PCB vertical and horizontal movement in agricultural soils of a highly contaminated site: Role of soil properties, cultivation history and PCB physico-chemical parameters

The agricultural areas of a historically contaminated National Relevance Site (SIN Brescia Caffaro) in Italy are an ideal case for studying the long term vertical and horizontal movement of polychlorinated biphenyls (PCBs) in soil. Here, a former large producer of PCBs (Caffaro S.p.A.) discharged its wastewaters, contaminated by PCBs and other chemicals, to a ditch used for about 80 years as source of irrigation waters for the adjacent agricultural areas. This caused a spread of contamination along both a vertical and a horizontal soil gradient. PCB concentrations of about 80 congeners, including PCB 209, peculiar of Caffaro production, were measured in three areas, selected for their different soil properties and cultivation history. The contamination levels with depth ranged from about 30 mg/kg dry weight (d.w.) of soil in the top (plow) layer to less than 0.1 mg/kg d.w. at the depth of 1 m. The concentrations varied also horizontally, since each field was surface irrigated from the short edge of each field, showing that PCBs could spread with length halving the initial concentrations in the topsoil only after about 30–35 m. The concentration gradients detected were explained considering the historic soil use and its change with time, the pedological properties as well as PCB physico-chemical parameters and halflives, developing equations which could be employed as guidance tools for evaluating PCBs (and similar chemicals) movement and direct further studies.