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SITE CONCEPTUAL MODEL AND REMEDIAL ACTION PLAN 4 7-Eleven Store No 14117 2725 Country Club Blvd, Stockton CA June 20, 2002 Assumptions included a hydraulic conductivity of 2 5 ft/day (typical of silty sand), with a groundwater gradient of 0 001 ft/ft with flow to the south/southeast (one of the typical flow directions for this site) For ' modeling purposes, the aquifer thickness was set at 18 5 feet, with a porosity of 20% Starting MTBE concentrations for the model were the maximum levels recorded for the site on January 8, 2002 The model was allowed to run for 428 days ' The model is 2-dimensional and assumes that the area modeled will exhibit steady state flow under homogeneous aquifer conditions Model limitations for this site are related to the Inconsistent groundwater ' flow direction (during early 2002 the groundwater flow direction has been to the southlsoutheast, but other monitoring data Indicates flow directions toward the west), and heterogeneous soil types (Figure 3) ' Despite limitations, the modeling results verify observed groundwater monitoring well trends the oxygenate plume remains relatively isolated to the immediate vicinity of the UST areas, with little offsite movement Attached in Appendix A are the WinTran data and a map of the lateral extent of the plume after 428 days without the influence of remediation Based on the modeling results, as of March 2003, the dissolved plume distribution is calculated to be contained to the area immediately surrounding the UST pit The plume edge is predicted to migrate slightly further south towards well MW-6, but offsite wells MW-6 and MW-5 are calculated to remain at non-detectable dissolved MTBE ' Therefore, modeling data Indicate that the risk of offsite migration is low even if no remediation activities take place 40 REMEDIATION ALTERNATIVE EVALUATION One of the remediation alternatives which is always evaluated for any site is the possibility that no further ' action is required It Is considered that no further action may not be approved by the SJCPHS at this time, based on existing dissolved MTBE levels relative to State of California secondary action levels of 5 ug/L ' MTBE Site-specific cleanup levels will be discussed with the SJCPHS once dissolved MTBE concentrations have been reduced in all source area wells to less than 100 ug/L, which represents an approximate 95% reduction from MTBE levels used In fate and transport modeling ' Site-specific response to the selected remediation process will be verified after treatment implementation If the remediation method requires modification based on site-specific data, the process will be adjusted to 1 achieve maximum effectiveness All remediation process adjustments and/or modifications will be discussed with the SJCPHS prior to implementation ' 14117Scrn2 doe