Techniques for evaluating the on-site bioavailability with passive samplers and/or bioaccumulation on benthic invertebrates: the case study of DDT and Mercury in the Toce River of Lake Maggiore basin

The EU Directive n. 2013/39/UE on priority substances in managing water policy recommends, at the article 18, the use of sampling methodologies as passive samplers or other promising methods for water monitoring, overcoming the practise of collecting single water samples. CNR-IRSA, supported by CIPAIS Commission, investigated the possibility of using specific passive samplers and bioaccumulation in benthic invertebrates as proficiency tools for monitoring DDT and mercury contamination in sediments. DDT and mercury contamination in the Toce River (North Italy) was caused by a factory producing technical DDT and using a mercury-cell chlor-alkali plant. In this study, DDT and Hg contamination and bioavailability were assessed in the Toce River using different approaches: 1) direct evaluation of sediment contamination; 2) assessment of bioaccumulation in native benthic invertebrates belonging to different taxonomic/functional groups; 3) evaluation of the on-site bioavailability of DDT and Hg for aquatic organisms using passive samplers. Polyethylene passive samplers were used to estimate DDT concentrations, while Diffusive Gradients in Thin Films passive samplers (DGT) were used to measure labile mercury. Sampling stations were selected upstream and downstream the industrial plant along the Toce River. Results showed an increasing DDT and mercury contamination in sediments and in benthic organisms from upstream to downstream sites. Benthic invertebrates exhibited Hg concentration up to 0.33 mg kg-1 d.w., with higher values for collectors and predators. The highest DDT contamination instead resulted in Gammaridae and Diptera with concentrations up to 300-400 ng g-1 d.w. The assessment of bioaccumulation allows to understand the real bioavailability of the contaminants and to evaluate a possible risk of secondary toxic effects. Concentrations in macroinvertebrates and sediments were used to calculate the Biota Sediment Accumulation Factor (BSAF); results showed high values of BSAF (higher than 10 for DDT contamination and up to 3.7 considering Hg) revealing the capacity to bioaccumulate of the investigated taxa. At the end, the on-site exposure of passive samplers showed increasing concentrations of both contaminants in pore water of sediments from upstream to downstream sites, confirming the bioavailability of DDT and Hg to benthic organisms.