Distribution And Migration Rules Of Nitrate In Vadose Zone In Shulehe River Basin And Its Effects On The Groundwater

Vadose zone is the key to connecting with the atmosphere and the aquifer, the spatial distribution characteristics and migration accumulation rule of nitrate in soil are essential to prevent nitrate pollution in groundwater. Exploring the nitrate correlation between soil and water has very important theoretical and realistic significance. By measuring the concentrations of Cland NO₃^- in seven typical soils, precipitation and groundwater collected from drought Shulehe river basin in northwest China, this study explored the spatial distribution characteristics and migration transformation rule of Cl^- and NO₃^-, judged the enrichment degree and accumulation characteristics of NO₃^-, analyzed the source of nitrate in different types soils and the factors that affect its vertical migration, explored the influence of soil nitrogen leaching to groundwater, and put forward some pollution prevention measures. The results indicated that:（1） Cl^- and NO₃^- mainly exist in 0-60 cm soil near the surface, the distribution characteristics of NO₃^-were similar to that of Cl^-, majority are "S" shape distribution, and both of them are well correlated with the soil moisture. With the increase in the depth of the unsaturated zone, the concentration of NO₃^- presents a decreasing trend of volatility. The intense evapotranspiration and microbial nitrification make the NO₃^-content to be relatively high in the soil surface, because NO₃^-is easily to be leached in sand soil, while water and nutrition uptake by plant roots reduces the concentration of NO₃^- in the root zone. This process causes some differences in the vertical distributions between NO₃^-and Cl^-. The concentration of NO₃^-showed significant differences for different soils with a decreasing order of saline > forest > farmland > grassland > wetland >wasteland > gobi.（2） The atmospheric precipitation is the main source of vadose nitrate in arid areas, the lower level of Cl^- and high level of NO₃^-/Cl^- in gobi and desert indicated that nitrate enrichment mainly occurs in the arid phase and nitrogen fixation of cyanobacteria is active. However, nitrate enrichment in farmland soil was significantly affected by anthropogenic activities, such as a large amount of nitrogen fertilizer and sewage irrigation. Nitrogen enrichment degree is not high inforest land, grassland and wetland soil. It may be due to the absorption of vegetation, both nitrification and denitrification are more severe, while nitrate in saline soil is mainly comes from the soil salinity caused by natural or man-made. Nitrogen mainly accumulated in the soil surface,the nitrate cumulative amount in 0-40 cm soil is accounting for above 80% of the total cumulative amount, it is closely related to soil properties and vegetation coverage.（3） NO₃^-has the wavy migration in the soil profile, it is the result of elution and thermal drive. Under natural conditions, relatively low pH with higher TDS and EC, the larger soil porosity and higher moisture and TOC content are conducive to vertical migration of NO₃^-. NO₃^-and Cl^- have the piston migration in the desert, both significant positive correlation and negative correlation with the water content. Short rains and strong evaporation lot nitrate enriched in the surface. The high content of deep soil shows the suction effect of nitrate leaching. The high NO₃^-/Cl^- descript that nitrate is high enriched during droughts.（4） The spatial distribution of the groundwater nitrate is consistent with the terrain structure,content increased from upstream to downstream. NO₃^- average concentration is 14.94 mg·L-1, and more than drinking water nitrate maximum allowable value 10 mg·L-1 provisionsed by WHO. The groundwater nitrate content overweight and exceeds bid badly accounted for 37% in all samples, it is seriously polluted. The groundwater nitrate content under different land types was positively correlated with the content of the soil, and increased with the increase of fertilizer rate,groundwater level, irrigation times, population density, and associated with fertilizer species, crop species and farming practiceat the same time. The groundwater nitrate pollution is particularly serious in farmland soil, excessive rate was 23.08%, and the seriously overweight rate was 53.85%.It should be carried out through the reasonable application of nitrogen fertilizer, use water-saving irrigation, adjust industrial restruction, strengthen pollution remediation and so on, thus comprehensive preventing soil and groundwater nitrate contamination.