PAH release from tar-oil contaminated soil material in response to forced environmental gradients: implications for contaminant transport

摘要

Laboratory test systems are frequently used to assess the release of pollutants from contaminated sites. To infer behaviour in the field, all factors that control the release of such pollutants should be considered in the experiment. We carried out column experiments with varying boundary conditions under saturated flow to identify the processes governing the release and to evaluate the effect of environmental conditions on several polycyclic aromatic hydrocarbons (PAHs). We compared the results with groundwater concentrations monitored in the field. The contaminated soil material originated from a former tar-processing site. The effluent was analysed in response to forced variations in flow velocity, residence time, ionic strength and temperature. Interruptions to the flow had no effect on concentrations, which were close to those predicted by Raoult's law. We conclude that release of PAHs is controlled by equilibrium dissolution according to Raoult's law at moderate ionic strength. Diminishing the ionic strength by a pulse of pure water, however, results in a marked increase in the concentrations of exported PAHs. We attribute this to PAHs being bound to mobile particles. The effect was larger in the column percolated with fast flow, suggesting that the release of carriers is controlled by shear stress. An increase of temperature by 10 K resulted in marked increases in concentrations of the PAHs in the outflow between 6 and 160%. Concentrations in the groundwater correlate well with those in the outflow from the columns with similar ionic strength and temperature. We were able to identify the processes governing the release of PAHs under various conditions and to explain the concentrations observed in the field. The study illustrates that column outflow experiments, which support decisions in risk assessment, must be designed appropriately.