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. Updated Corrective Action Plan—Tosco(76)Service Station No 11192, Stockton, California June 26 2002 3.0 CORRECTIVE ACTION PLAN COMPONENTS 3 1 Irn act Assessment 1. The physical and Chemical characteristics of the hazardous substance or its constituents, including their toxicity, persistence, and potential for migration in water, soil, and air: Based on the analytical results for both soil and groundwater samples collected to date, the primary contaminant appears to be gasoline (BTEX constituents and MtBE) Material Safety Data Sheets (MSDS) for unleaded gasolines are attached to this CAP in Appendix A The MSDS sheets describe the physical and chemical characteristics of gasoline and its components The analytical results of the groundwater samples collected from the monitoring wells at and in the vicinity of the site indicate that dissolved concentrations of petroleum hydrocarbons are present in the wells Free product has never been detected at the site It is GR's understanding that as of January 1, 2001, Tosco no longer delivers fuel containing MtBE to service stations in northern California The shallow MtBE plume beneath the site is predominantly defined Previous investigations at the site indicate that the plume is largely confined to the site,although detections of petroleum hydrocarbons in the onsite deep well and offsite Geoprobes indicate that additional vertical and lateral delineation is necessary in the downgradient direction The extent of the hydrocarbon impact to the deeper water bearing zone has not been fully delineated, additional deep monitoring wells are needed to complete the delineation GR proposes to install four double-cased monitoring wells to delineate the deeper hydrocarbon plume (see Section 4 0) Iso-concentration maps for TPHg,benzene, MtBE and TBA in shallow groundwater are presented on Figures 5 through S Isoconcentration maps for the deeper aquifer were not prepared due to insufficient data (only one data point in deeper aquifer) Historical tables of soil and groundwater data are attached in Appendix C Also included in Appendix C are figures showing the sample locations for the tabulated soil data GR prepared hydrocarbon mass calculations in groundwater for TPHg, benzene, MtBE and TBA The calculations are presented in Appendix D Results of the calculations indicate a very low residual mass of TPHg(0 18 lbs),benzene(0 01 lbs)and MtBE(3 69 lbs) Apparently the mass of MtBE previously present beneath the site has degraded to TBA, as the calculation for TBA mass equaled 929 lbs According to information provided by Tosco,current MtBE degradation experiments seem to indicate that dissolved oxygen is the controlling factor in the breakdown of MtBE to TBA Current theory holds that if enough dissolved oxygen is present in the groundwater, MtBE will degrade to TBA, and ultimately to carbon dioxide If the supply of dissolved oxygen is limited during this degradation sequence,then the process appears to halt In December of 2001,a DPE event was conducted at the site (see section 2 2) The groundwater sampling event conducted immediately after the DPE event revealed the apparent mass transformation of MtBE to TBA GR believes that the DPE event may have mobilized sufficient oxygen into the saturated zone to accelerate the degradation of MtBE to TBA,but • not enough oxygen to allow the TBA to be fully degraded It is expected that the introduction of 14024508 5 4 a