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i I Closure Report 19 Golden Grain Manteca BeanFacili!X,299 Moffat Boulevard,Manteca,Caidomia duty 16,19% Technical Basis Baseline Site Current Site Conditions Conditions I l TPH-g, Benzene, and Lead concentrations 3,200 ppb-TPH-g <50 ppb-TPH-g in the groundwater system influent 1,100 ppb-benzene 2 9 ppb-benzene 3 6 ppm-total lead <0 04-total Lead < 010-organic lead <0 2-organic lead ' TPH-g concentrations in the SVES influent 3,500 ppmv ND-TPH-g TPH-g and Benzene concentrations in MW-2 MW-2 monitoring wells MW-2,MW-5,and MW-7 200,000 ppb-TPH-g 600 ppb-TPH-g 2/20196 ' (Concentrations were nondetected in 18,000-ppb-benzene 11 -ppb-benzene MW-2 on 5x21196,therefore 2120196 concentrations are shown) Concentration MW-5 MW-5 ' versus time graphs are included in 940,000 ppb-TPH-g 1,500 ppb-TPH-g appendix F 4,900 ppb-benzene 14 ppb-benzene MW-7 MW-7 20,000 ppb-TPH-g 6,800 ppb-TPH-g 6,900-benzene 780-benzene The technical basis for requesting site closure as illustrated by the changes in subsurface conditions is ' summanzed as follows ' ■ Free-phase hydrocarbons are no longer present ■ Hydrocarbon mass in sal has been reduced to negligible levels ■ Hydrocarbon mass and detectable concentrations in groundwater have decreased by several orders of magnitude and do not extend off-site ' so HYDROGEOLOGIC MODELING ' 51 Modeling Approach ' The objective of the computer modeling was to evaluate the fate and transport of the existing benzene plume in groundwater beneath the site without further remedial action and to simulate the ' effect of natural attenuation on the plume The fate and transport model, BIOPLUME II (Rifai et al 1987), was used to simulate the natural attenuation of the plume BIOPLUME II, developed by Rice 4020213 CR FLUOR DANIEL 611