Distribution characteristics and sources of nitrate in the unsaturated zone and groundwater of farmlands in an area irrigated with reclaimed water

The present study aimed to presenting the source and environmental behavior of nitrate in various environmental media by sampling and analyzing the soil in the unsaturated zone, surface water, and groundwater of a typical area irrigated with reclaimed water in the North China Plain. This study also identified the sources of groundwater nitrate pollution in the area, and determined the effects of different irrigation conditions on nitrate migration in the soil in the unsaturated zone. In the Xiao River basin, which is seriously affected by reclaimed urban water, nitrate concentrations in groundwater ranged from 4.0 mg·L^-1 to 156.6 mg·L^-1, nitrate in the groundwater had formed a high-value area of 2 km from the river channel and 70 m in depth. And nitrate moved down 1-2 m every year. The correlation between nitrate and chloride ion indicated that reclaimed urban water was the main source of nitrate in the unsaturated zone, surface water, and groundwater of the area irrigated with reclaimed water. Geoprobe system was used to obtain continuous soil core samples from the typical unsaturated zones to study the influence of reclaimed water on the vertical distribution of NO₃^--N in thick unsaturated zone. The average contents of NO₃^--N in the reclaimed water and groundwater irrigation areas were 137.0 mg·kg^-1 and 107.7 mg·kg^-1, with the peak values of 523.2 mg·kg^-1 at a soil depth of 1.20 m and 725.9 mg·kg^-1 at a depth of 0.85 m, respectively. The correlation analysis between soil nitrate and chloride ion indicated that the NO₃^--N in the reclaimed water irrigation area was mainly affected by reclaimed urban water, whereas in the groundwater irrigation area it may because the use of agricultural nitrogen fertilizers. By comprehensively analyzing the relationship between groundwater age and nitric acid, the historical process of the nitrogen source causing the groundwater nitrate pollution could be reproduced on a time scale. Under the unique hydrogeological background of the North China Plain, the nonpoint source pollution of the farmland has a limited effect on groundwater pollution, while the risk of groundwater nitrate nitrogen pollution is relatively high around the reclaimed water river.