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An empirical technique was used to calculate the recharge to bedrock aquifers in northern Westchester County. This method requires delineation of ground-water divides within the aquifer area and values for (1) the extent of till and exposed bedrock within the aquifer area, and (2) mean annual runoff. This report contains maps and data needed for calculation of recharge in any given area within the 165square-mile study area. Recharge was computed by this technique for a 93-square-mile part of the study area and used a ground-water-flow model to evaluate the reliability of the method. A two-layer, steady-state model of the selected area was calibrated. The area consists predominantly of bedrock overlain by small localized deposits of till and stratified drift. Ground-water-level and streamflow data collected in mid-November 1987 were used for model calibration. The data set approximates average annual conditions. The model was calibrated from (1) estimates of recharge as computed through the empirical technique, and (2) a range of values for hydrologic properties derived from aquifer tests and published literature. Recharge values used for model simulation appear to be reasonable for average steady-state conditions. Water-quality data were collected from 53 selected bedrock wells throughout northern Westchester County to define the background ground-water quality. The constituents and properties for which samples were analyzed included major cations and anions, temperature, pH, specific conductance, and hardness. Results indicate little difference in water quality among the bedrock aquifers within the study area. Ground water is mainly the calcium-bicarbonate type and is moderately hard. Average concentrations of sodium, sulfate, chloride, nitrate, iron, and manganese were within acceptable limits established by the U.S. Environmental Protection Agency for domestic water supply.

Computation of bedrock-aquifer recharge in northern Westchester County, New York, and chemical quality of water from selected bedrock wells