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Government Document

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A plume of contaminated ground water has been delineated within an 11.4-square-mile area in east-central Nassau County, where residential neighborhoods surround an area zoned for industrial use. The industrial zone contains several firms that, in the past, have discharged effluent containing volatile organic compounds into the upper glacial aquifer through onsite recharge basins. The upper glacial aquifer is in direct hydraulic connection with the underlying Magothy aquifer; the first continuous formation that impedes downward movement of ground water is the Raritan confining unit, which is more than 500 feet below sea level.

The chemicals in ground water·and their distribution were identified through analysis of water samples collected from 56 monitoring wells and 11 industrial wells in the spring and fall of 1986 and 1987. Trichloroethylene and tetrachloroethylene were found near the water table at concentrations greater than 1,000 micrograms per liter and were detected more than 5,000 feet downgradient of the industrial zone. The distribution of several other volatile organic compounds indicates more than one contaminant source in the industrial area. The plume of contaminated ground water in 1987 was 12,000 feet long, 5,700 feet wide, and more than 500 feet thick.

In 1987, water was pumped from 14 industrial wells, completed in the Magothy aquifer, at an average rate of 8.05 million gallons per day, mainly for cooling purposes. The water was returned chemically unaltered to recharge basins from which it could percolate to the water table. Water-table mounding beneath basins and drawdowns near the pumped wells greatly increase the vertical component of ground-water flow beneath the industrial zone, which has increased the rate of advective movement of contaminated ground water downward toward the screened zones of pumped wells, which are 370 to 560 feet deep in the Magothy aquifer. The concentration of tetrachloroethylene decreases much more rapidly than that of trichloroethylene downgradient of the industrial zone, which indicates that sorption and (or) biodegradation may be occurring. The major effect of industrial activity on inorganic constituents is to decrease the concentration of constituents near the water table in the vicinity of the recharge basins by the addition of water from the Magothy aquifer, where the concentration of inorganic constituents is lower.