NATURAL ATTENUATION AS THE CORRECTIVE ACTION FOR LANDFILL GAS EFFECTS ON GROUNDWATER

摘要

Detections of volatile organic compounds (VOCs) in groundwater samples from a monitoring well located downgradient of the Forster Canyon Landfill led to an investigation. The objectives of the investigation were to evaluate the source of the VOCs in the groundwater (landfill gas or landfill leachate) and the extent of VOC-impacted groundwater. Landfill gas was suspected as the source of the VOC detections in the groundwater based on methane concentrations in the headspace of this well and based on the absence of semi-VOCs and other organic compounds in the groundwater samples. Historical groundwater data for this well were evaluated and it was found that increases in VOC concentrations were not accompanied by increases in sodium or chloride concentrations, ruling out a leachate source for the VOC detections. The stratigraphy around the well includes a sand lens in the unsaturated zone, approximately 5-feet thick, which is overlain and underlain by silt and clay layers. Pressure-driven landfill gas transport through the unsaturated sand lens directly into the well was suspected as the process by which the landfill gas affected groundwater. This "in-well" landfill gas effect was evaluated during the investigation. The monitoring well headspace gas and groundwater VOC concentrations were compared and it was found that the direction of the concentration gradient was consistent with landfill gas-to-groundwater transport of the VOCs. Fate and transport modeling of the VOCs in the groundwater phase was then performed using the US EPA model BIOSCREEN for natural attenuation. Because of the limited area of the in-well landfill gas effects, modeling results indicated that VOC concentrations decrease by natural attenuation to regulatory drinking water standards and to laboratory method detection limits within 135 ft of the impacted well. Because the distance to the closest potential receptor is over 3,000 ft natural attenuation was found to be an adequate corrective action for the VOC plume.