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15 October 2002 AGE-NC Project No 96-0232 Page 18 of 19 as 30 feet bsg, and approximately 80 feet to the east at depths as great as 30 feet bsg (Figures 8 and ' 11) The northern extent of the Flag City plume is uncertain at the present time because of the high concentrations of MTBE (up to 700 µg/l) detected in northernmost Flag City Chevron well MW-8 ' during the March, June and September 2002 monitoring events AGE estimates the arithmetic average concentration of MTBE in ground water extracted between ' 13 June and 05 September 2002 was 12 µg/l During this time, the volume of water processed through the ground water pump and treat system was recorded at 1,485,238 gallons (equivalent to 12,382,429 pounds) of water extracted yields 0 19 pounds of MTBE (equivalent to 86 grams) ' removed from the plume of impacted ground water (Appendix D), or approximately one gram per day ' Ground water elevations versus ground water monitoring dates (hydrograph) were plotted for monitoring wells MW-2, MW-6A and MW-6B, generally, all three plots depict an overall trend of decreasing ground water elevation from March 2002 to September 2002 (Appendix I) ' During September 2002,hexavalent chromium was detected by EPA Method 218 6 from the pump- and-treat effluent at a concentration of 0 30 µg11, a confirmation sample collected on 25 September 2002 was non-detect at a concentration of less than 0 5 gg/l,utilizing EPA Method 7199 It is AGE's understanding that hexavalent chromium is regulated under the maximum contaminant level(MCL) for total chromium Total chromium is considered primarily trivalent chromium (chromium-3) and hexavalent chromium Because chromium-3 is of low toxicity, the total chromium MCL is considered by many regulators to be the chromium-6 standard For Californa, the MCL for total ' chromium is 50 µg/l,the Federal MCL is 100 gg/l(California Dept of Health Services) However, as a precautionary measure, the ground water pump-and-treat system will be suspended until the RWQCB addresses the above referenced detection 8.4 RECOMMENDATIONS 1 Based upon data collected from the site, AGE recommends • Continued quarterly ground water monitoring and sampling of monitoring wells MW-1 through MW-13 • Continued coordinated quarterly ground water monitoring and sampling events with Three B's Truck Plaza • Continued ground water extraction from wells EW-4 and EW-5, extraction wells EW-7, EW-8, EW-9 and EW-10 should be alternated into the ground water extraction system • Due to the possible one time occurrence of hexavalent chromium,the constituent should be included with the monthly pump-and-treat effluent monitoring and sampling Advanced GeoEnvironmentai,Inc