Geohydrology and ground-water quality, Big Elk Creek Basin, Chester County, Pennsylvania, and Cecil County, Maryland

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 SMCL's. 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 MCL’s. 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 MCL’s.