Sources and transformation of nitrogen in shallow aquifers with progressive water table recovery using geochemical and isotopic approaches

Groundwater level recovery has a significant effect on the sources and transformation of nitrogen in groundwater, but there are still few studies on the influences of the water table on the sources and transformation of nitrogen in groundwater using field data. In this study, the changes in groundwater level, geochemical composition, and isotopic signatures of NO 3 - and NH 4 + during a period of groundwater level recovery at a pilot site were analyzed in detail. The water table underwent progressive recovery of almost 1.6 m in 16 months. At a depth of 5.5 m below the surface, both low NH 4 + and high NO 3 - appeared in the groundwater, whereas below that depth, both high NH 4 + and low NO 3 - simultaneously appeared in the groundwater. The main sources of NO 3 - were manure and septic waste, and NH 4 + fertilizers. The main sources of NH 4 + were mineral fertilizers. The main transformation process were nitrification and denitrification at a depth of 5.5 m below the surface; below that depth, the main transformation processes were denitrification and dissimilatory nitrate reduction to ammonium (DNRA). This study provides a theoretical understanding of the relationship between the changing water table and nitrogen in groundwater level recovery areas. Graphical abstract