Prepared in cooperation with the Pennsylvania Department of Environmental Protection,

Bureau of Mining and Reclamation

Factors Affecting Specific-Capacity Tests and their 

Application—A Study of Six Low-Yielding Wells in 

Fractured-Bedrock Aquifers in Pennsylvania

Scientific Investigations Series 2010–5212

U.S. Department of the Interior

U.S. Geological Survey
Cover.

Specific-capacity testing at Cambria County well CA 462. Photograph by Randall Conger, U.S. Geological Survey. 
Factors Affecting Specific-Capacity Tests 

and their Application—A Study of Six 

Low-Yielding Wells in Fractured-Bedrock 

Aquifers in Pennsylvania

By Dennis W. Risser

In cooperation with the Pennsylvania Department of Environmental Protection, 

Bureau of Mining and Reclamation

Scientific Investigations Report 2010–5212

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Risser, D.W., 2010, Factors affecting specific-capacity tests and their application—A study of six low-yielding wells in 

fractured-bedrock aquifers in Pennsylvania: U.S. Geological Survey Scientific Investigations Report 2010-5212, 44 p. 
iii

Contents

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

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

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

Fractured-Bedrock Aquifers...............................................................................................................2

Specific Capacity..................................................................................................................................4

Methods of Study..................................................................................................................................6

Specific-Capacity Tests ..............................................................................................................6

Water-Level Monitoring..............................................................................................................8

Geophysical Logging.................................................................................................................10

Specific-Capacity Tests at Six Low-Yielding Wells in Fractured Bedrock.........................................12

Well AR 110 ..........................................................................................................................................12

Well BV 156 ..........................................................................................................................................14

Well CA 462 ..........................................................................................................................................16

Well GR 541 ..........................................................................................................................................18

Well WS 155.........................................................................................................................................20

Well YO 1222 ........................................................................................................................................22

Factors Affecting Specific Capacity.........................................................................................................24

Pumping Duration ...............................................................................................................................25

Theory Shows Importance of Pumping Duration Decreases With Time..........................25

Tests Show Effect of Pumping Duration on Specific Capacity...........................................26

Pumping Rate.......................................................................................................................................28

Theory Shows Pumping Rate May Not Affect Specific Capacity......................................28

Essential to Know Pumping Rate and Hold Discharge Steady ..........................................29

Tests Show Differing Effects of Pumping Rate on Specific Capacity ...............................30

Wellbore Storage................................................................................................................................32

Theory Shows Importance of Wellbore Storage in Early Time..........................................32

Wellbore-Storage Correction Can Improve Estimates of Well Yield.................................33

Specific-Capacity Tests Clearly Show Effects of Wellbore Storage.................................34

Deviations in Borehole Diameter Can Make Test Results Difficult to Interpret..............35

Aquifer and Well Properties..............................................................................................................36

Wet and Dry Periods Substantially Affected Specific Capacity at Well YO 1222 ...........36

Water-Yielding Zones Can Be Dewatered During Pumping...............................................37

Change in Wellbore Skin May Change Well Yield................................................................37

Turbulent Flow.....................................................................................................................................38

Step-Drawdown Tests Can Evaluate Effects of Turbulent Flow on Drawdown ..............38

Turbulent Flow Was Not Significant During Step-Drawdown Test at YO 1222................38

Application of Test Results.........................................................................................................................39

Specific-Capacity Values Were Reproducible With Some Caveats ..........................................39

Documenting Baseline and Replacement Well Yield ...................................................................39

Specific Capacity Values Can Overestimate Well Yields ....................................................39

Recovery Data Are Useful for Estimating Well Yield ...........................................................40

Difficult to Use Analytical Models to Predict Well Yields From Test Results ..................41
iv

YO 1222................................................................................................................................41

WS 155................................................................................................................................42

Summary........................................................................................................................................................43

Acknowledgments.......................................................................................................................................43

References Cited..........................................................................................................................................44

Figures

1. Map showing location of six low-yielding wells in fractured-bedrock aquifers in

areas of coal or carbonate-rock mining in Pennsylvania selected for testing

for this study ..................................................................................................................................3

2. Schematic of components of drawdown caused by laminar (BQ) and turbulent

(CQ2

) flow of water to a pumped well in an ideal confined aquifer......................................5

3. Graphs showing water-level fluctuations and dates of pumping for six

low-yielding wells in fractured-bedrock aquifers in Pennsylvania......................................9

4. Graph showing variation in wellbore diameter (caliper log), range of natural

water-level fluctuations 2007–09, and locations of major water-yielding zones as

determined from geophysical logging and driller reports for six low-yielding wells

in fractured-bedrock aquifers in Pennsylvania .....................................................................10

5. Images from the borehole video log showing fractures intersected by wells

BV 156 and YO 1222 ....................................................................................................................11

6 Graphs showing water level (A) and specific capacity (B) during five

specific-capacity tests at well AR 110 in Armstrong County, Pennsylvania ....................13

7. Graphs showing water level (A) and specific capacity (B) during five

specific-capacity tests at well BV 156 in Beaver County, Pennsylvania ..........................15

8. Graphs showing water level (A) and specific capacity (B) during five

specific-capacity tests at well CA 462 in Cambria County, Pennsylvania ........................17

9. Graphs showing water level (A) and specific capacity (B) during five

specific-capacity tests at well GR 541 in Greene County, Pennsylvania ..........................19

10. Graphs showing water level (A) and specific capacity (B) during five

specific-capacity tests at well WS 155 in Washington County, Pennsylvania.................21

11. Graphs showing water level (A) and specific capacity (B) during five

specific-capacity tests at well YO 1222 in York County, Pennsylvania..............................23

12. Schematic diagram of a hypothetical well in an idealized homogeneous aquifer

used in the theoretical analysis of some factors affecting specific capacity..................24

13. Graphs showing change in drawdown and specific capacity with duration of

pumping for constant withdrawal of 2 gallons per minute from a hypothetical

6-inch diameter well in an idealized aquifer with transmissivity 10 feet squared

per day and storage coefficient of 0.0001...............................................................................25

14. Graphs showing drawdown (A) and specific capacity (B) with time during selected

specific-capacity tests at six low-yielding wells in fractured-bedrock aquifers in

Pennsylvania ...............................................................................................................................27

15. Graph showing theoretical specific capacity with and without turbulent flow for

pumping at rates from 2 to 10 gallons per minute for 180 minutes in a hypothetical

6-inch diameter well with wellbore storage ..........................................................................28

16. Graph showing variability of the pumping rate during the 2.5 gallon-per-minute

specific-capacity test at well WS 155 in July 2008 ...............................................................29
v

17. Graph showing theoretical drawdown for constant-rate pumping at 5 gallons per

minute and varied-rate pumping at 6 gallons per minute for 90 minutes followed

by 4 gallons per minute for 90 minutes at the hypothetical well.........................................29

18. Graph showing specific-capacity values for tests completed at different rates

after 60 minutes of pumping at six low-yielding wells in fractured-bedrock

aquifers in Pennsylvania ...........................................................................................................31

19. Graphs showing drawdown (A) and specific capacity (B) for well diameters of

4, 6, and 8 inches simulated with the hypothetical well.......................................................32

20. Graphs showing specific capacity adjusted by removing water from wellbore

storage compared to unadjusted specific capacity for (A) the 6-inch diameter

hypothetical well and (B) well CA 462 in Cambria County, Pennsylvania .........................33

21. Graph showing incremental percentage of pumped volume provided by wellbore

storage for three specific-capacity tests at rates of 1, 2, and 4.5 gallons per

minute at well CA 462 in Cambria County, Pennsylvania .....................................................34

22. Graph showing caliper log and drawdown and recovery of water-level resulting

from pumping at 1.5 gallons per minute on July 23 and November 25, 2008,

showing effects of deviations in wellbore diameter in well GR 541 in Greene

County, Pennsylvania.................................................................................................................35

23. Graph showing water-level fluctuations in well YO 1222 in York County,

Pennsylvania, during June to August 2009 showing change in recession slope at

depth of a water-yielding zone 27 feet below land surface.................................................36

24. Image from borehole video log showing a particulate flocculent floating in the

water in well AR 110...................................................................................................................37

25. Graph showing determination of turbulent-flow factors B and C in equation 4 from

the step-drawdown test at well YO 1222 in York County, Pennsylvania, in August

2008 by the method of Bierschenk...........................................................................................38

26. Graph showing drawdown and recovery of water level during the March 2008

specific-capacity test at well WS 155 in Washington County, Pennsylvania...................40

27. Graphs showing measured drawdown from three tests at well YO 1222 in York

County, Pennsylvania, and drawdown predicted with the analytical solution of

Papadopulos and Cooper with transmissivity of 33 feet squared per day,

storage coefficient of 1.75 × 10-7, and well diameter of 6.25 inches...................................41

28. Graph showing measured water-level drawdown and recovery from the May

2009 specific-capacity test at well WS 155 in Washington County, Pennsylvania,

compared to simulated water levels from the analytical solution of Barker for

one-dimensional flow to the well.............................................................................................42
vi

Tables

 1. Characteristics of six low-yielding wells in fractured-rock aquifers 

in Pennsylvania .............................................................................................................................6

 2. Summary of results from multiple specific-capacity tests completed at six 

low-yielding wells in fractured-bedrock aquifers in Pennsylvania ......................................7

 3. Comparison of specific-capacity values after 60 and 180 minutes of pumping ...............26

 4. Specific-capacity values from two or three tests completed at the same pumping 

rate at each of six low-yielding wells in fractured-bedrock aquifers 

in Pennsylvania ...........................................................................................................................31

 5. Specific-capacity values from tests completed at different pumping rates at six 

low-yielding wells in fractured-bedrock aquifers in Pennsylvania ....................................31

Conversion Factors and Abbreviations

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square foot (ft2

) 929.0 square centimeter (cm2)

square foot (ft2

) 0.09290 square meter (m2)

Volume

gallon (gal) 3.785 liter (L) 

Flow rate

gallon per minute (gal/min) 0.06309 liter per second (L/s)

Specific capacity

gallon per minute per foot 

[(gal/min)/ft)]

 0.2070 liter per second per meter [(L/s)/m]

Transmissivity*

foot squared per day (ft2

/d) 0.09290 meter squared per day (m2/d) 

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

*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.
Factors Affecting Specific-Capacity Tests and their 

Application—A Study of Six Low-Yielding Wells in 

Fractured-Bedrock Aquifers in Pennsylvania

By Dennis W. Risser

Abstract

This report by the U.S. Geological Survey, prepared 

in cooperation with the Pennsylvania Department of Environmental Protection, Bureau of Mining and Reclamation, 

evaluates factors affecting the application of specific-capacity 

tests in six low-yielding water wells in areas of coal mining 

or quarrying in Pennsylvania. Factors such as pumping rate, 

duration of pumping, aquifer properties, wellbore storage, 

and turbulent flow were assessed by theoretical analysis and 

by completing multiple well tests, selected to be representative of low-yielding household-supply wells in areas of 

active coal mining or quarrying. All six wells were completed 

in fractured-bedrock aquifers—five in coal-bearing shale, 

siltstone, sandstone, limestone, and coal of Pennsylvanian 

and Permian age and one in limestone of Cambrian age. The 

wells were pumped 24 times during 2007–09 at rates from 

0.57 to 14 gallons per minute during tests lasting from 22 to 

240 minutes. Geophysical logging and video surveys also 

were completed to determine the depth, casing length, and 

location of water-yielding zones in each of the test wells, and 

seasonal water-level changes were measured during 2007–09 

by continuous monitoring at each well.

The tests indicated that specific-capacity values were 

reproducible within about ± 20 percent if the tests were 

completed at the same pumping rate and duration. A change in 

pumping duration, pumping rate, or saturated aquifer thickness can have a substantial effect on the comparability of 

repeated tests. The largest effect was caused by a change in 

aquifer thickness in well YO 1222 causing specific capacity 

from repeated tests to vary by a factor of about 50. An increase 

in the duration of pumping from 60 to 180 minutes caused as 

much as a 62 percent decrease in specific capacity. The effect 

of differing pumping rates on specific capacity depends on 

whether or not the larger rate causes the water level in the well 

to fall below a major water-yielding zone; when this decline 

happened at well CA 462, specific capacity was reduced by 

about 63 percent.

 Estimates of the maximum yield for low-yielding wells 

that are computed by multiplying the available drawdown 

by the specific-capacity value may contain large errors if 

the wells were pumped at low rates that do not cause much 

water-level drawdown. The estimates of yield are likely to be 

too large because the effects of lowering the water level in 

the well below water-yielding zones have not been incorporated. Better yield estimates can be made by the use of stepdrawdown tests or by over-pumping at a rate large enough to 

dewater most of the wellbore. The maximum well yield, after 

overpumping, can be estimated from the rate of water-level 

recovery or by subtracting the incremental rate of change of 

borehole storage at the end of the test from the pumping rate.
2 Factors Influencing Specific-Capacity Tests and Their Application

Introduction

Pennsylvania Department of Environmental Protection 

(PaDEP) regulations require a mining company to restore 

or replace the water supply if a mining company diminishes 

the yield of a well to the point where the well is no longer 

adequate for the purposes served (Commonwealth of Pennsylvania, 1982, 1990, 1998, 2005). PaDEP typically has used 

specific-capacity tests to establish baseline well yields, to 

determine mining effects on domestic supply wells, and to 

verify that replacement wells (if needed) have adequate yields. 

Specific capacity is the ratio of pumping rate to drawdown 

determined at some specified time during the test. Although 

the specific-capacity test has proven useful, the procedure for 

determining mining effects is based on the assumption that test 

results are reproducible and that differences between successive tests are caused by mining activity. Factors other than 

mining can affect the results of specific-capacity tests, but the 

importance of those effects is not well understood, particularly 

with low-yielding wells.

PaDEP often needs to evaluate the results of specificcapacity tests at domestic-supply wells for differing durations, 

pumping rates, and hydrologic conditions. Domestic-supply 

wells in Pennsylvania are frequently low-yielding (often less 

than 5 gal/min), and most references to the use of specific 

capacity for estimating well yield are for high-yielding wells 

used for public water supply or industry. PaDEP needs to 

know the degree to which factors other than mining can affect 

the reproducibility of specific-capacity tests in the fracturedrock aquifers near areas of active coal mining or quarrying. 

Such information would allow a better assessment of the 

assumptions inherent in the procedure for determining preand post-mining yield of a well with specific-capacity values, 

provide a basis for a better understanding of the variability of 

specific-capacity test results on a single well and the conditions responsible for this variability, and result in better interpretations of pumping-test data. To address this need the U.S. 

Geological Survey (USGS), in cooperation with the PaDEP, 

Bureau of Mining and Reclamation, evaluated the reproducibility of specific-capacity values derived from pumping of 

low-yielding wells.

Purpose and Scope

This report describes the results of a study, in cooperation with the PaDEP, Bureau of Mining and Reclamation, to 

evaluate factors affecting the application of specific-capacity 

tests in six low-yielding water wells in areas of coal mining or 

quarrying in Pennsylvania. Factors affecting specific capacity 

such as pumping rate, duration of pumping, aquifer properties, 

wellbore storage, and turbulent-flow well losses were assessed 

by theoretical analysis and by completing multiple tests at 

differing rates and durations at six low-yielding wells during 

2007–09 (fig. 1). Pumping each well was done to determine 

specific capacity, and geophysical logging and video surveys 

were done to determine the depth, casing length, and location 

of water-yielding zones in each of the test wells. Seasonal 

water-level changes were measured by continuous monitoring 

at each well.

Fractured-Bedrock Aquifers

The wells selected for study were completed in fractured 

siliciclastic or carbonate-bedrock aquifers in mining areas. 

Five wells are in shale, siltstone, sandstone, limestone, and 

coal of Pennsylvanian and Permian age in the bituminous coal 

fields of western Pennsylvania and one is in carbonate rocks 

(limestone of Cambrian age) in south-central Pennsylvania 

that are quarried for industrial uses.

Siliciclastic rocks of the bituminous coal fields of western 

Pennsylvania yield water to wells mostly through fractures 

because the primary permeability of the rock matrix is small. 

These fractured rocks create a complex, heterogeneous aquifer 

that is not easily classified as confined or unconfined. Callaghan and others (1998, p. 2–11) describe the groundwater 

flow system in the bituminous coal area as a “shallow, unconfined (possibly including seasonally perched or semi-perched 

zones) system grading to a semi-confined system at intermediate depth.”

Carbonate rocks in south-central Pennsylvania yield 

water to wells from fractures (some widened by dissolution) 

and voids (Taylor and Werkheiser, 1984, p. 32). Wells in 

carbonate-rock aquifers are capable of producing thousands 

of gallons per minute, but yields vary widely depending if 

the well intercepts a permeable feature. Low yields are not 

uncommon when a well is drilled through only solid bedrock.

The water-yielding zones described in drilling logs of the 

bedrock aquifers represent one or more fractures containing 

groundwater. The water level in a well typically represents a 

composite hydraulic head of all the water-yielding fractures 

intersected by the well. The maximum yield of a well depends 

largely on how the water-yielding fractures are interconnected 

with other fractures outside the near-well environment. Fractures or voids that are not well connected may provide some 

water from storage, but cannot provide a sustained source of 

water to the well.
Introduction 3

WS 155

GR 541

CA 462 BV 156

AR 110

YO 1222

WS 155

GR 541

CA 462 BV 156

AR 110

YO 1222

80° 78° 76°

42°

40°

EXPLANATION

Coal-bearing rocks

Carbonate rocks

Well with identifier—Specific-capacity test for this study

0 50 100 MILES

0 50 100 KILOMETERS

Figure 1. Location of six low-yielding wells in fractured-bedrock aquifers in areas of coal or carbonate-rock 

mining in Pennsylvania selected for testing for this study.
4 Factors Influencing Specific-Capacity Tests and Their Application

Specific Capacity

Specific capacity of a well is defined as the pumping rate 

divided by drawdown at some time after pumping was started 

(Lohman and others, 1972, p. 11):

SC = Q/s, (1)

where

SC	 is specific capacity, in (gallons per minute) 

per foot;

Q	 is the pumping rate, in gallons per minute; 

and

s	 is drawdown in the pumped well, in feet;

and

s = d	–	d0	,	 (2)

where

d	 is the depth to water in the well, in feet; and

d0	 is the static depth to water when pumping 

begins, in feet.

Thus, for a given pumping rate Q, any factor affecting 

drawdown s also will affect the value of specific capacity. 

Therefore, the accuracy and reproducibility of specific capacity for well evaluation depends on accurate field measurements 

of pumping rate and drawdown, and on control of the factors 

(other than mining influences) that affect drawdown.

Drawdown in a well is a function of laminar and turbulent flow in the aquifer and in the well. Under conditions of 

laminar radial flow in an ideal confined aquifer, the drawdown 

in a pumped well having negligible wellbore storage is given 

by Cooper and Jacob (1946) for all but early time as:

35.3Q 0.3Tt

s = log , (3)

T r S2

where

	 T	 is transmissivity, in square feet per day;

t	 is time since pumping began, in days;

r	 is the radius of the pumped well, in feet; and

S	 is the storage coefficient (dimensionless).

 Note: the constant 35.3 accounts for the conversion 

of units of Q	in gallons per minute to cubic feet per day.

The drawdown caused by laminar flow is directly proportional to the pumping rate Q	(eq. 3). Thus, if all drawdown 

were caused by laminar flow, the specific capacity from two 

tests at different rates would not be affected by pumping rate 

because the numerator and denominator on the right-hand 

side of (eq. 1) would both differ proportionately. The overall 

magnitude of drawdown is inversely proportional to aquifer 

transmissivity. The change of drawdown in time is determined 

by the logarithm of the dimensionless time, which is the term 

inside the parentheses in (eq. 3). The dimensionless time is 

directly proportional to time and aquifer transmissivity and 

inversely proportional to the storage coefficient and the square 

of the radius of the pumped well.

Turbulent flow can increase the total drawdown in the 

well (fig. 2). The laminar BQ and turbulent CQ2

 components 

of drawdown in the aquifer and the well are shown in the 

formula of Jacob (1947) as:

s = BQ + CQ2

, (4)

where

	 B	 is the laminar-flow factor (equal to terms on 

the right-hand side of equation 3), in feet 

per (gallons per minute); and

C	 is the turbulent-flow factor, in feet per (gallon 

per minute) squared.

Drawdown from turbulent flow CQ2

 is not directly proportional to the pumping rate (as is drawdown from laminar 

flow); thus, specific-capacity values of tests completed at 

different pumping rates are different during turbulent flow. 

Fortunately, for pumping at low rates in low-yielding wells, 

drawdown from turbulent flow is usually small compared to 

drawdown caused by laminar flow.

A major factor affecting drawdown in low-yielding wells 

during short-duration tests is the water stored in the well bore. 

The “wellbore storage” can substantially lessen the drawdown 

(and increase specific capacity) in early parts of a specificcapacity test compared to predictions based on the assumption 

of negligible wellbore storage used in equation 3.
Introduction 5

Land surface

Pre-pumping static water level

Well

Drawdown from laminar flow

Drawdown from turbulent flow

Aquifer Pump

Figure 2. Schematic of components of drawdown caused by laminar (BQ) and turbulent (CQ2)

flow of water to a pumped well in an ideal confined aquifer.
6 Factors Influencing Specific-Capacity Tests and Their Application

Methods of Study

The evaluation of short-duration specific-capacity tests 

was done by pumping wells in the field and by theoretical 

analysis. Water levels in the pumped wells were monitored 

during 2007–09 and downhole geophysical logs and videos 

were collected.

Specific-Capacity Tests

Six wells, mostly in coal-mining areas, were selected for 

pumping to evaluate the reproducibility of specific capacity 

for tests in the same well at different times and to examine the 

factors affecting reproducibility of results. Five of the wells 

were in siliciclastic rocks of mixed lithologies in the bituminous coal area of western Pennsylvania and one well was in 

carbonate rocks in south-central Pennsylvania (fig. 1).

Wells were selected to be representative of low-yielding 

household-supply wells in areas of active coal mining or 

quarrying. Typical household-supply wells in Pennsylvania 

are usually 6- or 8-in. diameter wells, cased from 20 to 40 ft 

below land surface, and completed as open holes in bedrock at 

depths less than 300 ft. Unlike household-supply wells, the six 

wells selected for this study were unused at the time of testing. 

Unused wells allowed unobstructed access for experimentation throughout the study period. The wells were selected from 

a set of candidate wells that were either known or reported to 

yield less than about 5 gal/min. One well is owned by a coal 

company, one is privately owned, and four are USGS observation wells. Characteristics of the wells are shown in table 1.

Twenty-four specific-capacity tests were completed at 

the six wells in various seasons from August 16, 2007, to 

September 21, 2009, and data were available from five tests 

completed on four of the wells prior to this study (table 2). 

The wells were pumped at rates ranging from 0.57–14 gal/min 

and durations of 22 to 240 minutes to demonstrate, in practice, 

how those factors could affect the reproducibility of specificcapacity values. Drawdown was monitored with a vented pressure transducer (Freeman and others, 2004) and check measurements were made periodically with a graduated electric 

tape. Discharge was monitored with either a recording paddlewheel flowmeter or a non-recording flowmeter, which was 

read every few minutes during the test. Flowmeter readings 

were checked periodically during the tests with a volumetric 

measurement by using a 5-gal bucket and stopwatch. Some 

of the specific-capacity tests were completed with the use of 

a device called the WellTender, which includes a flowmeter, 

pump controller, and data logger in a single unit capable of 

automatically holding the pumping rate of the Grundfos RediFlo2 pump steady during the duration of pumping. The steady 

pumping rate made it easier to evaluate how factors other than 

the pumping rate affect specific capacity.

Table 1. Characteristics of six low-yielding wells in fractured-rock aquifers in Pennsylvania.

[Borehole-video surveys were conducted at all wells. All wells were cased with steel casing. USGS, U.S. Geological Survey; ft, foot; in., inch; ss, sandstone; 

st, siltstone; sh, shale; md, mudstone; ls, limestone; gal/min, gallon per minute; @, at]

Characteristics Well

USGS Well Identifier

County

Latitude 

Longitude

Owner

Well use

Date drilled

Well depth (ft)

Casing inside diameter (in.)

Casing length (ft)

Geologic unit

Lithology

Driller-reported estimates of 

water-yielding zones (gal/min) 

and depth of zone below land 

surface (ft)

Geophysical logs

AR 110

Armstrong

40° 54′ 13.6′′

79° 31′ 25.7′′

Coal Company

unused

1999

198

6

1

49

Allegheny 

Formation

sh/ss/coal

2.7 @ 167

no

BV 156

Beaver

40° 30′ 06′′

80° 25′ 22′′

USGS

observation

1967

101

6.25

25

Glenshaw 

Formation

sh/md/st

0.5 @ 30

4 @ 52

6 @ 67

yes

CA 462

Cambria

40° 27′ 44.8′′

78° 45′ 32.5′′

USGS

observation

2008

142

6

17

Glenshaw 

Formation

ss/sh/coal

not reported

yes

GR 541

Greene

39° 47′ 38′′

80° 07′ 20′′

Private

unused

pre 1980

62

6

20

Washington 

Formation

ss/st/ls

not reported

yes

WS 155

Washington

40° 02′ 33′′

80° 26′ 12′′

USGS

observation

1971

133

6

19

Washington 

Formation

ss/st/ls/sh

2 @ 45

yes

YO 1222

York

40° 12′ 50.24′′

76° 52′ 24.99′′

USGS

observation

2001

202

6.25

18

Epler 

Formation

ls

trace @ 120

1 @ 162

2 @ 168

yes

1

An inner plastic 4-in. casing is hanging in the well from about 5 ft below land surface to a depth of 90 ft below land surface.
Introduction 7

Table 2. Summary of results from multiple specific-capacity tests completed at six low-yielding wells in fractured-bedrock aquifers in 

Pennsylvania.

[min, minute; ft, foot; (gal/min)/ft, gallon per minute per foot; gal/min, gallon per minute; --, no data]

Well Date

Pumping 

rate

(gal/min)

Duration of 

pumping 

(min)

Drawdown 

at end of 

test

(ft)

Specific capacity

[(gal/min)/ft] Comments

At 60 min At 180 min At end

of test

AR 110 05-17-1999 3.5 60 47 0.074 -- 0.074 Historical test. Specific capacity at 49 min = 

0.088 (gal/min)/ft

AR 110 09-01-1999 2.0 60 42 .048 -- .048 Historical test. Specific capacity at 49 min = 

0.088 (gal/min)/ft

AR 110 03-12-2008 2.0 55 63.2 -- -- .032 Specific capacity at 49 min = 

0.035 (gal/min)/ft

AR 110 07-24-2008 2.0 49 62.5 -- -- .032 Pumped until water level reached transducer 

depth.

AR 110 05-27-2009 .88 60 31.1 .028 -- .028 Specific capacity at 49 min = 

0.034 (gal/min)/ft

BV 156 01-29-1969 7.9 60 81 .100 -- .10 Historical test

BV 156 05-21-2008 2.5 69 14.6 .27 -- .17

BV 156 05-21-2008 5.0 180 43.8 .17 0.11 .11 Began pumping 1 hour 47 min after end of 

2.5 gal/min test.

BV 156 07-23-2008 5.0 180 37.8 .19 .13 .13

CA 462 03-11-2008 2.0 180 109.5 .047 .019 .02

CA 462 08-12-2008 2.0 180 89.1 .058 .022 .02

CA 462 05-26-2009 1.0 180 10.7 .12 .099 .10

CA 462 09-21-2009 1.1 180 7.8 .15 .14 .14

CA 462 09-21-2009 4.5 45 89.8 -- -- .050 Began pumping 1 hour after end of 

1.05 gal/min test. Pumped until water level reached transducer 

depth. Estimated specific capacity at 

60 min = 0.01 (gal/min)/ft

GR 541 04-15-1980 .88 130 23.1 .059 -- .038 Historical test

GR 541 07-23-2008 1.5 70 22.3 .096 -- .067 Pumped until water level reached transducer depth. Flowmeter malfunctioned, pumping rate from volumetric 

measurements. Specific capacity at 

50 min = 0.10 (gal/min)/ft

GR 541 11-25-2008 1.5 50 16.5 -- -- .091 Pumped until water level reached transducer 

depth.

GR 541 05-28-2009 .57 60 6.1 .094 -- .094 Specific capacity at 50 min = 

0.11 (gal/min)/ft

WS 155 07-01-1971 2.0 90 17 .29 .12 Historical test

WS 155 03-13-2008 2.5 180 17.8 .37 0.14 .14

WS 155 07-22-2008 2.5 180 15.8 .41 0.16 .16

WS 155 05-27-2009 1.0 60 2.9 .34 -- .34

WS 155 05-27-2009 6.0 35 36.5 -- -- .16 Began pumping 60 minutes after end of 

1 gal/min test. Pumped until water level 

reached transducer depth.

YO 1222 08-16-2007 2.0 60 24 .083 -- .083

YO 1222 08-22-2007 6.0 63 1.2 5.1 -- 5.00

YO 1222 03-10-2008 5.0 180 1.2 5.5 4.20 4.20

YO 1222 07-18-2008 2.1 180 17.3 .12 .12 .12

YO 1222 08-07-2008 2.0 240 46.3 .11 .11 .11 Step-drawdown test—60-min steps at 2, 3, 

4, 5 gal/min

YO 1222 09-02-2009 14 22 116.6 -- -- .12 Pumped until water level reached transducer 

depth.
8 Factors Influencing Specific-Capacity Tests and Their Application

Water-Level Monitoring

Water levels were monitored with transducers or floats 

with digital encoders to establish the range of water-level 

fluctuation during the study period. Large seasonal water-level 

fluctuations have the potential to cause substantial changes 

in aquifer saturated thickness and, therefore, transmissivity, 

which would directly affect the specific capacity of the well.

Water-level hydrographs for all six wells that were 

pumped during the study are shown in figure 3. Dates at which 

specific-capacity tests were done are noted with a triangle. 

Water-level fluctuations were less than 5 ft during the study 

for all wells except YO 1222. The water level in this well, 

completed in carbonate bedrock, fluctuated about 17 ft during 

the study and had rapid water-level fluctuations in response to 

storms. Seasonal changes ranged from about 2 to 10 percent 

of the saturated thickness of the aquifer open to the wells. The 

general decline in water levels in most of the wells from about 

March through September 2008 was caused by seasonal differences in groundwater recharge.

Data

Logger Data Logger

Well

Water Float

Level

Counter

Weight

Pressure

Transducer

Encoder

Water-level monitoring with transducer or float system.
Introduction 9

BV 156 

WS 155 YO 1222 

CA 462 GR 541 

110 

111 

112 

113 

114 

115 

116 

AR 110 

Specific-capacity tests 

6 

7 

8 

9 

10 

11 

4 

5 

6 

7 

8 

9 

10 

32 

33 

34 

35 

36 

37 

38 

33 

34 

35 

36 

37 

38 

39 

15 

20 

25 

30 

35 

Depth to water below land surface, in feet

Aug-07

Oct-07

Dec-07

Feb-08

Apr-08

Jun-08

Aug-08

Oct-08

Dec-08

Feb-09

Apr-09

Jun-09

Aug-09

Aug-07

Oct-07

Dec-07

Feb-08

Apr-08

Jun-08

Aug-08

Oct-08

Dec-08

Feb-09

Apr-09

Jun-09

Aug-09

Aug-07

Oct-07

Dec-07

Feb-08

Apr-08

Jun-08

Aug-08

Oct-08

Dec-08

Feb-09

Apr-09

Jun-09

Aug-09

Aug-07

Oct-07

Dec-07

Feb-08

Apr-08

Jun-08

Aug-08

Oct-08

Dec-08

Feb-09

Apr-09

Jun-09

Aug-09

Aug-07

Oct-07

Dec-07

Feb-08

Apr-08

Jun-08

Aug-08

Oct-08

Dec-08

Feb-09

Apr-09

Jun-09

Aug-09

Aug-07

Oct-07

Dec-07

Feb-08

Apr-08

Jun-08

Aug-08

Oct-08

Dec-08

Feb-09

Apr-09

Jun-09

Aug-09

Figure 3. Water-level fluctuations and dates of pumping for six low-yielding wells in fractured-bedrock aquifers in Pennsylvania.
10 Factors Influencing Specific-Capacity Tests and Their Application

Geophysical Logging

Geophysical and video surveys were completed at the six 

low-yielding wells in fractured-bedrock aquifers to verify well 

and casing diameter, casing length, and to determine the location of water-yielding zones. Those water-yielding zones identified by logging were associated with identified fractures or 

fracture sets in the bedrock. Geophysical logs that were useful 

for identifying water-yielding zones included: caliper, natural 

gamma, single-point resistance, fluid temperature, and fluid 

resistivity (Keys, 1990). Fluid flow in the well was measured 

by use of a heat-pulse flowmeter, which helped identify the 

major water-yielding zones by comparing vertical flow in the 

well when the well was pumped and when the well was idle 

(Keys, 1990, p. 130). A borehole video survey was completed 

in all wells to investigate the condition of the casing and 

examine possible clogging of water-yielding zones by chemical precipitates. The major water-yielding zones as determined 

from geophysical logging and driller reports are shown in 

figure 4. Examples of images from the borehole video showing 

fractures intersected by two wells are shown in figure 5.

W

W

W

W

W

W

W

W

W

0 12 INCHES

0 

20 

40 

60 

80 

100 

120 

140 

160 

180 

200 

220

Depth to water below land surface, in feet 

AR 110 BV 156 CA 462 GR 541 WS 155 YO 1222

Well

casing

Caliper

log

No caliper log available

Six-inch hole verified

by video survey.

Inner poly vinyl

chloride (PVC)

casing

Water-yielding zone, reported or

determined from geophysical logging

(dashed line indicates range of zone).

Range of natural water-level

fluctuations during 2007–09

EXPLANATION

W

Figure 4. Variation in wellbore diameter (caliper log), range of natural water-level fluctuations 2007–09, and locations of major wateryielding zones as determined from geophysical logging and driller reports for six low-yielding wells in fractured-bedrock aquifers in 

Pennsylvania.
Introduction 11

FRACTURE

BV 156

YO 1222

FRACTURES

Figure 5. Images from the borehole video log showing fractures intersected by wells BV 156 and YO 1222.
12 Factors Influencing Specific-Capacity Tests and Their Application

Specific-Capacity Tests at Six LowYielding Wells in Fractured Bedrock

Well AR 110

Well AR 110 in Armstrong County is completed in fractured shale, sandstone, and coal of the Allegheny Formation 

to a depth of 198 ft (table 1). The geologic log shows that the 

well penetrates disturbed spoil from the mining of the Middle 

Kittanning coal to a depth of 34 ft and the Lower Kittanning 

coal from 74.5–77.5 ft below land surface. The well is cased 

with 6-in. diameter steel casing to 49 ft below land surface, 

and an inner 4-in. diameter plastic liner hangs inside the steel 

casing from 5 to 90 ft below land surface. One zone yielding 

2.7 gal/min of water from gray sandy shale was reported on 

the geologic log at a depth of 167 ft below land surface. The 

static water level in the well was deep—about 110 ft below 

land surface (fig. 3), which allowed about 88 ft of drawdown 

available for testing. Geophysical logging was not done at this 

well for this study.

Three specific-capacity tests were done by USGS at 

well AR 110 during 2008–09. The well was pumped twice at 

2.0 gal/min and once at 0.88 gal/min (table 2). Results also 

were available from two tests completed at rates of 2.0 and 

3.5 gal/min by a coal company in 1999 (Kenneth King, W.D. 

Mohney and Associates, written commun., 2008). The durations of the pumping were short for those tests, ranging from 

49 to 60 minutes, because the water level in the well declined 

rapidly in response to pumping. Recovery was monitored for 

60 to 120 minutes after each test completed by USGS. Drawdown and recovery of water levels from each test are shown in 

figure 6A. The change in specific capacity with time for each 

test is shown in figure 6B.

The most striking characteristic of the water-level drawdown and recovery curves during the specific-capacity tests 

Well

Specific-capacity testing at well AR 110. Photograph by Randall 

Conger, U.S. Geological Survey. 

completed during 2008–09 is that the data plot as straight 

lines on arithmetic axes (fig. 6A). The straight lines indicate 

that drawdown and recovery are proportional to time, and that 

most of the pumped water is derived either from water stored 

in the well and (or) from the aquifer at a rate independent 

of the water level in the well. The straight-line water-level 

decline during the pumping period indicates that the rate of 

withdrawal from wellbore storage was about 1.84 gal/min; 

after pumping ceased, the water-level recovery curve indicates 

a constant inflow of about 0.2 gal/min. Thus, the maximum 

well yield appears to be about only 0.2 gal/min for tests completed during 2008–09.

The two specific-capacity tests completed about 10 years 

previously in 1999 resulted in less drawdown and more rapid 

recovery than for the tests completed during 2008–09. The 

maximum well yield indicated by these tests is much greater 

than 0.2 gal/min. The static water levels were about 10 ft 

lower in 1999, therefore the greater yield cannot be explained 

by more saturated water-yielding zones connected to the well 

than in 2008–09. One possible explanation for the lower specific capacity in 2008–09 is that the water-yielding zone noted 

on the geologic log at 167 ft below land surface has become 

clogged during the past 10 years. The video survey was unable 

to identify any obvious fractures at or near the depth of the 

reported water-yielding zone, therefore direct proof of plugged 

fractures could not be verified; however, the water in the well 

bore contained abundant particulate matter that was apparently 

dislodged from the wall of the well, indicating that clogging is 

a possibility.

The change in specific capacity with time for each test is 

shown in figure 6B. The specific-capacity values determined 

from the July 2008 test at 2 gal/min and May 2009 test at 

0.88 gal/min were nearly identical. The March 2008 test completed at 2 gal/min gives similar values of specific capacity 

toward the end of the test, but deviates substantially during the 

early part of the test, because the pumping rate was initially 

not stable.
Specific-Capacity Tests at Six Low-Yielding Wells in Fractured Bedrock 13

Figure 6 Water level (A) and specific capacity (B) during five specific-capacity tests at well AR 110 in Armstrong 

County, Pennsylvania. Depth to water below land surface, in feet

May 1999 (3.5 gallons per minute) 

September 1999 (2 gallons per minute) 

March 2008 (2 gallons per minute) 

July 2008 (2 gallons per minute) 

May 2009 (0.88 gallon per minute) 

Specific capacity, 

in gallons per minute per foot

Time, in minutes since pumping began 

Time, in minutes since pumping began 

May 1999 (3.5 gallons per minute) 

September 1999 (2 gallons per minute) 

March 2008 (2 gallons per minute) 

July 2008 (2 gallons per minute) 

May 2009 (0.88 gallon per minute) 

100 

110 

120 

130 

140 

150 

160 

170 

180 

0 60 120 180 

0.01 

0.10 

1.00 

10.00 

0 20 40 60 

EXPLANATION

A EXPLANATION

B
14 Factors Influencing Specific-Capacity Tests and Their Application

Well BV 156

Well BV 156 in Beaver County is an observation well, 

maintained and operated by USGS for monitoring drought 

conditions. The well is completed in shale, mudstone, and 

siltstone of the Glenshaw Formation to a depth of 101 ft and 

is cased with 6.25-in. diameter steel casing to 25 ft below land 

surface (table 1). The yields and the depths of the major wateryielding zones were reported by the driller as 0.5 gal/min at 

30 ft, 4 gal/min at 52 ft, and 6 gal/min at 67 ft (table 1). The 

water-yielding zone at 52 ft corresponds to a fracture shown 

on the video survey at 51 ft below land surface (fig. 5). The 

zone at 67 ft is at a lithologic contact between mudstone 

and siltstone, according to the log of cuttings from the well. 

Geophysical logging indicated a major opening at about 57 ft 

below land surface (fig. 4). The flowmeter survey and temperature log indicated that the most productive water-yielding 

zone was at 30 ft, with additional yield contributed between 

50 and 80 ft below land surface, probably from fractures at 

52 and 67 ft below land surface. The static water level in the 

well was shallow–about 8 ft below land surface (fig. 3), which 

allowed about 93 ft of drawdown available for testing.

Well BV 156 was pumped on two occasions by USGS 

during 2008 (table 2). The well was pumped at 2.5 gal/min 

Specific-capacity testing at well BV 156, inside culvert 

pipe. Photograph by Brad Kuntz, U.S. Geological Survey

for 69 minutes in May 2008, then the rate was increased to 

5 gal/min, but the pump failed after 6 minutes. The water 

level was allowed to recover for 101 minutes then the well 

was pumped again at 5.0 gal/min for 180 minutes. A specificcapacity test at 5 gal/min was repeated in July 2008. Recovery 

was monitored until the water level had nearly returned to the 

static pre-pumping levels. A 60-minute test also was completed by USGS in 1969 at a rate of 7.9 gal/min.

Drawdown and recovery of water levels during the four 

tests are shown in figure 7A. The two specific-capacity tests 

completed at 5 gal/min showed similar curves for waterlevel drawdown and recovery. Drawdown was nearly steady 

after 69 minutes of pumping at 2.5 gal/min during the test in 

May 2008. The specific-capacity test in 1969 at 7.9 gal/min 

resulted in more drawdown than for the tests completed during 

2008 for this study because of the greater pumping rate. The 

drawdown for the 1969 test stabilized for about 20 minutes at 

a depth of about 57 ft below land surface. This depth corresponds to the depth of the opening shown on the caliper log 

(fig. 4). Possibly, a fracture is connected to a large void in the 

bedrock and this storage is being dewatered during this period, 

after which drawdown increased. The rate of drawdown 

increases after the water level is below this zone, indicating 

that this zone contributes some inflow, although it was not 

identified as a major water-yielding zone during logging. An 

inflection in the water-level curve at this depth can be seen 

during recovery.

The change in specific capacity with time for each test is 

shown in figure 7B. The specific-capacity values determined 

from the pumping at 5 gal/min in the May and July 2008 tests 

were nearly identical. The May 2008 test at 2.5 gal/min gives 

a greater value of specific capacity and nearly reaches equilibrium. Specific capacity was substantially less for the test 

completed at a rate of 7.9 gal/min. The decrease in specific 

capacity with increased pumping rate could be explained by 

turbulent flow in the aquifer and well, but turbulent flow is 

probably not the major cause. The more likely explanation is 

that the greater pumping rates caused the water level in the 

well to draw down below one or more of the major wateryielding zones at 30, 52, and 67 ft below land surface, thereby 

reducing the transmissivity of the aquifer near the well. The 

maximum yield for the well based on the tests completed in 

2008 seems to be about 5 gal/min.

Well
Specific-Capacity Tests at Six Low-Yielding Wells in Fractured Bedrock 15

January 1969 (7.9 gallons per minute) 

May 2008 (2.5 gallons per minute) 

May 2008 (5 gallons per minute) 

July 2008 (5 gallons per minute) 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100 

0 60 120 180 240 300 

Depth to water below land surface, in feet 

Time, in minutes since pumping began 

Time, in minutes since pumping began 

0.01 

0.10 

1.00 

10.00 

0 20 40 60 80 100 120 140 160 180 

Specific capacity, in gallons per minute per foot

January 1969 (7.9 gallons per minute) 

Depth of major fracture

shown on caliper log (fig. 4)

May 2008 (2.5 gallons per minute) 

May 2008 (5 gallons per minute) 

July 2008 (5 gallons per minute) 

EXPLANATION

EXPLANATION

A

B

0 

Figure 7. Water level (A) and specific capacity (B) during five specific-capacity tests at well BV 156 in Beaver 

County, Pennsylvania.
16 Factors Influencing Specific-Capacity Tests and Their Application

Well CA 462

Well CA 462 in Cambria County is an observation well, 

maintained and operated by USGS for monitoring drought 

conditions. The well is completed in the Glenshaw Formation 

to a depth of 142 ft and cased with 6-in. diameter steel casing 

to 17 ft below land surface (table 1). The log of drill cuttings 

indicated that most of the well was drilled into gray sandstone 

with minor amounts of shale. A coal bed was noted at 27 ft 

below land surface. The depths of the major water-yielding 

zones were not reported by the driller. Geophysical logging 

indicated a fracture at about 37 ft below land surface (fig. 4), 

and the flowmeter survey indicated that most of the water is 

yielded from 17–50 ft below land surface. The static water 

level in the well was shallow—ranging from about 6–10 ft 

below land surface (fig. 3), which allowed about 132 ft of 

drawdown available for testing.

Well CA 462 was pumped on four occasions by USGS 

during 2008–09 (table 2). The well was pumped at 2 gal/min 

for 180 minutes in March and August 2008 and then 

allowed to recover for 120 minutes. The well was pumped at 

1.0 gal/min for 180 minutes in May 2009, and the static water 

level recovered after about 60 minutes. The well was pumped 

at 1.1 gal/min in September 2009, allowed to recover for 

60 minutes, and pumped again at 4.5 gal/min for 45 minutes.

Drawdown and recovery of water levels for the five 

tests are shown in figure 8A. Similar to the results from tests 

at AR 110, drawdown for the tests completed at 2.0 and 

4.5 gal/min plotted as straight lines on arithmetic axes. The 

straight lines show that the water-level change is proportional 

to time, and that most of the pumped water is being derived 

from water stored in the well and (or) water contributed 

from the aquifer at a constant rate. The straight-line waterlevel drawdown curves during pumping for the 2 gal/min 

tests starting after about 60 minutes indicate that the rate of 

withdrawal from wellbore storage was about 0.7–0.8 gal/min 

Well

Specific-capacity testing at well CA 462. Photograph by Randall 

Conger, U.S. Geological Survey. 

(assuming a 6-in. diameter well); after cessation of pumping, 

the water-level recovery curves indicate a constant inflow of 

about 1.3 gal/min. The straight-line curves for the 4.5 gal/min 

test indicate a constant inflow of about 1.5 gal/min. Thus, the 

maximum well yield is about 1.3–1.5 gal/min.

Water levels during the 1 gal/min tests in May and 

September 2009 reached an apparent equilibrium after about 

120 minutes of pumping. Straight-line segments of waterlevel change observed during the other tests were not seen 

in these tests because the water level did not fall below the 

water-yielding zones from 17–50 ft below land surface. Both 

1 gal/min tests recovered to static water level within an hour 

of the end of pumping. These tests verified that the well had a 

maximum yield of at least 1 gal/min.

The change in specific capacity with time for each test is 

shown in figure 8B. The specific-capacity values determined 

from the pumping at 1.0 gal/min in May and September 2009 

were similar, but not identical. The May and September tests 

reached an apparent equilibrium, but the specific capacity is 

greater for the September 2009 test. Specific capacity was 

substantially less for the tests completed at rates of 2.0 and 

4.5 gal/min. The decrease in specific capacity with increased 

pumping rate could be explained by turbulent flow in the 

aquifer and well, but turbulent flow is probably not the major 

cause. The more likely explanation is that the greater pumping 

rates caused the water level in the well to draw down below 

the water-yielding zones from 17–50 ft below land surface, 

thereby, decreasing the aquifer transmissivity.

The 2 gal/min test completed in August 2008 had a 

greater specific capacity than the test in March 2008, and the 

specific capacity from the 1 gal/min test in September 2009 

was greater than for the test at the similar rate in May 2009. 

The increase indicates that there may have been a slight development of the water-yielding fractures each time the well was 

pumped.
Specific-Capacity Tests at Six Low-Yielding Wells in Fractured Bedrock 17

EXPLANATION

EXPLANATION

A

B

20 

40 

60 

80 

100 

120 

Depth to water below land surface, in feet 

0.01 

0.10 

1.00 

10.00 

Specific capacity, in gallons per minute per foot

0 60 120 180 240 300 360 

Time, in minutes since pumping began 

0 20 40 60 80 100 120 140 160 180 

Time, in minutes since pumping began 

March 2008 (2 gallons per minute) 

August 2008 (2 gallons per minute) 

May 2009 (1 gallon per minute) 

September 2009 (4.5 gallons per minute) 

March 2008 (2 gallons per minute) 

August 2008 (2 gallons per minute) 

May 2009 (1 gallon per minute) 

September 2009 (1.1 gallons per minute) 

September 2009 (4.5 gallons per minute) 

September 2009 (1.1 gallons per minute) 

0 

Figure 8. Water level (A) and specific capacity (B) during five specific-capacity tests at well CA 462 in Cambria 

County, Pennsylvania.
18 Factors Influencing Specific-Capacity Tests and Their Application

Well GR 541

Well GR 541 in Greene County is a privately owned well 

that was used to monitor water levels for a study of groundwater in Greene County in the 1980s (Stoner and others, 1987), 

and is currently (2009) unused. The well is completed in sandstone, siltstone, and limestone of the Washington Formation 

to a depth of 62 ft and cased with 6-in. diameter steel casing 

to 20 ft below land surface (table 1). Limestone was reported 

on the USGS well schedule from 20–23 ft below land surface. 

A driller’s report was not available, but geophysical logging 

indicated considerable variability in well diameter from 6 in. 

to more than 12 in. Major enlargements of the well diameter, 

probably caused by fractures, were detected at depths from 

33–40, 46, and 50 ft below land surface. Flowmeter surveys, 

though not definitive, indicated that water is probably contributed throughout the saturated thickness of the aquifer, with 

most of the water being yielded less than 50 ft below land 

surface. The static water level in the well ranged from about 

33–36 ft below land surface (fig. 3), which allowed only about 

30 ft of drawdown available for testing.

Well GR 541 was pumped on three occasions by USGS 

during 2008–09 (table 2). The well was pumped at 1.5 gal/min 

until the water level reached the depth of the transducer setting 

in July and November 2008 (after 70 and 50 min of pumping, 

respectively), then was allowed to recover for 120 minutes. 

The well was pumped at 0.57 gal/min for 60 minutes in May 

2009, and recovery was monitored for 60 minutes. Results 

also were available from a test completed by USGS in 1980 

for 130 minutes at 0.88 gal/min.

Drawdown and recovery of water levels for the four tests 

are shown in figure 9A. The curves for the tests completed 

at 1.5 gal/min are complex, showing multiple straight-line 

Well

Specific-capacity testing at well GR 541, near shed. Photograph by 

Randall Conger, U.S. Geological Survey. 

segments of differing slopes. The breaks in slope seem to be 

caused by boundary conditions, but the multiple tests show 

that the breaks are not at the same time after pumping was 

started for each test; rather, the breaks are at the same waterlevel depths. The tests are nearly impossible to interpret without knowledge of the well diameter from the caliper log. The 

steepest sections of the curves correspond to smallest diameter 

sections of the well bore from 40–45, 47–49, and 51–53 ft 

below land surface. The flat sections of the water-level curves 

at depths of 46 and 50 ft below land surface correspond to 

depths at which the well intersected enlarged fractures or 

voids. The slope of the drawdown and recovery curves change 

because most of the water being pumped is derived from storage in the well and voids. The voids provide water to the well 

from storage, but have small transmissivity as indicated by the 

slow recovery. The straight-line water-level curves for the last 

straight-line segment during drawdown and first segment of 

recovery from the 1.5 gal/min test in July 2008, indicate a rate 

of constant inflow of about 0.5 gal/min.

Drawdown from the 0.57 gal/min test in May 2009 

did not reach an apparent equilibrium after 60 minutes of 

pumping and the recovery was slow. Probably the maximum 

yield is less than 0.5 gal/min. Data for the test completed in 

April 1980 show that the static water level was about 10 ft 

higher than for the other tests and that the water level stabilized at the same depth (46 ft below land surface) as for the 

other 1.5 gal/min tests.

The change in specific capacity with time for each test 

is shown in figure 9B. The specific-capacity plots for all tests 

completed for this study during 2008–09 were nearly identical, 

indicating that pumping rate did not affect specific capacity; specific capacity of the test completed in April 1980 was 

notably less.
Specific-Capacity Tests at Six Low-Yielding Wells in Fractured Bedrock 19

EXPLANATION

A EXPLANATION

B

20 

25 

30 

35 

40 

45 

50 

55 

60 

Depth to water below land surface, in feet 

0 60 120 180 

Time, in minutes since pumping began 

April 1980 (0.88 gallon per minute) 

July 2008 (1.5 gallons per minute) 

November 2008 (1.5 gallons per minute) 

May 2009 (0.57 gallon per minute) 

April 1980 (0.88 gallon per minute) 

July 2008 (1.5 gallons per minute) 

November 2008 (1.5 gallons per minute) 

May 2009 (0.57 gallon per minute) 

0 20 40 60 80 

Time, in minutes since pumping began 

0.01 

0.10 

1.00 

10.00 

Specific capacity, in gallons per minute per foot

Figure 9. Water level (A) and specific capacity (B) during five specific-capacity tests at well GR 541 in Greene 

County, Pennsylvania.
20 Factors Influencing Specific-Capacity Tests and Their Application

Well WS 155

Well WS 155 in Washington County is an observation well, maintained and operated by USGS for monitoring drought conditions. The well is completed in sandstone, 

siltstone, limestone, and shale of the Washington Formation to 

a depth of 133 ft and cased with 6-in. diameter steel casing to 

19 ft below land surface (table 1). The driller reported water 

entering the well at 2 gal/min at a depth of 45 ft below land 

surface (table 1). Geophysical logging indicated openings at 

about 67, 80, 100, and 116 ft below land surface (fig. 4), and 

flowmeter surveys indicated that all the water is yielded from 

depths shallower than 48 ft below land surface. The water is 

contributed from sandstone according to the description of 

cuttings from the well. Limestone beds from 30–35, 65–75, 

93–105, 148–150, and 155–160 ft below land surface were 

not reported as water-yielding zones by the driller. The static 

water level in the well ranged from about 35–38 ft below land 

surface (fig. 3), which allowed about 92 ft of drawdown available for testing.

Well WS 155 was pumped on three occasions by USGS 

during 2008–09 (table 2). The well was pumped at 2.5 gal/min 

for 180 minutes in March and July 2008, and then allowed to 

recover for 120 minutes. The well was pumped at 1.0 gal/min 

in May 2009, allowed to recover for 40 minutes, and pumped 

again at 6.0 gal/min for 35 minutes. A test also was completed 

by USGS in 1971 for 90 minutes at 2.0 gal/min. Although only 

drawdown values were reported from that test, the data are 

plotted on figure 10A assuming that the change in slope of the 

water-level decline is at a depth of 45 ft.

Drawdown and recovery for the five tests are shown 

in figure 10A. The early-time drawdown data for all tests 

show the rate of water-level decline decreasing with time, as 

is characteristic for radial flow to a well. However, in latetime drawdown (greater than 120 minutes) and early-time 

recovery when the water level was below about 45 ft below 

land surface, the data plot as straight lines on arithmetic 

axes. The straight lines indicate that the water-level change 

is proportional to time, and that the pumped water is being 

derived from water stored in the well and contributed from 

the aquifer at a constant rate, independent of drawdown. The 

drawdown curves plot as straight lines when the water level 

is greater than 45 ft below land surface, which is the depth 

of the only water-yielding zone reported by the driller. When 

the water level is below 45 ft, water cascades into the well 

from the fracture at that depth at a rate independent on the 

water level in the well. The straight-line drawdown segment 

starting after about 140 minutes indicates that the rate of 

withdrawal from wellbore storage was about 0.3 gal/min for 

the 2.5 gal/min test in March 2008 (assuming a 6-in. diameter 

well) during pumping; the recovery curve indicates a constant 

inflow of about 2.4 gal/min after pumping stopped. Thus, the 

straight-line curves for the March 2.5 gal/min test indicate a 

constant inflow of about 2.2 to 2.4 gal/min, which is a reasonable estimate of the maximum well yield. The slope of the 

straight-line recovery curve indicated a constant inflow of 

3.0 gal/min for the 6 gal/min test. Thus, the yield of the well 

is probably about 2–3 gal/min. Data from the 1 gal/min test in 

May 2009 did not show any straight-line segment of waterlevel change as observed in the other tests because the water 

level did not fall below the major water-yielding zone at 45 ft 

below land surface.

The change in specific capacity with time for each of the 

five tests is shown in figure 10B. The specific-capacity values 

for all tests compare closely until the water level drops below 

45 ft below land surface. At that point, the rate of change 

for specific capacity with time increases because the rate 

of change in drawdown has changed (fig. 10A). Therefore, 

specific capacity is not dependent on pumping rate for tests 

completed when the water level is less than 45 ft below land 

surface. The pumping rate affects specific capacity, but not 

because of head losses caused by turbulent flow. Instead, the 

key factor is the depth of the major water-yielding zone with 

respect to the water level in the well.

Well

Specific-capacity testing at well WS 155, inside culvert pipe. 
Specific-Capacity Tests at Six Low-Yielding Wells in Fractured Bedrock 21

EXPLANATION

EXPLANATION

A

B

30 

35 

40 

45 

50 

55 

60 

65 

70 

75 

80 

0 60 120 180 240 300 

Depth to water below land surface, in feet 

Time, in minutes since pumping began 

0.1 

1.0 

10.0 

0 60 120 180 

Specific capacity, in gallons per minute per foot

Time, in minutes since puming began 

January 1971 (2 gallons per minute) 

March 2008 (2.5 gallons per minute) 

July 2008 (2.5 gallons per minute) 

May 2009 (1 gallon per minute) 

May 2009 (6 gallons per minute) 

January 1971 (2 gallons per minute) 

March 2008 (2.5 gallons per minute) 

July 2008 (2.5 gallons per minute) 

May 2009 (1 gallon per minute) 

May 2009 (6 gallons per minute) 

Figure 10. Water level (A) and specific capacity (B) during five specific-capacity tests at well WS 155 in Washington County, 

Pennsylvania.
22 Factors Influencing Specific-Capacity Tests and Their Application

Well YO 1222

Well YO 1222 in York County is an unused well at the 

USGS Pennsylvania Water-Science Center office. The well 

is completed in limestone of the Epler Formation to a depth 

of 202 ft and is cased with 6.25-in. diameter steel casing to 

18 ft below land surface. The driller reported a water-yielding 

zone of unspecified rate at 120 ft below land surface and 

1 gal/min at 162 and 2 gal/min at 168 ft below land surface 

(table 1). Drill cuttings were described as gray massive 

limestone throughout, with calcite veins. Geophysical logging indicated few obvious fractures except those fractures at 

about 27 ft below land surface (fig. 4), which also could be 

seen on the video log of the well (fig. 5). The static water level 

in the well ranged from about 18 to 35 ft below land surface 

(fig. 3), which allowed at least 167 ft of drawdown available 

for testing.

Well YO 1222 was pumped on six occasions by USGS 

during 2007–09 (table 2). The well was pumped at about 

2 gal/min in August 2007, July 2008, and August 2008; 

at 5–6 gal/min in August 2007 and March 2008; and at 

14 gal/min in September 2009. The test in August 2008 was 

a step-drawdown test in which the well was pumped in four 

60-minute steps at increasing rates of 2, 3, 4, and 5 gal/min. 

Recovery was measured for 60 to 120 minutes for all tests 

except for the 2 gal/min test in August 2007, because of 

equipment problems.

Well

Well YO 1222. 

Drawdown and recovery of water levels for the six tests 

are shown in figure 11A. The water-level drawdown data 

for all tests show the rate of water-level decline decreasing with time, as is characteristic for radial flow to a well. 

However, drawdown was slight during the 5–6 gal/min tests. 

The drawdown for specific-capacity tests completed at 5 or 

6 gal/min was less than for the tests completed at lesser rates, 

which seems impossible. The reason for this oddity is that the 

5–6 gal/min tests were completed during periods when the 

water level in the well was above the fractures at 27 ft below 

land surface, which is a major water-bearing zone, capable of 

providing substantial water to the well when saturated. When 

the zone is unsaturated, the well yield is considerably smaller, 

as indicated by the larger water-level drawdown during the 

2 gal/min tests.

The change in specific capacity with time for each of 

the six tests is shown in figure 11B. The specific capacity 

for the tests completed from 5 to 6 gal/min compares closely 

and after 60 minutes of pumping indicate a value of about 

5 (gal/min)/ft. The other tests show some variability, but after 

60 minutes of pumping indicate much smaller specific-capacity values from 0.08 to 0.12 (gal/min)/ft. The specific-capacity 

values determined at YO 1222 are the most varied among the 

six wells tested for this study. The major factor affecting the 

specific capacity is the relation of the water level in the well 

to the major water-yielding zone at 27 ft below land surface. 

The specific capacity is about 50 times greater when the water 

level is above that zone than when the water level is below 

that zone.
Specific-Capacity Tests at Six Low-Yielding Wells in Fractured Bedrock 23

EXPLANATION

EXPLANATION

A

B

0 

20 

40 

60 

80 

100 

120 

140 

160 

Depth to water below land surface, in feet 

0 60 120 180 240 300 

Time, in minutes since pumping began 

August 2007 (2 gallons per minute) 

August 2007 (6 gallons per minute) 

March 2008 (5 gallons per minute) 

July 2008 (2.1 gallons per minute) 

August 2008 (Step-drawdown test) 

September 2009 (14 gallons per minute) 

0.01 

0.10 

1.00 

10.00 

Specific capacity, in gallons per minute per foot

0 60 120 180 240 

Time, in minutes since pumping began 

August 2007 (2 gallons per minute) 

August 2007 (6 gallons per minute) 

March 2008 (5 gallons per minute) 

July 2008 (2.1 gallons per minute) 

August 2008 (Step-drawdown test) 

September 2009 (14 gallons per minute) 

Figure 11. Water level (A) and specific capacity (B) during five specific-capacity tests at well YO 1222 in York 

County, Pennsylvania.
24 Factors Influencing Specific-Capacity Tests and Their Application

Factors Affecting Specific Capacity

The factors affecting drawdown in a pumped well have 

been widely discussed in the literature (Driscoll, 1986; Kruseman and deRidder, 1990). The major factors are pumping 

duration, pumping rate, wellbore storage, aquifer properties, 

and turbulent flow influenced by the characteristics of the 

well screen, casing, and pump. The usefulness of a specificcapacity test is greatly enhanced if the pumping and wellconstruction details have been reported. Individual factors that 

could affect specific capacity are discussed in this section, by 

using examples from theoretical simulations and from tests at 

the six low-yielding wells. The theoretical analysis provides 

insights into the factors for a simple aquifer (fig. 12) for which 

each factor can be specified; whereas, the six well tests illustrate the additional complexities caused by the less than ideal 

fractured-bedrock aquifer and errors inherent in measuring 

real pumping rates and water levels.

The theoretical analysis of a hypothetical well in an idealized aquifer was made with the use of Aqtesolv, a computer 

program designed to solve analytical equations for drawdown 

in wells with different well-construction and aquifer characteristics (Duffield, 2007). The Papadopulos and Cooper 

(1967) analytical solution in Aqtesolv was used to simulate a 

uniform, laterally extensive, homogeneous, isotropic aquifer 

and a finite-diameter well with wellbore storage. A standard 

case was simulated for a 6-in. diameter well in an aquifer 

with transmissivity of T = 10 ft2

/d and storage coefficient of 

S = 0.0001. Sensitivity of specific capacity was tested for 

variations in the following properties: pumping rate, pumping 

duration, and well diameter.

Aquifer 

Pre-pumping static water level 

Land surface Well diameter = 6 inches 

Pump 

Figure 12. Schematic diagram of a hypothetical well in an idealized homogeneous aquifer used in 

the theoretical analysis of some factors affecting specific capacity.
Factors Affecting Specific Capacity 25

Pumping Duration

Specific capacity decreases as the duration of pumping increases because drawdown increases with time in the 

pumped well as shown in equation 3. For this reason, the 

duration of the test is always noted when reporting a specificcapacity value.

Theory Shows Importance of Pumping Duration 

Decreases With Time

The increase in drawdown and decrease in specific 

capacity with time is shown in figure 13 for pumping of a 

hypothetical 6-in. diameter well at 2 gal/min in an idealized 

aquifer with transmissivity of 10 ft2

/d and storage coefficient of 0.0001. The example shows that the rate of change 

of drawdown and specific capacity decreases with time, so 

specific-capacity values determined from tests of long duration, in general, are less likely to be affected by differences 

in test duration than are values derived from short-duration 

tests. For example, from the data in figure 13, compare the 

differences in specific capacity between two tests, one having 

a pumping duration twice as long as the other. For short-duration tests of 30 and 60 minutes, figure 13 shows the specific 

capacity is 0.088 (gal/min)/ft after 30 minutes of pumping and 

0.065 (gal/min)/ft after 60 minutes of pumping—a decrease 

of 26 percent for the test with longer duration. For two tests 

of longer duration, figure 13 shows that specific capacity is 0.050 (gal/min)/ft after 240 minutes of pumping and 

0.047 (gal/min)/ft after 480 minutes of pumping—a decrease 

of only about 6 percent.

 Drawdown will increase and specific capacity will 

decrease until the well reaches equilibrium. Equilibrium is 

only reached when the drawdown in the aquifer caused by the 

pumping decreases the natural discharge of the aquifer and 

(or) increases the natural recharge in an amount equal to the 

pumping rate. Equilibrium cannot be reached in the theoretical example because the only source of water is storage in the 

aquifer, and in reality, equilibrium usually is not achieved in 

short-term tests of low-yielding aquifers. However, as shown 

by the hypothetical example in figure 13, the rate of change of 

water-level drawdown decreases with time until an apparent 

equilibrium condition can be reached in which drawdown and 

specific capacity change only slightly with time.

0.01 

0.1 

0 60 120 180 240 300 360 420 480 

Specific capacity,

in gallons per minute per foot

Time, in minutes since pumping began

10 

20 

30 

40 

50 

0 60 120 180 240 300 360 420 480 

Depth to water below land surface, in feet 

Time, in minutes since pumping began 

0 

1 

A

B

Figure 13. Change in drawdown 

and specific capacity with duration 

of pumping for constant withdrawal 

of 2 gallons per minute from a 

hypothetical 6-inch diameter well in 

an idealized aquifer with transmissivity 

10 feet squared per day and storage 

coefficient of 0.0001.
26 Factors Influencing Specific-Capacity Tests and Their Application

Tests Show Effect of Pumping Duration on 

Specific Capacity

Results from all the tests were consistent with the theory 

that pumping duration is an influential factor controlling specific capacity. At least one test was completed at each of the 6 

test wells for 180 minutes or until the water level in the well 

declined to the depth of the transducer setting to evaluate the 

effect of pumping duration on specific capacity. Examples of 

pumping-test data from each of the 6 wells showed an increase 

in drawdown (fig. 14A) and decrease in specific capacity 

(fig. 14B) with time. Specific-capacity values declined quickly 

in the early part of all tests, illustrating the value of pumping 

for durations longer than 60 minutes. Specific-capacity values 

after 180 minutes of pumping decreased from 0 to 62 percent 

compared to the value after 60 minutes for the four wells that 

were pumped for the full 180 minutes (table 3). Comparisons 

for wells AR 110 and GR 541 are not shown in table 3 because 

the water levels reached the transducer setting near the bottom 

of the wells; therefore, those tests were terminated prior to 

180 minutes.

Drawdown increased with time in well BV 156, but at 

a decreasing rate, similar to the response to pumping at the 

hypothetical well (fig. 13). The specific capacity, in well 

CA 462, was more affected by duration of pumping when 

pumped at 2 gal/min than at 1 gal/min. Specific capacity 

decreased less than 20 percent between 60 and 180 minutes 

of pumping at the lower pumping rate, but at the higher rate, 

the decrease was about 60 percent (table 3). In well WS 155, 

at about 130 minutes, the rate of drawdown unexpectedly 

increased, which resulted in an increase in the rate of decline 

of specific capacity. The drawdown declined little after 

60 minutes of pumping in YO 1222, and stabilized at 80 minutes, indicating an apparent equilibrium condition had been 

reached, which resulted in a steady specific-capacity value 

of about 0.12 (gal/min)/ft (fig. 14 and table 2). Thus, for well 

YO 1222, after about 60 minutes, duration of pumping was 

not a factor affecting specific capacity. Drawdown at the end 

of the tests in wells AR 110, CA 462, GR 541, and WS 155 

were proportional to time (linear), indicating that most of the 

pumped water was being derived from wellbore storage or was 

cascading into the well at a rate independent of the water level 

in the well.

Table 3. Comparison of specific-capacity values after 60 and 180 minutes of pumping.

[gal/min, gallons per minute; (gal/min)/ft, gallons per minute per foot]

Well1 Pumping rate 

(gal/min)

Specific capacity 

[(gal/min)/ft] Percent 

difference2 After 60 minutes 

pumping

After 180 minutes 

pumping Difference

BV 156

BV 156

CA 462

CA 462

CA 462

CA 462

WS 155

WS 155

YO 1222

YO 1222

YO 1222

5.0

5.0

2.0

2.0

1.0

1.0

2.5

2.5

5.0

2.1

2.0

0.17

.19

.047

.058

.12

.15

.37

.41

5.5

.12

.11

0.11

.13

.019

.022

.10

.14

.14

.16

4.20

.12

.11

-0.06

- .06

- .028

- .036

- .02

- .01

- .23

- .25

-1.3

0

0

-35

-32

-60

-62

-17

-6.7

-62

-61

-24

0

0

1

AR 110 and GR 541 are not included because the wells were not pumped for 180 minutes during any of the tests.

2

Ratio of the difference divided by the specific capacity after 60 minutes of pumping, as percent.
Factors Affecting Specific Capacity 27

0.01 

0.10 

1.00 

10.00 

0 30 60 90 120 150 180 

Specific capacity, in gallons per minute per foot

Time, in minutes since pumping began 

0 

20 

40 

60 

80 

100 

120 

0 30 60 90 120 150 180 

Drawdown, in feet 

Time, in minutes since pumping began 

AR 110 

BV 156 

CA 462 

GR 541 

WS 155 

YO 1222 

AR 110 

BV 156 

CA 462 

GR 541 

WS 155 

YO 1222 

EXPLANATION

EXPLANATION

A

B

Figure 14. Drawdown (A) and specific capacity (B) with time during selected specific-capacity tests 

at six low-yielding wells in fractured-bedrock aquifers in Pennsylvania.
28 Factors Influencing Specific-Capacity Tests and Their Application

Pumping Rate

Specific capacity may be affected by pumping rate under 

some conditions, so the pumping rate is always reported. 

Generally, when comparing tests, a good practice is to repeat 

tests at the same pumping rate, but some tests completed at 

different pumping rates may be comparable.

Theory Shows Pumping Rate May Not Affect 

Specific Capacity

The general equation for drawdown in the pumped well 

(eq. 3) shows that drawdown is directly proportional to the 

pumping rate provided that the turbulent-flow factor (eq. 4) 

is small and the factors inherent in coefficient B (time, well 

radius, transmissivity, and storage coefficient) are constant. 

Specific capacity, in theory, will not depend on the pumping 

rate if all flow in the aquifer and well is laminar.

Drawdown simulated after 180 minutes of pumping from 

the 6-in. diameter hypothetical well at rates from 2–10 gal/min 

is shown for the well with and without turbulent flow (fig. 15). 

The plot shows that specific capacity remains a constant value 

of 0.052 (gal/min)/ft regardless of the pumping rate for a well 

with no turbulent flow. However, when turbulent flow is simulated, the drawdown is proportionally greater at higher pumping rates, which decreases specific capacity. Turbulent flow is 

not usually a major factor contributing to drawdown for wells 

pumped at low rates (2–10 gal/min). The turbulent-flow factor 

C (eq. 4) used to produce the plot in figure 15 was much larger 

than would be typical, therefore, the effect of turbulence for 

these low pumping rates is exaggerated.

0.038 

0.040 

0.042 

0.044 

0.046 

0.048 

0.050 

0.052 

0.054 

0 2 4 6 8 10 12 

Specific capacity, in gallons per minute per foot

Pumping rate, in gallons per minute

No turbulent flow 

With turbulent flow

EXPLANATION

(Turbulent flow factor equals

20 minutes squared per foot to

the fifth power)

Figure 15. Theoretical specific capacity with and without turbulent flow for pumping at rates 

from 2 to 10 gallons per minute for 180 minutes in a hypothetical 6-inch diameter well with wellbore 

storage.
Factors Affecting Specific Capacity 29

Essential to Know Pumping Rate and Hold 

Discharge Steady

According to the definition of specific capacity (eq. 1), 

any error in measuring the pumping rate Q translates directly 

to an error in the specific capacity SC; thus, pumping rate 

needs to be measured accurately during a test. For example, if 

the measured pumping rate was 4 gal/min and the actual rate 

was 5 gal/min, the 20 percent error in determination of the 

pumping rate would cause a 20 percent error in the computed 

specific capacity. Thus, a good practice would be to check 

the discharge rate from a pump by more than one method. 

If a flowmeter is used to measure discharge, readings can be 

checked against a volumetric measurement with a watch and a 

graduated or known-volume container.

The largest source of experimental error in computing specific capacity is usually from the measurement of the 

pumping rate because manual adjustments to a valve on the 

discharge pipe are typically needed to hold the rate constant as 

drawdown in the well increases. The precision of the pumping 

rates in this study were within about 1–5 percent of the mean 

value. An example of the variability in discharge rate is shown 

on the graph of discharge from the test at WS 155 in July 2008 

(fig. 16). The accuracy of the measurements was not assessed 

other than to note that the flowmeter readings corresponded 

to periodic volumetric measurements within about 5 percent, 

and that comparison readings from two different flowmeters 

on the same discharge line sometimes differed by as much as 

5 percent.

2.0 

2.2 

2.4 

2.6 

2.8 

3.0 

0 30 60 90 120 

WELL WS 155

July 2008 test

150 180 

Pumping rate,

in (gallons per minute)/foot

Time, in minutes since pumping began 

Maximum rate = 2.53 gallons per minute

Minimum rate = 2.47 gallons per minute

Figure 16. Variability of the pumping rate during the 2.5 gallonper-minute specific-capacity test at well WS 155 in July 2008.

Results from a specific-capacity test can be more accurately compared to subsequent tests if the pumping rate is held 

constant through the duration of the test. The effects of varied 

pumping rate are illustrated with results from two simulated 

tests at the hypothetical well (fig. 17). The pumping rate was 

held steady at 5 gal/min in one test, which resulted in a drawdown of 96 ft after 180 minutes of pumping and a specific 

capacity of 0.052 (gal/min)/ft. The pumping rate was 6 gal/min 

during the first 90 minutes and 4 gal/min during the final 90 

minutes in the other test, resulting in, after 180 minutes of 

pumping, a drawdown of 81 ft and specific capacity of 0.062 

(gal/min)/ft if the average pumping rate of 5 gal/min is used to 

compute specific capacity.

The total quantity of water withdrawn (900 gal) and average pumping rate (5 gal/min) were the same for both tests, but 

use of the average pumping rate to compute specific capacity 

did not produce the same result for both tests. Thus, a good 

practice would be to document variations in pumping rate 

throughout the duration of the test instead of relying solely 

on a computation of the average pumping rate determined 

from the total volume pumped during the test divided by the 

duration of pumping. In fact, in this example, the drawdown 

at the end of the test has nearly adjusted to the pumping rate 

of 4 gal/min; thus, a better estimate of the specific capacity 

would be obtained by dividing the drawdown of 81 ft into 

4 gal/min to yield 0.049 (gal/min)/ft. Commonly, the pumping rate will decline slowly during a test as the water level in 

the well falls, causing the drawdown to stabilize temporarily 

and leading to the false conclusion that equilibrium has been 

achieved.

0 

20 

40 

60 

80 

100 

Depth to water below land surface,

120 

in feet 

0 30 60 90 120 150 180 

Time, in minutes since pumping began

4 gallons per minute

5 gallons per minute

6 gallons per minute

EXPLANATION

81 feet

96 feet

Figure 17. Theoretical drawdown for constant-rate pumping 

at 5 gallons per minute and varied-rate pumping at 6 gallons 

per minute for 90 minutes followed by 4 gallons per minute for 

90 minutes at the hypothetical well.
30 Factors Influencing Specific-Capacity Tests and Their Application

Tests Show Differing Effects of Pumping Rate on 

Specific Capacity

Specific-capacity values from multiple tests are compared 

for the same duration of pumping (60 minutes for most wells) 

in figure 18. Values from tests completed at the same (or 

nearly same) pumping rate all differed by less than 36 percent, 

and generally agreed within about 20 percent (table 4). When 

tested at greater rates, specific-capacity values compared 

closely at wells AR 110, GR 541, and WS 155, decreased 

substantially at wells BV 156 and CA 462, and increased by 

a factor of about 50 at well YO 1222 (table 5). Details about 

the change in specific capacity with time during each test are 

illustrated in figures 6B–11B.

Specific-capacity values after 49 minutes of pumping for 

two tests completed at 2 gal/min and one test at 0.88 gal/min 

for well AR 110 compare within about 9 percent of the mean 

value (table 4 and fig. 18), showing that results from tests 

completed at the same rate and duration are reproducible to 

that extent and that differing pumping rates did not affect specific capacity. The specific-capacity value from the historical 

test completed in 1999 at 3.5 gal/min was substantially greater 

than for tests completed during 2008–09. The smaller specificcapacity values from the more recent tests may be the result of 

clogging of water-yielding fractures over time.

Specific-capacity values for well BV 156 after 60 minutes 

of pumping for two tests completed at 5 gal/min compare 

within 11 percent of the mean value (table 4 and fig. 18), 

showing that tests completed at the same rate and duration are 

reproducible to that extent. The substantially larger specificcapacity value for the 2.5 gal/min test and smaller value for 

the 7.9 gal/min test indicate that pumping rate does substantially affect specific capacity. The effect could be caused by 

turbulent head losses but is more likely caused by dewatering 

of shallow water-yielding fractures (causing a decrease in 

transmissivity) as the pumping rate is increased.

Specific capacity for well CA 462 after 60 minutes of 

pumping compares reasonably well (about 20 percent of the 

mean value) for tests completed at the same rate (either 1 

or 2 gal/min) (table 4 and fig. 18), showing that tests completed at the same rate and duration are reproducible to that 

extent. The subsequent test had the greater specific capacity 

for the tests repeated at the same rate, which may indicate 

a slight development of the well during each successive 

test. The decrease in specific-capacity values for tests completed at increasing rates from 1.0 to 4.5 gal/min indicates 

that pumping rate does substantially affect specific capacity. The higher pumping rates caused dewatering of the 

water-yielding zone from 20–50 ft below land surface, causing 

a decrease in transmissivity.

Specific capacity for well GR 541 after 50 minutes of 

pumping compares within about 9 percent of the mean value 

for the two tests completed at 1.5 gal/min (table 4 and fig. 18), 

showing that tests completed at the same rate and duration 

are reproducible to that extent. Specific capacity was only 

13 percent smaller for the tests completed at 1.5 gal/min than 

for a test completed at 0.57 gal/min (table 5), showing that 

pumping rate does not have much effect on specific capacity. The smaller specific-capacity value for the historical test 

completed in 1980 may indicate the well/aquifer connection 

has changed.

Specific-capacity values for well WS 155 after 

60 minutes of pumping for two tests completed at 2.5 gal/min 

compare within 10 percent of the mean value, showing that 

tests completed at the same rate and duration are reproducible to that extent (table 4 and fig. 18). The specific-capacity 

value for the 2.5 gal/min test is 15 percent larger than for the 

1.0 gal/min tests, indicating that pumping rate does not greatly 

affect specific capacity at these rates. How a lesser pumping 

rate could result in the smaller specific-capacity value is difficult to understand, so other factors such as measurement error 

may be involved.

Specific-capacity values for well YO 1222 are similar 

for tests conducted at similar rates (fig. 18 and table 4) but 

greatly different for tests completed at different pumping rates 

(fig. 18 and table 5). Specific-capacity values after 60 minutes of pumping for two tests completed at 5 and 6 gal/min 

compare within 8 percent of the mean value and three tests 

completed at 2.0–2.1 gal/min compare within 36 percent of the 

mean value (table 4), showing that tests completed at the same 

rate and duration can be reproduced to that degree. However, 

the specific-capacity values for the 5 to 6 gal/min tests are 

about 60 times greater than those values for tests completed 

at 2 gal/min (table 5). This difference seems to be caused by 

differences in pumping rate, but rate is not the major control. 

The large specific-capacity values are seen when the well is 

pumped during periods when the water level in the well is 

above the major water-yielding fractures at 27 ft below land 

surface, which greatly increase the aquifer transmissivity.
Factors Affecting Specific Capacity 31

Table 4. Specific-capacity values from two or three tests completed at the same pumping rate at each of six low-yielding wells in 

fractured-bedrock aquifers in Pennsylvania.

[gal/min, gallons per minute; min, minutes; (gal/min)/ft, gallons per minute per foot; --, no data]

Well Pumping rate 

(gal/min)

Duration of 

pumping

(min)

Specific capacity

[(gal/min)/ft] Maximum 

percent

difference1 Test 1 Test 2 Test 3 Mean 

(rounded)

Maximum

difference

AR 110 2.0 49 0.035 0.032 -- 0.034 0.003 9

BV 156 5.0 60 .17 .19 -- .180 .020 11

CA 462 2.0 60 .12 .15 -- .135 .030 22

CA 462 1.0 60 .047 .058 -- .053 .011 21

GR 541 1.5 50 .10 .091 -- .096 .009 9

WS 155 2.5 60 .37 .41 -- .390 .040 10

YO 1222 2.0 60 .083 .12 0.11 .104 .037 36

1

 Ratio of maximum difference to the mean value of specific capacity, as percent.

Table 5. Specific-capacity values from tests completed at different pumping rates at six low-yielding wells in fractured-bedrock 

aquifers in Pennsylvania.

[Underlined values represent the mean specific capacity from two or three tests. min, minutes; gal/min, gallons per minute; (gal/min)/ft, gallons per minute per 

foot]

Well

Duration of 

pumping 

(min)

Low 

pumping rate 

(gal/min)

High 

pumping rate 

(gal/min)

Specific capacity 

[(gal/min)/ft] Percent 

difference1

Low rate test High rate test Difference

AR 110 49 0.88 2.0 0.034 0.034 0.000 0

BV 156 60 2.50 5.0 .27 .18 - .09 -33

CA 462 60 1.00 2.0 .14 .052 - .088 -63

GR 541 50 .57 1.5 .11 .096 - .014 -13

WS 155 60 1.00 2.5 .34 .39 .05 15

YO 1222 60 2.00 6.0 .10 5.1 5.0 5,000

1

Ratio of the difference divided by the specific capacity from the low rate test, as percent. 0.88 2.0 2.0 2.0 2.5 5.0 5.0 7.9 1.0 1.1 2.0 2.0 4.5 0.57 

0.95 

1.5 

1.5 

1.0 

2.0 

2.5 

2.5 

2.0 

2.0 

2.1 

5.0 

6.0 

3.5

0.0 

0.1 

0.2 

0.3 

0.4 

0.5 

Specific capacity, 

in gallons per minute per foot

5.0 

6.0 

WELL 

AR 110 BV 156 CA 462 GR 541 WS 155 YO 1222

Red bars are results from historical tests 

Number above bar is pumping rate, in gallons per minute

EXPLANATION

Figure 18. Specific-capacity values for tests completed at different rates after 60 minutes of pumping (49 minutes at AR 

110 and 50 minutes at GR 541; estimated values as listed in table 2) at six low-yielding wells in fractured-bedrock aquifers 

in Pennsylvania.
32 Factors Influencing Specific-Capacity Tests and Their Application

Wellbore Storage

Some water is always removed from the wellbore when a 

well is pumped. The quantity of water stored in the well may 

not be a substantial source of water for some water-supply 

wells, but for low-yielding wells, wellbore storage may provide most of the water withdrawn during the course of a 1- to 

3-hour specific-capacity test. Thus, wellbore storage can have 

a substantial effect on specific capacity.

Theory Shows Importance of Wellbore Storage 

in Early Time

The drawdown of short-term specific-capacity tests in 

low-yielding aquifers is reduced because of water stored in the 

pumped well. As the duration of pumping increases, the percentage of water contributed from the aquifer usually increases 

and the effect of the wellbore storage diminishes. The length 

of time (t

c

) during which wellbore storage will have a substantial (more than 5 percent) effect on drawdown of an ideal well 

was given by Reed (1980, p. 39) from the work of Papadopulos and Cooper (1967) as:

t r

T c

c >

25 2

 , (5)

where

	 tc	 is the critical time, in days; and

	 rc	 is the radius of the well casing through which 

the water level fluctuates, in feet.

Equation 5 shows that the effect of wellbore storage will 

last longer for wells with large diameter casing and small 

transmissivity. The pumping rate does not affect the critical 

time (t

c

) at which wellbore storage becomes negligible.

The effect of wellbore storage on drawdown was simulated for hypothetical wells with diameters of 4, 6, and 8 in., 

pumping at 5 gal/min from an idealized aquifer with small 

transmissivity (10 ft2

/d) and storage coefficient of 0.0001 

(fig. 19A). Simulated drawdown is smallest in the 8-in. diameter well because the “extra” water from the large wellbore 

is greatest for that well compared to the 4- and 6-in. diameter 

wells. The differences in drawdown among the three wells are 

substantial throughout the 180 minutes of simulated pumping, 

causing differences in the theoretical specific-capacity values 

(fig. 19B).

Although the effects of wellbore storage on low-yielding 

wells may linger throughout the duration of the test, the reproducibility of specific-capacity values between successive tests 

at the same well should not be affected as long as the well 

diameter is not changing. For example, figure 15 shows simulations for the hypothetical well of 6-in. diameter, pumped at 

differing rates. The specific capacity after 1-hour of pumping 

(with no turbulent flow) shows a constant value of specific 

capacity of 0.052 (gal/min)/ft regardless of pumping rate.

0 

0.05 

0.10 

0.15 

0.20 

0.25 

0.30 

0 30 60 90 120 150 180 

Specific capacity,

in gallons per minute per foot

Time, in minutes since pumping began 

0 

20 

40 

60 

80 

100 

120 

0 30 60 90 120 150 180 

Depth to water below land surface, in feet 

Time, in minutes since pumping began 

A

B

EXPLANATION

8-inch 

WELL DIAMETER

6-inch 

4-inch 

EXPLANATION

8-inch 

WELL DIAMETER

6-inch 

4-inch 

Figure 19. Drawdown (A) and specific capacity (B) for well 

diameters of 4, 6, and 8 inches simulated with the hypothetical 

well (pumping rate = 5 gallons per minute, transmissivity = 10 feet 

squared per day, storage coefficient = 0.0001).
Factors Affecting Specific Capacity 33

Wellbore-Storage Correction Can Improve 

Estimates of Well Yield

Pennsylvania Department of Environmental Protection 

(1999) recommends that the pumping rate be adjusted for 

the volume of water removed from wellbore storage before 

using specific capacity to compute well yield. The correction 

is needed because tests of short duration at low-yielding wells 

may draw only a small fraction of the discharged water from 

the aquifer. Thus, specific-capacity values determined from 

these tests might overestimate the ability of the aquifer to 

provide water to the well.

Well yield is often estimated by multiplying the available drawdown in the well times the specific capacity (Heath, 

1983, p. 77). If the specific capacity for an ideal well has been 

computed from a test where the water level has come close 

to reaching an apparent equilibrium, that value can be multiplied by the available drawdown to provide an estimate of 

well yield. If the well has not reached equilibrium, the specific 

capacity from a short-term test needs to be reduced to compensate for the additional drawdown that would happen during 

longer pumping periods.

Adjusting a short-duration specific-capacity test to estimate long-term well yield is a challenge (Heath, 1983, p. 58), 

but for a test where wellbore storage is a substantial source of 

water, the compensation between early and late-time specificcapacity values needs to be greater than if wellbore storage 

were not a factor. For example, in figure 19B, the specific 

capacity for the 8-in. well after 30 minutes of pumping is 

120 percent greater than the value after 180 minutes; whereas, 

the specific capacity for the 4-in. well (less affected by wellbore storage) is only about 30 percent greater.

The adjustment of specific capacity recommended 

by Pennsylvania Department of Environmental Protection 

(1999) removes the volume of water withdrawn from wellbore storage prior to computing specific capacity as shown in 

equation 6.

SC adj

V V t

s

p b ( ) = ( ) − / , (6)

where

SC(adj)	 is the adjusted specific capacity, in gallons per 

minute per foot;

Vp	 is the total volume pumped (theoretically Qt), 

in gallons; and

Vb	 is the volume removed from wellbore storage, 

in gallons.

The intent is to provide a value that can be used to make 

better estimates of the yield of the aquifer. For example, 

adjusted specific capacity was computed for a simulation for 

a 6-in. diameter hypothetical well pumping at 5 gal/min from 

an idealized aquifer. The adjusted value of specific capacity 

(adjusted for wellbore storage continuously for each increment 

of time in figure 20A) is much less than the unadjusted value 

during early time. By removing wellbore storage, the adjusted 

specific-capacity value for early time is a good estimate of 

the specific capacity for later time; thus, when multiplied by 

available drawdown the adjusted specific capacity is a better 

representation of well yield than if the unadjusted specific 

capacity had been used.

Consider, for example, an estimate of well yield from 

the 2 gal/min test completed in March 2008 at well CA 462 

(fig. 8) during which about half of the water withdrawn during pumping is derived from wellbore storage. Pumping for 

1 hour resulted in a drawdown of 43 ft and specific-capacity 

value of 0.047 (gal/min)/ft. Multiplying the specific capacity by the available drawdown of 132 ft provides an estimated well yield of about 6 gal/min. However, water-level 

recovery after pumping had ceased indicated that water was 

being contributed to the well at a steady rate of only about 

1.3 gal/min—about five times less than the estimated well 

yield. The 60-minute specific-capacity value would need to be 

substantially adjusted to be used to estimate long-term well 

yield. The adjusted specific capacity for well CA 462 is shown 

in figure 20B. The adjusted value is about 0.02 (gal/min)/ft 

at 60 minutes, which, when multiplied by 132 ft of available 

drawdown is 2.6 gal/min—still larger than the apparent well 

yield of 1.3 gal/min, but a better estimate than was made with 

the unadjusted specific capacity.

0 

0.2 

0.4 

0.6 

0.8 

0 30 60 90 120 150 180 

Time, in minutes since pumping began 

0 

0.05 

0.10 

0.15 

0.20 

0.25 

10 100 1,000 

Specific capacity,

in gallons per minute per foot

Specific capacity,

in gallons per minute per foot

Time, in minutes since pumping began 

A EXPLANATION

B

SPECIFIC-CAPACITY FORMULA

Unadjusted

Adjusted

EXPLANATION

SPECIFIC-CAPACITY FORMULA

Unadjusted

Adjusted

HYPOTHETICAL WELL

WELL CA 462

Figure 20. Specific capacity adjusted by removing water from 

wellbore storage compared to unadjusted specific capacity for 

(A) the 6-inch diameter hypothetical well (pumping rate = 5 gallons 

per minute, T = 10 feet squared per day, S = 0.0001) and (B) well 

CA 462 in Cambria County, Pennsylvania.
34 Factors Influencing Specific-Capacity Tests and Their Application

Specific-Capacity Tests Clearly Show Effects of 

Wellbore Storage

One straightforward method of quantifying the effect of 

wellbore storage is to compute the volume of water provided 

by wellbore storage as a percentage of the pumped volume 

during small increments of time. The incremental percentage 

of water contributed by wellbore storage for an ideal aquifer 

should approach zero at late time. Deviations from the idealaquifer response can reveal information about the aquifer.

The drawdown data for specific-capacity tests at the 

six wells used in this study were usually reported every 

20 seconds and, knowing the radius of each well, the incremental volume of water removed from the well between 

water-level measurements could be calculated. An example 

showing the change in the incremental percentage of volume from wellbore storage over time is shown for three 

tests completed at well CA 462 (fig. 21). The influence of 

wellbore storage was greatest in the early-time data of each 

test regardless of pumping rate; however, the influence of 

borehole storage stabilized at different levels for the different 

pumping rates.

The incremental percentage of water contributed from 

wellbore storage approached zero at a withdrawal rate of 

1 gal/min, as expected by theory; at 2 gal/min the percentage stabilized at about 37 percent; and at 4.5 gal/min the 

contribution was from 60 to 70 percent. The 2 gal/min test 

indicated that about 0.7 gal/min of water was steadily derived 

from the borehole and 1.3 gal/min was cascading into the 

well at a steady rate from the aquifer regardless of additional drawdown. Obviously, this well will not achieve an 

equilibrium condition. At a withdrawal rate of 4.5 gal/min, 

about 2.9 gal/min was being removed from the borehole with 

1.6 gal/min from the aquifer—again indicating a situation in 

which the pumping rate could not be sustained.

0 20 40 60 80 100 120 140 160 180 

Time, in minutes 

0 

20 

40 

60 

80 

100 

Incremental percentage of water from

wellbore storage 

WELL CA 462

EXPLANATION

4.5 gallons per minute test, September 2009

2 gallons per minute test, August 2008

1 gallon per minute test, May 2009

Figure 21. Incremental percentage of pumped volume provided by wellbore storage for three specific-capacity tests 

at rates of 1, 2, and 4.5 gallons per minute at well CA 462 in Cambria County, Pennsylvania.
Factors Affecting Specific Capacity 35

Deviations in Borehole Diameter Can Make Test 

Results Difficult to Interpret

If a substantial amount of water is contributed by wellbore storage, deviations in the well diameter with depth can 

be reflected in the water-level drawdown and recovery curves. 

Water-level data from the 1.5 gal/min specific-capacity tests 

in July and November 2008 at GR 541 clearly show these 

effects (fig. 22). The multiple straight-line segments of various 

slope could be interpreted as a lateral boundary, except that the 

changes in slope are at the same depth (not the same elapsed 

time) when multiple tests are examined. When the caliper log 

is compared to the water-level changes during the test, the 

slope of the water-level change is inversely related to well 

diameter (steepest slopes of water-level decline happen over 

intervals of smallest well diameter). The flat slope at a depth 

of about 46 ft from 40–60 minutes of pumping is caused by a 

considerable volume of water stored in a void intersected by 

the well at that depth. Storage in the void is about 2–3 cubic 

feet (ft3

), based on the pumping rate of 1.5 gal/min and duration of the nearly straight-line drawdown segment for the tests 

completed in July and November 2008. Drawdown during 

the straight-line segment was about 0.2 ft, which could be 

interpreted as the approximate void height. Given that height, 

an effective void radius of 22–28 in. would be needed to store 

2–3 ft3

 of water. The caliper log showed an enlarged wellbore 

radius of about 10 in. at a depth of 46 ft below land surface; 

but the volume of the wellbore alone cannot fully account for 

the volume of water provided from storage at that depth. Thus, 

this test shows that estimates of wellbore storage based on the 

casing diameter may be in error if the diameter of the uncased 

part of the hole differs from the diameter of the casing.

0 

10 

20 

30 

40 

50 

60 

CALIPER 0 20 40 60 80 100 120 140 

Depth, in feet below land surface 

Time, in minutes since pumping began 

November 25, 2008, pumping test 

July 23, 2008, pumping test

WATER LEVEL

EXPLANATION

WELL GR 541

10-inch

diameter

Open hole

6-inch diameter

11-inch

diameter

CALIPER LOG

Well cased to 20 feet with

6-inch diameter casing.

Open hole from 20–60 feet

Figure 22. Caliper log and drawdown and recovery of water-level resulting from pumping at 1.5 gallons per minute on July 23 and 

November 25, 2008, showing effects of deviations in wellbore diameter in well GR 541 in Greene County, Pennsylvania.
36 Factors Influencing Specific-Capacity Tests and Their Application

Aquifer and Well Properties

Specific capacity of a well may vary over time if the 

aquifer properties change or if the hydraulic properties of the 

well change. Aquifer properties can be changed by nearby 

mining activities (Callaghan and others, 1998, p. 2–24), but 

other possible natural causes can exist. The specific capacity 

of a well can decrease when the water table is low because 

the saturated thickness (and transmissivity) of the aquifer is 

smaller than when the water table is high. Often, the most productive water-bearing fractures are in the shallowest parts of 

the aquifer, so when water levels decline the decrease in transmissivity can be abrupt. The available drawdown in the well 

also decreases when water levels decline, which will decrease 

estimates of well yield based on available drawdown.

Wet and Dry Periods Substantially Affected 

Specific Capacity at Well YO 1222

Seasonal fluctuations in the water table can cause changes 

in the aquifer saturated thickness (and transmissivity). Seasonal water-level fluctuations were small and did not seem to 

affect well yield for five of the six wells tested in this study. 

The exception was well YO 1222, where a substantial change 

was observed.

Well YO 1222 had the greatest range of specific-capacity 

values among five tests completed for durations of 60 minutes or longer during 2007–09 (table 2 and fig. 18). Specific 

capacity ranged from 0.083 to 5.5 (gal/min)/ft. Individual test 

results were near one of these extremes, with no test results 

at intermediate values. The different results can be explained 

by the depth to water in the well in relation to a major wateryielding zone intersected at 27 ft below land surface (fig. 4), 

which may represent an intermittently perched water table. 

During periods when the water level was above this zone, 

the yield of the well was increased substantially compared to 

drier periods when the water level was below this zone. This 

water-yielding zone was not reported by the driller because 

the water level was below the zone at the time of drilling. 

The caliper log shows a small enlargement of the wellbore at 

this depth, but the deviation was slight and not suggestive of 

a major water-yielding fracture, though the fracture is visible 

on the borehole video (fig. 5). The effect of the water-yielding 

zone can be seen in the long-term hydrograph. The water-level 

recessions after periods of recharge showed a sharp increase 

in slope from 28–32 ft below land surface, possibly indicating 

the dewatering of the perched water (fig. 23).

Dates of the specific-capacity tests are marked on the 

hydrograph of water-level fluctuations in YO 1222 (fig. 3). 

Tests on August 2007 and March 2008 were completed when 

the water-yielding zone 27 ft below the surface was saturated; thus, drawdown was small and the computed specific 

capacity was large—about 5 (gal/day)/ft. Other tests were 

completed when the water level was below the water-yielding 

zone; therefore, the specific capacity was much less—about 

0.1 (gal/min)/ft. The water level in the well changes rapidly, so 

situations when the water-yielding zone is active can happen 

any time of the year; thus, the differences in well yield are not 

strictly a function of season.

WELL YO 1222 

18 

20 

22 

24 

26 

28 

30 

32 

34 

36 

Depth to water below land surface, in feet 

JUNE

2009 

Shaded area indicates depth of shallow transmissive water-yielding zone 

JULY AUGUST

Figure 23. Water-level fluctuations in well YO 1222 in York County, Pennsylvania, during June to August 2009 

showing change in recession slope at depth of a water-yielding zone 27 feet below land surface.
Factors Affecting Specific Capacity 37

Water-Yielding Zones Can Be Dewatered During 

Pumping

Changes in the water level in the well relative to major 

water-yielding zones can affect drawdown and specific capacity during a test. When the water level in the well is above a 

water-yielding zone, the water contributed from the zone is 

related to the difference in head between the aquifer and well. 

However, when the water level in the well drops below the 

zone, the effective water level at the zone, that is the boundary condition for flow from the zone, becomes fixed at a level 

corresponding to the depth of the zone. As the water level in 

the well drops farther, water cascades or seeps from the wateryielding zone at a rate independent of the water level in the 

well. A substantial change in transmissivity also can happen 

if the zone being dewatered is a major contributor to the well 

yield, and a change in slope on the drawdown-time curve will 

be noticeable. The effect of dewatering major water-yielding 

zones was clearly observed in the water-level drawdown data 

during tests at wells BV 156 at 57 ft and WS 155 at 45 ft 

(figs. 7A and 10A).

The importance of knowing the depth of the major wateryielding zones is exemplified by the specific-capacity tests at 

well WS 155. Tests completed at pumping rates greater than 

1 gal/min all show a break in slope of water-level drawdown 

and recovery at a depth of 45 ft below land surface (fig. 10A). 

The specific capacity declines at the same rate for all tests at 

that well until water levels fall below 45 ft, at which time the 

rate of decline increases. Specific-capacity tests for WS 155 

completed at different rates are comparable only if the water 

level in the well remains above the water-yielding zone at 

45 ft below land surface.

Deviations in the plots of water-level drawdown and 

recovery caused by dewatering of water-yielding zones can be 

distinguished from the effect of lateral boundaries away from 

the well. Deviations in plots would be seen at the same times 

for the different tests (regardless of pumping rate) if lateral 

boundaries were the cause of the change in slope, all other 

things being equal (Theis, 1940). Breaks in slope happened at 

the same water-level depth below land surface, not the same 

time after pumping began for the tests in this study, indicating 

a change in transmissivity with water-level drawdown.

Change in Wellbore Skin May Change Well Yield

Wellbore skin is a general term used to describe the 

hydraulic connection between the well and the aquifer immediately outside of the well. A positive wellbore skin refers to 

a zone of lessened transmissivity near the well and a negative wellbore skin refers to a zone of enhanced transmissivity. A positive wellbore skin could be created by plugging of 

fractures with precipitates of iron oxides in fractured-bedrock 

wells in coal areas.

Comparison of results from specific-capacity tests from 

this study to those results from earlier tests indicates that the 

yield of the unused wells may have decreased over the years. 

The test completed in 1999 in well AR 110 at 3.5 gal/min 

resulted in specific capacity values about three times greater 

than for tests completed during this study during 2008–09 

(fig. 6B). The borehole video of AR 110 survey showed white 

flocculent floating in the well as the camera was lowered 

(fig. 24). Precipitates may have formed in small water-yielding 

fractures during the time between tests.

Specific-capacity values from historical tests at wells 

BV 156 and GR 541 were smaller than values computed 

during this study. These larger values today could be caused 

by an enhancement of the well yield by successive pumping. 

Fractures that were plugged during drilling or from precipitates could have been cleaned out during the pump tests during 

2008–09. The specific capacity at well CA 462 was larger 

for the second of the two tests completed at 1 gal/min and at 

2 gal/min (table 2).

FLOCCULENT

WELL AR 110

Figure 24. Image from borehole video log showing a particulate 

flocculent floating in the water in well AR 110.
38 Factors Influencing Specific-Capacity Tests and Their Application

Turbulent Flow

Step-Drawdown Tests Can Evaluate Effects of 

Turbulent Flow on Drawdown

Step-drawdown tests are a standard method for evaluating the effects of turbulent flow and estimating well yield 

(Driscoll, 1986, p. 555). A step-drawdown test is completed 

by pumping the well in steps at successively greater pumping rates and monitoring the drawdown in the well. The 

duration of the steps is usually short—on the order of 60–90 

minutes. Analysis of the drawdown for various pumping steps 

can be used to evaluate the magnitude of the turbulent-flow 

factor C. The effects of turbulent flow are influential because 

turbulent flow causes drawdown to increase at an exponential rate greater than would be expected if flow were strictly 

laminar (eq. 4).

Turbulent Flow Was Not Significant During StepDrawdown Test at YO 1222

The step-drawdown test was used to analyze the effect of 

turbulent flow on drawdown in well YO 1222. Well YO 1222 

was pumped for four 60-minute steps at rates of 2, 3, 4, 

and 5 gal/min in August 2008. The water-level drawdown 

and specific capacity during the test are shown in figure 11. 

Examination of the change in specific capacity with time in 

figure 11B shows how the computed specific capacity reaches 

a value of about 0.11 at the end of each 60-minute pumping 

period, regardless of pumping rate. The specific-capacity value 

computed at the end of each pumping step would have been 

decreasing as pumping rate increased if turbulent flow would 

have been a major factor in this test.

The effect of turbulent flow can be quantified by plotting 

drawdown/pumping rate compared to pumping rate (Bierschenk, 1963, in Driscoll, 1986). The slope of the line is the 

turbulent-flow factor, C, and the intercept is the laminar-flow 

factor B (eq. 4); thus, knowing C and B, the drawdown for any 

pumping rate can be computed by using equation 4. Plotting 

the data for the YO 1222 step-drawdown test gives a line with 

slope of 0.13 ft/(gal/min)2

, and y-intercept of 8.49 ft/(gal/min) 

(fig. 25). However, the slope cannot be statistically shown 

to differ significantly from zero using the t-ratio (Helsel and 

Hirsch, 1992, p. 238) because the p value of 0.32 was greater 

than the test criterion of 0.05. Thus, the results indicate that 

turbulent losses are not influential at pumping rates from 

1–5 gal/min, and that the well has a maximum yield in excess 

of 5 gal/min.

6 

7 

8 

9 

10 

11 

0 1 2 3 4 5 6 

Drawdown/pumping rate,

in feet per gallons per minute 

Pumping rate, in gallons per minute 

y = 0.13x + 8.49 

EQUATION OF LINE 

B = slope of line = 0.13 feet/(gallons per minute)

C = y intercept = 8.49 feet/(gallons per minute) 

2 

Figure 25. Determination of turbulent-flow factors B and C in equation 4 from the step-drawdown 

test at well YO 1222 in York County, Pennsylvania, in August 2008 by the method of Bierschenk (1963).
Application of Test Results 39

Application of Test Results

Specific-capacity tests are used by PaDEP to document 

mining effects on domestic supply wells, to establish baseline 

well yield, and verify that replacement wells (if needed) have 

adequate yields. The six tests completed during this study, 

although not a large sample, provide some insights into the use 

of specific-capacity values for these purposes.

Specific-Capacity Values Were Reproducible 

With Some Caveats

Specific-capacity values generally were reproducible at 

the same well within about ± 20 percent if the values were 

derived from tests completed at the same pumping rate and 

duration. The following caveats can apply regarding the application of specific-capacity values.

Accurate measurement of pump discharge is vital.—

Completing two tests at the same pumping rate requires 

accurate measurement of discharge for each test. The readings 

from two flowmeters can differ. Check flowmeter readings 

against a volumetric measurement. 

Compare specific-capacity values for tests of the same 

duration.—Specific capacity will decrease with time unless a 

test reaches equilibrium. Specific-capacity values computed 

after 180 minutes of pumping were 0 to 62 percent less than 

values after 60 minutes for tests that did not achieve equilibrium (table 3).

Sometimes the pumping rate does not affect specific 

capacity.—Generally, tests completed at different pumping 

rates from about 1 to 6 gal/min resulted in similar specific 

capacities unless the large pumping rate caused the water level 

in the well to fall below a major water-producing zone. Unfortunately, the depths of water-producing zones usually are not 

known without information from driller’s reports or geophysical logs, so a comparison of the results from tests completed at 

the same pumping rates would be prudent.

Wellbore skin can affect comparisons.—Water-yielding 

fractures in wells that are pumped infrequently may become 

clogged over time, thereby reducing the specific capacity of 

the well. Specific-capacity values from tests completed during 2008–09 at AR 110 may have been affected by plugging. 

Increases in specific yield at CA 462 for successive tests also 

are suggestive of the development of water-yielding fractures 

(unplugging) during the tests.

Depth of water relative to major water-producing zones 

is vital.—The most productive water-yielding zones tend 

to be shallow in stress-relief fractured bedrock, so natural 

water-level fluctuations in the aquifer possibly can cause large 

changes in aquifer transmissivity. This condition was documented at well YO 1222, where specific-capacity values varied by a factor of almost 50 during the study (table 5) because 

the water table fluctuated across a major water-yielding zone 

in the well.

Documenting Baseline and Replacement 

Well Yield

PaDEP determines if the supply of a well has been diminished by comparing the yield of a well tested after mining to 

the yield prior to mining. A replacement well, if needed, also 

is deemed adequate based on yield. Although well yield is 

simple to describe, difficulty arises in practice in determining 

and comparing yields from specific-capacity tests of differing rate and duration. Thus, the procedures for determining 

adequate well yield were outlined by PaDEP in the Technical 

Guidance Document for Water Supply Replacement and Compliance (Pennsylvania Department of Environmental Protection, 1999). That document defines well yield as the maximum 

pumping rate that can be sustained by a well without lowering 

the water level below the pump intake and recommends that 

well yield be determined from the specific capacity of the well 

measured during short-duration tests as:

WellYield = SC(adj) × AD (7)

where

	 Well Yield	 is in gallons per minute;

SC(adj) is the adjusted specific capacity, in gallons per 

minute per foot; and

	 AD	 is the available drawdown (distance from 

the static water level to a point about 5 ft 

above the pump), in feet.

Specific Capacity Values Can Overestimate 

Well Yields

The approach for estimating well yield by multiplying 

the adjusted specific capacity times the available drawdown 

assumes that the rate of water contributed by the aquifer is 

directly related to drawdown in the well throughout the length 

of the well. Unfortunately, in low-yielding wells completed in 

fractured-bedrock aquifers, the most productive fractures commonly are detected in the shallow part of the well and deeper 

parts of the well serve only for water storage. Specific-capacity values computed from tests completed at low rates that do 

not cause much water-level drawdown are likely to overestimate the yield of the well because the effect of dewatering the 

shallow fractures has not been established.

For example, consider the two tests completed at about 

1 gal/min for 180 minutes at well CA 462. The specific-capacity values at the end of the 180-minute tests were 0.10 and 

0.14 (gal/min)/ft (table 2 and fig. 8). Water derived from wellbore storage was small, about 8 percent of the total volume 

pumped, so the specific capacity adjusted to remove wellbore 

storage effects by the method of Pennsylvania Department of 

Environmental Protection (1999) results in adjusted values 

of about 0.09 to 0.13 (gal/min)/ft. The estimated well yield, 

assuming 100 ft of available drawdown, would be about 

9 to 13 gal/min. However, when the well was pumped at 
40 Factors Influencing Specific-Capacity Tests and Their Application

only 4.5 gal/min, the water level drew down about 90 ft in 

45 minutes and pumping at 2 gal/min caused drawdown of 

about 100 ft in 180 minutes. Clearly, the well could not sustain 

a pumping rate of 9–13 gal/min as indicated by the specificcapacity value multiplied by available drawdown.

A better approach is to complete a step-drawdown test or 

over pump at a constant rate great enough to cause the water 

level in the well to fall below the major shallow water-yielding 

zones, or if their depths are unknown, the well can be pumped 

nearly dry. The maximum well yield can be estimated from the 

rate of water-level recovery or by subtracting the percentage 

of incremental borehole storage at the end of the test from the 

pumping rate.

Recovery Data Are Useful for Estimating Well 

Yield

Recovery data for low-yielding wells were shown to be 

useful for estimating well yield. Much of the water discharged 

from the well is derived from wellbore storage during the 

pumping period; therefore, the contribution from the aquifer can be difficult to assess. However, for tests that show 

a straight-line plot of water-level drawdown and recovery 

when plotted on arithmetic axes, as seen during tests at wells 

AR 110, CA 462, GR 541, and WS 155, recovery data indicate 

the rate at which water is entering the well. The recovery, if 

the water level is pumped down to near the bottom of the well, 

should be the maximum rate that the well can produce. The 

rate could, however, slow down if the head in the water-yielding zone declines over time.

The inflow rate during the recovery period is computed 

as:

Ir = 7.48πr2Δwlr , (8)

where

	 Ir	 is the inflow during recovery, in gallons per 

minute;

r is the well radius, in feet; and

Δwlr	 is the water-level change rate during recovery, 

in feet per minute.

An example is given from the March 2008 specificcapacity test at well WS 155 (fig. 26). The well radius is 

0.25 ft and the linear change in water level during the first 

4 minutes of rapid recovery after pumping ceased was 

1.7 ft/min. The inflow rate, determined from equation 8, was 

about 2.4 gal/min. This inflow rate is the rate at which water 

was cascading into the well from the major water-yielding 

zone at 45 ft below land surface, and should be a good estimate of maximum well yield.

The yield estimate from recovery data can be compared 

to an estimate of inflow rate to the well during a straight-line 

drawdown period as:

Ip = Q - (7.48πr2Δwlp) , (9)

where

	 Ip	 is the inflow during pumping, in gallons per 

minute; and

Δwlp	 is the water-level change rate during pumping, 

in feet per minute.

For the period of water-level drawdown during the March 

2008 test at well WS 155, the pumping rate was 2.5 gal/min, 

and drawdown rate from 140 to 180 minutes after pumping 

started was about 0.16 ft/min. From equation 9, the computed 

inflow during the pumping period would be about 2.3 gal/min, 

which agrees closely to the rate of 2.4 gal/min computed from 

the recovery data.

Compare the estimate of well yield from the recovery 

data to an estimate derived by multiplying specific capacity (adjusted for wellbore storage) by available drawdown. 

Specific capacity at the end of the 180-minute test was about 

0.13 (gal/min)/ft and available drawdown was about 95 ft for 

well WS 155. Multiplied together, these numbers indicate a 

well yield of about 12 gal/min, which is much larger than the 

actual maximum well yield.

Figure 26. Drawdown and recovery of water level during 

the March 2008 specific-capacity test at well WS 155 in 

Washington County, Pennsylvania.

Depth to water below land surface, in feet 

Time, in minutes since pumping began 

Recovery rate = 1.7 feet 

per minute 

Drawdown rate = 0.16 foot 

per minute 

54

52

50

48

46 

44 

42 

120 140 160 180 200 220
Application of Test Results 41

Difficult to Use Analytical Models to Predict Well 

Yields From Test Results

Mathematical models are powerful tools for estimating aquifer properties and making predictions about aquifer 

response to pumping. The models, under certain conditions, 

can predict drawdown in a well for any pumping rate, allowing the maximum yield of the well to be estimated. In practice, 

however, the use of analytical aquifer-test models to estimate 

well yields in low-yielding fractured-bedrock aquifers can be 

difficult, primarily because the aquifer properties and boundary conditions can change as drawdown increases. The use 

of automated aquifer-test software can get a good fit to the 

water-level data measured during a specific-capacity test, but 

if the model used to analyze the test does not incorporate the 

changing aquifer properties or boundary conditions predictions 

made for other pumping rates the model results will probably be erroneous. Specific-capacity tests at wells YO 1222 

and WS 155 are two examples where changes in aquifer 

conditions caused the drawdown to be difficult to predict for 

different tests.

YO 1222

 A good fit of the data for the step-drawdown test at well 

YO 1222 completed in August 2008 was obtained by use of 

the analytical solution of Papadopulos and Cooper (1967) with 

transmissivity of 33 ft2

/d, storage coefficient of 1.75 × 10-7, 

and well diameter of 6.25 in. (fig. 27A). Turbulent-flow effects 

were not included in the model. The same model, if the aquifer 

was uniform and homogeneous, would accurately predict 

drawdown from the other aquifer tests with the use of the 

same aquifer properties determined from the step-drawdown 

test. However, this model does not accurately predict drawdown during the September 2009 test at 14 gal/min or the 

August 2007 test at 5 gal/min (fig. 27B and C).

The model prediction closely matches the measured 

drawdown for the September 2009 test at 14 gal/min until 

about 70 ft of drawdown is achieved (fig. 27B). This result 

indicates that the transmissivity does not change substantially 

for pumping that causes less than 70 ft of drawdown, which 

was true of the step-drawdown test. The observed drawdown 

becomes substantially greater than that predicted by the model 

for drawdown greater than 70 ft. The deviation is probably 

caused by a reduction in transmissivity of the aquifer caused 

by dewatering of fractures. The analytical model results show 

a large discrepancy between predicted and measured drawdown values for the August 2007 test at 5 gal/min. The model 

predicted 40 times more drawdown than was measured during 

180 minutes of pumping (fig. 27C). The water level was high 

during this test, causing a shallow, major water-yielding zone 

at 27 ft below land surface to be saturated. The addition of this 

producing zone caused the transmissivity of the aquifer to be 

much greater in August 2007 than when the step-drawdown 

test was completed in August 2008.

0 

10 

20 

30 

40 

50 

0 50 100 150 200 250 300 350 

Drawdown, in feet 

Time, in minutes since pumping began 

0 

20 

40 

60 

80 

100 

120 

0 10 20 30 40 50 60 70 80 90 100 

Drawdown, in feet 

Time, in minutes since pumping began 

0 

20 

40 

60 

0 10 20 30 40 50 60 70 80 90 100 

Drawdown, in feet 

Time, in minutes since pumping began 

Step-drawdown test data

(Pumping rates of 2, 3, 4 and 

5 gallons per minute) 

Model fit 

EXPLANATION

EXPLANATION

B 

14 gallons per minute test data 

Predicted using model fit to

step-drawdown test 

5 gallons per minute test data

Predicted using model fit to 

step-drawdown test 

EXPLANATION

C 

A 

Figure 27. Measured drawdown from three tests at well YO 1222 

in York County, Pennsylvania, and drawdown predicted with 

the analytical solution of Papadopulos and Cooper (1967) with 

transmissivity of 33 feet squared per day, storage coefficient of 

1.75 × 10-7, and well diameter of 6.25 inches.
42 Factors Influencing Specific-Capacity Tests and Their Application

WS 155

Water-level drawdown and recovery from the May 2009 

pumping at well WS 155 were simulated by use of the analytical solution of Barker (1988) with transmissivity of 64 ft2

/d, 

storage coefficient of 8.7 × 10-5, and well radius of 0.25 ft. The 

analytical solution simulates one-dimensional linear flow to 

the well as applied for this test, which, in reality, could be representative of flow in a vertical fracture intercepting the well. 

The well was tested by pumping at 1 gal/min for 60 minutes 

followed by a 40-minute recovery period, then pumped for 

35 minutes at 6 gal/min. When the water level in the well was 

above about 45 ft below land surface, the analytical solution 

provides a good fit to the data (fig. 28). Thus, based on the 

1 gal/min pumping and recovery period (first 100 minutes of 

the test), the analytical solution seems to be a good tool for 

predicting water-level change in response to pumping. At the 

higher pumping rate of 6 gal/min, however, the predictive 

capability of the analytical solution breaks down when water 

level is drawn down below 45 ft below land surface, at which 

point water begins cascading from that major water-yielding 

fracture. The estimated yield would be too large if the analytical solution was used to estimate a maximum well yield from 

the results of the 1 gal/min test.

35 

40 

45 

50 

55 

60 

65 

70 

75 

80 

0 20 40 60 80 100 120 140 160 180 200 

Depth to water below land surface, in feet 

Time, in minutes since pumping began 

Measured—May 2009 test

Analytical solution for

one-dimensional flow

EXPLANATION

Pumping 1 gallon per minute Pumping

6 gallons per minute

No pumping No pumping

Aquifer properties used for analyical solution of Barker (1988):

Transmissivity = 64 feet squared per day

Storage coefficient = 0.000087 (dimensionless)

Flow dimension = 1 (represents linear flow to well)

Well radius = 0.25 foot

Figure 28. Measured water-level drawdown and recovery from the May 2009 specific-capacity test at well WS 155 in 

Washington County, Pennsylvania, compared to simulated water levels from the analytical solution of Barker (1988) for 

one-dimensional flow to the well.
Acknowledgments 43

Summary

Pennsylvania Department of Environmental Protection 

(PaDEP) regulations require the mining company to restore or 

replace the water supply if a mining company diminishes the 

yield of a well to the point where the well is no longer adequate for the purposes served. PaDEP typically has used the 

specific-capacity test to determine mining effects on domesticsupply wells and to verify that replacement wells (if needed) 

have adequate yields. Factors other than mining can affect the 

results of specific-capacity tests, but the importance of those 

effects is not well understood, particularly with low-yielding 

wells in fractured-bedrock aquifers.

This report, by the U.S. Geological Survey, in cooperation with the PaDEP, Bureau of Mining and Reclamation, 

evaluates factors affecting the reproducibility and application 

of specific-capacity tests in six low-yielding water wells in 

Pennsylvania. The wells were completed in fractured-bedrock 

aquifers in mining areas—five in siliciclastic rocks of Pennsylvania and Permian age in the bituminous coal fields of western 

Pennsylvania, and one in limestone of Cambrian age in southcentral Pennsylvania that are quarried for several industrial 

uses. Factors affecting specific capacity such as pumping rate, 

duration of pumping, aquifer properties, wellbore storage, 

and turbulent-flow well losses were assessed by theoretical 

analysis and by completing multiple tests. Specific-capacity 

tests, lasting from 22 to 240 minutes, were completed 24 times 

at the 6 wells during 2007–09 and data were available from 

5 tests completed on four of the wells prior to this study. In 

addition to pumping each well, video surveys were completed 

in each well and geophysical logging was done in five of six 

wells to determine the depth, casing length, and location of 

water-yielding zones; and seasonal water-level changes were 

measured by continuous monitoring at each well.

Specific-capacity values were reproducible at the same 

well within about ± 20 percent if the values were derived 

from tests completed at the same pumping duration and 

rate. The tests showed that unless the water level in the well 

reaches equilibrium, specific capacity will decrease with 

time. Specific-capacity values computed after 180 minutes of 

pumping decreased as much as 62 percent compared to the 

value after 60 minutes. Tests completed at different pumping 

rates (from about 1 to 6 gal/min) compared closely unless the 

larger pumping rate caused the water level in the well to fall 

below a major water-yielding zone. Turbulent flow during the 

tests did not seem to substantially affect specific capacity at 

the rates tested.

The depth to water in the well relative to the major wateryielding zones was a major control on the reproducibility of 

specific-capacity tests. The most productive water-yielding 

fractures tend to be shallow in stress-relief fractured bedrock, 

so natural water-level fluctuations or pumping can cause large 

changes in specific capacity when the water level is drawn 

below a major water-yielding zone. At well YO 1222, specificcapacity was about 50 times larger when the water level was 

above a major water-yielding zone 27 feet below land surface 

than when the water was below this level. During pumping 

tests at high rates in wells BV 156, CA 462, and WS 155, 

the water level dropped below major water-yielding zones, 

causing specific-capacity values to be smaller by as much as 

63 percent compared to tests at the same wells completed at 

pumping rates that allowed the water level to remain above the 

major water-yielding zone.

Water-yielding fractures in wells that are not frequently 

pumped may become clogged over time, thereby reducing the 

specific capacity of the well. Thus, specific-capacity values 

from historical tests may not be valid for evaluating mining 

effects if specific capacity has been reduced by plugging. 

Whether the smaller specific-capacity values from this study 

compared to historical tests at AR 110 have been affected by 

plugging or mining is unclear. The opposite effect, increases 

in specific yield, as shown in successive tests at CA 462, 

indicate a development of water-yielding fractures (unplugging) by pumping.

Pre- and post-mining specific-capacity tests on low-yielding wells in fractured-bedrock aquifers will be most comparable if the tests have been completed at the same pumping 

rate and duration. Ideally, pumping is held steady and the 

rate is carefully monitored. Interpretation of the tests will be 

facilitated if the depths of the major water-yielding zones are 

known and the tests are completed when water levels are at 

similar depths with respect to those zones.

Estimates of well yield determined by multiplying 

specific capacity by the available drawdown are likely to be 

too large when computed from tests completed at low rates 

that do not cause much water-level drawdown. Therefore, the 

effects of dewatering the shallow water-yielding zones do not 

become incorporated in the yield estimate. A better approach 

for estimating yield is to complete a step-drawdown test or 

over-pump at a constant rate until the water in the wellbore is 

nearly evacuated. The maximum well yield can be estimated 

from the rate of water-level recovery on the straight-line plot 

of water-level recovery when plotted on arithmetic axes or by 

subtracting the percentage of incremental wellbore storage at 

the end of the test from the pumping rate.

Acknowledgments

This project was the outgrowth of many discussions with 

Keith Brady of the PaDEP Bureau of Mining and Reclamation 

about the interpretation of specific-capacity tests at lowyielding wells. The test pumping and other field work was 

greatly facilitated by the assistance of Brad Kuntz of USGS. 

Jack Felbinger and Jay Hawkins, Office of Surface Mining, 

provided the borehole video for well GR 541. Thanks to Kenneth King of W.D. Mohney and Associates for data and access 

to well AR-110.
44 Factors Influencing Specific-Capacity Tests and Their Application

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For more information concerning this report, contact:

Director

U.S. Geological Survey

Pennsylvania Water Science Center

215 Limekiln Road

New Cumberland, PA 07070

dc_pa@usgs.gov

or visit our Web site at:

http://pa.water.usgs.gov
D.W. Risser—Factors Affecting Specific-Capacity Tests and Their Application—A Study of Six Low-Yielding Wells in Pennsylvania—SIR 2010–5212