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2, <br />ENGEO <br />Expect Excellence <br />GEOTECHNICAL <br />ENVIRONMENTAL <br />WATER RESOURCES <br />CONSTRUCTION SERVICES <br /> <br />Project No. <br />12066.002.000 <br />November 14, 2018 <br />Ms. Linda Turkette <br />San Joaquin County Environmental Health Department <br />1868 E HazeIton Ave <br />Stockton, CA 95205 <br />Subject: Mountain House Neighborhood A, Units 3 and 4 <br />Central Parkway <br />Mountain House, California <br />SOIL GAS CHARACTERIZATION WORK PLAN <br />References: 1. Wallace Kuhl Associates, Monitoring Well Sampling Report — Second Quarter <br />2014, Soil Vapor Investigation Report, PCCP MH Tuso Property, Mountain <br />House, California; June 13, 2014. <br />Wallace Kuhl Associates, Soil Vapor Investigation Report, PCCP Mountain <br />House Tuso and Rivani Properties, Mountain House, California; July 31, 2014 <br />ENGEO, Phase ll Environmental Site Assessment PCCP Mountain House <br />Property, Mountain House, California; September 1,2015. <br />Dear Ms. Turkette <br />As requested, ENGEO has prepared this Supplemental Soil Gas Characterization Work Plan <br />(Work Plan) for the subject property (Property) in Mountain House, California. The purpose of <br />the additional characterization is to evaluate the soil gas concentrations along the known <br />Crimson Pipelines (formerly Chevron) petroleum pipeline bordering the Property to the south. <br />BACKGROUND <br />The Property is located east of South Central Parkway, in Mountain House, San Joaquin <br />County, California (Figure 1). The Property consists of undeveloped residential lots, appurtenant <br />streets, and a proposed community park. Previous environmental investigations have identified <br />subsurface impacts resulting from a petroleum release associated with the Chevron pipeline. <br />Previous studies conducted by others identified the presence of soil impacts, but did not assess <br />the vertical or horizontal limits of impact that remain in place. Additionally, the previous limited <br />soil gas assessment has not adequately determined if a vapor intrusion risk may exist for the <br />future residential development. <br />As presented in the Wallace-Kuhl & Associates (Wallace-Kuhl) June 2014 Monitoring Well <br />Sampling Report (Reference 1), petroleum hydrocarbon-impacted soil was encountered during <br />the installation of an underground stormwater utility near the KLM pipeline in 2006. Subsequent <br />testing of excavated and stockpiled soil material identified the presence of gasoline-range total <br />petroleum hydrocarbons (TPH-g), TPH-diesel (TPH-d), TPH-motor oil (TPH-mo) and selected <br />BTEX constituents (xylene(s) and ethylbenzene). Five groundwater wells were installed in 2007 <br />2010 Crow Canyon Place, Suite 250 • San Ramon, CA 94583 • (925) 866-9000 • Fax (888) 279-2698 <br />www.engeo.corn