Leaching of weathered polychlorinated biphenyls (PCBs) obtained from a contaminated site: role of dissolved organic carbon and saturation conditions in a soil column experiment

Exposure assessment in contaminated soils is usually performed by the evaluation of the total residual concentrations of target pollutants in soils (e.g. ?PCBs). This approach does not take into account the actual mobility and bioavailability of the contaminants and may be inappropriate especially for historically contaminated soils where sorption processes may be scarcely reversible due to the presence of bound/weathered residues. Therefore, assuming that equilibrium conditions would occur using standard partitioning coefficients (e.g. Koc/Kd) may lead to misleading results in modelling attempts. Therefore, the estimation of the contaminant fraction available for leaching and transport in different soil conditions is recommended. During the last decades, several authors studied the release of pollutants from soil by using soil column leaching tests but the laboratory experiments were often performed in scarcely realistic conditions (e.g. fresh spiked contaminants). Additionally, often some variables influencing the mobility of contaminants were neglected, as well as the statistical scheme was poor (e.g. lack of replicates). In this context, a soil column leaching experiment was performed to evaluate: 1) the effects of dissolved organic carbon (DOC) content in the leaching solutions, 2) equilibration time, 3) soil saturation conditions on leaching fluxes of selected weathered PCBs (PCB 28, 52, 101, 138, 153, 180, 209) present in an historically contaminated soil. These effects were evaluated collecting leached samples at different contact time (2, 5, 7, 48 days), in flow vs. no flow conditions and in saturated vs. field capacity conditions. The results show that the most influential factors are the DOC content in non-equilibrium conditions (flow condition samples) and the soil saturation conditions.