EQS for a metal : Is it possible to determine a lonely legal value? Interest of the Biotic Ligand Model (BLM) ?
Résumé
Due to several physicochemical processes, metals exist in different chemical forms which might differ in bioavailability. As geographically distinct watersheds show distinct geochemical characteristics, the degree to which different aquatic systems can safely accommodate metal loadings will vary. The Water Framework Directive (WFD; 2000/60/EC) explicitly acknowledges the issues of bioavailability and naturally occurring concentrations for metals. According to the technical guidance for deriving environmental quality standards (EC, 2009), the derivation of EQSs for metals requires an explicit consideration of bioavailability using biotic ligand models (BLM) or, failing that, to utilise dissolved concentrations instead of total concentrations. Background concentrations may also need to be taken into account. With this work, EQSs were set considering three main steps as described in the guidance document (EC, 2009). EQS for metal as Zn or Ni were derived with taking into account bioavailability correction in function of the variations of physicochemical parameters in water and in a site specific approach. We chose to consider the variation of parameters in the same watershed to assess the range of variation of EQS in linked waters. Thus, this study could emphasize the potential distribution of EQS within the watershed. In fact, that represents an element of questioning in connection with the regulatory presentation of EQS for metals. Indeed, at a regulatory level, EQS should be set and expressed within a clear physicochemical domain of validity.