3D DISTRIBUTION OF ARSENIC CONTAMINATION IN GROUNDWATER FROM DIRECT-PUSH INTERVAL SAMPLING AT A LANDFILL IN NORTH-CENTRAL MASSACHUSETTS

Shepley's Hill Landfill (SHL) is a closed landfill overlying thick glacial deposits (50 to 100 ft) in North-Central Massachusetts with known disposal activity that lasted from 1917 until 1992 when the landfill was closed and capped by an impermeable cover. Its associated contaminant leachate plume is particularly noted for surprisingly high levels of arsenic concentration in the affected ground waters. Data from 15 years of monitoring and groundwater sampling show arsenic levels that occasionally exceeded 5,000 ppb of arsenic in groundwater. During summer of 2010 a comprehensive study of the landfill area included direct-push methods and roto-sonic drilling during which soil samples were obtained as well as information on the vertical profiles of groundwater. The study was conducted by the Army Corps of Engineers at 18 separate locations within and near SHL at regular 10 ft vertical intervals. Groundwater samples collected from each interval were analyzed by Ion Chromatography for major ions (Na⁺, K⁺, Ca⁺², Mg⁺², NH₄⁺, Cl^-, NO₃^-, and SO₄^-2), and for arsenic by Hydride Generation Atomic Fluorescence Spectrometry (HG-AFS). The results indicate in 3D the regions of elevated arsenic concentrations and more importantly provide a unique opportunity to delineate arsenic pathways in 3D space.

With few exceptions, all groundwater samples from the direct-push wells located within the SHL have arsenic concentrations that are above 10 ppb (the MCL limit for As in groundwater). More striking is the vertical distribution of arsenic in groundwater within a zone that lies below the landfill bottom. This zone extends from the landfill bottom toward the basal glacial till and encompasses a lens of significantly elevated arsenic concentrations in groundwater with a strong concentration gradient toward the center and lower limit of the zone. The central zone of the lens which appears to be less than 10 ft thick yields excessively elevated arsenic concentrations (as much as and above 10,000 ppb of As in groundwater).