CHARACTERIZATION OF LEACHATE DISTRIBUTION, REDOX CONDITIONS, AND THE PERSISTENCE OF ARSENIC IN GROUNDWATER AT THE AUBURN RD. LANDFILL SITE, LONDONDERRY, NEW HAMPSHIRE

The landfill site occupies a 180 acre parcel containing streams, ponds, wetlands and former gravel pits. Four areas totaling 14 acres have been used for waste disposal. Due to the discovery of volatile organic compounds associated with industrial waste dumping, the site was closed in 1980, and capped in 1996. Temporally persistent elevated arsenic concentrations occur in groundwater affected by the landfill leachate. In 2011, the U.S. Geological Survey, New Hampshire Department of Environmental Services and U.S. Environmental Protection Agency began an investigation to refine the hydrogeologic framework at the site and improve understanding of related in-situ geochemical processes.

Impermeable caps on landfills at the site prevent infiltration of precipitation. However groundwater may interact with landfill materials, creating additional leachate and ultimately reducing conditions in down-gradient groundwater. Elevated concentrations of total organic carbon from the leachate may be an important driver of reducing conditions that can liberate naturally occurring arsenic from glacial sediments and bedrock. Wetland areas that are found throughout the site also may be a source of carbon. Red-stained sediments are indicative of discharging iron-rich groundwater, which is often associated with areas of elevated arsenic. Groundwater has been observed at discrete locations discharging directly to wetland, stream and pond surface waters and in toe-slope seeps down gradient from the landfills.

Understanding the potential movement of leachate in groundwater is complicated by the presence of preferential flowpaths between three of the capped landfills and down-gradient surface waters and wetlands. Surface and borehole geophysical data collected in 2011 were used to identify zones of high hydraulic conductivity. Borehole data provide the opportunity to compare geophysical data to existing water quality information, which provides the basis for understanding geochemical processes that may be affecting arsenic mobility. Some areas in the overburden where the leachate plumes were present had lower electrical resistivity, low dissolved oxygen, and high arsenic concentrations. Low electrical-resistivity zones in the bedrock are associated with fractures that may also contain leachate.