The impact of in-the-field organic fraction on the interpretation of sediment threshold values under the Water Framework Directive

Abstract : In the context of the Water Framework Directive, the EU Technical Guidance Document on Environmental Quality Standard (EQS) recommends in certain circumstance to apply the equilibrium partitioning (EqP) method to derive a standard in sediment based on the standard derived in water. It assumes that the bioavailability of the substance depends on the chemical compound itself and the organic fraction of the sediment. Indeed, a chemical substance with a high soil organic carbon-water partitioning coefficient (Koc) is expected to bind strongly with sediment. Furthermore, a chemical substance will bind more easily with the fraction of organic carbon in the sediment (foc).The formula uses some generic value and organic carbon content in sediment is one of them (generic Total Organic Carbon or TOC = 5 and generic foc=5%). In 2012, an exceptional monitoring campaign in France gave the opportunity to investigate if bioavailability in sediment is efficiently taken into account by the default application of EqP method when compared to-field data. On 110 sampling point, TOC has been analyzed along with the concentration of 85 substances. The investigation reveals that the generic foc used in the formula may not be sufficiently protective in a majority of the station sampled and conducts to underestimate the bioavailable fraction of the pollutant. As a conclusion, it is recommended to consider measured TOC in poor organic matter station for substances that are not expected to be bioavailable.