Contrasting sources and fate of ammonium in groundwater systems of two large alluvial-lacustrine basins

Groundwater ammonium (NH₄⁺) contamination can threaten human health and disrupt surface ecological balance. Although the sources and fate of NH₄⁺-N in groundwater have been widely studied, comparative research on this issue between different large basins under contrasting sediment sources and hydrogeological structures is weak. In this study, we used a combination of water chemistry, stable isotopes (H/O/C/N), excitation-emission-matrix spectra and 16 S rRNA gene sequencing to reveal the differences in the sources and fate of NH₄⁺-N in groundwater between Poyang Lake Plain (PYP) and Dongting Lake Plain (DTP), two large alluvial-lacustrine plains in central Yangtze River. The results showed that NH₄⁺-N in DTP groundwater occurred in a more closed and reduced environment, with stronger water-rock interaction and richer organic matter compared to that in PYP groundwater. NH₄⁺-N in DTP groundwater mainly originates from the intensive degradation of organic matter up to methanogenic stage, while NH₄⁺-N in PYP groundwater is affected by the coupled influence of organic matter fermentation before the methanogenic stage and anthropogenic activities. In addition, the specific hydrogeochemical environment in PYP groundwater is favorable to the natural removal of NH₄⁺-N caused by processes such as anammox and Feammox. The sediment provenance and hydrogeological structure of the two large plains are the key factors determing the contrasting sources and fate of NH₄⁺-N in groundwater.