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San Joaquin County Environmental Health Department 12066.002.000 Mountain House Neighborhood A, Units 3 and 4 January 3, 2019 SOIL GAS CHARACTERIZATION WORK PLAN ADDENDUM Page 3 PROPOSED SCOPE OF WORK An ENGEO representative will contact the USA North Service Alert to facilitate notification to operators of underground utilities at the Property. Drilling activities will be performed by a C-57-licensed drilling contractor, utilizing Geoprobe® direct push technology. All work will be performed by an ENGEO environmental professional under the supervision of a registered Professional Geologist or Professional Engineer. ENGEO will prepare a drilling permit application and submit it to the San Joaquin Environmental Health Department. Standard operating procedures and methodologies for the soil gas sampling are provided in Appendix A. Soil Gas Assessment In order to further delineate the extent of VOCs in soil gas on the Property, sampling will be performed at three locations (SG-5, SG-11, and SG-12 @ 5' and 10') (Figure 2). The soil gas samples will be recovered in accordance with Department of Toxic Substances Control (DTSC) Final Advisory Active Soil Gas Investigations (July 2015). The soil gas samples will be recovered from each of 3 co-located locations (6 separate soil gas wells); one well sampling point at 5 feet below the ground surface and one at 10 feet below the ground surface, for a total of 6 soil gas samples. The 5- and 10-foot soil gas wells will be approximately 2 feet apart. Specific soil gas sampling procedures are provided in Appendix A. General procedures are described as follows. The soil gas monitoring well casings will consist of 1/4-inch-diameter Teflon® tubing equipped with a filter at the base of the tubing. The wells will be installed with a direct push probe rig, which will advance an approximately 3-inch-diameter boring. At each location, the first well boring will extend to a depth of approximately 5 feet below the ground surface; the second well boring will extend to a depth of approximately 10 feet below the ground surface. The bottoms of the well casings will be equipped with a filter that will be situated at depths of 4.5 and 9.5 feet below the ground surface, respectively, centered in the middle of a 1-foot layer of No. 3 sand. Twelve inches of dry granular bentonite will be installed on top of the sand, and the remaining annular space will be filled with hydrated bentonite grout to 6 inches below grade. Soil gas samples will be recovered using a sample train consisting of a stainless steel twin Summa manifold with built-in flow controller. The samples will be recovered using 1-liter Summa canisters. Following recovery, we will label each sample canister with a unique identification number, sampling time, and pre- and post-sample vacuum readings. The soil gas samples will be submitted to Torrent Laboratories, a State-certified laboratory located in Milpitas, California, under documented chain-of-custody for analysis of VOCs by EPA Test Method TO-15. The target laboratory reporting limit for soil gas is less than the respective site-specific soil gas screening levels. Sample results will be reported in micrograms per cubic meter (ug/m3). The six wells will be maintained for a period of approximately 10 days to allow for review of analytical data. Upon review of data and approval of SJCEHD, the sampling train will be removed and the soil gas wells will be drilled out and grouted in accordance with SJCEHD standards.