As an important food production area in the north of China, the Hetao plain is extremely vulnerable to nitrate pollution caused by agricultural production activities and additional factors. Thus, it is of great significance for the environmental protection and rational use of groundwater to detect the current situation of groundwater nitrate pollution, temporal and spatial evolution characteristics, and main influencing factors in the Hetao irrigation district. We selected the Wualte irrigation area as the study area, and the characteristics of temporal and spatial changes in groundwater nitrate concentration and the main influencing factors in this area were explored. We used statistical analysis to analyze the nitrogen content of groundwater in the study area, and the Piper three-line diagram was used to explore the characteristics of chemical composition and evolution; furthermore, we used ion ratio and correlation analysis methods to explore the source of NO3--N in groundwater. The results showed that NO3--N was the main existing form of nitrogen in the Wulate irrigation area, and its concentration varied from 0.01 to 60.00 mg·L-1, with an exceeding standard rate of 10.50%. In terms of time, the characteristic of time change was that the NO3--N concentration in August of groundwater was the highest (average 6.61 mg·L-1), followed by that in October (6.22 mg·L-1) and November (6.25 mg·L-1), and that in March (average value of 1.77 mg·L-1) was the lowest. With the influence of rainfall and irrigation, the NO3--N in the soil was infiltrated into the groundwater, showing the characteristic that wet season and concentrated irrigation periods were higher than those in other periods. Spatially, it appeared as southwest (8.87 mg·L-1)>northwest (4.25 mg·L-1)>east (0.89 mg·L-1), mainly due to the original geological conditions, land use, and domestic waste stacking. In addition, the concentration of NO3--N of groundwater in the study area was closely related to the depth of groundwater and redox conditions but was relatively less affected by the concentration of water chemical ions. Therefore, identifying the temporal and spatial distribution characteristics and main sources of groundwater nitrogen pollution can provide a scientific basis for scientific fertilization, groundwater nitrate pollution control, and water safety.
Keywords: influencing factors; nitrate; shallow groundwater; temporal and spatial changes; water chemistry.