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4 <br /> GERAGHTY & MILLER, INC <br /> boundary of the hydrocarbon plume surrounding the ARCO service station located across <br /> Benjamin Holt Drive from the site The farthest downgradient well, MW-8, had a TPH-as- <br /> ' gasoline concentration of 90 gg/L in May 1997, no BTEX constituents were reported above <br /> the laboratory method-detection hmit. <br /> ' MTBE has been detected in all monitoring wells except Monitoring Well MW-7. The cause <br /> of an increase in the concentration of MTBE in Monitoring Well MW-4 from 16 gg/L in <br /> ' Vnt�n <br /> 50I)ry February 1997 to 3,600 gg/L during May 1997 has not been determined The concentration of <br /> °` MTBE in Monitoring Well MW-9 has vaned from 1,200 gg/L to 3,500 gg/L during the most <br /> ' M ref <br /> cr,&rgae recent year of sampling The source of this MTBE is unknown, but may be attributable to <br /> commingling with the ARCO plume Continued groundwater monitoring will determine <br /> ' whether additional lateral delineation will be required in the future <br /> MASS OF HYDROCARBONS IN SUBSURFACE <br /> ' <br /> Geraghty &Miller has made a gross estimate of the mass of petroleum hydrocarbons in soil <br /> g ty g <br /> ' and groundwater in the subsurface. Numerous assumptions were made to estimate the total <br /> mass in the subsurface at slightly over 6,000 pounds A summary of assumptions and calcula- <br /> tions is included in Attachment 5 10' a ctv t� — <br /> IFIEASIBI LITY ANALYSIS <br /> Geraghty & Miller has reviewed the feasibility and cost-effectiveness of remediation <br /> methods that reduce concentrations of petroleum hydrocarbons in the subsurface An accept- <br /> able solution must address the impacts of petroleum hydrocarbons in the vadose zone, capillary <br /> 1 fringe, and saturated zone A previous analysis of the feasibility of implementing various <br /> remedial technologies was provided by Groundwater Technology, Inc (GTI) in a report dated <br /> March 16, 1994 <br /> ANALYSIS OF SVE SYSTEM <br /> The installed SVE system was started on September 10, 1997 Vacuum-influence tests <br /> were conducted on September 26, 1997, to determine the effectiveness of this system These <br /> ' tests indicate that the installed vapor-extraction wells have an effective capture zone extending <br /> beyond the installed well network A summary of these vacuum-influence tests is presented in <br /> lie Attachment 6 Low hydrocarbon concentrations in the influent of the SVE system and in soil <br /> Project No RC0309 010 <br /> 1 $� <br />