SITE INFORMATION AND CORRESPONDENCE_2001-2018

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/ Mr. Jeff Goold - 3 - 10 October 2008 Shell Stockton Terminal • • t The Pilot Test Report presents the results of DPE pilot testing conducted between 12 and 16 May 2008. DPE extraction was conducted from three newly installed extraction wells, including MW-37 in the MW-1 hot spot, MW-38 in the MW-13R hot spot, and MW-39 in the MW-10 hot spot. The extraction wells were installed to total depths ranging between 26 and 30 feet bgs. Observation wells P-1 and P-2 were installed in the MW-1 and MW-10 hot spots, respectively. Soil samples were collected for laboratory analysis from the five well borings. Results showed that the concentrations in soil samples from all borings were generally low to ND for BTEX and oxygenates. Maximum concentrations of TPHg and TPHd were in P-1 at 660 mg/kg and 180 mg/kg, respectively. During the DPE pilot test, the groundwater level was lowered in each extraction well using a pneumatic pump at rates that averaged about two gallons per minute (gpm) from MW-37, three gpm from MW-38, and one gpm from MW-39. Maximum petroleum hydrocarbon concentrations were present in groundwater extracted from MW-37. TPHg was detected at 2,000 pg/L, benzene at 110 pg/L, MTBE at 100 pg/L and TBA at 2,700 pg/L. The extraction test was repeated in MW-37 using a submersible electric pump to evaluate the maximum groundwater extraction rate. Groundwater extraction averaged about three gpm during this phase of the DPE pilot test, and about two feet of drawdown was observed in a monitoring well located about 18 feet from extraction well MW-37. Vapor phase extraction was performed in stepped rates consisting of two two-hour steps and one seven-hour step. Sustained applied vacuums varied between 17 inches of mercury (in Hg) to 24 in Hg. A maximum sustained flow rate of 48 standard cubic feet per minute (scfm) was attained from MW-38. Maximum vapor concentrations were obtained from MW-37 during the extra extraction event at a flow rate of 32 scfm. At this flow rate, TPHg was detected at ' 24,000 parts per million by volume (ppmv). BTEX was detected at 220, 57, 70, and 185 ppmv, respectively, and MTBE was detected at 28 ppmv. By observing a pressure response that was about 3% of the applied vacuum in a well located about 11 feet from extraction well MW-39, Shell calculated that a radius of influence (ROI) of about 12 feet could be used as a design basis for all three hot spots. Shell recommends implementing DPE to remediate the three hot spots and is prepared to submit a Remedial Action Plan (RAP) within 60 days of approval of the Pilot Test Report. Our comments are presented below. 1. Except for the detection of 150 mg/kg of TPHg in the MW-39 borehole, the results of the Investigation Report show that petroleum contamination in vadose zone soils has been delineated. The laboratory analytical results for groundwater samples obtained from off-site borings SB-25 to SB-26 show that the leading edge of the plume also has been delineated to ND. The grab groundwater samples from hot spot borings SB-22 through SB-24 all' detected petroleum hydrocarbons exceeding their respective WQOs in groundwater beneath the continuous sand unit present between 70 and 80 feet bgs. Therefore, the installation of monitoring wells screened below the continuous sand unit is necessary to monitor potential plume migration and the progress of site cleanup. 2. We do not concur that the ND results reported in the Soil Vapor Report for the outdoor ambient air samples provide any data useful for evaluating potential indoor air intrusion from the elevated petroleum hydrocarbons concentrations in soil vapor. Flowing outddor air