Digital-simulation model of the Wenonah-Mount Laurel Aquifer in the coastal plain of New Jersey

Abstract

A digital computer-simulation model of the Wenonah-Mount Laurel aquifer is used to evaluate the aquifer's capabilities of meeting the projected future demands and to study the cause of the rapidly declining water levels. The modelled area includes 1,500 square miles (3,885 square kilometres) of the New Jersey Coastal Plain and includes all the important centers of pumping in Monmouth, Burlington, and Ocean Counties from the Wenonah-Mount Laurel aquifer. The Wenonah Formation and Mount Laurel Sand of Late Cretaceous age are exposed in the western part of the New Jersey Coastal Plain along a belt trending northeast-southwest from Raritan Bay to Delaware Bay. The formations typically consist of poorly sorted silty to fine quartz sand, the Wenonah Formation, and a coarse clastic quartz sand unit, the Mount Laurel Sand. The Wenonah-Mount Laurel aquifer is composed of the sandy part of the geologic units. Transmissivity of the aquifer ranges from 360 square feet per day (33.4 square metres per day) to 1,430 square feet per day (132.8 square metres per day); the estimated hydraulic conductivity ranges from about 1.5 x 10 -4 feet per second (4.6 x 10 -5 metres per second) to 2.2 x 10 -4 feet per second (6.7 x 10 -5 metres per second); and the storage coefficient varies from about 1.5 x 10 -5 to 3.5 x 10 -4 . The annual average rate of withdrawal from the Wenonah-Mount Laurel aquifer in Monmouth, Burlington, and Ocean Counties increased from about 1 million gallons per day (44 cubic decimetres per second) in 1959 to slightly more than 2 million gallons per day (88 cubic decimetres per second) in 1970. Near the pumping centers in Monmouth County, the water level declined as much as 100 feet (30.5 metres) between 1959 and 1970. In the subjacent Englishtown aquifer underlying the lower confining unit (Marshalltown Formation), the annual average rate of withdrawal in all of Monmouth and Ocean Counties increased from 5.5 million gallons per day (241 cubic decimetres per second) in 1959 to 9.5 million gallons per day (416 cubic decimetres per second) in 1970. Water-level declines for the same period are as much as 140 feet (42.7 metres) near centers of pumping. The digital-simulation model was calibrated by matching computed declines in the Wenonah-Mount Laurel aquifer with historic water-level declines over the 12-year period, 1959-70. The results of the modelling show that recharge to the aquifer occurs as leakage from the upper confining unit owing to withdrawals from the aquifer. Of equal significance is the effect of water-level declines in the Englishtown aquifer, which generate leakage from the Wenonah-Mount Laurel aquifer through the lower confining unit, which in turn generates leakage (recharge) to the aquifer from the upper confining unit. The rapid declines of water levels in the Wenonah-Mount Laurel aquifer in the northern part of the New Jersey Coastal Plain, hence, are caused directly by the withdrawals from the Wenonah-Mount Laurel aquifer and indirectly by withdrawals from the underlying Englishtown aquifer.