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San Joaquin Environmental Health Department 12066.002.000 Mountain House Neighborhood A, Units 3 and 4 November 14, 2018 SOIL GAS CHARACTERIZATION WORK PLAN Page 2 in the vicinity of the identified impact. Following a cessation of sampling due to economic reasons, sampling resumed in 2014. The 2007 and 2014 sampling events identified the presence of TPHg, TPH-d, TPH-mo, benzene, ethylbenzene, and xylene(s). As presented in the Wallace-Kuhl July 2014 Soil Vapor Investigation Report (Reference 2), the following occurrences of ruptures, confirmation of subsurface impact, and pipeline repairs have been reportedly identified at or in the immediate vicinity of the Property: The pipeline was ruptured in December 2003 by a farmer at a location approximately 300 feet east of the Tuso property. Records indicated that 31,500 gallons of crude oil were released, although it was opined at the time that the release had not caused the Tuso or adjacent Rivani properties to be environmentally impacted. Petroleum hydrocarbon-impacted soils were encountered near the pipeline during stormwater utility installation at the property in 2006. Repairs of the KLM pipeline in October 2007 as mandated by the United States Department of Transportation (DOT). The repairs reportedly consisted of re-welding of joints to maintain pipeline integrity. Petroleum hydrocarbon-impacted soils were reportedly encountered at the KLM pipeline during the installation of a Mountain House Community Services District (CSD) potable water pipeline in the western portion of the Tuso property. Visibly stained soil was reportedly identified within the pipeline easement, and found to extend below the pipeline. This investigation focused on potential impacts to the residential lots closest to the highest levels of previously identified soil As presented in the ENGEO September 2015 Phase II Environmental Site Assessment (Reference 3), four soil gas wells were installed and sampled in the vicinity of noted areas of elevated passive soil gas concentrations, that were previously identified during the Wallace-Kuhl June 2014 Monitoring Well Sampling Report (Reference 1). ENGEO identified TPH-g, VOCs (including 2-propanol, hexane, 4-methyl 2-pentanone), and TPH-d at low concentrations, all well below their corresponding residential screening levels. Soil samples (Si through S4 @ 5', S1 through S4 @ 10', 51 through S4 @ 15' and Si through S4 @ 20') collected from 5, 10, and 15 feet were analyzed by the laboratory. VOCs and TPH-g were non-detect in any of the soil samples collected from the portion of the Property closest to the pipeline. TPH- d and TPH-mo were detected at trace concentrations of 3.4 and 14 milligrams per kilograms (mg/kg), respectively, in Sample S-4, which was collected at a depth of 5 feet below ground surface. These concentrations are well below the corresponding Regional Screening Levels (RSLs) established by the United States Environmental Protection Agency (USEPA) Region IX for residential land use1. No other samples exhibited detectable concentrations of TPH-d or TPH-mo. The deepest samples collected at 20 feet below ground surface were not analyzed, since the results of the shallower samples did not indicate any impacts. In addition, any potential impacts at a depth of 20 feet would not represent an exposure concern to the future residents. SAMPLING RATIONALE Soil Gas Based on review of previous soil and groundwater data collected, San Joaquin County Environmental Health Department (SJCEHD) has concerns that there may be residual soil gas impacts, due to leaks from the Chevron petroleum pipeline. Soil gas samples will be recovered from depths of 5 and 10 feet. The intent of this soil gas characterization is to further delineate the extent of VOC impacts in soil gas at 5 feet and 10 feet below ground surface.