The Key Factor Toxicity: development and application of a toxic pressure characterization tool for surface and drinking water

The European Water Framework Directive (WFD) aims to ensure a status with sufficient good-quality water to sustain ecosystems and drinking water production. Water quality is, however, affected by complex mixtures of polluting substances, whose combined effects are poorly captured by traditional single-compound monitoring. This limits our ability to characterize ecological and human health risks and hampers effective water quality management. We developed and applied the Key Factor Toxicity (KFTox) tool, integrating two complementary approaches for chemical water quality assessment: a Chemistry Track and a Bioassay Track. The Chemistry Track calculates mixture toxic pressures using Species Sensitivity Distributions and mixture modelling, resulting in multi-substance potentially affected fractions of species. The Bioassay Track measures biological activities in water samples using a comprehensive panel of bioassays, benchmarked against effect-based trigger values relevant to ecological and human health. Both tracks use colour-coded impact-related pollution classes to support interpretation by water quality authorities. A nine-month case study applied the KFTox tool to samples from surface water abstraction points, process water, and produced drinking water. The Chemistry Track indicated low toxic pressure from over 500 monitored substances, with metals and nutrients contributing most. The Bioassay Track applied bioassays measuring general toxicity, endocrine disruption, reactive modes of action (induction of oxidative stress response and genotoxicity), and xenobiotic metabolism. The latter revealed multiple activities exceeding trigger values, highlighting potential risks. The selected bioassays proved relevant for water quality assessment and provided information complementary to the Chemistry Track. Differences between both tracks illustrated the importance of combining approaches to detect unmonitored compounds and mixture effects. The KFTox tool provides an integrated and practice-oriented approach for assessing chemical water quality and prioritizing mitigation measures. This aligns with the objectives of the WFD and its latest amendment, encouraging the development of quantitative methods for chemical pollution assessment.