SITE INFORMATION AND CORRESPONDENCE (2)

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Ranch Market - 3 - 10 October 2016 23569 South Santa Fe Road Riverbank, San Joaquin County capacity by the property owners. While the SVE wells are screened between 13-30 feet bgs, and are not representative of shallow soil gas concentrations, they do indicate the presence of remaining hydrocarbon vapors in the subsurface, subsequent to shut-down of the SVE system. In order to properly evaluate the vapor intrusion risk at the Site, soil gas samples should be collected adjacent to the Site structure. As soil gas samples are affected by atmospheric and meteorological fluctuations, permanent soil gas wells should be installed to allow for multiple rounds of sampling. By 9 December 2016, please submit a Work Plan which proposes the installation and sampling of a sufficient number of permanent soil gas wells to adequately evaluate vapor intrusion risk at the Site. Proposed installation and sampling procedures should be in accordance with the Department of Toxic Substances Control (DTSC) guidance document Advisory—Active Soil Gas Investigations dated July 2015. 4. The Site does not appear to meet the LTCP criteria for Direct Contact and Outdoor Air Exposure. As noted in Comment 3 above, 225 mg/kg benzene was reported in a sample from 9.5 feet bgs in the former tank pit, and no confirmation sampling has been performed. Additionally, no samples have been collected from shallower than 9.5 feet bgs at the Site historically. In order to adequately evaluate the Direct Contact and Outdoor Air Exposure risk, shallow soil samples should be collected between 0-5 and 5-10 feet bgs. In the Work Plan due 9 December 2016, please include a proposal for the collection of a sufficient number of shallow soil samples to adequately assess remaining hydrocarbon concentrations in soil between 0-5 and 5-10 feet bgs. In summary, the Central Valley Water Board requests the following: • By 30 October 2016, please submit a semi-annual Groundwater Monitoring Report summarizing third quarter 2016 groundwater monitoring and sampling activities. These activities are to include: o Water level gauging of all Site wells. o Groundwater sampling of wells MW-1, MW-2, MW-3, MW-5, MW-5D, and MW-7. • By 9 December 2016, please submit a Work Plan to include: o A proposal for the installation and sampling of a sufficient number of permanent soil gas wells to adequately assess the vapor intrusion risk at the Site. Wells should be installed and sampled in accordance with DTSC guidance. c A proposal for the collection of a sufficient number of samples to adequately assess remaining hydrocarbon concentrations in shallow soil between 0-5 and 5-10 feet bgs. • By 30 April 2017, please submit a semi-annual Groundwater Monitoring Report summarizing first quarter 2017 groundwater monitoring and sampling activities. These activities are to include: o Water level gauging of all Site wells. o Groundwater sampling of wells MW-1, MW-2, MW-3, MW-5, MW-5D, and MW-7, and domestic well DW-3.