Natural Attenuation of Polycyclic Aromatic Hydrocarbon-Contaminated Sites: Review

Natural attenuation is currently being applied as a remedial technology at many petroleum hydrocarbon and chlorinated compound contaminated sites. Although information on the use of natural attenuation at these sites is abundant, information on sites contaminated with polycyclic aromatic hydrocarbons (PAH) is limited. An assessment report by the National Research Council on natural attenuation indicates that the current understanding of the fate and transport of PAH compounds at contaminated sites is "moderate" and the likelihood of success in the application of natural attenuation at these sites is expected to be "low," given the current level of understanding. The purpose of this paper is to review documented work on natural attenuation of PAH-contaminated sites and summarize information to improve our level of understanding and address important issues for the implementation of natural attenuation at these sites. The main processes affecting the attenuation of PAH compounds are sorption and biodegradation. The relative contribution of each of the two attenuation processes is unclear. The few studies available tend to focus on the degradation of low molecular weight PAHs such as naphthalene, acenaphthylene, and phenanthrene. The estimated first-order decay rates of naphthalene, acenaphthylene, and phenanthrene from the various studies were 0.00057-0.0063 day−1, 0.00027 day−1, and 0.000027 to 0.063 day−1, respectively. Some of the issues that need further investigation include: (1) an understanding of the solubility and dissolution of PAH NAPLs; (2) the interactions and effects of the more soluble low molecular weight PAHs on the sparingly soluble high molecular weight PAHs; and (3) the utilization of electron acceptors other than oxygen during microbial degradation of PAHs under complex mixture conditions. Overall, the natural attenuation of low molecular weight PAHs appears to be promising for the sites investigated.