Refining environmental risk assessment of diltiazem in groundwater through better surrogate selection and sublethal endpoints

Pharmaceutical pollution is a growing concern for aquatic ecosystems, yet the extent of contamination in groundwater and its ecological consequences remain poorly understood. This knowledge gap stems in part from the lack of mandatory monitoring of pharmaceutical compounds in groundwater across most regions of the world and from the lack of ecotoxicological tests on obligate groundwater-dwelling fauna. In this study, we applied the European Medicines Agency guidelines for groundwater environmental risk assessment (ERA) of diltiazem - a calcium channel blocker – introducing methodological adaptations to better reflect the characteristics of groundwater communities. We compared the sensitivity of the standard test species Daphnia magna and that of the facultative groundwater copepod Diacyclops crassicaudis crassicaudis, and we validated the predicted no-effect concentration (PNEC) of diltiazem by assessing its sublethal effects on oxygen consumption rates. We also reviewed the global literature to compile measured environmental concentrations of diltiazem in freshwater systems and, finally, estimated the potential risk to groundwater. Our study showed that D. crassicaudis crassi caudis is more sensitive to diltiazem than D. magna, supporting its use as surrogate species for groundwater ERA. Moreover, short-term exposure (48 h) to a sublethal concentration (54 μg/L) significantly increased oxygen consumption in D. crassicaudis crassicaudis, indicating the need for a groundwater PNEC at least one order of magnitude lower than that applied to surface waters. These findings highlight a negligible environmental risk from diltiazem in groundwater and stress the need to revise current regulatory thresholds by incorporating sublethal endpoints, ultimately promoting more realistic ERA for groundwater ecosystems.