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URS • Mr Mike Infuma Page 3 San Joaquin County Environmental Health Department June 6, 2003 In November 1993,two additional off-site monitoring wells(MW-8 and MW-10)were installed MW-8 was subsequently abandoned in 1996 An SVE system that utilized 12 SVE wells began operating in August 1995 and was operated continuously through November 1997 EMCON performed monitoring,operation,and maintenance of the system During the operation of the system,EMCON reported that extraction rates for TPHg and benzene decreased by 78% and 99 9%,respectively As of June 1998,EMCON reported that the system was inoperable Remediation of groundwater continued by injecting ambient air at low flow rates into selected wells to enhance biodegradation Rising groundwater levels have caused the screens of the site vapor extraction wells to become partially or fully submerged As of September 2000, EMCON concluded that insufficient screen is available to effectively operate the SVE system Groundwater elevations remain elevated and ARCO does not intend to use the SVE system in the future unless site conditions change HYDROCARBON DISTRIBUTION Hydrocarbons in the Unsaturated Zone Hydrocarbons in the unsaturated zone appear to be confined primarily to the former UST area Soil samples collected at 16 feet bgs during UST removal activities in August 1993 contained up to 4,000 mg/kg TPHg and up to 2 7 mg/kg benzene Soil samples collected beneath the piping in August 1993 contained maximum concentrations of 1,600 mglkg for TPHg and 0 089 mg/kg for benzene Soil samples collected during drilling activities conducted between 1991 and 1993 indicated that hydrocarbons in soil were primarily confined to the vicinity of the UST complex Figures 4 and 5 show the lateral extent of impacted soil prior to SVE at depths of 15 to 40 feet bgs,and 40 to 60 feet bgs,respectively These figures show that the lateral extent of soil impacts increases with depth, particularly in the area beneath the pump islands However,because the water table has risen to approximately 40 feet bgs,the soil impacts shown in Figure 5 are now below the water table Influent analytical data for the SVE system, which operated from August 1995 through November 1997, indicated that extraction rates for TPHg and benzene decreased by 78%and 99 9%,respectively during the period of operation These data indicate that hydrocarbon concentrations,and the lateral extent of impacted soil were reduced by the SVE system Tables summarizing historical soil analytical data,prior to operation of the SVE system, are included as Appendix D Irk P(.VC4 f-�,u, �rq r�J sL.,U noJ� Hydrocarbons in Groundwater TPHg concentrations reported in groundwater during the December 2002 monitoring event are shown on Figure 6 TPHg impacted groundwater is generally confined to the vicinity of the dispenser islands and the • K 1WpmcecA5 BY ARC01021301Sne Conceptual Model Report12130 Site Conceptual Model doe