Fe and Mn biogeochemical cycling associated with basin-scale redox dynamics traced by DOM degradation in different alluvial aquifers.

Iron (Fe) and manganese (Mn) contamination in groundwater has emerged as a global health challenge, primarily influenced by the degradation pathways of organic matter. However, the understanding of Fe and Mn biogeochemical behaviors, particularly the release mechanisms driven by the redox dynamics of aquifers at the watershed scale remains limited. This investigation employed a multi-method framework integrating hydrogeochemical-isotopic analyses with DOM molecular characterization (FT-ICR MS) to elucidate DOM degradation processes along the groundwater flow paths and their driving effects on Fe and Mn mobilization. The findings revealed that DOM degradation significantly modulates the redox zoning in porous aquifers, thereby governing the release patterns of Fe and Mn. In the weakly oxidizing environment (Zone I), DOM derivatives exhibited intricate molecular structures, characterized by higher relative abundances of saturated compounds, aliphatic species, and polyphenols compared to the downstream area. Fe and Mn primarily originate from the water-rock interactions, and tend to form stable DOM-metal complexes under oxidizing aquifers that constrain the concentration of dissolved metals. As groundwater flows into the plain area (Zone II) where the aquifers gradually become anaerobic, enclosed sedimentary aquifers and sluggish groundwater runoff intertwine highly mineralized DOM with biogeochemical processes. The preferential utilization of DOM with higher NOSC values drives sequential anaerobic respiration from sulfate reduction to dissimilatory metal reduction. This redox cascade promoted extensive dissolution of Fe and Mn (oxy)hydroxides. Intriguingly, methanogenic-phase DOM fermentation in Zone II-XKR activated anaerobic methane oxidation, generating a secondary Fe and Mn mobilization pathway. This process augmented the efficiency of metal release, resulting in Fe and Mn concentration in the Zone II-XKR being 2-3 times higher than those in the WLR subzones. Our findings establish DOM molecular signatures coupled with δC-DIC isotopic tracers as indicators for deciphering redox gradient biogeochemistry. The proposed model deepens the understanding of metal-cycling mechanisms and provides an informative framework for the genesis of high Fe and Mn groundwater in alluvial plains.