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.