GROUND-WATER RESOURCES
OF THE ELK CREEK WATERSHED
This project was done in cooperation with the
Chester County Water Resources
Authority and the
Chester County Health
Department.
FINAL REPORTS
FACT SHEET:
Ground-Water Resources of Big Elk Creek Basin,
Pennsylvania and Maryland [PDF File]
INTERPRETIVE REPORT:
Geohydrology and Ground-Water Quality,
Big Elk Creek Basin, Chester County, Pennsylvania,
and Cecil County, Maryland [PDF File]
NEED FOR THE PROJECT
The rural Big Elk Creek and Little Elk Creek watersheds in
southwestern Chester County are beginning to undergo urbanization
similar to the rest of the county. Comprehensive
ground-water-quality data and water-budget analyses, which provide a
foundation for managing water resources, did not exist for these basins.
Water budgets are essential in the
management of water withdrawals to achieve a sustainable water cycle
balance. Because
Big Elk Creek is a tributary to the
Chesapeake Bay,
water withdrawals
in the Big Elk and Little Elk Creek watersheds are not regulated by the
Delaware River Basin Commission or the Susquehanna River Basin
Commission.
PROJECT OBJECTIVES
- Develop a water budget for the Big Elk Creek watershed
- Estimate ground-water availability
- Describe the chemical quality of ground water
ABSTRACT
A study of ground-water quantity and quality
was conducted in the Big Elk Creek Basin, a rural
area undergoing rapid growth. The 79.4-square
mile study area is in the Piedmont Physiographic
Province and is underlain almost entirely by crystalline
rocks. Most of the basin in Pennsylvania is
underlain by Wissahickon Schist, a fractured crystalline-rock
aquifer. Yields of wells in the Wissahickon
Schist range from 5 to 200 gal/min (gallons
per minute); the median yield is 15 gal/min. Specific
capacity ranges from 0.03 to 15 (gal/min)/ft
(gallons per minute per foot) of drawdown; the
median specific capacity is 0.4 (gal/min)/ft.
Recharge to the basin occurs by infiltration of
precipitation, and ground water discharges locally
to streams. The median annual ground-water discharge
to streams (base flow) for 1933-99 was
10.79 in. (inches) or 0.518 (Mgal/d)/mi2 (million
gallons per day per square mile), which was
63 percent of the median annual streamflow. The
median annual ground-water discharge to streams
ranged from 5.32 in. or 0.255 (Mgal/d)/mi2 in 1966
to 17.98 in. or 0.863 (Mgal/d)/mi2 in 1972.
Estimated ground-water availability ranges from
0.127 to 0.535 (Mgal/d)/mi2, depending on the estimation
method used.
Annual water budgets were calculated for the
Big Elk Creek Basin for 1998-99. The 1998-99
average annual streamflow was 15.38 in., change
in ground-water storage was an increase of
1.32 in., ground-water exports were 0.03 in., and
estimated evapotranspiration (ET) was 30.5 in.
Despite a 12.27-in. difference in precipitation
between 1998 and 1999, the percentage of precipitation
as ET (65.6 and 64 percent, respectively)
is similar. Estimated average annual recharge for
1998-99 was 12.12 in. [0.580 (Mgal/d)/mi2].
For this study, water samples from 20 wells in
the Big Elk Creek Basin were collected for analysis
for inorganic constituents and pesticides. In addition,
data were available from 44 additional wells.
Major ions, in order of decreasing concentration,
based on median concentrations for the Wissahickon
Schist, are silica, calcium, chloride, sodium,
sulfate, magnesium, and potassium. The Wissahickon
Schist and Peters Creek Schist have similar
water types; ground water from serpentinite, the
basal unit of the Baltimore Mafic Complex that
straddles the Pennsylvania-Maryland border, is distinctly
different. For the Wissahickon Schist and
Peters Creek Schist, no cation is predominant; calcium,
magnesium, and sodium are in nearly equal
concentrations expressed in milliequivalents per
liter. Bicarbonate is the dominant anion. Water
from serpentinite is of the magnesium bicarbonate
type; magnesium is the dominant cation, and bicarbonate
is the dominant anion.
Water from 2 percent of sampled wells
exceeded the U.S. Environmental Protection
Agency (USEPA) secondary maximum contaminant
level (SMCL) for total dissolved solids. None
of the chloride or sulfate concentrations exceeded
the USEPA SMCL. Water from 10 percent of sampled
wells exceeded the USEPA maximum contaminant
level (MCL) of 10 mg/L (milligrams per
liter) nitrate as nitrogen. All of those wells are in the
Wissahickon Schist. The median concentration of
nitrate in water samples from the Wissahickon
Schist was 3.6 mg/L, and the maximum concentration
was 36 mg/L. Except for iron and manganese,
metals and other trace inorganic constituents do
not appear to pose a water-quality problem. Fourteen
percent of water samples analyzed for iron
and 29 percent of water samples analyzed for
manganese exceeded the USEPA SMCLs. The
median activity of radon-222 for all formations was
2,400 pCi/L (picoCuries per liter). The median
activity for water from 35 wells sampled in the Wissahickon
Schist in the Big Elk Creek Basin was
2,500 pCi/L. Water from 94 percent of sampled
wells exceeded the proposed USEPA MCL of
300 pCi/L, and water from 25 percent of sampled
wells exceeded proposed USEPA alternate MCL of
4,000 pCi/L.
In addition to the 20 wells sampled for pesticides
for this study, data were available for 20 other
wells sampled for pesticides. The most commonly
detected pesticides in the Big Elk Creek Basin are
deethyl atrazine (71 percent of sampled wells),
atrazine (35 percent of sampled wells), metolachlor
(32 percent of sampled wells), carbaryl (19 percent
of sampled wells), picloram (14 percent of sampled
wells), simazine (13 percent of sampled wells), and
carbofuran (11 percent of sampled wells). Most
concentrations are extremely low and are in the
parts per trillion range. Concentrations of pesticides
detected did not exceed USEPA MCLs. Out
of 43 volatile organic compounds analyzed, only
4 were detected: chloroform, total phenols, tert butyl
methyl ether (MTBE), and toluene. None of
the concentrations exceeded USEPA MCLs.
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This page was last modified January 17, 2003 by R. A. Sloto (rasloto@usgs.gov)