Rainstorm‑driven highly chlorine‑reactive DOM increases disinfection byproduct risks in drinking water.

More frequent rainstorm events under global climate change substantially alter dissolved organic matter (DOM) composition in source waters via runoff, exacerbating disinfection byproduct (DBP) risks in drinking water. Here, we provide a molecular- level elucidation of how rainstorm-altered DOM regulates DBP formation and associated cytotoxicity in drinking water. Results show that rainstorm events increased DOM concentrations and shifted DOM composition toward higher chlorine reactivity, leading to an approximately 46% increase in total organic halogen formation (TOX) and an approximately four-fold increase in water cytotoxicity after disinfection. Post-rainstorm DOM showed increased contributions of heteroatom-containing and less saturated molecular formulas, which were associated with higher TOX yields and enhanced apparent chlorine reactivity. A mass-difference linkage analysis indicated a 52% increase in possible precursor-product formula pairs after the rainstorm, with formula-level mass differences related to chlorine addition and chlorine substitution accompanied by oxygen incorporation accounting for 63% of the increase in links. Importantly, coagulation-precipitation process and O3-BAC exhibited limited effectiveness in removing chlorine-reactive DOM components induced by the events, resulting in 19-86% higher DBP formation in BAC filter effluents compared with pre-rain waters. These findings provide molecular-level evidence that rainstorm-responsive DOM increases the treatment burden and residual DBP formation potential in full-scale drinking water treatment systems, highlighting the urgent need for proactive stormwater management and effective DBP mitigation strategies.