SITE INFORMATION AND CORRESPONDENCE

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Fernando's Place -4 - 29 September 2016 1201 South Center Street Stockton, San Joaquin County However, no soil gas samples were collected near the shop building in the middle of the Site property and the L-shaped building in the southwest corner of the Site property, which the 2Q16 Report describes as a single-story multi-family residence. While depth to groundwater is greater than 10 feet (approximately 20 feet bgs), the Site does not meet the LTCP vapor intrusion criteria because benzene concentrations in shallow groundwater underlying the Site property are greater than 1,000 ug/L (22,100 ug/L in MW-1), and concentrations of TPHg in the top 10 feet of soil have been reported above 100 mg/kg (8,480 mg/kg at 10 feet bgs in MW-1). While significant mass (22,715 pounds of TPHg) has been removed via soil vapor extraction (SVE) at the Site, no confirmation soil samples have been collected from 5-10 feet bgs subsequent to shut- down of the SVE system in 2011. As such, soil gas samples should be collected to assess the vapor intrusion risk to structures on your property. As soil gas sampling can be affected by changes in climatic and atmospheric conditions, a single soil gas sample is not adequate to evaluate vapor intrusion risk. An additional round of soil gas samples should be collected near the 21, 27, and 36 West Jackson Street and 21 East Jackson Street residences in order to confirm the vapor intrusion risk to occupants. In the Work Plan due 30 November 2016, please include a proposal for the installation and sampling of a sufficient number of permanent soil gas wells near the shop building and L-shaped building in the southwest corner to adequately evaluate potential vapor intrusion risk. Please also include a proposal for the installation and sampling of permanent soil gas wells near the residences at 21, 27, and 36 West Jackson Street and 21 East Jackson Street. Installation and sampling of permanent soil gas wells should be performed in accordance with the Department of Toxic Substances Control (DTSC) guidance document Advisory—Active Soil Gas Investigations dated July 2015, and collected samples should be analyzed, at a minimum, for TPHg, BTEX, naphthalene, oxygen, and a selected leak detection compound. As soil gas sampling is affected by changes in climatic and atmospheric conditions, permanent soil gas wells are requested so that repeatable samples may be collected. In summary, the Central Valley Water Board requests the following: • By 30 November 2016, please submit a Work Plan to include: o A proposal for the collection of a sufficient number of groundwater samples to adequately delineate groundwater pollution laterally and vertically to the north, northeast, east, and southeast of the Site. o A proposal for the installation and sampling of permanent soil gas wells near Site structures, and residences at 21, 27, and 36 West Jackson Street and 21 East Jackson Street, in accordance with DTSC guidance. o By 30 November 2016, please submit a Feasibility Study to include: o Evaluation of at least three (3) alternative groundwater remedial options. o Estimates for cost and time to reach Water Quality Objectives for each evaluated option. o A recommendation for the preferred remedial action.