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San Joaquin County Environmental Health Department 12066.002.000 <br />Mountain House Neighborhood A, Units 3 and 4 January 3, 2019 <br />SOIL GAS CHARACTERIZATION WORK PLAN ADDENDUM Page 2 <br />1650 ug/m3 (SG-G @ 10 feet). These concentrations are below applicable RWQCB Tier 1 <br />Environmental Screening Levels (ESLs). Multiple samples reported low-level concentrations of <br />several VOCs. The following are the maximum concentrations observed for these analytes: <br />Acetone (400 ug/m3), Carbon Disulfide (12 ug/m3 ), Benzene (33 ug/m3), Toluene (36 ug/m3), <br />4-Methyl-2-Pentanone (MIBK) (17 ug/m3 ), Xylenes (14.6 ug/m3 ), Styrene (4.9 ug/m3), Hexane <br />(40 ug/m3), 2-Hexanone (9.9 ug/m3), 2-Butanone(MEK) ( 110 ug/m3), Tetrahydrofuran (240 <br />ug/m3), Napthalene (3.0 ug/m3 ), and tert-Butanol (13 ug/m3). All of these concentrations <br />reported are below applicable RWQCB Tier 1 ESLs, DTSC Modified Screening Levels, and <br />USEPA RSLs. <br />SAMPLING RATIONALE <br />r--.Based on review of the 2018 ENGEO soil gas data, San Joaquin County Environmental Health <br />Department (SJCEHD) expressed concerns regarding a leak check compound, <br />1,1-difluoroethane (DFA), detected at sample location SG-5. The reported DFA concentrations <br />were greater than 10 times the detection limit for Benzene. Based on DTSC guidelines, this <br />could suggest a potential bias in the soil gas data for SG-5. The SJCEHD has requested <br />retesting of the SG-5 location to confirm soil gas results. In addition, it was agreed that two <br />additional locations would be sampled northeast of the pipeline ROW, proximate to the graded <br />residential lots, to determine if potential lateral migration of soil gas has occurred (Figure 2). <br />PROJECT TEAM AND CONTACT INFORMATION <br />Environmental Consultant — ENGEO <br />Client — Century Communities <br />Mr. Jeff lnabnit <br />Century Communities <br />99 Almaden Boulevard, Suite 400 <br />San Jose, CA 95113 <br />Jeff.lnabnit@centurycommunities.com <br />Owner — Mountain House Investors LLC <br />Mr. Don Johnson <br />Mountain House Investors LLC <br />11249 Gold Country Blvd, Suite 190 <br />Gold River, CA 95670 <br />DJohnson@kamilos.com <br />Regulatory Oversight <br />Ms. Linda Turkette <br />San Joaquin County Environmental Health <br />Department <br />1868 E Hazelton Ave <br />Stockton, CA 95205 <br />Iturkatte@sjcehd.com <br />Principal-in-Charge <br />Shawn Munger, CHG <br />2213 Plaza Drive <br />Rocklin, CA 95765 <br />916-580-6535 <br />smunger@engeo.com <br />Project Manager <br />Cody Johnson <br />6399 San Ignacio Avenue, Suite 150 <br />San Jose, CA 95119 <br />925-786-7895 <br />cjohnson@engeo.com <br />Field Manager/Health and Safety Manager <br />Cody Johnson <br />6399 San Ignacio Avenue, Suite 150 <br />San Jose, CA 95119 <br />925-786-7895 <br />cjohnson@engeo.corn