U.S. Department of the Interior

U.S. Geological Survey

AQUIFER TESTS AND SIMULATION OF GROUND-WATER FLOW

IN TRIASSIC SEDIMENTARY ROCKS NEAR COLMAR,

BUCKS AND MONTGOMERY COUNTIES, PENNSYLVANIA

by Dennis W. Risser and Philip H. Bird

Water-Resources Investigations Report 03-4159

In cooperation with the

U.S. ENVIRONMENTAL PROTECTION AGENCY

New Cumberland, Pennsylvania

2003
ii

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

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

not constitute endorsement by the U.S. Geological Survey.

For additional information Copies of this report may be

write to: purchased from:

District Chief U.S. Geological Survey

U.S. Geological Survey Branch of Information Services

215 Limekiln Road Box 25286, Denver Federal Center

New Cumberland, Pennsylvania 17070-2424 Denver, Colorado 80225-0286

Email: dc_pa@usgs.gov Telephone: 1-888-ASK-USGS

Internet Address: http://pa.water.usgs.gov
iii

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

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

Purpose and scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Previous investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Well-identification system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Hydrogeologic setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Ground water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Occurrence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Water-yielding fractures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Recharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Aquifer tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Design of tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Well NP–21 and NP–87. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Observation wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

NP–21 test of June 20–27, 2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Climatic conditions and resultant water-level trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Precipitation during test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Changes in atmospheric pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Pre-test trends. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Withdrawals from other wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Drawdown caused by pumping NP–21 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

NP–87 test of May 7–10, 2002 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Climatic conditions and resultant water-level trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Precipitation during test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Changes in atmospheric pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Pre-test trends. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Withdrawals from other wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Drawdown from pumping NP–87 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Simulation of ground-water flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Description of the model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Finite-difference grid and model structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Model boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Recharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Wells. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Aquifer properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Model adjustments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Method and data used for model adjustments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Model parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

CONTENTS

Page
iv

Simulation of ground-water flow—Continued

Adjusted model results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Model assumptions and limitations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Simulated flow paths and area contributing recharge to wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Conditions in 2000. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Pumping from NP–87. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

New recovery well in the southern area of ground-water contamination . . . . . . . . . . . . . . . . 64

New recovery well in the northern area of ground-water contamination . . . . . . . . . . . . . . . . 64

All remediation wells and NP–87. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

References cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

CONTENTS—Continued

Page
v

Figure 1-2. Maps showing:

1. Location of study area near Colmar, geology, and ground-water-flow

model domain, Bucks and Montgomery Counties, Pennsylvania . . . . . . . . . . . 3

2. Location of selected wells in the study area near Colmar . . . . . . . . . . . . . . . . . . . . . . 4

3. Graph showing depth to bedrock reported from 56 wells in the Colmar study area. . . . . 9

4. Sketch showing ground-water-flow paths in dipping geologic units of differing

hydraulic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5. Graph showing water-level fluctuations in observation well W–6 and changes in

atmospheric pressure from June 27 to July 5, 2000, Colmar study area . . . . . . . . . . . 10

6. Photograph showing water flowing from well NP–87 on June 24, 2002 . . . . . . . . . . . . . . 11

7. Schematic diagram of bedding-plane and high-angle fractures identified by

use of the acoustic televiewer in the Colmar study area. . . . . . . . . . . . . . . . . . . . . . . . 12

8. Photograph showing sedimentary bedrock of Triassic age similar to bedrock in

the Colmar area showing (1) thin beds of highly fractured siltstone, (2) thicker

siltstone beds where fractures are less well developed, and (3) interbeds of

soft low-permeability shale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

9. Graph showing density of water-yielding fractures with depth determined from

flowmeter surveys in the Colmar study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

10. Map showing altitude and configuration of water levels in selected shallow wells

less than 100 feet deep in the Colmar study area, June 19-20, 2000. . . . . . . . . . . . . . . 14

11. Sketch showing well construction and (A) driller-reported yield of supply well

NP–21 and (B) specific capacity of isolated intervals in NP–87 . . . . . . . . . . . . . . . . . . 16

12. Map showing observation wells monitored and drawdown after 7 days of

pumping from supply well NP–21 during the June 2000 aquifer test. . . . . . . . . . . . . 20

13-16. Graphs showing:

13. Water levels in wells where drawdown was observed during the aquifer test

at well NP–21 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

14. Water levels in wells where drawdown was not observed during the aquifer

test at well NP–21 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

15. Atmospheric pressure during the aquifer test of supply well NP–21 . . . . . . . . . . . 27

16. Effect of pumping from BAE Systems supply well on water levels

in monitor wells W–16, RW–1, RW–2, and RW–3 during the aquifer test

of supply well NP–21. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

17. Cross section showing pumped well NP–21 and observation wells monitored

during the June 2000 aquifer test, Colmar study area, Bucks and

Montgomery Counties, Pennsylvania . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

18-19. Maps showing:

18. Drawdown in shallow wells (less than 100 feet deep) after 3 days

of pumping from well NP–87 during the May 2002 aquifer test . . . . . . . . . . . 34

19. Drawdown in deep wells (at least 200 feet) after 3 days of pumping

from well NP–87 during the May 2002 aquifer test . . . . . . . . . . . . . . . . . . . . . . 35

20. Cross section showing pumped well NP–87 and observation wells monitored

during the May 2002 aquifer test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

ILLUSTRATIONS

Page
vi

Figure 21-24. Graphs showing:

21. Water levels in wells where drawdown was monitored during

the May 2002 aquifer test at well NP–87. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

22. Atmospheric pressure during the May 2002 aquifer test of well NP–87 . . . . . . . . . 41

23. Drawdown in observation well RI–27S showing adjustments for changes

in atmospheric pressure during and after the May 2002 aquifer

test at NP–87 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

24. Stream stage in a tributary to the West Branch Neshaminy Creek near

well NP–87 and in the Little Neshaminy Creek at Valley Road, monitored

during the May 2002 aquifer test at well NP-87 . . . . . . . . . . . . . . . . . . . . . . . . . 43

25. Map of modeled area showing finite-difference grid, domain boundary,

stream cells, and location of Lansdale-area model of Senior and Goode (1999),

Bucks and Montgomery Counties, Pennsylvania . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

26. Schematic cross section along the model column through well NP–87 showing

model layers and thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

27. Structure-contour map of northwest-dipping surface representing top of model

layer 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

28-30. Graph showing

28. Water-level fluctuations, January 2000 through July 2002 in U.S. Geological

Survey observation well MG–917, 4 miles northwest of Colmar, Pa.. . . . . . . . 50

29. Composite-scaled sensitivity of hydraulic parameters used in

the ground-water-flow model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

30. Observed values of (A) water-level altitude and (B) drawdown

in relation to values simulated by the ground-water-flow model . . . . . . . . . . 54

31. Map showing simulated steady-state water-level altitude in model layer 1

and residual between simulated and observed values . . . . . . . . . . . . . . . . . . . . . . . . . 55

32. Hydrographs comparing observed drawdown in selected observation wells

to drawdown simulated by the ground-water-flow model . . . . . . . . . . . . . . . . . . . . . 57

33-40. Maps showing:

33. Simulated drawdown in model layer 1 after 72 hours of pumping well NP–87

at 400 gallons per minute and measured drawdown in observation

wells less than 100 feet deep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

34. Simulated ground-water-flow paths and areas contributing recharge

to wells, near Colmar, Pa., for pumping conditions in 2000 . . . . . . . . . . . . . . . 63

35. Simulated ground-water-flow paths and area contributing recharge

to wells, near Colmar, Pa., for hypothetical pumping from NP–87 . . . . . . . . . 65

36. Simulated area contributing recharge to wells near Colmar, Pa., for

hypothetical pumping from a new recovery well in the southern area

of ground-water contamination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

37. Simulated area contributing recharge to wells near Colmar, Pa., for hypothetical

pumping from a new recovery well in the southern area of ground-water

contamination and NP–87. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

38. Simulated area contributing recharge to wells, near Colmar, Pa., for hypothetical

pumping from two new recovery wells near the former Stabilus facility . . . . 68

ILLUSTRATIONS—Continued

Page
vii

Figure 33-40. Maps showing—Continued:

39. Simulated area contributing recharge to wells, near Colmar, Pa., for

hypothetical pumping from two new recovery wells near the former

Stabilus facility and NP–87 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

40. Simulated area contributing recharge to wells, near Colmar, Pa., for

hypothetical pumping from new recovery wells near BAE Systems,

the former Stabilus facility, and NP–87. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

TABLES

Table 1. Record of selected wells in the study area near Colmar, Bucks and Montgomery

Counties, Pennsylvania . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2. Summary of observations from wells monitored during the aquifer test at

well NP–21, Colmar study area, Bucks and Montgomery Counties,

Pennsylvania . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3. Pumping of NWWA–22 from June 17–30, 2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4. Summary of observations from wells monitored during aquifer test at well NP–87 . . . . 32

5. Initial estimates of aquifer properties for the model of ground-water flow in the

Colmar study area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

6. Steady-state water-level altitude and drawdown used for

calibration of the ground-water-flow model in the Colmar area. . . . . . . . . . . . . . . . . 51

7. Definition of parameters used to represent aquifer properties in the groundwater flow model of the Colmar area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

8. Final aquifer-property values used for predictive simulations with the

ground-water-flow model of the Colmar area, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

9. Measured and simulated steady-state water levels from the ground-water-flow

model of the Colmar area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

10. Wells and withdrawal rates in 2000 simulated in the ground-water-flow model of the

Colmar area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

ILLUSTRATIONS—Continued

Page
viii

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29);

horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27).

Other abbreviations:

μg/L, micrograms per liter

Multiply By To obtain

Length

inch (in.) 25.4 millimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

Volume

gallon (gal) 3.785 liter

Flow rate

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

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

Hydraulic conductivity

foot per day (ft/d) 0.3048 meter per day

CONVERSION FACTORS, DATUMS,

AND ABBREVIATED WATER-QUALITY UNIT
1

ABSTRACT

This report presents the results of a study by

the U.S. Geological Survey in cooperation with the

U.S. Environmental Protection Agency to evaluate

ground-water flow in Triassic sedimentary rocks

near Colmar, in Bucks and Montgomery Counties,

Pa. The study was conducted to help the U.S. Environmental Protection Agency evaluate remediation

alternatives at the North Penn Area 5 Superfund

Site near Colmar, where ground water has been

contaminated by volatile organic solvents (primarily trichloroethene). The investigation focused on

determining the (1) drawdown caused by separately pumping North Penn Water Authority wells

NP–21 and NP–87, (2) probable paths of groundwater movement under present-day (2000) conditions (with NP–21 discontinued), and (3) areas contributing recharge to wells if pumping from wells

NP-21 or NP–87 were restarted and new recovery

wells were installed. Drawdown was calculated

from water levels measured in observation wells

during aquifer tests of NP–21 and NP–87. The

direction of ground-water flow was estimated by

use of a three-dimensional ground-water-flow

model.

Aquifer tests were conducted by pumping

NP–21 for about 7 days at 257 gallons per minute

in June 2000 and NP–87 for 3 days at 402 gallons

per minute in May 2002. Drawdown was measured in 45 observation wells during the NP–21

test and 35 observation wells during the NP–87

test. Drawdown in observation wells ranged from

0 to 6.8 feet at the end of the NP–21 test and 0.5 to

12 feet at the end of the NP–87 test. The aquifer

tests showed that ground-water levels declined

mostly in observation wells that were completed in

the geologic units penetrated by the pumped

wells. Because the geologic units dip about

27 degrees to the northwest, shallow wells up dip

to the southeast of the pumped well showed a

good hydraulic connection to the geologic units

stressed by pumping. Most observation wells

down dip from the pumping well penetrated units

higher in the stratigraphic section that were not

well connected to the units stressed by pumping.

The best hydraulic connection to the pumped wells

was indicated by large drawdown in observation

wells that penetrate the water-bearing unit

encountered below 400 feet below land surface in

wells NP–21 and NP–87. The hydraulic connection

between wells NP–21 (or NP–87) and observation

wells in the southern area of ground-water contamination near the BAE Systems facility is good

because the observation wells probably penetrate

this water-bearing unit.

A 3-dimensional, finite-difference, groundwater-flow model was used to simulate flow paths

and areas contributing recharge to wells for current (2000) conditions of pumping in the Colmar

area and for hypothetical situations of pumping

suggested by the U.S. Environmental Protection

Agency that might be used for remediation. Simulations indicate that under current conditions,

ground water in the northern area of contamination near the former Stabilus facility moves to the

northwest and discharges mostly to West Branch

Neshaminy Creek; in the southern area of contamination near BAE Systems facility, ground water

probably moves west and discharges to a tributary

of West Branch Neshaminy Creek near well NP–21.

Model simulations indicate that if NP–21 or NP–87

are pumped at 400 gallons per minute, groundwater recharge is likely captured from the southern

area of contamination, but ground-water recharge

from the northern area of contamination is less

likely to be captured by the pumping. Simulations

also indicate that pumping of a new recovery well

near BAE Systems facility at 8 gallons per minute

and two new recovery wells near the former Stabilus facility at a total of about 30 gallons per minute

probably would capture most of the ground-water

recharge in the areas where contamination is

greatest.

Aquifer Tests and Simulation of Ground-Water Flow

in Triassic Sedimentary Rocks near Colmar,

Bucks and Montgomery Counties, Pennsylvania

by Dennis W. Risser and Philip H. Bird
2

INTRODUCTION

The U.S. Environmental Protection Agency

(USEPA) is in the process of characterizing groundwater contamination and evaluating remediation

alternatives for the North Penn Area 5 Superfund

Site, Montgomery and Bucks Counties, in southeastern Pennsylvania. The U.S. Geological Survey

(USGS) is assisting USEPA by providing geophysical and hydrologic investigations to evaluate the

fractured-rock aquifer. The North Penn Area 5

includes numerous industries and commercial

operations near Colmar, Pa., where ground water

has been contaminated by volatile organic compounds (VOCs). The area of study near Colmar

described in this report is shown in figure 1.

North Penn Area 5 is one of seven Superfund sites in the Lansdale area where VOCs have

contaminated ground-water resources used for

public supply. In August 1979, ground-water contamination became evident near Colmar when the

North Penn Water Authority (NPWA) found

trichloroethene (TCE), 1,1,1-trichloroethane (TCA),

and tetrachloroethene (PCE) in water samples collected from municipal-supply well NP–21 (Earth

Technology Corporation, 1993). Through subsequent sampling by industry, utilities, and regulatory agencies, an area of ground-water

contamination was identified, and in 1989, the

USEPA added North Penn Area 5 to the National

Priorities List (NPL).

Since 1998, ground-water sampling for the

Remedial Investigation/Feasibility Study (RI/FS)

by USEPA and its contractors has verified two general locations of ground-water contamination near

well NP–21—one near the BAE Systems facility

and one near the former Stabilus facility. These

areas, respectively, are referred to as the southern

and northern areas of contamination in this report

and are indicated by the well symbols shown in

red on figure 2. Specifically, red symbols indicate

wells where TCE in ground water exceeded

300 μg/L in at least one sample between 1998 and

2002. The concentration of 300 μg/L was chosen to

illustrate the two areas of greatest contamination in

the study area and does not denote the extent of

known ground-water contamination nor represent

a specific health-criteria level.

The major contaminant in ground water at

both areas is TCE. In the southern area of contamination near BAE Systems, TCE was detected in

ground water from well A–12 at a concentration of

1,200 μg/L in 1998, and in the same general area,

ground-water samples from RI–20S contained

320 μg/L of TCE (Tetra Tech/Black & Veatch,

2002). To remediate the southern area of contamination, a pump-and-treat system has been in operation since 1986, using well A–10 (fig. 2) as the

recovery well. In the northern area of contamination near the former Stabilus facility, ground-water

samples from well RI–27S contained 3,800 μg/L of

TCE in 2002 (Tetra Tech/Black & Veatch, 2002).

Concentrations of TCE in excess of 300 μg/L were

measured in water samples from seven other

shallow wells in the same general area and at

RI–17 since 1998 (fig. 2).

Purpose and Scope

This report presents the results of a study by

the USGS to evaluate ground-water flow in the

vicinity of well NP–21 near Colmar, Pa. (fig. 2).

Specific objectives were to (1) determine the drawdown caused by pumping NPWA wells NP–21 or

NP–87, (2) evaluate probable paths of groundwater flow under present-day conditions (2000)

(with NP–21 pumping discontinued), and

(3) determine the areas contributing recharge if

pumping from NP–21 or NP–87 was restarted and

new recovery wells were installed. Drawdown was

calculated from water levels measured in observation wells during aquifer tests at wells NP–21 and

NP–87. Because the dipping geologic units yielding water to the pumping wells impart heterogeneity and anisotropy with respect to hydraulic

conductivity, ground-water-flow paths may be difficult to infer directly from the water-table configuration. Ground-water-flow paths and areas

contributing recharge were therefore estimated by

use of a ground-water-flow model.

The scope of the report is limited to an analysis of ground-water flow near Colmar under current conditions and those imposed by hypothetical

pumping of supply well NP–21 or NP–87 and proposed recovery wells; the effects of historical

ground-water withdrawals on ground-water flow

were not evaluated. Although ground-water flow

in the Colmar area was the focus of the study, to

ensure that arbitrary boundaries in the groundwater-flow model were sufficiently distant from

the Colmar area, the domain of the ground-waterflow model encompassed the larger area shown in

figure 1.
3

Figure 1. Location of study area near Colmar, geology, and ground-water-flow model domain, Bucks and Montgomery

Counties, Pennsylvania.
4

Figure 2. Location of selected wells in the study area near Colmar, Bucks and Montgomery Counties,

Pennsylvania.
5

Previous Investigations

A number of previous studies provide information about the ground-water resources and

geology in the Colmar area. Rima (1955) described

the occurrence and movement of ground water in

the Lansdale area. Longwill and Wood (1965) and

Newport (1971) provided information about well

characteristics, ground-water-quality data, and

general ground-water resources in Montgomery

County. The study by Longwill and Wood (1965)

focused on ground water in the Brunswick Formation and contained a geologic map, which in the

Lansdale area was based almost entirely on

unpublished manuscripts by Dean B. McLaughlin,

Pennsylvania Topographic and Geologic Survey.

Lyttle and Epstein (1987) compiled a geologic map

of the Newark 1° × 2° Quadrangle that updated

and revised the geologic nomenclature for the area.

Biesecker and others (1968) described the water

resources of the Schuylkill River Basin, which

drains part of the Lansdale area. Goode and Senior

(1998) reviewed aquifer tests conducted in the

Lansdale area, including a test at supply well

NP–21 in 1980. Senior and Goode (1999) described

the ground-water system and the effect of pumping on ground-water flow in the Lansdale area.

Their work included an analysis of regional

ground-water-flow directions by the use of a

ground-water-flow model. The results of geophysical logging conducted in support of the regional

study in the Lansdale area were reported by Conger (1999).

Investigations of ground-water contamination at North Penn Area 5 are summarized by

Earth Technology Corporation (1993). Their work

describes previous ground-water sampling results,

remediation actions, and sources of contamination

and outlines the approach for the RI/FS study. The

most comprehensive description of site characteristics, delineation of ground-water contamination,

and discussion of contaminant fate and transport

are contained in the RI/FS report by Tetra Tech/

Black & Veatch (2002). USGS work in support of

the RI/FS is reported in Bird and Conger (2002).

They present an analysis of borehole geophysical

logging and packer tests, list results of water-quality analyses, and map water levels from wells in

North Penn Area 5.

Well-Identification System

Well identifiers used by USGS consist of a

county-abbreviation prefix preceding a sequentially assigned well number. The prefix BK denotes

a well in Bucks County; the prefix MG denotes a

well in Montgomery County. Local site identifiers

given to wells by water utilities, local property

owners, or contractors are cross-referenced to

USGS well identifiers in table 1. In this report,

wells are identified by their local identifier to

accommodate readers who are familiar with the

site and are accustomed to the established local

well names. Locations of wells in the Colmar study

area described in this report are shown in figure 2.

Acknowledgments

The authors acknowledge the individuals

and companies that provided data and allowed

access to their wells. Thanks to the NPWA for use

of wells NP–21 and NP–87 for aquifer testing.

Appreciation is extended to Patrick Till of BAE

Systems; David Fennimore of Earth Data Northeast; Kevin McLaughlin of North Wales Water

Authority; and Gregory Cavallo of the Delaware

River Basin Commission for providing water-use

data and information about wells. Thanks also to

Tad Yancheski and Dana Small of TetraTech/Black

& Veatch for providing data during the study and

reviewing this report.

HYDROGEOLOGIC SETTING

The Colmar study area is in the Piedmont

Physiographic Province and is underlain by Triassic sedimentary rocks of the Brunswick and Lockatong Formations (fig. 1). Beds in the Brunswick

and Lockatong Formations are part of a homoclinal

structure, striking NE-SW and dipping about 20 to

30° NW. The dipping bedrock is thinly mantled at

the site with soil and unconsolidated weathered

rock. The terrain varies from flat to low rolling hills

and is drained by the West Branch Neshaminy

Creek and its tributaries.

Geology

The Brunswick Formation underlies the

northwestern half of the Colmar study area (fig. 1).

The Brunswick Formation consists of several thousand feet of primarily reddish brown mudstone,

shale, and siltstone, which interfinger with the

underlying Lockatong Formation. The contact

between the Brunswick and the Lockatong Formations generally is placed where the total thickness
6

Table 1. Record of selected wells in the study area near Colmar, Bucks and Montgomery Counties, Pennsylvania

[NGVD 29, National Geodetic Vertical Datum of 1929; Deg, degree; Min, minute; Sec, second; O, observation;

N, industrial; P, public supply; R, remediation; U, unused; W, withdrawal; Z, destroyed; —, no data]

Local

well

identifier1

U.S.

Geological

Survey

well

identifier

Latitude Longitude Primary use Measuring

point

elevation2

(feet

above

NGVD 29)

Well

depth

(feet

below

land

surface)

Well

diameter

(inches)

Top of

open

interval

(feet

below

land

surface)

Bottom

of open

interval

(feet

below

land

surface)

Deg Min Sec Deg Min Sec Site Water

A–3 MG–1762 40 16 4.62 75 14 40.74 O U 317.22 11 6 0 11

A–10 MG–1195 40 16 5.23 75 14 41.75 R U 315.14 104 6 4 104

A–12 MG–1797 40 16 5.34 75 14 43.26 O U 316.15 24 6 0 24

A–13 MG–1763 40 16 5.27 75 14 43.08 O U 314.83 19 6 0 19

A–18 MG–1764 40 16 5.56 75 14 42.86 O U 315.96 13 6 0 13

BAE Supply MG–876 40 16 8 75 14 40 W N 318 300 6 — 300

MW–1 MG–1921 40 16 1.67 75 14 54.96 O U 297.23 50 6 19 50

MW–2 MG–1922 40 16 5.59 75 14 58.70 O U 295.56 50 6 19 50

MW–3 MG–1923 40 16 .23 75 15 4.25 O U 289.52 42 6 19 42

MW–5 MG–1924 40 16 5.20 75 14 53.38 O U 302.58 50 6 19 50

NP–12A MG–1765 40 16 11.82 75 15 16.20 U U 300.42 46 6 17.5 46

NP–12B MG–1691 40 16 11.89 75 15 15.91 U U 300.19 115 6 22 115

NP–12C MG–914 40 16 12.32 75 15 16.60 W U 302.52 620 8 43 620

NP–21 MG–924 40 16 6.38 75 15 1.48 W U 290.21 500 12 50 500

NP-613 MG-1125 4— —— — — — W P 331 400 10 60 400

NP–75 BK–1292 40 16 40.01 75 14 56.08 O U 280.31 67 8 20 67

NP–87 MG–1693 40 16 6.46 75 15 1.98 U U 288.06 476 12 103 476

NWWA–16 MG–1048 40 16 18 75 14 28 Z U — 502 12 60 502

NWWA–173 MG–875 4— — — — — — W P 332 500 12 60 500

NWWA–22 MG–1051 4— — — — — — W P 308 500 8 — 500

R&B–1 MG–186 40 16 24.60 75 15 .58 U U 282.23 286 10 44 286

R&B–2 MG–187 40 16 26.94 75 15 2.99 U U 279.55 302 10 58 302

R&B–3 MG–188 40 16 29.64 75 15 5.18 U U 276.81 300 10 58 300

RI–1D BK–3001 40 16 32.12 75 14 10.46 O U 296.65 108 2 98 108

RI–1S BK–3002 40 16 32.12 75 14 10.46 O U 296.75 66 2 56 66

RI–2D BK–3003 40 16 20.75 75 14 15.79 O U 309.94 139 2 129 139

RI–2S BK–3004 40 16 20.75 75 14 15.79 O U 309.96 48 2 38 48

RI–4D MG–1768 40 15 34.81 75 15 2.27 O U 341.61 146 2 126 146

RI–4S MG–1769 40 15 34.81 75 15 2.27 O U 341.64 39 2 20 40

RI–5D MG–1770 40 15 54.50 75 15 10.94 O U 296.46 85 2 45 55

RI–5S MG–1771 40 15 54.50 75 15 10.94 O U 296.47 33 2 20 30

RI–6D MG–1772 40 16 26.69 75 14 46.86 O U 285.23 84 2 64 84

RI–6S MG–1773 40 16 26.69 75 14 46.86 O U 285.20 28 2 18 28

RI–7D MG–1774 40 16 2.96 75 15 14.76 O U 309.15 96 2 86 96

RI–7S MG–1775 40 16 2.96 75 15 14.76 O U 309.09 66 2 56 66

RI–8D MG–1776 40 16 16.36 75 15 19.40 O U 302.39 125 2 115 125

RI–8S MG–1777 40 16 16.36 75 15 19.40 O U 302.48 90 2 80 90

RI–9D MG–1902 40 16 32.66 75 14 53.30 O U 272.62 136 2 126 136

RI–9S MG–1903 40 16 32.66 75 14 53.30 O U 272.67 48 2 18 48

RI–10D BK–3016 40 16 51.46 75 14 22.20 O U 269.99 63 2 46 61
7

RI–10S BK–3015 40 16 51.46 75 14 22.20 O U 270.13 30 2 18 28

RI–11D MG–1906 40 16 44.62 75 15 13.93 O U 308.05 150 2 138 148

RI–11S MG–1907 40 16 44.62 75 15 13.93 O U 308.09 115 2 80 115

RI–12D MG–1925 40 16 36.01 75 15 10.76 O U 273.38 140 2 120 140

RI–12S MG–1926 40 16 36.01 75 15 10.76 O U 273.38 30 2 20 30

RI–13D MG–1778 40 16 31.73 75 15 10.66 O U 270.65 129 2 112 129

RI–13S MG–1779 40 16 31.73 75 15 10.66 O U 270.63 54 2 40 50

RI–14D MG–1927 40 16 31.62 75 15 19.44 O U 292.96 123 2 125 135

RI–14S MG–1928 40 16 31.62 75 15 19.44 O U 292.99 58 1 48 58

RI–15D BK–3005 40 16 41.45 75 14 25.98 O U 292.90 145 2 120 145

RI–15S BK–3006 40 16 41.45 75 14 25.98 O U 292.96 65 2 55 65

RI–16D BK–3007 40 16 45.77 75 14 37.14 O U 272.07 153 2 130 150

RI–16S BK–3008 40 16 45.77 75 14 37.14 O U 271.70 40 2 18 38

RI–17D BK–3009 40 16 36.30 75 14 36.64 O U 286.26 194 2 178 188

RI–17S BK–3010 40 16 36.30 75 14 36.64 O U 286.20 65 2 43 63

RI–18D MG–1780 40 16 22.62 75 14 32.82 O U 293.97 95 2 85 95

RI–18S MG–1781 40 16 22.62 75 14 32.82 O U 294.01 64 2 44 64

RI–19D MG–1782 40 16 16.90 75 14 44.38 O U 295.06 80 2 70 80

RI–19S MG–1783 40 16 16.90 75 14 44.38 O U 295.03 58 2 46 56

RI–20D MG–1784 40 16 7.18 75 14 48.34 O U 304.68 110 2 100 110

RI–20S MG–1785 40 16 7.18 75 14 48.34 O U 304.72 80 2 70 80

RI–23 MG–1786 40 16 21.22 75 14 40.20 O U 297.14 23 3.5 4 16

RI–25 MG–1787 40 16 20.96 75 14 37.57 O U 296.09 14 3.5 4 14

RI–27D MG–1766 40 16 23.20 75 14 40.88 O U 298.80 313 2 303 308

RI–27S MG–1767 40 16 23.20 75 14 40.88 O U 298.78 55 2 40 55

RW–1 MG–1153 40 16 5.30 75 14 43.58 O U 313.41 70 6 23 70

RW–2 MG–1154 40 16 5.20 75 14 43.58 O U 314.96 150 6 80 150

RW–3 MG–1155 40 16 5.12 75 14 43.22 O U 315.30 210 6 150 210

RW–4D MG–1929 40 16 17.94 75 14 46.18 O U 293.46 300 2 150 300

RW–4I MG–1930 40 16 17.90 75 14 46.07 O U 293.47 150 2 50 150

RW–4S MG–1931 40 16 17.87 75 14 45.96 O U 293.31 50 6 20 50

RW–5D MG–1932 40 16 20.64 75 14 43.55 O U 296.81 300 2 150 300

RW–5I MG–1933 40 16 20.57 75 14 43.48 O U 296.99 150 2 50 150

RW–5S MG–1934 40 16 20.53 75 14 43.37 O U 296.83 50 6 20 50

RW–6D MG–1935 40 16 18.66 75 14 40.20 O U 299.92 300 2 150 300

RW–6I MG–1936 40 16 18.77 75 14 40.38 O U 296.36 150 2 50 150

RW–6S MG–1937 40 16 18.62 75 14 40.13 O U 297.80 50 6 20 50

W–1 MG–1788 40 16 5.23 75 14 41.89 O U 314.51 40 3.5 9 40

W–3 MG–1789 40 16 14.09 75 14 54.96 O U 301.39 23 3.5 12 23

W–4 MG–1790 40 16 18.19 75 14 46.68 O U 293.43 30 3.5 16 30

Table 1. Record of selected wells in the study area near Colmar, Bucks and Montgomery Counties, Pennsylvania—

Continued

[NGVD 29, National Geodetic Vertical Datum of 1929; Deg, degree; Min, minute; Sec, second; O, observation;

N, industrial; P, public supply; R, remediation; U, unused; W, withdrawal; Z, destroyed; —, no data]

Local

well

identifier1

U.S.

Geological

Survey

well

identifier

Latitude Longitude Primary use Measuring

point

elevation2

(feet

above

NGVD 29)

Well

depth

(feet

below

land

surface)

Well

diameter

(inches)

Top of

open

interval

(feet

below

land

surface)

Bottom

of open

interval

(feet

below

land

surface)

Deg Min Sec Deg Min Sec Site Water
8

W–5 MG–1791 40 16 21.50 75 14 43.30 O U 296.98 30 3.5 9 30

W–6 MG–1792 40 16 17.47 75 14 36.06 O U 300.56 32 3.5 10 32

W–8 MG–1793 40 16 17.18 75 14 48.26 O U 290.84 30 3.5 15 30

W–9 MG–1794 40 16 13.44 75 14 47.26 O U 298.01 45 3.5 13 45

W–12 MG–1796 40 16 7.50 75 14 51.97 O U 298.97 40 3.5 7 40

W–13 MG–1661 40 16 6.56 75 15 .76 O U 288.29 44 3.5 10 44

W–14 MG–1798 40 16 7.28 75 14 50.39 O U 302.83 36 3.5 6 36

W–16 MG–1662 40 16 2.06 75 14 36.60 O U 319.05 47 3.5 9 47

W–17 MG–1799 40 16 5.74 75 14 58.56 O U 293.09 44 3.5 6 44

1 RI-series wells were drilled by U.S. Environmental Protection Agency for Remedial Investigation/Feasibility Study; A-series,

MW-series, RW-series, and W-series wells were drilled by industry for ground-water monitoring or recovery; BAE Supply, NP, NWWA, and

R&B wells were drilled for public- or industrial-supply wells. 2 Measuring-point elevation is for top of casing, from survey, unless reported to nearest integer, which is land surface from

topographic maps.

3 Well is located outside of the Colmar study area but is used in the model of ground-water flow; location is shown in figure 25.

4 Geographic coordinates of public-supply wells are known but not reported.

Table 1. Record of selected wells in the study area near Colmar, Bucks and Montgomery Counties, Pennsylvania—

Continued

[NGVD 29, National Geodetic Vertical Datum of 1929; Deg, degree; Min, minute; Sec, second; O, observation;

N, industrial; P, public supply; R, remediation; U, unused; W, withdrawal; Z, destroyed; —, no data]

Local

well

identifier1

U.S.

Geological

Survey

well

identifier

Latitude Longitude Primary use Measuring

point

elevation2

(feet

above

NGVD 29)

Well

depth

(feet

below

land

surface)

Well

diameter

(inches)

Top of

open

interval

(feet

below

land

surface)

Bottom

of open

interval

(feet

below

land

surface)

Deg Min Sec Deg Min Sec Site Water
9

of interfingered red beds of the Brunswick equals

the total thickness of interfingered gray and black

beds of the Lockatong, which causes some ambiguity in mapping the stratigraphy. The interfingered

thin beds that underlie the southeastern part of the

site were mapped as the Lockatong Formation by

Willard and others (1959) but are considered the

lower beds of the Brunswick Formation by Lyttle

and Epstein (1987). The lower beds of the Brunswick Formation contain detrital cycles of medium

to dark gray and olive to greenish-gray, thin-bedded and evenly bedded shale and siltstone that are

similar to but not as continuous as those in the

underlying Lockatong Formation (Lyttle and

Epstein, 1987). The mean strike of the Brunswick

Formation determined from the acoustic televiewer logs at North Penn Area 5 is N. 62° E., and

the dip is 31° NW. (Bird and Conger, 2002, p. 24).

The Lockatong Formation as mapped by

Willard and others (1959) underlies the southeastern half of the Colmar study area (fig. 1). The

Lockatong Formation consists mainly of thick-bedded argillite, siltstone, mudstone, and shale that

underlie and interfinger with the Brunswick Formation. The argillite is a tough, firmly cemented,

non-fissile rock that is very resistant to weathering.

Drilling of monitor wells in the Colmar area indicates that the upper beds of the Lockatong Formation are mostly dark red shale and siltstone that

interfinger with gray beds that appear very similar

to the Brunswick Formation. The strike of the

Lockatong Formation as mapped by Willard and

others (1959) changes direction across the Colmar

study area from about N. 35° E. in the southern

part to N. 70° E. in the northern part (fig. 1). The

mean strike of the Lockatong Formation determined from the acoustic televiewer logs at North

Penn Area 5 is N. 67° E., and the dip is 19° NW.

(Bird and Conger, 2002, p. 24).

Regolith consisting of soil and unconsolidated weathered bedrock mantles the dipping bedrock formations. Thickness of the regolith varies

considerably. On the basis of drilling records from

56 monitor wells in the Colmar study area,

reported depth to bedrock ranges from 3 to 45 ft,

and median depth is 10 ft (fig. 3). Fifty percent of

the reported depths to bedrock were between 8

and 13 ft.

Ground Water

Ground water in the Colmar area originates

from infiltration of local precipitation, moves

through regolith and fractured bedrock, and discharges to streams and wells. A generalized conceptual model of natural flow as it moves through

dipping geologic units is illustrated in figure 4.

Although greatly simplified, the conceptual model

illustrates the potential for preferential flow within

units of large hydraulic conductivity and across

the interbeds of small hydraulic conductivity.

Occurrence

Ground water in rocks of the Brunswick and

Lockatong Formations may occur under unconfined or confined conditions. Unconfined conditions usually are assumed for the shallowest part

of the aquifer, but confined conditions are indicated at many locations by monitor wells that display barometric efficiencies from 50 to 80 percent.

For example, in monitor well W–6, which is open

to the aquifer from 10 to 36 ft below land surface,

the magnitude of water-level fluctuation is about

50 percent of the change in barometric pressure as

expressed in feet of water (fig. 5). This indicates a

50-percent barometric efficiency for well W–6,

which probably is the result of confinement of the

shallow, fractured bedrock by clayey soil. Ground

water in the deeper part of the fractured-bedrock

aquifer is confined or partially confined, resulting

in local artesian conditions. Well NP–87, open to

the fractured bedrock at 82 ft below land surface

and from 104 to 500 ft below land surface is a flowing artesian well for many days during the year.

(fig. 6).

Figure 3. Depth to bedrock reported from

56 wells in the Colmar study area, Bucks

and Montgomery Counties, Pennsylvania.
10

Figure 4. Ground-water-flow paths in dipping geologic units of differing hydraulic properties.

Figure 5. Water-level fluctuations in observation well W–6 and changes in atmospheric pressure from June 27 to July 5, 2000, Colmar study area, Bucks and

Montgomery Counties, Pennsylvania. (Note that the y-axis showing atmospheric

pressure is inverted and the full scale covers twice the range of the y-axis showing

water level so that the graphical traces could be directly compared for a well efficiency

of 50 percent.)
11

Water-Yielding Fractures

After infiltrating through soil and regolith,

ground water moves preferentially through fractures in the bedrock because the hydraulic conductivity of the rock matrix is very low. Bird and

Conger (2002) used acoustic-televiewer and heatpulse-flowmeter surveys in 31 boreholes to investigate the fractured-bedrock system. They identified

two populations of fractures in the Triassic bedrock—(1) low-angle fractures that are probably

bedding-plane partings and (2) high-angle fractures. The bedding-plane partings had a mean

strike of N. 62° E. and mean dip of 27° NW. The

high-angle fractures had a mean strike of N. 58° E.,

which was nearly parallel to the strike of the bedding-plane partings, and mean dip of 72° SE.,

which was almost perpendicular to the dip of the

bedding-plane partings. These fracture populations are shown schematically in figure 7.

The bedding-plane and high-angle fractures

can be seen in outcrops of the Brunswick and

Lockatong Formations, but in the Colmar area the

rocks are poorly exposed. Although not taken in

the study area, a photograph of Triassic bedrock

from Wood (1980) illustrates the nature of fractures

in the dipping, layered, sequences of siltstone and

shale similar to rocks in the Colmar area (fig. 8). As

seen in outcrop, thin siltstone beds tend to be more

highly fractured than the shale and thicker siltstone beds. High-angle fractures may be more

abundant in siltstone beds because they are more

brittle than shale; however, they undoubtedly also

are present throughout the shale beds. The highangle fractures shown in figure 8 tend not to be

extensive features; they are abundant within individual beds but terminate at the contacts between

beds.

Results of heatpulse-flowmeter surveys indicate that only 18 percent of the fractures yielded

water to wells (Bird and Conger, 2002, p. 24).

Eighty-three percent of the water-yielding fractures were high-angle fractures. Because the highangle fractures probably are best developed in the

brittle siltstone, those beds are probably the most

permeable water-bearing zones. Where a highly

fractured, dipping siltstone bed is bounded above

and below by less permeable shale, ground water

will move preferentially through the fractured siltstone bed. Consequently, ground-water movement

probably is affected significantly by the orientation

and lateral extent of the dipping beds as illustrated

schematically in figure 4. Vertical fractures in the

shale probably provide hydraulic connections

between siltstone beds.

Figure 6. Water flowing from well NP–87 on June 24, 2002,

Colmar study area, Bucks and Montgomery Counties,

Pennsylvania.
12

Figure 7. Schematic diagram of bedding-plane and high-angle fractures identified by use of the acoustic

televiewer in the Colmar study area, Bucks and Montgomery Counties, Pennsylvania.

72° 27°

BEDDING-PLANE PARTINGS

HIGH-ANGLE

FRACTURES

N. 58° E.

N. 62° E.

Figure 8. Sedimentary bedrock of Triassic age similar to bedrock in the

Colmar area showing (1) thin beds of highly fractured siltstone, (2) thicker

siltstone beds where fractures are less well developed, and (3) interbeds of

soft low-permeability shale. Bed in top left corner of photograph is 4 ft thick.

(from Wood, 1980, fig. 2).
13

The distribution of the water-yielding fractures with depth in the Colmar area was evaluated

from results of heatpulse-flowmeter surveys

(fig. 9). Flowmeter surveys in 30 wells showed that

the highest density of water-bearing zones was in

the upper 40 ft of the wells. However, drilling

reports from the RI–series wells seemed to contradict this finding because they describe almost no

water produced in the 0-40-ft depth interval. Possibly, the weathered bedrock from land surface to a

depth of 40 ft has a large number of water-yielding

fractures (identified by flowmetering) that yield

small quantities of water not identified on drilling

reports. The low yields from 0-40 ft below land

surface may be the result of clay that has filled fractures in the most intensely weathered surficial

parts of the aquifer. Driller’s reports indicate that

the increase in well yield appears to be greatest

from about 80 to 90 ft below land surface. The

increase in yield at that depth may represent an

optimum depth where weathering has enhanced

fracture openings and interconnections without

filling them with clay.

Recharge

Ground-water recharge to Triassic rocks in

Pennsylvania ranges from about 6 to 10 in/yr on

the basis of base-flow analysis of streamflow

hydrographs (Wood, 1980; Sloto and Schreffler,

1994). The magnitude of annual, average recharge

in the Colmar area probably is similar to that estimated by Senior and Goode (1999, p. 78); they

found the optimum value of recharge to be

8.3 in/yr from calibration of a model to simulate

ground-water flow in the vicinity of Lansdale.

Movement

Ground water moves from areas of high

hydraulic head to areas of low hydraulic head.

A map shows the configuration of water levels in

wells less than 100 ft deep in the Colmar area on

June 19-20, 2000 (fig. 10). Wells NP–21 and NP–87

were not pumping. The map shows ground-water

levels are lowest near Neshaminy Creek and its

tributaries; thus, the hydraulic gradient is driving

ground water toward the streams.

If the aquifer system is homogeneous and

isotropic, ground water will move in a direction

orthogonal to contours of hydraulic head and can

be inferred directly from water-level maps. However, in the Colmar area, the hydrogeologic framework consisting of multiple, dipping, fractured,

water-yielding zones produces an aquifer that is

heterogeneous and anisotropic with respect to

hydraulic conductivity. The movement of ground

water through such a layered multi-aquifer system

is shown schematically in figure 4. Note how

ground water moves preferentially along the dipping high-permeability fractured units and cuts

perpendicularly across the less permeable interbedded shales. Figure 4 also illustrates that the vertical hydraulic gradients are upward in most all of

the eastern half of the cross section (east of West

Branch Neshaminy Creek) where beds dip to the

west. Measurement of vertical hydraulic gradients

in nested piezometers and vertical flow of water in

wells having long open intervals confirms that

hydraulic gradients mostly are upward in the Colmar area east of the creek and downward west of

the creek (Bird and Conger, 2002; Tetra Tech/ Black

& Veatch, 2002). Figure 9. Density of water-yielding fractures

with depth determined from flowmeter

surveys in the Colmar study area, Bucks

and Montgomery Counties, Pennsylvania.
14

Figure 10. Altitude and configuration of water levels in selected shallow wells less than 100 feet deep in

the Colmar study area, June 19-20, 2000.
15

Discharge

Ground water in the Colmar area discharges

to Neshaminy Creek and its tributaries or to wells.

Ground-water discharge to streams provides their

base flow; however, some stream reaches in the

study area have periods when they gain water and

other periods when they lose water.

Currently, the only known pumping wells

are ground-water remediation well A–10 at BAE

Systems, the BAE Systems supply well, and North

Wales Water Authority (NWWA) well NWWA–22.

In 2000, withdrawals from A–10 averaged about

15 gal/min, withdrawals from the BAE Systems

supply well averaged about 13 gal/min (Patrick

Till, BAE Systems Inc. written commun., 2001), and

withdrawals from NWWA–22 averaged about

20 gal/min (North Wales Water Authority, written

commun., 2001). Previously, large ground-water

withdrawals were made by the NWWA and the

NPWA from former public-supply wells NP–12C,

NP–21, and NWWA–16 in the Colmar area. Wells

NP–12C, NP–21, and NWWA–16 were taken out of

service in March 1999, December 1995, and September 1994, respectively. Well NP–21 has not been

in operation since December 1995.

AQUIFER TESTS

Aquifer tests were conducted in June 2000 at

well NP–21 and May 2002 at well NP–87 to identify hydraulic connections and to quantify the

drawdown caused by pumping. The tests also supported efforts to simulate areas contributing

recharge to wells NP–21 and NP–87, which, in

turn, will support management decisions regarding ground-water remediation. The tests were performed primarily to determine if the area

influenced by pumping reached the northern and

southern areas of ground-water contamination

shown in figure 2. Drawdown observed in areas of

contamination indicates a hydraulic connection to

the pumped well and that pumping could alter the

direction of ground-water flow in the vicinity of

the contamination. However, measurement of

drawdown at an observation well does not necessarily indicate the movement of ground water

from that observation well to the pumped well.

Ground-water flow direction is controlled by the

hydraulic gradient (not drawdown), aquifer

heterogeneities, and boundary conditions of the

hydrologic system.

Design of Tests

The aquifer test in June 2000 was designed

with the intention of pumping NP–21 at

400 gal/min for 3 days. However, soon after the

pumping began, it was apparent that a rate of

400 gal/min could not be obtained with the existing pump without lowering the water level below

the pump intake. Thus, the test of NP–21 was conducted at an average rate of 257 gal/min, and as a

result of a large rainstorm and a lack of stabilization of water levels, the duration of pumping was

extended to 7 days. Subsequent to completion of

the aquifer test of NP–21 in June 2000, interest in

determining the drawdown caused by pumping at

a greater rate (about 400 gal/min) continued.

Because obstructions in NP–21 made it impossible

to set a large capacity pump deeper, a second aquifer test was conducted by pumping nearby well

NP–87 at 402 gal/min for 3 days. The two wells

(NP–21 and NP–87) are only about 40 ft apart and

are similar in depth and construction; thus, it was

assumed that pumping either well would result in

drawdown of a similar magnitude and extent.

Wells NP–21 and NP–87

Wells NP–21 and NP–87 are near an

unnamed tributary to the West Branch Neshaminy

Creek (fig. 2). Well NP–21 is an inactive publicsupply well owned and operated by NPWA. The

well is 500 ft deep and is cased to 50 ft below land

surface. The interval from 50 to 500 ft below land

surface is completed as an open hole (fig. 11). The

diameter of the well is 12 in. from land surface to a

depth of 250 ft below land surface and 6 in. from

250 ft below land surface to the bottom. Well

NP–87 is 476 ft deep, is cased to 103 ft below land

surface, and is completed as a 12-in. open hole.

A broken casing joint in NP–87 at 82 ft below land

surface allows water to enter (or exit) the well at

that location.

On the basis of geologic maps and an average dip of 27° NW., wells NP–21 and NP–87 penetrate the Brunswick Formation in the upper 70 to

90 ft and the Lockatong Formation in the remainder of the well. The driller’s log of the well indicates gray and brown rock throughout, with no

clear differentiation between the Brunswick and

Lockatong Formations. Geophysical logs show a

good correlation of rock units between NP–21 and

NP–87 (Bird and Conger, 2002).
16

The driller’s log of well NP–21 indicated a

yield of 300 gal/min; three principal water-yielding zones were at 90-140, 300-350, and 400-450 ft

below land surface (fig. 11A). On the basis of the

driller’s log, about 40 percent of the pumped water

enters above 140 ft below land surface, and 60 percent enters from 300 to 450 ft below land surface.

Well inspection conducted by a down-hole

video camera during the aquifer tests allowed for

improved understanding of the hydraulic connections of the major water-producing zones between

NP–21 and NP–87. Video logs in well NP–87

recorded during the June 2000 aquifer test of

NP–21 showed a large quantity of water cascading

from a break in the casing at 82 ft below land surface. By trolling the video camera downward at a

velocity equal to the downward velocity of the

smallest visible particulates in the well (assumed

to be moving at the velocity of water), a vertical

downward flow rate of about 100 gal/min was

estimated. The downward movement ended at

425 ft below land surface, indicating water

appeared to be leaving the well through the highangle fractures at 417-435 ft below land surface.

NP–87 represents a “short-circuit” that facilitates

vertical flow in the natural flow system from 82 to

417-435 ft below land surface when well NP–21 is

pumped and potentially improves the yield of

NP–21 by as much as 100 gal/min.

In NP–87, packers were used to isolate and

test the water-yielding properties of different

depth intervals. The 13 zones tested had specific

capacities ranging from 0.04 to 3.1 (gal/min)/ft of

drawdown (fig. 11B). The uppermost zone tested

was the break in the casing at 82 ft below land surface. The principal water-yielding zones determined from the testing of isolated intervals were

the break in casing at 82 ft and the interval from

412 ft to the bottom of the well at 476 ft, which are

shown with shaded patterns in figure 11B. The

interval from 300 to 350 ft below land surface that

was identified as a major water-yielding zone in

the drilling report of NP–21 was not shown to

yield much water to well NP–87 based on its specific capacity of 0.09 (gal/min)/ft of drawdown.

Figure 11. Well construction and (A) driller-reported yield of supply well NP–21 and (B) specific capacity of isolated

intervals in NP–87. Shaded areas represent major water-yielding zones.
17

Taken together, the driller’s reported yield

data from NP–21 and results from testing of isolated intervals in NP–87 indicate most water probably enters both wells from a shallow interval

about 50 to 140 ft below land surface and from a

deep interval below 400 ft below land surface.

When one well is pumped, the wells act together

as if each were being pumped because of the good

hydraulic connection between the wells within the

deep interval below 400 ft below land surface.

Observation Wells

Water levels were monitored at 45 observation wells during the June 2000 aquifer test of

NP–21 and in 34 observation wells during the May

2002 test of NP–87. Most of the wells were drilled

specifically to monitor water levels and chemical

quality of the shallow ground-water flow system.

The RI–series wells were drilled by USEPA contractors for the current RI/FS; the A–series,

MW–series, RW–series, and W–series wells were

drilled by private industry for ground-water monitoring or recovery. Wells RW–1, RW–2, and RW–3

were drilled as possible recovery wells but were

not used for that purpose. Other observation wells

(NP, NWWA, and R&B notation) are inactive public- or industrial-supply wells. An active recovery

well (A–10) and industrial-supply well (BAE Systems supply well) also were monitored during the

May 2002 test of NP–87. Not all the same wells

were available for monitoring during each aquifer

test. Wells MW–1, MW–2, MW–3, and MW–5 (all

of which are in the immediate vicinity of well

NP–21); RI–27S; RI–27D; and well clusters RW–4,

RW–5, and RW–6 (shallow, intermediate, and deep

wells at each cluster) were installed after the June

2000 aquifer test. Thus, these newer wells were

monitored only during the May 2002 test of NP–87.

Water levels were monitored using pressure

transducers and electric water-level tapes. Water

levels in most wells were monitored for at least

5 days prior to the start of the aquifer test.

Atmospheric pressure was monitored prior

to and during the aquifer test to determine if measured drawdown should be adjusted for changes

in atmospheric pressure. In confined aquifers, an

increase in atmospheric pressure can cause water

levels in wells to decline; a decrease in atmospheric

pressure can cause water levels to rise. The waterlevel response is typically 20 to 70 percent of the

change in atmospheric pressure (Todd, 1980,

p. 236). For some observation wells, sufficient pretest water-level measurements were available to

establish a barometric efficiency. The barometric

efficiency was multiplied by the change in atmospheric pressure during the aquifer test (expressed

as feet of water) to arrive at the water-level change

caused by the change in pressure. Pressure was

measured in inches of mercury and multiplied by

1.13 to convert units to feet of water. When barometric efficiency was not determined, an efficiency

of 50 percent was assumed for purposes of computing drawdown unless those adjustments produced drawdown values that seemed anomalous

when plotted, which was the case for wells RW–6S

and W–8. For wells where a large drawdown was

observed, the maximum potential effect of the

barometric-pressure adjustment was small, so the

barometric efficiency of the well was not determined.

NP–21 Test of June 20–27, 2000

The aquifer test was conducted by pumping

well NP–21 and measuring drawdown in the

pumped well and 45 observations wells. Supply

well NP–21 was pumped at an average rate of

257 gal/min. The rate determined from periodic

readings of a totalizing flowmeter ranged from 281

to 256 gal/min. During the aquifer test, ground

water pumped from NP–21 was discharged directly into the sewer system to avoid recharging the

aquifer. Pumping began at 9 a.m. on June 20, 2000,

and ended 7 days and 6 hours later at 3 p.m. on

June 27. Wells that were monitored and drawdown

values measured at the end of the 7 days of pumping are shown in table 2 and figure 12. Hydrographs of water levels monitored before, during,

and after the aquifer test are shown in figures 13

and 14.

Climatic Conditions and Resultant

Water-Level Trends

Prior to the aquifer test, water levels in

pumped well NP–21 and observation wells were

monitored to determine climatic effects on water

levels, prepumping trends, and effects of any

nearby pumping wells. Probably the greatest factor

affecting the measured water levels (other than the

pumping from well NP–21) was ground-water

recharge from precipitation that began about

9:45 p.m. the night of June 21, 2000. Because of the

storm, atmospheric-pressure changes were large

enough to cause water-level fluctuations of several

tenths of a foot in observation wells during the

pumping period. Ground-water withdrawals from

well NWWA–22 and the BAE Systems supply well

also caused significant water-level fluctuations in

several observation wells during the aquifer test.
18

Table 2. Summary of observations from wells monitored during the aquifer test at well NP–21, Colmar study area, Bucks and Montgom

[—, not determined; <, less than; ?, reported drawdown may be partially or entirely caused by natural ground-water recession]

Local well

name

Distance from

pumped well

NP–21,

in feet

Drawdown after

7 days of

pumping,

in feet

Direction of

pre-test waterlevel trend

Water-level rise

caused by

rainstorm

during test,

in feet

Comments

WELLS WITH MEASURABLE DRAWDOWN DURING AQUIFER TEST OF NP–21

A–13 1,428 2.4 None 0 Barometric efficiency 50 percent, no water-level recovery.NP–21 0 200 None 0 Pumped well.NP–87 40 95 None 0 Water cascades from break in casing at 82 feet below land surfaceRI–5D 1,408 1.2 Up .4 Affected by pumping from NWWA–22; reported drawdown has e

removed; assumed barometric efficiency of 50 percent.

RI–5S 1,408 .9 Up .2 Affected by pumping from NWWA–22; reported drawdown has e

removed; barometric efficiency 50 percent.

RI–7D 1,086 1.4 None .2 Barometric efficiency 70 percent.RI–7S 1,086 1.4 None .2 Barometric efficiency 50 percent.RI–19D 1,697 .4? Down <.1 Drawdown may be natural water-level recession; barometric efficRI–19S 1,697 .8? Down Slight Drawdown may be natural water-level recession.RI–20D 1,020 4.0 None .4 Barometric efficiency 70 percent.RI–20S 1,020 2.8 None .1 Barometric efficiency 50 percent.RW–1 1,391 3.9 None .2 Water levels in RW–1, RW–2, and RW–3 were affected by pumpin

during early part of test; drawdown reported here was adjusted teffects. Barometric efficiency 50 percent.

RW–2 1,392 about 3.5 None .3 RW–2 not monitored for entire period of aquifer test; barometric eRW–3 1,418 about 3.5 None .3 RW–3 not monitored for entire period of aquifer test; barometric eW–1 1,520 4.0 None .5 No water-level recovery apparent after pumping ceased; large res

because surface water flows into flush-mounted well. No barome

W–6 2,265 .5? None .3 Slow water-level recovery after pumping ceased. Barometric efficW–9 1,312 .9 to 1.8 Down 0 Drawdown of 0.9 ft incorporates effect of pre-test water-level rece

adjustment made for recession; barometric efficiency 50 percent; pceased.

W–12 745 3.5 None 0 Barometric efficiency 50 percent.W–13 58 6.8 None 1.8 Manual measurements only from June 23 to July 6, 2000; cause un

measured at 200 to 500 minutes into the test; barometric efficiency 

W–14 863 3.1 None .1 Barometric efficiency 75 percent.W–16 1,977 2.8 None .5 Early drawdown affected by pumping from BAE supply well; bar

50 percent; poor recovery after pumping ceased.

W–17 234 about 5.3 — .2 Transducer installed June 22, 2000; some manual water-level mea

shows apparent recharge boundary.
19

WELLS WITH NO MEASURABLE DRAWDOWN DURING AQUIFER TEST OF NP–21

A–3 1,616 0 Down 1.6 Shows large response to precipitation.A–18 1,444 0 Down 4.8 Shows large response to precipitation. Well dry except after precipR&B–1 1,843 — Down — Data not recoverable from data logger memory.R&B–2 2,081 0 Down .7R&B–3 2,369 0 None .8NP–12A 1,266 0 None <.1 Water level was affected by unknown pumping source after NP–2NP–12B 1,251 0 None <.1 Water level was affected by unknown pumping source after NP–2NP–12C 1,318 0 None <.1 Water level was affected by unknown pumping source after NP–2RI–2D 3,825 0 None —RI–2S 3,825 0 None .4RI–4D 3,196 0 Up .8 Affected by pumping from NWWA–22.RI–4S 3,196 0 Up .8 Affected by pumping from NWWA–22.RI–6D 2,345 0 None .4RI–6S 2,345 0 None .4RI–8D 1,717 0 None 0RI–8S 1,717 — — — Miscellaneous manual measurements during test.RI–13D 2,661 0 Down 1.0RI–17S 3,587 — — —RI–18D 2,761 0 None .3RI–18S 2,761 0 None .3RI–23 2,230 0 Down .3RI–25 2,368 0 — — Monitored only during recovery period.W–4 1,657 0 Down .2 Barometric efficiency assumed 50 percent.W–5 2,080 0 Down — Barometric efficiency assumed 50 percent.W–8 1,497 0 None .3 May show drawdown of <0.3 ft; no apparent water-level recovery 

barometric efficiency assumed 40 percent.

Table 2. Summary of observations from wells monitored during the aquifer test at well NP–21, Colmar study area, Bucks and Montgom

[—, not determined; <, less than; ?, reported drawdown may be partially or entirely caused by natural ground-water recession]

Local well

name

Distance from

pumped well

NP–21,

in feet

Drawdown after

7 days of

pumping,

in feet

Direction of

pre-test waterlevel trend

Water-level rise

caused by

rainstorm

during test,

in feet

Comments
20

Figure 12. Observation wells monitored and drawdown after 7 days of pumping from supply well NP–21

during the June 2000 aquifer test, Colmar study area, Bucks and Montgomery Counties, Pennsylvania.
21

Figure 13. Water levels in wells where drawdown was observed during the aquifer test at well

NP–21, Colmar study area, Bucks and Montgomery Counties, Pennsylvania.
22

Figure 13. Water levels in wells where drawdown was observed during the aquifer test at well

NP–21, Colmar study area, Bucks and Montgomery Counties, Pennsylvania—Continued.
23

Figure 13. Water levels in wells where drawdown was observed during the aquifer test at well

NP–21, Colmar study area, Bucks and Montgomery Counties, Pennsylvania—Continued.
24

Figure 14. Water levels in wells where drawdown was not observed during the aquifer test at well

NP–21, Colmar study area, Bucks and Montgomery Counties, Pennsylvania.
25

Figure 14. Water levels in wells where drawdown was not observed during the aquifer test at well

NP–21, Colmar study area, Bucks and Montgomery Counties, Pennsylvania—Continued.
26

Figure 14. Water levels in wells where drawdown was not observed during the aquifer test at well NP–21,

Colmar study area, Bucks and Montgomery Counties, Pennsylvania—Continued.
27

Precipitation during test

Water levels in many of the observation

wells were affected quickly by ground-water

recharge from precipitation. Rainfall of 0.98 in. was

measured in the Colmar area from 9:45 p.m. on

June 21, 2000, through 2:45 a.m. on June 22, 2000

(Kevin McLaughlin, North Wales Water Authority,

written commun., 2001). The rain began about

1.5 days (2,205 minutes) into the aquifer test. Water

levels in some wells rose, and in others, the rate of

decline caused by the pumping of well NP–21 was

reduced. Measured water-level rises ranged from 0

to about 9.5 ft. The recharge caused water levels in

both shallow and deep monitor wells to rise. However, by the end of the test, the effect of the precipitation was fairly negligible on the total drawdown

computed after 7 days of pumping.

Changes in atmospheric pressure

During the aquifer test, the atmospheric

pressure changed a maximum of about 0.5 ft of

water (fig. 15). From June 20–22, 2000, the atmospheric pressure decreased as a result of the storm

that came through the night of June 21, then

increased until June 24. Atmospheric pressure

slowly fell from that time until the end of the test at

3 p.m. on June 27. Thus, in wells where only a few

tenths of a foot of drawdown were measured

during the aquifer test, changes in atmospheric

pressure could significantly affect the interpretation of drawdown, depending on the barometric

efficiency of the well.

The effect of atmospheric-pressure changes

on water levels was evaluated, and drawdown

values were corrected for some wells as noted in

table 2. Barometric efficiency of wells ranged from

50 to 75 percent for most wells. Water levels in both

shallow and deep monitor wells showed the effects

of changes in atmospheric pressure, indicating that

shallow parts of the “water-table” aquifer are confined to some extent by the overlying clayey

regolith and soil.

Pre-test trends

Pre-test water-level measurements indicated

that trends could be defined in 15 wells (table 2).

The upward trend in wells RI–5S, RI–5D, RI–4S,

and RI–4D was caused by cessation of pumping

from supply well NWWA–22. The downward pretest trend in water levels measured in wells A–3,

A–18, R&B–1, R&B–2, RI–13D, RI–19S, RI–19D,

RI–23, W–4, W–5, and W–9 is probably the natural

ground-water recession subsequent to a 0.2-in.

rainfall on June 18. Drawdown at the end of the

7-day test was adjusted for the natural pre-test

water-level trend only at well W–9, because water

levels at this well appeared to be affected by both

pumping and the natural recession. The difficulty

in making this correction for a 7-day aquifer test is

that the trend is assumed to continue at a constant

rate for the entire 7-day period.

Figure 15. Atmospheric pressure during the aquifer test of supply well NP–21, Colmar study

area, Bucks and Montgomery Counties, Pennsylvania.
28

Withdrawals from other wells

Ground-water withdrawals from wells other

than NP–21 caused water-level fluctuations during

the aquifer test. The most significant effect was

from pumping of well NWWA–22 and the industrial-supply well at the BAE Systems facility. Water

levels in monitor wells RI–4S, RI–4D, RI–5S, and

RI–5D (figs. 13 and 14) were affected by pumping

from NWWA–22 at a rate of about 230 gal/min.

NWWA–22 was pumped twice—about 8 hours on

the evening of June 25 and 4 hours on the morning

of June 26 (table 3). At the start of the aquifer test

on June 20, water levels in the deep and shallow

RI–4 and RI–5 well clusters were recovering from

the pumping of NWWA–22 on June 19.

Water levels in monitor wells W–16, RW–1,

RW–2, and RW–3 were affected by pumping from

the BAE Systems supply well during the aquifer

test (fig. 16). The cyclic pumping of the supply well

is evident on hydrographs of RW–2 and RW–3,

indicating a good hydraulic connection between

these monitor wells and the supply well. The

hydrographs indicate that the supply well was in

operation for the first 100 minutes of the aquifer

test beginning at 9 a.m. on June 20, 2000. Monitor

wells RW–1 and W–16 are not as well connected

hydraulically to the supply well, so water levels in

these wells do not fluctuate with the cyclic pumping but they are affected gradually by changes in

the supply-well pumping. When the BAE Systems

supply well was shut off, water levels in all four

monitor wells recovered from 0.2 to about 2 ft for

the next 100 to 300 minutes. Thus, water levels

during the aquifer test were affected by pumping

from the BAE Systems supply well for about the

first 200 to 400 minutes of the test.

Table 3. Pumping of NWWA–22 from June 17–30,

2000, Colmar study area, Bucks and Montgomery

Counties, Pennsylvania

[Data from Kevin McLaughlin, North Wales Water

Authority, oral commun., 2001]

Time pumping began Time pumping ended

6/17/00 at 12:45 p.m. 6/17/00 at 10 p.m.

6/19/00 at 8:30 a.m. 6/19/00 at 1:30 p.m.

6/25/00 at 6:45 p.m. 6/26/00 at 2:15 a.m.

6/26/00 at 8:15 p.m. 6/27/00 at 12:45 a.m.

6/30/00 at 8:15 p.m. 6/30/00 at 9:45 p.m.

Figure 16. Effect of pumping from BAE Systems

supply well on water levels in monitor wells W–16,

RW–1, RW–2, and RW–3 during the aquifer test of

supply well NP–21, Colmar study area, Bucks and

Montgomery Counties, Pennsylvania.
29

Pumping from an unknown source caused

water levels in wells NP–12A, NP–12B, and

NP–12C to decline from July 1–3, 2000, as shown

on the hydrographs of those wells in figure 14. The

pumping did not affect water levels during the

aquifer test of supply well NP–21, which ended on

June 27, 2000.

Drawdown Caused by Pumping NP–21

Pumping of well NP–21 caused measurable

drawdown in the pumped well and 21 observation

wells monitored during the aquifer test. Drawdown at the end of 7 days of pumping from well

NP–21 was 200 ft in the pumped well and 95 ft in

well NP–87 and ranged from 0 to 6.8 ft in the other

observation wells (fig. 12 and table 2). Drawdown

values were adjusted for pre-test trends, changes

in atmos-pheric pressure, and effects of pumping

from NWWA–22 and the BAE supply well as summarized in table 2. Water-level hydrographs for

wells where drawdown was observed are shown

in figure 13.

Most of the observation wells monitored

during the aquifer test are shown in cross section

on figure 17. The wells are projected along the

strike of the Lockatong Formation to a section

through pumped well NP–21 shown in figure 12.

Figure 17 shows that most observation wells are

monitoring the shallow part of the aquifer less

than 150 ft deep. Only well NP–12C, drilled to

620 ft below land surface, penetrates rocks at

depths below 400 ft where a considerable wateryielding zone was encountered in NP–21. However, well NP–12C is far enough downdip from

NP–21 that the water-yielding zone is probably

stratigraphically below the bottom of NP–12C.

The water-level response to pumping from

well NP–21 was obvious in some observation

wells, but in many others, the response was subtle

and is certainly open to various interpretations. In

observation wells NP–87, RI–5D, RI–5S, RI–7D,

RI–7S, RI–20D, RI–20S, RW–1, RW–2, RW–3, W–1,

W–12, W–13, W–14, W–16, and W–17, drawdown

from NP–21 pumping was clearly indicated (fig. 2).

However, for observation wells W–6, W–9, RI–19D,

and RI–19S, the water-level decline measured during the 7-day pumping period could be interpreted

as the result of NP–21 pumping or the natural

ground-water recession because the water-level

decline was not large and the recovery was not

clearly evident after the pumping stopped. The

water-level response in well A–13 is especially difficult to explain because the water level appeared

to clearly decline in response to pumping of NP–21

but did not recover at all after pumping stopped.

The 0.98 in. of precipitation the night of June 21

also complicates the interpretation because

recharge caused water levels to rise in some wells.

The drawdown in wells and the projected

outcrop of rocks intercepted by supply well NP–21

are shown in figure 17. The outcrop band was projected assuming an average dip of 20° NW. Waterlevel measurements during the aquifer test

showed that, except at RI–7, ground-water levels

declined almost exclusively in observation wells

completed within or updip of the geologic units

penetrated by the pumped well NP–21 (figs. 12

and 17). Because the geologic units become deeper

to the northwest, shallow wells updip to the southeast of the pumped well showed more drawdown

than observation wells an equal distance but

downdip from the pumping well. Although this

observation fits well with the conceptual model of

the hydrogeologic framework, it must be acknowledged that few observation wells were available

for monitoring immediately downdip (north and

northwest) of pumped well NP–21. Regardless, a

good hydraulic connection (2.4 to 4.0 ft of drawdown) was shown for observation wells updip in

the southern area of contamination near the BAE

Systems facility. The northern area of contamination in the vicinity of the former Stabilus facility

did not appear to be within the area of influence of

the supply well NP–21 after 7 days of pumping at

257 gal/min.
30

Figure 17. Cross section showing pumped well NP–21 and observation wells monitored during the June 2000 aquifer test, Colmar st

Montgomery Counties, Pennsylvania. (Blue lines indicate wells where drawdown was observed.)
31

NP–87 Test of May 7–10, 2002

Well NP–87 was pumped for 72 hours at an

average rate of 402 gal/min beginning on 11 a.m.

on May 7, 2002. The aquifer test was conducted to

evaluate the drawdown caused by pumping at a

greater rate than was possible during the previous

test of NP–21 in June 2000. The test also allowed

monitoring of drawdown in several deep observation wells (RI–27D, RW–4D, RW–5D, and RW–6D)

in or near the northern area of contamination that

were drilled subsequent to the NP–21 aquifer test.

Computed drawdown values for shallow and deep

wells monitored during the May 2002 aquifer test

of NP–87 are shown in table 4 and figures 18

and 19. The wells are shown in a cross section

trending perpendicular to the strike of beds in

figure 20.

Water levels were measured in the pumped

well NP–87 and 35 observation wells listed in

table 4. Stream stage was measured with a pressure transducer installed in the western tributary

of the West Branch Neshaminy Creek in the vicinity of NP–87. Water levels were monitored by the

use of pressure transducers except at the BAE Systems supply well where the pressure gauge

attached to an air line was read periodically. Water

levels were checked periodically during the test

with electric tapes, and it was found that the transducer readings needed adjustment to match the

manual measurements in some wells. Most of the

manual measurements differed from the transducer readings by a constant amount for each well.

Most likely, the transducers were bumped during

the many visits by USGS and other personnel to

make water-level measurements and collect samples prior to the start of the aquifer test. Water levels were monitored for 20 days prior to the start of

the aquifer test and at least 6 days after pumping

stopped. Hydrographs of water levels monitored

before, during, and after the aquifer test are shown

in figure 21.

Climatic Conditions and

Resultant Water-Level Trends

The period prior to the test was wet for several weeks, which helped ease the drought conditions that were prevalent in southeastern Pennsylvania during 2002. Ground-water levels were

rising but had leveled off at most observation wells

before the test. During the 3-day aquifer test,

streamflow in the unnamed tributary of the West

Branch Neshaminy Creek declined, and soil dried

out considerably until the rainstorm that began

about 6 a.m. on May 9.

Precipitation during test

As with the June 2000 aquifer test of NP–21,

a rainstorm occurred during the May 2002 test of

NP–87. Precipitation of about 0.7 in. fell mostly

between 6 to 10 a.m. on May 9, 2002. The rain

began about 43 hours (2,580 minutes) into the aquifer test. Water levels in many of the observation

wells rose in response to the precipitation, and

stream stage increased quickly. Measured waterlevel rises ranged from 0 to about 3.0 ft, but most

were only several tenths of a foot. As with the

NP–21 test, by the end of the 3-day pumping

period, the effect of the precipitation was probably

fairly negligible on the total drawdown.
32

Table 4. Summary of observations from wells monitored during aquifer test at well NP–87, Colmar study area, Bucks and Montgomery 

[—, not determined; <, less than; >, greater than; ?, reported drawdown may be partially or entirely caused by natural ground-water rec

Local well

name

Distance from

well NP-87,

in feet

Drawdown

after

3 days of

pumping,

in feet

Direction of

pre-test

water-level

trend

Water-level

rise caused

by rainstorm

during test,

in feet

Comments

WELLS WITH MEASURABLE DRAWDOWN DURING AQUIFER TEST OF NP–87

A–10 1,572 7.0 None 0.7 BAE recovery well; not in operation during pumping of NP–87; responds 

receive direct runoff from parking lot because of flush-mount constru

BAE Supply 1,693 11.5 — — Monitored water level with pressure gage on air line.MW–1 728 3.1 Down .2 Well drilled after NP–21 aquifer test was conducted; shows apparent rechMW–2 269 6.3 None .3 Well drilled after NP–21 aquifer test was conducted; shows apparent rechMW–3 654 1.6 None .3 Well drilled after NP–21 aquifer test was conducted.MW–5 679 3.0 None .2 Well drilled after NP–21 aquifer test was conducted; shows apparent rechNP–21 40 225 None 0 Water cascades during pumping of NP–87.NP–87 0 343 None 0 Pumped well. Water cascades from break in casing at 83 feet below land sRI–19D 1,724 .8 None <.1 Drawdown from pumping is evident, but recovery incomplete; barometriRI–19S 1,724 .8 None .1 Drawdown from pumping is evident, but recovery incomplete.RI–20D 1,059 5.5 None .1 Barometric efficiency 70 percent.RI–20S 1,059 3.0 None .1 Barometric efficiency 50 percent.RI–27D 2,355 1.6 None .1 Well drilled after NP–21 aquifer test was conducted; shows effects of earth RI–27S 2,355 .9 ? None .1 Well drilled after NP–21 aquifer test was conducted. Incomplete water-levRW–1 1,432 4.2 Up <.1 Barometric efficiency 50 percent.RW–3 1,459 12.0 Up 0 Shows strong response to pumping from BAE Systems supply well.RW–4D 1,665 2.0 None .1 Well drilled after NP–21 aquifer test was conducted. Barometric efficiency RW–4I 1,667 .9 None .1 Well drilled after NP–21 aquifer test was conducted. Water runs into casin

water-level recovery after pumping ceased. Barometric efficiency ass

RW–4S 1,695 .6 ? Down .1 Well drilled after NP–21 aquifer test was conducted. No water-level recov

ceased. Barometric efficiency assumed 50 percent.

RW–5D 2,024 1.2 None .2 Well drilled after NP–21 aquifer test was conducted. Barometric efficiency RW–5I 2,025 1.0 None .6 Well drilled after NP–21 aquifer test was conducted. Surface water pours i

Barometric efficiency assumed 50 percent.

RW–5S 2,028 1.0 None .1 Well drilled after NP–21 aquifer test was conducted. Barometric efficiency RW–6D 2,091 3.7 None .1 Well drilled after NP–21 aquifer test was conducted.RW–6I 2,086 .8 None 0 Well drilled after NP–21 aquifer test was conducted. Barometric efficiency RW–6S 2,093 .6? None <.1 Well drilled after NP–21 aquifer test was conducted. No water-level recov

ceased. Barometric efficiency assumed 20 percent.

W–3 947 1.4 None <.1 Barometric efficiency assumed 70 percent.
33

W–4 1,680 .6 Down 0.1 No water-level recovery apparent after pumping ceased. Barometric efficiW–5 2,102 1.0 None .1 Slow water-level recovery after pumping ceased. Barometric efficiency assW–6 2,297 .5 Down .2 Slow water-level recovery after NP–21 and NP–87 pumping ceased. Barom

50 percent.

W–8 1,520 .6 ? Down .1 Slow water-level recovery after pumping ceased. Barometric efficiency assW–9 1,343 .9 None <.1 Drawdown of 0.9 ft incorporates effect of pre-test water-level recession; 1.

for recession; barometric efficiency 50 percent.

W–13 96 6.0 None .1 Manual measurements only from June 23 to July 6, 2000; cause unknown f

200 to 500 minutes into test; barometric efficiency 50 percent.

W–16 2,018 5.2 Up 0 Barometric efficiency 50 percent.W–17 275 6.0 None .2 Shows apparent recharge boundary.

WELLS WITH NO MEASURABLE DRAWDOWN DURING THE AQUIFER TEST OF NP–87

A–3 1,657 0 Down 3.0 Shows large response to precipitation.A–13 1,469 0 None <.1 Barometric efficiency 50 percent.

Table 4. Summary of observations from wells monitored during aquifer test at well NP–87, Colmar study area, Bucks and Montgomery 

Continued

Local well

name

Distance from

well NP-87,

in feet

Drawdown

after

3 days of

pumping,

in feet

Direction of

pre-test

water-level

trend

Water-level

rise caused

by rainstorm

during test,

in feet

Comments
34

Figure 18. Drawdown in shallow wells (less than 100 feet deep) after 3 days of pumping from well

NP–87 during the May 2002 aquifer test, Colmar study area, Bucks and Montgomery Counties,

Pennsylvania.
35

Figure 19. Drawdown in deep wells (at least 200 feet) after 3 days of pumping from well NP–87

during the May 2002 aquifer test, Colmar study area, Bucks and Montgomery Counties,

Pennsylvania.
36

Figure 20. Cross section showing pumped well NP–87 and observation wells monitored during the May 2002 aquifer test, Colmar stuMontgomery Counties, Pennsylvania. (Blue lines indicate wells where drawdown was observed.)
37

Figure 21. Water levels in wells where drawdown was monitored during the May 2002 aquifer test at well NP–87, Colmar stuand Montgomery Counties, Pennsylvania.
38

Figure 21. Water levels in wells where drawdown was monitored during the May 2002 aquifer test at well NP–87, Colmar Bucks and Montgomery Counties, Pennsylvania—Continued.
39

Figure 21. Water levels in wells where drawdown was monitored during the May 2002 aquifer test at well NP–87, Colmar Bucks and Montgomery Counties, Pennsylvania—Continued.
40

Figure 21. Water levels in wells where drawdown was monitored during the May 2002 aquifer test at well NP–87, Colmar studand Montgomery Counties, Pennsylvania—Continued.
41

Changes in atmospheric pressure

During the 3-day period when NP–87 was

pumping, the atmospheric pressure changed a

maximum of about 0.25 ft of water (fig. 22). Thus,

as in the NP–21 test, only in wells where drawdown was small (generally less than 1 ft) did

adjustments for atmospheric pressure make much

difference. Some of the water-level rise measured

in wells following the precipitation on May 9 could

have been attributed to the 0.2-ft decline in atmospheric pressure associated with the storm. The

major effect of atmospheric pressure was after the

pumping stopped on May 10. The rise in pressure

from about 33.8 to 34.2 ft after the pump was shut

off at 11 a.m on May 10 may have caused water

levels to continue to decline instead of begin to

recover. A hydrograph of RI–27S shows both the

measured drawdown and drawdown that was

adjusted assuming a 70-percent barometric efficiency (fig. 23). The adjustment makes it more

apparent that the water level began to recover

shortly after the pumping of NP–87 ceased. The

adjusted water-level data show partial recovery of

Figure 22. Atmospheric pressure during the May 2002 aquifer test of well NP–87,

Colmar study area, Bucks and Montgomery Counties, Pennsylvania.

Figure 23. Drawdown in observation well RI–27S showing adjustments for changes in

atmospheric pressure during and after the May 2002 aquifer test at NP–87, Colmar study

area, Bucks and Montgomery Counties, Pennsylvania.
42

about 0.2 ft from the time that pumping stopped

(4,320 minutes) to about 7,000 minutes (fig. 23).

Water levels did not return to pre-test levels until

after the area received precipitation on May 13

(about 9,000 minutes after the pumping began).

Pre-test trends

Pre-test water-level measurements indicated

that 9 of the 35 observation wells showed a clearly

defined trend (table 4). The slight upward trend in

wells RW–1 and W–16 was caused by cessation of

pumping from the nearby BAE Systems remediation and supply wells about 24 hours prior to the

start of the aquifer test on May 7, 2002. The slight

downward pre-test trend in water levels measured

in A–3, A–13, MW–1, RW–4S, W–4, W–6, and W–8

is probably the natural ground-water recession. An

adjustment of drawdown for a pre-test water-level

trend was not made for any of the wells because

the trends were so slight.

Withdrawals from other wells

Significant drawdown caused by withdrawals from wells other than NP–87 was not observed

during the May 2002 aquifer test. A slight effect

was the upward pre-test water-level trend at wells

RW–1 and W–16 attributed to cessation of pumping from the BAE Systems supply and remediation

wells (fig. 21). The BAE Systems supply well was

off from May 6 at 9 a.m. to May 14 at 9:05 a.m.

except for about twice a day when it pumped for

less than 5 minutes to refill a pressure tank. Those

small pumping episodes show up on the hydrograph of RW–3 (fig. 21) but are not a significant

influence compared to the drawdown caused by

the pumping of NP–87. The BAE Systems recovery

well (A–10) was off from May 6 at 10:40 a.m. to

May 13 at 4:05 p.m., and NWWA–22 was not in

operation from May 3 to 13, 2002.

Drawdown from Pumping NP–87

Pumping of well NP–87 at 402 gal/min for

3 days caused measurable drawdown in all 34 of

the observation wells monitored during the aquifer

test except A–3 and A–13. Drawdown at the end of

3 days of pumping from well NP–87 was 343 ft in

the pumped well and 225 ft in NP–21. Drawdown

in the other observation wells ranged from about

0.5 to 12.0 ft. Drawdown is shown for shallow

wells (less than 100 ft) and deep wells (at least

200 ft) in figures 18 and 19. At the same location,

drawdown is greater in deep wells than shallow

wells because the major water-yielding zone was

encountered at 400 ft below land surface in NP–21

and NP–87.

The test of NP–87 confirmed the general pattern of drawdown observed during the test of well

NP–21; however, water-level declines were

observed further to the northeast near the former

Stabilus facility because of the greater pumping

rate and availability of additional observation

wells (clusters at RW–4, RW–5, RW–6, and nested

wells at RI–27). Outside of the immediate area of

the NP–87 well field, the greatest drawdowns

(7-12 ft) were observed near the BAE Systems facility in wells A–10, RW–3, and the BAE Systems supply well. The drawdown confirms the existence of

a good hydraulic connection between NP–87 and

wells completed within the deepest part of the geologic units penetrated by the pumped well. These

units correspond to water-yielding zones encountered below 400 ft in NP–21 and NP–87 (fig. 20).

Near the former Stabilus facility, drawdown

was indicated most clearly in deep wells RI–27D,

RW–4D, RW–5D, and RW–6D. The greatest drawdown in that area (3.7 ft) was measured in the

deepest well in the RW–6 cluster, which is the well

farthest updip and completed most closely to the

deep water-producing zone penetrated by well

NP–87 as shown in figure 20. The shallow wells

less than 50 ft deep in the area of the former Stabilus facility probably showed some drawdown

from the pumping of NP–87, but the response was

indicated less clearly than for the deep wells. After

pumping ceased from NP–87, the water-level

recovery in shallow wells RI–27S, RW–4S, RW–5S,

RW–6S, W–3, W–4, W–5, W–6, W–8, and W–9 was

slow. The small apparent drawdown and poor

recovery indicate that the pumping has less effect

on water levels and direction of ground-water flow

in the shallow part of the aquifer than the deep

part, at least for the duration of the pumping test.

Streamflow in the tributary of the West

Branch Neshaminy Creek near NP–87 appeared to

be affected by pumping from NP–87 during the

aquifer test. Measurements of stream stage show a

downward deflection of the hydrograph recession

slope corresponding to the start of pumping from

NP–87 at 11 a.m. on May 7, 2002 (fig. 24). The

change in recession slope could be interpreted as a

capture of ground-water discharge to the stream or

induced infiltration of stream water caused by the

pumping. Although streamflow measurements

were not sufficient to construct a relation between

stage and streamflow, miscellaneous measure-
43

ments showed that streamflow decreased during

the aquifer test from 380 to 170 gal/min between

May 7 at 1 p.m. to May 8 at 7 p.m. However, it is

not clear how much of the decrease was caused by

the pumping and how much was part of the natural streamflow recession because precipitation during the morning of May 9 caused the stream to rise,

masking any increase in streamflow that might

have been noted when the pumping ceased on

May 10. The hydrograph from nearby USGS gaging station Little Neshaminy Creek at Valley Road

(station 01464907), located about 5 mi southeast of

the Colmar area, shows a similar recession prior to

the start of pumping (fig. 24) but does not show a

downward deflection of the recession slope on

May 7 at 11 a.m., which provides evidence that

streamflow in the tributary near NP–87 may have

been affected by pumping.

Figure 24. Stream stage in a tributary to the West Branch Neshaminy Creek near well NP–87 and in

the Little Neshaminy Creek at Valley Road, monitored during the May 2002 aquifer test at well NP-87,

Colmar study area, Bucks and Montgomery Counties, Pennsylvania.
44

SIMULATION OF GROUND-WATER FLOW

A numerical model was developed to simulate ground-water flow in the Colmar area. The

model was constructed with dipping layers to

characterize the dipping geologic units in the

vicinity of wells NP–21 and NP–87. Specifically, the

model was used to evaluate (1) ground-water-flow

paths under current (2000) conditions and (2) flow

paths and areas contributing recharge to wells for

hypothetical pumping scenarios that include withdrawals from well NP–21 or NP–87.

Model simulations were conducted under

steady-state and transient conditions. Simulations

to estimate ground-water-flow paths and contributing areas were conducted under steady-state

conditions. Steady-state simulations give results

that represent the average position of flow paths

and contributing areas on the basis of the values of

average annual ground-water recharge and pumping assigned to the model. Changes caused by seasonal variations in recharge or operational variability in pumping were not simulated. Transient

model runs simulated the drawdown caused by

pumping during the aquifer test of NP–87, which

helped guide adjustments of hydraulic properties

used in the model, but transient simulations of

flow paths and contributing areas were not made.

Description of the Model

A 3-dimensional, finite-difference model

was used to simulate steady-state and transient

ground-water flow for a region of about 12 mi2,

which is much larger than the area of interest in the

Colmar area (fig. 25). This large area was simulated

to assure that significant effects from pumping in

the Colmar area would not reach model boundaries that define the limits of the model domain.

The numerical model constructed for the Colmar

area was based partially on the MODFLOW model

developed by Senior and Goode (1999), which simulated regional ground-water flow in the Lansdale

area. The Lansdale regional model included the

headwaters of the West Branch Neshaminy Creek

and extended into the Colmar area (fig. 25). Withdrawals from NP–21 were included in that model

on its northeastern boundary. For this study of the

Colmar area, the regional model of Senior and

Goode (1999) was extended to the northeast to

include the area of interest in this study; the area

outside of the Neshaminy Creek watershed was

excluded. The regional model was further modified by adding dipping layers to incorporate more

detail about the local-scale geologic structure in the

vicinity of wells NP–21 and NP–87.

The 3-dimensional, finite-difference computer code MODFLOW–2000 (Harbaugh and others, 2000) was used with the particle-tracking

program MODPATH (Pollock, 1994) to compute

ground-water-flow paths and contributing areas.

A graphical user interface linked to Argus Numerical Environments was used for pre- and post-processing of data (Winston, 2000).

Finite-Difference Grid and Model Structure

The modeled area was divided into a finitedifference grid with 80 rows and 123 columns

(fig. 25). The horizontal dimensions of the cells

were varied so that small cells were in the Colmar

area, which is the focus of the study. The smallest

cells, 66 ft by 66 ft, surround wells NP–21 and

NP–87 where the largest effects of pumping were

expected; maximum cell size was 328 ft by 328 ft in

horizontal dimension. The model grid was oriented N. 62° E. to align with the strike of beds in the

Colmar area as determined by orientation of bedding-plane partings interpreted from acoustic-televiewer surveys (Bird and Conger, 2002, p. 24). The

orientation of the grid along bedding strike is

important because the assumed principle direction

of horizontal hydraulic conductivity is along the

strike of beds, which was simulated as 11 times

greater than across strike in the Lansdale-area

model of Senior and Goode (1999).

The bottom of the model is set at 736 ft

below land surface everywhere, which is assumed

to be the depth below which very little groundwater movement occurs. The depth at which flow

becomes negligible is not well known, but Senior

and Goode (1999) used 696 ft for the base of their

model. In this study, the base of the model was initially set at 696 ft below land surface, but 40 ft was

added to the thickness of the uppermost model

layer during the process of model adjustment,

which lowered the bottom of the model to 736 ft

everywhere.

The vertical structure of the model consists

of eight layers as shown in figure 26. Layers 1, 2,

and 8 are horizontal layers and layers 3, 4, 5, 6, and

7 are dipping layers. Layer 1 represents the weathered part of the aquifer that drapes over the area

and layers 2 and 8 represent the regional extent of

the aquifer. Layers 3 through 7 represent dipping

geologic units in the vicinity of wells NP–21 and
45

Figure 25. Modeled area showing finite-difference grid, domain boundary, stream cells, and location of

Lansdale-area model of Senior and Goode (1999), Bucks and Montgomery Counties, Pennsylvania.
46

NP–87. To simulate the dipping units, the model

layers were assigned three zones—active zone,

inactive zone, and pseudo-active zone. The active

zone is the part of the layer representing the geologic unit where it is present beneath layer 1 until

its downdip extent reaches the base of the model

(736 ft). The inactive zone of the layer represents

the geologic unit where it extends below 736 ft.

The pseudo-active zone represents the geologic

unit where it extends above the base of layer 1.

Pseudo-active zones are present in layers 2-7. The

geologic unit is not physically present where the

pseudo-active zone exists, but the unit needs to be

included in the model so that continuity exists

between layer 1 and deeper layers; thus, the

pseudo-active zone consists of thin cells (0.01 ft

thick) that are assigned a horizontal hydraulic conductivity of 0.1 ft/d and a vertical hydraulic conductivity of 100 ft/d, allowing water to pass

vertically from model layer 1 to the model layer

representing the geologic unit physically present

beneath layer 1.

Layer 1 is 80 ft thick and represents the

uppermost weathered part of the aquifer. The top

of layer 1 is land surface and the bottom is

80 ft below land surface everywhere. Layer 1 is

simulated as a confined aquifer; thus, transmissivity does not vary as the saturated thickness of the

layer changes. Because most of the water-yielding

fractures are in the lower 40 ft of the layer, dewatering of the upper part of the layer probably does

not affect transmissivity greatly. In addition, the

clayey regolith in the upper 40 ft of the layer partially confines water in the weathered zone.

Layers 2 and 8 are horizontal layers that

represent the regional aquifer. These layers have a

maximum thickness of 656 ft but become thinner

where they truncate into the sequence of dipping

layers (layers 3 through 7).

Layers 3 through 7 are dipping layers that

represent the dipping structure intercepting wells

NP–87 and NP–21. Dipping layers 4 and 6 are a

maximum of 100 ft thick and represent the two

major water-yielding zones intercepted by NP–21

and NP–87 (fig. 11). Layers 3, 5, and 7 each have a

Figure 26. Schematic cross section along the model column through well NP–87 showing model

layers and thickness.

Layer 1 (80)

Layer 2

Layer 4 (100)

Layer 8

WELL

NP-87

PSEUDO-ACTIVE ZONE—Layers 2-7 are 0.01-foot thick in this area.

INACTIVE ZONE—Layers 3-8 are 0.1-foot thick in this area; hydraulic conductivity is 0.

1 (80) LAYER NUMBER AND THICKNESS—Number in bold is model layer number; maximum

 thickness, in feet, is in parentheses.

EXPLANATION

 (656) (656)

ACTIVE ZONE—All of layer 1 and parts of layers 2-8.

736

80

DEPTH

 IN

 FEET

0

Layer 3 (250)

Layer 5 (250)

Layer 7 (250)

Layer 6 (100)

LAND SURFACE NW SE
47

maximum thickness of 250 ft. The top and bottom

of each dipping layer is tied to the structure-contour surface shown in figure 27. This surface is an

estimate of the altitude of an arbitrary horizon

within the sequence of rocks that strike 35 to

70° NE. and dip 10 to 30° NW. The top of the horizon was established to pass through the top of the

open interval in NP–21, 50 ft below land surface.

The horizon represents the top of layer 4 in the

model. Where the horizon is projected above land

surface, layer 4 does not exist but the elevation of

the horizon is used to compute the top and bottom

elevations of lower layers 3 through 7.

Model Boundaries

The lateral extent of the model area is

defined by an impermeable boundary represented

in each layer by inactive cells or limit of the model

grid (fig. 25). The southern and western boundaries of the model extend to the surface-water

divide of the West Branch Neshaminy Creek. The

northern and most of the eastern boundary follow

the main stem or tributaries of Neshaminy Creek.

All of these boundaries are far removed from the

Colmar area to assure that simulated pumping of

NP–21 or NP–87 has little effect on ground-water

levels near model boundaries and that simulated

contributing areas are totally within the model

area. The base of the model, 736 ft below land surface everywhere, is simulated as an impermeable

boundary.

Streams

Streams are simulated in the model by use of

the stream package (Prudic, 1989) as modified for

MODFLOW–2000. The stream package allows simulated streams to gain or lose water and accounts

for the flow in each stream cell so that losses cannot exceed the simulated base flow of the stream.

The altitude of the stream bottom was assigned

from altitudes estimated from 7.5-minute topographic maps. Stream stage was assigned an altitude 1 ft higher than the streambed and thickness

of the streambed was assumed to be 1 ft. Stream

width was assigned a value that increased from 10

to 30 ft depending on the position of the stream in

the watershed. First-order and second-order headwater streams are 10 ft wide, mid-basin streams are

20 ft wide, and near the confluence of the West

Branch and main branch Neshaminy Creek,

streams are 30 ft wide. The hydraulic conductivity

of the streambed was assigned a value of 0.16 ft/d.

This is the value of the upper 40 ft of regolith used

in the Lansdale area regional model. It represents

the clayey and relatively impermeable weathered

rock near land surface. The location of stream cells

is shown in figure 25.

Recharge

Recharge to the model is added as a constant

flux to the top of each active cell in layer 1. An

average, annual recharge rate of 8.3 in/yr estimated by Senior and Goode (1999) for the Lansdale

area also was used in this model of the Colmar

area. Recharge was not varied spatially according

to differences in land use or topography.

Wells

Ground-water withdrawals from wells are

represented by a constant flux of water removed

from the model using the well package in

MODFLOW–2000. Withdrawals from each well

were assigned to the model layer representing the

geologic unit yielding water to the well. In cases

where a well receives water from more than one

model layer, the total withdrawal is apportioned to

each layer according to estimates from drillers

records, flowmeter surveys, or isolated-interval

testing. Where yield information is not available,

the apportionment was arbitrarily divided equally

between layers. Location and pumping rates of

wells used in model simulations are listed in later

sections of this report where the results of simulations are discussed.

Aquifer Properties

Initial estimates of aquifer properties used in

the model were taken from the regional modeling

study of the Lansdale area by Senior and Goode

(1999); the values subsequently were changed during the model-adjustment procedure described in

the following section. In the Lansdale area model,

the hydraulic conductivity along model rows was

5.35 ft/d and the Lockatong Formation was

1.12 ft/d. The hydraulic conductivity of both formations was 11 times greater along rows than

across rows. A hydraulic-conductivity value of

0.16 ft/d was used to represent a relatively impermeable weathered zone in the upper 40 ft of both

Lockatong and Brunswick Formations. Specific

storage was assumed to be 0.000001 per foot of

aquifer thickness. Initial values for the Lansdale

area model are listed in table 5.
48

Figure 27. Structure-contour map of northwest-dipping surface representing top of model layer 4, Colmar

study area, Bucks and Montgomery Counties, Pennsylvania.
49

Model Adjustments

Model adjustment is a process in which

aquifer properties are changed until the model

adequately simulates the measured response of the

physical hydrologic system. This process commonly is called model calibration, but it is not

intended to establish with certainty the accuracy or

precision of model predictions. Rather, the process

is used to refine concepts until an adjusted model

is constructed that can reproduce observed conditions in the real hydrologic system. The model of

the Colmar area was adjusted by matching as

closely as possible the water levels in monitoring

wells during (1) average, non-pumping, steadystate conditions and (2) transient pumping of well

NP–87 during the May 2002 aquifer test.

Method and Data Used for Model Adjustments

The aquifer properties in the model were

adjusted by use of the parameter-estimation program that is integrated into MODFLOW–2000 (Hill

and others, 2000). The parameter-estimation program adjusts aquifer properties in the model to

optimum values that minimize the difference

between simulated and measured steady-state

water levels and transient drawdown in wells.

Water-level altitudes from 54 wells representing

the average, non-pumping, steady-state conditions

and drawdown measured in 30 wells during the

3-day aquifer test of NP–87 were used by the

parameter-estimation program to adjust aquifer

properties. Drawdown data from the NP–87 aquifer test were used (instead of the NP–21 aquifer

test) because (1) the greater pumping rate during

the NP–87 test made it easier to differentiate drawdown from natural trends, (2) drawdown caused

by pumping from other wells in the area was not

significant, (3) rainfall during the NP–87 test

caused less ground-water recharge than during the

NP–21 test, and (4) the pumping rate of NP–87 was

monitored more closely than that of NP–21.

Water levels used to adjust the model for

steady-state non-pumping conditions need to represent the average annual position of the water

table to the best extent possible. Water-level measurements made prior to the aquifer test of well

NP–21 during June 2000 probably are fairly representative of average conditions based on long-term

water-level fluctuations at the USGS observation

well MG–917 in Hatfield, about 4 mi northwest of

Table 5. Initial estimates of aquifer properties for the model of ground-water flow in the Colmar study area,

Bucks and Montgomery Counties, Pennsylvania

Model

Layer

Geologic

unit

Hydraulic

conductivity

in direction of

strike of beds

(feet per day)

Geometric

mean hydraulic

conductivity

(feet per day)

Horizontalanisotropy

(ratio of hydraulic

conductivity along

model columns to

along rows)

Vertical

anisotropy

(ratio of hydraulic

conductivity in

the horizontal

direction to the

value in the

vertical direction)

Specific

storage

(1/foot)

1 Weathered

Zone

0.16 0.16 1 1 0.000001

2 Brunswick

Formation

5.35 1.16 .09 1 .000001

3 Brunswick

Formation

5.35 1.16 .09 1 .000001

4 Brunswick

Formation

5.35 1.16 .09 1 .000001

5 Lockatong

Formation

1.12 .34 .09 1 .000001

6 Lockatong

Formation

1.12 .34 .09 1 .000001

7 Lockatong

Formation

1.12 .34 .09 1 .000001

8 Lockatong

Formation

1.12 .34 .09 1 .000001
50

Colmar (fig. 28). The water level in MG–917 during

the period of the June 2000 aquifer test was near

the 3-year median water level for that well; the

water level during the May 2002 test was about 1 ft

higher. Thus, water levels measured prior to the

aquifer test from June 19–20, 2000, were used to

calibrate the model under steady-state nonpumping conditions.

Some wells monitored during the May 2002

aquifer test of NP–87 were not drilled until after

the June 2000 test was conducted, so average,

steady-state water levels were estimated for those

wells on the basis of the water level measured

prior to the May 2002 test and an adjustment factor, determined by comparing water levels in other

wells that were monitored during both periods.

Water levels measured in May 2002 in the

MW–series wells were adjusted by lowering them

0.8 ft on the basis of the water levels measured in

nearby wells W–13 and W–17 during both periods.

Water levels in the RW–4, RW–5, and RW–6 well

clusters and RI–27 nested well pair measured in

May 2002 (but not in June 2000) were lowered 1.4 ft

on the basis of the average difference in 14 wells

monitored during both periods. The final set of

water levels used for model adjustments is shown

in table 6. All values, measured and adjusted, were

considered to be known with the same degree of

accuracy for purposes of parameter estimation.

The drawdown values used for model

adjustment are listed in table 6. Drawdown values

from 30 monitor wells were determined after 1, 4,

10, 24, and 43 hours of pumping well NP–87 from

May 7–8, 2002. Drawdown values from 43 to

72 hours of pumping were affected to varying

degrees by the rainstorm and were not used for

calibration. Drawdown was computed after adjusting the water-level measurements for transducer

drift and changes in atmospheric pressure.

Aquifer properties that were adjusted were

the horizontal hydraulic conductivity, horizontal

anisotropy, vertical anisotropy, and specific storage

of the active parts of all model layers. The inactive

parts of model layers 3 through 8 at the base of the

model were assigned a hydraulic conductivity of 0;

thus, cells in the inactive parts are not used in

model computations.

Figure 28. Water-level fluctuations, January 2000 through July 2002 in U.S.

Geological Survey observation well MG–917, 4 miles northwest of Colmar,

Pennsylvania.
51

Table 6. Steady-state water-level altitude and drawdown used for calibration of the ground-water-flow

model in the Colmar area, Bucks and Montgomery Counties, Pennsylvania

[—, not measured; NGVD 29, National Geodetic Vertical Datum of 1929]

Well Steady-state water-level altitude

(feet above NGVD 29)

Drawdown in feet, after pumping from NP–87

1 Hour 4 Hours 10 Hours 24 Hours 43 Hours

A–10 298.27 0.04 1.00 2.97 5.27 6.49

A–18 303.21 — — — — —

BAE Production 298.27 .50 1.20 5.00 7.20 12.50

MW–1 291.48 1.01 1.90 2.33 2.64 3.10

MW–2 285.20 4.00 5.37 5.75 6.13 6.36

MW–3 287.80 .34 .90 1.18 1.45 1.67

MW–5 290.78 .60 1.49 1.99 2.55 2.93

NP–75 260.20 — — — — —

R&B–2 267.90 — — — — —

R&B–3 266.69 — — — — —

RI–1S 263.88 — — — — —

RI–2S 297.25 — — — — —

RI–4S 330.13 — — — — —

RI–5S 291.74 — — — — —

RI–6S 280.75 — — — — —

RI–7S 290.17 — — — — —

RI–8D 273.19 — — — — —

RI–9S 262.04 — — — — —

RI–10D 256.04 — — — — —

RI–11S 267.45 — — — — —

RI–12D 263.29 — — — — —

RI–13D 263.74 — — — — —

RI–15D 259.02 — — — — —

RI–16D 256.96 — — — — —

RI–18D 287.42 — — — — —

RI–19D 290.09 .00 .04 .21 .45 .62

RI–19S 289.93 .00 .05 .20 .43 .62

RI–20D 294.12 .59 1.98 3.11 4.39 5.20

RI–20S 291.72 .28 .84 1.35 2.13 2.65

RI–23 275.02 — — — — —

RI–27D 284.40 .01 .08 .34 .89 1.28

RI–27S 285.66 .01 .00 .09 .33 .61

RW–1 295.10 .04 .61 1.57 2.81 3.76

RW–3 304.88 1.09 3.61 6.24 9.23 11.23

RW–4D 285.06 -.01 .02 .30 1.03 1.69

RW–4I 286.41 .02 .03 .06 .44 .74

RW–4S 286.97 .01 .04 .12 .27 .50

RW–5D 286.36 .01 .02 .18 .61 .95

RW–5I 285.83 .01 .02 .15 .63 1.24

RW–5S 285.95 .01 .02 .12 .42 .69

RW–6D 297.84 .00 .56 1.51 2.64 3.27

RW–6I 288.73 .01 .02 .19 .53 .68

RW–6S 289.95 .01 .02 .10 .27 .43

W–3 295.40 .01 .03 .13 .49 1.04

W–4 286.03 .01 .04 .11 .25 .47

W–5 285.36 .01 .02 .08 .38 .63

W–6 291.77 .01 .05 .14 .25 .36

W–8 286.41 .04 .06 .15 .33 .55

W–9 290.05 .01 .04 .21 .52 .70

W–10 286.91 — — — — —

W–12 290.58 — — — — —

W–13 283.77 — — — — —

W–14 290.16 — — — — —

W–16 303.30 .01 .25 1.09 2.71 4.22
52

Model Parameters

Fifteen parameters were used in

MODFLOW–2000 to represent hydraulic properties of the active parts of layers 1 through 8. The

parameters are described in table 7. Each parameter was assigned a value or it was optimized by the

parameter-estimation process in MODFLOW–

2000. The decision of which parameters to allow

the program to optimize was based on the sensitivity of simulated hydraulic head to changes in each

parameter. The composite-scaled sensitivity of the

15 parameters is shown in figure 29. The composite-scaled sensitivity is computed as defined by

Hill (1998, p. 15). It can be interpreted as the average amount that the simulated head or drawdown

will change, expressed as a percent of the standard

deviation of the measurement error, if the parameter value changes 1 percent. In general, a change in

a parameter with a large composite sensitivity

causes a greater proportional change in simulated

water level than a change in a parameter with

smaller sensitivity. Thus, aquifer properties for five

of the seven most sensitive parameters [K(1),

K(dip), K(4&6), SS(1), SS(23578)] were optimized

(black bars in figure 29) and the properties of the

other parameters were assigned a value (white and

grey bars in figure 29). The value of the parameter

with the largest normalized sensitivity, VANI(dip),

was assigned because it was highly correlated with

parameter K(dip). Because of the correlation, many

combinations of VANI(dip) and K(dip) could produce essentially the same results, so only the value

of K(dip) was optimized.

Final values of aquifer properties used in the

model are shown in table 8. The model-adjustment

procedure used to arrive at the values for aquifer

properties indicated that, given the dipping bed

structure, the two producing intervals (layers 4

and 6) must be separated by geologic units (layers

3, 5, and 7) with much lower hydraulic conductivity than the units supplying water to the well. The

low hydraulic conductivity is needed to prevent

drawdown from the deep-producing interval from

propagating to shallower units where little drawdown was observed. However, the low hydraulic

conductivity units represented by K(dip), which

were layers 3, 5, and 7, restrict simulated regional

ground-water flow so that simulated steady-state

water levels are higher than observed. From the

parameter-estimation process in MODFLOW, the

Table 7. Definition of parameters used to represent aquifer properties in the ground-water-flow model of the Colmar

area, Bucks and Montgomery Counties, Pennsylvania

Parameter

name

Description of model characteristic represented by parameter

Aquifer property Active part of

model layer(s) Geologic unit

K(1) Horizontal hydraulic conductivity along model rows 1 Weathered zone

K(brun) Horizontal hydraulic conductivity along model rows 2 Brunswick Formation

K(dip) Horizontal hydraulic conductivity along model rows Dipping layers

3, 5, and 7

Brunswick and Lockatong Formations

K(4&6) Horizontal hydraulic conductivity along model rows Dipping layers

4 and 6

 Shallow and deep water-producing

zones in wells NP–21 and NP–87

K(lock) Horizontal hydraulic conductivity along model rows 8 Lockatong Formation

HANI(1) Ratio of hydraulic conductivity along model columns

to along rows

1 Weathered zone

HANI(2&8) Ratio of hydraulic conductivity along model columns

to along rows

2 and 8 Brunswick and Lockatong Formations

HANI(dip) Ratio of hydraulic conductivity along model columns

to along rows

Dipping layers

3, 4, 5, 6, and 7

Brunswick and Lockatong Formations

VANI(1) Ratio of horizontal to vertical hydraulic conductivity 1 Weathered zone

VANI(2&8) Ratio of horizontal to vertical hydraulic conductivity 2 and 8 Brunswick and Lockatong Formations

VANI(dip) Ratio of horizontal to vertical hydraulic conductivity Dipping layers

3, 4, 5, 6, and 7

Brunswick and Lockatong Formations

SS(1) Specific storage 1 Weathered zone

SS(23578) Specific storage 2, 3, 5, 7 and 8 Brunswick and Lockatong Formations

SS(4) Specific storage Dipping layer 4 Shallow water-producing zones in

wells NP–21 and NP–87

SS(6) Specific storage Dipping layer 6 Deep water-producing zone in wells

NP–21 and NP–87
53

optimum hydraulic conductivity for the weathered

zone was 7.4 ft/d along model rows (geometric

mean of 3.3 ft/d), which is larger than previously

thought reasonable. Because of this modeling

result, slug tests were conducted at eight shallow

observation wells (MW–1, MW–2, MW–5, RI–2S,

RI–5S, RI–9S, W–3, and W–17) completed to depths

of less than 50 ft to provide an estimate of hydraulic conductivity of the weathered zone. Hydraulic

conductivity from slug tests ranged from 1.6 to

68 ft/d; the median was 7.7 ft/d, and geometric

mean was 8.4 ft/d. This small testing effort shows

that the hydraulic conductivity is highly variable,

but the value of 7.4 ft/d determined from the

parameter optimization procedure may be reasonable.

Table 8. Final aquifer-property values used for predictive simulations with the ground-water-flow model of the Colmar

area, Bucks and Montgomery Counties, Pennsylvania

[Shading pattern indicates method that parameter was assigned; dark shading, value estimated using parameterestimation program; medium shading, value assigned from Lansdale area model of Senior and Goode (1999);

no shading, parameter set at reasonable assumed value]

Layer

Parameter name and value

Hydraulic conductivity in

direction of strike of beds

(feet per day)

Horizontal anisotropy

(ratio of hydraulic

conductivity in the

direction of dip to the value

in the direction of strike)

Vertical anisotropy

(ratio of hydraulic conductivity

in the horizontal direction

to the value in the

vertical direction)

Specific storage

(per foot)

1 K(1)

7.4

HANI(1)

0.2

VANI(1)

1

SS(1)

0.000012

2 K(brun)

5.2

HANI(2&8)

.09

VANI(2&8)

1

SS(2,3,5,7,8)

.0000039

3 K(dip)

.081

HANI(dip)

.2

VANI (dip)

10

SS(2,3,5,7,8)

.0000039

4 K(4&6)

5.3

HANI(dip)

.2

VANI (dip)

10

SS(4)

.00000031

5 K(dip)

.081

HANI(dip)

.2

VANI (dip)

10

SS(2,3,5,7,8)

.0000039

6 K(4&6)

5.3

HANI(dip)

.2

VANI (dip)

10

SS(6)

.00000031

7 K(dip)

.081

HANI(dip)

.2

VANI (dip)

10

SS(2,3,5,7,8)

.0000039

8 K(lock)

1.1

HANI(2&8)

.09

VANI(2&8)

1

SS(2,3,5,7,8)

.0000039

Figure 29. Composite-scaled sensitivity of hydraulic parameters used in the

ground-water-flow model. (See table 7 for definition of parameters.)
54

Adjusted Model Results

The ability of the adjusted model to simulate

steady-state water-level altitude is illustrated in

figures 30A and 31. The model-simulated steadystate water levels are compared to levels measured

in observation wells in figure 30A. The difference

between simulated and observed steady-state

water levels at 54 wells ranged from -12.66 to

14.9 ft. The largest differences were at wells RI–1S,

RI–11S, and W–3 (table 9). The root-mean-square

difference between simulated and observed

steady-state water levels was 5.0 ft.

The relations between simulated and measured drawdown and time since pumping began

show the degree to which the model can simulate

drawdown and the shape of the drawdown curves

at selected observation wells (fig. 30B and 32).

In general, the drawdown curves are reasonably

reproduced except for the MW–series wells and

some of the W–series wells. The MW–series wells

are near NP–87 and tributary streams to West

Branch Neshaminy Creek. The reason for the poor

simulation of measured drawdown may be attributed to an inaccurately simulated hydraulic connection between the stream and aquifer. In

addition, lowering the specific storage of layer 1

produced a better simulation of measured drawdown in the MW–series wells but produced too

much drawdown in other wells.

The ability of the adjusted model to simulate

drawdown at the end of the 3-day aquifer test of

NP–87 is shown in figure 33. The spatial pattern of

simulated and measured drawdown at the end of

3 days pumping is reasonably simulated with the

model. The model structure of dipping beds striking to the northeast produced a pattern of drawdown in the shallow part of the aquifer system that

was similar to that observed in observation wells.

The east-west trend of maximum observed drawdown between the pumping well and BAE Systems (shown for both the June 2000 aquifer test of

NP–21 and the May 2002 test of NP–87 in

figures 12 and 18) was simulated fairly well. The

simulated area of 5 to 7 ft of drawdown in the BAE

Systems area occurs because the deep producing

zone in wells NP–21 and NP–87 crops out in that

vicinity. The area of largest drawdown extending

from the pumped well to the BAE area appears to

be the result of east-west anisotropy but can be

produced from a model constructed with the

NE-SW trending structure of dipping beds.

Figure 30. Observed values of (A) water-level altitude

and (B) drawdown in relation to values simulated by the

ground-water-flow model.
55

Figure 31. Simulated steady-state water-level altitude in model layer 1 and residual between

simulated and observed values.
56

Table 9. Measured and simulated steady-state water levels from the ground-water-flow model of the Colmar area,

Bucks and Montgomery Counties, Pennsylvania

[NGVD 29, National Geodetic Vertical Datum of 1929]

Well identifier Measured1 steady-state water level,

in feet above NGVD 29

Simulated steady-state water level,

in feet above NGVD 29

Residual, simulated

minus observed, in feet

A–10 298.27 303.66 5.39

A–18 303.21 302.21 -1.00

BAE Supply 303.37 298.27 5.10

R&B–2 267.90 265.46 -2.44

R&B–3 266.69 263.34 -3.35

MW–1 291.48 293.78 2.30

MW–2 285.20 287.25 2.05

MW–3 287.80 289.08 1.28

MW–5 290.78 292.57 1.79

NP–75 260.20 262.84 2.64

RI–1S 263.88 278.78 14.90

RI–2S 297.25 298.70 1.45

RI–4S 330.13 342.83 12.70

RI–5S 291.74 294.19 2.45

RI–6S 280.75 272.83 -7.92

RI–7S 290.17 287.40 -2.77

RI–8D 273.19 272.62 -.57

RI–9S 262.04 263.68 1.64

RI–10D 256.04 257.05 1.01

RI–11S 267.45 278.09 10.64

RI–12D 263.29 268.02 4.73

RI–13D 263.74 263.98 .24

RI–15D 259.02 263.62 4.60

RI–16D 256.96 258.99 2.03

RI–18D 287.42 284.33 -3.09

RI–19D 290.09 286.30 -3.79

RI–19S 289.92 286.44 -3.48

RI–20D 294.12 298.65 4.53

RI–20S 291.72 295.93 4.21

RI–23 275.02 283.38 8.36

RI–27D 284.40 289.31 4.91

RI–27S 285.66 280.59 -5.07

RW–1 295.10 301.87 6.77

RW– 3 304.88 305.36 .48

RW–4D 285.07 286.45 1.38

RW–4I 286.41 284.70 -1.71

RW–4S 286.98 284.45 -2.53

RW–5D 286.36 284.41 -1.95

RW–5I 285.83 283.14 -2.69

RW–5S 285.95 282.65 -3.30

RW–6D 297.84 292.53 -5.31

RW–6I 288.73 287.27 -1.46

RW–6S 289.95 286.72 -3.23

W–3 295.40 282.74 -12.66

W–4 286.03 283.63 -2.40

W–5 285.36 281.50 -3.86

W–6 291.77 291.63 -.14

W–8 286.41 283.85 -2.56

W–9 290.05 288.88 -1.17

W–10 286.91 289.79 2.88

W–12 290.58 291.82 1.24

W–13 283.77 284.71 .94

W–14 290.16 293.45 3.29

W–16 303.30 312.95 9.65

1 Some water levels were adjusted as described on page 50.
57

 .

Figure 32. Hydrographs comparing observed drawdown (solid line) in selected observation wells to

drawdown simulated (points) by the ground-water-flow model.
58

Figure 32. Hydrographs comparing observed drawdown (solid line) in selected observation wells to

drawdown simulated (points) by the ground-water-flow model—Continued.
59

Figure 32. Hydrographs comparing observed drawdown (solid line) in selected observation wells to

drawdown simulated (points) by the ground-water-flow model—Continued.
60

Figure 33. Simulated drawdown in model layer 1 after 72 hours of pumping well NP–87 at

400 gallons per minute and measured drawdown in observation wells less than 100 feet deep.
61

Model Assumptions and Limitations

The ground-water flow model is based on a

generalized conceptualization of ground-water

flow in a heterogeneous, dipping, fractured, bedrock aquifer. Some of the major assumptions and

limitations of the model are discussed below:

• The fractured bedrock is modeled as an

equivalent porous medium. This approach

assumes that the hydraulic properties of fractures can be represented by an equivalent set

of hydraulic properties representing a continuous porous medium. The approach is usually

adequate if fractures are numerous but may be

invalid at the local scale if a few discrete fractures control ground-water flow paths. In the

vicinity of well NP-21 and NP-87, fractures

intercepted by the wells at depths of 50 to

140 feet and below 400 feet below land surface

contribute the majority of the water to the

wells and probably have a strong local effect

on ground-water flow. Therefore, these fractures were simulated in the model as layers of

high hydraulic conductivity separated by layers of low hydraulic conductivity.

• The interbedded, dipping, geologic units are

assumed to be laterally extensive in the model

and were simulated as layers having uniform

hydraulic properties. In reality, units of shale

and siltstone interfinger and pinch out,

undoubtedly causing considerable variability

of hydraulic properties within a layer.

• Recharge to the ground-water system was

assumed to be spatially and temporally uniform. Ground-water recharge rates probably

vary in this urban area because of impermeable surface cover, detention basins, and

sewers.

• The model simulates only the advective flow

of ground water. In addition to advective flow,

the transport of a contaminant in the groundwater system also may be affected by the density of the contaminant. TCE, the principal

contaminant in the study area, has a specific

gravity of 1.6, and could move as a non-aqueous phase in a direction different than the

advective flow of ground water. In addition,

the effects of dispersion, diffusion, dilution,

chemical reactions, and biological transformations are not simulated.

• Given the simplifying assumptions noted

above, undoubtedly ground-water flow within

the fractured-bedrock aquifer is more complex

than shown by illustrations of simulated flow

paths and areas contributing recharge to wells.

The simulated areas contributing recharge are

approximations that are useful for comparing

the potential effects of alternative pumping

scenarios and general characteristics of areas

contributing recharge to pumping wells.

Simulated Flow Paths

and Area Contributing Recharge to Wells

The ground-water-flow model was used to

simulate flow paths and to determine the area contributing recharge to wells for pumping conditions

in the Colmar area in 2000 and for hypothetical

situations of pumping suggested by USEPA in the

proposed plan for North Penn Area 5, dated July

2002, that might be employed to limit contaminant

migration (U.S. Environmental Protection Agency,

2002). The focus was on the northern area of contamination near the former Stabilus facility and the

southern area of contamination near the BAE

Systems facility. Ground-water withdrawals were

simulated from the wells listed in table 10. Withdrawals from wells NP–61, NWWA–17, and

NWWA–22 were included in all simulation

scenarios because those wells represent existing

withdrawals for public supply. NP–61 and

NWWA–17 are outside of the Colmar area but

within the model area. Their locations are shown

in figure 25. Withdrawals from the other wells in

table 10 were simulated only in scenarios used to

illustrate the effect of adding that pumpage.

The area contributing recharge was determined by using the forward-tracking option in

MODPATH (Pollock, 1994). One particle was

placed at the center of the top face of each cell in

model layer 1 and tracked to its discharge location.

The starting location of each particle that terminated in a cell representing a pumping well was

then plotted on a map. Particles were allowed to

pass through weak sinks, where a simulated

stream does not discharge at a rate large enough to

remove all the water entering a model cell, if

40 percent ar less of the flow entering the cell was

discharged to the sink. The location of the plotted

particles represents the land area contributing

recharge to the aquifer that is captured by the

pumping well. The particles are shown as open circles to allow the features of the base map to be visible beneath the area contributing recharge.
62

Conditions in 2000

Current (2000) ground-water-flow conditions in the Colmar area were simulated by including pumping from the BAE Systems supply well

and recovery well A–10 and from NP–61,

NWWA–17, and NWWA–22 at the rates shown in

table 10. The area contributing recharge to BAE

Systems supply well and recovery well (A–10)

within the study area is shown in figure 34. The

contributing area does not surround the BAE supply well because the withdrawal was simulated

from layer 6, which is 130 to 230 ft below land surface. The simulated capture zone extends to the

east slightly outside of the study area in a long,

narrow band because of the steep hydraulic gradients in this area.

Ground-water-flow paths were tracked from

land surface in the northern and southern areas of

contamination to discharge locations. Flow paths

were simulated by placing particles on the top face

of cells in layer 1 and using the forward-tracking

flow-path option in MODPATH (Pollock, 1994).

Simulations indicate that ground water near BAE

Systems moves west and discharges to a tributary

of West Branch Neshaminy Creek (fig. 34). Ground

water in the northern area of contamination moves

north and west to discharge in West Branch

Neshaminy Creek and tributaries. The wide range

of flow paths taken by the ground water originating from the northern area is partially a result of

the anisotropy with respect to hydraulic conductivity (fig. 34).

Simulated flow paths shown on figure 34 do

not correlate very well with the distribution of

ground-water contamination found in the vicinity

of the northern area of contamination near the

former Stabilus facility. Simulated flow paths show

that contaminated ground water should be moving

northwest toward West Branch Neshaminy Creek,

but contamination was not found in downgradient

wells RI–6S, RI–6D, RI–9S, and RI–9D (see figure 2

for well locations). Flow paths do show, however,

that contamination potentially could move northward to well RI–17, where TCE has been detected

in ground water at concentrations greater than

300 μg/L. The apparent lack of contamination

along simulated flow paths could be partly the

result of past large-scale pumping in the Colmar

area for public supply. Supply well NP–21 was in

service until 1995 and supply well NWWA–16 was

in service until 1994. Withdrawals from each of

those wells, southwest and southeast of the former

Stabilus facility (fig. 2), probably would have

inhibited the northwestern movement of ground

water from the northern area of contamination.

Another possibility is that the ground-water model

does not adequately incorporate the aquifer heterTable 10. Wells and withdrawal rates in 2000 simulated in the ground-water-flow model of the Colmar area,

Bucks and Montgomery Counties, Pennsylvania

Well

Well status

(existing or

hypothetical)

Model cell Simulated

withdrawal rate

(gallons per

minute) Layer Row Column

Wells included in all simulation scenarios

NP–61 Existing 1 64 23 28.5

4 64 23 28.5

NWWA–17 Existing 4 60 35 5.5

6 60 35 5.5

NWWA–22 Existing 1 58 45 10

6 58 45 10

Wells simulated only in scenarios to illustrate effect of that withdrawal

NP–87 Existing 4 43 65 100

6 43 65 300

BAE supply Existing 6 49 80 13

BAE Recovery (A–10) Existing 1 51 78 15

New BAE Recovery

(80 feet deep)

Hypothetical 1 47 73 8

New Stabilus Recovery

(80 feet deep)

Hypothetical 1 36 88 15

New Stabilus Recovery

(80 feet deep)

Hypothetical 1 38 87 15
63

Figure 34. Simulated ground-water-flow paths and areas contributing recharge to wells, near Colmar,

Pa., for pumping conditions in 2000. (NP–87, no pumping; BAE supply well, 13 gal/min; A–10,

15 gal/min)
64

ogeneity caused by the fractured, dipping siltstone

and shale. Use of a more extreme value of horizontal anisotropy in the model, stipulating that

hydraulic conductivity be even larger along the

strike of beds than along the dip direction, would

cause the flow paths to all align in a more northerly

direction toward well RI–17 but would allow much

more drawdown in the vicinity of the northern

area of contamination than was observed during

either aquifer test.

Pumping from NP–87

Ground-water flow in the Colmar area was

simulated for current conditions with the hypothetical addition of pumping NP–87 at

400 gal/min. Pumping was simulated from the

BAE Systems supply well and recovery well A–10,

NP–87, NP–61, NWWA–17, and NWWA–22 at the

rates shown in table 10. The areas contributing

recharge to NP–87 and the two wells at BAE Systems are shown in figure 35. The simulation indicates that pumping from NP–87 is likely to capture

recharge from the southern area of ground-water

contamination but is less likely to capture recharge

from the northern area of contamination.

Simulated ground-water-flow paths are

shown in figure 35 that represent water entering

well NP–87 from the deep water-yielding zone

400-500 ft below land surface represented in the

model by layer 6. Flow paths were simulated by

placing particles on the four lateral faces of the

model cell in layer 6 representing pumping from

well NP–87 and using the backward-tracking flowpath option in MODPATH (Pollock, 1994). The

flow paths show that the surface area contributing

recharge to the deep water-bearing zone in NP–87

is likely not coincident with the subsurface extent

through which ground water moves to the well.

Thus, if contaminants occurred at depths of several

hundred feet in the northern area of contamination, they could be captured by the NP–87 pumping.

New Recovery Well in Southern Area

of Ground-Water Contamination

Ground-water flow in the Colmar area was

simulated for current conditions with the hypothetical addition of one new recovery well pumping 8 gal/min from the southern area of contamination. Pumping also was simulated from the BAE

Systems supply and recovery well, NP–87, NP–61,

NWWA–17, and NWWA–22 at the rates shown in

table 10. The area contributing recharge to the

three wells is shown in figure 36. The simulation

indicates that an additional recovery well situated

west of the existing recovery well (A–10) would

likely improve the chances of capturing the contamination.

Ground-water flow was simulated for the

conditions shown in figure 36 with the hypothetical addition of NP–87 pumping at 400 gal/min.

The contributing areas to the two wells at BAE Systems, the hypothetical new recovery well, and

NP–87 are shown in figure 37. The simulation indicates that the new recovery well would probably

capture recharge from the southern area of contamination where TCE concentrations exceed

300 μg/L, and NP–87 would capture recharge in

the surrounding area. Recharge from the northern

area of contamination is less likely to be captured.

New Recovery Wells in the Northern Area

of Ground-Water Contamination

Ground-water flow in the Colmar area was

simulated for the hypothetical addition of two new

recovery wells, each pumping 15 gal/min from the

northern area of contamination. Pumping also was

simulated from wells NP–61, NWWA–17, and

NWWA–22 at the rates shown in table 10. The area

contributing recharge to the two hypothetical new

wells in the northern area of contamination is

shown in figure 38. The simulation indicates that

pumping from the two wells would capture

recharge from the northern area of contamination

near the former Stabilus facility where TCE in

excess of 3,000 μg/L has been found in groundwater samples. Contamination at RI–17 would not

be captured because it is too far downgradient

from the contributing area.

Ground-water flow was simulated for the

conditions shown in figure 38 with the hypothetical addition of NP–87 pumping at 400 gal/min.

The contributing areas for hypothetical pumping

from the two new recovery wells near the former

Stabilus facility and NP–87 are shown in figure 39.

The simulation indicates that the pumping from

NP–87 probably will make the two new recovery

wells more effective at capturing recharge from the

northern area of contamination by moving their

contributing area to the north. The southern area of

contamination is within the area contributing

recharge to NP–87.
65

Figure 35. Simulated ground-water-flow paths and area contributing recharge to wells, near Colmar,

Pa., for hypothetical pumping from NP–87. (NP–87, 400 gal/min; BAE Supply, 13 gal/min; A–10,

15 gal/min)
66

Figure 36. Simulated area contributing recharge to wells near Colmar, Pa., for hypothetical pumping

from a new recovery well in the southern area of ground-water contamination. (NP–87, 0 gal/min;

BAE Supply, 13 gal/min; A–10, 15 gal/min; new BAE recovery well, 8 gal/min)
67

Figure 37. Simulated area contributing recharge to wells near Colmar, Pa., for hypothetical pumping

from a new recovery well in the southern area of ground-water contamination and NP–87. (NP-87,

400 gal/min; BAE Supply, 13 gal/min; A-10, 15 gal/min; new BAE recovery well, 8 gal/min)
68

Figure 38. Simulated area contributing recharge to wells, near Colmar, Pa., for hypothetical pumping

from two new recovery wells near the former Stabilus facility. (NP-87, 0.0 gal/min; new Stabilus

recovery wells, 30 gal/min)
69

Figure 39. Simulated area contributing recharge to wells, near Colmar, Pa., for hypothetical

pumping from two new recovery wells near the former Stabilus facility and NP–87. (NP-87,

400 gal/min; new Stabilus recovery wells, 30 gal/min)
70

Figure 40. Simulated area contributing recharge to wells, near Colmar, Pa., for hypothetical pumping

from new recovery wells near BAE Systems, the former Stabilus facility, and NP–87. (NP-87,

400 gal/min; BAE Supply, 12 gal/min; A-10, 15 gal/min; new BAE recovery well, 8 gal/min; new

Stabilus recovery wells, 30 gal/min)
71

All Remediation Wells and NP–87

Ground-water flow in the Colmar area was

simulated to evaluate the combined effect of all the

previously described current and hypothetical

pumping. The simulation used withdrawals from

all nine wells pumping at the rates listed in

table 10. The wells in the Colmar area included the

hypothetical addition of two new recovery wells in

the northern area of contamination, one new recovery well in the southern area, and pumping from

NP–87 at 400 gal/min. The areas contributing

recharge to the wells are shown in figure 40. The

simulation indicates that this hypothetical pumping scenario would capture recharge from both the

northern and southern areas of contamination,

with the exception of contamination at RI–17.

SUMMARY AND CONCLUSIONS

The U.S. Environmental Protection Agency

(USEPA) is in the process of characterizing groundwater contamination and evaluating remediation

alternatives for the North Penn Area 5 Superfund

Site near Colmar, Pa. This report presents the

results of a study by the U.S. Geological Survey in

cooperation with USEPA to determine the

(1) drawdown caused by separately pumping

North Penn Water Authority wells NP–21 and

NP–87, (2) probable paths of ground-water flow

under present-day (2000) conditions (with NP–21

pumping discontinued), and (3) areas contributing

recharge to wells if pumping from NP–21 or NP-87

were restarted and new recovery wells were

installed. Drawdown was estimated from water

levels measured in observation wells during aquifer tests at supply well NP–21 and NP–87. Groundwater-flow paths and areas contributing recharge

were estimated by use of a ground-water-flow

model.

Aquifer tests were conducted at wells NP–21

and NP–87 to identify hydraulic connections and

to quantify the drawdown caused by pumping

these wells. Wells NP–21 and NP–87 are only 40 ft

apart and are similar in depth and construction.

Aquifer-isolation tests indicated that the wells are

connected hydraulically through fractures below

400 ft below land surface. Thus, when one well is

pumped, the wells act together as if each were

being pumped, so the effect on the aquifer is similar regardless of which well is pumped. Well

NP–21 was pumped at the maximum rate possible

(average of 257 gal/min) for about 7 days in June

2000. Well NP–87 was pumped at an average rate

of 402 gal/min for 3 days in May 2002.

Water-level measurements made during the

aquifer test at NP–21 showed that, except at RI–7,

ground-water levels declined almost exclusively in

observation wells completed within or updip of

the geologic units penetrated by the pumped well.

Because the geologic units become deeper to the

northwest, shallow wells updip to the southeast of

the pumped well showed more drawdown than

observation wells an equal distance but downdip

from the pumping well. A good hydraulic connection (2.4 to 4.0 ft of drawdown) was shown for

observation wells updip in the southern area of

ground-water contamination near the BAE Systems facility. Shallow wells in the northern area of

ground-water contamination (RI–23, RI–25, and

RI–17S) near the former Stabilus facility did not

appear to be within the influence of well NP–21

after 7 days of pumping at 257 gal/min.

Pumping of supply well NP–87 at

402 gal/min for 3 days confirmed the general pattern of drawdown observed during the test of well

NP–21. However, during the pumping of supply

well NP-87, water-level declines were observed

further to the northeast near the northern area of

ground-water contamination because of the

greater pumping rate and availability of additional

observation wells. Outside of the immediate area

of well NP–87, the greatest drawdowns (7-12 ft)

were observed near the BAE Systems facility in

wells A–10, BAE Systems supply well, and RW–3.

The drawdown confirms the existence of a good

hydraulic connection between NP–87 and wells

completed within the deepest part of the geologic

units penetrated by the pumped well. Near the

former Stabilus facility, the greatest drawdown

(3.7 ft) was measured in RW–6D, which is the well

farthest updip and completed most closely in the

deep water-producing zone penetrated by well

NP–87. After pumping from NP–87 stopped, the

water-level recovery in shallow wells RI-27S,

RW–4S, RW-5S, RW–6S, W-3, W–4, W–5, W–6,

W–8, and W-9 was slow. The small drawdown and

poor recovery indicates that the pumping has less

effect on water levels and direction of groundwater flow in the shallow part of the aquifer than

in the deep part. Streamflow in the tributary of the

West Branch Neshaminy Creek near NP–87

appeared to be affected by pumping from NP–87.

Miscellaneous measurements showed that streamflow decreased during the aquifer test from 380 to

170 gal/min; however, it is not clear how much of

the decrease was caused by the pumping and how

much was part of natural streamflow recession.
72

A 3-dimensional, finite-difference, groundwater-flow model was used to simulate flow paths

and contributing areas to wells for current (2000)

conditions of pumping in the Colmar area and for

hypothetical situations of pumping suggested by

USEPA that might be used for remediation. The

focus was on principal areas of contamination near

the former Stabilus facility (northern area of contamination) and BAE Systems facility (southern

area of contamination) where TCE has been

detected at concentrations greater than 300 μg/L.

The numerical model constructed for the Colmar

area was based partially on the MODFLOW model

developed to analyze regional ground-water flow

in the Lansdale area by Senior and Goode (1999)

but was modified by adding dipping layers to

incorporate more detail about the local-scale geologic structure in the vicinity of wells NP–21 and

NP–87.

Current ground-water-flow conditions simulated by the model in the Colmar area indicate that

ground water in the southern area of contamination moves west and discharges to a tributary of

West Branch Neshaminy Creek near well NP–21,

and ground water in the northern area of contamination moves northwest to discharge to West

Branch Neshaminy Creek and tributaries. Model

simulations indicate that if NP–21 or NP–87 are

pumped at 400 gal/min, ground-water recharge is

likely to be captured from the southern area of contamination, but ground-water recharge from the

northern area of contamination is less likely to be

captured by the pumping. Simulations also indicate that a new recovery well pumping about

8 gal/min near RI–20 and two new recovery wells

pumping a total of 30 gal/min near the former

Stabilus facility probably would capture groundwater recharge in most of the area where contamination exceeds 300 μg/L.

REFERENCES CITED

Biesecker, J.E., Lecinsky, J.B., and Wood, C.R., 1968,

Water resources of the Schuylkill River

Basin: Pennsylvania Department of

Environmental Protection Water Resources

Bulletin no. 3, 198 p.

Bird, P.H., and Conger, R.W., 2002, Evaluation of

borehole geophysical logging, aquiferisolation tests, distribution of contaminants,

and water-level measurements at the North

Penn Area 5 Superfund Site, Bucks and

Montgomery Counties, Pennsylvania:

U.S. Geological Survey Water-Resources

Investigations Report 01-4261, 79 p.

Conger, R.W., 1999, Evaluation of geophysical logs

at North Penn Area 6 Superfund Site,

Lansdale, Montgomery County,

Pennsylvania: U.S. Geological Survey

Open-File Report 99-271, 149 p.

Earth Technology Corporation, 1993, Final North

Penn Area 5 RI/FS Work Plan, Project

No. 591546, September 1993, 119 p.

Goode, D.J., and Senior, L.A., 1998, Review of

aquifer test results for the Lansdale area,

Montgomery County, Pennsylvania, 1980-

95: U.S. Geological Survey Open-File

Report 98-294, 70 p.

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

McDonald, M.G., 2000, MODFLOW–2000,

the U.S. Geological Survey modular groundwater flow model—User guide to

modularization concepts and the groundwater flow process: U.S. Geological Survey

Open-File Report 00-92, 121 p.

Hill, M.C., 1998, Methods and guidelines for

effective model calibration: U.S. Geological

Survey Water-Resources Investigations

Report 98-4005, 90 p.

Hill, M.C., Banta, E.R., and Harbaugh, A.W., 2000,

MODFLOW–2000, the U.S. Geological

Survey modular ground-water flow model—

User guide to the observation, sensitivity,

and parameter-estimation processes and

three post-processing programs:

U.S. Geological Survey Open-File Report

00-184, 209 p.
73

REFERENCES CITED—Continued

Longwill, S.M., and Wood, C.R., 1965, Groundwater resources of the Brunswick Formation

in Montgomery and Berks Counties,

Pennsylvania: Pennsylvania Geological

Survey, 4th ser., Water Resource Report 22,

59 p.

Lyttle, P.T., and Epstein, J.B., 1987, Geologic map of

the Newark 1° × 2° Quadrangle, New Jersey,

Pennsylvania and New York: U.S. Geological Survey Miscellaneous Investigations

Map I-1715, scale 1:250,000.

Newport, T.G., 1971, Ground-water resources of

Montgomery County, Pennsylvania:

Pennsylvania Geological Survey, 4th ser.,

Water Resource Report 29, 83 p.

Pollock, D.W., 1994, User’s guide for MODPATH/

MODPATH-PLOT, VERSION 3—A particle

tracking post-processing package for

MODFLOW, the U.S. Geological Survey

finite-difference ground-water flow

model: U.S. Geological Survey Open-File

Report 94-464, 249 p.

Prudic, D.E., 1989, Documentation of a computer

program to simulate stream-aquifer relations

using a modular, finite-difference, groundwater flow model: U.S. Geological Survey

Open-File Report 88-729, 113 p.

Rima, D.R., 1955, Ground-water resources of the

Lansdale area, Pennsylvania: Pennsylvania

Geological Survey, 4th, ser., Progress Report

146, 24 p.

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

system, estimation of aquifer hydraulic

properties, and effects of pumping on

ground-water flow in Triassic sedimentary

rocks in and near Lansdale, Pennsylvania:

U.S. Geological Survey Water-Resources

Investigations Report 99-4228, 112 p.

Sloto, R.A., and Schreffler, C.L., 1994, Hydrogeology and ground-water quality of northern

Bucks County, Pennsylvania:

U.S. Geological Survey Water-Resources

Investigations Report 94-4109, 85 p.

Tetra Tech/Black & Veatch, 2002, Draft Remedial

Investigation Report, Revision 1, Remedial

Investigation and Feasibility Study, North

Penn Area 5 Site, Colmar, Montgomery

County, Pennsylvania: Work Assignment

No. 006-RICO-03W, 128 p.

Todd, D.K., 1980, Groundwater hydrology

(2nd ed.): New York, John Wiley and Sons,

525 p.

U.S. Environmental Protection Agency, 2002,

North Penn Area 5, Colmar and Hatfield,

Montgomery County, Pennsylvania:

U.S. Environmental Protection Agency

Superfund Program Proposed Plan, July 26,

2002, 60 p.

Willard, Bradford, Freedman, Jacob, McLaughlin,

D.B., Ryan, J.R., Wherry, E.T., Peltier, L.C.,

and Gault, H.R., 1959, Geology and mineral

resources of Bucks County Pennsylvania:

Pennsylvania Geological Survey, 4th ser.,

Bulletin C9, 243 p.

Winston, R.B., 2000, Graphical User Interface for

MODFLOW, Version 4: U.S. Geological

Survey Open-File Report 00-315, 27 p.

Wood, C.R., 1980, Groundwater resources of the

Gettysburg and Hammer Creek formations,

southeastern Pennsylvania: Pennsylvania

Geological Survey, 4th ser., Water Resource

Report 49, 87 p.