Identifying groundwater ammonium hotspots in riverside aquifer of Central Yangtze River Basin.

Elevated ammonium (NH-N) contents in groundwater are a global concern, yet the mobilization and enrichment mechanisms controlling NH-N within riverside aquifers (RAS) remain poorly understood. RAS are important zones for nitrogen cycling and play a vital role in regulating groundwater NH-N contents. This study conducted an integrated assessment of a hydrochemistry dataset using a combination of hydrochemical analyses and multivariate geostatistical methods to identify hydrochemical compositions and NH-N distribution in the riverside aquifer within Central Yangtze River Basin, ultimately elucidating potential NH-N sources and factors controlling NH-N enrichment in groundwater ammonium hotspots. Compared to rivers, these hotspots exhibited extremely high levels of NH-N (5.26 mg/L on average), which were mainly geogenic in origin. The results indicated that N-containing organic matter (OM) mineralization, strong reducing condition in groundwater and release of exchangeable NH-N in sediment are main factors controlling these high concentrations of NH-N. The Eh representing redox state was the dominant variable affecting NH-N contents (50.17 % feature importance), with Fe and dissolved organic carbon (DOC) representing OM mineralization as secondary but important variables (26 % and 5.11 % feature importance, respectively). This study proposes a possible causative mechanism for the formation of these groundwater ammonium hotspots in RAS. Larger NH-N sources through OM mineralization and greater NH-N storage under strong reducing condition collectively drive NH-N enrichment in the riverside aquifer. The evolution of depositional environment driven by palaeoclimate and the unique local environment within the RAS likely play vital roles in this process.