The model developed in this study simulates .recharge to, flow through, and discharge from the water-table aquifer in the upper Pickering Creek basin, a 5.98-square-mile basin representative of most of Chester County, Pennsylvania. The two-dimensional finite-difference model of Trescott, Pinder, and Larson was used with slight modification. The way ground-water evapotranspiration varies with depth was modified, and a minimum transmissivity was used to prevent the model from "going dry" during water-table simulations.
Tests of the model showed good correlation with field data, but also showed that certain additional types of information would be useful to make the model more reliable. Particularly helpful would be more water-level and evapotranspiration data, synoptic base-flow measurements in the subbasins, and better information on the variation of permeability and specific yield with depth.
Predictive simulations made by the model show the effects of imposing various changes on the hydrologic system. Residential development was simulated in 1 square mile of the basin by combinations of the following conditions: 1-acre, 1/2-acre, and 1/4-acre lot size; wastewater disposal by septic systems or by public sewers; domestic wells or public-supply wells for water supply; and 0, 1, and 5 percent reduction in recharge in the developed area. Where water is supplied locally by wells, decreased lot size and reduced recharge result in increased drawdowns and reduced base flow. When wastewater is exported by sewers, these effects are magnified. The simulations by the model show the relative severity of the effects of various development schemes.
