Human and Ecological Risk Assessment - Earthjustice

Section 2.0Problem FormulationAs shown in Figure 2-2, EPA focused full-scale modeling on groundwater-to-drinkingwaterand groundwater-to-surface-water exposure pathways for the national risk assessment.This groundwater pathway analysis evaluated exposures through drinking water ingestion andsurface water contamination from groundwater discharge. For the groundwater-to-surface-waterpathway, the analysis assumed that human exposure occurs through the consumption ofcontaminated fish and that ecological exposure occurs through direct contact with contaminatedsurface water and sediment or from the consumption of aquatic organisms.2.2.2 Conceptual Site LayoutsThis risk assessment was based on site layouts that are conceptual rather than sitespecific.Although EPA had plant locations and some site-specific data on WMUs, we did nothave the exact locations of each WMU or the residential wells surrounding each facility.Therefore, EPA had to develop conceptual layouts to place receptors around each WMU.The conceptual site layouts capture possible relationships between a WMU and humanand ecological receptors by locating, with respect to the WMU boundary, the geographicfeatures (i.e., receptor wells, waterbodies) that are important for determining human andecological exposures to chemicals released from CCW landfills and surface impoundments.Two site layouts were used in the full-scale analysis to model the land use scenarios ofmost concern for CCW disposal facilities:• Residential groundwater ingestion scenario• Recreational fisher and aquatic ecological risk scenario.These two conceptual site layouts are shown in the following two subsections, includingWMU boundaries, waterbodies, and residential wells modeled in this analysis. In the conceptualsite layouts, the WMU is represented as a square source. The size of the source was determinedby the surface area of the WMU (CCW WMU areas were collected from the EPRIcomanagement survey, as described in Appendix B). The WMU was assumed to be located atthe property line of the facility to which it belongs.Adjacent to the WMU is a buffer area within which there was assumed to be no humanactivity that would present human risk (i.e., there are no residences or waterbodies in the buffer).The buffer area lies between the WMU boundary and the residential well or waterbody, andrepresents the distance to well or waterbody discharge point modeled by the groundwater model.Each site layout must also be oriented in terms of direction.2.2.2.1 Residential Groundwater-to-Drinking-Water ScenarioThe residential groundwater-to-drinking-water scenario, shown in Figure 2-3, calculatedexposure through residential use of well water as drinking water. In the Monte Carlo analysis,the receptor well was randomly placed up to 1 mile downgradient from the edge of the WMU(this radial well distance is labeled R rw in Figure 2-3), based on a nationwide distribution ofnearest downgradient residential wells from Subtitle D municipal landfills (U.S. EPA, 1988a;this distribution is provided in Appendix C). EPA assumed that this distribution was relevant toApril 2010–Draft EPA document. 2-9