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The valley-fill and carbonate-rock aquifer system near Long Valley is an important source of ground water in southwestern Morris and northeastern Hunterdon Counties, where demand for water is increasing. Through this study, the hydrogeology of the area was assessed, and numerical-modeling techniques were used to evaluate the ground-water-flow system and the factors that limit water-supply availability. The effects of recent and anticipated withdrawals on stream base flows, water levels, and the overall water budget were estimated. Simulation results indicate that recent withdrawals have resulted in water-level declines of up to 35 feet near pumping centers. Under conditions of projected increases in ground-water withdrawals of 121 percent, average water levels in the carbonate-rock aquifer would decline up to 28 feet, but water levels in two public supply wells in the affected area during pumping would not approach the depths of present pump intakes. The magnitude of predicted average base-flow depletion, when compared with historic low flows, indicates that projected increases in pumpage may substantially deplete seasonal low flow of Drakes Brook and the South Branch Raritan River. Average base flow of Drakes Brook at Bartley would decrease from 20.5 cubic feet per second by as much as 5.3 cubic feet per second, or 26 percent. Historically, low flows at this location have been less than 5.3 cubic feet per second. Water-budget changes that would result from increased withdrawals from the carbonate-rock aquifer include (1) decreased discharge to rivers from the aquifer system, (2) increased downward flow from and decreased upward flow to the lower valley-fill aquifer, (3) increased lateral flow of ground water into the South Branch Raritan River Basin from the Lamington River Basin, and (4) a slight increase in tributary-stream leakage to the aquifer system. These water-budget changes are indicative of the sources of water to additional supply wells. Water-quality data indicate that human activities are affecting ground water, particularly in the northern and central parts of the study area. With the exception of an elevated iron concentration in water from one well, concentrations of inorganic constituents in water from 75 wells sampled did not exceed New Jersey primary or secondary drinking-water regulations. Volatile organic compounds were detected in water from several wells; in two samples, concentrations of specific compounds exceeded drinking-water regulations. A data-collection program designed for early detection of potential adverse effects of watersupply development would include (1) continuous gaging of streamflow in the South Branch Raritan River near Naughright, (2) frequent measurement of water levels in nests of wells open to each aquifer, and (3) water-quality sampling in nests of wells open to each aquifer. INTRODUCTION A valley-fill and carbonate-rock aquifer system in the New Jersey Highlands, extending from Califon in the south to Picatinny Arsenal in the north, is an important source of ground water in a region that is experiencing increasing demand for water. The aquifer system and adjacent upland areas cover an area of about 98 mi2 (fig. 1). Aquifers within the valley-fill sediments supply water to Picatinny Arsenal and the communities of Succasunna and Kenvil. The carbonate-rock aquifer supplies water to communities from Califon to Kenvil, inclusive. Prior to the study reported here, the hydrogeology of some parts of the area had been investigated, but the regional flow system, particularly the interactions between the aquifers and the interactions between the aquifers and surface waters, had not been evaluated. On the basis of historical well yields and water-level drawdowns, the potential for additional ground-water development is substantial. Wells tapping the confined valley-fill aquifer and wells tapping the carbonate-rock aquifer have produced yields of up to 1,700 and 2,000 gal/min, respectively. Total withdrawals from the aquifers averaged about 5 Mgal/d in 1989 and were increasing in response to residential and commercial growth. In order to meet increasing water-supply needs, planning agencies have recommended that new well fields be developed to yield large volumes of potable water (Killam Associates, Inc., 1982, p. 6-15). This growth has resulted in concern about the adequacy of the aquifers in the area to meet the increasing demand for water and about hydrogeologic issues such as streamflow reduction, well-field interference, and contaminant migration. Although stream depletion and well-field interference are not known to be problems at present, several public supply and domestic wells tapping valley-fill sediments have become unusable recently as a result of organic contamination. Effective water-resources planning and management, which minimize the potential for these problems, require the definition of the regional hydrogeologic framework and flow system and an understanding of the operation of the flow system and its response to withdrawal stress. During 1987-90, the U.S. Geological Survey (USGS) in cooperation with the New Jersey Department of Environmental Protection, conducted a hydrogeologic study to obtain information and conduct analyses that would address these concerns to facilitate effective water-resources planning and management. Purpose and Scope This report describes the hydrogeology of the valley-fill and carbonate-rock aquifer system near Long Valley in the New Jersey Highlands and presents the results of a numerical ground-water-flow modeling analysis. Specific effects of pumpage that potentially could limit the availability of ground water for water supply, such as stream base-flow depletion, wellfield interference, and vulnerability to contamination (McAuley and others, 1992, p. 12), are addressed. The hydrologic assessment includes the interpretation of data on the hydrogeologic framework, aquifer and confining-unit characteristics, water levels, ground-water withdrawals, stream discharge, and water quality. Most of the hydrogeologic-framework information is based on work done by the New Jersey Geological Survey (NJGS) (L. J. Nicholson, New Jersey Geological Survey, written commun., 1990; Robert Canace, New Jersey Geological Survey, written commun., 1990). Interpretations of the water quality and geochemistry of the aquifer system include a discussion on the effects of land use on water quality. A conceptual model of the ground-water system is presented, and the mathematical representation of the aquifer system used to simulate groundwater flow is described. Throughout the report, results of ground-waterflow simulations are presented together with qualitative interpretations of hydrogeologic data. Numerical modeling techniques were used for the following analyses: (1) a steady-state analysis of recent (1988-89) conditions to estimate water-transmitting properties of aquifers and confining units, to test and refine concepts of flow and boundary conditions, and to determine the water budget; (2) a transient analysis of flow conditions during aquifer tests, in which water-transmitting properties and boundary conditions were tested further and refined; (3) an evaluation of conditions that prevailed prior to large groundwater withdrawals from the aquifer system; and (4) an evaluation of the simulated hydrologic effects of additional ground-water withdrawal from the carbonate-rock aquifer. Simulated hydrologic effects include base-flow depletion, waterlevel declines, and increased flow between aquifers. 74°52'30" 74°45' 74°37'30" Flanders ff Alamatong Well Field/. Flanders Valley Figure 1. Location of the study area in the New Jersey Highlands. Use of Simulation in Hvdrogeoloqic Analysis The numerical model developed for the purpose of analyzing the flow system is a simplified mathematical representation of the physical system. The model consists of a series of equations governing the flow of ground water in a discretized (gridded) representation of the system. The equations are solved simultaneously for head and flow at discrete points within the system. During the modeling process, concepts of flow are tested, watertransmitting properties are estimated, and various flow conditions are evaluated. Throughout this process, the model representations of hydrogeologic geometries, water-transmitting properties, flow-system boundaries, and imposed stresses reflect and integrate the concepts of the ground-water-flow system. Model output is the simulated distribution of hydraulic head and flow. This output is compared with observed data in order to evaluate the accuracy of the conceptual model and values of hydrogeologic properties. During calibration, the model is refined until simulated results are reasonably consistent with observed data, at which point the model is considered to represent the ground-water-flow system accurately. The degree to which the model accurately simulates historical conditions indicates the degree to which the model is expected to simulate accurately the effects of projected increases in ground-water withdrawals on the ground-water-flow system. Model output from simulations of specific hydrologic conditions provides insight into the operation of the ground-water system and its response to increased withdrawals for water-supply. Description of the Study Area The study area encompasses about 98 square miles in Morris and Hunterdon Counties in the central part of the New Jersey Highlands. It includes parts of the drainage areas of three major rivers: the Lamington River, the South Branch Raritan River, and the Rockaway River (fig. 1). In the Lamington River drainage area, the study area extends from the headwaters north of Kenvil to above Milltown. In the South Branch Raritan River drainage basin, the study area extends from the headwaters near Budd Lake to Hoffmans, 2 miles southwest of Califon. The entire extent of the Drakes Brook drain

Hydrogeology of, and ground-water flow in, a valley-fill and carbonate-rock aquifer system near Long Valley in the New Jersey Highlands