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t 6 Closure Report Golden Grain Manteca Bean Facility,299 Moffat Boulevard,Manteca,California July 16.1M 1 In summary, the estimated hydrocarbon mass In vadose zone sod (includes adsorbed phase and ' vapor phase) was approximately 3,000 lbs ' 2 2.2 Extent of Hydrocarbon Impacted Groundwater Results of groundwater samples collected during the Initial site assessment activities Indicated the presence of free-phase petroleum hydrocarbons and an associated dissolved-phase groundwater ' plume The distribution of total petroleum hydrocarbons as gasoline (TPH-g) and benzene detected in groundwater in June 1988 is illustrated in figures 10 and 11, respectively Free-phase hydrocarbons were measured in monitoring wells MW-1 through MW-6 on June 21, 1988, with a maximum thickness of 2 97 feet measured in monitoring well MW-2,which is adjacent to the former underground tank location Removal of the free-phase product from wells MW-1, MW-2, MW-4, and MW-5 began in June 1989 and continued until prior to implementation of the groundwater remediation system The removal of the free-phase product at the site was accomplished through a manual well bailing program, and through use of pilot test equipment used during the remedlation feasibility tests conducted at the site The manual bailing program was continued until free phase was no longer present at the site The manual bailing of free-phase product began in June 1989 and continued to April 1991 At that time, no measurable amounts of free-phase existed, however, a ' sheen was noted intermittently in some site monitoring wells Free-phase product was removed and placed in an aboveground storage tank at the site prior to proper disposal ' Figures 12 and 13 illustrate the distribution of TPH-g and benzene detected in groundwater as of June 1990 These figures illustrate the baseline conditions of the dissolved groundwater plume prior to implementation of site remediation work Dissolved-phase mass in groundwater at that time was estimated at 35 pounds At that time, the lateral extent of dissolved hydrocarbons in groundwater was concentne around the location of the former underground gasoline tank, with a radius of approximately 100 feet The highest dissolved petroleum hydrocarbon concentrations were detected in the on-site wells, with an average decrease in concentrations of approximately 730 parts per billion (ppb) per foot away from the former underground tank location The hydraulic gradient pnor to groundwater extraction was relatively low(0 001 ft/ft),which contributed to the low mobility of the ' groundwater plume The vertical hydrocarbon concentration gradient was determined from groundwater samples ' collected in June 1990 Monitoring well MW-2, screened in the upper sand zone, contained 73,000 ppb TPH-g and 28,000 ppb benzene Monitoring well MW-7, adjacent to MW-2, and screened in the FLUOR DANIEL GTI I