Human and Ecological Risk Assessment - Earthjustice

Section 3.0Analysisobtained from a study of industrial surface impoundment membrane liner leak rates by TetraTech (2001).3.5.4 Surface Impoundment Model OutputsFor each year in the simulation, the surface impoundment source-term model uses theaverage annual leachate concentration and calculates an infiltration rate to estimate theconstituent flux through the bottom of the impoundment. This time series was used as an input tothe EPACMTP unsaturated zone model.3.6 Groundwater ModelThis section describes the methodology and the models that were used to predict the fateand transport of chemical constituents in soil and groundwater to determine impacts on drinkingwater wells and surface water that is connected to groundwater. The surface water model used toaddress the groundwater-to-surface water pathways is described in Section 3.7.3.6.1 Conceptual ModelThe groundwater pathway was modeled to determine the receptor well concentrationsand contaminant flux to surface water resulting from the release of waste constituents from aWMU. The release of a constituent occurs when liquid percolating through the WMU becomesleachate as it infiltrates from the bottom of the WMU into the subsurface. For landfills, the liquidpercolating through the landfill is from water in the waste and precipitation. For surfaceimpoundments, the percolating liquid is primarily the wastewater managed in the impoundments.Waste constituents dissolved in the leachate are transported through the unsaturated zone(the soil layer under the WMU) to the underlying saturated zone (i.e., groundwater). Once in thegroundwater, contaminants are transported downgradient to a hypothetical receptor well orwaterbody. For this analysis, the groundwater concentration was evaluated for two receptorlocations, each at a specified distance from the downgradient edge of the WMU:• The intake point of a hypothetical residential drinking water well (the receptor well),which was used for the residential drinking water pathway• A nearby river, stream, or lake, which is modeled as a fully penetrating surfacewaterbody and was used for the fish ingestion and ecological pathways.Figure 3-6 shows the conceptual model of the groundwater fate and transport of contaminantreleases from a WMU to a downgradient receptor well.April 2010–Draft EPA document. 3-32