Ground-water flow and solute transport at a municipal landfill site on Long Island, New York – Part 1: Hydrogeology and water quality

Abstract

As part of a study on the transport of contaminants in ground water, the hydrogeology and chemical quality of ground water in a 4-square-mile area surrounding a municipal sanitary landfill in the Town of Brookhaven, Long Island, N.Y., were investigated during 1981-83. The landfill is excavated in glacial-outwash deposits that form part of the upper glacial aquifer and is lined with a polyvinyl chloride membrane. The aquifer in that area consists mostly of highly permeable coarse sand and gravel and is a principal source of water supply for the Town. The saturated thickness of the aquifer ranges from 100 to 120 feet. Ground water moves southeastward at about 1.1 feet per day. Although the landfill is lined, water samples from wells downgradient contained elevated concentrations of several inorganic ions, which indicates that leachate has entered the aquifer. A leachate plume 3,700 feet long, 2,400 feet wide, and at least 90 feet thick was delineated from specific-conductance data from monitoring-well samples. Water quality in the Magothy aquifer and in Beaverdam Creek, a stream fed by ground water 2,000 ft southeast of the landfill site, does not appear to be contaminated by leachate. The areal distribution of sodium and chloride is similar to that of specific conductance. Both sodium and chloride appear to be conservative and are attenuated by dispersive mixing only. Bicarbonate, as measured by total alkalinity, is attenuated by several chemical processes,including oxidation of reduced species in leachate. Ammonium concentrations decrease downgradient of the landfill through ion exchange and, along the edges of the plume, by oxidation of ammonium ion to nitrate. INTRODUCTION Burial of municipal waste in sanitary landfills is the primary means of solid-waste disposal on Long Island. The high permeability of the glacial outwash deposits in the southern part of Long Island allows leachate from unlined sanitary landfills to easily enter the water-table aquifer. A recent study by Kimmel and Braids (1980) described the degradation of water quality downgradient from two unlined landfills in the Towns of Babylon and Islip (fig. 1). To minimize the potential for ground-water contamination, newly constructed sanitary landfills on Long Island must be underlain by impermeable