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Workplan for Additional Assessment and Subslab Gas Sampling Tracy Office Plaza 324 E. 11th Street, Tracy,CA November 22,2013 Site Geology & Hydrogeology Soil from site borings consisted primarily of silty clay to a depth of approximately 5 to 9 ft bgs, underlain by more permeable sandy clays or clayey sands observed to a depth of approximately 25 ft bgs. Wet soil(possible groundwater) was encountered in the more permeable sandy clay or clayey sand layers encountered at approximately 8 to 11 ft bgs. Soil contamination is primarily located between 1 and 10 ft bgs based on field observations and analytical results. PROPOSED INVESTIGATION To identify the appropriate next steps at the site, Pangea discussed site conditions with the SJCEHD and reviewed the recently implemented Low Threat Closure Policy(LTCP)adopted by the State Water Resources Control Board. SJCEHD geologist Nuel Henderson requested additional source area assessment near boring B-I and the apparent subsurface piping from the former service station operations. Mr. Henderson also expressed concern that hydrocarbons could have migrated offsite and around prior offsite monitoring well MW-1 at the Tracy High School site. The prior benzene impacted detected in groundwater at the subject site and Tracy High School is illustrated on Figure 1.Additionally,monitoring well data forthe former Tracy High monitoring wells is included in Appendix B. If a significant source area is present,and if the residual impacts poses a vapor intrusion risk to the existing site building,Mr.Henderson believes source area removal could be appropriate. Our proposed scope of work to address these concerns is provided below. The objective of the proposed investigation is to install three onsite borings to define the lateral extent of contamination in soil and groundwater near boring B-1, and a contingent boring west of former Tracy High School well MW-1. To provide additional vertical delineation, one boring (B-7) will be continued to about 25 to 30 ft depth. To evaluate the potential for vapor intrusion into the onsite building, subslab gas sampling is proposed at two locations. Results of this investigation will be discussed with the SJCEHD and compared to media-specific criteria of the LTCP. Task 1 - Pre-Field Activities Prior to initiating field activities, Pangea will conduct the following tasks: • Obtain encroachment and excavation permits from the City of Tracy and drilling permits from SJCEHD as necessary; 2