Two-Year Water Quality Monitoring of Drinking Water Treatment Plants in Biscay (Basque Country)

Drinking water is one of the many potential routes of human exposure to environmental contaminants, as it often originates from anthropogenically impacted sources. This two-year study monitored contaminants of emerging concern (CECs) in five drinking water treatment plants (DWTPs) across the Basque Country. A complementary analytical approach combining target analysis and non-target screening (NTS) was employed to assess the presence of CECs in both non-treated and treated waters. DWTPs sourcing water from surface waters exhibited the highest number of CECs, with compounds such as metformin and didecyldimethylammonium (DDAC) frequently detected (> 30%). The presence of CECs in treated water overall suggests that the implemented treatment technologies are not fully effective in removing them. The detected, non-eliminated contaminants were prioritised based on a human health risk assessment (HHRA) and several compounds, including norgestimate, tris(2-butoxyethyl)phosphate (TBOEP), and DDAC, were identified as posing the highest potential risk to human health. In addition, an effect-based methodology (EBM) with bioassays targeting cytotoxicity and mutagenicity was applied to provide a more comprehensive evaluation of mixture effects. No detectable biological activity was observed in any of the samples, suggesting a potentially low risk for human health regarding the evaluated endpoints. The divergence between concentration-based prioritisation and bioassay results suggests the need to expand experimental testing across a broader range of biological endpoints, such as endocrine disruption, which may not be captured by the applied assays.