Attenuation of petroleum hydrocarbons in smear zones: A case study

Soil and ground water in a shallow aquifer were contaminated by petroleum hydrocarbons, mostly comprised of toluene. The degrees of soil and ground-water contamination were closely correlated. The vertical profile of soil contamination showed an exponential or semilinear decrease both upward and downward from the mean water table, and this was closely related to water table fluctuation. Vertical and seasonal variations in the concentrations of petroleum hydrocarbon contaminants and their degradation products were analyzed in detail. Total natural attenuation rate and the contribution of biodegradation to the rate were estimated. Biodegradation was found to be an effective attenuation process at this site.