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

Elevated ammonium (NH₄-N) contents in groundwater are a global concern, yet the mobilization and enrichment mechanisms controlling NH₄-N within riverside aquifers (RAS) remain poorly understood. RAS are important zones for nitrogen cycling and play a vital role in regulating groundwater NH₄-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 NH₄-N distribution in the riverside aquifer within Central Yangtze River Basin, ultimately elucidating potential NH₄-N sources and factors controlling NH₄-N enrichment in groundwater ammonium hotspots. Compared to rivers, these hotspots exhibited extremely high levels of NH₄-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 NH₄-N in sediment are main factors controlling these high concentrations of NH₄-N. The Eh representing redox state was the dominant variable affecting NH₄-N contents (50.17 % feature importance), with Fe²⁺ 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 NH₄-N sources through OM mineralization and greater NH₄-N storage under strong reducing condition collectively drive NH₄-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.