Groundwater compliant with drinking water standards poses a "silent risk" of cumulative health hazards.

Groundwater is the primary source of drinking water for private well users. Traditional safety assessments and the classification of groundwater bodies (GWBs) typically rely on regulatory standards for individual contaminants, thereby overlooking mixture toxicity. This study aims to conduct a comprehensive health risk assessment (HRA) and evaluate existing regulatory standards within the framework of the "silent risk" phenomenon. Monitoring data spanning 2010, 2016, and 2022 for 174 GWBs in Poland were analysed. Non-carcinogenic (hazard index, HI) and carcinogenic risks (total cancer risk, TCR) were evaluated for both adults and children using deterministic and probabilistic (Monte Carlo simulation) approaches. The carcinogenic risk assessment conservatively assumed that all measured total chromium occurred in the form of Cr(VI).The results were compared against Polish, European Union (EU), and World Health Organization (WHO) standards. Although manganese exceeded regulatory limits in over 60% of monitoring points due to the geogenic background, it was not the primary driver of health risk. Instead, extreme hazards (HI > > 1) were driven by localized, anthropogenic spikes in nitrate and boron concentrations. Notably, children faced health risks more than twice as high as adults. Crucially, the study identified conservative, model-derived a "silent risk" phenomenon, where water fully compliant with regulatory standards for individual contaminants still posed a screening-level warning of unacceptable cumulative health risks (HI > 1) due to the additive effects of chemical mixtures. Furthermore, aggregate GWB classifications masked localized hotspots of critical health hazards. These findings indicate that, within the limits of our conservative modeling assumptions, selected contaminants and ingestion-only exposure framework for the selected contaminants, regulatory compliance does not inherently ensure chemical safety. This screening-level silent risk highlights how current frameworks fail to account for mixture toxicity and the heightened vulnerability of children. Consequently, HRA should be integrated into routine monitoring programs, and GWB status assessments must incorporate consumption-based metrics to effectively safeguard the health of private well users.