Biodegradation of polycyclic aromatic hydrocarbons in soils containing weathered crude oil

A laboratory treatability study was conducted to measure the biodegradation of polycyclic aromatic hydrocarbons (PAHs) in soil containing weathered crude oil residues from three abandoned sites. Total petroleum hydrocarbon (TPH) levels before treatment ranged from 3 to 16%; after treatment, residual TPHs of 1.2 to 2.5% remained. All PAH data were normalized to hopanes as an internal standard. Losses of dibenzothiophenes, phenenthrenes, pyrenes and chrysenes did not conform to first order kinetics. Loss of three-ring dibenzothiophenes exceeded 95% for all three soils. Losses of four-ring pyrenes ranged from 71 to 87%, while loss of chrysenes ranged from 63 to 88%. Dimethyl homologues of pyrene and chrysene were the most persistent. Losses of these compounds ranged from 54 to 62%, with residual levels of 1 to 5 mg/kg remaining after treatment. A risk assessment at one of the sites showed an excess cancer risk to humans of less than 10(-6) from potentially carciogenic PAHs. In spite of this, all three sites will require require remediation because their high soil TPH levels represent a threat to surface water. Active land treatment bioremediation followed by low maintenance, plant-mediated bioremediation has been implemented at one of the sites, and is proposed for the others.