Prioritizing groundwater management in chemical industrial parks by achievable risk reduction measures

Groundwater pollution control commonly prioritizes contaminants and locations according to observed concentrations or associated risks, implicitly assuming that high risk corresponds to high mitigation potential. However, this assumption may not hold where groundwater quality is jointly controlled by anthropogenic inputs and natural geochemical backgrounds. This study developed a natural-background-constrained framework to distinguish current groundwater risk from achievable risk reduction and applied it to 121 monitoring wells in a 1.54 km² chemical industrial park in eastern China. Individual and mixture ecological risks were integrated with deterministic and probabilistic human health risks across multiple receptor groups into an environmental health risk index (EHRI). Idealized contaminant-removal scenarios were first used to identify dominant risk contributors, followed by natural background levels (NBLs)-constrained scenarios to quantify potential risk reduction. The two scenarios revealed a marked decoupling between risk contribution and achievable risk reduction. As was the dominant contributor to baseline EHRI, but reducing it to its NBL produced almost no EHRI decrease because its natural background level approached or exceeded the health-risk-based concentration threshold. In contrast, Cl^-, NH₄⁺, and Na⁺ yielded substantially greater reductions, resulting in a control priority of Cl^->NH₄⁺>Na⁺>As. Spatial EHRI reduction further identified zones with the greatest risk-reduction potential, while residual risk was used to prioritize post-control monitoring. The proposed monitoring scheme would reduce annual sampling events by 14.7% while maintaining more frequent sampling at high-priority locations. These findings provide insights into groundwater pollution prioritization by distinguishing controllable from persistent background risk.