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' Pombo Property Corrective Action Plan Page: 6 ' The executive officer of the State Water Resources Control Board in December 1995 provided guidance on the disposition of impacted shallow, near surface ' groundwater contamination,. The guidance document states that "passive bioremediation should be considered the primary remediation tool". "For cases affecting low risk groundwater (for instance, shallow groundwater with a ' maximum depth to water less than 50 feet and no drinking water wells screened in -the shallow aquifer with 250 feet of the leak) we recommend that active remediation be replaced by with monitoring to determine if the plume is stable". tStable plumes should be considered for closure. Clean up levels for this site are dependent on whether the SWRCB 12195 site ' conditions are present and passive bioremediation and plume monitoring are appropriate. If the SWRCB 12195 site conditions are present (maximum depth to water less than 50 feet and no drinking water wells screened in the shallow Z aquifer with 250 feet of the leak) clean up levels as described by the state board are present numerical clean up values would not be appropriate. ' if the SWRCB 12195 site conditions are not present than clean up values as described for domestic or municipal beneficial are appropriate. 4.0 MITIGATION ALTERNATIVES Preliminary fate and transport modeling of the identified contaminates of concern (volatile aromatic hydrocarbons and methyl tert butyl ether) under of four ' possible mitigation scenarios using the USEPA model Bioscreen4." Corrective action scenarios for low risk closure, excavation of source material, enhanced aerobic biodegradation, and excavation of source material with enhanced aerobic ' biodegradation were evaluated. Model input parameters for "Bioscreen4" are presented in Table 3, Bioscreen ' Input Parameters. Hydrogeologic, dispersion, absorption, and biodegradation parameters were provided from onsite groundwater monitoring data and published values. 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 2. ' 4.1 Low Risk Closure The mitigation strategy consists of the evaluation of the site to confirm the low risk nature of the site as determined by the State Waters Resources Control Board ' (SWRCB) Policy of December 1995. Monitoring of the onsite domestic well to determine screened interval and collection of water samples for laboratory analysis s would be conducted. If the site met the requirements of the SWRCB ' Policy of December 1995, the site would be proposed for closure. 1