| Summary: | Since 2005, an assessment of freshwater fish contamination by polychlorobiphenyls (PCBs), polychlorodibenzo-dioxins (PCDDs) and -furans (PCDFs) has been carried out in the Rhone basin (France). A large database of surface sediment contamination by PCBs is also available, opening the way to a study of biota to sediment accumulation factors (BSAFs) distribution throughout this basin. The ultimate goal of the study was to determine a sediment quality guideline (SQG) corresponding to the regulatory fish consumption limit. A bootstrapping procedure for determining BSAFs was applied to a dataset matching the available databases of sediment and fish contamination by PCBs in the Rhone river basin. The SQG is obtained by combining the current tissue-based regulatory threshold with a characteristic BSAF value, for a species particularly prone to accumulate these compounds. As the current regulatory threshold refers to dioxins and related compounds, while most available sediment data deal with non-dioxin-like PCBs, a set of correlations is used to derive a SQG for the sum of seven indicator PCBs. To assess the reliability of this SQG pairs, of actual fish and sediment concentrations were classified into four categories, according to a comparison with the regulatory threshold, for fish data, and the calculated SQG. BSAFs could be determined for 11 species. The barbel and the European eel had the highest BSAFs. The common carp, a benthic species, had surprisingly low BSAFs, as low as pelagic and omnivorous species such as the chub. The dataset was also split in two parts, one constituted of fish samples at or above the regulatory limit and the other by samples below this threshold; sediment concentrations and BSAFs were higher in the former group. A SQG was derived on the basis of the barbel's BSAFs distribution, and equaled 26.6 ng.g-1 [15.6-38.6]. When tested against the same database, this SQG displayed an overall efficiency of about 60% resulting from the limited reliability of sediment data and the factors influencing PCB bioavailability. In the perspective of regulatory frameworks such as the European Water Framework Directive, a tiered monitoring strategy combining sediments (first tier) and biota (second tier) could be more relevant than a single compartment approach. In this perspective, a SQG such as the one we determined would trigger the second tier.
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