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Soil Boring Results and VEG/ISCO Work Plan Field Maintenance Shop #24, Stockton, California October 2016 Ethylbenzene ranged from a concentration of 0.61 mg/kg at 31-32 feet bgs (silt) to below the laboratory MDL at 19-20 feet bgs (well graded sand with silt). Total Xylenes ranged from a concentration of 0.23 mg/kg at 31-32 feet bgs (clay) to below the laboratory MDL 19-20 feet bgs (well graded sand with silt) and at 25-26 feet bgs (clay). Naphthalene ranged from a concentration of 0.64 mg/kg at 31-32 feet bgs (silt) to 0.020 mg/kg at 14-15 feet bgs (poorly graded sand with silt). 2.2. Soil Sample Data FMS-MW6, FMS-MW7, FMS-MW8 For comparison purposes soil sample results collected by OTIE during the installation of wells FMS-MW6, FMS-MW7, and FMS-MW8 are presented in Table 3. Soil results from both FMS- MW7 and FMS-MW8 were all below laboratory MDLs with the exception of some low level detections of TPH-d. Soil results collected during the installation of FMS-MW6 generally show increasing concentrations of TPH-g, TPH-d, benzene, and other VOCs with depth from 15 to 35 feet bgs. 2.3. Conclusions Vadose zone impacts extending significantly north beneath the former UST concrete pad were not found during this soil investigation. However, the current soil data (SB-4, SB-5, and SB-6) combined with historical soil analytical results (FMS-MW6) indicates that the area in the vicinity of the dispenser island (near FMS-MW6) is likely the source of impacts observed in soils and groundwater. Moreover, lithological results and assessment data indicate a potential for downward migration through interconnected and partially interconnected fine to coarse-grained sand lenses and stringers. To reduce vadose zone source impacts and limit petroleum contaminants from leaching and contributing to groundwater concentrations, the proposed vadose zone treatment area will be centered around FMS-MW6 (Figure 6). 3.0 REMEDIAL ACTION WORK PLAN Two remedial action measures are proposed to target impacted soils and groundwater within the source area. ISCO has been implemented at the site three times before (2013, 2014, and 2015); however, in-situ thermal treatment via VEG technology has not yet been implemented at the site. VEG treatment is discussed herein and will be added to the waste discharge application. 3.1. Waste Discharge Requirements and Permitting Previous ISCO injections have been performed under CVRWQCB General Order No. R5-2008- 0149-045. In February 2015 the CVRWQCB updated the Waste Discharge Requirements (WDRs) for In-situ Remediation and Discharge of Treated Groundwater to Land (General Order No. R5-2015-0012). Per paragraph No. 1 of the updated WDRs, "General Order R5-2008-0149 and the (Notices of Applicability) NOAs issued pursuant to that Order are still in effect. No additional NOAs will be issued under that Order. The Order will be proposed for rescission once all of the projects under that Order are completed and the NOAs issued have been terminated. This updated version of the Order is not more restrictive to the projects covered under Order No. R5-2008-0149 and thus it is not necessary to enroll them under this Order." Since the scope of this work plan is a continuation of ISCO injections and in-situ thermal treatment of vadose zone soils utilizing only steam (VEG) as described below, the CA ARNG is seeking to perform the proposed work under the existing General Order No. R5-2008-0149-045. Additional permitting requirements for the project include VEG injection/recovery wells and ISCO injection point permits and an air quality permitting/exemption for VEG implementation. All Page 5 •