Human and Ecological Risk Assessment - Earthjustice

Section 2.0Problem Formulationpotential for cumulative exposure from both contaminated fish and groundwater was notconsidered in the CCW risk assessment. One reason is that the exposures are likely to occur overdifferent timeframes because of differences in transit time of the contaminant plume to wellsversus surface waterbodies. As described in Section 3.6.3, for each model run in the MonteCarlo analysis, the distances to the downgradient well and surface water were independentlysampled from national distributions presented in Appendix C, Tables C-1 (wells) and C-2(surface waterbodies). Also, these exposures may involve different receptors because a residentexposed via groundwater may not be a recreational fisher. Thus, adding risks across pathwayswould not likely change the results.The waterbody was assumed to be a stream or lake located downgradient from the WMU,beginning where the buffer area ends (see Figure 2-4), and was also used as the most impactedaquatic system for the ecological risk assessment. Waterbody characteristics were determinedbased on a combination of site-specific, regional, or national data (as described in Appendix C),except for the length of the stream impacted by the plume, which was determined by the width ofthe plume as it intersects the waterbody.The downgradient distance to the surface water body was determined from a nationaldistribution developed by measuring this distance (using scaled U.S. Geological Survey [USGS]maps and aerial photographs obtained from the Terraserver Web site [http://terraserver.usa.com/geographic.aspx]) at 59 CCW landfill and surface impoundment sites randomly selected from alarger data set of 204 CCW WMUs, including those modeled in this risk assessment.Appendix C presents that distribution and further details on how the distribution was developedand the sample of 59 facilities used to develop the distribution.Figure 2-4. Conceptual site layout for residential fisher andaquatic ecological risk scenario.April 2010–Draft EPA document. 2-11