Scientific opinion on "Draft Environmental Quality Standards for Priority Substances under the Water Framework Directive" : tributyltin Compounds

The dossier on Environmental Quality Standards for "tributyltin" is reviewed by the SCHEER according to the general mandate on EQS dossiers. For TBT, the request to the SCHEER was limited to scrutinizing the QS for benthic organisms. The SCHEER does not endorse the suggested value of 0.3 μg TBT kgdw-1 for QSsediment,fw,eco, which is calculated by applying an AF of 50 to the lowest experimental toxicity value of 15.8 μg kgdw-1. Instead, the SCHEER calculates a rounded value of 1.6 μg TBT kgdw-1 for QSsediment,fw,eco, by applying an AF of 10, according to the TGD. The SCHEER endorses the value of 1.6 μg TBT kgdw-1 also as QSsediment,sw,eco, but considers this value preliminary until marine data for the most sensitive taxonomic group (sediment-dwelling molluscs) are available. Marine and freshwater test systems differ in their sensitivity, likely due to a lower bioavailability of TBT in marine sediments. This prohibits the pooling of marine and freshwater data for the purpose of estimating a joint SSD. As the individual data collections for marine and freshwater organisms, respectively, are insufficient for an SSD estimation, the SCHEER is of the opinion that currently no probabilistic QS values can be derived for TBT in sediments. The most critical EQS value for sediments is the deterministic value of 1.6 μg TBT kgdw-1 for QSsediment,fw,eco as well as QSsediment,sw,eco. In line with the mandate, no further comparison was drawn with other QS values.