Salinity–Organic Matter Coupling Facilitates Geogenic Ammonium Enrichment in Coastal Aquifer Systems

Excessive geogenic ammonium (NH₄⁺-N), derived from the mineralization of naturally nitrogen-containing organic matter (OM), has gained increasing attention, particularly in coastal aquifers. However, the mechanisms linking groundwater NH₄⁺-N enrichment to salinity and its interaction with dissolved organic matter (DOM) and soluble organic matter (SOM) remain unclear. In this study, the molecular characteristics of DOM and SOM in the Pearl River Delta aquifer systems were analyzed by using ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry. Comparative incubation experiments were conducted to investigate DOM/SOM degradation pathways, their contribution to NH₄⁺-N enrichment, and effect of salinity on DOM/SOM degradation. Results showed that DOM and SOM degradation involved the progressive degradation of aliphatic compounds, highly unsaturated compounds (HUC), and polyphenols (PPE) with lower O/C to HUC and PPE with higher O/C. Four major deamination reactions (hydrolytic deamination, oxidative deamination, reductive deamination, and ammonia elimination) were identified, and both DOM and SOM contributed similar to NH₄⁺-N enrichment. Notably, salinity was found to influence the degradation pathways, facilitating the deamination of DOM/SOM by altering the molecular composition and microbial community structure. This study enhances the understanding of geogenic NH₄⁺-N enrichment mechanisms in coastal groundwater and underscores the potential pollution risks associated with groundwater salinization.