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Gealaglcat T" Ma 14c Page 17 Site Conceptual Model Report(2006 Revision) Valentine(Castle Motors) Project No 723 2 June 16,2006 1 3.0 POTENTIAL OFF-SITE SOURCES OF CONTAMINATION ' Mr Jeff Won of the SJC/EHD supplied GTI staff with a co of a groundwater monitoring Wong PP PY report (July 2003 sampling event) for the Shell Service Station facility across N El Dorado ' Street The report indicates that the wells on the northern portion of the Shell site contain low levels of TPH-G Since these monitoring wells are 50 feet deep, it is likely that the subject plume is diluted in these wells and mixing with shallower clean water The Shell site ' report also indicates that the two wells in the southern portion of the site (near Shell's existing USTs) have MTBE above the laboratory detection linuts The two northern wells that lie along the subject site plume axis do not contain MTBE above the laboratory reporting ' linut (0 5 ug/1) This suggests that the Shell facility has an MTBE problem not related to the subject site A copy of the July 2003 report is included in Appendix G The Shell site consultant has coordinated quarterly groundwater monitoring with GTI's sub- contractor (Del Tech Geotechnical Support Services) and both sites were monitored concurrently on April 17, 2006 The Shell data was included in drafting the contours for the ' shallow aquifer plume in Figure 12 The April 17, 2006 event data from Blaine Tech Services Inc for the Shell site at 2320 N El Dorado Street is also included in Appendix G with the 2003 data The May 10, 2006 report was provided to GTI by Ms Anni Kreml of Cambria Environmental Technology Inc ' 4.0 DISCUSSION AND CONCLUSIONS ' 4.1 Discussion In GTI's March 15, 2005 "Additional Site Characterization" work plan we identified two areas of concern relevant to completing the Site Conceptual Model for the site 1) The five foot sampling methodology utilized at this site may have failed to intercept lithologic units of greater hydraulic conductivity 2) The plume remains undefined down gradient of the MW-8/-1081-208 well cluster ' The February2006 CPT field investigation was intended to complete the definition of the g P groundwater plume and update this SCM The second concern noted above was resolved and ' as stated in the conclusion section of our May 9, 2006 "Additional Site Characterization Sod do Groundwater Investigation" report, the down gradient lateral extent of the plume probably ' attenuates to non-detect levels within 50 feet of the CPT-3 location The vertical extent of the plume has already been completed by the on site wells MW-101 and MW-201 ' The first concern noted above was also sufficiently resolved The highest TPH-G detected (16,000 ug/1) in the MW-8/108/208 well set for the April 17, 2006 monitoring event was in the deeper section (MW-208 screened @ 76 feet bgs) This supports the conclusion that the sandy units occurring at 60 — 75 feet bgs and shown in Figure 4 are acting as a preferential