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Gosure Report 23 Golden Grain Manteca Bean Facility,299 Moffat Boulevard,Manteca,California July 16,19% 5.2.4 Biodegradation ' Aerobic biodegradation Is the process of hydrocarbon removal by microorganisms in-the presence of oxygen Aerobic biodegradation can degrade benzene if dissolved oxygen is present In sufficient concentrations, usually greater than 1 to 2 mg/L(Chiang et al 1989) Dissolved oxygen concentrations in groundwater downgradient of the plume at this site range from 3 to 4 mg/L Data suggests that natural attenuation from aerobic biodegradation can occur at rates of 0 3 to 1 3 percent per day for BTEX compounds if an active microbial population is present and acclimated to the subsurface contaminant It can be assumed that intrinsic aerobic biodegradation is occurring at the site based upon evidence of reduced dissolved oxygen concentrations within the plume Dissolved oxygen concentrations in groundwater within the plume range from 1 to 2 mg/L A difference in pH between contaminated and uncontaminated groundwater is another indicator of biological activity ' (Buscheck and O'Reilly 1995) The pH of groundwater samples from within the plume is consistently lower than pH from samples downgradient of the plume Furthermore high concentrations of heavy metals and salts which may inhibit biodegradation are not present The average total dissolved ' solids concentration of groundwater at the site is 300 mg/L The BIOPLUME model can simulate the source of oxygen Into an aquifer in three ways the Initial dissolved oxygen in the uncontaminated aquifer, natural recharge of oxygen across the model y boundaries, and vertical exchange of oxygen from the vadose zone (reaeration) For the model ' simulations it was assumed there was no recharge across the model boundaries This is a conservative assumption because even though the groundwater seepage velocity is quite small, over ' time the groundwater beneath the site will be recharged by inflow from upgradient, oxygenated groundwater For the model simulations it was assumed there was no reaeration This is a conservative assumption because the seasonal fluctuation of the shallow water table beneath the site suggests that recharge from the surface does occur For the model simulations it was assumed there was a constant dissolved contaminant source of 780,ug/L benzene This is a conservative assumption because although there may be residual contamination in the vadose zone beneath the site, this source would also be diminishing over time ' due to biodegradation BIOPLUME is a saturated flow and transport model and cannot simulate conditions in the vadose zone Model simulations were run to evaluate the effect of a constant ' source versus no source Recent research has also indicated that biodegradation can occur under anaerobic conditions (Kuhn ' et al 1985 and McCarty 1994) Groundwater samples were collected at the site and analyzed for dissolved inorganic constituents such as nitrate, sulfate, and alkalinity, which can be used as ' 42020213 GR FLUOR DANIEL GTI �5-