Nitrogen cycling regulates arsenic mobilization in alluvial-lacustrine aquifer systems: DOM molecular signatures and thermodynamic constraints.

High arsenic (As) groundwater commonly co-occurs with elevated ammonium (NH4+) in aquifers worldwide. However, the underlying nitrogen cycling processes regulating As mobilization in groundwater remain elusive. Here, hydrochemistry, stable isotopes, FT-ICR MS, and thermodynamic modelling were integrated to investigate the NH4+ production pathways and their impacts on As mobility within the alluvial-lacustrine aquifer. Hydrochemical and isotopic data indicated that groundwater NH4+ was primarily produced through OM mineralization and dissimilatory nitrate reduction to ammonium (DNRA), and both processes contributed to As enrichment which reached up to 1603 μg/L. FT-ICR MS analysis revealed that OM mineralization depleted CHON2 and highly unsaturated compounds, while DNRA preferentially consumed labile aliphatic compounds. Thermodynamic potential factors (FT) further evidenced that OM mineralization faced thermodynamic constraints (0.26 < FT < 0.99) that are highly dependent on the molecular properties of OM and the crystallinity of Fe(III) oxides, whereas DNRA experienced less thermodynamic limitations (FT > 0.97). Notably, thermodynamically favorable organic compounds such as highly unsaturated compounds and N-containing OM could promote OM mineralization, during which NH4+ is produced and the reductive dissolution of Fe(III) oxides is simultaneously facilitated, thereby releasing As into groundwater. These findings could provide novel insights into the role of coupled C-N-Fe cycling in As mobilization within alluvial-lacustrine aquifer systems.