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Idlewdd Market May 23, 1996 Page -5- 5 4 Ethylene Dibromide (EDB) EDB was modeled using a 20 year simulation without applying a degradation rate SESOIL predicted that it would take approximately 8 years for EDB to reach the capillary fringe under the conditions described above The predicted concentration of EDB at the capillary fringe is 2 0 µg/ml dissolved, 0 48 fig/g adsorbed, and 0 10 ttg/cm' air in pores (see Run 5 - Ethylene Dibromide, Appendix B) The primary MCL for EDB, as defined in the California Code of Regulations, Title 22, Drinking Water Regulatory Criteria, is 0 00002 µg/ml I 5 0 CONCLUSIONS IThe results of the SESOIL modeling indicate that without degradation, xylene and EDB would reach the capillary fringe, within approximately 10 to 20 years, at concentrations exceeding their respective primary MCL's as defined in the California Code of Regulations, Title 22, Drinking Water Regulatory Criteria The results of the modeling are based on the assumption that no surface pavement exists at the site The SESOIL modeling results for toluene and ethyl benzene, without applying a degradation rate, indicate that upon reaching the capillary fringe, the constituents will be at concentrations below their respective MCL's The soil type used in the model (generic-sand) is considered to be conservative because of the presence of fine-grained, well indurated, silt and silty sands at the site In addition, a majority of the site is paved with asphalt and concrete, which is not taken into account by SESOIL A sensitivity analysis for a biological degradation rate was run using a conservative degradation rate of 0 10 of the published rate for xylene The results yielded a very low concentration of xylene at the capillary fringe, well below the primary MCL A degradation rate for EDB could not be located According to Ms Anne Happel, Ph D , Lawrence Livermore National Laboratory, EDB does degrade abiotically and is heat sensitive The limited data available suggests that EDB degrades at a moderate rate in soil and water Therefore, usil,g no degradation rate for EDB is considered to be conservative Based on the information presented above, EDB is the gasoline constituent detected in the subject area that has the potential to impact groundwater at the site The modeling results indicate that the other I constituents will not impact groundwater underlying the site at levels exceeding the MCL's, considering conservative degradation rates and soil parameter assumptions Without applying a degradation rate for EDB, the results of the SESOIL modeling are likely to be exaggerated Sensitivity analysis indicates that even with a conservative degradation rate, such as the rate used for xylene, constituent concentrations are decreased dramatically I7 i) RECOMMENDATIONS Condor recommends that the excavated area be paved over with asphalt or concrete The pavement will limit infiltration and subsequent migration of surface water through the vadose zone thereby mimimizmg 1 the potential for mobilization of contaminates I ap CONDOR