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ENGEO <br />Expect Excellence <br />GEOTECHNICAL <br />ENVIRONMENTAL <br />WATER RESOURCES <br />CONSTRUCTION SERVICES <br />Project No. <br />12066.002.000 <br />January 3, 2019 <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 ADDENDUM <br />References: 1. ENGEO, Soil Gas Characterization Work Plan, Mountain House <br />Neighborhood A, Units 3 and 4, Central Parkway, Mountain House, <br />California; November 14, 2018. <br />2. ENGEO, Soil Gas Characterization Report, Mountain House Neighborhood <br />A, Units 3 and 4, Central Parkway, Mountain House, California; <br />November 30, 2018. <br />Dear Ms. Turkette <br />As requested, ENGEO has prepared this Soil Gas Characterization Work Plan Addendum <br />(Addendum) for the subject property (Property) in Mountain House, California (Figure 1). The <br />purpose of the additional characterization is to further evaluate the soil gas concentrations <br />within the northern area of the subject Crimson petroleum pipeline alignment, bordering the <br />planned residential property located approximately 150 feet northeast of the pipeline right of <br />way (ROW). <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 soil gas <br />assessments have not adequately determined if a vapor intrusion risk may exist for the future <br />residential development. <br />As presented in the ENGEO November 2018 Soil Gas Characterization Report (Reference 2), <br />five "nested" soil gas wells were installed and sampled in the vicinity of noted areas of elevated <br />passive soil gas concentrations, that were previously identified during a previous Wallace-Kuhl <br />June 2014 Monitoring Well Sampling Report. Seven samples reported detectable <br />concentrations of TPH-gasoline, ranging from 210 micrograms per cubic meter (ug/m3 ) to <br />2010 Crow Canyon Place, Suite 250 • San Ramon, CA 94583 • (925) 866-9000 • Fax (888) 279-2698 <br />www.engeo.com