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1 <br /> 16 <br /> As summarized in the table above, and illustrated on the isoconcentration maps provided in Appendix B (i.e.; <br /> Figures 7, 10, 12 and 14), these dissolved petroleum hydrocarbons are detected in the area of the former <br /> transmix UST; however, are commingled with petroleum hydrocarbon impacts located upgradient of the former <br /> UST (e.g.; MW-7A located north of the loading racks). The petroleum hydrocarbon concentrations drop <br /> significantly at the downgradient monitoring well MW-44A. <br /> B Zone <br /> The 4Q05 groundwater monitoring results for monitoring wells MW-23B and MW-65B were compared to <br /> evaluate groundwater quality upgradient and downgradient of the former transmix UST location. The table <br /> below summarizes the TPHd, TPHg, total BTEX, TBA, and MtBE concentrations reported for the 4Q05 <br /> monitoring event: <br /> Distance/Location Relative to Tot <br /> Well ID Former UST' TPHd TPH6 BTEX TBA MtBE <br /> MW-65B Within UST Pit 220 <50 <0.5 <5 <0.5 <br /> MW-23B 260 feet/Down radient 550 <50 <0.5 <51 <0.5 <br /> Notes: <br /> All results reported in Ng/I. <br /> - Result for 2Q05, MW-23B not sampled for TBA during 4Q05. <br /> As summarized in the table above, and illustrated on the isoconcentration map provided in Appendix B (i.e.; <br /> Figure 8), TPHd is detected in the area of the former transmix UST; however, is commingled with petroleum <br /> hydrocarbon impacts located downgradient of the former UST (e.g.; MW-23B located north of the loading racks <br /> and AST #134 and AST #135). The TPHd concentration dropped to 170 µg/L for the groundwater sample <br /> collected during 4Q05 from downgradient monitoring well MW-5013, located approximately 170 feet northeast <br /> of MW-2313. <br /> 3.3 Remediation <br /> Previous remedial actions have included dual extraction (DE), limited soil excavation, and ISO-GEN TM pilot <br /> testing. A summary of previous remedial efforts are summarized below. <br /> • In January 1994, a groundwater remediation system was installed at the site for SPH recovery and <br /> hydraulic containment. The remedial system was initiated in February 1994 and continued until <br /> February 1996. During this time period, groundwater was extracted from wells MW-18 and MW-19 <br /> ' (for hydraulic containment of groundwater) and SPH was recovered from wells, MW-14, MW-15, and <br /> MW-16 (PEG, 1995). <br /> • On March 25, 1996, a DE system began continuous operation. The DE system was comprised of 14 DE <br /> ' wells (MW-1, MW-2, MW-7, MW-8, MW-14, MW-15, MW-16, and DE-1 through DE-7), a water <br /> knockout tank, an oil/water separator, four granular activated carbon vessels arranged in series, and a <br /> thermal oxidizer. In May 1997, site data suggested the onsite SPH plume had been substantially <br /> diminished by the DE system. In addition, groundwater influent concentrations of TPHd had declined <br /> from 69,000 pg/L to 5,300 and influent vapor concentrations had declined from 300 parts per million by <br /> . volume (ppmv) to 21 ppmv (SECOR, 1997). Temporary shutdown of the DE system was approved by <br /> BLASLAND, BOUCK &LEE, INC. <br /> 724106 _ — engineers,scientists,economists 3-4 <br /> J:1D0006144634 00161022 Conceptual Site Model.doc <br />