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Evaluation of Natural Attenuation: 7500 West Eleventh Street,Tracy, C4. Page 33 oxidative reactions yield electrons which, through a series of enzyme-catalyzed electron transport steps, produce the energy the biota need for maintenance and growth. For the P p � p gY electrons to pass through the energy-generating steps, an electron sink is required- i < typically this electron sink (or receptor) is molecular oxygen (02). That process is called aerobic respiration. However, biodegradation of components of fuel hydrocarbons can also occur in oxygen-starved environments where nitrate (NO3'), manganese (Mn+4), ferric iron (Fe+3), sulfate (SO4'2), or carbon dioxide (CO2) may serve, if available, as electron acceptors. The latter processes are known as anaerobic respiration. r The biogenetic processes, either aerobic or anaerobic, that act on the components of fuel hydrocarbons change the geochemistry of the groundwater when they are active so that measurements of the appropriate geochemical parameters can be used to indicate which type, if any, of those processes are active at a site. Following is a discussion of the direct and indirect physical and geochemical indicators measured in the groundwater at the Navarra Site that evidence active bio-remediation of groundwater. 7.2.1 Evidence for Aerobic Biodegradation The effect of biological transformation of components of fuel hydrocarbons into innocuous carbon dioxide and water causes the oxygen content of groundwater within a contaminant plume to be reduced relative to the ambient concentrations of oxygen in unaffected groundwater. The same processes also affect the oxygen reduction potential (redox potential) and that parameter can be monitored to distinguish between areas within the plume that are under reducing as opposed to oxidizing conditions. 7.2.1.1 Dissolved Oxygen in Groundwater r.. Figures 31-33 show plots of the distribution of BTEX concentration and dissolved oxygen concentration along the length of sections A-A', B-B' and C-C',respectively (see _ Figure 3 for section line locations). In each case, the plots show that there is an inverse k;: relationship between BTEX concentrations and dissolved oxygen concentrations in the groundwater at the Navarra Site. In Figure 31, it can be seen that dissolved oxygen is depleted in the area around the source of the fuel discharge and down-gradient from there to a point a short distance beyond Monitoring Well MW-7, the location of which is also shown on Figure 3. Beyond that point, the dissolved oxygen in the groundwater increases rapidly. It rises to 11.6% at Monitoring Well MW-19, which is beyond the northern limit of the plume. Of particular interest is the sharp depression in the oxygen content of the groundwater in the area of Monitoring Well MW-7, where the concentrations of components of fuel hydrocarbons are higher than at any other area within the plume. The plots of BTEX and dissolved oxygen concentrations along section B-B' shown on Figure 32 illustrate a classic inverse relationship between BTEX and dissolved oxygen concentrations. At the western periphery of the plume at Monitoring Well MW-2, dissolved oxygen in the groundwater is much greater than it is in the center of the plume i sic kr.-,