Characterizing nitrogen distribution, source and transformation in groundwater of ecotone of agriculture–animal husbandry: an example from North China

The ecological environment of the ecotone of agriculture–animal husbandry in North China is fragile and the groundwater resources are in short supply, in particular, nitrogen contamination is serious. So, the nitrogen source, distribution and transformation in groundwater need to be investigated. A case study was conducted in Tuoketuo County, Inner Mongolia. Hydro-geological, hydro-geochemical, and quality monitoring of groundwater were investigated in the study area. Altogether, 66 groundwater samples were taken with an equal number from both the unconfined and confined aquifers in the study area. The results show that the ammonium compound (NH₄⁺) was the most prevalent species of nitrogen contamination. Both the irrigations of farmland with contaminated surface water and the excessive usage of chemical fertilizers were probably two major non-point sources of nitrogen contamination. The large-scale dairy farm effluents also play an important role in nitrogen contamination in local areas of an unconfined aquifer. The nitrogen contamination in the confined aquifer was mainly due to the leakage recharge of the contaminated unconfined water. The mean concentrations of NO₃⁻, NO₂⁻ and NH₄⁺ in confined aquifer were significantly lower and the areas exceeding the water quality standard were similar but smaller than those in an unconfined aquifer. The spatial distributions, migration and transformation of ammonium (NH₄⁺), nitrate (NO₃⁻) and nitrite (NO₂⁻) in the investigation area were closely related to the characteristics of the groundwater flow system and the hydro-geochemical environment. In the groundwater recharge area, nitrification plays a leading role, with NO₃⁻ as the major contaminant. Along the groundwater discharge zone, the denitrification, anammox and dissimilatory nitrate reduction to ammonium (NDRA) processes may occur, causing NH₄⁺ to become the main contaminant. The Cl^– contamination was also severe both in unconfined and confined aquifers. High concentrations of NO₃⁻ and NH₄⁺ were mainly observed in the groundwater of Ca·Mg–Cl and Na–Cl types. The background type of water for unconfined and confined aquifers was mainly Ca–HCO₃. This study will provide helpful information to identify the most contaminated locations in the study area for environmental management in future and provides a reference for preserving the environment of ecologically fragile areas.