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r <br /> - k •-ik ;C <br /> 1 <br />' 17 September 2003 <br /> + AGE-NC Project No 98-0448 <br />' Page 8 of 9 <br />' Laboratory results of the water sample collected from well MW-4B (screened 45 to 50 feet <br /> bsg)suggest that only residual TPH-gBTEX constituents and residual MTBE remain in the <br /> dissolved petroleum hydrocarbon plume at deeper depth, an over-all trend of decreasing <br />' concentrations from MW-4B is evident for nine consecutive quarters u <br /> • Between the March 2003 and the June 2003 ground water monitoring events, a significant <br /> rebound was observed in the dissolved TPH-gBTEX concentrations at well MW-2, this <br />' rebound effect may be related to alternating IAS activities on 20 March 2003 from IAS well <br /> MW-4B (screened 45 to 50 feet)to IAS wells AW-1 through AW-4(screened 25 to 30 feet) <br />' Conversely,dissolved TPH-gBTEX concentrations continued to decline significantly in the <br /> ground water sample collected from MW-4A, the decreasing concentrations may be <br /> attributed to air sparging activities conducted at AW-1 through AW-4 Additionally, <br /> Idowngradient well MW-6 has not detected any petroleum hydrocarbon constituents of <br /> concern for four successive quarters <br /> Based on SVE/1AS remediation data collected between 20 March 2003 and 23 June 2003, AGE <br /> concludes <br /> • •' The volume of hydrocarbons extracted by the SVE system was calculated to be <br /> approximately 25 42 pounds, or equivalent to an approximate volume of 4 06 gallons of <br /> gasoline extracted (Appendix F) <br /> The average SVE inlet flow rate was estimated to be 34 cubic feet per minute (cfm) at an <br /> average differential pressure of 0 38 inches of water(Table 6) <br /> • Since initiation of the SVE system(April 2001),the total volume ofpetroleum hydrocarbons <br /> extracted is estimated to be approximately 235 gallons (Table 7) <br /> r <br />' 6.0. RECOMMENDATIONS <br /> Based upon the results of this investigation, AGE recommends <br /> I • Continued quarterly ground water monitoring and sampling of wells MW-1, MW-2, <br /> MW-4A, MW-4B and MW-6, ground water sampling results will be used to monitor <br /> I progress of the SVE and IAS remediation system ' <br /> • Continued operation of the on-site SVE and IAS remediation system, however, to optimize <br /> the effective radius of influence in the `core' area of the plume, AGE recommends only <br /> Advanced GeoEnvironmental,Inc <br />