Source and enrichment mechanism of ammonium in shallow confined aquifer in the west of Dongting Plain

Shallow confined aquifer is the main groundwater exploitation layer in the West of Dongting Plain, but it is faced with serious water shortage owing to worse water quality, among which ammonium anomaly is the most typical.However, its source and enrichment mechanism has been poorly understood at present.Taking the west of Dongting Plain as the study area, the hydrogeochemical analysis of groundwater samples along the direction of regional groundwater flow was carried out to find out the source of ammonium in groundwater and reveal the controlling mechanism of groundwater flow to the enrichment of ammonium.The results showed that the concentration of NH₄-N was 0.05~16.75 mg/L, and had good positive correlations with DOC, HCO₃^-, As, Fe²⁺, Mn and P, while the high concentration of NH₄-N corresponded to very low concentrations of Cl^-, SO₄^2-, NO₃^-, and very low Cl/Br ratio, it can be speculated that ammonium in shallow confined aquifer was produced by the mineralization of natural organic matter rather than anthropogenic input.Along the groundwater flow direction, the concentrations of NH₄-N, As, Fe²⁺ and Mn increased significantly, indicating that with the more sluggsih groundwater flowed, the particles of water-bearing media were becoming finer and finer, and the organic matter in sediments was more and more enriched, thus the mineralization of nitrogen-bearing organic matter was gradually enhanced and the concentration of NH₄-N increased gradually, forming a gradually reduced groundwater environment.As a result, the reductive components (arsenic, iron, manganese, etc) produced from related geochemical processes were also gradually enriched.The study further enriches the genetic theory of geogenic ammonium in groundwater and provides theoretical basis for the safety and security of local water supply.