U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2007–3002

January 2007

Ground-Water Recharge in Minnesota

Printed on recycled paper

Figure 1.  Ground-water recharge is an important part of the hydrologic cycle in Minnesota.

What is Ground-Water Recharge?

“Ground-water recharge” broadly 

describes the replenishment of water to 

a ground-water flow system (Winter and 

others, 1998). Recharge, an integral part 

of the hydrologic cycle, is the process 

by which water moves to the water table 

and then away from that area through 

saturated materials. Figure 1 shows how 

water recharges an unconfined aquifer 

and moves toward a river where it discharges. 

The process of ground-water 

recharge is like refilling a leaky tank. 

Over the long term, recharge is generally 

balanced by discharge to surface waters 

or deeper parts of the ground-water 

system or by uptake by plants. However, 

this balance can be altered locally by 

pumping, impervious surfaces, land use, 

or climate changes resulting in increased 

or decreased recharge. 

Where Does Recharge Occur?

Recharge to the water table occurs 

in most areas of the landscape but commonly at varying rates. Given the same 

precipitation, recharge rates in areas 

where sediments are sandy are greater 

than in areas where sediments are finer 

grained. In addition, surface-water runoff 

to low lying depressions in the land-surface generally promotes recharge (fig. 1). 

Surface-water bodies can be a source of 

recharge in areas where the water level 

(hydraulic head) in the underlying aquifer 

is below the lake or river water level. 

Water in confined glacial and bedrock aquifers is typically replenished by 

leakage of water through overlying 

“Ground-water recharge” broadly describes the addition of water to the ground-water system. Most water recharging the groundwater system moves relatively rapidly to surface-water bodies and sustains streamflow, lake levels, and wetlands. Over the long term, 

recharge is generally balanced by discharge to surface waters, to plants, and to deeper parts of the ground-water system. However, this 

balance can be altered locally as a result of pumping, impervious surfaces, land use, or climate changes that could result in increased 

or decreased recharge.

Recharge rates to unconfined aquifers in Minnesota typically are about 20–25 percent of precipitation.

Ground-water recharge is least (0–2 inches per year) in the western and northwestern parts of the State and increases to greater 

than 6 inches per year in the central and eastern parts of the State.

Water-level measurement frequency is important in estimating recharge. Measurements made less frequently than about once 

per week resulted in as much as a 48 percent underestimation of recharge compared with estimates based on an hourly measurement frequency.

High-quality, long-term, continuous hydrologic and climatic data are important in estimating recharge rates. 

•

•

•

•

The first section of this fact sheet 

introduces the process of ground-water 

recharge, including definitions of 

related terminology and clarification of 

common misconceptions. The second 

section describes how ground-water 

recharge rates vary in Minnesota. The 

third and final section is a more technical overview of several methods used to 

estimate recharge rates in Minnesota.

Pumped 

well 

Surface 

runoff 

Precipitation 

River 

Unsaturated 

sediment or rock 

Recharge to unconfined aquifer 

Confined 

aquifer 

Leakage to and from confined aquifer 

Confining layer 

Lowland 

Evaporation 

Direction of ground-water flow 

Water table 

Transpiration 

Wetland 

Unconfined 

aquifer 

Photograph by Geoffrey Delin, U.S. Geological Survey
confining layers. Leakage is a process 

similar to but not the same as recharge 

(fig. 1). Rates of leakage to confined 

aquifers are generally less than rates of 

recharge to unconfined aquifers. Upward 

leakage also occurs from a confined aquifer in areas where hydraulic head in the 

confined aquifer is greater than that in the 

overlying formation; water moves from 

areas of greater to lower hydraulic head. 

Confined aquifers also are replenished in 

areas where the confining layer is permeable or absent. 

Figure 3a illustrates the temporary 

effect recharge water has on the water 

table. The water level in a well installed 

in an unconfined aquifer generally rises 

rapidly in response to recharge from 

precipitation. In the absence of recharge, 

the water table declines in response to 

movement of water away from the area.

 When Does Recharge Occur?

The amount of ground-water 

recharge varies seasonally in response to 

precipitation; spring and fall are the times 

of greatest ground-water replenishment in 

Minnesota (fig. 3b). Evidence of smaller 

recharge events also can be seen in the 

figure 3b hydrograph. During winter, 

water levels generally decline because 

recharge rates are negligible due to snow 

cover and frozen soil. During the growing 

season, plant transpiration rates generally exceed precipitation rates. A large 

proportion of the water that infiltrates 

at land surface, therefore, is returned to 

the atmosphere before it can reach the 

water table. Occasionally, as shown in 

figure 3b, summer rainfall in excess of 

ET rates results in net recharge. 

Why is Knowledge of Ground-Water 

Recharge Rates Important?

Knowledge of ground-water 

recharge rates is important to studies of 

water availability, sustainability, wellhead protection, contaminant transport, 

ground-water and surface-water interactions, effects of urbanization, and aquifer 

vulnerability to contamination (Scanlon 

and others, 2002). Estimates of recharge 

rates are necessary to quantify the amount 

of water moving through near-surface 

ground-water systems and are important 

to understanding the water balance of an 

area and the ways that human activities, 

such as landscape practices, affect 

Figure 2.  Much ground water recharge in

Minnesota ends up in streams, lakes, and

wetlands. Photograph by Lee Grim, National Park Service

Why Do Recharge Rates Vary?

Numerous factors, including physical characteristics of the soil, vegetation 

cover, land use, topography, water content of surface materials, and the presence 

and depth of the confining layers, influence the spatial variability of recharge 

rates. Weather patterns, including the 

timing and intensity of spring snowmelt, 

spring rains, evapotranspiration (ET) 

during the summer growing season, and 

fall rains, play a major role in controlling spatial and temporal variability in 

recharge rates. 

What Happens to the Water After it is 

Recharged?

In semihumid regions like Minnesota, most water recharging the groundwater flow system moves relatively 

rapidly to surface-water bodies (fig. 1) 

and sustains streamflow, lake levels, and 

wetlands (fig. 2). Smaller portions of the 

recharged water move to deeper confined 

aquifers, are extracted by plants, or are 

withdrawn from aquifers by wells for 

irrigation, public supply, or industrial and 

other uses. 

Ground-water recharge is not...

...equivalent to “infiltration” of 

water at the land surface. Most water 

that infiltrates at the land surface 

is returned to the atmosphere by 

plant transpiration and evaporation 

from soil and water surfaces (fig. 1). 

Recharge is typically only a small 

fraction of infiltration.

...equated to the process of “percolation;” instead, percolation refers 

to the movement of water through 

unsaturated sediments. The percolating water can be viewed as potential 

recharge, however.

...to be confused with the term 

“aquifer yield.” This term refers to 

the amount of water that an aquifer 

can yield to pumping. 

...the same as “sustainable yield.” 

Recharge is less than sustainable 

yield. If all recharge water was 

utilized, stream, lake, and wetland 

levels could be adversely affected. 

Furthermore, it cannot be assumed 

that pumping at less than the 

recharge rate will not cause waterlevel declines and ground-water 

storage depletions. 

Recharge = specific yield x

 water-level rise 

 = 0.23 x 2.1 feet

 = 0.483 feet per year

Depth to water, feet below land surface

61

60

59

58

57

56

Jan

Feb

Mar

Apr

May

June

July

Aug

Sept

Oct

Nov

Dec

Projected water-level decline

R

e

c

h

a

r

2.1 feet

Depth to water, feet below measuring point

17.0

16.6

16.2

15.8

15.4

Jan

Feb

Mar

Apr

May

June

July

Aug

Sept

Oct

Nov

Dec

15.0 0

0.4

0.8

1.2

1.6

Precipitation, inches

ET effects

Figure 3a.  Example recharge estimation

based on the water-table fluctuation

method using hypothetical data with only

one recharge event in a year (modified from

Delin and others, 2007).

Figure 3b.  Hydrograph for calendar year

2000 from a well near Windom, Minn.,

completed in an unconfined aquifer,

showing the effects of evapotranspiration

(ET) and of multiple recharge events

(modified from Cowdery, 2005).
that balance. For example, by estimating the seasonal and spatial distribution 

of recharge rates, one can estimate the 

total volume of water entering a system. 

Recharge is a sensitive component of 

ground-water flow models and is the one 

that is least understood (Delin and others, 

2007). 

How are Recharge Rates Estimated?

There is no “recharge gage” analogous to a rain gage, but many methods 

have been developed for estimating 

recharge rates. Selection of the appropriate method for a given study is important 

and can be challenging. Recharge rates 

are best estimated by use of multiple 

methods and the results compared. Accurate long-term measurements of streamflow, ground-water levels, and climate are 

needed to estimate recharge rates. The 

final section of this fact sheet provides a 

general description of several methods 

that can be used to estimate recharge in 

Minnesota. 

How do Recharge Rates Vary 

Across Minnesota?

The statewide basin-scale estimates 

of ground-water recharge rates in Minnesota (fig. 4) are based on the regional 

regression recharge (RRR) method of 

Lorenz and Delin (2007). Stream baseflow estimates in selected basins and 

statewide climate and soil data are incorporated in the RRR method. The results 

illustrate the spatial variability of annual 

recharge rates to surficial materials across 

the State. An abbreviated description of 

the RRR method is in the Regional-Scale 

Methods section of this fact sheet. The 

recharge estimates shown in figure 4 are 

representative of average recharge rates 

for 1971–2000 because they are based on 

data from that time period. 

EXPLANATION

Water

0 20 40 60 80 M ILES

0 20 40 60 80 KI LOMETERS

Base from U.S. Geological Survey digital data

1:2,000,000, 1972, Universal Transverse Mercator Projection

Recharge in 

 inches per year

 Greater than 12

 10.01 to 12

 8.01 to 10

 6.01 to 8

 4.01 to 6

 2.01 to 4

 0 to 2

 Unclassifiable

94o

92o

90o

96o

48o

46o

44o

NORTH DAKOTA 

CANADA

IOWA

Lake Superior 

Anoka

Sand

Plain

SOUTH DAKOTA 

Mississippi

River

WISCONSIN

Minnesota

River

MINNESOTA

Figure 4.  Average annual recharge rate to surficial materials in Minnesota (1971–2000)

estimated on the basis on the regional regression recharge method (modified from Lorenz

and Delin, 2007).

Spatial trends in recharge in Minnesota (fig. 4) reflect general trends in 

precipitation. In the western and northwestern parts of the State where precipitation is least (20–25 in/yr (inches per 

year)), recharge also is least (0–2 in/yr). 

In contrast, recharge increases in the eastern part of the State to greater than 6 in/yr 

as precipitation increases to greater than 

about 30 in/yr. 

In addition to being affected by 

precipitation, recharge rates are locally 

affected by soil properties. Sandy areas, 

such as the Anoka sand plain and others 

in the central and east-central parts of 

the State, have coarse-textured soils that 

correspond well with RRR rates in the 

6–10 in/yr range (fig. 4). Clay-rich soils, 

such as those in the western parts of the 

State, correspond well with RRR rates in 

the 0–4 in/yr range. The “Unclassifiable” 

areas in figure 4 represent primarily peatlands, where the organic content is too 

great to estimate RRR rates accurately. 

Fine-scale variability in weather 

patterns, soil properties, land use, and 

topography were not included in the RRR 

model of the State. Low permeability 

units at or near land surface, ET in areas 

of shallow ground water, and impervious 

surfaces could effectively reduce the 

Recharge rates to unconfined 

aquifers in Minnesota typically are 

about 20–25 percent of precipitation. A crude, preliminary estimate 

of recharge rate is sometimes 

made on the basis of this assumption. Recharge rates where glacial 

clays or till are present, however, 

generally are less than 10 percent 

of precipitation. By comparison, 

leakage rates to confined aquifers 

generally are less than 1 percent of 

precipitation.
estimated recharge rates locally. Focused 

recharge to depressional areas by runoff 

from upland areas also may increase 

recharge locally. In addition, 30-year 

averages of precipitation and other data 

were used to construct the RRR model 

(Lorenz and Delin, 2007). Thus, actual 

recharge rates may vary during a given 

year as well as from year to year, depending on weather patterns. Nonetheless, 

the RRR recharge estimates (fig. 4) can 

be a useful source of input for regional 

ground-water flow models and should be 

helpful to resource managers in developing water-management plans at State and 

regional scales. 

Methods Used to Estimate 

Recharge Rates in Minnesota

Ground-water recharge rates cannot 

easily be measured, thus recharge rates 

must be estimated using indirect methods. 

Recharge rates in Minnesota have been 

estimated by means of many methods. A 

detailed description of all recharge-estimation methods and their limitations is 

beyond the scope of this fact sheet. The 

reader is referred to Scanlon and others 

(2002) for a thorough review of recharge 

estimation methods and their limitations. 

The methods commonly used in Minnesota are briefly described below and 

grouped on the basis of spatial scale: 

local, representing areas up to thousands 

of square feet; basin, representing tens to 

hundreds of square miles, and regional, 

representing hundreds to thousands of 

square miles. 

Local-Scale Methods

The water-table fluctuation (WTF) 

method may be the most widely used 

method for estimating recharge rates in 

humid regions (Healy and Cook, 2002). 

The WTF method relates changes in measured water-level elevation to changes in 

the amount of water stored in the aquifer. 

Recharge is assumed to be equal to the 

product of water-table rise and specific 

yield (fig. 3a). Specific yield is the 

amount of water a unit volume of saturated permeable material will yield when 

drained by gravity. Several approaches 

have been used in Minnesota for estimating the peak water-table rise attributed 

to the recharge event (Delin and others, 

2007). The method is relatively easy to 

use but is limited by the accuracy with 

which specific yield can be determined; 

it is also affected by water-level measurement frequency and accuracy. 

Delin and others (2007) 

showed that water-level measurement frequency is important in 

applying the water-table fluctuation method. Measurements made 

less frequently than about once 

per week resulted in as much as 

a 48 percent underestimation of 

recharge based on an hourly measurement frequency (fig. 5).

The age dating of ground water 

method uses estimated ground-water 

ages (the time elapsed since the water 

entered the aquifer as recharge) and welldepth information to obtain an estimate 

of vertical ground-water velocity. The 

velocity is then multiplied by aquifer 

porosity to obtain an estimate of recharge 

rate. This method is limited primarily 

by the accuracy with which the groundwater age is determined and by the accuracy with which aquifer porosity can be 

determined. Environmental tracers such 

as chlorofluorocarbons and sulfur-hexafluoride (as described by Busenberg and 

Plummer, 1992 and 2000, among others) 

are used to estimate ground-water age to 

within about 1 to 2 years. 

The chloride tracer method is used 

to estimate recharge rates as the product 

of precipitation and the ratio of chloride 

concentration in wet and dry deposition 

on land surface to chloride concentration in the unsaturated or saturated zones 

(Scanlon and others, 2002). The unsaturated zone is that part of the subsurface 

from land surface down to the water 

table. Chloride is conservative (not prone 

to change in concentration as a result of 

chemical reactions), and its mass inflow 

must be balanced by mass outflow or by 

a change in storage. In areas where total 

infiltration is less than total precipitation, 

because of surface runoff, the chloride 

tracer method will overestimate actual 

recharge. 

(a)

Recharge / precipitation = 5.3 percent

(b)

Recharge / precipitation = 7.8 percent

Recharge / precipitation = 3.6 percent

(c)

Depth to water, feet below measuring point

Jan

Feb

Mar

Apr

May

June

July

Aug

Sept

Oct

Nov

Dec

31.6

31.8

32.0

32.2

32.4

31.6

31.8

32.0

32.2

32.4

31.6

31.8

32.0

32.2

32.4

Data point

Figure 5.  Hydrographs and recharge

estimates for a well near Bemidji, Minn., for

calendar year 2002 based on the watertable fluctuation method and a water-level

measurement frequency of once every

(a) day, (b), month, and (c) two months.

The unsaturated-zone water 

balance (UZWB) method (Delin and 

others, 2000) is based on the premise that 

soil water moves upward in response to 

ET above the zero-flux plane boundary in 

the unsaturated zone and that water below 

that boundary eventually percolates 

downward to the water table. Recharge 

is estimated by measuring the change in 

water stored below the boundary over 

time. This method, sometimes called 

the zero-flux plane method, is limited 

primarily by the requirement of intensive 

collection of soil-moisture data from the 

unsaturated zone. 
The unsaturated-zone drainage 

method can be used to estimate recharge 

rates by directly measuring the vertical 

flow of water from gravity lysimeters 

installed in the unsaturated zone. This 

drainage from the lysimeters represents 

water that has not yet reached the water 

table and has been termed “potential 

recharge” by Scanlon and others (2002). 

Lysimeters are not routinely used to estimate recharge because they are expensive 

and difficult to construct, and they also 

require substantial maintenance. 

The Darcian flux method is used 

to estimate recharge rates on the basis 

of Darcy’s Law, by using estimated 

hydraulic head gradient and estimated 

hydraulic conductivity. Several variants 

of this method can be used with data 

from saturated and unsaturated materials. In the unit gradient variation of this 

method, one assumes the matric potential 

in the unsaturated zone is constant with 

depth and gravity is the only driving force 

(Nimmo and others, 2003). 

Seepage meters can be used to 

estimate seepage to or from surfacewater bodies (Scanlon and others, 2002). 

A seepage meter typically consists of a 

cylinder that is pushed into the bottom of 

a stream or lake; an attached reservoir, 

commonly a plastic bag, collects water. 

A ground-water recharge rate can be estimated from the rate at which water in the 

cylinder infiltrates, as determined from 

changes in the reservoir volume. This 

method is inexpensive and easy to apply; 

however, numerous measurements may 

be necessary because of the point nature 

of the estimate. 

Basin-Scale Methods

RORA (Rutledge, 1998) is an automated method for estimating the average 

recharge rate in a basin from analysis of 

streamflow records, and is based on the 

recession-curve-displacement method of 

Rorabaugh (1960). RORA accounts for 

the effects of ET, underflow (the flow 

of ground water beneath and bypassing 

a stream), and other losses or gains of 

ground water after a precipitation event. 

High-quality, long-term, continuous hydrologic and climatic data 

are important in estimating recharge 

rates. High-quality streamflow 

(fig. 6), ground-water level (fig. 7), 

and climate data are required for some 

recharge estimation methods. Missing data add an additional degree of 

uncertainty to the results.

In the streamflow-hydrograph 

separation methods, base flow is used 

as a proxy for recharge. Base flow is 

defined as ground water that discharges 

to a stream and sustains the flow during 

dry periods. In using these methods, one 

assumes that ground-water underflow, 

ET, and other losses of ground water 

are minimal. PART (Rutledge, 1993), 

HYSEP (Sloto and Crouse, 1996), and 

BFI (Wahl and Wahl, 1988) are computer 

programs that separate base flow from a 

streamflow hydrograph on the basis of 

different criteria.

Regional-Scale Methods

The regional regression recharge 

(RRR) method of Lorenz and Delin 

(2007) yields an estimate of spatial variability of annual recharge rates within a 

region. Many other methods are documented in the literature, and the reader is 

referred to Lorenz and Delin (2007) for 

a review of these references. The RRR 

method is based on a regression of RORA 

recharge rate estimates with climate and 

soil data for the region. The accuracy of 

the RRR estimates are representative of 

the soils data, which are collected over 

areas ranging from about 2 to 150 square 

miles. Recharge rates estimated on the 

basis of the RRR method (fig. 4) compared favorably with results from localand basin-scale methods in Minnesota. 

The RRR rates on average were about 

41 percent less than UZWB rates, ranged 

from 35 percent greater to 12 percent less 

than WTF rates, were about 12 percent 

less than age-dating-method rates, and 

were about 5 percent greater than RORA 

Figure 6.  High-quality, continuous

streamflow data are essential for accurate

recharge estimation using basin-scale

methods. The USGS maintains a nationwide

network of stream-gaging stations, with

stream water level and flow data served in

real-time to the Internet. Photograph by Charlie Smith, U.S. Geological Survey

rates (Delin and others, 2007). RRR 

rate estimates could be a useful source 

of input for regional ground-water flow 

models. 

The water-balance equation is 

a common approach for estimating 

recharge rates at regional scales. A general form of a water budget is:

P + Qon = ET + Qoff +∆S 

where P is precipitation (including irrigation), Qon and Qoff are water flow onto and 

off the site respectively, ET is evapotranspiration, and ∆S is change in water 

storage (Scanlon and others, 2002). Many 

other variations of this equation can be 

formulated by incorporating subcomponents of each variable. All ground-water 

and surface-water flow models fit under 

this method because they are based on a 

water-balance equation. Although listed 

here as a regional-scale method, a water 

budget estimate of recharge can also be 

made on the basis of local- or basin-scale 

data. 
References

Busenberg Eurybiades, and Plummer 

L.N., 1992, Use of chlorofluorocarbons (CCl3

F and CCl2F2) as hydrologic 

tracers and age-dating tools—The 

alluvium and terrace system of Central 

Oklahoma: Water Resources Research, 

v. 28, no. 9, p. 2257–2283. 

Busenberg, Eurybiades, and Plummer, 

L.N., 2000, Dating young groundwater 

with sulfur hexafluoride—Natural and 

anthropogenic sources of sulfur hexafluoride: Water Resources Research, 

v. 36, no. 10, p. 3011–3030.

Cowdery, T.K., 2005, Hydrogeology and 

ground-water/surface-water interactions in the Des Moines River Valley, 

southwestern Minnesota, 1997–2001: 

U.S. Geological Survey Scientific 

Investigations Report 2005–5219, 51 p.

Delin, G.N., Healy, R.W., Landon, M.K., 

and Böhlke, J.K., 2000, Effects of 

topography and soil properties on 

recharge at two sites in an agricultural 

field: Journal of the American Water 

Resources Association, v. 36, no. 6, 

p. 1401–1416. 

Delin, G.N., Healy, R.W., Lorenz, D.L., 

and Nimmo, J.R., 2007, Comparison 

of local- to regional-scale estimates of 

ground-water recharge in Minnesota, 

USA: Journal of Hydrology, v. 334, 

no. 1-2, p. 231–249.

Healy, R.W., and Cook, P.G., 2002, 

Using groundwater levels to estimate 

recharge: Hydrogeology Journal, v. 10, 

p. 91–109. 

Lorenz, D.L., and Delin, G.N., 2007, A 

regression model to estimate regional 

ground-water recharge in Minnesota: 

Ground Water, v. 45, no. 2, 10.1111/

j.1745-6584.2006.00273.x. 

Nimmo, J.R., Stonestrom, D., and Healy, 

R.W., 2003, Aquifer recharge in Encyclopedia of Water Science: Stewart, 

B.A., and Howell, T.A., eds., New 

York, Marcel Dekker, p. 1– 4.

Rorabaugh, M., 1960, Use of water levels 

in estimating aquifer constants in a 

finite aquifer: International Association 

of Scientific Hydrology Commission 

of Subterranean Waters, publication 

52, p. 314–323.

Rutledge, A.T., 1993, Computer programs for describing the recession of ground-water discharge and 

for estimating mean ground-water 

recharge and discharge from streamflow records: U.S. Geological Survey 

Water-Resources Investigations Report 

93–4121, 45 p. 

Rutledge, A.T., 1998, Computer programs for describing the recession of 

ground-water discharge and for estimating mean ground-water recharge 

and discharge from streamflow 

records—Update: U.S. Geological 

Survey Water-Resources Investigations 

Report 98–4148, 43 p.

Scanlon, B.R., Healy, R.W., and Cook, 

P.G., 2002, Choosing appropriate techniques for quantifying groundwater 

recharge: Hydrogeology Journal, v. 10, 

p. 18–39. 

Sloto, R.A., and Crouse, M.Y., 1996, 

HYSEP—A computer program for 

streamflow hydrograph separation 

and analysis: U.S. Geological Survey 

Water-Resources Investigations Report 

96–4040, 46 p.

Wahl, K.L., and Wahl, T.L., 1988, Determining the flow of Comal Springs at 

Braunfels, Texas, in Texas Water ’95, 

San Antonio, Texas, August 16-17, 

1995, Proceedings: American Society 

of Civil Engineers, p. 77–86.

Winter, T.C., Harvey, J.W., Franke, O.L., 

and Alley, W.M., 1998, Ground water 

and surface water—a single resource: 

U.S. Geological Survey Circular 1139, 

79 p. 

By G.N. Delin1

 and J.D. Falteisek2

Information on the 1U.S. Geological Survey Minnesota Water Science 

Center can be obtained at following 

web site http://mn.water.usgs.gov/ 

and information on the 2Minnesota 

Department of Natural Resources 

Division of Waters can be obtained at 

http://www.dnr.state.mn.us/waters/

Figure 7.  High quality, continuous groundwater level data are essential for accurate

recharge estimation with the water-table

fluctuation method. In this picture a USGS

hydrologist is programming dataloggers to

measure ground-water levels in Otter Tail

County. The schematic diagram beneath

the picture illustrates an observation well

completed in a glacial aquifer. Photograph by Jeffrey Stoner, U.S. Geological Survey

SOIL

SAND AND

GRAVEL

AQUIFER AQUIFER

WATER TABLE

CLAY