Stable isotopic study of groundwater arsenic contaminated plume at Shepley's Hill Landfill

In the northeast United States, arsenic (As) contamination in groundwater is frequently associated with historical landfill leachate plumes. Based on the history of Shepley's Hill Landfill (SHL) in Devens, MA, solid waste disposal activities spanned nearly a century of landfilling with little or no documentation of when or what waste material was disposed. Past geochemical investigations proved the presence of high levels of As in groundwaters within and around the SHL region. A total of 114 samples were collected from the SHL region and analyzed for their hydrogeochemistry and isotopic signature. Since the isotopic ratios of aD and a18O can potentially be influenced by the mobilization process of As, this study attempts to identify any correlations between the stable isotopic ratios and the hydrogeochemistry of SHL waters. The results of the groundwater hydrogeochemical analysis show multiple relationships between metal concentrations and As concentration levels, typical of groundwater undergoing redox reactions. The result of the stable isotope analysis show significant fractionation of stable isotope ratios away from the meteoric water line. However, the role of strong redox gradients and various redox ladder reactions involving water did not produce a significant correlation with the isotopic fractionations present within different zones of the landfill. In most cases, the fractionations stand independent of the increase/decrease in As concentration and can be attributed to either unrelated chemical reactions within groundwater or evaporation.