sedimentary aquifers. Current approaches 

to characterizing heterogeneity in 

clastic sedimentary aqui­fers are relatively 

simplistic, and efforts will need to be 

made to under­stand how the distribution 

of hydraulic con­ductivity is determined 

by grain-size distributions in space, the sorting of grain sizes, 

the arrangement of sedimentary facies, and post-depositional 

burial- and exhumation-related diagenesis.

Research will need to focus on understanding the control 

that fault zones and other structures, such as folds, have on a 

ground-water flow sys­tem. The identification of faults in the 

Figure 3. An alluvial deposit near Sacramento, California, that exhibits heterogeneity 

and vertical anisotropy where a 1- to 2-meter-thick, silty-clay layer at the ground 

surface is underlain by sand, sandy gravel, and gravelly sand layers. (Photograph from 

Boulanger and Duncan, 2000)

 Research Opportunities in Interdisciplinary Ground-Water Science in the U.S. Geological Survey
subsur­face, their geometry, variations in hydraulic properties, 

and the boundary conditions that they impose on a flow 

system are poorly understood. Predicting hydraulic properties 

caused by faults is particularly difficult. A framework for 

understanding the effects of faulting on hydraulic properties 

would include alternative and com­bined approaches, such 

as (1) extrapolation of structures from the surface to the 

subsurface, (2) delineation of the geometry of faults in three 

dimensions, (3) determination of the perme­ability distribution 

of faults, and (4) delineation of the geologic processes that 

currently affect fault-related hydraulic properties, such as 

stress fields, heat, and diagenetic processes.

Information synthesized from multiple characterization 

methods is needed to develop a conceptual understanding of 

aquifer hetero­geneity and its control on ground-water flow 

and contaminant movement. The extremely heterogeneous 

nature of frac­tured rock makes it difficult to apply con­ceptual 

approaches and field techniques that have been used with 

more homogeneous unconsolidated aquifers. In fractured rock, 

historical and current regional and local stress fields control 

the occurrence of geologic structures and fractures; fractures 

are the pre­dominant pathways for fluid movement. Joints, 

bedding-plane partings, and sometimes faults commonly 

are referred to as fractures and are not distributed uniformly 

through rock, thus making assumptions that commonly are 

applied to unconsolidated porous media, such as homogeneity 

and anisotropy, inappropriate. Also, because individual 

fractures can have spatially varying characteristics and the 

con­nectivity of fractures can be highly complex, hydraulic 

properties of fractured rock do not vary smoothly in space. 

Because of the extreme heterogeneity in fractured rock, no 

single technique or interpretive approach can be used to 

explicitly and unambiguously map the spatial distribution of 

hydraulic properties that control fluid movement and chemical 

migra­tion.

Crystalline rocks and some low-permeability sedimentary 

fractured rocks are characterized by low porosity and low 

specific storage, which imply that the volume of water in 

bedrock aquifers is not necessarily suf­ficient for sustained 

water supply. Sufficient well yields require drawing water 

from either a huge volume of the bedrock or from adjoining 

formations with higher porosity, such as over­lying unconsolidated geologic materials (typ­ically not suitable in mountainous 

and upland areas where few deposits of this type are present). 

Thus, the ability to sustain a water sup­ply from a fracturedrock aquifer depends as much on the surficial geology and 

topographic controls as on the presence of highly perme­able 

fractures in the bedrock. Identifying the factors that control 

the interdependence of surficial geology on bedrock aquifers 

is of great importance in managing ground-water resources. 

Identifying the potential water supply from a bedrock aquifer 

frequently is approached at the scale of a well field, where 

fractures of suitable transmissivity are sought in production 

wells. Well yields depend to a large extent, however, on the 

surface area of high permeability fractures and their ability to 

garner water from more perva­sive, less permeable fractures in 

the formation. 

Geologic controls on ground-water flow over increasingly 

larger dimensions must be investigated in different geologic 

settings, and methods of inferring hydraulic properties of bedrock with confidence need to be developed over increasingly 

larger scales through a com­bination of geologic, geophysical, 

geochemi­cal, and hydrologic methods. Several theoretical 

investigations have proposed that the hydraulic conductivity 

of bedrock aquifers increases with increasing scale; however, 

field evidence indicates oth­erwise. 

A better understanding of the magnitude of the contaminant mass that is constrained within the intrinsic porosity of 

rocks (and bound to fracture surfaces) could lead to more 

realistic ground-water remediation approaches that do not 

necessarily rely on ground-water with­drawal. Highly heterogeneous aquifers commonly con­tain zones of relatively stagnant 

water. These stagnant zones can influence chemical transport, 

because chemicals can dif­fuse from active flow zones into the 

stagnant zones. This is analogous to diffusive mass transport 

between fractures and the unfrac­tured rock matrix—a process 

often referred to as matrix diffusion. In situations when 

frac­tured-rock environments are considered as sites for waste 

isolation, matrix diffusion is regarded as a positive attribute. 

In contrast, matrix diffusion is regarded as a negative attribute 

when ground-water restoration is considered, because removal 

of a contaminant from the primary porosity is limited by the 

rate of diffusion; thus, the achievement of acceptably low 

concentrations of the contaminant could take centuries. 

Additional emphasis needs to be given to more 

comprehensive and quantitative methods for characterizing 

and predicting flow and transport in karst terrains. Although 

tracer tests are valuable for locating potential flow paths, they 

have limited transfer value to other locations or for covering 

larger regions under different hydrologic conditions. One 

promising modeling approach has been to incorporate the 

cave or fracture network directly into deterministic flow and 

transport models by treating the network as a special type of 

boundary condition. A combination of open-channel flow 

in underground streams and porous-media flow in the rock 

matrix also can be represented using a coupled ground-water 

and surface-water model. Geophysical science has provided 

useful tools, including ground-penetrating radar, for field 

mapping and detecting trans­missive caves, fractures, and 

fracture networks in locations that are otherwise inaccessible. 

Combining different field and(or) modeling techniques also 

is possible. For example, tracer tests could be combined with 

electrical resistivity surveys, or tracer break­through curves 

could be used to help calibrate parameters in transport models. 

Work also needs to continue in recognizing karst features and 

characterizing the degree to which different types of features 

affect flow in the aquifer system. Characterizing karst aquifers 

remains one of our most difficult challenges. Developing a 

variety of techniques and improved understanding of where 

each technique works best is likely to bring the most progress 

in improving characterization of aquifer heterogeneity.

Ground-Water Research Opportunities 
Measuring Recharge to Ground-Water Flow Systems

Issues

The sustainability of ground water as a resource is an 

important issue for many communities throughout the Nation. 

Ground water is recharged naturally by rainfall and sur­face 

water. Under steady-state conditions, the amount of recharge 

to an aquifer system bal­ances the discharge, and the amount of 

recharge determines the rate at which water and any associated 

contaminants move through the system. Thus, determining the 

amount of recharge is useful for determining water availability 

and transport rates in ground-water systems. 

Two critical factors of recharge are the temporal and 

spatial distributions of recharge. With quantitative values 

of these factors, one can estimate the total volume of water 

entering a system. In the more arid, western parts of the 

coun­try, the temporal and spatial distributions of recharge 

are more difficult to quantify. Recharge also is a function of 

evapo­transpiration, which makes the accurate measurement 

of evapo­transpiration critical. In the past, we have relied on 

monitoring ground-water levels as an indicator of recharge; 

for a more com­plete characterization in the future, we need to 

focus on developing methods to measure recharge fluxes to 

ground-water systems.

Current State

Recharge is estimated by using a climato­logical and 

hydrologic budget approach in which rainfall, runoff, 

evapotrans­piration, and changes in soil moisture and groundwater storage are measured, and recharge is calculated as the 

residual. Because errors in the measured terms accumulate 

into the recharge term, this approach is inaccu­rate.

 Methods for measuring recharge based on the physics 

of water movement in porous media have been available for 

decades. These meth­ods measure the water content or waterbear­ing properties of subsurface materials but tend to provide 

point measurements that do not nec­essarily reflect subsurface 

conditions at regional scales. Lysimeters can be used to keep 

track of the total water budget in an isolated volume of the 

subsurface. Borehole geophysical tools can be used to measure 

the water content of a vertical profile through time; these data 

then can be interpreted to determine net downward water flux. 

Water levels in shallow wells can be analyzed to estimate 

recharge from rainfall and evapotranspiration from ground 

water. Finally, the unsaturated hydraulic conductivity of rock 

or soil can be measured at very low saturations using centrifuges, and if a unit hydraulic gradient is assumed, the values 

can be converted to recharge rates. 

On a more regional scale, stream hydrograph separation 

has been used for decades to estimate recharge; geophysical 

techniques, such as microgravity, electromagnetics, and 

ground-penetrating radar, have been used only more recently. 

Another approach has been to use ground-water models to 

estimate the recharge based on some degree of certainty 

assumed for the other model parameters. Recharge estimates 

for such models improve when ground-water age data are 

available to constrain ground-water fluxes. Rainfall-runoff 

watershed models also have been used to estimate recharge 

and the effects of climate change on recharge. Research efforts 

that are currently underway to couple watershed models 

with variably saturated subsurface-flow models will improve 

estimates of ground-water recharge rates and our understanding of the effects of climate change on the timing and amounts 

of ground-water recharge.

Both naturally occurring and anthropogenic environmental tracers have been useful in providing a means of 

dating ground water, determining recharge rates, and in some 

cases, determining recharge source areas. The mass balance 

of chloride in precipitation and soils has proven to be useful 

in determining recharge on an areal basis. The natural radioactive tracer 14C is useful in dating ground water in the range 

of 1,000 to 50,000 years, and the radioactive isotope 36Cl is 

useful in dating ground water in the range of thousands to 

hundreds of thousands of years. Anthropogenic environmental 

tracers, introduced into the atmosphere by above-ground 

testing of nuclear weapons (3H, 3He, 36Cl, 14C, and 85Kr, for 

exam­ple) and by industrial activities (chlorofluoro­carbons), 

are useful in dating ground water that is less than 50 years old.

Research Opportunities

The USGS will need the expertise to build upon the 

results of research in meteorological and agricultural disciplines to improve estimations of recharge for ground-water 

modeling and other hydrologic applica­tions. Remote-sensing 

imagery has a role in identify­ing, delineating, and quantifying 

variability in land cover, soil moisture, and vegetation. The 

U.S. Department of Agriculture and the academic community 

have conducted substantial research in the use of remote 

sensing to esti­mate the energy and water fluxes that occur 

between land and the atmosphere. Landsat satellite data are 

used to identify and delineate land-cover types and vegetation 

types. Geostationary Operational Environmental Satellite 

(GOES) data are used to estimate insolation and energy fluxes. 

Surface-energy balance compo­nents have been evaluated by 

combining satel­lite remote-sensing data with on-site measurements of solar radiation, air tempera­ture, wind speed, and 

vapor pressure.

Research is needed to understand better the various physical and chemical processes in the variably saturated zone. The 

variably saturated zone between land surface and the water 

table is a critical region in which infiltration mobilizes and 

spreads contaminants into the ground-water system. Understanding a variety of different physical and chemical processes 

in the vari­ably saturated zone is an essential prerequisite for 

the design of cost-effective remediation efforts on contamination from differ­ent sources, including chemical spills, leaking 

underground storage tanks, drainage of leachate from waste-

 Research Opportunities in Interdisciplinary Ground-Water Science in the U.S. Geological Survey
80°45’ 80°30’ 80°15’

26°00’

25°45’

25°30’

25°15’

Figure 4. Simulated values of average annual net ground-water recharge in Southeastern Florida (from 

Langevin, 2001).

Direct measurements of recharge usually are made at the local scale at individual field sites. Groundwater models can provide valuable regional information, however, by estimating the magnitude and spatial 

distribution of ground-water recharge and discharge.

Ground-Water Research Opportunities 
dis­posal sites, and applications of agricultural chemicals. 

Another environmental concern is the long-term migra­tion of 

radionuclides from low- and high-level nuclear waste-disposal 

facilities. Variably satu­rated conditions in fractured rock 

present an even more complicated flow-path system than in 

unconsolidated rock, which can lead to transport calculations 

based on different conceptual models. The development of 

more sophisti­cated flow and transport processes in variably 

saturated flow models will be a key require­ment for designing 

remediation and containment strategies. 

Improved efficiency and capability in col­lecting and 

analyzing ground-water age tracers will lead to improved estimates of recharge rates. One of the most promising techniques 

for quanti­fying recharge is using ground-water age tracer data 

in an inverse-modeling approach. Currently, age determination 

is limited to ground water either less than 50 or between about 

1,000 and 50,000 years old. Use of the radioisotopes 39Ar 

and 32Si has the potential to aid in filling the ground-water 

age gap between 50 and 1,000 years, but further research on 

both techniques is needed to improve corrections for potential 

natural sources and to reduce the volume of water needed 

for analysis. Other techniques are on the horizon for dating 

ground waters older than 50,000 years. The radioisotope 36Cl 

continues to be investigated, but more research is needed to 

quantify better its natural sources and long-term changes in 

atmospheric deposition. The radioisotope 81Kr has a half life 

of 225,000 years, and challenges in the analyti­cal detection 

of this element are just beginning to be overcome. Finally, 

radioisotope 129I has a half life of 16 million years; thus, its 

detection has the most potential for improving the accuracy of 

age dating deep-basin waters.

Stochastic approaches are needed to address the spatial 

variability in soil and rock types and the resulting variations in 

recharge rates. Recharge fluxes are important quantities, not 

only for water-resource planning but to help predict potential 

contamination of water supplies from nonpoint sources. 

Agricultural contaminants, for example, move through the 

variably saturated zone before reaching the water table, and 

the arrival time at the water table is directly dependent on 

the vertical flux of water through the variably saturated zone. 

These approaches involve the use of Monte-Carlo techniques 

to simulate a wide range of parameter values, with the result 

being a related range of chemical concentrations. This range of 

predicted concentrations then can be quantified in terms of the 

uncertainty or likeli­hood that a given chemical will contaminate a given site by a specified time. Ongoing developments in 

coupled watershed and subsurface-flow models will improve 

estimates of spatially variable fluxes of water and contaminants through the unsaturated zone, which will help to provide 

a basis for stochastic analysis of agricultural contamination of 

ground-water systems. Another potentially powerful modeling 

approach is the combination of GIS and soil-type data bases 

with nonpoint-source contamination studies. Progress is likely 

to occur over the next several years as we begin to combine the 

stochastic approach for predic­tions with GIS-based models.

The long-term hydraulic response of the aquifer system 

and the geochemical effects of mixing water of differ­ent 

chemistries need to be studied at pilot test sites and through 

numerical simulation. In order to supplement the natural 

recharge to aquifer systems, water managers and engineers 

have proposed for some time the artificial recharging of 

aquifers using either ground water or surface water. Most such 

projects are char­acterized as aquifer storage and recovery 

(ASR) and involve injecting or infiltrating water into permeable aquifers and recovering the water later during times of 

high demand. The use of ASR still poses many technical challenges, however. A large-scale ASR project currently is being 

pro­posed as a partial solution to the water needs of southern 

Florida and the Everglades (National Research Council, 2001). 

If this ASR project goes forward, more than 1.7 billion gallons 

per day of surface water will be injected into a permeable 

saline aquifer through more than 300 wells.

Methodologies are needed to incorporate predictions 

of changes in rain­fall and evapotranspiration made with 

glo­bal climate models into recharge calculations for groundwater models. The amount of water available to recharge a 

ground-water system is strongly dependent on rainfall and 

evapotranspiration, which in turn are dependent on climate. 

Long-term changes in climate are, therefore, likely to affect 

recharge to aquifer systems and eventually long-term watermanagement plans. As population growth is expected to stress 

the world’s water resources, decreases in rain­fall are predicted 

to be greatest in the mid to low latitudes, where most of the 

world’s population lives. Predictions of changes in rainfall, 

evapotranspiration, and recharge can be made with global 

climate models linked with watershed and subsurface-flow 

models. Additionally, because rainfall intensity affects runoff, 

climate and watershed models can be used to predict changes 

in runoff, which also affect ground-water recharge rates. 

Water managers need accurate forecasts for water availability 

to plan for growth and water use. Given an understanding 

of the spatial variability of recharge, models that address 

water-availability trends can be developed, and forecasts with 

such models can improve as our ability to predict potential 

climate change improves. Ground-water studies also can help 

us understand changes in recharge that occurred during past 

climate changes, thus improving our understanding of the 

response of recharge to climate change. 

Quantifying Ground-Water and Surface-Water Interaction

Issues

Under natural conditions, most ground water discharges 

into a body of surface water—a stream, river, lake, or ocean. 

The spatial distribution and magnitude of the discharge can 

have a large effect on the surface-water body and its ecosystem. Recognition is growing that ground water and surface 

water must be treated as a single system, because stress that is 

applied to either one is likely to affect the other. For example, 

10 Research Opportunities in Interdisciplinary Ground-Water Science in the U.S. Geological Survey
pumping wells can lower stream levels, river floods can affect 

ground-water levels, and chemical contamination can move 

from ground water to surface water or the reverse. Evapotranspiration is a major component of the hydrologic budget at sites 

of ground-water and surface-water interac­tion, and our ability 

to quantify this component affects our ability to quantify the 

interac­tion. 

Current State

Although past studies have focused on understanding 

the theory and potential for ground-water and surface-water 

interaction, more recent studies have focused on try­ing to 

measure fluxes. A standard method for identifying regions of 

interaction has been to monitor ground-water lev­els in shallow 

observation wells near surface water. Differences between 

ground-water and surface-water levels indicate discharge or 

recharge conditions. To quantify flux, however, a value for the 

vertical permeability of the sediment—a value that is not easy 

to measure accurately—is required. In an attempt to measure 

flux directly, seepage meters were developed to collect a 

volume of water in a submerged bag attached to a cylinder that 

captures the discharge flowing through the sedi­ment. Although 

seepage meters will continue to be useful in measuring 

ground-water discharge to surface-water bodies, the disadvantage of this technique is that many point measurements are 

required to obtain an integrated representative value.

In the measurement of surface- and ground-water 

interaction, temperature recently has been a useful property. 

By recording temperature profiles 

beneath streams, the transient 

diurnal temperature sig­nal in 

the surface water, which reflects 

the air temperature, can be 

observed moving down through 

the subsurface. The shape and lag 

time of this signal are dependent 

on the downward water flux. 

Chemical tracers also are used to 

identify regions of ground-water 

and surface-water interaction. The 

natural isotopes of hydrogen and 

oxygen, for example, commonly 

indicate differences between the 

water bodies that can be used to 

identify the different sources of 

water. Neither temperature nor 

isotope techniques, however, 

quantify the fluxes. More recently, 

the dating of young ground water 

has been used to help quantify 

fluxes from sur­face-water bodies.

Much work has gone into 

trying to estimate or measure 

evapotranspiration. Most studies 

have focused on one of three 

methods—the chamber method, the energy-budget method, or 

the eddy-correlation method. These methods are applied at a 

local scale and can be useful in measuring evapotranspiration 

from different types of veg­etative cover at different sites. 

Research Opportunities

Improved expertise in bridging soil, vegetation, and 

atmospheric modeling with hydrologic model­ing would be 

useful in achieving a fuller understanding of the ground-water 

component of the hydro­logic cycle. Recent attempts have been 

made to couple climate models with water-resources models. 

Coupling these models requires evapotranspiration rates that 

are appropriate for the subregional to regional scale. 

Advancements in a variety of techniques to measure 

flux and identify discharge areas need to be applied to 

ground-water studies. Efforts continue to improve the standard 

seepage meter and expand its use to new envi­ronments. Some 

promise is being shown in coupling borehole magnetometers 

to seepage meters, whereby continuous-flux measure­ments 

can be made remotely for longer times. Additional advances 

also are being made in the use of temperature as a tracer. For 

example, temperature signals can be recorded using small 

inexpensive temperature loggers that can fit in a 1- to 2-inchdiameter borehole, which makes it possible to instrument 

an entire site at a reasonable cost. This technique especially 

is useful in arid and semi­arid regions where recharge often 

is concentrated in perennial or ephemeral streams (fig. 5), 

although applications in more humid regions also show 

In the western United States, arid conditions often lead to ground-water recharge being 

concentrated along mountain fronts in losing reaches of perennial or ephemeral streams. 

Estimating recharge from these streams is important for water-availability studies, and the use 

of temperature as a tracer is proving to be a valuable method.

Figure 5. USGS scientists installing instrumentation in an ephemeral stream at Abo Arroyo, 

New Mexico, to monitor streamflow and calculate recharge using temperature as a tracer. 

(Photograph courtesy of David Stonestrom, USGS, Menlo Park, California)

Ground-Water Research Opportunities 11
promise. It is likely that this technique will be used in more 

studies as details of its efficiency emerge and as tools are 

developed to analyze the large amounts of data that typically 

are collected. For example, an automatic parameter-estima­tion 

program and a one-dimensional transport model show promise 

in estimating recharge when time-series data are entered.

Thermal-infrared imaging can be effective in identifying and delineating areas of ground-water discharge. The 

temperature of ground water discharging into a surface-water 

body—a stream, lake, marsh, or bay—commonly is different 

from the surface-water temperature. Ground-water discharge 

to mountainous streams and lakes can be detected by largescale thermal imagery because of the temperature differ­ence 

between warm ground water and cooler surface water. 

Thermal-infrared images include tonal differences that aid in 

delineating the location and extent of diffuse and concentrated 

ground-water discharge. Thermal-infrared imaging can help in 

interpreting zones of ground-water discharge to surface-water 

bodies when more conventional, intru­sive techniques are not 

available. Thermal-infrared imaging can be used to detect 

ground-water discharge to coastal, saline waters when suitable 

temperature contrasts and sufficient rates of ground-water 

discharge occur. The feasibility of this technique, however, has 

many constraints. The imaging must be done when weather 

conditions maximize tem­perature contrasts between discharging ground water and surface water. Thermal sensors respond 

to energy from only the uppermost portion of an emitting body 

and cannot penetrate vegetation. Only an unobstructed view 

of the ground allows correlation of the detected radiant flux to 

temperatures on the land surface. Over water, thermal imag­ery 

records the radiant flux of approximately the upper millimeter 

of the water surface.

The focus of research in evapotranspiration must shift 

from making local measurements to obtaining average 

regional values. Remote sensing has been used by the USGS 

in Nevada to extrapolate evapotranspiration measure­ments on 

a regional basis (Laczniak and others, 1999). In another study, 

an approach that correlates ground-water evapotranspiration 

with a modified soil-adjusted vegetation index derived from 

Land­sat data was used to develop a method for esti­mating the 

magnitude and distribution of ground-water evapotranspiration 

at a regional scale (Nichols, 2000). The area of each vegetative 

cover type was multiplied by the evapotranspiration measured 

at a site representative of the vegetative cover type. In Florida, 

evapotranspiration measure­ments made as part of the South 

Florida Eco­system Initiative have been used to correlate 

evapotranspiration 

rates with various 

discrete wave bands 

of remotely sensed 

data (Desmond and 

others, 1999). 

Substantial 

research external 

to the USGS needs 

to be coupled with 

USGS investigations to develop 

a comprehensive 

set of methods 

and approaches 

with regard to 

evapotranspiration 

rates derived from 

remotely sensed 

data (fig. 7). 

Researchers in other 

government agencies and academia 

have investigated 

the use of remote sensing to estimate energy and water fluxes 

between the land and atmosphere. In one such study, a method 

for esti­mating regional evapotranspiration by using synoptic 

weather data and satellite imagery was explored (Sucksdorff 

and Ottle, 1990). Soil moisture and evapotranspi­ration 

correlate highly with the radiometric sur­face-temperature component of the energy balance at the land-atmosphere interface. 

Soil moisture and evapotranspiration can be esti­mated by 

combining thermal infrared imagery, in-situ measure­ments of 

surface and atmospheric properties, and surface-energy flux 

modeling.

Rigorous research is needed for both the benefits and 

constraints of using remote sensing for determining evapotranspiration. Study results have shown direct correlations 

between evapotranspiration and density of photosynthetically 

active vegetation, as defined by the leaf-area index. Leaf-area 

index, as expressed by the Normalized Differ­ence Vegetation 

Index (NDVI) data from sat­ellite imagery, however, apparently 

Figure 6. Remote sensing image showing cold (blue) temperatures that reveal ground-water discharge to 

the Umatilla River in Oregon, August 10, 1998. (Photograph courtesy of Watershed Science, Inc., under contract 

to the Oregon Department of Environmental Quality)

The temperature of discharging ground water often differs from the temperature of surface water, which 

makes temperature an obvious indicator of discharge locations. Temperature also is an important 

environmental factor controlling fish habitat.

12  Research Opportunities in Interdisciplinary Ground-Water Science in the U.S. Geological Survey
does not correlate directly to all ecosystem landscapes. A 

USGS study in the Florida Everglades demonstrated that 

NDVI and evapotranspiration appear to be inversely related 

(German, 2000).

Understanding the Relation of Ground-Water Flow 

Systems to Aquatic Ecosystems

Issues

Ground water is an important component of the water 

budget in aquatic ecosystems, including wetlands, rivers, 

streams, lakes, and coastal areas. Ground water also is criti­cal 

in determining water quality and has an important role in 

habitat development and maintenance for a variety of aquatic 

and semiaquatic organisms. As noted in the National Research 

Council (2000) report on ground-water systems, research is 

leading to a better understanding of the interactions of ground 

water and surface water, and the USGS has led much of the 

research. Until recently, however, ground-water research has 

focused on hydrology and geology. Ecologists have been slow 

to embrace ground water as an integral component of their 

inter­disciplinary research, or have had difficulty incorporating 

ground-water studies into their ecological research. Ecologists 

generally are not prepared to conduct ground-water studies, 

and hydrologists generally are not prepared to conduct 

ecological studies. Collaboration, therefore, is critical in 

the research of aquatic ecosystems, from understanding the 

Figure 7. Four images of different sections of the San Pedro River basin from a thermal 

infrared multispectral scanner (TIMS) used to calculate evapotranspiration and recharge 

at land surface. (The colors represent reflected infrared (thermal) wavelengths.) (Images 

provided by National Aeronautics and Space Administration/Advanced Spaceborne Thermal 

Emission and Reflection Radiometer Science Team)

Evapotranspiration is an important component of the energy budget at land surface. The 

evapotranspirative flux can create temperature anomalies that can be detected by remote 

sensing and used to estimate fluxes and their spatial distribution, especially when the 

remotely sensed data are combined with field measurements.

Ground-Water Research Opportunities 13
underlying geology and associ­ated hydrology to the biological 

and biogeochemical systems they support.

Current State

In 1975, the eminent river ecologist H.B.N. Hynes 

challenged the scientific com­munity to relate the biota and 

processes occur­ring in streams and rivers to the physical 

processes linking surface waters with their watersheds, including surface-water and ground-water interactions. In subsequent 

studies, scientists recognized that the distribution of biota is 

related to ground-water discharge to surface water and surfacewater recharge to ground water. At about the same time, other 

work was laying the founda­tion for increased understanding of 

the interac­tion of lakes and ground water. The results of this 

work caught the attention of ecolo­gists, particularly those who 

realized the need to understand the hydrologic characteristics 

and processes of lakes and ground water in order to understand 

better the biogeochemical processes and biota in these waters. 

Wet­land studies conducted by ecologists 20 years ago rarely 

included detailed examina­tion of hydrologic processes, 

including inter­action with ground water. Over the decades, 

some ecologists and hydrologists began to col­laborate to gain 

a better understanding of wet­land ecosystems. These studies 

have advanced the knowledge of wetland functions and 

pro­cesses and could serve as models for how such interdisciplinary work can be achieved.

The scientific hydrologic community increasingly 

recognizes that ground water and surface water are a 

single resource. A recent USGS report (Winter and others, 

1998) serves as a guide on this subject for wetland and 

aquatic professionals, the general public, and governmental 

decisionmakers. The report addresses ground-water and 

surface-water interactions in terms of natural processes, 

such as the hydrologic cycle, chemical interactions, and 

landscape features; and in terms of human activities, such 

as agriculture, urban and industrial development, drainage, 

and modifi­cations to river valleys and the atmosphere. Each 

of these topics has impli­cations for aquatic ecosystems and 

biota. Determining the source of water, the resultant water 

chemistry, and the ultimate response of biological organisms 

in any aquatic system requires an understanding of the 

ground-water component of the hydrologic cycle, which often 

varies according to landscape setting. Moreover, in many 

settings, the biological pro­cesses can appreciably influence 

the hydro­logic components (for example, evapotranspiration 

and enhanced recharge as a result of bioturbation). Thus, 

ecological understanding is needed to understand fully the 

hydrology. Evaluating the effects of human activities that alter 

watersheds, airsheds, landscapes, and individual wetland and 

aquatic settings requires an understanding of the contribution 

of ground water to both the natural and modified conditions. 

Factors ranging from changes in water level to contaminant 

trans­port can greatly affect wetland and aquatic biota and are 

often dependent on ground-water flows. 

Research incorporating ground-water hydrology in 

ecological studies related to human activities generally has 

been site spe­cific. Examples include studies of the dewatering effects of ditching wetlands or pumping nearby wells; 

aquifer-contamination effects from agricultural, industrial, and 

residen­tial development; effects of water and rock inter­actions 

in metal-rich, mineral deposits that load aquatic ecosystems 

with highly acidic waters and toxic metals; and landscapelevel effects from clearcutting to saltwater intrusion and 

land-use change in a water­shed. Not only can human activities 

reduce or increase exchange between ground water and surface 

water, but they can reverse or sever the exchange altogether. 

In most local studies, the specific nature of the ground-water 

component of hydrology is unique, and it is the fine-scale 

aspect of hydrology that often dic­tates the ecological effects. 

Therefore, generaliza­tions about ground-water flow are not 

adequate to explain site-specific ecological conditions. Other 

applications of ground-water hydrology to ecology include 

wetland restoration, construction of wetlands for miti­gation 

purposes, and development of an “abi­otic” wetlandclassification system, such as the hydrogeomorphic (HGM) 

approach. Other studies have focused on a variety of methods 

employed in under­standing the relation of ground water to 

aquatic ecosystems and on developing models. Although 

ecological research that incorporates studies of ground-water 

hydrology is increas­ing and includes wetlands, rivers, streams, 

lakes, and coastal areas, the research generally is localized and 

specific, and results are not linked to results of other studies. 

Research Opportunities

The challenge for the USGS in integrating ground-water 

studies with ecological studies is in fostering collaborative 

research opportunities among scientists from different 

disciplines. Specific environmental concerns need to be 

identified that require integrated research related to groundwater and ecological science. Initially, studies of many 

aquatic ecosystems could be reviewed to identify the geologi­c 

framework, including the ground-water and surface-water 

hydrologic components of the framework, as a means of 

enhancing understanding of the biological and bio­geochemical 

systems that are an integral part of the land and water. A 

further progression would be to generate useful products from 

these aquatic studies, including models and maps showing 

3–D relations among the interactive components replete 

with enough data to include as many variables and processes 

as needed for relevance to the ecosystems. Where data are 

not available, scientists from all disciplines could generate 

conceptual models that could be used to guide research efforts 

targeted at filling the gaps in our understand­ing. In all cases, 

studies of the relation of ground water to aquatic ecosystems 

should have both a temporal and spatial per­spective and be 

geared to improve forecasting whenever possible.

Research is needed to understand better the role of 

various hydrologic components at various scales in order to 

forecast changes in water budgets of local aquatic ecosystems 

14  Research Opportunities in Interdisciplinary Ground-Water Science in the U.S. Geological Survey
affected by human activities. Recent work has 

improved the understanding of evapotranspiration 

as a com­ponent of water budgets, but more work 

is nec­essary. Additional information is needed to 

understand the effects of ground-water withdrawals 

and artificial recharge on wetlands, rivers, lakes, 

and other aquatic resources. Urban development and 

land­scape-scale paving of surfaces reduce groundwater recharge and increase stormwater runoff and 

related sediment transport. The resulting redistribution 

of water is affecting aquatic ecosystems at discharge 

locations. Alteration of surface-water hydrology (for 

example, dams, diversions, and channel modifications 

in fluvial systems) similarly can affect recharge, 

dis­charge, and aquatic ecosystems. The greatest issue 

related to water budgets, however, is the potential 

effects of climate change. For example, the amount of 

precipitation reaching different geographic regions is 

likely to change, thus affecting recharge and discharge 

ecosys­tems. Recent history indicates an increase in the 

frequency of extreme precipitation events. Thus, within 

any given range of precipitation, stormwater runoff is 

likely to increase, recharge of ground water is likely to 

decrease, and discharge of ground water subsequently 

is likely to decrease. In addition to the immediate 

ecological effects of altered surface-water hydrology, 

this scenario has implications for ground-water hydrology, especially in wet­lands. Warm air temperatures 

enhance evapotranspiration. This, along with reduced 

precipitation, can rapidly affect wet­lands dependent 

on surface water, including those that recharge ground water. 

Wetlands supplied by ground water, however, can remain wet 

during short-term changes and con­tinue to support biological 

communities. Long-term reductions in recharge, however, 

ultimately affect the ability of these wet­lands to serve as 

refuges during warm cli­matic periods when evapotranspiration 

is greatest. In general, little is known about the ground-water 

component of flow and the ecological dependence on this flow 

for most of the Nation’s wetlands.

Forecasting changes in water quality in aquatic ecosystems that are influenced by human activities requires a better 

understanding of ground-water interactions. Ground-water 

withdrawal along or near coastal areas may result in saltwater 

intrusion into freshwater aquifers, which may affect the 

distribution and quality of aquatic ecosystems. Recharge areas, 

which can be as important to ecosystems as discharge areas, 

can be contaminated directly by indus­tries, mining, spills, or 

urbanization. Other more pervasive influences require further 

investigation to gain a holistic concept of potential effects, 

including (1) contributions of nutri­ents to ground water from 

agriculture, septic fields, and lawns in urban areas; (2) contributions of pesti­cides from agricultural and residential sources 

and concentrations of dissolved constit­uents in return flow 

from irrigation and water-reuse systems; and (3) deposition 

of a variety of contaminants from the atmosphere. More­over, 

these chemical stressors rarely occur in isolation, and the 

effects of combinations of stressors are not well understood. 

More research is needed to determine the ability of 

wetland and riparian areas to improve surface-water quality. 

Knowledge of hydrologic sources, transformations, and 

processes is fundamental in understanding, assessing, and 

monitoring the health of aquatic ecosystems. For example, 

ground-water inflows in a natu­ral or constructed wetland 

system have been related to root-zone temperature, time and 

duration of freezing, water levels and range of fluctuation, and 

carbon profiles. Yet little informa­tion is available. Although 

ground water may be considered in water-management 

plans (for example, source water for filling and sustaining 

impound­ments) and in plans for construc­tion or restoration 

of wetlands, a greater understanding of hydrologic processes 

and dependencies is needed before ecological studies can fully 

integrate ground-water hydrology. Many bio­logical organisms 

are affected by water-quality conditions in areas where ground 

water is a dominant forcing function (for example, salinity). 

Finally, more information is needed on ground-water flows, 

biogeochemistry, and bio­logical communities associated with 

the inter­facial “hyporheic” zone between ground-water and 

surface-water systems. In many cases, the chemical transformations that occur in the interface zone over a few centimeters 

can overwhelm the transformations that occur in kilometers of 

upgradient ground-water flow.

Figure 8. The Texas Blind Salamander, Typhlomolge rathbuni, lives 

only in the caves of the Edwards aquifer, an important source of water, 

and is listed as endangered by both the State of Texas and the Federal 

government. (Photograph courtesy of Joe N. Fries, U.S. Fish and Wildlife 

Service)

Endangered species of fish, birds, reptiles, and amphibians inhabit 

waterways throughout the Nation. More and more habitats are coming 

into conflict with increasing stresses on our finite water resources.

Ground-Water Research Opportunities 15
Developing More Effective Computer Models

Issues

The first five research topics in this report are focused 

on developing a better understanding of the framework and 

processes of ground-water systems, their relations to aquatic 

ecosystems, and ways of adapting and improving methods to 

extract information from these systems. Additional research on 

any of these topics can contribute to improving the capabilities 

of ground-water simulation models. Although assumptions 

must always be made in con­structing simulation models and 

the amount of data will always be limited, no other tool can 

integrate the available knowledge of a system to the same 

extent. As computer speed and storage continue to increase, 

modeling programs will be developed to represent more 

effectively the structural framework and dynamic processes 

of ground-water systems. Future modeling efforts also need 

to quantify better the uncertainty inherently associated with 

forecasting. In order to manage the resource for sustainability, 

society needs to be able to forecast responses of ground-water 

systems to the ever-increasing stresses being imposed on them. 

Hydrologic forecasting has been listed as one of the most 

important challenges in the environmental sciences. To use 

ground-water resources more efficiently, we need to be able to 

predict changes in water levels, discharge, and the movement 

of contaminants. To make informed forecasts, all available 

data regarding a ground-water system must be brought 

together and analyzed simultaneously in a framework that 

defines the physics and chemistry of how water and solutes 

move in the subsurface. This is best done by using computer 

simulation models. 

Current State

The USGS is a leader in the develop­ment of groundwater simulation computer codes. MODFLOW, the USGS 

modular ground-water model (Harbaugh and others, 2000; 

Hill and others, 2000), is one of the most widely used groundwater models in the world. Ground-water modeling software 

has improved from describing predominantly two-dimensional 

(2–D) ground-water flow or geochemical equi­libria in the 

1970s to 3–D flow and 2–D transport of chemicals (or 

water-rock interactions) in the 1980s to 3–D flow and transport, one-dimensional (1–D) reactive transport, and inverse 

modeling and parameter estimation in the 1990s. As the 21st 

century begins, computer modeling has expanded to include 

sim­ulation of 3–D transport for studies involv­ing seawater 

intrusion and reactive contaminants. For water-resource 

planning, ground-water flow and transport-simulation models 

also have been coupled with management (or optimization) 

models from the field of operations research. Such simulationoptimization models simultaneously account for the physical 

processes of a ground-water system and the water-resource 

management aspects of the system.

Another aspect of modeling in which simulation advancements have been made is the coupling of ground-water and 

surface-water models. Traditionally, simulation models of 

hydrologic systems have focused on either the ground-water or 

surface-water systems. Ground-water models did not account 

for land-surface hydrologic processes, such as precipitation, 

evapotranspiration, and overland runoff; whereas, surfacewater models either did not account for the contributions of 

ground water to surface-water flow or, in the case of watershed 

models, accounted for ground-water storage and discharge 

using very simple formulations. An important exception to this 

generalization is the development of computer programs that 

simulate leakage between ground-water systems and hydraulically connected surface-water features by applying Darcy’s 

Law. These programs range in complexity from simple 

approaches that do not include flow in the surface-water 

features to more detailed approaches that either track flow 

in a network of streams that interact with the ground-water 

system or that formally couple 1–D, unsteady streamflowrouting models to a ground-water flow model. Although these 

programs are extremely useful for simulating flows between 

ground water and surface water, they do not account for 

the important land-surface processes typically simulated by 

watershed models, such as the USGS Precipitation-Runoff 

Modeling System (PRMS; Leavesley and others, 1983).

One of the most challenging aspects of coupling groundwater and surface-water models has been the representation 

of water flow through the unsaturated zone. Typically, 

models of subsurface flow assume instantaneous recharge of 

water infiltrating the land surface to the water table, even in 

areas that have a thick unsaturated zone. Recently, however, 

investigators have made progress in developing numerical 

methods that account for the storage and transit time of water 

through the unsaturated zone, and one of these methods is 

being implemented in the development of a coupled watershed 

and ground-water flow code using the PRMS and MODFLOW 

models.

The trial-and-error approach often used in the past to 

calibrate ground-water models is ill-suited for calculating 

optimal parameter values, model uncertainty, and the relative 

importance of individual observations. Param­eter-estimation 

models that use nonlinear regression or related methods 

better address these calibration issues. Parameter-estimation 

codes were developed initially with the intent that the type of 

observations that could be used in the estimation would be 

limited to water-level and flow data. These data usually are 

sufficient for convergence of the nonlinear regression rou­tines; 

in actual case studies, however, the data often are dominated 

by water-level information, yielding high parameter correlation and uncer­tainty. 

More recent development of universal inverse modeling 

programs, such as the USGS program UCODE (Poeter 

and Hill, 1998), make it possible to calibrate ground-water 

mod­els using, for example, geo­physical or geochemical data. 

Because inverse methods require a large number of individual 

model simulations, the computational speed requirements are 

16  Research Opportunities in Interdisciplinary Ground-Water Science in the U.S. Geological Survey
appreciably greater than the computation speed required for 

a single for­ward-model simulation. To date, this require­ment 

has limited the size and complexity of a model that can be 

calibrated using automated calibration. Calibrations have 

been confined mostly to ground-water flow models, as 

transport models require finer grids for stable and realistic 

simulations and, thus, more computations. This limitation is 

slowly but steadily being overcome by increasing speeds and 

storage capabilities of computers. Therefore, it is probable 

that over the next several years it will be possible to apply 

inverse methods to models that simulate multidimensional 

transport and dispersion of solutes at real field sites using 

realistic ranges of transport properties. Such calibrations will 

include other computationally demanding processes, such as 

geochemical reactions or multiphase flow and transport.

Another valuable piece of information that comes from 

inverse modeling is the identifica­tion of the relative value of 

individual observa­tions. Sensitivity analyses from parameterestimation methods reveal not only the observations that give 

the most information regard­ing the different parameters of 

the model, but also the new observations that give the most 

valuable information for improv­ing parameter estimates. 

These results also can be related to parameters that give the 

most information regarding the model predictions. This is 

critical information in the design of data-collection or monitoring networks. Financial resources are always limited, but 

cost effectiveness and value will be enhanced by knowledge of 

the types of data that are needed and where data will be more 

useful. Long-term costs associated with monitoring hydrologic 

systems can be minimized and the benefit maximized by 

using such an analysis from the inverse-modeling approach. 

Currently, these types of analyses are in the early stages of 

development, and additional development and application with 

test cases is likely to be of great benefit to society over the 

next several years.

Research Opportunities

Hydrologic modeling and 

hydrologic research go hand-in-hand. 

Simulation of ground-water flow 

systems, including surface-water and 

ground-water interactions, their relation to ecosystems, and simulation 

of the fate of contaminants in ground 

water, require enormous amounts of 

data and sophisticated algorithms 

to approximate the real world, 

especially in complex hydrogeologic 

settings. The input demand for 

modern computer ground-water 

programs is a major driver for the 

directions of hydrologic research. 

Modeling results indicate the 

limitations and gaps in data where 

additional research is needed, and 

computer simulation provides insight 

that leads to a better understanding 

of the hydrogeologic system and the 

fundamental processes. Similarly, 

as hydrologic research provides 

a better understanding of the real 

world and the tools and techniques 

to provide better data, the usefulness 

and accuracy of hydrologic modeling 

improves. Thus, advancements in 

hydrologic modeling are both a 

research opportunity and a requirement for the societal needs for better 

hydrologic forecasting. 

Figure 9. Rankings based on sensitivities of water-level observations and future forecasts 

of radionuclide transport paths to parameters of a regional model around Yucca Mountain, 

Nevada (modified from Hill and Tiedeman, in press). 

Ground-water models are used frequently by water-resource managers to forecast the 

effects of stresses on water supplies and the movement of contaminants. Ground-water 

modeling tools are being used to incorporate procedures for calculating sensitivities to 

model parameters and uncertainties of model predictions.

Ground-Water Research Opportunities 17
Hydrologic modeling development over the next several 

years needs to be especially strong in several directions. 

Ground-water optimization models need to expand beyond 

applications of managing water-level declines to contaminantplume con­tainment, conjunctive ground-water and surfacewater use, and saltwater-intrusion abatement.

Fully coupled ground-water and surface-water models 

need to be developed beyond the current prototypes to more 

fully developed, field-tested tools. Such integrated models 

will be in high demand for use in better understanding such 

processes as the rainfall-runoff response, ground-water level 

fluctuations near rivers, the effects of ground-water withdrawals on stream levels, and the contributions of ground 

water to streams. The newly coupled models also need to 

account for the transport and reactions of chemicals that 

are released at land sur­face, move into the subsurface, and 

dis­charge into streams. Use of new USGS models involving 

the coupling of surface-water, unsaturated-zone, and groundwater codes is expected at the watershed scale—that is, on 

the scale of 100 square kilometers (km2) or larger—where the 

issues of water supply and conjunctive use are addressed. The 

watershed scale is ideal for broadly integrating knowledge of 

how surface-water and ground-water interactions are affected 

under different scenarios of water-resources management. 

Additionally, it is important that USGS research 

focuses on further development of coupled surface and 

subsurface models with a goal of increasing understanding 

of biogeochemical and ecological aspects of the interaction 

between surface water and ground water, and applying these 

models to water-quality, terrestrial, and aquatic ecologicalresources management. Extension of watershed-scale models 

for fine-scale application to water-quality and ecological 

modeling needs to be addressed. An example of a fine-scale 

application is the development of coupled models that focus 

on reach-scale processes. Reach-scale models simulate solute 

and sediment transport processes in river corridors and the 

effects and interactions between subenvironments, such as 

hyporheic and riparian zones, on material transport, storage, 

and processing in the river corridor as a whole.

More widely field-tested 3–D variable-density flow 

models are needed. The intrusion of seawater into coastal aquifers has become an increasingly important issue as populations 

increase in coastal areas. The 3–D aspects of coastal aquifer 

sys­tems can be significant, because pumping cen­ters can 

produce cones of depression within a regional flow field that is 

perpendicular to the coastline. Computers continue to become 

more powerful, and 3–D codes that have been developed by 

the USGS (SUTRA3D, Voss and Provost, 2002; SEAWAT2000, Guo and Langevin, 2002; Langevin and others, 2003) 

are just now being tested with data from real aquifer systems. 

Additional testing can produce a better understanding of the 

true capabilities and limita­tions of 3–D modeling codes under 

full regional-scale modeling situations.

Com­patibility is needed between software packages that 

create input for ground-water models and those that create 

output from 3–D geo­logic models so that geologic information 

can be incorporated into the model quickly and efficiently. As 

we continue to develop and apply larger, more complex and 

more refined models to specific field sites, we will become 

more dependent on graphical user interfaces to pro­cess the 

input data sets necessary to adequately define the hydrogeologic system. A more realistic represen­tation of a hydrologic 

boundary or geologic feature always requires detailed 

information to define the feature. Land-surface elevations, 

land-use and soil-type distributions, exact positions of rivers 

and lakes, and contacts between geologic units are examples 

of some of these features. Viewing the results in 3–D also is an 

essential requirement for analyzing results from 3–D models, 

and recent model-visualization software is proving to be 

greatly beneficial in interpreting model output. Ani­mation of 

time-dependent output sequences also is needed. All of these 

graphical interface tools need to be made compatible with new 

and improved models as they are developed.

Studies are needed to define the types of additional data 

that can be incor­porated, the types of hydrogeologic frameworks in which these data are most beneficial, and the best 

methods for including these data in the parameter-estimation 

process. Progress in model calibration over the next 10 years 

is promising because of the avail­ability of new, universal, 

parameter-estima­tion software and the increasing speed and 

storage of computers at decreasing cost. No technical barriers 

exist to carry out such stud­ies, but appropriate sites need to be 

identified and the resources obtained to gather the appro­priate 

data. Two of the most promising types of data are from 

chemical tracers and geophys­ics. For example, environmental 

tracer data, such as ground-water age or travel path, provide 

direct infor­mation on the speed and direction of ground-water 

flow in the system. These tracer data are being used to help 

calibrate models and are beginning to be used in automatic 

calibration approaches. 

Society needs models that not only can help forecast 

hydrologic events but also can estimate the uncertainty of 

these fore­casts. Although nonlinear regression methods can 

give values for confidence intervals, the cal­culation of these 

intervals is based on the measurement error associated with 

obser­vations, and such measurement errors are assumed to be 

greater than those associated with how the model represents 

the aquifer framework. As the error in the latter often is 

greater than in the former, new methods are needed that 

include the potential error in boundary locations or zonations 

in the statistical calculations that yield uncertainties. In 

addition to parameter uncertainties, there are uncertainties in 

the predictions themselves. Advancements appear promising 

in develop­ing statistics that relate the prediction uncer­tainty 

to the individual parameter uncertainties. Further testing is 

needed to evaluate the credibility of such values of uncertainty 

assigned to model predictions.

18  Research Opportunities in Interdisciplinary Ground-Water Science in the U.S. Geological Survey
Summary

Advances in the six ground-water science research topics 

described in this report could greatly improve our ability to 

forecast ground-water availability to sustain human use and 

support aquatic ecosystems. These topics do not cover all 

aspects of ground-water science that the USGS could pursue 

but were derived from interactions with colleagues internal 

and external to the USGS. The topics also are meant to foster 

discussion and contin­ued evolution of research opportunities 

on which USGS scientists can focus in the coming years.

A major challenge for the USGS is to integrate the 

ground-water science summarized in these six topics with 

other USGS research that bears on understanding and forecasting ground-water sustainability. These topics and the sci­ence 

behind them are complex and highly technical, representing 

specialized research and expertise developed over many years. 

Integrating the science specializations and syn­thesizing the 

results of research to support decisionmaking for important 

societal issues pose important challenges to biologists, 

geographers, geologists, and hydrologists in the USGS.

A major goal of the USGS ground-water program is to 

provide a national and regional understanding of ground-water 

resources suitable for forecasting a sus­tainable level of human 

use and support to aquatic ecosystems. Underlying a national 

and regional understanding of ground-water sus­tainability is 

the need for the collection, com­pilation, and analysis of basic 

hydrologic, geologic, biologic, and geographic data. Therefore, it is critical to continue a commit­ment to monitoring and 

collecting long-term basic data at long-established sites and at 

new sites established to improve the science. This commitment 

will cultivate the growth of new programs that will incorporate 

lessons from the past while allowing for the flexibility to 

respond to the ever-changing needs of our growing population.

The research opportunities identified in this report for 

USGS ground-water science are summarized for each of the 

six topics as fol­lows:

Mapping in Three Dimensions—Geology and Geophysics

Research and development are needed to bring 3–D 

capabilities to a level at which geologic models are 

routinely integrated with hydrologic models.

The abil­ity to determine the relation of geophysical 

responses to hydraulic properties and the means to 

quantitatively incorporate this geophysical information 

and interpretation into hydrologic models are needed.

Improvements in geophysical methods that result from 

advancements in mathematical algorithms, visualization, and instrumentation are needed for ground-water 

studies.

•

•

•

Advancements in modeling capabili­ties, such as 

coupling 3–D seismic and other geophysi­cal data and 

interpretation with hydraulic parameterization routines, 

are needed for ground-water models.

Characterizing Aquifer Heterogeneity

A better understanding of depositional environ­ments 

and processes is needed to improve characterization of 

sedimentary aquifers.

Research that focuses on understanding the control that 

fault zones and other structures, such as folds, have on 

a ground-water flow sys­tem is needed.

Information synthesized from multiple aquifercharac­terization methods is needed to develop a 

conceptual understanding of aquifer heterogeneity 

and its control on ground-water flow and con­taminant 

movement.

Geologic controls on ground-water flow over increasingly larger dimensions needs to be investigated in 

different geologic settings, and methods of inferring 

hydraulic properties of bedrock with confidence needs 

to be developed over increasingly larger scales through 

a com­bination of geologic, geophysical, geochemi­cal, 

and hydrologic methods.

A better understanding of the magnitude of the contaminant mass that is constrained within the intrinsic 

porosity of rocks (and bound to fracture surfaces) is 

needed, which could lead to more realistic approaches 

to ground-water remediation.

Additional emphasis needs to be given to more comprehensive and quantitative approaches for characterizing and predicting flow and transport in fractured-rock 

and karst ter­rains.

Measuring Recharge to Ground-Water Flow Systems

The USGS needs to build upon the results of research 

in meteorological and agricultural disciplines to 

improve recharge estimation. 

Research is needed to understand better the various 

physical and chemical processes in the variably saturated zone.

Improved efficiency and capability of col­lecting and 

analyzing ground-water age tracer data is needed to 

improve estimates of recharge rates.

•

•

•

•

•

•

•

•

•

•

Summary 19
Stochastic approaches are needed to address the spatial 

variability in soil and rock types and the resulting 

variations in recharge rates.

The long-term hydraulic response of the aquifer system 

and the geochemical effects of mixing water of different chemistries need to be studied at pilot test sites and 

through numerical simulation. 

Methodologies are needed to incorporate predictions of 

changes in rain­fall and evapotranspiration made with 

glo­bal climate models into recharge calculations for 

ground-water models.

Quantifying Ground-Water and Surface-Water Interaction

The USGS needs to improve expertise in bridging 

soil-vegetation-atmospheric mod­eling with hydrologic 

modeling for a fuller understanding of the groundwater component of the hydrologic cycle. 

Advancements in a variety of techniques to measure 

flux and identify discharge areas need to be applied to 

ground-water studies.

The focus of research in evapotranspiration needs to 

shift from making local measurements to obtaining 

average regional values.

Substantial research external to the USGS with regard 

to evapotranspiration derived from remotesensing data needs to be coupled with USGS 

investigations to develop a comprehensive set of 

methods and approaches.

Rigorous research is needed for both the benefits and 

constraints of using remote sensing for determining 

evapotranspiration.

Understanding the Relation of Ground-Water Flow 

Systems to Aquatic Ecosystems

Mechanisms that foster collaboration among scientists 

from different disciplines are needed to ensure an integrated approach to ground-water and ecological studies 

in the USGS.

•

•

•

•

•

•

•

•

•

Research is needed to better understand the role of 

various hydrologic components at various scales in 

order to forecast changes in water budgets of local 

aquatic ecosystems affected by human activities.

Forecasting changes in the quality of water in aquatic 

ecosystems that are influenced directly by human 

activities is needed to understand better ground-water 

interactions.

More research is needed to determine the ability of 

wetland and riparian areas to improve surface-water 

quality.

Developing More Effective Computer Models

Ground-water optimization models need to expand 

beyond applications of managing water-level declines 

to contaminant-plume con­tainment, conjunctive 

ground-water and surface-water use, and saltwaterintrusion abatement.

Fully coupled ground-water and surface-water models 

need to evolve beyond the current prototypes into fully 

developed, field-tested tools.

USGS research needs to focus on further development 

of coupled surface-subsurface models with a goal to 

increase understanding of biogeochemical and ecological aspects of the interaction between surface water 

and ground water.

3–D variable-density flow models need to be tested 

more widely in field settings. Compatibility is needed 

between software packages that create input to groundwater models and those that create input to 3–D 

geologic models.

Studies are needed to define the types of additional 

data that can be incorporated, the types of hydrogeologic frameworks and degree of heterogeneity in which 

these data are most beneficial, and the best methods 

for including these data in the parameter-estimation 

process.

Society needs models that not only can help forecast 

hydrologic events but also can esti­mate the uncertainty 

of these predictions.

•

•

•

•

•

•

•

•

•

20 Research Opportunities in Interdisciplinary Ground-Water Science in the U.S. Geological Survey
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Sanford and others—Research Opportunities in Interdisciplinary Ground-Water Science in the U.S. Geological Survey—USGS Circular 1293

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