COMPLIANCE INFO_PRE 2019 - Laserfiche WebLink

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15135 Eight Mile Road -7- 1 October 2018 Stockton, San Joaquin County levee is likely to be expanded vertically above areas with impacted soil and, given the Site's proximity to the Little Potato Slough, land is unlikely to be rezoned as residential. The Site is currently zoned "General Agriculture" (AG-40)which San Joaquin County defines as a zone "established to preserve agricultural lands for the continuation of commercial agriculture enterprises." The Site meets Petroleum Vapor Intrusion to Indoor Air Criterion (b). AGE performed a human health risk assessment and found there to be a 4.7 x 10-6 cancer risk and 3.3 x 10-' hazard index for a commercial worker who might occupy a hypothetical building cited over soil sampling location BC2-5 where 11.6 mg/kg of benzene were detected in shallow soil. The current(2016) maximum concentrations of petroleum constituents detected in soil between 0 and 10 feet bgs (Table 1) meet the commercial LTCP criteria for Direct Contact and Outdoor Air Exposure for both shallow and deep soils, which are appropriate criteria for this commercial facility. GENERAL SITE CONDITIONS The western portion of the Site is a levee comprised of up to 20 feet of fill material, which is comprised of poorly sorted sands. The eastern portion of the Site is composed of approximately 6 to 20 feet of peat in the shallow zone underlain by interbedded layers of sand and finer grained sediments to a depth of 102 feet bgs. Groundwater flow is generally to the east-southeast. Depth to groundwater fluctuates between about 2 and 9 feet bgs. H&H removed approximately 1,020,000 gallons of petroleum impacted groundwater, containing approximately 5 pounds of MTBE through operation of GWETS. An additional 4,500 gallons of petroleum impacted groundwater and 14.5 tons of petroleum impacted soil were removed from the Site associated with the 2007 vandalism and release. Though AGE has calculated that approximately 1.6 pounds (0.26 gallon) of TPHg remain in shallow groundwater beneath the Site, these petroleum hydrocarbons are expected to attenuate below their respective WQOs within a reasonable timeframe. Between 18 June and 20 June 2018, AGE oversaw the proper destruction of the remaining groundwater monitoring wells at the Site. On 2 August and 16 August 2018, AGE insured that all investigation derived waste was removed from the Site and properly disposed. CONCLUSIONS A combination of excavation, active remediation and natural attenuation have reduced hydrocarbon concentrations in groundwater beneath the Site and residual petroleum constituents in groundwater are expected to continue to attenuate within a reasonable time. A surface water body adjacent to the Site, Little Potato Slough, has been monitored regularly, and is no longer threatened by the Site release. The attenuating petroleum hydrocarbon plume is not likely to pose a threat to human health or waters of the State. Additionally, the Site meets the State Water Board's LTCP criteria for case closure. Central Valley Water Board staff recommends that the case be closed.