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levels substantially higher than the top of the screen interval result in unreliable analytical data This is especially true at the subject site where groundwater recharge is strong and groundwater drawdown during purging is minimal On Novemebr 13 and 14, 1997, the five original monitoring wells and the two deep vapor extraction wells were sampled To confirm the presence and concentration of MTBE and determine if any other oxygenate or volatile organic compound was present in groundwater, each of the samples was analyzed by EPA Method 8260 Groundwater in the deep vapor extraction well VW-2D contained 1,700 µg/l of MTBE and 190 µg11 of 1,2-Dichloroethane MTBE was also detected in MW-3 (1,400 µg/l), MW-2 (230 µg11) and VW-1 D (60 µg/1) The volatile organic compound 1,2-Dichioroethane was also detected in MW-3 (230 µg/Q, MW-2 (120 µg/I) and VW-1 D (14 µg/!) No other alcohol-or ether-based oxygenate compound was detected Historically, gasoline hydrocarbons have been detected in MW-1, MW-2, MW-3, VW-1D and ' VW-2D During one sampling event on November 13, 1997, groundwater from MW-5 contained minor detectable concentrations of BTEX Groundwater samples from monitoring well MW-4 have never contained detectable concentrations of TPH-as-Gasoline, BTEX or MTBE The historical analytical results are summarized in Table 4 MOn May 1, 1998, groundwater was sampled from the newly-installed wells (MW-6 and MW-7) and from VW-1 D and VW-2D The analytical results are summarized in Table 4 and the laboratory reports are included in Appendix E Grou !ed from VW-2D contained 2,200 µg11 of TPH-as-Gasoline, 300 µg11 of benzene and 600`µg/l o `T7E Groundwater sampled from VW-1 D contained 90 µg/I of TPH-as-Gasoline, 7 µg/I of TBE and no detectable concentrations of BTEX Groundwater sampled from MW-6 anmidlW--7/did not contain detectable concentrations of TPH-as-Gasoline, BTEX or MTBE The distribution of gasoline hydrocarbons detected in groundwater is illustrated in Figure 8 3.3 Remediation Feasibility Test Results The feasibility of using soil vapor extraction as a remediation alternative was evaluated Only soil vapor extraction was evaluated, since the vapor extraction well network had already been installed and pilot tested and since other soil remediation options (e g , excavation)would be obviously less cost-effective The feasibility of remediating groundwater directly was not evaluated The treatment of impacted soil by soil vapor extraction will promote the greatest reduction in hydrocarbon mass and indirectly improve groundwater quality • 11