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N . •./ 2.2.6 Advective Groundwater Velocity The advective velocity ofgroundwater in the direction parallel to groundwater flow is given by the following relationship: -K v = vh 77 where: v =average advective groundwater velocity(seepage velocity) ` K=hydraulic conductivity Vh = groundwater gradient q=effective porosity Using this relationship in conjunction with site-specific data presented above,the average advective groundwater velocity at the site is 0.055 ft/day or approximately 20.1 feet per year. ZZ7Inorganic Groundwater Chemistry and Geochemical Indicators of Biodegradation In September 1997,a variety of water quality parameters were analyzed from groundwater samples collected from wells MW-2A,MW-7,and MW-9 due to their position relative to the source area. These parameters are important for evaluating water quality conditions and as geochemical indicators of biodegradation offuel hydrocarbons. For the biodegradation of fuel hydrocarbons and additives(benzene, toluene,ethylbenzene,and total xylenes[BTEX])to occur through the action ofindigenous microbes,an adequate supply ofcarbon,electron acceptors,essential nutrients,and proper pH,temperature,and redox potential must be available. Naturally occurring organic material and petroleum hydrocarbons supplymost of the carbon. Organic material also tends to enhance sorption and decrease the mobility of BTEX compounds. Dissolved oxygen,nitrate,ferrous iron,sulfate,and carbon dioxide are the more important sources of electron acceptors. Fuel degrading microbes thrive under a wide range oftemperatures and pH ` and are ubiquitous in the environment. Because the rate of natural biodegradation is generally limited by a lack of electron acceptors,oxygen can be introduced into the contaminant plume to increase the rate of biodegradation. _ 2.2.7.1 Dissolved Oxygen Oxygen is required for aerobic biodegradation ofhydrocarbons. During aerobic biodegradation,dissolved oxygen concentrations decrease. Anaerobic bacteria generally cannot function at dissolved oxygen ` concentrations greater than approximately 0.5 milligrams per liter(mg/L). Measured dissolved oxygen concentrations were generally low and ranged from 0.96 to 1.12 mg/L. The low oxygen concentrations are a strong indication that aerobic biodegradation ofBTEX compounds has occurred at the site,and that anaerobic bacteria are not active. _ 5 _ _