Geohydrology and simulation of ground-water flow in the Red Clay Creek Basin, Chester County, Pennsylvania, and New Castle County, Delaware

Abstract

The 54-square-mile Red Clay Creek Basin, located in the lower Delaware River Basin, is underlain primarily by metamorphic rocks that range from Precambrian to Lower Paleozoic in age. Ground water flows through secondary openings in fractured crystalline rock and through primary openings below the water table in the overlying saprolite. Secondary porosity and permeability vary with hydrogeologic unit, topographic setting, and depth. Thirty-nine percent of the water-bearing zones are encountered within 100 feet of the land surface, and 79 percent are within 200 feet. The fractured crystalline rock and overlying saprolite act as a single aquifer under unconfined conditions. The water table is a subdued replica of the land surface. Local groundwater flow systems predominate in the basin, and natural groundwater discharge is to streams, comprising 62 to 71 percent of streamflow. Water budgets for 1988-90 for the 45-square-mile effective drainage area above the Wooddale, Del., streamflow-measurement station show that annual precipitation ranged from 43.59 to 59.14 inches and averaged 49.81 inches, annual streamflow ranged from 15.35 to 26.33 inches and averaged 20.24 inches, and annual evapotranspiration ranged from 27.87 to 30.43 inches and averaged 28.98 inches. The crystalline rocks of the Red Clay Creek Basin were simulated two-dimensionally as a single aquifer under unconfined conditions. The model was calibrated for short-term steady-state conditions on November 2,1990. Recharge was 8.32 inches per year. Values of aquifer hydraulic conductivity in hillside topographic settings ranged from 0.07 to 2.60 feet per day. Values of streambed hydraulic conductivity ranged from 0.08 to 26.0 feet per day. Prior to simulations where groundwater development was increased, the calibrated steady-state model was modified to approximate long-term average conditions in the basin. Base flow of 11.98 inches per year and a groundwater evapotranspiration rate of 2.17 inches per year were simulated by the model. Different combinations of groundwater supply and wastewater-disposal plans were simulated to assess their effects on the stream-aquifer system. Six of the simulations represent an increase in population of 14,283 and water use of 1.07 million gallons per day. One simulation represents an increase in population of 28,566 and water use of 2.14 million gallons per day. Reduction of average base flow is greatest for development plans with wastewater removed from the basin through sewers and is proportional to the amount of water removed from the basin. The development plan that had the least effect on water levels and base flow included on-lot wells and on-lot septic systems. Five organochlorine insecticides lindane, DDT, dieldrin, heptachlor, and methoxychlor were detected in ground water. Four organophosphorus insecticides malathion, parathion, diazinon, and phorate were detected in ground water. Four volatile organic compounds-benzene, toluene, tetrachloroethylene, and trichloroethylene were detected in ground water. Phenol was...