Estimating Areas 

Contributing Recharge to Wells 

Lessons from Previous Studies 

By O.L. Franke, T.E. Reilly, D.W. Pollock, and J.W. LaBaugh 

U.S. GEOLOGICAL SURVEY CIRCULAR 1174 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, Director 

The use of firm, trade, and brand names in this report is for identification purposes only and 

does not constitute endorsement by the U.S. Government 

First printing, April 1998 

Revised edition, July 1998 

Third printing, November 1999 

RESTON, VIRGINIA: 1999 

Free on application to the 

U.S. Geological Survey 

Branch of Information Services 

Box 25286 

Denver, CO 80225-0286 
CONTENTS 

Introduction ......... ... .. ..................................... ...... ...... .. ..................... ... ...................................................................... ............. 2 

Pathways of Contaminated Ground Water to Discharging Wells....... ......... .............. ..... .. ....... ....... .. .................................... . 3 

Significance of the Ground-Water Flow System... ................. ...................................................... ......... .. ..... .. .......... ............. 4 

Ground-Water Travel Times...... .......... .. ................. ......... ...... ... ... .................... .......... ....... .. .. ..... ... .. ............. ........ ............... ... 6 

Effects of Changing Hydrologic Conditions on Contributing Areas...... ........................ .. .. .. ........ .. .. ........ .. ........ ........ ...... .. ... 7 

Effects of Well-Screen Locations and Pumping Rates on Contributing Areas.... .................. ...... .................. .. .. .. ...... ............ 7 

Uncertainty in Estimating Contributing Areas.............. .. ........ .. .................. ........ .. .. ... ........ ................................................... 9 

In Conclusion-A Point of View... .. .. ............................ ....... .. ... ....... ...... ... ... ........ .......... ..... ........... ... .. ...................... .. .......... 12 

Selected References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 

FIGURES 

1. Cross section and map showing area contributing recharge to a single discharging well in a 

simplified hypothetical ground-water system........ .. .. .. .. ................. .. ... .. .. .................. .. ............. ..... ...... ............... ....... . 2 

2. Cross section showing pathways for movement of contaminated ground water into a well, 

when the source of contaminants is at or near the water table .. ........ .......... ...... .............................................. .. .......... 3 

3. Maps and diagram showing long-term (steady-state) model-calculated contributing recharge 

areas for wells 11 and 26 near Rochester, Minnesota, which are screened in the St. PeterPrairie du Chien-Jordan aquifer ............................... .. ..................... .... .. ...... ...... .... .. .. .. ................................. ............... 4 

4. Map showing long-term (steady-state) model-calculated contributing recharge areas for 

and ground-water travel times to the Independence, Missouri, well field .. ............................................................. ... 6 

5. Cross section showing degrees of separation between the water table and the well screen 

of a cased well or the open hole of an uncased well............................ .. .......................................... .. .. ............ ........ ... 8 

6. Cross section showing hypothetical model-calculated ground-water flowpaths at equilibrium 

from the water table to a nearby stream and to a discharging well screened at the bottom 

of the aquifer.......... ..... ..... ................................................... ............. ... ........................................................................ 9 

7. Maps showing simulated effects of increasing horizontal hydraulic conductivity of moraine 

sediments on the water-table configuration and ground-water flowpaths near the Landfill-1 

contaminant plume, western Cape Cod, Massachusetts................................................ ............ .. .. .... ......................... 10 

8. Maps showing comparison of areas contributing recharge to seven public-supply wells as 

determined by two numerical models, Cape Cod, Massachusetts................................................. ........ ..................... 11 
Introduction 

About one-third of the population of the United 

States obtains drinking water from public-supply systems 

that rely on ground water. Because of the large number of 

people involved, public health officials, regulators, waterresource planners, and the public are concerned about the 

quality of ground water pumped by public-supply wells. 

Most human-derived contaminants in ground water are 

related to activities at the land surface and enter the groundwater flow system at the water table after passing through 

the unsaturated zone. A second important location of contaminant entry, which is of much smaller areal extent than 

the water table, is the beds and banks of streams, reservoirs, 

lakes, and wetlands. Given that most human-derived contaminants enter the ground-water flow system directly 

or indirectly from the land surface, one approach for protecting public ground-water supplies is to estimate areas 

contributing recharge to public-supply wells and then to 

implement ground-water protection practices on the overlying land surface. 

A 

Areal recharge 

Area 

contributing 

recharge 

Area 

contributing 

recharge 

~ 

Discharging 

well 

Discharging 

well 

~ 

8 Not to scale 

2 

The ultimate source of ground water 

discharging from a well is recharge 

at the water table and, possibly, inflow 

from a surface-water body. 

The U.S. Geological Survey (USGS) has taken an 

active role in studies involving the estimation of areas 

contributing recharge to wells (fig. 1), particularly publicsupply wells, both in providing analysis tools for such studies (McDonald and Harbaugh, 1988; Pollock, 1994) and 

in applying these tools within the context of cooperative 

studies with States and other public entities in different 

hydrogeologic settings throughout the Nation (see Selected 

References). General outcomes of these studies include 

(1) identification of the source of water to public-supply 

wells, including recharge at the water table and induced 

infiltration from surface-water bodies; (2) comparisons 

of estimated contributing areas determined by different 

Figure 1. Area contributing recharge to a single 

discharging well in a simplified hypothetical 

ground-water system. 

A, Cross-sectional view. B, Map view. For threedimensional systems, the area contributing recharge 

to a discharging well is defined in this report as the 

surface area at the water table where water entering 

the ground-water system eventually flows to the well. 

This area must provide an amount of recharge that 

balances the amount of water being discharged from 

the well. Thus, lower areal recharge rates result in 

larger contributing areas of wells. If a nearby surfacewater body also contributes water to the discharging 

well, the area contributing recharge is reduced and 

is a function not only of the areal recharge rate but 

also of the amount of water obtained from the surfacewater body. Depending on factors that describe the 

three-dimensional flow system and the placement 

of the well, the area contributing recharge to a well 

does not necessari ly have to include the location of 

the wel l. (Modified from Reilly and Pollock, 1993, 

fig. 1.} 
techniques; (3) depiction of model-calculated two- and 

three-dimensional ground-water flowpaths and times of 

travel along these flowpaths; ( 4) estimation of times of 

travel from potential sources of contamination to pumping 

wells; and (5) proposed design of ground-water-quality 

monitoring networks based on numerical model results. 

The purpose of this report is to provide a brief review 

of selected concepts, assumptions, and uncertainties related 

to estimating areas contributing recharge to wells. Emphasis 

is on estimating contributing areas by numerical simulation 

and on what has been learned from previous studies by the 

USGS using this approach. 

Pathways of Contaminated 

Ground Water to Discharging Wells 

Several common pathways by which shallow ground 

water contaminated by human activities can reach a discharging well are illustrated in figure 2. However, only the 

pathway "normal flow of ground water from the aquifer 

to a discharging well" (1) (fig. 2) is amenable to analysis 

by standard analytical approaches. Because the ultimate 

source of the normal flow of ground water to a discharging 

well is recharge at the water table and possibly inflow from 

a surface-water body, the usual analytical approaches for 

estimating the contributing recharge area of a discharging 

well generally assume that contamination related to human 

activities is derived from one or both of these sources. 

Naturally occurring contaminants such as trace elements, 

radionuclides, and ground water with high dissolved-solids 

concentrations cannot be taken into consideration unless the 

location of specific source areas and subsurface volumes for 

these contaminants can be identified. 

A strong suspicion that water is entering a well along 

pathways (2) or (3) arises when human-derived contaminants are detected in water samples from fully cased wells 

that are screened only in deep, confined aquifers. However, 

such detections do not necessarily reflect pathways (2) or 

(3). When water samples are analyzed for parts per billion 

of some constituents (primarily synthetic organic compounds), minor sources of contaminants within the well 

may be detected; for example, a grease layer on installed 

casing, glue from glued joints of plastic casing, oil and 

grease on equipment installed within the well, and so on. 

Possible additional contaminant pathways other than 

those illustrated in figure 2 are related to the normal groundwater flow system and also to the properties of the fluid. For 

example, consider a leak or spill of a contaminant with a 

density greater than water near a pumping well. The fl uid 

would sink vertically by gravity across deeper and deeper 

flowpaths and also possibly be carried laterally by the existing ground-water flow system toward the well. 

3 

(3) 

Flow down 

annular 

space 

(1) 

Normal flow 

from aquifer 

discharging 

well 

Discharging well 

(2) 

Flow through 

hole, crack, 

or casing joint 

tr~tt--- Well 

casing 

Annular space 

filled with grout, 

bentonite, or 

other material 

Figure 2. Pathways for movement of contaminated ground water into a well, when the source 

of contaminants is at or near the water table. 

Analytical techniques for estimating areas 

contributing recharge to discharging wells, including 

numerical simulation, consider only pathway (1 ), 

normal flow of ground water from the aquifer 

to the discharging well. Pathways (2) (flow through 

a hole, crack, or casing joint) and (3) (flow down the 

annular space) can permit entry of adjacent ground 

water into the well from any depth above the well 

screen. The prevention of contamination along these 

pathways depends on the design and construction of 

the well and on maintenance of the well through time. 

Unfortunately, no analytical tools or simple predictors 

are available to identify if or when these pathways are 

or will become active. 
Significance of the 

Ground-Water Flow System 

Ground-water flow systems contain water that 

moves continuously along three-dimensional flowpaths 

from points of recharge to points of discharge. As 

hydrologic stresses on a flow system change, the threedimensional flowpaths also can change. Thus, in order 

to determine which flowpaths lead to a specific well, the 

ground-water system must be evaluated on a systemwide 

basis for a particular set of conditions. In other words, a 

contributing area for a particular well is unique only for 

one specific discharge rate of that well and for specific 

The location and shape of areas 

contributing recharge to individual 

wells reflect the composite effects of 

many interacting hydrologic phenomena 

associated with the surrounding 

ground-water flow system. 

discharge rates for all other wells and stresses that influence 

the ground-water system near the particular well. As pumping rates are varied at other wells in the vicinity, the area 

contributing recharge to any particular well, even if its 

discharge is constant, can change (fig. 3). 

Figure 3. Long-term (steady-state) A 

model-calculated contributing .------.:;:~---------.:~~----c--..,------, 

recharge areas for wells 11 and 26 

near Rochester, Minnesota, which 

are screened in the St. Peter-Prairie 

du Chien-Jordan aquifer. 

(A) Wells 11 , 26, and all other 

nearby high-capacity wells are pumping. 

(8) Only wells 11 and 26 are pumping. 

(C) Conceptualized patterns of threedimensional ground-water flow 

from the top of the St. Peter-Prairie 

du Chien-Jordan aquifer to well 11 . 

These figures depict the complicated 

and discontinuous spatial patterns of 

long-term contributing recharge areas 

in systems that are pumped by 

numerous nearby wells with open 

inteNals at various depths below the 

water table and how these patterns 

change as pumping wells are added 

to (or subtracted from) the groundwater system. 

In particular, (C) also depicts the 

irregularly shaped volumes in the 

subsurface that contain the many 

flowpaths that originate at the water 

table and discharge at well 11 . Any 

additional discharging wells (one 

shown) would capture their own 

subsurface flowpaths and related 

contributing recharge areas at the 

water table, thereby changing local 

flow patterns in the surrounding groundwater flow system. (Modified from Delin 

and Almendinger, 1993, figs. 10, 11 , 

and 12.) 

4 

CJ 

- 0 2 3 4 MILES 

0 2 3 4 KILOMETERS 

EXPLANATION 

Area contributing recharge to well 26 

Area contributing recharge to well 11 

Well location and number 

Other high-capacity well (discharge exceeds 200 

gallons per minute) 
EXPLANATION 

CJ Area contributing recharge to well 26 

- Area contributing recharge to well 11 

• 26 Well location and number 

0 2 3 4 MILES 

0 2 3 4 KILOMETERS 

EXPLANATION 

Model computed areas contributing recharge and subsurface 

volumes containing flowpaths that discharge to well 11 

Ground-water flowpaths that discharge to well 11 , dashed 

where flow is not along face of block diagram 

Other ground-water flowpaths 

5 
Ground-Water Travel Times 

A useful product of numerical-simulation studies, in 

addition to the estimation of steady-state contributing areas 

in figure 3, is model-calculated travel times from different 

parts of the steady-state contributing area to discharging 

10" 

09' 

wells (fi g. 4). Although these travel times are calculated 

at steady state and are valid only for one particular stress 

pattern on the ground-water system, they do provide 

valuable insight for planning and design purposes, for 

example, in the design of wellhead-protection programs 

(U.S. Environmental Protection Agency, 1987). 

EXPLANATION 

TRAVEL TIMES FROM AREAS 

CONTRIBUTING RECHARGE 

0 6 Months 

0 1 Year 

0 2Year 

0 3Year 

0 4Year 

CJ 5Year 

0 10Year 

0 25Year 

0 50Year 

0 100Year 

II 250 Year 

DISCHARGING WELL 

Existing } * Collector Independence well field 

Planned 

1 KILOMETER 

Figure 4. Long-term (steady-state) model-calculated contributing recharge areas for and ground-water 

travel times to the Independence, Missouri, well field. 

In this setting, large amounts of water are being pumped from the alluvial aquifer along the Missouri River. The 

ultimate source of water to the discharging wells is a combination of areal recharge at the water table and induced 

inflow from the Missouri River and its local tributaries. The area contributing recharge has an irregular shape, is 

almost continuous, is extensive across the river from most of the pumping, and extends to part of the boundary of 

the alluvial aquifer on both sides of the river. An additional feature of this figure, compared to figure 3, is modelcalculated travel times from the various parts of the contributing recharge area to the discharging wells. An 

important caveat in using these travel times is that they are calculated for steady-state conditions and for a single 

assumed pattern of pumping. Despite this caveat, these travel times provide valuable information for planning nearand long-term pumping from the well field and possible ground-water-quality monitoring programs in critical parts of 

the aquifer. (Modified from Kelly, 1996b, fig. 58.) 

6 
Effects of Changing Hydrologic 

Conditions on Contributing Areas 

In nature, ground-water systems are always changing 

to some degree. Ground-water flow patterns are continually 

adjusting to natural and human-induced changes in the surrounding hydrologic environment. Although areas contributing recharge also change to reflect changes in groundwater flow patterns, the nature of how they change and the 

time scale over which that change occurs can be complex 

and difficult to analyze. 

An important characteristic of areas contributing 

recharge to discharging wells is that usually these areas 

respond very slowly to changes in stresses on the groundwater flow system. The introduction of a new well in a 

shallow aquifer may cause temporal changes in the rate 

and direction of ground-water flow in the vicinity of the 

well that require only several months or a few years to 

approach a new steady state. However, the areas contributing recharge to discharging wells that are located at some 

distance from these wells may require several decades to 

adjust their shape and distribution to reflect the new hydraulic conditions. Contributing areas represent the source location of water discharging to wells at any instant in time. 

Thus, the contributing areas reflect the integrated effects 

of the movement of water through the system from the time 

and point at which the water entered the system to the time 

that it discharges to wells. Because ground-water velocities 

are usually small, the journey from recharge location to a 

well can take many decades. At any instant in time, contributing recharge areas are complex composites that integrate 

the changes and variations in ground-water flow patterns 

that have occurred in the system over the many years or 

decades that the water has been in the ground-water system 

on its journey to discharging wells. 

An interesting consequence of the slow response 

time of contributing areas is that they are relatively insensitive to short-term cyclic changes in ground-water flow patterns, such as seasonal cycles. The extremes of these shortterm cycles do not last long enough to influence the size and 

shape of contributing areas. Instead, the contributing areas 

tend to reflect longer term, average flow patterns that 

smooth out short-term cyclic variations. 

An important characteristic of areas 

contributing recharge to discharging wells 

is that usually these areas respond very 

slowly to changes in stresses on the 

ground-water flow system. 

7 

The water discharging from a 

well is usually a mixture of waters 

of different ages and from 

different source areas. 

Effects of Well-Screen Locations and 

Pumping Rates on Contributing Areas 

The primary perspective of the discussion thus far 

has been that of the ground-water flow system. Here, the 

perspective shifts briefly to the discharging well. Deeper 

placement of the open interval of the well (greater separation between the water table and the open interval of the 

well) (fig. 5) generally results in longer and more complex 

three-dimensional flowpaths from areas contributing 

recharge to the well and in longer travel times along these 

flowpaths. 

Initiating pumping at a new well or increasing the 

pumping rate at an existing well causes the convergence and 

capture of many new flowpaths to the well-flowpaths that 

formerly would have ended at other discharge points (figs. 3 

and 6). The conclusion from figures 3- 6 and the associated 

discussion is that the water discharging from a well is usually a mixture of waters of different ages and from different 

source areas. Furthermore, increasing the discharge rate at 

a well will increase the size of the contributing area from 

which flowpaths to the discharging well originate and, 

therefore, will increase the possibility for encountering 

sources of contaminated water. 
A 

(1) (2) (3) (4) (5) 

Land surface 

Unsaturated zone 

Water table 

Saturated zone 

Well 

casing 

Slit with fine sand 

Fine to medium sand 

Fine to coarse 

sand with 

some silt 

8 

(6) 

Land surface 

Soil and weathered 

bedrock 

Water table 

Bedrock 

Figure 5. Degrees of separation between the water table and the well screen of a cased well or the open 

hole of an uncased well. 

The placement of the well screen or open interval of a discharging well can greatly influence areas contributing 

recharge to the well and vulnerability of the well to contamination. The "degree of separation" refers to a combination 

of two factors. The first is the distance between the water table and the top of the well screen or open interval of 

the well (greater distance implies greater separation). The second is the thickness and vertical and horizontal 

conductivities of confining units and aquifers between the water table and open interval of the well (in particular, 

greater thickness and lower hydraulic conductivity of low-conductivity confining units imply greater separation). 

The wells in part A illustrate various degrees of separation in a layered aquifer. In general, greater degrees of 

separation lead to a more complex three-dimensional configuration of ground-water flowpaths, longer travel 

times from the water table or surface-water body to a discharging well, extension of the contributing area farther 

from the discharging well, and possibly a more discontinuous and complex shape of the contributing area (see fig. 3). 

A separate sketch for bedrock wells is provided in part 8 (well 6) to illustrate a practice in some areas to install well 

casing only to the top (surface) of consolidated rock. As a result, the bottom of the well casing can be above or only 

a few feet below the water table. In these situations, the well can be particularly vulnerable to possible contamination 

from ground water near the water table; that is, no separation exists between the water table and the top of the open 

interval of the bedrock well. Homeowner wells and virtually all public-supply wells tapping unconsolidated deposits 

(wells 1-5) usually are screened some distance below the water table. 

8 
Figure 6. Hypothetical model-calculated groundwater flowpaths at equilibrium from the water table 

to a nearby stream and to a discharging well screened 

at the bottom of the aquifer. 

Before pumping of the well, all flow in the groundwater system discharged to the stream. The sources of this 

ground-water flow were inflow from the stream-valley walls 

and areal recharge at the water table. When the groundwater system reached equilibrium with the discharging well, 

a portion of both sources of water to the stream are diverted 

to the well. The result of this partitioning of flow between 

the stream and the discharging well is a flowpath divide, 

a complexly shaped surface below the water table. This 

flowpath divide (a line in vertical section) between the two 

subsystems extends below the area contributing water to 

the stream at the water table. The location of this flowpath 

divide is highly sensitive to the discharge rate of the well. 

Furthermore, it can be inferred from figure 6 that the ground 

water enteri ng the well could represent a considerable 

variety of source areas above the water table and a 

considerable range in age. (Modified from Reilly and 

Pollock, 1993, fig. 7.) 

Trace of flowpath divide 

between subsystems discharging 

to the stream and to the well 

I 

D 

D 

EXPLANATION 

Open interval of 

discharging well 

Water that originates in the 

area contributing recharge 

to well 

Water that originates as 

lateral inflow and flows 

to well 

~ Ground-water flowpath 

I Location of 

lateral inflow 

to well 

from stream valley walls 

0 1,000 FEET 

l....w.....u... Vertcal scale greatly exaggerated 

9 

Many uncertainties exist in 

estimating areas contributing 

recharge to discharging wells-for 

example, uncertainty in quantifying 

the physical properties that describe 

the ground-water flow system . . . 

Uncertainty in Estimating 

Contributing Areas 

Many uncertainties exist in estimating areas 

contributing recharge to discharging wells. These 

uncertainties have been alluded to in the previous 

discussion about stresses on the system-locations 

and pumping rates of wells and areal recharge 

rates. Other critical properties that are needed to 

quantify the ground-water flow system include 

hydraulic conductivities of aquifers and confining 

units. Hydraulic conductivities of the various 

hydrogeologic units in a ground-water system 

are always a major control on the distribution 

of ground-water heads in the system. The threedimensional distribution of ground-water heads, 

in turn, is a major control on the three-dimensional 

configuration of flowpaths in the system (fig. 7). 

Particularly in systems with low hydraulic gradients, small changes in the distribution of groundwater heads can have a significant influence on 

the configuration of ground-water flowpaths. Thus, 

uncertainty in quantifying the physical properties 

that describe the ground-water flow system results 

in uncertainty in delineating ground-water flowpaths and in estimating the resulting contributing 

areas of wells. 

In general, complexity of ground-water fl ow 

systems and uncertainty in quantifying properties 

that describe the flow system are related-greater 

complexity implies greater uncertainty. Factors that 

increase the complexity of ground-water systems 

include complex geometry of hydrogeologic units, 

aquifers with well-developed fractures or solution 

cavities, placement of open intervals of numerous 

wells at various depths and in different aquifers, 

areal recharge that varies significantly in space, and 

surface-water bodies that contribute water to wells. 

Relative complexity is an important factor in deciding which analytical approach is appropriate for 

estimating areas contributing recharge to wells 

(fig. 8). In many of the studies conducted by the 

USGS (see Selected References) that estimate 

contributing areas of wells, multilayer models 

were needed to adequately represent the threedimensional features of the ground-water flow 

system. 
- 10 -

8 

EXPLANATION 

Areal extent of contaminant plume--Shows known extent of 

Landfill-1 plume as of January 12, 1994 (National Guard Bureau 

Installation Restoration Program, written commun., 1994) 

Model-calculated flowpaths 

Model-calculated water-table contour--Shows altitude 

of water table. Contour interval is 5 feet. Datum is sea level 

2 KILOMETERS 

Figure 7. Simulated effects of increasing horizontal hydraulic conductivity 

of moraine sediments on the water-table configuration and ground-water 

flowpaths near the Landfill-1 contaminant plume, western Cape Cod, 

Massachusetts. 

Hydraulic conductivity of the moraine sediments is (A) 50 feet per day and 

(B) 150 feet per day. In the first simulation (A), flowpaths split in two directions, 

west and south, but predominantly to the south. In the second simulation (B), 

although the configuration of the water table (ground-water head surface) changed 

very little at this scale, virtually all of the flowpaths moved to the west and followed 

the known configuration of the contaminant plume. The significant conclusion from 

these simulations is that small changes in ground-water heads can substantially 

change the local configuration of flowpaths in the ground-water flow system. 

(Modified from Masterson and others, 1997, fig. 8.) 

10 
Base from U.S. Geological Survey 

Digital Line Graphs, 1 :24,000 

State Plane Projection, 

Zone 5176 

A 

0 1 MILE r-------,------'----,----' 

0 1 KILOMETER 

EXPLANATION 

- Wastewater-treatment-facility infitration beds 

C=:J Areas contributing recharge to public-supply wells 

- 24- Water-table contour-Shows calculated altitude of 

water table. Contour interval, in feet, is variable. 

Datum is sea level 

• sFo-1 Public-supply well and local well number 

B 

0 1 MILE f------,------'----,----' 

0 1 KI LOMETER 

Figure B. Comparison of areas contributing recharge to seven public-supply wells as determined 

by two numerical models, Cape Cod, Massachusetts. 

Results from (A) two-dimensional (single-layer) model and (B) three-dimensional (eight-layer) model. 

The ground-water flow system consists of a thick (250-500 feet) multilayered sequence of unconsolidated 

deposits or materials that range in grain size from gravel and sand through silt and clay and includes numerous 

overlying ponds and streams and variable recharge rates from precipitation. More than 30 public-supply wells, 

screened at various depths, withdraw water from the system at widely differing rates. The contributing recharge 

areas in A are fairly typical of the simple ellipsoidal shapes that are delineated by two-dimensional analytical 

and numerical modeling techniques. In comparison, the shapes of the contributing recharge areas in 8 are 

much more complex. (Modified from Barlow, 1994, fig. 6.) 

11 
In ConclusionA Point of View 

Ground water moves in complex, three-dimensional 

patterns that change with time. Nearly all of what we know 

about estimating contributing recharge areas comes from 

simulation studies of both real and hypothetical groundwater flow systems because it is impractical to perform field 

experiments on real ground-water systems that respond over 

a period of years to decades. The strong reliance on simulation poses a dilemma when analyzing real systems because 

there is no proof or test to verify the results, especiall y since 

we have relatively incomplete knowledge of the transient 

hydraulic history of most systems. However, it is because 

of the complexity of real systems and our inability to fieldverify analytical results that simulation analysis is, in fact, 

the best means to obtain physically based estimates of 

contributing recharge areas. Systems with complex spatial 

or time-dependent characteristics usually require the application of numerical models (for example, finite-difference 

or finite-element models) to approximately represent key 

features. Other, less complex systems sometimes can be 

simulated adequately using analytical methods. Regardless 

of the specific simulation method, the process of simulation 

provides the means to systematically integrate the important, interacting hydrologic phenomena that control groundwater flow and, thereby, makes possible physically based 

estimates of contributing recharge areas for discharging 

wells. 

However, it is because of the 

complexity of real ground-water flow 

systems and our inability to field-verify 

analytical results that simulation is, 

in fact, the best means to obtain 

physically based estimates of 

contributing recharge areas. 

All simulation studies require the construction of 

a simplified representation of the real system. The key to 

successful simulation is to recognize controlling geologic 

and hydrologic features and processes and represent them 

as accurately as possible by using an appropriate simulation 

technique. To achieve such success requires broad-based 

experience and expertise in geology, ground-water hydrodynamics, and simulation methods. The case studies 

presented here and additional studies listed in the Selected 

References provide examples of the kind of integrated, 

simulation-based studies conducted by the USGS in 

recent years. 

12 

Selected References 

Almendinger, J.E. , 1994, The travel-time ellipse-An approximate zone of transport: Journal of Hydrology, v. 161, 

p. 365-373. 

Arihood, L.D. , and Cohen, D.A., in press, Geohydrology 

and simulated ground-water flow in northwestern Elkhart 

County, Indiana: U.S. Geological Survey Water-Resources 

Investigations Report 97-4238. 

Bailey, Z.C., 1993, Hydrology of the Jackson, Tennessee, 

area and delineation of areas contributing ground water to 

the Jackson well fields: U.S. Geological Survey WaterResources Investigations Report 92-4 146, 54 p. 

Bair, E.S., Sheets, R.A., and Eberts, S.M., 1990, Particletracking analysis of flow paths and traveltimes from 

hypothetical spill sites within the capture area of a 

wellfield: Ground Water, v. 28, no. 6. 

Barlow, P.M., 1994, Two- and three-dimensional pathline 

analysis of contributing areas to public-supply wells of 

Cape Cod, Massachusetts: Ground Water, v. 32, no. 3, 

p. 399-410. 

--1997, Particle-tracking analysis of contributing areas of 

public-supply wells in simple and complex flow systems, 

Cape Cod, Massachusetts: U.S. Geological Survey WaterSupply Paper 2434, 66 p. 

Barlow, P.M., and Hess, K.M., 1993, Simulated hydrologic 

responses of the Quashnet River stream-aquifer system to 

proposed ground-water withdrawals, Cape Cod, Massachusetts: U.S. Geological Survey Water-Resources Investigations Report 93-4064, 52 p. 

Bayless, E.R., and Arihood, L.D., 1996, Hydrogeology and 

simulated ground-water flow through the unconsolidated 

aquifers of northeastern St. Joseph County, Indiana: 

U.S. Geological Survey Water-Resources Investigations 

Report 95-4225 , 47 p. 

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13 

--1996b, Design of a monitoring well network for the 

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51 p. 

Edited by: Carol L. Anderson and Mary A. Kidd 

Graphics by: John M. Evans 

Manuscript preparation by: Joy K. Monson 

For more information, please contact: 

U.S. Geological Survey 

Office of Ground Water 

12201 Sunrise Valley Drive, 

Mail Stop 411 

Reston, VA 20192 

(703) 648-5001 

14 

ISBN 0-607-89779-1 

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