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Mr. Jim Barton RWQCB March 29, 2013 Page 5 Various VOCs (Isopropyl Benzene, 1,2,4-Trimethylbenzene and/or Tert-Butylbenzene) were detected at low,levels in both soil and groundwater samples from boring B-33. None of the VOCs detected exceeded California Environmental Screening Levels (ESLs) for soil or California drinking water quality standards for groundwater. No constituents of concern were detected in soil or groundwater samples from boring B-35. Site-Specific Lithology The subsurface to a depth of approximately 20 feet bgs is dominated by interbedded silt and clay deposits, as observed during drilling of the shallow borings in the southeast and southwest areas of the site in 2005. Thin (2 to 3 feet) sand deposits occur at approximately 15 to 18 feet bgs in the B-7 area, and at 20 feet and deeper in the B-15 area. The lack of sand deposits in the,vadose zone .and shallow groundwater zone suggests that migration of fuel hydrocarbons from USTs was slowed by low hydraulic conductivities. As part of URS' investigation .in the southwest portion of the site, boring B-41 was drilled to a depth of,.approximately 57 feet bgs and logged to provide information regarding the underlying geology. Lithology of the southern portion of the site consists of interbedded silts and clays from ground surface to the maximum depth explored of 57 feet bgs with the exception of a sand layer present from approximately 43 feet to 50 feet bgs. The sand layer was reported using cone penetrometer testing (CPT) technology which was used during the drilling of this boring because the larger rig allowed for deeper penetration depths than other direct push technologies. A geologic cross-section of the investigation area is included in Attachment B. PROPOSED SCOPE OF WORK Investigation Schedule I URS will complete the proposed field activities andsubmit a report summarizing the results to the RWQCB by June 30, 2013. The report will also include an updated CSM addressing the RWQCB comments presented in the February 27, 2013, letter. Agency Notification and Permits Th q County The Sari Joaquin Coun Environmental Health Department(SJCEHD)requires drilling permits to be obtained prior to the commencement of field activities. URS will submit appropriate permits and fees with the approved workplan to the SJCEHD. Following approval of the permits, URS will commence field activities. URS will notify the RWQCB at least 48 hours prior to any field activities. S