Drought-Sensitive Aquifer Settings in 

Southeastern Pennsylvania

by Tammy M. Zimmerman and Dennis W. Risser 

U.S. Department of the Interior 

U.S. Geological Survey

In cooperation with the 

Pennsylvania Department of Conservation and Natural Resources, 

Bureau of Topographic and Geologic Survey 

Scientific Investigations Report 2005-5169
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: 2005

For sale by U.S. Geological Survey, Information Services 

Box 25286, Denver Federal Center 

Denver, CO 80225

For more information about the USGS and its products: 

Telephone: 1-888-ASK-USGS 

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report.

Suggested citation:

Zimmerman, T.M., and Risser, D.W., 2005, Drought-sensitive aquifer settings in southeastern Pennsylvania: U.S. Geological Survey Scientific Investigations Report 2005-5169, 21 p.
 iii

Contents

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

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

Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Description of Study Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Previous Studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Hypothesized Drought-Sensitive Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Height of Water Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Rate of Decline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Water-Table Decline (WTD) Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Settings Categorized by Water-Table Decline Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Settings Categorized by Topographic Position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Documenting Drought-Affected Wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Well-Owner Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Well-Driller and Agency Surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Comparing Hypothesized Drought-Sensitive Settings to Location of Drought-Affected Wells . . . . . . . . . . . . . . . 13

Water-Table Decline Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Topographic Position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Combined Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

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

Figures

1. Map showing study area as defined by the Piedmont and New England Physiographic 

Provinces and South Mountain Section of the Ridge and Valley Physiographic Province of 

southeastern Pennsylvania . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

2. Sketch showing idealized strip aquifer of width (2a) and infinite length, with constant 

recharge (W), showing height of water table (ho) above draining stream. . . . . . . . . . . . . . . . . . . . . . . . . . . .5

3. Graph showing simulated water-level declines in wells at upland, mid-slope, and nearstream locations for the idealized aquifer shown in figure 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

4. Map showing location of drought-affected wells in southeastern Pennsylvania during 

1998-2002 as determined from results of well-owner, well-driller, and agency surveys . . . . . . . . . . . . 11

5. Graphs showing drought-affected wells reported by Chester and Montgomery County 

Health Departments and cumulative departure from normal precipitation, 1998-2002. . . . . . . . . . . . . . 13

6. Maps showing density of drought-affected wells during the 1999 and 2002 droughts in 

(A) Chester County and (B) Montgomery County, Pennsylvania. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

7. Graph showing relation between water-table decline index (low, moderate, high) and 

incidence of drought-affected wells reported by Chester and Montgomery County Health 

Departments, southeastern Pennsylvania, during the 1999 and 2002 droughts and as a 

percentage of Water Well Inventory (WWI) wells.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
iv 

8. Graph showing relation between topographic position (valley, slope, upland) and 

incidence of drought-affected wells reported by Chester and Montgomery County Health 

Departments, southeastern Pennsylvania, during the 1999 and 2002 droughts and as a 

percentage of Water Well Inventory (WWI) wells.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

9. Map showing drought-sensitive settings in southeastern Pennsylvania determined by 

water-table decline index of geologic units and topographic position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Tables

1. Generalized rock types in southeastern Pennsylvania . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2. Households with private wells in the study area, southeastern Pennsylvania, 1990 . . . . . . . . . . . . . . . . . 4

3. Streamflow-recession index for three streams in basins underlain by different 

generalized rock types in southeastern Pennsylvania . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4. Categorization of aquifers in southeastern Pennsylvania by water-table decline index . . . . . . . . . . . . . 8

5. Results from the 35 responses to the well-owner survey showing characteristics of 

domestic-supply wells in southeastern Pennsylvania affected by drought in 2002 . . . . . . . . . . . . . . . . . 12

6. Dry wells in each aquifer in Chester and Montgomery Counties, Pennsylvania, 

expressed per 100 square miles and as percentage of Water Well Inventory wells. . . . . . . . . . . . . . . . 16

7. Comparison of drought-affected wells (1999 and 2002 droughts) in Chester and Montgomery 

Counties, Pennsylvania, in nine settings defined as the intersection of water-table decline 

index and topographic position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Conversion Factors

*Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times 

foot of aquifer thickness [(ft3/d)/ft2]ft. In this report, the mathematically reduced form, foot 

squared per day (ft2/d), is used for convenience.

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

Area

square mile (mi2) 2.590 square kilometer (km2)

Flow rate

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

Transmissivity*

foot squared per day (ft2/d) 0.09290 meter squared per day (m2/d)
Drought-Sensitive Aquifer Settings in Southeastern 

Pennsylvania

by Tammy M. Zimmerman and Dennis W. Risser 

Abstract 

This report describes the results of a study conducted by 

the U.S. Geological Survey, in cooperation with the Pennsylvania Department of Conservation and Natural Resources, Bureau 

of Topographic and Geologic Survey, to determine droughtsensitive aquifer settings in southeastern Pennsylvania. 

Because all or parts of southeastern Pennsylvania have been in 

drought-warning or drought-emergency status during 6 of the 

past 10 years from 1994 through 2004, this information should 

aid well owners, drillers, and water-resource managers in guiding appropriate well construction and sustainable use of Pennsylvania’s water resources.

“Drought-sensitive” aquifer settings are defined for this 

study as areas unable to supply adequate quantities of water to 

wells during drought. Using information from previous investigations and a knowledge of the hydrogeology and topography 

of the study area, drought-sensitive aquifer settings in southeastern Pennsylvania were hypothesized as being associated 

with two factors – a water-table decline (WTD) index and topographic setting. The WTD index is an estimate of the theoretical 

water-table decline at the ground-water divide for a hypothetical aquifer with idealized geometry. The index shows the magnitude of ground-water decline after cessation of recharge is a 

function of (1) distance from stream to divide, (2) ground-water 

recharge rate, (3) transmissivity, (4) specific yield, and (5) duration of the drought. WTD indices were developed for 39 aquifers that were subsequently grouped into categories of high, 

moderate, and low WTD index. 

Drought-sensitive settings determined from the hypothesized factors were compared to locations of wells known to 

have been affected (gone dry, replaced, or deepened) during 

recent droughts. Information collected from well owners, drillers, and public agencies identified 2,016 wells affected by 

drought during 1998-2002. Most of the available data on the 

location of drought-affected wells in the study area were from 

Chester and Montgomery Counties because those counties have 

well-construction regulations that identify wells that failed during drought. The locations of drought-affected wells in Chester 

and Montgomery Counties indicated the most highly sensitive 

settings are uplands and slopes in aquifers with high WTD 

index and uplands in aquifers with moderate WTD index. The 

least sensitive settings are in aquifers with low WTD index, in 

valleys, or on slopes. A map was developed showing the relative drought sensitivity (low, moderate, and high) of aquifers in 

southeastern Pennsylvania.

Study results were limited by the inability to obtain much 

information about the location of drought-affected wells, with 

the exception of Montgomery and Chester Counties. Also, the 

construction characteristics (particularly depth) of droughtaffected wells generally were not available. Well depth could be 

used to distinguish between problems caused by shallow well 

depth (generally less than 100 ft) and those caused by deficiency of the aquifer to supply water. With the exception of 

owner-derived information from a public survey on droughtaffected wells (35 wells), depth data were not obtained. Data 

from the 35 drought-affected wells indicated most were drilled 

(not dug) and were completed to depths greater than 100 feet. 

This finding indicates that the affects of recent droughts in 

southeastern Pennsylvania were not restricted to shallow dug 

wells, but also affected deeper drilled wells. 

Introduction 

All or parts of southeastern Pennsylvania have been in 

drought-warning or drought-emergency status during 6 of the 

past 10 years from 1994 through 2004 (Pennsylvania Department of Environmental Protection, 2004). In response to the 

precipitation deficits during these droughts, ground-water levels throughout most of southeastern Pennsylvania fell below 

seasonal average levels and, in some cases, all-time historic low 

levels. The low ground-water levels have caused hundreds of 

domestic-supply wells to go dry or become unusable because of 

decreased yield (Kyle Schmeck, Montgomery County Health 

Department, written commun., 2002; Benjamin Aller, Chester 

County Health Department, written commun., 2002). 

“Drought-sensitive” aquifer settings are defined for this 

study as areas unable to supply adequate quantities of water to 

wells during dry periods. This is a loose definition aimed to 

broadly identify areas that might be most problematic for well 

owners during droughts. Whether or not a well “goes dry” dur-
2 Drought Sensitive Aquifer Settings in Southeastern Pennsylvania

ing a drought depends on the severity of drought, the inherent 

sensitivity of the aquifer setting to reduced recharge, and the 

characteristics of the well—predominantly its depth. During 

droughts, anecdotal and media reports frequently document the 

occurrence of wells going dry but whether they are related to a 

site-specific well-construction problem or an insufficient supply of water within the larger aquifer setting is usually not clear. 

Information about the location and extent of drought-sensitive aquifer settings in southeastern Pennsylvania is needed to 

identify areas where ground-water shortages may occur during 

times of drought. Such information would assist well owners to 

obtain wells of adequate depth, allow developers to more accurately plan for adequate water supplies, and help planners and 

water-resources managers guide the sustainable use of Pennsylvania’s water resources. In 2002, the U.S. Geological Survey 

(USGS), in cooperation with the Pennsylvania Department of 

Conservation and Natural Resources, Bureau of Topographic 

and Geologic Survey (PaGS), began a study to identify droughtsensitive aquifer settings in southeastern Pennsylvania where 

ground-water shortages may result during drought.

Purpose and Scope

This report presents and tests a hypothesis about the conditions creating drought-sensitive aquifer settings in southeastern Pennsylvania. Probable drought-sensitive aquifer settings 

are identified on the basis of a water-table decline (WTD) index 

and topography. The drought-sensitive aquifer settings are 

compared to locations of drought-affected wells in Chester and 

Montgomery Counties identified by the County Health Departments during the two most recent droughts (December 1998 

through September 1999 and November 2001 through November 2002) to confirm or refute the hypothesis that wells located 

in aquifers categorized as drought sensitive are problematic 

during drought. Overall, drought sensitivities are defined on the 

basis of hypothesized factors that may be useful for identifying 

the spatial distribution of drought-affected wells in southeastern 

Pennsylvania. The scope of the report is limited to evaluating 

the effect of drought on wells; the effect on base flow to streams 

is not discussed.

Description of Study Area

The study area is the Piedmont and New England Physiographic Provinces and South Mountain Section of the Ridge and 

Valley Physiographic Province of southeastern Pennsylvania 

(fig. 1). The area is approximately 5,100 mi2 and spans all or 

parts of Adams, Berks, Bucks, Chester, Cumberland, Dauphin, 

Delaware, Franklin, Lancaster, Lebanon, Lehigh, Montgomery, 

Northampton, and York Counties.

The physiography of the study area is diverse. Topography 

varies from the pronounced ridges and deep valleys of the South 

Mountain Section of the Ridge and Valley Physiographic Province to the broad valleys separated by low hills of the Piedmont 

Lowland Section of the Piedmont Physiographic Province 

(Sevon, 2000). Geology varies from igneous and metamorphic 

rocks of Precambrian age that have undergone multiple periods 

of intense deformation to sedimentary rocks of Triassic age that 

have undergone relatively little deformation compared to the 

igneous rocks. 

Characteristics of aquifers within the study area also are 

diverse, but aquifers generally can be categorized according to 

generalized rock type (table 1). Ground water is present and 

moves in crystalline (igneous and metamorphic), siliciclastic 

(sandstone and shale), and carbonate (limestone and dolomite) 

aquifers primarily within fracture space in the rock, although in 

some valley settings the saturated regolith can store and transmit water. The frequency of water-bearing fractures tends to 

diminish with depth, so ground-water-flow paths are local and 

shallow—generally less than about 400 ft deep (Low and others, 2002). In the Gettysburg-Newark Lowland Section of the 

Piedmont Physiographic Province, water-producing fractures 

sometimes are encountered at depths as great as 600 ft. Deep 

water-producing zones are a characteristic that would make 

these aquifers inherently less susceptible to dewatering of 

water-bearing fractures during drought.

Table 1. Generalized rock types in southeastern Pennsylvania. 

Generalized rock type Specific rock types within each generalized rocktype category Principal physiographic setting

Crystalline Igneous and metamorphic rocks predominate. 

Major lithologies are schist, gneiss, diabase, 

quartzite, phyllite, and metavolcanics.

South Mountain Section of Ridge and Valley Province, Reading Prong Section of New England 

Province, and Upland Section of Piedmont Province, Gettysburg-Newark Lowland Section of 

Piedmont Province (diabase only)

Siliciclastic Sandstone, siltstone, shale, and conglomerate. Gettysburg-Newark Lowland Section of Piedmont 

Province

Phylitic shale. Lowland Section of Piedmont Province

Carbonate Limestone and dolomite with some interbedded 

shale.

Lowland Section of Piedmont Province
Figure 1. Study area as defined by the Piedmont and New England Physiographic Provinces and South Mountain Section of the Ridge and Valle

ince of southeastern Pennsylvania. 
4 Drought Sensitive Aquifer Settings in Southeastern Pennsylvania

The study area contains a large rural population that 

depends on ground water from private on-lot wells as a daily 

source of drinking water and a source of water for livestock. 

These private on-lot wells are more vulnerable to drought conditions because they typically are shallower and yield less water 

than wells drilled for public-supply, commercial, or industrial 

uses. About 385,000 households in the study area rely on 

ground water from private on-lot wells for daily water supply 

(U.S. Bureau of the Census, 1992). Southeastern Pennsylvania 

has a greater density of private on-lot wells compared to Pennsylvania as a whole (Fleeger, 1999). Reliance on private wells 

is particularly great for residents of Adams County, where 

56 percent of households have private wells (table 2). 

Previous Studies

The hydrogeology of southeastern Pennsylvania has been 

extensively studied and is documented in numerous reports. 

Most of these studies provided information about ground-water 

availability and well yields, but few specifically characterized 

drought-sensitive aquifer settings in southeastern Pennsylvania. 

Using the information from these previous studies, Low and 

others (2002) described the hydrogeology of the study area and 

analyzed well data from over 12,000 wells in the USGS 

Ground-Water Site Inventory (GWSI) database and the PaGS 

Water Well Inventory (WWI) database.

The hypothetical effects of drought on the ground-water 

resources in parts of southeastern Pennsylvania were estimated 

in past studies by the use of ground-water-flow models. Gerhart 

and Lazorchick (1984) simulated effects of a winter-spring 

drought in parts of Berks and Lancaster Counties and showed 

that ground-water levels declined most in the Triassic-age sedimentary rocks and least in the carbonate rocks. In a related 

study, Gerhart and Lazorchick (1988) simulated ground-water 

declines throughout the lower Susquehanna River Basin in 

response to basinwide ground-water withdrawals. Model-simulation results from the two studies can be viewed as showing the 

effect of drought if simulated ground-water withdrawals are 

interpreted as a reduction in recharge instead of an increase in 

pumping. In Chester County, Sloto (2004) simulated the effects 

of drought and ground-water withdrawals in the French Creek 

Basin, with an emphasis on estimating the reduction of base 

flow to streams. Ground-water declines at various locations 

within the watershed were shown for a simulated drought of 

90 days.

Table 2. Households with private wells in the study area, southeastern Pennsylvania, 1990.

[From U.S. Bureau of Census, 1992]

County

(Fig. 1)

Number of 

households 

Number of 

households with 

private wells 

Total estimated 

water usage1

(gallons per day) 

in households 

with private wells

Percentage of 

households 

with private 

wells 

Adams2 28,066 15,655 3,804,000 56

Berks 127,849 38,847 9,440,000 30

Bucks 190,312 45,507 11,058,000 24

Chester2 133,592 49,316 11,984,000 37

Cumberland 73,506 19,587 4,760,000 27

Dauphin 95,123 21,655 5,262,000 23

Delaware 201,618 6,757 1,642,000 3

Franklin 45,642 14,455 3,513,000 32

Lancaster 151,352 50,966 12,385,000 34

Lebanon 42,7098 13,034 3,167,000 31

Lehigh 112,552 17,465 4,244,000 16

Montgomery 254,596 30,716 7,464,000 12

Northampton 90,619 17,456 4,242,000 19

York2 128,764 43,441 10,556,000 34

Total 1,676,299 384,857 93,520,000 23

1

Based on an average of 243 gallons of water per day per average household of four persons.

2

The county is entirely within the study area. All other counties are only partially in the study area.
Hypothesized Drought-Sensitive Settings 5

 A recent study was conducted as a direct result of the 

drought of 1998-99 when more than 20 private wells went dry 

in the Borough of Carroll Valley in Adams County. Low and 

Conger (2002) used data from 369 wells throughout an area of 

about 5 mi2 to show the crystalline-rock aquifers—metabasalt, 

metarhyolite, and greenstone schist—yielded only small 

amounts of water to the wells. The low yields, coupled with 

shallow water-bearing zones, probable poor connection to 

ground-water storage in the regolith, and seasonal water-level 

declines of as much as 40 ft, illustrated the susceptibility of 

these aquifers to drought.

Hypothesized Drought-Sensitive Settings

Drought-sensitive aquifer settings are defined in this study 

as those that fail to supply adequate quantities of water to wells 

during dry periods (droughts). Assuming well-construction 

characteristics are not variable, the most problems with water 

wells usually occur during drought in areas where the decline in 

water-table altitude is greatest or where the water table falls 

below major water-producing fractures. Theoretically, the magnitude of water-table decline during drought should be related 

to the initial height of the water table and the rate at which the 

water table declines when recharge ceases. 

Theory

Consider the case of an idealized strip aquifer of infinite 

length receiving uniform recharge as shown in figure 2. In this 

case, the aquifer transmissivity and stream stage are assumed to 

be constant. The distance from the stream to the ground-water 

divide is a, and the steady-state height of the water table above 

the stream is ho for any distance x from the stream.

The decline of the water table through a 90-day period 

without recharge was simulated by the use of MODFLOW 

(Harbaugh and others, 2000) for the idealized aquifer shown in 

figure 2. For the simulation, the aquifer was assumed to be 

2,000 ft wide (2a), recharge prior to the drought was 12 in/yr, 

transmissivity of the aquifer was 100 ft2/d, and specific yield 

was 0.01. Water-level declines in three wells tapping this idealized aquifer at various distances (x) from the stream are shown 

in figure 3. The magnitude of water-level decline was greatest 

(13 ft) at the upland well 1,950 ft from the stream and smallest 

(3.8 ft) at the near-stream well 150 ft from the stream. The magnitude of decline appears to be a function of (1) initial height of 

the water table prior to the drought (ho) and (2) rate of watertable decline during the drought. Understanding the aquifer and 

basin characteristics that control the water-table height and 

decline rate will help identify the location of drought-sensitive 

aquifer settings.

Figure 2. Idealized strip aquifer of width (2a) and 

infinite length, with constant recharge (W), 

showing height of water table (ho) above draining 

stream. 

Figure 3. Simulated water-level declines in wells at upland, 

mid-slope, and near-stream locations for the idealized 

aquifer shown in figure 2. 
6 Drought Sensitive Aquifer Settings in Southeastern Pennsylvania

Height of Water Table 

The initial height of the water table for an idealized unconfined aquifer (fig. 2) of large thickness was given by Jacob 

(1944, eq. 2). Assuming the water table is in equilibrium with 

steady recharge, the water-table profile is described by

h , (1) o

W

T

-----     ax

x

2

2 = ----–

where

ho is the height of the water table above the draining 

stream, in feet;

W is the steady pre-drought recharge rate, in feet per 

day;

T is the transmissivity, in feet squared per day;

a is the distance from the stream to the ground-water 

divide, in feet; 

and

x is the distance from the stream to any location on the 

water table, in feet.

Equation 1 indicates the height of the water table is 

directly related to a, W, and x and is inversely related to T. The 

effect of well location (x) within a basin can be seen in the differing initial water levels in the upland, mid-slope, and nearstream wells (fig. 3) for the drought conditions simulated for the 

idealized aquifer in figure 2. In actual basins, a is difficult to 

determine, but it can be approximated as half the reciprocal of 

drainage density (D), where drainage density is the total length 

of streams divided by the basin area (Carlston, 1963) and given 

as

a . (2) 1

2D = -------

At the ground-water divide, where x = a, equation 1 simplifies to

ho . (3)

a

2

W

2T ----------=

Rate of Decline

The approximate rate of water-table decline from a steadystate condition for the ideal aquifer in figure 2 is an exponential 

function of transmissivity, specific yield, and distance from the 

stream to the ground-water divide. These properties define the 

hydraulic response time for the aquifer (Alley and others, 2002) 

and control how quickly water levels decline during drought 

periods. The water-table height above the stream at some time 

after cessation of recharge is given approximately by Jacob 

(1944, eq. 9) rewritten in base 10 notation as

h h , (4) o10

–tT

0.933a

2

Sy

-------------------------    

=

where

h is the height of the water table at time t;

t is time after cessation of recharge, in days; 

and 

Sy is the specific yield.

Although the variables T, Sy, and a are difficult to quantify, analysis of streamflow records can provide an index that 

lumps these variables together. Rorabaugh and Simons (1966) 

showed that the streamflow–recession index (K) derived from 

analysis of the master recession curve equals (0.933 a2Sy/T). In 

the study area of southeastern Pennsylvania, the streamflowrecession index was determined for various streams representative of basins underlain by aquifers composed primarily of 

either siliciclastic, crystalline, or carbonate rocks (table 3). The 

exponent (-tT/0.933a2Sy) of equation 4 can be replaced by 

(-t/K), so it follows that the smaller the value of K in table 3, the 

greater the rate of water-table decline during drought. Thus, in 

general, the rate of water-table decline should be greatest for 

siliciclastic rocks, moderate for crystalline rocks, and least for 

carbonate rocks. 

Table 3. Streamflow-recession index (K) for three streams in basins underlain by different generalized rock types 

in southeastern Pennsylvania.

U.S. Geological Survey 

streamflow-gaging station 

and number

Years 

of record

 Generalized 

rock type 

in basin 

Streamflow-recession 

index (K), 

in days 

West Conowago Creek near Manchester 

(01574000)

73 Siliciclastic 32

French Creek near Phoenixville 

(01472157)

33 Crystalline 78

Valley Creek near Turnpike Bridge 

(01473169)

19 Carbonate 116
Hypothesized Drought-Sensitive Settings 7

Water-Table Decline (WTD) Index 

Combining equations 3 and 4 gives an approximate 

expression (equations below) for the decline in water table (∆h), 

in feet, at the ground-water divide of the idealized aquifer during a drought of given duration t

∆h (5) a

2

W

2T ---------- a

2

W

2T ---------- 10

–tT

0.933a

2

Sy

-------------------------    

–=

From equation 5, it is apparent that, even for the idealized 

aquifer, the magnitude of ground-water decline (∆h) after cessation of recharge is a function of at least these five variables—(1) distance from stream to divide (a), (2) ground-water 

recharge rate (W), (3) transmissivity (T), (4) specific yield (Sy), 

and (5) duration of the drought (t). In this report, the approximate expression for the magnitude of ground-water decline during drought for the idealized aquifer will be termed the WTD 

index. Because the WTD index describes the water-table 

decline at the divide, it can be thought of as a basin index, not 

affected by well location (x), which should be useful for comparing the relative drought sensitivity of different aquifers.

Aquifers with a large WTD index have some combination 

of small T, small Sy, large a, and large W. In actual aquifers with 

non-ideal properties, prediction of water-table declines is 

dependent on even more variables than for idealized aquifers. 

For example, in fractured-rock aquifers, T may not be constant 

in time. This condition is especially important for aquifers with 

small T and Sy that yield only marginally sufficient quantities of 

water to household-supply wells during periods of normal 

recharge. During times of drought, those marginal yields may 

drop at rates much greater than predicted because T is decreasing as water-bearing fractures are dewatered. Practical observations regarding the fractured-rock aquifers in southeastern 

Pennsylvania are:

• Aquifer properties are not spatially uniform—Rocks in 

upland settings are more resistant to erosion and tend to 

be less fractured than in valleys; thus, transmissivity 

and specific yield values are not spatially uniform. 

Transmissivity commonly is smaller for aquifers 

beneath hills (uplands) than valleys. 

• Aquifer properties are not constant with depth—The 

density of water-bearing fractures tends to be greatest 

near land surface and decreases with depth below land 

surface, so wells in upland settings (where the water 

table is deep) may be more vulnerable to the dewatering of a fracture when the water table falls during 

drought.

• Upland streams may not be perennial—The watertable decline during drought is greatest in upland settings most distant from streams, and the rate of decline 

accelerates when headwater streams become dry as 

drought conditions progress.

• Storage in saturated regolith is variable—The thickness of saturated regolith usually is thin to zero in 

upland settings but can be an important source of 

ground-water storage in valleys because the regolith 

typically has a greater capacity to store water than the 

underlying bedrock. 

From the preceding theory and practical observations, a 

plausible hypothesis for the conditions creating drought-sensitive aquifer settings in southeastern Pennsylvania is that the 

intersection of aquifers having (1) a large WTD index and 

(2) upland topographic settings should provide a good first estimate of the most drought-sensitive settings in the study area. 

The aquifers underlying the study area were characterized using 

this approach.

Settings Categorized by Water-Table Decline Index 

Aquifers in the study area were classified according to 

their WTD index (table 4). The aquifer names used in this study 

correspond to the nomenclature of geologic units used by the 

PaGS for the geologic map of Pennsylvania (Berg and others, 

1980). Derivation of the WTD index from equation 5 involved 

estimating characteristic values of T, Sy, a, and W for each aquifer. Values of T, Sy, and W were mostly taken from published 

studies involving model simulation of ground-water flow (Gerhart and Lazorchick, 1984, 1988; Sloto, 1990, 2004; Vogel and 

Reif, 1993; Senior and Goode, 1999). Model-simulation studies 

were used because aquifer properties and recharge rates were 

derived from model calibration against observed ground-water 

levels and base flow. In some cases, the same aquifer was 

assigned appreciably different properties in different studies, so 

a subjective determination was made of which values would be 

used based on the areal extent of the aquifer and scale of the 

study area. Generally, values from detailed studies of small 

areas were assumed to be better estimates of aquifer properties 

than values from regional studies. The approximate distance 

from stream to ground-water divides (a) was derived from 

stream-density for generalized rock types given in Gerhart and 

Lazorchick (1988, table 10).

Settings Categorized by Topographic Position

The topography of the study area was categorized as valley, slope, or upland by the use of a geographic information system (GIS) analysis based on the location of streams and basin 

divides. The valley topographic setting was defined as the area 

represented within a buffer of uniform width surrounding all 

streams classified as 3rd order or greater. Similarly, uplands 

were defined as the area bounded within a uniform buffer surrounding basin divides. Slopes were the areas not defined as 

valleys or uplands, which was most of the terrain. 

 Buffer widths for use in the analysis initially were 

assumed to be 300 ft on each side of the stream or basin divide. 

Although this method of classification oversimplifies the com-
Table 4. Categorization of aquifers in southeastern Pennsylvania by water-table decline (WTD) index. [The distance from stream to divide was taken from Gerhart and Lazorchick (1988, p. 23). If more than two references for aquifer properties are given, first is for recharge and trafor specific yield. If aquifer was anisotropic, the geometric mean of the maximum and minimum transmissivity values was used.] References for aquifer properties: 1, Gerhart and Lazorchick (1984); 2, Vogel and Reif (1993); 3, Werkheiser (1995); 4, Gerhart and Lazorchick (1988); 5, Senior and Goode ((2004); 8, Assumed same as plutonic; 9, Transmissivity estimated from Low and others (2002, p. 269), recharge from Low and Conger (2002), Sy assumed same as schist (Gerh

10, Assumed same as metarhyolite and greenstone schist; 11, Assumed same as Chickies Formation Aquifer Generalized rock type Recharge (W), in inches per year Distance from stream to divide (a), in feet Transmissivity (T), in feet squared per day Specific yield (Sy), dimensionless Water-table decline predicted from equation 5, in feet Water-t

decline (W

index cat

Stonehenge Formation and Beekmantown Group1 Carbonate 18.3 2,640 25,560 0.090 0.6 Low 

Cockeysville Marble Carbonate 8.3 2,640 912 .090 2.2 Low Gettysburg Formation Siliciclastic 7.1 1,940 1,202 .007 2.6 Low Brunswick Formation Siliciclastic 8.2 1,940 1,050 .007 3.4 Low Millbach Formation Carbonate 10.7 2,640 900 .070 3.5 Low Ledger Formation Carbonate 15.1 2,640 2,100 .080 3.5 Low Conestoga Formation Carbonate 15.3 2,640 2,010 .080 3.6 Low Ontelaunee and Richland 

Formations

Carbonate 11.3 2,640 750 .070 3.8 Low 

Zooks Corner Formation Carbonate 11.3 2,640 450 .070 4.1 Low Buffalo Springs Formation Carbonate 11.1 2,640 330 .070 4.1 Low 

Snitz Creek Formation Carbonate 11.1 2,640 330 .070 4.1 Low Kinzers Formation Carbonate 11.3 2,640 300 .070 4.2 Low Epler, Annville, and 

Hershey Formations

Carbonate 13.9 2,640 1,500 .060 4.3 Low 

Serpentinite Crystalline 8.4 1,780 576 .020 4.7 Low Vintage Formation Carbonate 13.2 2,640 300 .070 4.9 Low Elbrook Formation Carbonate 13.2 2,640 1,860 .080 5.0 Low Octoraro Formation Crystalline 6.9 1,780 150 .020 7.4 ModerStockton Formation Siliciclastic 15.5 1,940 900 .005 7.4 ModerPeach Bottom Slate and 

Cardiff Conglomerate

Crystalline 9.7 1,780 268 .020 8.4 Moder

Marburg Schist Crystalline 9.7 1,780 268 .020 8.4 ModerPeters Creek Schist Crystalline 9.7 1780 268 .020 8.4 Moder
Sams Creek Metabasalt Crystalline 9.7 1,780 268 0.020 8.4 ModerWissahickon Formation Crystalline 8.3 1,780 162 .020 8.7 ModerSetters Quartzite Crystalline 8.3 1,780 144 .020 9.0 ModerHammer Creek Formation Siliciclastic 15.5 1,940 700 .005 9.5 Moder

Plutonic Rocks2 Crystalline 8.3 1,780 35 .020 11.0 High 

Granitic Rocks Crystalline 8.3 1,780 35 .020 11.0 High New Oxford Formation Siliciclastic 8.4 1,940 223 .012 11.4 High Cocalico Formation Siliciclastic 11.6 2,080 300 .015 12.9 High Lockatong Formation Siliciclastic 8.2 1,940 217 .007 14.1 High Triassic Conglomerate3 Siliciclastic 4.2 1,940 39 .007 14.2 High Metarhyolite and Greenstone Schist Crystalline 14.8 1,780 150 .020 15.8 High 

Weverton and Loudon 

Formations

Crystalline 14.8 1,780 150 .020 15.8 High 

Gneissic Rocks4 Crystalline 15.5 1,780 170 .010 24.3 High 

Antietam and Harpers 

Formation

Crystalline 15.5 1,780 90 .010 31.5 High 

Chickies Formation Crystalline 15.5 1,780 90 .010 31.5 High Hardyston Formation Crystalline 15.5 1,780 90 .010 31.5 High Metabasalt Crystalline 14.8 1,780 150 .001 35.7 High Diabase Crystalline 15.5 1,940 135 .001 49.5 High 

1Properties of Beekmantown Group assumed same as Stonehenge Formation.2Plutonic rocks include quartz monzonite, granodiorite, anorthosite, gabbro and gabbroic gneiss, metadiabase, metagabbro, and pegmatite.3Triassic conglomerate includes conglomerate, limestone conglomerate, and fanglomerates.4Gneissic rocks include granite gneiss, granodiorite gneiss, quartz monzonite gneiss, hornblende- and pyroxene-bearing felsic gneisses, graphitic gneiss, hornblende gneiss, h

ing mafic gneisses, and gabbroic gneiss.

Table 4. Categorization of aquifers in southeastern Pennsylvania by water-table decline (WTD) index. [The distance from stream to divide was taken from Gerhart and Lazorchick (1988, p. 23). If more than two references for aquifer properties are given, first is for recharge and trafor specific yield. If aquifer was anisotropic, the geometric mean of the maximum and minimum transmissivity values was used.]References for aquifer properties: 1, Gerhart and Lazorchick (1984); 2, Vogel and Reif (1993); 3, Werkheiser (1995); 4, Gerhart and Lazorchick (1988); 5, Senior and Goode ((2004); 8, Assumed same as plutonic; 9, Transmissivity estimated from Low and others (2002, p. 269), recharge from Low and Conger (2002), Sy assumed same as schist (Gerh

10, Assumed same as metarhyolite and greenstone schist; 11, Assumed same as Chickies Formation

Aquifer Generalized rock type Recharge (

W), 

in inches per 

year

Distance from 

stream to divide 

(a), 

in feet 

Transmissivity 

(T), 

in feet squared 

per day 

Specific yield 

(Sy), 

dimensionless

Water-table 

decline 

predicted from 

equation 5, 

in feet

Water-t

decline (W

index cat
10 Drought Sensitive Aquifer Settings in Southeastern Pennsylvania

plexities inherent in the actual terrain, the method provided a 

straightforward and consistent approach for categorizing the 

topography that was considered adequate for testing the hypothesis regarding the drought sensitivity of differing topographic 

positions. 

Documenting Drought-Affected Wells 

Information about the status of individual and public 

ground-water supplies during recent droughts was surveyed 

from various sources to determine the location and construction 

characteristics of wells that went dry or became unusable in 

southeastern Pennsylvania. For this study, any well that went 

dry or was replaced or deepened as a direct result of drought is 

termed “drought-affected.” Information on drought-affected 

wells was solicited by (1) a well-owner survey of individual private well owners and (2) surveys of water purveyors, drillers, 

and government agencies such as the Pennsylvania Department 

of Environmental Protection (PaDEP), PaGS, county agricultural extension offices, and county health departments. The surveys were able to identify 2,016 wells affected by recent 

droughts (1998–2002 time period). The locations of the 

drought-affected wells identified by the surveys are shown in 

figure 4. The distribution of drought-affected wells shown in 

figure 4 is an indication of where data concerning droughtaffected wells were available and should not be interpreted as 

illustrating drought-sensitive settings throughout southeastern 

Pennsylvania. Because of the uneven distribution of available 

data for drought-affected areas across the entire study area, 

much of this report focuses on data from Chester and Montgomery Counties. These counties have well-construction regulations and data considered representative of the occurrence and 

distribution of problem areas in those counties. Location data 

were used from 1,182 drought-affected wells (December 1998 

through September 1999, and November 2001 through November 2002) in Chester and Montgomery Counties to determine 

drought-sensitive settings.

The construction characteristics of drought-affected wells 

were sought to determine if they were mostly old, shallow, dug 

wells or if deeper drilled wells of modern construction also 

were affected. This information could help determine if the 

cause of well failure was the result of site-specific, well-construction issues or a systemic inadequacy of the aquifer to supply water during drought. However, with the exception of the 

well-owner survey, only information on the location of the well 

drilled to replace or deepen the drought-affected well was 

readily available. Construction characteristics of the replacement wells commonly were available from well-completion 

reports, but that information was not useful for indicating why 

the original well was adversely affected during the drought. 

Well-Owner Survey

Individual well owners were surveyed from March to 

October 2003 to identify private wells affected by the drought 

of 2002. The survey was published in local newspapers and also 

posted on the USGS Pennsylvania Water Science Center Web 

pages (http://water.pa.usgs.gov). Because the survey was conducted only a few months after drought-emergency status had 

been lifted in southeastern Pennsylvania, a significant public 

participation to the survey was anticipated. Contrary to expectations, public participation was small—only 35 well owners in 

the study area, mostly from York County, responded to the survey. The owners provided information on type of well (drilled 

or dug), well depth, construction date, topographic setting 

(upland, slope, valley), and whether or not the well had gone dry 

(table 5). The locations of the drought-affected wells identified 

by the well-owner survey are shown in figure 4. 

Results from the well-owner survey indicated most 

drought-affected wells were drilled (not dug) within the past 

40 years, were deeper than 100 ft, and were on upland or slope 

settings. About 64 percent of drought-affected wells were over 

100 ft deep (all drilled), 24 percent were from 51 to 100 ft deep 

(2 dug, 6 drilled), and 12 percent were from 0 to 50 ft deep 

(2 dug, 2 drilled). Construction dates of the drought-affected 

wells ranged from 1854 to 1998; the median construction date 

was 1974. About 57 percent of drought-affected wells were 

reported to be in upland settings, 37 percent on slopes, and 

6 percent in valleys. 

Well-Driller and Agency Surveys

Telephone surveys were conducted during March to October 2003 by calling local well drillers and regulatory agencies 

in an attempt to locate information about wells affected by 

recent droughts. Agencies contacted included PaDEP, PaGS, 

county agricultural extension offices, and county health departments. Data were difficult to obtain from local well drillers and 

regulatory agencies because there currently (2004) is no statewide mechanism for recording information about droughtaffected wells. 

 One well driller provided records showing the location of 

39 drought-affected wells—37 in Adams County, 1 in Cumberland County, and 1 in York County (fig. 4). The driller’s records 

noted if the drought-affected well was replaced or deepened and 

provided construction information for the replacement well or 

modified well—but not the original drought-affected well. 

The largest source of information about drought-affected 

wells came from the Chester and Montgomery County Health 

Departments. The information was available because both 

counties have well-construction regulations enforced through 

their respective health departments (Chester County Health 

Department, 2001; Montgomery County Health Department, 

1997). As with the driller records, the information was for wells 

that replaced or deepened a well that went dry or became unusable during the drought; thus, the location of drought-affected 
Figure 4. Location of drought-affected wells in southeastern Pennsylvania during 1998-2002 as determined from results of well-owner, well-dsurveys. 
12 Drought Sensitive Aquifer Settings in Southeastern Pennsylvania

Table 5. Results from the 35 responses to the well-owner survey showing characteristics of domesticsupply wells in southeastern Pennsylvania affected by drought in 2002. 

[--, unknown]

Well Identifier County Well depth

(feet)

Well 

construction

Topographic 

setting

Year well 

constructed

394849077252901 Adams Over 100 drilled upland 1980

394950077133901 Adams Over 100 drilled upland 1994

394401077043101 Adams 51-100 drilled upland 1960

394330077000801 Adams Over 100 drilled slope 1979

402838075074501 Bucks Over 100 drilled upland 1986

400029075455801 Chester Over 100 drilled slope 1976

395805075475301 Chester Over 100 drilled slope 1970

400054076275801 Lancaster -- -- upland 1970

400554075582101 Lancaster Over 100 drilled upland 1983

400220076130101 Lancaster 0-50 dug valley 1929

395624076243101 Lancaster 51-100 dug upland --

395917076200601 Lancaster Over 100 drilled slope 1963

395539076183901 Lancaster 0-50 dug upland 1854

400818075045101 Montgomery 0-50 drilled valley --

395455076390701 York Over 100 drilled upland 1996

395320076442901 York Over 100 drilled slope 1960

400008076324501 York Over 100 drilled upland 1998

395249076541501 York Over 100 drilled valley 1985

394508076534301 York Over 100 drilled upland 1974

394536076531601 York Over 100 drilled upland 1976

394714076391501 York -- -- upland --

394916076454001 York Over 100 drilled slope --

401000076494101 York 51-100 drilled upland 1955

394331076381401 York Over 100 drilled upland 1978

395012076540201 York Over 100 drilled upland 1969

394703076412501 York Over 100 drilled slope 1976

395422076444101 York 51-100 drilled upland 1973

400543076431901 York Over 100 drilled upland --

400700076515501 York 51-100 dug slope 1960

395153076510001 York Over 100 drilled upland 1988

395647076314501 York 51-100 drilled upland 1940

395035076583801 York 51-100 drilled slope --

395129076443401 York Over 100 drilled slope 1991

394750076570001 York 51-100 drilled slope 1959

394536076511701 York 0-50 drilled slope 1967
Comparing Hypothesized Drought-Sensitive Settings to Location of Drought-Affected Wells 13

wells could be inferred from the health-department data but not 

the construction characteristics.

Information was obtained on 1,443 wells in Chester 

County and 499 wells in Montgomery County that were 

affected from 1998 through 2002. The number of droughtaffected wells reported each month is compared to the cumulative departure from normal precipitation (1960-89) for each 

county in figure 5. The factors causing more wells to be 

reported in Chester County than in Montgomery County is not 

known, but the magnitude of the precipitation deficit (especially 

during 2001- 2002) may have been greater in Chester County 

than Montgomery County (fig. 5).

Comparing Hypothesized Drought-Sensitive 

Settings to Location of Drought-Affected 

Wells

The hypothesized drought-sensitive settings were compared to the documented location of drought-affected wells to 

test the hypothesis that the intersection of aquifers with large 

WTD index with upland topographic positions should provide 

a good estimate of drought-sensitive settings in the study area. 

The analysis could not be conducted for the entire study area 

because information on drought-affected areas was only available in some areas. However, data from 1,182 drought-affected 

wells provided by the Chester and Montgomery County Health 

Departments were considered to be fairly representative of the 

occurrence and distribution of problem areas in those counties 

during the two most recent periods of declared drought emergency in southeastern Pennsylvania (December 1998 through 

September 1999, hereafter termed the 1999 drought, and 

November 2001 through November 2002, hereafter termed the 

2002 drought).

A bias is present in the dataset of drought-affected wells 

associated with the uneven distribution of the population of 

wells that potentially could become affected during drought. 

For example, most residents in urban areas are served by pubic 

water supplies, so there are few wells affected during drought, 

and even in rural areas, the distribution of wells cannot be 

assumed to be uniform. Thus, the reported number of droughtaffected wells from the county health departments was used in 

two ways to determine if the hypothesized drought-sensitive 

settings differ. The number of drought-affected wells for each 

WTD index category (high, moderate, low) and topographic 

setting (valley, slope, upland) were reported on the basis of 

(1) wells per square mile and (2) percentage of wells from the 

PaGS WWI. Although the WWI does not contain the entire 

population of wells in the study area, the WWI should represent 

the relative distribution of wells among aquifers. 

The spatial density of drought-affected wells in Chester 

and Montgomery Counties was evaluated as a preliminary analysis to illustrate potential drought-sensitive areas, strictly on the 

basis of the locations of wells obtained from the county health 

departments. A continuous-density surface was created with a 

GIS for the drought-affected wells during recent periods of 

declared drought in Pennsylvania (1999 and 2002 droughts). In 

Chester County, areas with a high density of drought-affected 

wells occurred in a widely distributed splotchy pattern (fig. 6A). 

The largest area of drought-affected wells was northwest of 

South Coatesville, which may be associated with the gneissic 

bedrock. In Montgomery County, the highest density of 

drought-affected wells was east of Green Lane and in an area 

north of Pottstown (fig. 6B). These areas appear to be associated with diabase bedrock and the associated hornfels (baked 

shales) within the Brunswick and Lockatong Formations.

Figure 5. Drought-affected wells reported by Chester and Montgomery County Health Departments and 

cumulative departure from normal precipitation, 1998-2002.
14 Drought Sensitive Aquifer Settings in Southeastern Pennsylvania

Figure 6. Density of drought-affected wells during the 1999 and 2002 droughts in (A) Chester County and (B) Montgomery County, 

Pennsylvania.

A. Chester County 
Comparing Hypothesized Drought-Sensitive Settings to Location of Drought-Affected Wells 15

Figure 6. Density of drought-affected wells during the 1999 and 2002 droughts in (A) Chester County and (B) Montgomery County, 

Pennsylvania.—Continued

B. Montgomery County 
16 Drought Sensitive Aquifer Settings in Southeastern Pennsylvania

Water-Table Decline Index

The hypothesis that drought-sensitive settings were related 

to the location of aquifers with high WTD index was tested by 

comparing areas of Chester and Montgomery Counties underlain by aquifers of low, moderate, and high WTD index to the 

locations of drought-affected wells obtained from the county 

health departments. The number of drought-affected wells per 

100 mi2 from the 1999 and 2002 droughts was determined for 

each aquifer having an area greater than 5 mi2 in Chester and 

Montgomery Counties (table 6). The highest frequency of 

drought-affected wells (173 wells /100 mi2) was in the diabase 

and hornfels, and the lowest frequency was documented in the 

Elbrook Formation (15 wells/100 mi2). 

The frequency of drought-affected wells reported during 

the 1999 and 2002 droughts from the Chester and Montgomery 

County Health Departments was compared to the WTD index 

of aquifers in figure 7. The comparison showed a strong relation 

between WTD-index category of the aquifer and number of 

drought-affected wells per 100 mi2—aquifers categorized as 

having low WTD index had about 62 dry wells per 100 mi2

compared to about 125 wells per 100 mi2

 for the high WTDindex category. When drought-affected wells are viewed on 

the basis of percentage of WWI wells in each WTD-index category (using WWI wells as a surrogate for the total number of 

wells in Chester and Montgomery Counties), there is a clear difference between the percentage of dry wells for aquifers categorized as low WTD index (4 percent) and high WTD index 

(11 percent). However, there is little difference between the 

moderate and high WTD index categories.

For some of the aquifers, the number of drought-affected 

wells reported in table 6 was greater or less than expected for its 

assigned WTD index category. For example, there were more 

drought-affected wells reported in the Cockeysville Marble 

than expected for an aquifer with low WTD index, more in the 

Setters Quartzite and Peters Creek Schist than expected for 

aquifers with moderate WTD, and fewer in the Lockatong Formation and Plutonic rocks than expected for aquifers with high 

WTD index. Some of the discrepancy probably is caused by the 

topographic setting of the drought-affected wells, which is not 

accounted for in the WTD index. However, in some cases, the 

Table 6. Dry wells in each aquifer in Chester and Montgomery Counties, Pennsylvania, expressed 

per 100 square miles and as percentage of Water Well Inventory (WWI) wells.

[Aquifer with an area of 5 square miles or less in the study area are not shown. Water-table decline index categories are described in table 4.]

Aquifer

Water-table 

decline 

index 

Area of 

aquifer 

(square 

miles)

WWI 

wells

Drought-affected wells

Number 

reported1

Per 100 

square 

miles

Percentage 

of WWI 

wells 

Elbrook Formation Low 13.7 30 2 14.6 6.7

Ledger Formation Low 22.8 159 6 26.3 3.8

Conestoga Formation Low 24.7 123 13 52.7 10.6

Brunswick Formation Low 196.9 3,797 135 68.6 3.6

Cockeysville Marble Low 10.2 41 11 107.7 26.8

Stockton Formation Moderate 106.7 877 52 48.7 5.9

Hammer Creek Conglomerate Moderate 8.8 91 6 68.2 6.6

Wissahickon Formation Moderate 215.0 1,728 172 80.0 10.0

Octoraro Formation Moderate 51.3 464 51 99.4 11.0

Setters Quartzite Moderate 11.8 65 14 118.5 21.5

Peters Creek Schist Moderate 68.4 594 88 128.7 14.8

Lockatong Formation High 58.8 570 31 52.7 5.4

Plutonic Rocks High 31.1 266 23 74.0 8.6

Gneissic Rocks High 276.6 2,565 346 125.1 13.5

Antietam and Harpers High 9.0 75 12 133.4 16.0

Chickies Formation High 34.7 427 54 155.6 12.6

Diabase + Hornfels High 94.0 1,858 163 173.4 8.8

1

The number of wells reported by the Chester and Montgomery Health Departments during the 1999 and 2002 droughts.
Comparing Hypothesized Drought-Sensitive Settings to Location of Drought-Affected Wells 17

WTD index may not be representative of the aquifer or the number of drought-affected wells might have been affected by factors such as nearby ground-water withdrawals or land-use 

changes that were not considered in this analysis. 

Topographic Position 

The hypothesis that drought-sensitive settings are related 

to the topographic position was tested by comparing valley, 

slope, and upland settings in Chester and Montgomery Counties 

to the locations of drought-affected wells obtained from the 

well owner, driller, and agency surveys. The settings initially 

were delineated by classifying valleys with a 300-ft buffer 

around streams and uplands with a 300-ft buffer around basin 

divides. All other areas were classified as slopes.

To determine if this arbitrary topographic classification 

could be improved, the width of the stream buffer identifying 

the valley position was varied by the use of a GIS analysis, and 

the number of drought-affected wells per 100 mi2 was recorded. 

Streams were buffered at 100 ft increments up to 2,000 ft and at 

500 ft increments from 2,000 to 5,000 ft. The number of dry 

wells increased noticeably between the 300 and 400 ft streambuffer widths. This increase indicates that wells are more vulnerable to drought if located more than 300 ft from a stream. 

This analysis indicates the use of the arbitrarily proposed 

300-ft buffer surrounding streams is probably optimal for defining the valley setting (in this simple topographic classification 

scheme) for purposes of identifying drought-sensitive settings. 

The comparison of valley, slope, and upland topographic 

positions in Chester and Montgomery Counties to the locations 

of drought-affected wells reported by the county health departments during the 1999 and 2002 droughts showed the number 

of drought-affected wells per 100 mi2 appears to be strongly 

related to topographic position (fig. 8). Upland settings have 

about 154 drought-affected wells per 100 mi2 compared to 

about 57 wells per 100 mi2 in valleys. The same strong relation 

also holds if drought-affected wells are expressed as a percentage of wells in the WWI database (fig. 8).

Combined Factors

Nine possible drought-sensitive settings were identified 

for southeastern Pennsylvania by combining the three categories of drought-sensitive aquifer (high, moderate, and low) with 

the three categories of topographic position (valley, slope, and 

upland). The hypothesis that the intersection of aquifers having 

high WTD index with upland topographic positions provides a 

good estimate of the most drought-sensitive settings in the study 

area was tested by comparing the nine settings to the locations 

of drought-affected wells during the 1999 and 2002 droughts 

obtained from the Chester and Montgomery County Health 

Departments. The settings were compared on the basis of 

(1) wells per 100 mi2 and (2) percent of wells from WWI in 

table 7. 

The results shown in table 7 differ depending on the basis 

of the comparison—wells per square mile or percentage of 

wells from the WWI. In each table, the nine settings were 

divided equally into categories of low, moderate, or high 

drought sensitivity. The three most highly sensitive settings, 

on the basis of either criteria, were (1) uplands in aquifers with 

high WTD index, (2) slopes in aquifers with high WTD, and (3) 

uplands in aquifers with moderate WTD index (cells with red 

shading in table 7). The two least-sensitive settings, as determined from either criterion, were valleys and slopes in aquifers 

Figure 7. Relation between water-table decline 

index (low, moderate, high) and incidence of 

drought-affected wells reported by Chester and 

Montgomery County Health Departments, 

southeastern Pennsylvania, during the 1999 and 

2002 droughts and as a percentage of Water 

Well Inventory (WWI) wells.
18 Drought Sensitive Aquifer Settings in Southeastern Pennsylvania

of low WTD index (cells with green shading in table 7). Settings 

of moderate drought sensitivity could be found in aquifers of 

any WTD-index category or topographic position depending on 

the basis of comparison (drought-affected wells per square mile 

or percentage of wells in WWI). 

The hypothesis that the WTD index and topographic position can be used to estimate the relative drought sensitivity of 

settings in southeastern Pennsylvania seems to be supported by 

data on drought-affected wells in Montgomery and Chester 

Counties from table 7. The relative sensitivity of the settings 

derived from table 7 is generalized for the entire study area in 

figure 9. It is difficult to draw conclusions more specific than 

those presented here for three major reasons listed below:

1. The WTD index was developed for an idealized aquifer 

that is a greatly simplified representation of the 

fractured-rock aquifers in southeastern Pennsylvania. It 

requires estimates of four physical parameters—(1) 

distance from stream to divide, (2) ground-water 

recharge rate, (3) transmissivity, and (4) specific yield 

that are not well known; thus, there is large uncertainty in 

the determination of WTD index.

2. Data on drought-affected wells were not available for the 

Figure 8. Relation between topographic position (valley, slope, upland) and incidence of 

drought-affected wells reported by Chester 

and Montgomery County Health Departments, southeastern Pennsylvania, during 

the 1999 and 2002 droughts and as a percentage of Water Well Inventory (WWI) wells.

Table 7. Comparison of drought-affected wells (1999 and 2002 droughts) in Chester 

and Montgomery Counties, Pennsylvania, in nine settings defined as the intersection 

of water-table decline (WTD) index and topographic position. 

[ , low sensitivity; , moderately sensitive; , highly drought sensitive]

Topographic position

WTD INDEX 

Low Moderate High

Basis of Comparison—Drought-affected wells per 100 square mile

Valley 44 54 69

Slope 63 81 118

Upland 74 141 195

Basis of Comparison—Drought affected wells as a percentage of 

Water Well Inventory wells.

Valley 2.9 9.9 8.0

Slope 4.2 9.8 10.3

Upland 4.5 13.4 15.3
Figure 9. Drought-sensitive settings in southeastern Pennsylvania determined by water-table decline index of aquifers and topographic posit
20 Drought Sensitive Aquifer Settings in Southeastern Pennsylvania

entire study area. Thus, the hypothesized drought 

sensitivity of many aquifers in southeastern Pennsylvania 

could not be compared to the occurrence of droughtaffected wells during recent droughts.

3. It is difficult to know how to interpret the information on 

drought-affected wells provided by Montgomery and 

Chester County Health Departments. In addition to WTD 

index and topographic position, factors that could affect 

the number of reported drought-affected wells are 

(a) uneven distribution of the population of wells 

available to go dry during drought, (b) unequal 

distribution of shallow wells, which are more likely to go 

dry than deep wells, (c) spatial differences in 

precipitation deficit during the drought, (d) possible 

differences in the degree of consistency among drillers in 

reporting drought-affected wells, and (e) factors such as 

nearby ground-water withdrawals or land-use changes 

that were not part of this analysis.

Summary and Conclusions

All or parts of southeastern Pennsylvania have been in 

drought-warning or drought-emergency status during 6 of the 

past 10 years from 1994 through 2004. As a result, low domestic ground-water levels have caused hundreds of domestic-supply wells to go dry or become unusable. Therefore, a study was 

conducted by the U.S. Geological Survey, in cooperation with 

the Pennsylvania Department of Conservation and Natural 

Resources, Bureau of Topographic and Geologic Survey, to 

determine drought-sensitive aquifer settings in southeastern 

Pennsylvania where ground-water shortages may occur during 

drought. 

The study area is approximately 5,100 mi2 and spans all or 

parts of Adams, Berks, Chester, Cumberland, Dauphin, Delaware, Franklin, Lancaster, Lebanon, Lehigh, Montgomery, 

Northampton, and York counties. Geology varies from igneous 

and metamorphic rocks of Precambrian age to sedimentary 

rocks of Triassic age. Aquifer characteristics within the study 

are diverse but the fractured-rock aquifers can be categorized 

according to crystalline, siliciclastic, and carbonate-rock types. 

The aquifer names used in this study correspond to the nomenclature of geologic units used by the Pennsylvania Geologic 

Survey. Ground water in these aquifers moves primarily within 

fracture space in the rock, although in some valley settings the 

saturated regolith can store and transmit water. The frequency 

of water-bearing fractures tends to diminish with depth. Therefore, ground-water flow paths are local and shallow—generally 

less than 400 ft deep.

Drought-sensitive aquifer settings were first defined conceptually using information from previous investigations and 

knowledge of the hydrogeology and topography of the study 

area. It was hypothesized that wells will be most affected by 

drought in aquifer settings having the factors of high WTD 

index and upland topographic positions. WTD index was developed from equations of Jacob to estimate the maximum decline 

of the water table at a basin divide for a hypothetical aquifer 

with idealized geometry. The WTD index shows that the magnitude of ground-water decline after cessation of recharge is a 

function of (1) distance from stream to ground-water divide, (2) 

ground-water recharge rate, (3) transmissivity, (4) specific 

yield, and (5) duration of the drought. Aquifers in the study area 

were divided into categories of low, moderate, and high WTD 

index, and the topography was classified into valley, slope, and 

upland positions. 

 The hypothesized factors affecting drought were compared to locations of drought-affected wells to confirm or refute 

the hypothesis that these factors are problematic during 

drought. The information about drought-affected wells was 

obtained from surveys of well owners, drillers, and public agencies. Data were obtained from 2,016 wells in southeastern Pennsylvania that went dry or were problematic during the drought 

in 1998-2002. With the exception of the owner-derived information from 35 wells from a well-owner survey, well-construction data were only obtained for the replacement or modified 

wells and not the original wells that went dry. The depths of 

drought-affected wells generally were not available, which 

made it impossible to determine if the drought-related well failure was caused by an appreciable deficiency of the aquifers to 

supply water or if the well was too shallow to tap a significant 

portion of the saturated thickness of the aquifer.

An analysis of the drought-affected wells could not be conducted for the entire study area because information on droughtaffected areas was only obtained in some areas, but the occurrence and distribution of drought-affected wells in Chester and 

Montgomery Counties were considered to be fairly representative of problem areas in those two counties associated with the 

two declared periods of drought from December 1998 through 

September 1999 (termed “the 1999 drought” in this report) and 

November 2001 through November 2002 (termed “the 2002 

drought” in this report). Thus, the locations of 1,182 droughtaffected wells from those counties during the two periods of 

declared drought were used to evaluate drought-sensitive aquifers and topographic positions

In general, the occurrence of drought-affected wells was 

found to be greater in aquifers having high or moderate WTD 

index in Chester and Montgomery Counties compared to aquifers with low WTD index. The occurrence of drought-affected 

wells also appears to be strongly related to topography; upland 

settings have about 154 drought-affected wells per 100 mi2

compared to about 57 wells per 100 mi2 in valleys.

The three most drought-sensitive settings were (1) uplands 

in aquifers with high WTD index, (2) slopes in aquifers with 

high WTD index, and (3) uplands in aquifers with moderate 

WTD. The two least sensitive settings were valleys and slopes 

in aquifers of low WTD index.
Acknowledgments 21

Acknowledgments

This project was developed initially by William 

Werkheiser of USGS and Samuel Berkheiser of PaGS. Datasets 

on drought-affected wells were provided by Kyle Schmeck of 

the Montgomery County Health Department and Benjamin 

Aller of the Chester County Health Department. Review of the 

report was provided by Gary Fleeger and Thomas McElroy of 

T&GS, and Lisa Senior, Curtis Schreffler, and Kim Otto of 

USGS.

References Cited

Alley, W.H., Healy, R.W., LaBaugh, J.W., and Reilly, T.E., 

2002, Flow and storage in groundwater systems: Science, 

v. 296, p. 1985-1990.

Berg, T.M., Edmunds, W.E., Geyer, A.R., Glover, A.D., Hoskins, D.M., MacLachlan, D.B., Root, S.I., Sevon, W.D., and 

Socolow, A.A., comps., 1980, Geologic map of 

Pennsylvania: Pennsylvania Geological Survey, 4th ser., 

map 1, scale 1:250,000, 3 sheets. 

Carlston, C.W., 1963, Drainage density and streamflow: U.S. 

Geological Survey Professional Paper 422-C, 8 p.

Chester County Health Department, 2001, Rules and regulations—Water well construction, monitoring wells, and individual semi-public water supplies: chap. 500, sec. 501, 24 p.

Fleeger, G.M., 1999, The geology of Pennsylvania’s 

groundwater: Pennsylvania Geological Survey, 4th series, 

Educational Series 3, 34 p.

Gerhart, J. M., and Lazorchick, G. J., 1984, Evaluation of the 

ground-water resources of parts of Lancaster and Berks 

Counties, Pennsylvania: U.S. Geological Survey WaterResources Investigations Report 84-4327, 135 p.

Gerhart, J. M., and Lazorchick, G. J., 1988, Evaluation of the 

ground-water resources of the Lower Susquehanna River 

Basin, Pennsylvania and Maryland: U.S. Geological Survey 

Water-Supply Paper 2284, 128 p.

Harbaugh, A.W., Banta, E.R., Hill, M.C., and McDonald, M.G., 

2000, MODFLOW-2000, the U.S. Geological Survey modular ground-water model—User guide to modularization concepts and the ground-water flow process: U.S. Geological 

Survey Open-File Report 00-92, 121 p.

Jacob, Charles E., 1944, Correlation of ground-water levels and 

precipitation on Long Island, New York; part 1, theory; part 

2 correlation of data: Transactions of the American Geophysical Union, part 2, part 6 (Jan.), p. 564-573.

Low, D.J., and Conger, R.W., 2002, Ground-water availability 

in part of the Borough of Carroll Valley, Adams County, 

Pennsylvania, and the establishment of a drought-monitor 

well: U.S. Geological Survey Water-Resources Investigations Report 02-4273, 78 p.

Low, D.J., Hippe, D.J., and Yannacci, D., 2002, Geohydrology 

of southeastern Pennsylvania: U.S. Geological Survey 

Water-Resources Investigations Report 00-4166, 347 p.

Montgomery County Health Department, 1997, Montgomery 

County Public Health Code—Division of Water Quality 

Management individual water supply system 

regulations: chap. 17, sec. 17, 9 p.

Pennsylvania Department of Environmental Protection, 2004, 

Drought information center: accessed December, 1, 2004, at 

http://www.dep.state.pa.us/dep/subject/hotopics/drought/.

Rorabaugh, M.I., and Simons, W.D., 1966, Exploration of 

methods relating ground water to surface water, Columbia 

River basin—second phase: U.S. Geological Survey OpenFile Report, 62 p.

Senior, L.A., and Goode, D.J., 1999, Ground-water system, 

estimation of aquifer hydraulic properties, and effects of 

pumping on ground-water flow in Triassic sedimentary rocks 

in and near Lansdale, Pennsylvania: U.S. Geological Survey 

Water-Resources Investigations Report 99-4228, 112 p.

Sevon, W. D., 2000, Physiographic provinces of Pennsylvania 

(4th ed.): Pennsylvania Geological Survey, 4th ser., map 13, 

scale 1:2,000,000. 

Sloto, R. A., 1990, Geohydrology and simulation of groundwater flow in the carbonate rocks of the Valley Creek basin, 

eastern Chester County, Pennsylvania: U.S. Geological 

Survey Water-Resources Investigations Report 89-4169, 

60 p.

Sloto, R. A., 2004, Geohydrology of the French Creek Basin 

and simulated effects of drought and ground-water withdrawals, Chester County, Pennsylvania: U.S. Geological Survey 

Water-Resources Investigations Report 03-4263, 82 p.

U.S. Bureau of the Census, 1992, Census of population and 

housing, 1990: Washington, D.C., Summary Tape File 3 on 

CD-ROM [machine-readable data files].

U.S. Geological Survey, 2004, National elevation dataset: U.S. 

Geological Survey data available on the Web, accessed January 10, 2005, at http://ned.usgs.gov/.

Vogel, K. L., and Reif, A.G., 1993, Geohydrology and simulation of ground-water flow in the Red Creek Basin, Chester 

County, Pennsylvania and New Castle County 

Delaware: U.S. Geological Survey Water-Resources Investigations Report 93-4055, 111 p.

Werkheiser, W.H., 1995, Geohydrology of the Hockessin area 

with emphasis on the Cockeysville Aquifer, in Woodruff, 

K.D., and Plank, M.O., eds., Geology and hydrogeology of 

the Cockeysville Formation Northern New Castle County, 

Delaware: Delaware Geological Survey Bulletin, no. 19, 

p. 26-48.