Patterns and causes of ammonia nitrogen enrichment in groundwater of a typical agricultural irrigation area in the northern Henan Plain, China.

Groundwater ammonium (NH4+) enrichment in agricultural plains is often attributed to anthropogenic pollution, yet geogenic contributions remain poorly understood. This study investigates NH4+ distribution and genesis in the North Henan Plain, China, using hydrochemical, isotopic, and sediment analyses. Results show that NH4+ concentrations increase with depth in the unconfined aquifer, while confined aquifers exhibit localized enrichment within clay lenses. Dual-nitrate isotopes (δ15N-NO3 and δ18O-NO3) reveal significant anthropogenic nitrogen inputs that are effectively attenuated by denitrification under reducing conditions. Critically, δ15N-NH4 values (+ 1.3‰- + 7.2‰) provide strong evidence that NH4+ primarily originates from the mineralization of buried organic nitrogen rather than fertilizers. The correlation between Fe2+ and NH4+ further supports a coupled release mechanism during organic matter degradation. Sediment leaching experiments demonstrate that clay layers, characterized by high adsorption and bound water retention, act as primary "ammonium reservoirs". In contrast, sandy strata facilitate migration without significant accumulation. This research emphasizes that elevated NH4+ levels are predominantly shaped by specific sedimentary environments and lithological structures. The findings offer novel insights into the geogenic origins of nitrogen in agricultural alluvial-fluvial systems, highlighting the necessity of integrating geological controls into groundwater management frameworks.