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Work Plan for Groundwater Investigation and
<br />Groundwater Monitoring Well Installation
<br />Field Maintenance Shop #24, Stockton, California
<br />each event, groundwater elevations were gauged in, and groundwater samples were collected
<br />from each of the five site groundwater monitoring wells. Key findings from the quarterly
<br />monitoring program were as follows:
<br />• Overall, direction of groundwater flow at the site is generally northeasterly, with a
<br />relatively flat gradient;
<br />• No evidence of SPH has been identified in site groundwater; and
<br />• Groundwater at the site has been impacted with petroleum hydrocarbons and related
<br />VOCs at concentrations that exceed potentially applicable regulatory standards.
<br />Over the course of quarterly sampling, TPH-g, TPH-d, and select VOCs, including naphthalene
<br />and BTEX, were consistently reported in the groundwater samples. Concentrations of all
<br />contaminants are generally lowest (trace to non -detect) in FMS-MW1, which is generally
<br />upgradient from the former USTs, and highest in FMS-MW5, which is considered to be within
<br />the primary source area for COPCs. TPH-g and TPH-d were reported in all five wells during
<br />each event. TPH-g values ranged from 25 pg/L ("J -flagged", FMS-MW1; April 2010) to 34,000
<br />pg/L (FMS-MW5; November 2010). TPH-d values ranged from 140 pg/L ("Y -flagged", FMS-
<br />MW1; February 2011) to 6,800 pg/L (FMS-MW5; November 2010). Other analytes regularly
<br />detected above potentially applicable water quality standards are 1,2 -DCA (all wells); and
<br />benzene, ethylbenzene, and naphthalene (FMS-MW2 to FMS-MW5). 1,3,5-trimethylbenzene
<br />(FMS-MW3, FMS-MW4, and FMS-MW5), toluene (FMS-MW5), and xylenes (FMS-MW5) have
<br />also regularly been detected at concentrations above potentially applicable water quality
<br />standards (OTI E, 2010a; 2010b; 2011 a; 2011 b).
<br />2.7. 2011-2012 SEMIANNUAL GROUNDWATER MONITORING (OTIE)
<br />In December 2011 the CVRWQCB approved the Work Plan for Implementing an Air Sparge,
<br />Soil Vapor Extraction, and Dual -Phase Extraction Treatability Study for In -Situ Petroleum
<br />Hydrocarbon Remediation (AS/SVE/HVDPE Work Plan; OTIE, 2011c). In addition to detailing
<br />the scope associated with the May 2012 pilot -scale SVE, AS, and HVDPE testing, the Work
<br />Plan also defined a semiannual groundwater monitoring program. The first semiannual
<br />groundwater monitoring event occurred in December 2011 (OTIE, 2012a), and the second in
<br />July 2012 (OTIE, 2012c). OTIE conducted groundwater monitoring at site wells FMS-MW1
<br />through FMS-MW5 during the first semiannual event, and incorporated new wells installed as
<br />part of the 2012 treatability study (FMS-DPE1 and —DPE2; see Section 2.8 below) during the
<br />second event. During each of the semiannual events, groundwater elevations were gauged in,
<br />and groundwater samples were collected from site groundwater monitoring wells. The results of
<br />these groundwater monitoring events were generally consistent with previous events performed
<br />prior to the treatability study in May 2012, indicating the rebound in concentrations post-
<br />AS/SVE/HVDPE.
<br />2.8. MAY 2012 SVE, AS, AND HVDPE TREATABILITY STUDY (OTIE)
<br />Between May 7 and May 10, 2012, OTIE advanced four soil borings that were subsequently
<br />converted into an SVE well (FMS-SVE1), a dual -nested AS well (FMS-AS1A/B), and two
<br />HVDPE wells (FMS-DPE1 and FMS-DPE2) in association with implementation of the
<br />AS/SVE/HVDPE Work Plan. The borings were advanced using a HSA drill rig and soil samples
<br />were collected from each borehole at five foot intervals beginning 15 feet bgs and extending
<br />downward to the first occurrence of groundwater. Soil samples collected during advancement of
<br />the soil borings were analyzed for TPH-g, TPH-d, VOCs including BTEX, fuel oxygenates, and
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<br />OTIE
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