Environmental quality standards for diclofenac derived under the European Water Framework Directive: 1. Aquatic organisms

Diclofenac is a nonsteroidal anti-inflammatory human and veterinary medicine widely detected in European surface waters, especially downstream from Wastewater Treatment Plants (WWTPs). Veterinary uses of diclofenac in Europe are greatly restricted, so wastewater is the key exposure route for wildlife. Proposed Environmental Quality Standards (EQS) which include an assessment of toxicity to aquatic organisms are under consideration by the European Commission (EC) to support the aims of the Water Framework Directive (WFD). The EC approach favours the use of a deterministic (single test value and an assessment factor) approach to the derivation of a direct toxicity EQS for diclofenac, resulting in an EQS of 0.040 µg L−1 based on a single mesocosm study. In this paper, we discuss potential issues with this approach with respect to the EC’s own guidance on EQS derivation and derive an evidence-driven alternative EQS of 0.126 µg L−1 using a probabilistic (species sensitivity distribution) approach that accounts for all of the reliable and relevant data and is in accordance with the guidance. Europe-wide freshwater monitoring data for diclofenac are used in an indicative compliance assessment using the EC and the alternative evidence-driven EQS. The implications of using only some data to derive an EQS that does not adhere to the guidance, compared to a guidance-compliant approach that uses all the data available are also discussed.