Sixty-eight percent of the 22.6-square-mile Valley Creek basin is underlain by Cambrian and Ordovician limestone and dolomite. Ground water flows through a network of interconnected secondary openings; primary porosity is virtually nonexistent. Some of these openings have been enlarged by solution. Secondary porosity and permeability exhibit great spatial variability, and the yield and specific capacity of wells are highly variable. The number of water-bearing zones decreases with depth. Fifty percent of water-bearing zones are encountered within 100 feet of the land surface, and 81 percent are within 200 feet. Most ground-water flow in the Valley Creek basin is local and discharges to nearby streams. Ground-water discharge comprised an average of 76 percent of the flow of Valley Creek during 1983--87, including both natural ground- water discharge and quarry pumpage discharged to Valley Creek. Discharge from the Cedar Hollow quarry comprised 21 to 26 percent of the base flow of Valley Creek; the average was 23 percent. The average natural base flow of Valley Creek would be 8 percent lower if the quarry were not operating. Regional ground-water flow is to the northeast to the Schuylkill River. On the western side of the Valley Creek basin, the ground-water divide is 1/2 mile west of the surface-water divide. An estimated 0.75 million gallons per day of ground water flows from the adjacent West Valley Creek basn eastward into the Valley Creek basin. A ground-water divide is not present on the eastern side of the basin; the water table slopes gently eastward toward the Schuylkill River. On the northeaster side, an estimated 1.76 million gallons per day of ground water flows northeastward out of the basin to the Schuylkill River beneath the surface-water divide. On the southeaster side, an estimated 0.85 million gallons per day of ground water flows beneath the surface-water divide into the basin. Annual water budgets and an average water budget were calculated for 1983-87 for the 20.8-square-mile area bove the streamflow-gaging station. Annual precipitation for 1983-87 ranged from 40.61 to 56.55 inches and averaged 47.25 inches; annual streamflow ranged from 15.55 to 28.57 inches and averaged 22.31 inches; annual evapotranspiration ranged from 18.21 to 24.83 inches and averaged 22.90 inches; and annual recharge ranged from 15.89 to 26.84 inches and averaged 21.04 inches. The Valley Creek basin was modeled as a two-dimensional water-table aquifer. Recharge to, ground-water flow through, and discharge from the rocks of Chester valley were simulated. In order to include the natural hydrologic boundaries of the ground-water-flwo system, the 66.4-square-mile area between the Brandywine Creek and the Schuylkill River was modeled. The model was calibrated under stead-state conditions using avareage recharge and evapotranspiration rates. Aquifer hydraulic conductivity was estimated from specific-capacity and quifer-test data. The average (1983-87) annual water budget for hte Valley Creek basin was simualted. The effect of increased ground-water development on base flow and underflow was simulated by locating a hypothetical well field produceing 4 million gallons per day in different parts of the basin. Pumpage from a well field near surface-water divides would induce as much as an additional 1.41 inches per year of underflow from an adjacent surface-water basin. Pumpage from a well field near the center of the basin would affect base flow more than underflow. Increased seepage of ground water into quarries as a result of their expansion was simulated as increased withdrawal by pumping. A 100-percent increase in the pumping rate of the Cedar Hollow quarry, from 3.93 to 7.86 million gallons per day, owuld reduce the natural base flow of Valley Creek by 18 percent. However, the quarry pumpage would be discharged to Valley Creek, thereby increasing the base flow at the gaging station by