The European water-based environmental quality standard for pentachlorophenol is NOT protective of benthic organisms

Risk management of toxic substances is often based on Environmental Quality Standards (EQS) set for the water compartment, assuming they will also protect benthic organisms. In the absence of experimental data, EQS for sediments can be estimated by the equilibrium partitioning approach. The present study investigates whether this approach is protective of benthic organisms against pentachlorophenol (PCP), a legacy contaminant and EU priority substance still used in some parts of the world. Three freshwater species of invertebrates with different life cycles and feeding behaviors (the oligochaetes Lumbriculus variegatus, Tubifex tubifex and the dipteran insect Chironomus riparius) were exposed to PCP spiked sediments (2.10-46.03mgPCP/kg d.w. plus controls) in laboratory standard tests. Exposure duration was 28days for T. tubifex and L. variegatus and 10 and 28days for C. riparius; according to the corresponding OECD guidelines. For each investigated end-point, dose-response data were normalized to the mean control and fitted to a four-parameter log-logistic model for calculating the corresponding EC₅₀ and EC₁₀. The ranges for EC₅₀ and EC₁₀ estimates were 4.39 (Chironomus riparius-emergence)-27.50 (Tubifex tubifex-cocoon) and 0.30 (T. tubifex-young worms) -16.70 (T. tubifex-cocoon) mg/kg d.w., respectively. The EC₅₀ and the EC₁₀ values of L. variegatus were within these ranges. Following the EU Technical Guidance for deriving EQS, the lowest EC₁₀ value of 0.30mg/kg (T. tubifex-young worms) resulted in a PCP quality standard (QS) for sediments of 30ng/g, about one fourth of the tentative QS of 119ng/g estimated by the equilibrium partitioning (EqP) approach. The response of benthic biota to PCP varied across organisms and across end-points for the same organism, so that the use of sediment PCP-QS calculated using the EqP-approach may be under-protective of the most sensitive organisms. Information on the possible effects of PCP on resident organisms must therefore be collected for appropriately managing aquatic systems.