Adaptive Groundwater Management: Tiered Environmental Health Risk Assessment and Zonation at Complex Contaminated Sites

“One toxicant at a time” regulations overlook the joint risks posed by realistic coexposure to complex toxicant mixtures in groundwater. Here, 1,016,590 quarterly concentration records for 277 toxicants from 367 monitoring points were integrated into an adaptive zonation and classification framework for mixture-oriented risk management. Ecological mixture risk was quantified with species-sensitivity-distribution models combined in a tiered concentration addition → response addition (CA → RA) scheme, while Monte Carlo simulations captured probabilistic carcinogenic and noncarcinogenic human health risks. These dimensions were merged into an environmental health risk index (EHRI; 0.42 ecology: 0.58 health). Quadrant analysis identified four coexposure patterns and showed that ≤10% of detected toxicants─chiefly inorganics, metals, BTEXs, haloalkanes, and phenols─drive >95% of cumulative EHRI. Spatial interpolation converted point-level EHRIs to grid-scale and, when embedded in a “source–plume–buffer” treatment train, reduced the great concern zone (EHRI > 0.60) from 19 km 2 to