Human and Ecological Risk Assessment - Earthjustice

Appendix CSite DataC.3 Soil DataThe groundwater model used in the CCW risk assessment—EPA’s Composite Model forLeachate Migration with Transformation Products (EPACMTP)—requires soil properties for theentire soil column to model leachate transport through the vadose zone to groundwater. As withaquifer type, soil data were collected within a 5-km radius of each CCW plant. A GIS was usedto identify soil map units within a 20-mile radius around each meteorological station. Databaseprograms were then used to assemble and process soil texture, pH, and soil organic matter datafor these map units from the State Soil Geographic (STATSGO) database. Both pH and soilorganic matter were processed and indexed by the soil textures present within the 5-km radius.Soil properties are listed by texture for each of the 181 CCW plants in Attachment C-1.C.3.1 Data SourcesThe primary data source for soil properties was the STATSGO database. STATSGO is arepository of nationwide soil properties compiled primarily by the U.S. Department ofAgriculture (USDA) from county soil survey data (USDA, 1994). STATSGO includes a1:250,000-scale GIS coverage that delineates soil map units and an associated databasecontaining soil data for each STATSGO map unit. (Map units are areas used to spatiallyrepresent soils in the database.)In addition, two compilations of STATSGO data, each keyed to the STATSGO map unitGIS coverage, were used in the analysis as a convenient source of average soil properties:• USSOILS. The USSOILS data set (Schwarz and Alexander, 1995) averages STATSGOdata over the entire soil column for each map unit.• CONUS. The Conterminous United States Multi-Layer Soil Characteristics (CONUS)data set (Miller and White, 1998) provides average STATSGO data by map unit and a setof 11 standardized soil layers.Soil organic matter and pH were derived directly from USSOILS and STATSGO data. Acomplete set of hydrological soil properties 2 was not available from STATSGO. To ensureconsistent and realistic values, EPACMTP relies on established, nationwide relationshipsbetween hydrologic properties and soil texture. Peer-reviewed publications by Carsel and Parrish(1988) and Carsel et al. (1988) provide a consistent set of correlated hydrologic properties foreach soil texture. Soil texture data for the entire soil column were collected from the CONUSdatabase.C.3.2 MethodologyThe soil data collection methodology began with GIS programs (in Arc Macro Language[AML]). These programs overlaid a 5-km radius around each CCW plant location on theSTATSGO map unit coverage to determine the STATSGO map units and their area within theradius. These data were then passed to data processing programs that derive soil properties for2 Hydrological soil properties required by EPACMTP include bulk density, saturated water content, saturatedhydraulic conductivity, and the van Genuchten soil moisture retention parameters alpha and beta.April 2010–Draft EPA document. C-5