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San Joaquin County <br /> Environmental Health Department DIRECTOR <br /> Donna Heran,REHS <br /> a 600 East Main Street <br /> m Stockton, California 95202-3029 PROGRAMCOORS <br /> Robert McClellon, REHS REHS <br /> Jeff Carruesco,REHS, RDI <br /> • <br /> 0.- Website:Website: www.sjgov.org/ehd Kasey Foley,REHS <br /> /ppR <br /> Phone: (209) 468-3420 <br /> Fax: (209)464-0138 <br /> January 13, 2010 <br /> Denis L. Brown Hung Tran & Mary Nguyen <br /> Shell Oil Products US Pacific Avenue Shell Station <br /> 20945 S. Wilmington Blvd. 6131 Pacific Avenue <br /> Carson, CA 90810-1039 Stockton, CA 95207 <br /> Subject: Shell-Branded Service Station <br /> 6131 Pacific Avenue <br /> Stockton, CA 95207 <br /> The San Joaquin County Environmental Health Department (EHD) has reviewed Corrective <br /> Action Plan and Site Investigation Work Plan (CAP) dated December 9, 2009, submitted on <br /> your behalf by Conestoga-Rovers &Associates (CRA) and has the following comments. <br /> CRA proposes in the CAP to conduct an investigation utilizing cone penetrometor testing <br /> (CPT) methodology and to install additional shallow- and deep-screened groundwater <br /> monitoring wells. They also present a feasibility study of remedial alternatives to address the <br /> groundwater contamination, recommending insitu chemical oxidation (ISCO). <br /> A CPT investigation was conducted in May 2000 to define the vertical extent of the <br /> contamination at this site. Two borings, SB-5 and SB-6 were advanced to total depths of 100 <br /> and 80 feet below surface grade (bsg), respectively. SB-5 was located in the former tank pit <br /> area. Please refer to the map in Fourth Quarter 2001 Quarterly Monitoring and Site <br /> Investigation Report (Report), dated January 11, 2002, for the correct location of SB-5. <br /> Boring SB-6 was located near the northern dispenser islands, next to boring SB-4. The CPT <br /> tool encountered refusal at 80 feet bsg, but subsequent assessment borings were extended <br /> to 100 feet bsg. In the Report, the conclusion was made that in soil, total petroleum <br /> hydrocarbons as gasoline (TPHg) was defined at 60.5 feet bsg and benzene was defined at <br /> 80.5 feet bsg; MtBE was not detected in SB-5. TPHg was defined at 66.5 feet bsg in SB-6, <br /> benzene was defined at 70.5 feet bsg and MtBE was defined at 61.5 feet bsg. The data <br /> support this conclusion. <br /> Grab groundwater samples collected from SB-5 and SB-6 were reported to be impacted by <br /> petroleum hydrocarbons. The EHD directed that the vertical extent of the reported <br /> contamination be defined, and CRA/Shell proposed and installed one deep-screened <br /> groundwater monitoring well downgradient of the former tank pit area, MW-13. This well is <br /> located approximately 55 feet from VEW-7, is screened across a sand interval identified in <br /> both SB-5 and SB-6 sampling borings at 75-80 feet bsg, and provides sufficient information <br /> on the condition of the groundwater underlying and down-gradient of VEW-7. <br /> CRA proposes to install three CPT borings between VEW-7 and MW-4 to investigate the <br /> cause of increased contaminant concentrations reported in groundwater samples collected <br /> from MW-4 since 2004. They did not explain how CPT borings would provide this <br />