Human and Ecological Risk Assessment - Earthjustice

Appendix CSite DataQuality assurance/quality control (QA/QC) measures included independent review of theassignments by other geologists with expertise in assigning settings.C.4.3 Data ProcessingHGDB hydrogeologic environment fractions (i.e., the portion of the region assigned toeach of the 12 hydrogeological environments) were defined and used in the CCW riskassessment as follows. If the 5-km radius around a site contained only one HGDB environment,the fraction assigned was 1.0 and all groundwater model runs for that location were associatedwith that hydrological environment. If more than one HGDB environment was present, eachenvironment was assigned a fraction based on the areal percentages of each setting within the5-km radius.These fractions were used to generate the hydrogeologic environment for that location foreach iteration of the Monte Carlo groundwater modeling analysis. For example, if twohydrogeologic environments were assigned to a CCW site with a fraction of 0.5, half of therealizations were modeled with the first hydrogeologic environment and half with the second.Once the hydrogeologic environments were assigned, a preprocessing run of EPACMTPwas conducted to construct a set of randomly generated but correlated hydrogeologic parametervalues for each occurrence of the hydrogeologic environments in the source data files. Missingvalues in the HGDB data set were filled using correlations, as described in U.S. EPA (1997).C.4.4 ResultsAttachment C-2 lists the hydrogeologic environment assignments for each CCWdisposal site. Table C-4 summarizes these results, showing the crosswalk between GroundwaterRegions, hydrogeologic settings, and hydrogeologic environments used to make the assignments,along with the number of CCW sites for each setting. Table C-5 totals the number of CCWdisposal sites for each hydrogeologic environment sent to EPACMTP.Hydrogeologic SettingAlluvial BasinsTable C-4. Groundwater Regions, Hydrogeologic Settings, andHydrogeologic Environments: CCW Disposal SitesHydrogeologicEnvironmentNumber ofCCW Sites2C Alluvial Fans 5 12E Playa Lakes 5 12Ha River Alluvium With Overbank Deposits 6 1Colorado Plateau and Wyoming Basin4B Consolidated Sedimentary Rock 2 74C River Alluvium 7 3High Plains5Gb River Alluvium Without Overbank Deposits 7 1(continued)April 2010–Draft EPA document. C-9