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Mr. Thomas Black Page 2 of 4 <br /> Tracy Office Plaza March 15, 2016 <br /> 324 E. Eleventh St., Tracy CA <br /> groundwater samples previously collected on the THS site (315 E. Eleventh St.) on the north <br /> side of Eleventh Street. In ASAR, Pangea concluded that impacted soil on the subject site is <br /> limited to the area around borings B-1 and B-7, and that the impact to groundwater is primarily <br /> in the area of borings B-1 and B-7 with reduced or lack of impact at borings B-4 and B-6. <br /> Pangea recommended considering the site for case closure for appearing to meet the California <br /> State Water resources Control Board's (SWRCB) Low-threat Underground Storage Tank Case <br /> Closure Policy (LTCP), and conducting a well survey for the area within 1,000 feet of the subject <br /> site. <br /> The EHD has reviewed the site reports cited above and data from the THS site and has <br /> concluded the following: <br /> • Based on data from the former groundwater monitoring site at the THS site, the depth to <br /> water on the site is on the order of 8 to 14 feet bsg, and the inferred flow direction is <br /> northerly, placing the THS site downgradient from the subject site and potentially <br /> capable of receiving impacted groundwater from the subject site. <br /> • Contaminants detected in monitoring well MWA on the THS site did not appear to be <br /> related to the investigated release from the THS UST system and were attributed by the <br /> consultant working on the THS site to the subject site, which would be consistent with <br /> the groundwater flow direction. <br /> • Pangea states that borings B-1 and B-4 are in the suspected UST farm area, with boring <br /> B-1 adjacent to the suspected former fuel line that ran to the UST farm (tank pit); <br /> however, there are no site maps or documented indication of the gas station layout <br /> showing where the tanks were actually located, so it is also possible that B-1 is in a <br /> former dispenser area and the USTs were elsewhere, and in the EHD's opinion, it is also <br /> possible that the geophysical anomaly may not even be piping associated with the <br /> former UST system. <br /> • The soil samples from borings B-1 and B-7 were more intensely impacted at 1 foot <br /> below surface grade (bsg) with impact intensity generally declining with increasing depth <br /> (a slight increase at 10 feet bsg), which would be more consistent with a dispenser or <br /> product line leak or surface release than with a UST leak. <br /> • Pangea noted that none of the recovered soil samples appeared to be fill material, <br /> suggesting none of the borings actually penetrated the former tank pit. <br /> • Soil data from B-1 indicated soil impact extends to at least 15 feet below surface grade, <br /> but lack of odor in soil from 24 feet bsg from subsequent boring B-7 near the B-1 <br /> location indicates impacted soil appears to not penetrate to 24 feet bsg. <br /> • The lateral extent of significantly impacted soil around B-1 appears to be limited, <br /> potentially extending at reduced concentration or plume thickness from B-1 to B-7, B-4, <br /> B-8 and B-6. <br /> • In soil, benzene was detected only samples from B-1 and B-7, and TPHg was detected <br /> in soil samples from B-1, B-6, B-7 and B-8, demonstrating a limited lateral distribution of <br /> both analytes. <br /> • The soil analytical data, supplemented with photo-ionization detector (PID) and odor <br /> observation data shows that, with the exceptions of the 1-foot bsg samples described <br /> above, the most intensely impacted subsurface soil was encountered in all borings at 10 <br />