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Ripona Market October 28,2004 Corrective Action Plan 4.0 MITIGATION ALTERNATIVES Preliminary fate and transport modeling of benzene therimary contaminate of cnc� oinder of two possible mitigation sic—enari os using the A model "Bioscreen4 " The modeling was based on average values and site conditions but without site specific biodegradation indicators The corrective action scenario for low risk closure (no action) was modeled Model input parameters for "Bioscreen4" and-belie glass balance calculations are presented in Appendix B, Bioscreen Input Parameters Hydrogeologic, dispersion, absorption, and biodegradation parameters were provided from onsite groundwater monitoring data and published values The model is limited in the soluble mass value that can be used in the model An estimate of the contaminant mass balance is presented in Appendix B Approximately 0 3 kilograms of MTBE are estimated to present at the site The minimum value that can be used in the model is 1 kilogram which greatly over estimates the magnitude and potential extent of the plume migration Model runs were evaluated to determine the maximum extent of plume migration and remaining contamination at plume stabilization or end of modeled time period Model results are presented in Appendix C • The model was calibrated for existing conditions using recent monitoring data Groundwater flow directions have remained consistently to the northeast The plume appears to be stable and under going degradation / dispersion along the expected pattern for a first order decay pattern with long solute half life as expected with MTBE The source contaminant concentrations have dropped from Y-1117Upf s f L 6300 to 450 in 4 years for an estimated source half life of 1 year �ol�te yd hk, Under existing conditions the plume is predicted to degrade to an MTBE concentration of 0 ug/l in 18 years from a current concentration of 450 ug/1 , at the identified receptor at 1000 feet down gradient Assuming no degradation, it is estimated that simple dilution and attenuation will reduce the plume to 10 ug/1 at 1000 feet 4.1 Low Risk Closure The mitigation strategy consists of the evaluation of the site to confirm the low I risk nature of the site as determined by the State Waters Resources Control Board (SWRCB) Policy of December 1995 Monitoring of the site monitoring wells and domestic wells is conducted to verify the natural breakdown of the contamination I plume I 9