Distribution characteristics and genesis of nitrate in nested groundwater flow system in northern Ordos Basin

Under the situation of water shortage and increasingly serious groundwater nitrate pollution in Ordos Basin, it is helpful for rational utilization and protection of groundwater resources to identify the distribution and causes of nitrate in different groundwater flow systems in Ordos Basin, so this research was carried out in the typical groundwater-fed lakes area within the basin.Hydrochemistry and multivariate statistical analysis (i.e., hierarchical cluster analysis and principal component analysis) were combined to identify the nested groundwater flow systems.Based on this, the distribution characteristics of nitrate in different groundwater flow systems were compared and analyzed.And both hydrochemistry and environmental isotopes were considered to further identify the sources and potential evolution processes of nitrate in nested groundwater flow systems.The results show that the groundwater samples with nitrate content exceeding the groundwater quality standard (GB/T 14848-2017) were taken from the local and intermediate groundwater flow system, with an over-standard rate reached 28%, and the mean nitrate content in the regional groundwater flow system is about 1 mg/L.The distribution of nitrate in different groundwater flow systems was dominated by the extent of human activities, while the influence of evaporation enrichment and denitrification attenuation processes can be ignored.And within the nested groundwater flow systems, the local and intermediate ones were significantly affected by pollution from human activities, with inorganic ammonium fertilizer and manure and sewage the main pollution sources of nitrate, but the regional one has not been polluted, with mineralization of natural organic nitrogen as its nitrate source.