Environmental risk of diclofenac sodium in European groundwaters: implications for setting environmental quality standards

Abstract Groundwater harbors unique species adaptated to perpetual darkness. Groundwater fauna plays a crucial role in global ecosystem services, but contamination poses a threat to this keystone ecosystem. Diclofenac sodium is a common non-steroidal anti-inflammatory drug of particular concern, due to its presence in both surface and groundwater. We assess the environmental risk of diclofenac sodium in European groundwaters using different scenarios, analyzing Measured Environmental Concentrations (MECs) of diclofenac sodium and estimated the Predicted No Effect Concentration (PNECs) through two approaches: considering the sensitivity of the groundwater crustacean Proasellus lusitanicus, and using surface water species as proxies. Our results show that scenarios based on surrogate species predict that groundwater ecosystems are at risk, and the MECs of diclofenac sodium were consistently lower than the PNEC of P. lusitanicus, suggesting that the current MECs do not pose a significant threat to this groundwater-adapted species. However, risk scenarios differ considering the sensitivity of other groundwater species, emphasizing the importance of considering multiple species’ sensitivities in risk assessment. Therefore, we recommend establishing an environmental quality standard for diclofenac sodium in groundwater at 5 ng/L, value that accounts the need for precautionary measures to safeguard groundwater ecosystems, essential for preserving their unique biota and services.