Identifying carbon processing based on molecular differences between groundwater and water-extractable aquifer sediment dissolved organic matter in a Quaternary alluvial-lacustrine aquifer

Dissolved organic matter in groundwater (GDOM) plays an important role in many biogeochemical processes. The molecular composition of GDOM varies with the source and processing pathways, and also affects its reactivity. Sedimentary organic matter (SOM) is a key carbon source in groundwater, but its association with GDOM remains unclear. In this study, Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) and comprehensive data analysis were used to identify the molecular characteristics of SOM and the fate of DOM in groundwater by comparing the differences between the GDOM with depth-matched water-extractable aquifer SOM (WEOM; the most bioavailable and dynamic pool of organic carbon in SOM) in a Quaternary alluvial-lacustrine aquifer with abundant SOM in Jianghan Plain, central China. The results show that WEOM is a key carbon source in groundwater as reflected by the strong correlation (rₛ = 0.76, P = 0.028) of the DOC content between GDOM and WEOM, and the large proportion (70.64 ± 14.09%) of common compounds present both in GDOM and WEOM. However, the molecular compositions of GDOM and WEOM are significantly different even for common compounds when considering the relative abundance. WEOM contains relatively high abundance of low molecular weight and thermodynamically more favorable components for microbial oxidation (aminosugars and carbohydrates), making WEOM an energetic carbon source for groundwater that can fuel reduction half-reactions such as iron oxide reduction. Although the hydrochemical compositions and the DOC content of groundwater are quite different, the molecular composition of GDOM is relatively similar and enriched in oxygen-poor highly unsaturated and phenolic compounds compared with WEOM. This indicates that organic matter is gradually processed into stable components with relatively medium aromaticity, low average nominal carbon oxidation state, low O/C and medium H/C; these transformations occur through adsorption to minerals and biodegradation after being mobilized from aquifer sediment into groundwater. We identified the processing pathways of GDOM based on comprehensive comparisons of molecular compositions between aquifer WEOM and GDOM, and highlight the combined influence of aquifer SOM and processing pathways on the GDOM molecular compositions in the alluvial-lacustrine aquifer with abundant aquifer SOM.