Antiestrogenicity as a residual biological activity in treated drinking water: Evidence from passive sampling and...

Drinking water treatment plants (DWTPs) are designed to protect public health; however, residual endocrine-disrupting activity may persist in treated water and remain undetected by conventional monitoring. This study presents the first systematic assessment of both estrogenic and antiestrogenic activities in raw and treated waters from multiple DWTPs using a combined passive sampling-bioassay approach. Time-integrated Polar Organic Chemical Integrative Samplers (POCIS) were deployed at six DWTPs in Czechia and analyzed using a recombinant yeast estrogen receptor assay. Estrogenic activity was detected in raw waters reaching 4.25 ng estradiol equivalents per sampler (EEQ/POCIS) but was consistently eliminated during treatment. In contrast, antiestrogenic activity persisted in treated waters, reaching 3.19 ng antiestradiol equivalents per sampler (anti-EEQ/POCIS), and in some cases increased after treatment. Antiestrogenic effects were quantified using a transparent framework that expresses both agonistic and antagonistic activities in the same estradiol-equivalent units, allowing direct comparison of their relative magnitudes. These results indicate that antiestrogenicity represent a biologically relevant effect comparable to, or even exceeding, estrogenicity and may dominate residual endocrine activity in treated water. The persistence and occasional increase in anti-EEQ after treatment underscore the importance to monitoring both agonistic and antagonistic endpoints and highlight an underexplored dimension of drinking water quality.