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' During the 4Q05 groundwater monitoring event,the highest reported concentrations of TPHd, TPHg, and MtBE
<br /> in groundwater at the site were 190,000 gg/L (MW-40UA), 21,000 µg/L (EW-1), and 3,900 gg/L (MW-3UA),
<br /> respectively. These concentrations were detected in monitoring wells that are screened the shallow groundwater
<br /> ' in the Upper A zone. The 4Q05 groundwater elevation, maps are presented in Figures 5 through 7, and
<br /> isoconcentration maps are presented in Appendix B.
<br /> SPH has historically been detected in wells MW-IUA, MW-3UA, MW-7A, MW-8A, MW-14A, MW-15A,
<br /> MW-16, and MW-36UA through MW-42UA. During the 4Q05, SPH was only detected in monitoring well
<br /> MW-40UA located north of the loading racks associated with the onsite fueling station(SECOR, 2005c).
<br /> In the Upper A zone, concentrations of TPHd in groundwater is not defined at the site except to the southeast
<br /> and southwest of the site. TPHg in groundwater are not defined to the north, northeast, northwest and south of
<br /> the site. MtBE and TBA in groundwater are not defined to the north, northwest,northeast and south of the site.
<br /> In the A zone, concentrations of TPHd in groundwater are not defined to the north, northeast, south and east;
<br /> and MtBE and TBA in groundwater are not defined to the north(SECOR 2005c).
<br /> ' The vertical extent of hydrocarbons in groundwater is defined by B zone wells MW-12B, MW-13B, MW-22B,
<br /> MW-23B, MW-50B, and MW-64B. These wells are screened from approximately 55 feet bgs to 65 feet bgs.
<br /> During 4Q05, highest concentrations of TPHd, TPHg, and MtBE in the B zone were 550 µg/L (MW-23B), <50
<br /> gg/L, and <0.5 gg/L, respectively (SECOR, 2005c). As summarized in Table 2, concentrations of TPHd for
<br /> ' groundwater samples collected from the downgradient B zone well (MW-50B) have periodically exceeded the
<br /> RWQCB Water Quality Objective (WQO) of 100 gg/L, as outlined in the CAO. The concentration of TPHd
<br /> exceeded the WQO at MW-50B during the 2005 quarterly monitoring program at 170 gg/L(4Q05).
<br /> 3.2.1 2005 Offsite Assessment
<br /> Based on data collected during the 2005 offsite investigation, concentrations of TPHg, MtBE and TAME for
<br /> groundwater samples collected at offsite boring locations exceed the RWQCB WQOs provided within the CAO.
<br /> ' The horizontal extent of TPHd and TPHg in groundwater is delineated north of the site; however, MtBE is not
<br /> delineated north and northeast of the site. Concentrations of TPHd and MtBE in groundwater decrease to the
<br /> northeast from the site based on the results of the D and H-series borings. Data collected during the 2004 and
<br /> ' 2005 offsite investigation indicate TPHd concentrations have decreased over time. MtBE concentrations in
<br /> groundwater samples collected from offsite borings installed between 2001 and 2004 indicate the offsite MtBE
<br /> concentrations have not increased significantly, suggesting a stable plume. Further information regarding the
<br /> offsite investigation is presented in the 2005 Offsite Assessment Report(SECOR,2005b).
<br /> 3.2.2 Former Transmix UST
<br /> Groundwater monitoringassociated with the e former transmix UST has been ongoing since late 2003 when
<br /> monitoring well MW-67UA was installed. The results of the 4Q05 groundwater monitoring event have been
<br /> ' used for this evaluation, as these analytical results are representative of the dissolved petroleum hydrocarbon
<br /> concentrations reported for wells near the former UST location, as summarized in Tables 2 and 3, illustrated on
<br /> Figures 3 and 4, and presented on the isoconcentration maps presented in Appendix B. Groundwater monitoring
<br /> ' results for the Upper A zone,A zone and B zone in the area of the former transmix UST are described below.
<br /> BLASLAND,BOUCK&LEE,INC.
<br /> J:\D0006\446346 — engineers,scientists,economists
<br /> J:\D0006\44634-00161022—conceptuaI Site Model.doc 3-2
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