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L & M OPERABLE UNIT- FIRST QUARTER 2012 SITE STATUS, REMEDIAL SUMMARY, <br />AND GROUNDWATER MONITORING REPORT <br />Soil Vapor Sampling <br />April 30, 2012 <br />I:\Tidewater\TW Stockton (P2A)\QMRs\2012\1Q12\1Q12 QSSR QRSR QMR Stockton_FINAL.doc 4.2 <br />4.2.2 APSV Soil Vapor Concentrations <br />Soil vapor sampling data collected during 1Q12 sampling event is provided in Table 25 and <br />shown on Figure 35. TPHg was detected in samples collected from APSV-10 at a concentration <br />of 12.0 parts per million by volume (ppmV). No additional requested analytes were detected at <br />or above the MRL in the APSV wells during 1Q12. <br />4.2.2.1 APSV Trends <br />During 1Q12, TPHg concentrations decreased in samples collected from APSV-10 and <br />decreased below the MRL in samples collected from APSV-9 in comparison to 4Q11 sampling <br />event. During 1Q12, methane concentrations decreased below the MRL in samples collected <br />from APSV-9 and APSV-10 in comparison to 4Q11. All other sampled vapor points have been <br />below the MRL for all requested analytes since fourth quarter 2010. Concentrations are shown <br />in Table 25. <br />Except for well APSV-10, there were no detections of the requested analytes at or above the <br />detection limits in the samples collected from the APSV wells during 1Q12. This is consistent <br />with the previous quarter (4Q11) data (Table 25, Figure 36 through 38). <br />4.2.3 UPSV Soil Vapor Concentrations <br />Soil vapor sampling data collected during 1Q12 sampling event is provided in Table 25 and <br />shown on Figure 35. TPHg was detected in samples collected from UPSV-5 at a concentration <br />of 587 ppmV. No additional requested analytes were detected at or above the detection limits in <br />the samples collected from the UPSV wells during 1Q12. There have been no fuel oxygenates <br />detected in UPSV wells since their installation. <br />4.2.3.1 UPSV Trends <br />During 1Q12, TPHg concentrations decreased in samples collected from UPSV-5 in comparison <br />to 4Q11 sampling event. There were no detections of the requested analytes at or above the <br />detection limits in the remaining samples collected from all UPSV wells. UPSV-1 and UPSV-6 <br />have had no detections of the requested analytes at or above the detection limits since first <br />quarter 2008. For two consecutive quarters, UPSV-2 and UPSV-3 have had no detections of <br />the requested analytes at or above the detection limits. As shown in Table 25, TPHg has <br />historically decreased in wells UPSV-1 and UPSV-6 since base line sampling in July 2005 and <br />since start-up of the on-site remediation system on May 31, 2007. Additionally, benzene, <br />ethylbenzene, and total xylenes have decreased to below detection limits which are <br />complimentary to the decreasing trend of TPHg observed in well UPSV-6 (Figure 39). There <br />have been no other apparent trends associated with reported concentrations in the UPSV wells.