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Revised Soil Gas Sampling I r VVorkplan September 8, 2017
<br />CA ARNG, FMS #24, Stocktu.., CA Page 3
<br />Petroleum hydrocarbon constituents were also found in groundwater collected from all five
<br />wells, with maximum concentrations of TPHg (23,000 micrograms per liter [pg/L]) and benzene
<br />(5,400 pg/L) found in FMS-MW-5.
<br />OTIE performed three more quarters of groundwater monitoring and sampling on wells FMS-
<br />MW-1 through FMS-MW-5 from July 2010 to February 2011. The direction of groundwater flow
<br />at the site continued to be northeasterly, with a shallow hydraulic gradient. Concentrations of
<br />contaminants of concern (COCs), TPHd, TPHg, and select VOCs, were generally lowest (trace
<br />to non-detect) in FMS-MW-1, which is generally upgradient from the former USTs, and highest
<br />in FMS-MW-5, which is located within the likely source area for COCs.
<br />In December 2011, the CVRWQCB approved the Work Plan for Implementing an Air Sparge, Soil
<br />Vapor Extraction, and Dual-Phase Extraction Treatability Study for In-Situ Petroleum Hydrocarbon
<br />Remediation (OTIE, 2011). In addition to detailing the scope associated with a May 2012 pilot-
<br />scale soil vapor extraction (SVE), air sparge (AS), and high-vacuum, dual-phase extraction
<br />(HVDPE) testing, the approved workplan also defined a semiannual groundwater monitoring
<br />program. The first semiannual groundwater monitoring event was conducted in December 2011,
<br />and the second semiannual groundwater monitoring event was conducted in July 2012. The results
<br />of these groundwater monitoring events were generally consistent with previous events (OTIE,
<br />2012b).
<br />OTIE advanced four soil borings and converted them into an SVE well (FMS-SVE-1), a dual-
<br />nested AS well (FMS-AS1A/B), and two HVDPE wells (FMS-DPE-1 and FMS-DPE-2) in May
<br />2012. These locations are shown on Figure 2. Soil samples collected during advancement of
<br />the soil borings were analyzed for TPHg, TPHd, VOCs, and total lead. Following installation
<br />and development of the wells, OTIE conducted a treatability study to evaluate the feasibility of
<br />using SVE, AS, and HVDPE technology to remediate the petroleum hydrocarbon plume.
<br />Results from the soil boring samples and detailed procedures and results of the feasibility
<br />testing are provided in the Treatability Study Report and Feasibility Evaluation for In Situ
<br />Petroleum Hydrocarbon Remediation (OTIE, 2012a).
<br />In February and March 2013, OTIE conducted a membrane interface probe (MIP) investigation
<br />(15 MIP borings), installed three new groundwater monitoring wells (FMS-MW-6 through FMS-
<br />MW-8), conducted a soil and groundwater investigation, and completed specific capacity
<br />testing. The well locations are shown on Figure 2. Soil and groundwater samples were
<br />analyzed for TPHd, TPHg, VOCs, and total lead. Groundwater samples were additionally
<br />analyzed for semi-volatile organic compounds, dissolved metals, total dissolved solids, total
<br />organic carbon, sulfate, and hexavalent chromium. Results are presented in a letter report
<br />entitled MIP Groundwater Investigation, Groundwater Monitoring Well Installation, Soil and
<br />Groundwater Sampling, and Specific Capacity Testing, Field Maintenance Shop #24, Stockton
<br />California Army National Guard Complex (OTIE, 2013). OTIE concluded that The MIP
<br />investigation adequately delineated VOC distribution in site soil in the vertical and horizontal
<br />directions.
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