The Nitrogen Loads and Its Distribution of Shallow Aquifer for Groundwater-Dominated Agricultural and Pastoral Transition Region in Inland River Basin

Excessive nitrate loads from intensive agricultural and pastoral production have been led to groundwater pollution in many groundwater-dominated urban and rural in North China, especially in semi-arid grassland. Hydro-geological units and watershed-based nutrient reduction planning have been proposed for the impacted grassland ecological system in Inland River Basin all around the Inner Mongolia, in China. Taking Damao County as typical area, an integrated modeling approach was developed to simulate and predict the nitrate loads and the temporal and spatial distribution characteristic of nitrate concentration in the different representative sub-units during 2013–2016, then the concentration of nitrogen is estimated that accounts for long-term nitrate storage in the aquifer and losses from the recharge to the discharge zones. The calibration and verification statistics and sensitivity analysis show that the model can provide accurate nitrate concentration predictions for watersheds in semi-arid grassland that is a suitable tool for controlling the nitrate emission. The result shows that annual nitrate losses, including aquifer long-term storage, transport, biological uptake, and discharge were estimated to be 0.65 mg N L^-1, corresponding to 3.00 kg N ha^-1. The total nitrate loads from unsaturated zone to the shallow aquifer were predicted to an optimal emission concentration range between 0.35 mg N L^-1 and 1.29 mg N L^-1 under varies nutrient reduction planning in Damao grassland.