Plumes of leachate-enriched ground water extend 10,600 and 5,000 ft (3,200 and 1,500 m) downgradient from landfills in the towns of Babylon and Islip, respectively, and extend vertically beneath the landfills to the base of the upper glacial aquifer, whose thickness ranges from 71 to 77 ft (22-24 m) at the Babylon site and is 170 ft (52 m) at the Islip site. The Babylon and Islip landfills were started in 1947 and 1933, respectively. The quantities of ground water in the plumes are 2X109 gal (8X106 m3) at Babylon and 1X109 gal (4X106 m3) at Islip. Differences in quantity of water in the plumes and length of the plumes may result partly from a slower rate of refuse accumulation at Islip than at Babylon; however, the rate of travel for the front of the Islip plume is considerably less than the prevailing groundwater velocity would indicate. Distance traveled by the front of the Islip plume was roughly one-third the distance indicated by the calculated minimum ground-water velocity and may result from greater vertical travel distance in the upper glacial aquifer at Islip. Concentrations of HCO3-, Cl-, and SO4-2 in the leachate-enriched ground water are higher than those in ambient ground water. Concentrations of Fe and Mn in the plumes are as much as 400,000 μg/1 and 200,000 μg/1, but concentrations of other metals, including Zn (220 μg/1), do not exceed the U.S. Public Health Service drinking-water standards. The concentration of NH4+, the predominant nitrogen species in plume water, is usually more than 10 percent of the total cations in plume water and is greater than NH4+ concentrations in ambient ground water; concentrations of NO3- in plume water are less than those in ambient ground water at Babylon and at some locations in Islip.
