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Revised Soil Gas Sampling H Workplan September 8, 2017 CA ARNG, FMS #24, Stocktoii, CA Page 4 Star was contracted to conduct four quarterly groundwater monitoring and sampling events on wells FMS-MW-1 through FMS-MW-8, and the first event was completed May 2013. Star was additionally contracted to install and sample well FMS-MW-9 for performance monitoring in support of in-situ chemical oxidation (ISCO) remediation efforts. The first performance monitoring event was the baseline event, completed September 2013. Details of the well installation are provided in Second and Third Quarters 2013 Groundwater Monitoring Report, Field Maintenance Shop #24, submitted November 15, 2013 (Star, 2013). One- and 3-month- post-injection monitoring results were presented in the fourth quarter 2013 groundwater monitoring report (Star, 2014a). Six-month-post-injection monitoring results were included in the first quarter 2014 report (Star, 2014b). OTIE was contracted to conduct performance monitoring under the MRP on wells FMS-MW-1 through FMS-MW-8; therefore, Star was contracted to conduct performance monitoring only on FMS-MW-9 and standard quarterly groundwater monitoring on all wells (FMS-MW-1 through FMS-MW-9). Star was contracted to conduct four additional quarters of groundwater monitoring through second quarter 2015. ECM Consultants (ECM), formerly Environmental Cost Management, was contracted by CA ARNG to conduct four additional quarters of groundwater monitoring starting with the third quarter 2015 event. In February 2014, ECM completed a second MIP investigation (15 MIP borings and 2 soil borings) to further delineate the horizontal and vertical impacts to soil and groundwater in the source area. Results of the 2014 MIP, soil and groundwater investigations were presented in the Final Summary Letter Report for Additional Groundwater and MIP Investigation (ECM, 2014). ECM then performed two more rounds of ISCO injections and post-injection monitoring at the site. In July and August 2014, ECM performed a total of 36 ISCO injection borings in the source area (Figure 3). ECM also completed 30-day, 90-day, and 180-day post-injection monitoring to comply with MRP requirements. A complete summary of the 2014 ISCO injections and post-injection monitoring results were presented in a Remedial Action Completion Report (ECM, 2015). The second round of ISCO injections were completed in July and August 2015. ECM completed 28 ISCO injections borings in the source area (ECM, 2016a). ECM also performed post-injection monitoring to comply with the site MRP requirements (ECM, 2016b). Between October through December 2016, Adanta-ECM Joint Venture completed additional remedial actions consisting of Vapor Energy Generator (VEG) technology and ISCO for further remediation of soil and groundwater. Twenty-three ISCO injection points were used in three areas within the source area, and VEG technology was intermittently operated in the same area between November 7 and December 8, 2016 (Adanta-ECM Joint Venture, 2017). See Attachment 3 for figures related to these activities that show locations of remedial efforts and recent groundwater flow direction and gradient. The soil gas sampling proposed in this workplan will help determine if previous remedial activities have mobilized VOCs in soil gas.