Biodegradation of low molecular weight organic compounds driving arsenic mobilization in groundwater systems

Although organic matter (OM) has been intensively investigated in high arsenic (As) groundwater, the biodegradable OM molecules contributing to As enrichment is poorly understood. To address this, two multilevel wells (K1 & K2 being located in flat plain and alluvial fan, respectively) from the Hetao Basin were setup to investigate lipid biomarkers and their compound-specific carbon isotopes of dissolved organic matter (DOM) and depth-matched sedimentary organic matter (SOM). We revealed that groundwater As and DOC concentrations exhibited greater in K1 from the flat plain than those in K2 from the alluvial fan. The flat plain aquifer had higher microbe-derived OM inputs in both SOM and DOM, since groundwater and sediment OM from K1 was characterized by lower C/N ratios, lower terrigenous/aquatic ratio values, and higher proportion of low molecular weight (LMW) n-alkane compounds (≤C24) than that from K2. The stronger extent of OM biodegradation appeared in the flat plain aquifers, which was supported through greater contrast of δ13C between short-chain n-alkanes and long-chain n-alkanes in groundwater (Δδ13CDOM) and in sediments (Δδ13CSOM), higher thermal maturity of SOM, less odd-to-even predominance and aquatic alkane alkanol values of DOM and SOM from K1. The DOM of high-As groundwater showed more microbe-derived OM than the depth-matched SOM. Arsenic levels showed positive associations with LMW n-alkanes and Δδ13CDOM, suggesting that LMW n-alkanes were electron donors driving As mobility. More specifically, C22-alkane and C18-alkanoic acid were identified as the primary drivers of As mobility, being evidenced by positive relationships between their concentrations and dissolved As levels. Furthermore, a linear correlation between C22-alkane δ13C and As concentrations reinforced this conclusion. This study provides insights into specific LMW compounds, i.e., C22-alkane and C18-alkanoic acid, elevating dissolved As concentrations in groundwater systems.