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%. ✓ Microorganisms obtain energy for reproduction and cell maintenance by expediting the transfer ofelectrons from electron donors to electron acceptors. This results in the oxidation of the electron donor and the reduction of the electron acceptor. Electron donors at the site are natural organic carbon and fuel hydrocarbons. Fuel hydrocarbons are completely degraded if they are utilized as the primary electron donor for microbial metabolism. Electron acceptors are typically elements or compounds that occur in relatively higher oxidation states and include oxygen,nitrate,ferrous iron,sulfate,and carbon dioxide. Microorganisms preferentially utilize electron acceptors while metabolizing fuel hydrocarbons. Dissolved oxygen is utilized first as the primary electron acceptor. After the dissolved oxygen is consumed,anaerobic ►- microorganisms use electron acceptors in the following order ofpreference:nitrate,ferrous iron,sulfate, and finally, dissolved carbon dioxide. Depending on the type of electron acceptors present,pH conditions, and redox potential,anaerobic biodegradation can occurby denitrification,ferrous iron reduction,sulfate reduction,or methanogenesis. Other,less common anaerobic degradation mechanisms,such as manganese reduction,may dominate if the physical and chemical conditions in the subsurface favor use of these electron acceptors. 4.2.3.2 Evaluation Evidence supporting the effectiveness ofintrinsic remediation off iel hydrocarbons at the site is supported by trends in benzene and TPH-g concentrations in groundwater from source wells MW-2 and MW-2A (Figure 4-1). There has been limited sampling in monitoring well MW-2 in the past due to problems with water levels in the well during drought conditions;however,available data show a 23 percent decrease in benzene and a 25 percent decrease in TPH-g from December 1988 to December 2005. Monitoring well MW-2A shows a 98 percent decrease in benzene and a 95 percent decrease in TPH-g from July 1995 to December 2005. In Figure 4-1,die curve representing MW-2A follows a typical logarithmic function, with a significant drop in contaminants over a short time period followed by a steady decrease and leveling off. The curve representing MW-2 does not show an initial decrease quite as significant as in MW-2A; however,this well shows a sharp rebound before beginning its steady,downward trend. It is also apparent in both wells that benzene and TPH-g levels have leveled offsince August 2000. Minor fluctuations are still occurring,but a dramatic downward trend in recent years is not apparent. Additional information supporting the effectiveness ofintrinsic remediation offuel hydrocarbons at the site is that all other monitoring wells consistently show non-detect or low levels of BTEX compounds and TPH-g(Table 3-2). Intrinsic remediation is not appropriate where there are imminent risks to human health and the environment; however,this is not the case at the site. The site is located in an urban environment where the existing contamination in the groundwater is under city streets,sidewalks,or buildings. Underground access is ` restricted by permit applications through the City of Stockton and San Joaquin County. The nearest surface water is the Mormon Slough,which is located approximately 0.5 mile south(crossgradient)ofthe ` site. There is no current groundwater usage within approximately 1 mile downgradient ofthe site,and 12