The Biodegrability of Petroleum Hydrocarbons and Their Effects on the Environment
27/03/14
Word Count: 1413
Introduction
Many tons of petroleum hydrocarbons have been spilled and have contaminated the sediments of coastal and inland regions. The effects of these hydrocarbons in the soil can be devastating to the vegetation and the wildlife. The petroleum fuels do not decompose, and remain saturated in the soil for many years, and in many cases indefinitely, preserving traces of the fuel in the sedimentary rock record. Reddy CM, et al., (2002), found that the hydrocarbons in the No.2 fuel, dumped by the barge Florida in 1969, were still present in the marsh sediments of the bay thirty years after the initial contamination. Only the n- alkanes were completely degraded, contrary to previous studies that stated that other branched alkanes had degraded as well. Their studies concluded that at this site in Falmouth the hydrocarbons would persist indefinitely, and would be preserved in the rock record. When heavier petroleum hydrocarbons are spilled and absorbed into the soil they will not completely dissolve unless under certain conditions, and will have a long-term effect on the surrounding environment.

Biodegrability of Oil
Venosa AD, et al., (2010), researched the biodegrability of the 19 year lingering oil on the beaches of Prince William Sound from the Exxon Valdez oil spill in 1989. They found that although a substantial amount of weathering had occurred, high concentrations of hydrocarbons remained in the sediment, and could still pose a threat to organisms that encountered it. For the biodegradation of oil to occur there must be a competent population of oil degrading bacteria, and nutrients present. Unavailability of these factors may have been a reason why the oil on the beaches lingered for such a long period of time. A gas chromatogram of hydrocarbons extracted from the sediments at West Falmouth one year after the spill showed a loss of oil compounds due to evaporation, water washing, and the loss of n-alkanes due to microbial degradation (Reddy CM, et al., 2002). Venosa AD, et al., (2010), calculated that approximately 0.6-1.3 g N/kg sediment and 44-99 g O2/kg sediment would be required to completely biodegrade the oil. Seawater contains only 0.2 mg N/L and about 8-9 mg O2/L. Thus about 3 000- 11 000 L of water must come into contact with each kilogram of contaminated sediment in order to provide enough nutrients and oxygen for complete biodegradation of the oil to occur. Hydrocarbon biodegradation occurs at the hydrocarbon- water interface. Thus the surface area to volume ratio of the oil can impact the biodegradation rate (Venosa AD, et al., 2010). Many physical boundaries can affect this ratio, such as rock beds with very low permeability and porosity. An intertidal zone of course- grained gravel shores and armoring from boulders and cobbles can inhibit disturbance by waves, thus reducing the hydrocarbon- water interface (Peterson CH, et al., 2003). It is physically impossible for the oil to completely dissolve out of the soil before any long term problems occur. Not all fuels are this difficult to erode, lighter crudes contain simpler, lighter hydrocarbons than heavy crude and are therefore more easily biodegraded.

Effect of Lingering Hydrocarbons on Environment
Oil that is spilled into the ocean and washed on the shore can have a serious impact on the surrounding environment. The oil that is washed into rivers impacts populations of fish, and the oil that is washed ashore and absorbed into the soil affect vegetation and animals. Some oil spilled during the Exxon Valdez spill was trapped under mussel beds providing a route of entry into many food chains. The subsurface grains of stream banks harbored the oil, exposing and killing salmon embryos (Peterson CH, et al., 2003). Chronic exposure to the oil persisting in the sediments were evident in fish, sea otters, and seaducks, all who rely on the sediments for