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P0 availability of other nutrients for microbial metabolism, the water solubility of the chemical and possibly other factors Biodegradation in SESOIL is modeled mathematically as a simple ' first order decay function for both the liquid phase and the solid phase SESOIL does not attempt to model the extremely complex process of biodegradation The chemical specific decay rates for both the liquid phase and the solid phase applicable to the given site are input ' Biodegradation rates are rarely measured for the liquid phase and the solid phase. The liquid phase and the solid phase biodegradation are generally lumped together and the over-all biodegradation rate is reported Biodegradation rates for benzene, toluene and xylenes are calculated from conservative biodegradation half-life's for natural sites reported in Table 15 2 8 of reference no. 3 A biodegradation rate for ethylbenzene in soil and groundwater was located in Table 9.9 of reference no 6 ' 5.4 Results of Analysis 5.4.1 SESOIL Output Files ' Using the input parameters discussed in Section 5 3, four output runs for benzene, toluene, ethyl benzene, and xylenes were generated to assess the potential for volatilization and leaching of aromatic ' constituents under a conservative model scenario Model input parameters for these runs are shown on the output files, as well as monthly and annual summaries of the output The model for benzene, the most soluble of the aromatic hydrocarbons,was run for a period of ten years The complete output file for this model run is contained in Appendix A The models for toluene, ethylbenzene, and xylenes were each run for 99 years and the output files are contained in Appendices B, C, and D, respectively Due to the size of these output files these appendices only contain the input parameter summary and the annual summaries for years 1, 5, 10, 25, 50, and 99 The output of annual summary contains the total pollutant mass inputs, hydrologic cycle components, the total pollutant mass removed from each layer, the average pollutant concentration distributions for each layer, and the maximum pollutant depths The pollutant mass in the soil moisture, adsorbed on soil and in soil air and the total maximum pollutant depth are summarized in Tables 6 through 9 ' 4 2 Groundwater Impact Assessment 5. . p ' After 10 years of simulated transport the maximum pollutant depth of benzene below the site is 9 8 feet (3 meters) At this time the residual concentrations of benzene are all less than 7 x 10-9 ppm In fact ' after the second year concentrations have decreased to values less than the detection limit for benzene in soil The model runs for toluene, ethylbenzene and xylenes produced similar results The maximum pollutant depth of these constituents was seen for toluene which reached a depth of 26 5 feet at concentrations less than 1 x 10'10 Based upon the SESOIL- results presented in the output files, no----= _ - 1 leaching of hydrocarbon constituents from the impacted soils to groundwater at the site is anticipated and constituents will naturally degrade to nondetectable levels within two to five years 1 w 194368tireportslsesod doc 10 SMTH 1