A Scenario-Based Probabilistic First-Tier Screening of Potential Chemical Risk to European Groundwater Ecosystems

Groundwater supplies much of the freshwater used by humans while supporting highly specialized biological communities characterized by high endemism and limited taxonomic, functional, and phylogenetic redundancy. Yet European Union groundwater legislation remains resource-oriented and addresses the ecological consequences of chemical contamination only indirectly. We conducted a continent-wide, first-tier screening of potential chemical risk to European groundwater ecosystems. Harmonized European surface water and groundwater monitoring data were combined with geostatistical interpolation and transport modeling to estimate contaminant concentrations at the groundwater table, while uncertainty in hydrogeological conditions, chemical fate, and preferential transport was propagated through Monte Carlo simulations. We compared a Surface Water Baseline scenario, based on model-derived groundwater concentrations and existing freshwater regulatory thresholds, with a Groundwater-Informed Conservative scenario integrating measured groundwater concentrations, groundwater-fauna toxicity data, and precautionary adjustments where groundwater-specific evidence was unavailable. Potential risk was summarized as the average frequency of exceedance (AFE) across contaminants and uncertainty realizations; its maps indicate the frequency and spatial distribution of threshold exceedances, not their magnitude or the severity of ecological damage. Mean AFE was low under the baseline scenario but increased 5-fold under the conservative scenario, revealing areas in the medium-to-very-high screening classes in the Pyrenean–Catalan sector, Sierra Nevada, and the eastern Balkan–Carpathian region. The framework provides a replicable basis for prioritizing monitoring, ecotoxicological testing, and management.