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- REMEDIAL lON WORK PLAN, 1645 CHEROKEE R090, STOCKTON, CA 1.4 Regional Geologic Setting The subject property is situated at an elevation of 21 feet Mean Sea Level (MSL)and lies within the Great Valley Province, which is dominated by the alluvial basin of the Sacramento Valley. An area topographical map is provided as Figure 2. This region is composed primarily of Quaternary aged alluvium, mainly in the form of unconsolidated flood plain deposits. The main soil types that exist in this type of deposit are sand, silt, gravel and clay that are irregularly interstratified. The primary mechanism for sedimentation in this area has been fluvial. 1.5 Site Geologic Subsurface Conditions The subject property is underlain by layers of silt, sandy silt, and silty very fine grained sand (URS Greiner-Woodward Clyde Report dated September 1999). Groundwater has been encountered locally at approximate depths between 45-feet and 50-feet bgs and flows in a northeasterly trend. 2.0 PROPOSED REMEDIAL ACTION 2.1 Summary of Fieldwork and Rationale for Remedial Action Previous subsurface investigation efforts completed by URS Greiner-Woodward Clyde and Hydro Environmental Technologies between August 1999 and June 2004 have concluded that significantly detectable concentrations of residual petroleum hydrocarbon compounds remain in the native soils and/or shallow groundwater beneath the site. The residual contamination imbedded within the native soils located adjacent to the former tank excavation(s) have been determined to be a "secondary" source of ongoing contaminant migration and potential leaching of hydrocarbon constituents into the groundwater. Furthermore, evaluation of the geological conditions of the site and hydrocarbon concentrations -vs- time, indicates that additional mitigation of the contaminants of concern by in-situ means nor the �- ability of the affected soil and groundwater to naturally attenuate without interference is not effectual. Therefore, it is to this conclusion"that a remedial corrective action consisting of the removal of the accessible hydrocarbon affected soil considered as the secondary source, is hereby proposed. No definitive cleanup goals have been established for this site by any of the regulatory communities governing this site. Rather, it has been suggested by the Central Valley Regional Water Quality Control Board (CVRWQCB) and the SJCDEH that Risk Based Corrective Action (RBCA), risk �. screening guidance combined with residual mass contaminant levels be used to establish remedial goals. The American Society of Testing and Materials (ASTM) standard for RBCA, ASTM E-1739- 95 establishes provisions for determining "risk levels" which may increase the concentrations of specific constituents that may be safely left in place without producing a continual exposure concern to human health and/or the environment. Utilizing determining factors such as the presence of ecological receptors and site specific exposure pathways, and considering the residual contaminate mass and the threat potential of the contaminant to significantly impact groundwater, a realistic and reasonable remedial Site Specific Target Level(SSTL)can be established. The Preliminary Remedial Goal guideline established by the United Sates Environmental Protection Agency, Region IX for diesel in soil is 360 mg/kg. However, in consideration of the aforementioned factors and in consideration of the residual mass to produce a continual environmental liability and exposure concern, a SSTL of 750 mg/kg for the residual diesel in soil has been explicated and is hereby proposed for the remedial action to be completed at the subject site. 2 Sohm Environmental Solutions