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Ci Report 7 Thea Southland Corporation,9410 Thornton Road Stockton, CA December 5, 1995 petroleum hydrocarbons, and lack of significant adsorbed petroleum hydrocarbons, it can be concluded that all LPH has been removed from the subsurface 4.2 Historical and Current Distribution in Soil Historical distribution of petroleum hydrocarbons is illustrated on Figures 14-16 and summarized in Table 5 The initial amount of adsorbed petroleum hydrocarbons is unknown The sod vapor extraction system which operated from February 1987 to January 1991 removed an equivalent of 576 gallons of gasoline An unknown percentage of the original amount has undergone natural biodegradation Of the 48 confirmation samples submitted for analysis from the 16 soil confirmation bonngs, BTEX or TPH-G was detected in seven samples (fable 6) Benzene was detected in only one sample (boring K at 40 feet) ata concentration of 0 010 mglkg TPH-G was detected in only samples from boring P (4 samples),with a maximum detected concentration of 2,600 mg/kg at a depth of 20 feet BGS or approximately 20 feet above the current water table Residual distribution of adsorbed petroleum hydrocarbons is illustrated on Figures 17-19 Laboratory reports of soil analysis from the latest round of soil confirmation sampling is provided in Attachment 5 Mass calculations estimating total residual petroleum hydrocarbons in soil, based on data presented in • Tables 5 and 6 and Figures 17-19 are provided in Attachment 6 These calculations reveal that approximately 10 gallons of gasoline remain in soil Assuming that the amount of gasoline removed through soil vapor extraction plus the amount remaining is equal to the initial amount in soil, at least 98 percent of petroleum hydrocarbons in soil have been removed (Table 4) 43 Historical and Current Distribution in Groundwater Historical distribution of dissolved petroleum hydrocarbons is illustrated on Figure 13 Vertical definition of petroleum hydrocarbons is provided by the MW-17 well cluster Historical and current groundwater analytical data is provided in Table 7 Calculations estimating the initial dissolved hydrocarbon volume based on Figure 13 are provided in Attachment 6 These calculations indicate that approximately 20 gallons of dissolved petroleum hydrocarbons were initially present The groundwater extraction system which operated between May 1986 and August 1990 removed and treated approximately 1 6 million gallons of groundwater No dissolved petroleum hydrocarbons were detected in domestic wells during sampling in July/September 1995 Current groundwater analytical data from domestic well sampling are provided in Table 8 The residual distribution of dissolved petroleum hydrocarbons based on the September 1995 quarterly monitoring and sampling event is illustrated on Figure 20 Calculations estimating residual dissolved hydrocarbon volume based on Figure 20 are provided in Attachment 6 These calculations indicate Closure2 rpt(STFIL.B 1) =1141GUOUNIAVATFR �:]! = TECHNOLOGY