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Additional Site Assessment Report 324 East 11 th Street Tracy,CA October 8, 2014 Site Geology & Hydrogeology Soil from site borings consisted primarily of silty clay to a depth of approximately 17 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 at approximately 17 ft bgs.Soil contamination is primarily located between 1 and 10 ft bgs based on field observations and analytical results. SITE ASSESSMENT PROCEDURES 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-1 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. If a significant source area is present,and if the residual impact poses a vapor intrusion risk to the existing site building, Mr. Henderson believes source area removal could be appropriate. The investigation involved drilling three onsite borings to define the lateral extent of contamination in soil and groundwater near boring B-1,and drilling one off site boring west of former Tracy High School well MW-1. To evaluate the vertical extent of contamination, one onsite boring(B-713)was continued to 24 ft depth. Pre-Drilling Activities A comprehensive site safety plan was prepared to protect site workers and the plan was kept onsite during all field activities. A drilling permit was obtained from the San Joaquin County Environmental Health Department. A copy of the permit is presented in Appendix A.The proposed drilling locations were marked and Underground Service Alert was notified at least 48 hours before the proposed field activities. Drilling Procedures On July 10,2014,Pangea coordinated the drilling of four soil borings(B-7S,B-71), B-8 and B-9)onsite and one soil boring (B-10) offsite at Tracy High School (Figure 1). Soil borings B-7S, B-7D and B-8 were advanced near the potential former piping location, while B-9 was installed northwest (downgradient) of former boring B-1,within the planter along E 11th Street.Boring B-10 was installed within the planter on the north side of E. 11th Street, adjacent to the Tracy High School parking lot. 2