Anaerobic decomposition of oil in bottom sediments.

Abstract

The American Petroleum Institute has estimated the national pollution potential from oil to be approximately 450 mil gal (1,700 mil 1) annually.1 A sig nificant portion of oil pollutants discharged into aquatic environments is incorporated into bottom sediments of receiving waters. The decomposition of oils and other or ganics in these sediments occurs both aerobically and anaerobically in a process referred to as benthal decomposition. If the water overlying the oil-bearing sediments contains dissolved oxygen (do), aerobic conditions will prevail at the sur face of the sediments. Oxygen will pene trate into the sediment only as deeply as the balance between oxygen diffusion and oxygen consumption will permit. Usually, downward diffusion of oxygen is not vigor ous enough to maintain aerobic conditions below the superficial layers.2 Thus, in all but the uppermost mud strata, the available supply of do is ex hausted quickly and anaerobic conditions result. The anaerobic use of oils in sediments was previously thought to be so slow as to be negligible or not to occur at all. The reasoning was that, if this were not true, the reservoirs of crude oil underground would not have been found because the oil would have been attacked and transformed by microorganisms. Petroleum microbiolo gists today, however, state that, in a closed system such as that which existed during the formation of petroleum deposits, an aerobic bacteria probably attacked hydro carbons at the oil-water interface. How ever, the extent of their activity is prob ably limited by toxic metabolic products and competition for essential nutrients. As a result, the bacteria reach a condition of dynamic equilibrium in which multiplica tion is limited by the rate of diffusion of toxic products away from, and that of essential nutrients into, the microenviron ment.2 It is now widely accepted that oil is decomposed by microbial action under anaerobic conditions throughout the en vironment at the expense of oxygen in ni trates and sulfates, leading to the formation of nitrites, free nitrogen, and hydrogen sulfide.3~n Tauson,6 on the basis of a ther mochemical relationship between the pro cess of reduction of sulfates and oxidation