Fate of anthracene in contaminated soil: Transport and biochemical transformation under unsaturated flow conditions

To obtain reliable estimates for the loss of polycyclic aromatic hydrocarbons (PAHs) from contaminated soils, one has to distinguish between (i) losses due to release and solute transport and (ii) losses resulting from degradation. We studied the interplay of these processes in a column experiment representing a typical soil contamination scenario: in the upper part of the column was a contaminated layer, spiked with 9-¹³C-labelled anthracene, and beneath it uncontaminated pristine soil. The experimental course comprised a steady-state flow phase (constant irrigation for 4 months) followed by several periods during which flow was halted. The effects of varied residence time on anthracene biodegradation and on anthracene mass transfer were investigated. We monitored labelled anthracene and its transformation products, dissolved organic carbon, electric conductivity (EC), pH, and inorganic carbonate content in the column effluent, and the CO₂ evolved. Under steady-state flow, pH, dissolved organic C, and EC approached steady states after 350 pore volumes. Concentrations of anthracene in the effluent, however, increased continuously and levelled off after 800 pore volumes. This marked retardation reflects the great affinity of anthracene to soil organic matter. The response to interruptions in the flow revealed that mass is transferred without equilibrium between solid and liquid phase for both anthracene and dissolved organic C. Thus, residence time is one factor controlling the concentration of anthracene in the effluent and therefore the export of contaminant to the aquifer. In the course of the experiment several labelled anthracene degradation products appeared in the effluent. At least three of them were identified as transformation products showing a dramatic increase in mobility relative to the parent compound. A third of the overall anthracene loss from the column was due to solute transport, and biodegradation was responsible for the remaining two thirds. The incomplete degradation of anthracene leads to the formation of highly mobile transformation products and thus promotes the export of carbon, derived from the contaminant.