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C?cokgtcjt re CAM CS the Page 2 Morita Risk Evaluation Work Plan Project No 724 2 March 18, 2002 It appears that the soil plume is defined by e ❑ MW-3 and MW-5 to the north and northwest ❑ MW-2, MW-6 and MW-106 to the east ❑ MW-1 to the south ' u MW-4 is interpreted to be on the edge of the soil plume on the west under Grant Street The shallow groundwater plume is evident in well MW-4 and defined laterally to the east (down gradient) by MW-5, MW-6 and MW-7 The presence of TPH-G contamination in intermediate wells MW-106 and MW-107 suggests that a submerged plume is being sourced by residual soil contamination beneath the present water table (at 35 feet below surface in December 2001) It is noted that the screened intervals of these wells (55-60 feet bgs) are placed near the elevation of the groundwater table in 1992 — approximately 54 feet bgs The contamination plume as observed from these wells is stable and does not appear to be sourcing a significant off site plume The levels of contamination present do not warrant an engineered clean up ' 2.0 PROPOSED WORK The work proposed in this plan is intended to meet the requirements for obtaining site closure GTI proposes to perform the following work to prepare this site for closure ❑ Contaminant mass estimate calculations for the soil and groundwater plumes ❑ Limited fate and transport calculations ❑ Completion of the Regional Water Quality Control Board (Board) — Central Valley Region's Appendix B Site Closure Checklist This documentation is necessary to receive site closure concurrence from the Board ' 2.1 Contaminant Distribution & Mass Estimate Calculations ' Through interpretation and extrapolation of the data generated through the history of the project, the distribution of the contamination, both laterally and vertically, will be detennined The contaminant distribution information will then be used in estimation of the contaminant mass Mass estimate calculations will be performed to determine the mass of gasoline present at ' this site To accomplish this task, the soil analytical data and the groundwater monitoring data obtained from previous investigations will be used The calculations divide up the contaminated zone into a series of layers according to geology A cylindrical shape is used to estimate the volume of soil that has been impacted in each layer To account for differences in contaminant distribution, an inner core and an outer ring will be used on several of the layers 1 r