SITE INFORMATION AND CORRESPONDENCE

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1160 West Yosemite Avenue Page 2 of 3 • The fate and transport model predicts the concentrations for petroleum hydrocarbon contaminants in groundwater will increase considerably over the next twenty years. • Areas where the greatest potential for finding high concentrations for chemicals of concern (COC) are currently covered by buildings and have not been investigated. EHD will not comment whether the collection of soil vapor samples is warranted until the additional site investigation is completed and results are known. Cambria presented assumptions and calculations to determine that the estimated mass of TPHg remaining at this site is approximately 200 pounds in soil and 2.25 pounds in groundwater; and the estimated mass of benzene remaining at this site is approximately 0.04 pounds. Please submit the plus or minus twenty percent error calculation to the EHD. Cambria states that the estimated mass of tertiary butyl alcohol (TBA) remaining at this site was not calculated because "the detection of TBA in soil and groundwater samples collected from beneath the site is attributed to an offsite source as the underground storage tanks were removed from the subject site during 1983 and Shell did not begin blending fuel oxygenates into gasoline until 1991." While the EHD is not concerned with the masses of TBA at this time, which appears to be quite small, the EHD disagrees with this assumption since TBA was detected in soil samples collected from boring B-2 at 5.5 feet below ground surface (bgs), boring B-1 at 20.5 feet bgs, boring MW-4 at 20.5 feet bgs, and boring MW-5 at 10.5 feet bgs; and TBA is intermittently being detected in groundwater collected quarterly from monitoring wells MW-1 and MW-3. Shell may not have added TBA to gasoline prior to 1991, but the EHD has not been presented documentation that demonstrates that TBA has never been present in the feed stocks used to produce the gasoline formerly dispensed at this site. As stated above, Cambria proposes the advancement of two CPT borings, one onsite to investigate the vertical extent of impacted groundwater and one offsite to investigate the lateral and vertical extent of impacted soil and groundwater east of this site. Cambria proposes to advance both CPT borings to approximately 40 feet bgs, as recommended by EHD by letter dated July 21, 2006, but upon further review EHD recommends these two borings should be advanced to at least 50 feet bgs to ensure the sand layer identified at approximately 35 feet bgs in boring logs for MW-1 and MW-3, a potential contaminant migration pathway, is sampled and characterized, and the vertical extent of impacted media is delineated. Although Cambria acknowledges that "the primary impact to groundwater appears to be north of the former dispenser islands, in the vicinity of MWA and MW-3," an area where the groundwater flow direction tends to be predominately towards the north-northeast, the work plan does not address investigating the area north of MW-3 and MW-4, two monitoring wells where petroleum hydrocarbon contaminants are detected quarterly in groundwater samples. Additional borings may be necessary to adequately define the lateral extent of your dissolved petroleum hydrocarbon plume to the north. EHD agrees that collecting soil samples from CPT-1 is not necessary; however, EHD believes the proposed soil sample collecting from CPT-2 at five foot intervals, should extend to total depth instead of terminating at first encountered groundwater.