Proportional contributions to organic chemical mixture effects in groundwater and surface water

•GC–MS and LC-MS were used to quantify organic chemical mixtures.•Chronic SSD HC50s were used as the hazard metric to predict mixture effect.•Increased hazard was found from the presence of a cocktail of substances.•The most toxic chemical contributed ≥ 20% of mixture effect in >99% of cases.•Mixture complexity was only weakly associated with increased mixture effect.