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1 <br /> r <br /> 31 January 2003 <br /> AGE-NC Protect No 96-0232 <br />' Page 16 of 17 <br /> r <br /> I i <br /> Relative to the Inferred source area near the Flag City Chevron UST area (EW-1), the Flag City <br />' Chevron plume appears to have extended approximately 200 feet to the west at depths as great as <br /> 75 feet bsg, AGE suspects that the plume may pinched off and/or separated at Thornton Road <br /> (Figures 8 and 11) The plume extends approximately 30 feet to the south at depths as great as 30 <br />' feet bs and approximately 80 feet to the east at depths asf eat as 30 feet bs (Figure's 8 and 11 <br /> g� pp Y p �' g ( g ) <br /> The northern extent of the Flag City plume is uncertain at the present time because of the high <br />' concentrations of MTBE (up to 700 pg/l)detected in northernmost Flag City Chevron well MW-8 <br /> during the March and June 2002 monitoring events ' <br />' AGE estimates the arithmetic average concentration of MTBE in ground water extracted between <br /> 05 September and 05 December 2002 was 14 µg/l During this time,the volume of water processed <br /> f, through the ground water pump and treat system was recorded at 1,169,835 gallons (equivalent to <br />' 9,752,914 pounds), yielding 0 12 pounds of MTBE (equivalent to 55 grams) from the plume of <br /> impacted ground water (Appendix D) - <br />' Ground water elevations versus ground water monitoring dates (hydrograph) were plotted for _ <br /> monitoring wells MW-2, MW-6A and MW-6B, generally, all three plots depict an overall trend of <br /> decreasing ground water elevation since May 2000,' the increase in ground water elevation of <br />' approximately 4 41 feet between September 2002 and December 2002 may be due to seasonal <br /> fluctuation (Appendix 1) <br /> 1 <br /> 7 4 RECOMMENDATIONS <br /> -Based upon data collected from the site, AGE recommends <br /> • Continued quarterly ground water monitoring and sampling of monitoring wells MW-1 <br />' through MW-18 <br /> • Continued coordinated quarterly ground water monitoring and sampling events with Three <br />' B's Truck Plaza <br /> • Continued ground water extraction from wells EW-4 and EW-5, extraction wells EW-7, <br /> EW-8, EW-9,andEW-10 should be alternated into the ground water extraction system i <br /> • ' Due to the possible one time occurrence of hexavalent chromium, the constituent,will be <br /> included with the monthly pump-and-treat,effluent monitoring and sampling <br /> A work plan addendum ,was submitted to the RWQCB on 09 May 2002 and approved by the <br /> RWQCB by letter dated 20 May 2002 The scope of work for the work plan addendum included <br /> installation a cluster of ground water monitoring wells (MW-14 through MW-18) near the former <br /> . Rocky's Restaurant domestic water well, installation of a triple completed ozone sparging well <br /> 1 r <br /> Advanced GeoEnvironmental,Inc <br /> i I <br /> 1 j <br /> I ' <br />