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1 <br /> t <br /> 05 September 2002 <br />' AGE-NC Project No. 95-0100 <br /> Page 8 of 9 <br />' During the last sampling event,MTBE was detected in MW-1 and MW-3 by EPA method 8260B at <br /> 0.73 dug/1)and 41 pg/l, respectively. No other target analytes were detected in the samples analyzed <br /> (see Appendix C). <br />' MTBE concentrations have generally been declining since the February 2001 monitoring event. <br /> During the sampling event on 01 November 2000, MTBE was detected at a maximum concentration <br />' of 1,500 p.g/I in the sample from well MWA, at the former UST area. MTBE was detected at a <br /> maximum concentration of 350 gg/l in the sample from well MW-3, down-gradient of the former <br /> UST area. <br /> 1 The down-gradient extent of MTBE-impacted ground water has been defined. <br /> No target analytes were detected in the domestic samples analyzed, except for the low' detection of benzene in domestic well DW-1 during the final sampling event. The final sample from well DW-1 <br /> was obtained by purging the well with a down-hole pump. This benzene detection may have <br /> originated from the down-hole pump. <br /> ' AnalyticaI results of ground water samples are summarized in Table 6. <br /> 4.0. CONCLUSIONS and RECOMMENDATIONS <br /> Based on the findings of this and previous investigations, the following conclusions and <br /> ' recommendations are presented for consideration. <br /> • Field observations, analytical results of the final vapor samples collected from the vapor <br /> extraction system and the results of the confirmatory borings suggest that the vapor extraction <br /> system was successful in reducing the concentrations ofhydrocarbons in the vadose zone. The <br /> highest concentrations of petroleum hydrocarbons in soil remain at depths of 10 feet and 15 <br /> feet bsg below the former fuel dispenser area, however, the reduction of hydrocarbon <br /> ' concentrations ranged between 46 % and 77 % at a depth of 15 feet bsg and approximately <br /> 60 % in the soil above 10 feet bsg. Benzene was not detected in the samples collected near <br /> former sampling points of benzene detection. The reduction of benzene concentrations at a <br /> ' depth of 10 feet to 15 feet was virtually 100%, while the estimated reduction of benzene at <br /> a depth of 25 feet to 30 feet bsg is approximately 95 %. MTBE was detected at 22 mg/kg in <br /> ' <br /> a soil sample at a depth of 15 feet bsg, and at lower concentrations (0.65 mg/kg and 0.097 mg/kg) in soil above or near the ground water table. <br /> • The highest concentrations of hydrocarbons are located within the upper 20-feet of the soil <br /> zone. Based on the ground water depth, approximately 30-feet of soil exists between the <br /> round water table and the highest concentrations of hydrocarbons in.soil. <br /> g <br /> Due to the detection of MTBE in a grab ground water sample from a confirmatory boring, <br /> six monitoring wells were eventually installed at the site. The maximum concentrations of <br /> ' MTBE detected have been in monitoring wells MW-1 and MW-3 only, at concentrations of <br /> 1,500 µg/1 in samples MW-1 in November 2000 and 350µg/l in sample MW-3 in May 2000. <br /> . The concentrations of MTBE have generally declined since the February 2001 monitoring <br /> event. The concentration of MTBE in MW-1 and MW-3 during the last monitoring period <br /> Advanced GeoEnvirowmental,Inc. <br /> 1 <br />