Reconnaissance of hydrogeology and ground-water quality in Pennsauken Township and vicinity, Camden County, New Jersey, 1996-98

The Potomac-Raritan-Magothy aquifer system in the New Jersey Coastal Plain is the primary source of potable water supplies in northwestern Camden County. In Pennsauken Township and vicinity, local drinking-water supplies from this aquifer system have been affected by contaminants identified in the soils and ground water. The discovery of contaminated water in wells in Camden City's Puchack well field in the early 1970's led to the cessation of all pumping from the well field in April 1998. Contaminants identified at the well field include chromium, mercury, and organic compounds (principally trichloroethylene), all of which were detected at concentrations that exceeded the respective New Jersey Department of Environmental Protection (NJDEP) maximum contaminant levels. Hexavalent chromium, the most toxic and mobile form, represented about 90 percent of the chromium detected. Ground water at some nearby well fields also was contaminated with volatile organic compounds. By 1996, as part of its Site Remediation Program, NJDEP had identified more than 38 contaminant sites in Pennsauken Township, most of which are in the outcrop area of the PotomacRaritan-Magothy aquifer system. Many watersupply wells also are in the outcrop area of the aquifer system. The juxtaposition of the wells and contaminant sites maximizes the vulnerability of the ground-water resources to contamination in the Pennsauken Township area. Hydraulic gradients induced by pumping have facilitated the transport of shallow contaminants into the aquifer system, and the southeasterly direction of ground-water flow indicates that contaminants in the outcrop area eventually could reach other, downgradient water-supply wells. In 1996, the U.S. Geological Survey (USGS), in cooperation with the NJDEP, began a ground-water investigation that included collecting new hydrogeologic and water-quality information in order to describe local conditions in the Potomac-Raritan-Magothy aquifer system in the Pennsauken Township area. Water levels in 128 wells were used to prepare potentiometric-surface maps, and analyses of water-quality samples from 55 wells were used to describe the nature and extent of the ground-water contamination. Hydrogeologic and geophysical data obtained from 72 wells, including the 12 new monitoringwell clusters installed in 1997, were used to refine the concept of the local hydrostratigraphic framework of the aquifer system, which previously had been described as Upper, Middle, and Lower aquifers separated by confining units. These analyses were done using data obtained from the USGS's National Water Information system (NWIS) database in 1998. Local discontinuities in the permeable sand and gravel aquifers and low-permeability silt and clay confining units were identified during the study. Discontinuities in the confining units increase the likelihood of contaminant transport within the aquifer system. An Intermediate Sand layer within the confining unit between the Middle and Lower aquifers is connected hydraulically to the Lower aquifer and likely is an important pathway through which chromium-contaminated water reaches the Lower aquifer. To date (1998), high concentrations of chromium (as high as 3,320 ng/L) deep in the Lower aquifer have been found only near the Puchack well field, indicating that the historical pumping stress at the well field has contributed to the transport of chromium to the Lower aquifer in this area. Analysis of ground-water samples revealed the widespread presence of volatile organic compounds, principally dissolved trichloroethylene and tetrachloroethylene, in both the Middle and Lower aquifers. Although concentrations of these and other chlorinated compounds typically were less than 100 micrograms per liter, concentrations of at least one compound in most of the samples exceeded the NJDEP maximum contaminant levels for drinking water. Delineation of the extent of both inorganic and organic contamination is limited by the number and distribution of existing wells. Available data are insufficient to define the actual extent of the contamination or to determine its source(s).