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i Closure Report 24 Golden Gram Manteca Bean Facility,299 Moffat Boulevard,Manteca,California July 16, 1996 Indicators of anaerobic biodegradation A cursory evaluation of nitrate, sulfate, and alkalinity concentrations in groundwater upgradient, downgradient, and within the plume suggest anaerobic biodegradation maybe taking place Table 10 presents analytical results for inorganic constituents in groundwater samples collected at the site The lower nitrate and sulfate concentrations for ' groundwater within the plume (see MW4 results) relative to downgradlent concentrations suggest anaerobic biodegradation is occumng, as does the Increase in alkalinity concentrations within the plume The BIOPLUME model has the capability of simulating anaerobic biodegradation For the model simulation, However, It was assumed there was no anaerobic biodegradation which is therefore a conservative assumption 1 53 Results of Wydrogeological Model A number of BIOPLUME model scenanos were run Model output is included in appendix G All scenarios assumed advection, dispersion, sorption, active aerobic biodegradation, and no groundwater extraction Results of the modeling simulations indicate that under the scenano of initial dissolved oxygen concentrations, no reaeration, no upgradient recharge, and a constant 780,tcg/L source of benzene,the plume would maintain its current size and concentrations through about the fifth year(figure 23) By the tenth year the plume will have degraded After ten years, the ' model shows the plume would again begin to expand (due to the constant source term) but never reaches the present magnitude of extent of the plume A discussion before using a constant source term for long penods is unrealistic because biodegradation Is also occumng in the vadose zone If ' the constant source is maintained but reaeration is simulated, the plume is completely degraded between five and ten years If the benzene source term is eliminated, the plume shnnks and is completely degraded within two year and does not reappear The overall effect of a low groundwater velocity and high sorption suggests the benzene plume ' beneath the site is only moving about 33 ft/yr Measurement of dissolved oxygen, nitrate, sulfate, and alkalinity concentrations in groundwater samples collected at the site suggests intnnsic ' biodegradation Is occumng, both aerobically and anaerobically The intrinsic biodegradation results In degradation of the dissolved-phased hydrocarbons and shnnkage of the plume over time Model simulations from the BIOPLUME model clearly show the combined effects of advection, dispersion, 1 sorption, and biodegradation result In the natural attentuation of the plume ' 42020213 CR I ' FLUOR PAN19L G1r1