Rainfall-driven shifts in disinfection byproduct precursors in eutrophic lakes: From algal to terrestrial sources

In subtropical monsoon regions, eutrophic lakes frequently experience concurrent heavy rainfall and algal blooms during the wet seasons. However, the combined effects of these events on source-water quality remain poorly understood. This study investigated the influence of rainfall on disinfection byproduct (DBP) formation potentials during algal blooms in Chaohu Lake-a critical water source servicing approximately 9 million people-and elucidated the dissolved organic matter (DOM)-mediated mechanisms underlying DBP variability. Following rainfall, total organic halogen concentrations in chlorinated lake waters increased from 537.6 ± 107.1 μg/Lto 716.5 ± 187.7 μg/L as Cl, accompanied by a 1.5- to 3.3-fold rise in DBP-associated cytotoxicity and a marked shift in DBP composition. Notably, post-rainfall concentrations of haloacetaldehydes (HALs) and haloacetic acids (HAAs) increased significantly, whereas haloacetonitriles (HANs) decreased. This pattern aligns with a shift in DOM composition-from predominantly algal-derived constituents before rainfall to terrestrial-dominated components afterward, which explains the observed changes in DBP formation potential. Specifically, the post-rainfall decline in tyrosine-like proteins-key precursors of HANs-was associated with reduced HAN formation. Conversely, the increase in fulvic and humic acids, strongly promoted HAL and HAA formation. Multiple linear regression identified water temperature, dissolved organic carbon, dissolved organic nitrogen, the biological index, and the humification index as key predictors of major DBP formation. These findings underscore the urgent need for targeted management of key water-quality parameters to mitigate DBP precursor levels in eutrophic lakes, particularly following rainfall events.