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' Report—Fist Phase Remedial System Page 3 Former Manna Pro Site ' Protect No 207 3 . June 18,2002 ' Corrective Action Plan was approved by the LOP agency and the anticipated costs were pre-approved for reimbursement by the Underground Storage Tank Clean-up Fund on June 19, 2001 1.2 Scope of Work ' The goal of the work described herein was to construct and operate a portion of the complete remedial system envisioned in the CAP This limited part of the system ' cnsisted of two sparge wells and one vapor extraction well The decision was made to complete only a limited part of the system in order to verify and refine the engineering details The full-scale system is costly, and a conservative approach was selected to ' minimize the engineering risk and avoid possible costly mistakes 1.2.1 Conceptual Basis ' In recent years, groundwater at the site has been recorded at historically high levels The j surface of groundwater has recently been as high as 25 feet bgs, but as late as 1994, the ' groundwater surface was as low as about 40 feet bgs This rise in groundwater elevation was coeval with six continuous years of greater than average rainfall in the northern San • Joaquin Valley The highest concentrations of gasoline are known to lie from about 35 to ' 45 bgs or about 10 to 25 below the surface of groundwater Due to the fact that the bulk of the contaminant plume lies 10 to 25 feet below the groundwater surface, we postulated that remedial measures based principally on vapor extraction would not be sufficient to fmaterially reduce the contaminant mass below groundwater in a reasonable period of time Air sparging to introduce oxygen into the subsurface and mobilize contaminants to ' the groundwater surface was posited as an attractive supplement to enhance the effectiveness of vapor extraction It was anticipated that injected air would encourage the growth of aerobic bacteria and would also mobilize light hydrocarbons and cause them to rise to the groundwater surface As these mobilized hydrocarbons moved through, and up, the water column, down-gradient vapor extraction wells could be used to extract them The pilot testing generally validated these concepts, and the work described below ' was intended to construct and operate a remedial system designed in accordance with this conceptual model and the first phase of the CAP ' 1.2.2 Scope of Work In order to complete the first phase of the remedial system described in the CAP, the following steps were completed • Install and operate a vapor extraction / air sparge system The system will consists of two sparge wells and one vapor extraction well Two of these wells were already in ' place, but it was necessary to drill one additional sparge well ' 2937 Veneman Ave , #B240 Geo-Phase Environmental Inc Phone(209)569-0293 Modesto, CA 95356 Fax(209)569-0295 I