Organic matter-driven methane and carbon dioxide enrichment in groundwater systems: Molecular and isotopic evidence

Dissolved organic matter (DOM) plays a crucial role in methane (CH₄) and carbon dioxide (CO₂) enrichment in groundwater systems, but the underlying mechanism regulating CH₄ and CO₂ dynamics across diverse DOM degradation pathways at molecular-level remains elusive. To fill this knowledge gap, stable carbon isotopes (δ¹³C-CH₄, δ¹³C-CO₂, and δ¹³C-DIC) coupled with DOM optical properties and molecular signatures were employed to elucidate the spatial heterogeneity of groundwater CH₄ and CO₂ within the alluvial-lacustrine aquifer along the middle reaches of the Yangtze River. Hydrochemical and stable carbon isotopic fingerprints revealed that CH₄ and CO₂ production was governed by two distinct OM degradation stages, the fermentation stage and the methanogenic stage. DOM optical properties and molecular signatures further unraveled the unique DOM composition governing the divergence between CH₄ and CO₂ production pathways. Degradation of labile DOM such as aliphatic compounds fueled early CO₂ production at the fermentation stage. Recalcitrant molecules such as polyphenols and highly unsaturated compounds with higher NOSC values were thermodynamically more favorable to be utilized, which was conducive to CH₄ enrichment as DOM degradation progressed toward the methanogenic stage. These findings could enhance the understanding of the links between DOM degradation and greenhouse gas emissions from an integrated isotopic and molecular-level perspective, and provide insights into carbon cycling and greenhouse gas emission budgets, thus might be applicable to similar alluvial-lacustrine aquatic environments.