Application of different scenarii for the impact assessment of micropollutants in mixtures under the Water Framework Directive
Résumé
The assessment of chemical status under the WFD is currently made based on a limited number of chemicals, most of them being assessed individually. In a few cases, chemicals are evaluated as a family (e.g. PBDEs) and the environmental quality standard (EQS) is compared to the sum of the compounds defined in the family, assuming that it is the addition of these compounds effect that affects a specific target. In the surface waters, chemicals occur as mixtures, and these mixtures composition varies as a function of anthropogenic pressures. In order to propose a methodology taking into account the co-occurrence of chemicals, several approaches are applied on the chemicals measured on 8 sampling points and 2 genuine stations. The sampling points have been selected as a function of pressure (agricultural, industrial, and residential). As a part of a national monitoring campaign, 182 chemicals were monitored in these stations. In order to estimate the risk at a local scale, alternative approaches are investigated, including the following ones. The taxonomic group approach is founded on the fact that some substances may all affect a specific organism or taxon. This approach consists in identifying substances that are the most dangerous for a taxon (algae, invertebrate and fish) in order to group them together and sum up the measured concentrations. The mode of action (MoA) approach is a tentative to sum up the measured concentrations of substances that have the same effect on organisms and to calculate a threshold for the most frequently encountered MoAs. The pragmatic approach is founded on the fact that the bad water status may sometimes be linked to one or a few substances which are particularly toxic and that these contribute mainly to the toxicity of the mixture. Comparison of these alternative approaches is done, in relation to the anthropogenic pressures applied at each station, in order to highlight the approach leading to the most realistic results, in other words to evaluate if one of these approaches allows an assessment more consistent with the observed ecological status than the traditional individual or additive methods.