Computer simulation model of the Pleistocene valley-fill aquifer in southwestern Essex and southeastern Morris counties, New Jersey

A finite-difference digital computer model was developed to simulate a buried valley-fill aquifer consisting of outwash sand and gravel deposited in a series of valleys cut into bedrock of Triassic age. Till, clay, silt, and muck function as an overlying semiconfining layer. The bedrock which is represented as an unconfined aquifer and the valley fill are in hydraulic connection. Calibration of the model was achieved by comparing model-computed water-level declines with measured declines at 12 observation wells during the period 1953-71. During calibration, changes in several hydraulic properties were tested. The most significant changes were changes in hydraulic conductivity of the semiconfining layer. The amount of water available from the valley-fill aquifer on a continuing basis, determined using the criterion that water levels would not decline below 30 feet above the base of the aquifer, is approximately 40 Mgal/d or about 40 percent more than the 1972-73 withdrawal rates. (Woodard-USGS)