A Geochemical and Isotopic Approach to Delineate Landfill Leachates in an RCRA Study

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Abstract
Under many circumstances during an RCRA investigation, the EPA will not allow intrusive sampling of an SWMU (Solid Waste Management Unit, or “waste dump”). This presented difficulties in characterizing the waste within a closed landfill identified as an SWMU located on land owned by Purdue University, 3 kilometers northwest of the West Lafayette (Indiana) campus because there were few records of what was discarded into this public dump during its operation from 1958–87. Chemical and isotopic data of shallow ground water and surface runoff provided a realistic means to infer movement of contaminants away from the landfill.Leachate is characterized by high concentrations of Na+ and Cl− (∼ 4mN), sulfate (∼ 8 mN), total hardness (Ca+2+ Mg+2∼ 19 mN), and alkalinity (∼ 12mN). Disposal of road salt is responsible for the NaCl concentrations, while dissolution of concrete explains the high Ca+2, Mg+2, and alkalinity values for waters having a pC02 only slightly above the regional level for ground waters (10−1.82atm).Deep and shallow uncontaminated ground waters in the vicinity of the SWMU have an average δ34S value of 0.5 per mil (CDT). Since the δ34S of leachate lies between + 10 and + 17 per mil (CDT), the high sulfate concentrations are most likely due to dissolution of interred plasterboard consisting primarily of gypsum.Tritium activities for the leachate in two shallow wells emplaced 15 meters from the toe of the SWMU ranged from 55 to 85 TU. Correcting for half‐lives and correlating these activities to historical records of tritium in rainfall as measured at Chicago, these 3H activities suggest that the travel time between the SWMU interior and these wells is between 15 and 20 years.