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

Abstract

A finite-difference digital computer model was developed to simulate a buried valley-fill aquifer in southwestern Essex and southeastern Morris Counties, N.J. Withdrawal from this aquifer and from the adjacent consolidated-rock aquifer has increased from an estimated 5 million gallons per day (0.22 cubic metres per second) during the period 1900-29 to 28.5 million gallons per day (1.25 cubic metres per second) during the period 1972-73. The valley-fill aquifer consists chiefly of outwash sand and gravel deposited in an interconnected series of valleys during the last glaciation, A total length of about 20 miles (32 kilometres) of valley-fill aquifer has been simulated. The aquifer is typically 0.5 to 1.5 miles (0.8 to 2.4 kilometres) wide and ranges in thickness from 0 to 100 feet (30 metres), Glacial till, lacustrine clay and silt, and swamp muck ranging in thickness from about to 80 feet (3 to 24 metres) overlie the valley-fill aquifer and function as a confining layer. The bedrock underlying and adjacent to the valley-fill aquifer belongs to the Newark Group of Triassic age. It consists of lava flows, referred to as Watchung Basalt, interbedded with shale and sandstone of the Brunswick Formation. The bedrock and valley-fill aquifer are in hydraulic connection. The model simulates the valley-fill material as an artesian aquifer overlain by a semiconfining layer, but it allows for conversion to watertable conditions when the water level falls below the top of the aquifer. The bedrock between the valley-fill deposits is represented aa an unconfined aquifer in which saturated thickness remains much greater than drawdown and Its transmissivity can therefore be considered constant. It is assumed that a lateral hydraulic connection exists between the bedrock aquifer and the valley-fill aquifer along the valley walls but that bedrock beneath the valley-fill aquifer is impermeable. Values of hydraulic properties of t^he valley-fill/aquifer used in the model are: hydraulic conductivity, 3 x 10 to 4 x feet petsecond (78 to 105 metres per day) and specific storage, 4 x 10 ft (1.2 x 106 m1). A specific yield of 0.16 is used if the simulated water level drops below the top of the aquifer during computer runs. Hydraulic conductivity of the semiconfining layer overlying the valley-fill aquifer, as used in the model, ranges from 7 x 10 to 4.9 x feet per second (1.8 x to 1.3 x metres per day). Release of water from storage in the semiconfining layer was not simulated. Values of hydraulic properties of the bedrock aquifer used in the model are: hydraulic conductivity, 3.6 x to 6.0 x 10 feet per second (0.94 to 1.58 metres per day); thickness, 500 feet (152.4 metres); and coefficient of storage, or specific yield, 0.12. The model was calibrated by simulating the pumpage from 1900 through 1971. For purposes of simulation this time interval was divided into seven pumping periods ranging from 3 to 19 years in duration. Calibration was based on comparison of computed water-level declines with declines measured in 12 observation wells during the latter part of the pumping history. Calibration of the model was more successful at some localities than at others. The model is adequately calibrated to be used for planning and predictive purposes for valley-fill aquifers in the East Hanover, Chatham, and Southern Millburn Valleys. The model is not calibrated or is poorly calibrated for valley-fill aquifers in the Northern Millburn, Slough Brook, and Canoe Brook Valleys. The model has been used to determine pumpage available from the valley-fill aquifer, based upon the criterion that water levels would stabilize at least 30 feet (9.1 metres) above the base of the aquifer. On this basis, the model indicates that pumpage of approximately 40 million gallons per day (1.8 cubic metres per second) or about 40 percent more than the 1972-73 rates could be obtained on a continuing basis. All this increase would have to occur in the East Hanover and Chatham Valleys. In the other valleys, the amount of water pumped during 1972-73 either equals (Southern Millburn Valley) or exceeds the anticipated pumpage availability (Northern Millburn, Slough Brook, and Canoe Brook Valleys). INTRODUCTION Purpose and Scope Sand and gravel deposits of Pleistocene age have been an important source of water for communities and industries in southwestern Essex and southeastern Morris Counties (fig. 1) for several decades. Withdrawal from these deposits has increased from an estimated 5 Mgal/d (million gallons per day) [0.22 m /s (cubic metres per second)] during the period 1900-29 to approximately 28.5 Mgal/d (1.25 m3/s) during the period 1972-73. Yet