Investigation of Hydrogeologic Impacts at Two Historic Landfills Overlying Fractured Bedrock in the Lower Hudson Valley, NY

Investigation of two types of landfills, a construction and demolition (C&D) debris landfill and a municipal solid waste landfill, overlying two separate bedrock lithologies provides evidence for the ability of these types of groundwater regimes to mitigate and prevent the migration of contaminants to the underlying bedrock aquifer. A marble quarry turned municipal landfill for 50 years was investigated as part of the New York State Department of Environmental Conservation (NYSDEC) Brownfield Cleanup Program (BCP). An illegally created C&D landfill overlying metamorphic rock in close proximity to a New York City Department of Environmental Protection (NYCDEP) reservoir was investigated as part of a proposed landfill closure plan. The investigations included installation of multiple monitor wells in the landfill material (overburden aquifer) and the underlying regional bedrock aquifers. Both investigations included coring the underlying bedrock to determine the relationship of secondary permeability features in the bedrock to the overlying landfill material. Primary constituents of concern in the municipal landfill material included volatile organic compounds (VOCs [PCE and TCE]), semi-volatile organic compounds (SVOCs) and heavy metals while constituents of concern in the C&D landfill included SVOCs and metals. Extensive groundwater monitoring and sampling of the monitor well arrays was completed to establish vertical and horizontal gradients between the two aquifers and groundwater flow direction across the landfills. The results of the two investigations indicated that constituents of concern in the landfill material were predominantly not found in the groundwater moving through the bedrock, nor was there evidence that contaminants travelled through the bedrock to the adjacent reservoir near the C&D landfill. The information garnered in both investigations was used to work with the regulatory agencies in the design of a landfill cap and BCP cleanup.