Provided in cooperation with New Jersey Department of Environmental Protection
Program to Maintain and Update Groundwater Models
NJCP Sharp (Coastal Plain optimization)
Simulation of groundwater flow and movement of the freshwater-saltwater interface in the New Jersey Coastal Plain
By Daryll A. Pope and Alison D. Gordon
The confined aquifers of the New Jersey Coastal Plain are sands that range in thickness from 50 to 600 feet and are separated by confining units. The confining units are composed of silts and clays that range in thickness from 50 to 1,000 feet. The aquifers are recharged by precipitation on their outcrop areas. This water then flows laterally downdip and vertically to the deeper confined aquifers. The confined aquifers ultimately discharge to the Raritan and Delaware Bays and to the Atlantic Ocean.
In 1988, groundwater withdrawals from confined and unconfined New Jersey Coastal Plain aquifers were approximately 345 million gallons per day, more than 75 percent of which was pumped from the confined aquifers. These withdrawals have created large cones of depression in several Coastal Plain aquifers near populated areas, particularly in Camden and Monmouth Counties. The continued decline of water levels in confined aquifers can cause saltwater intrusion, reduce stream discharge near the outcrop areas, and threaten the quality of the groundwater supply.
SHARP, a quasi-three-dimensional finite-difference computer model that can simulate freshwater and saltwater flow, was used to simulate the groundwater flow system in the New Jersey Coastal Plain, including the location and movement of the freshwater-saltwater interface in nine aquifers and eight intervening confining units. The freshwater-saltwater interface is defined as the hypothetical line seaward of which the chloride concentration is equal to or greater than 10,000 milligrams per liter. Model simulations were used to estimate the location and movement of the freshwater-saltwater interface resulting from (1) eustatic sea-level changes over the past 84,000 years, (2) groundwater withdrawals form 1896 through 1988, and (3) future groundwater withdrawals from 1988 to 2040 from Coastal Plain aquifers. Simulation results showed that the location and movement of the freshwater-saltwater interface are more dependent on the historical sea level than on the stresses imposed on the flow system by groundwater withdrawals form the Coastal Plain aquifers from 1896 to 1988.
Results of a predictive simulation in which pumpage from existing wells was increased by 30 percent indicate that additional withdrawals from each of the eight confined aquifers in the Coastal Plain would broaden and deepen the existing cones of depression and result in significant drawdowns from the 1988 potentiometric surfaces. Drawdowns of 30 feet were simulated at the center of the cone of depression in the Upper, Middle, and Lower Potomac-Raritan-Magothy aquifers in Camden and Ocean Counties. Simulated drawdowns exceeded 80 feet at the center of the cone of depression in the Wenonah-Mount Laurel and Englishtown aquifers in Monmouth County. Drawdowns of 30 feet were simulated in the lower Kirkwood-Cohansey and confined Kirkwood aquifers in Cape May County. Simulation results showed that the increase in groundwater withdrawals would result in only minimal movement of the freshwater-saltwater interface by 2040, despite large drawdowns.