Multi-Phase mobilization and degradation pathway of natural organic matter within alluvial-lacustrine aquifer sediments

Alluvial-lacustrine sediments are crucial organic matter pools in the subsurface, fueling biogeochemical processes in aquifers through the release of dissolved organic matter (DOM). However, little is known about the detailed mobilization and degradation pathways of organic matter from sediments to groundwater. Here, we examined porewater bridging sediments and groundwater and characterized the fluorescence properties and molecular composition of DOM in porewater with varying binding strengths to sediments. Results showed that bound water DOM primarily contained polycyclic aromatics, polyphenols, and highly unsaturated compounds with high O/C ratios and humification indices. Free water DOM, by contrast, was rich in aliphatic compounds with high H/C ratios and biological indices. Unique molecular formulae analysis revealed that in bound water, CHO + nN polyphenolic compounds were consumed, and highly unsaturated CHO + 2 N compounds were produced; in free water, highly unsaturated CHO + 2 N compounds were consumed, and CHO + 1 N aliphatic compounds were produced. This molecular fractionation of organic matter is likely controlled by abiotic diffusion and adsorption as well as by biotic microbial activity, with the former dominating in bound water and the latter in free water. This may relate to the stable double-layer structure in bound water and the increased exposure to electron acceptors in free water. Together, these dual mechanisms produce a high proportion (>50 %) of biodegradable DOM in free water, potentially explaining the geogenic enrichment of ammonium in local aquifers. Our results highlight the strong influence of local mobilization and degradation processes on the molecular characteristics of DOM, bearing significant environmental implications for the evolution of groundwater quality.