Characteristics and potential sources of nitrate pollution in surface water and groundwater systems in Taizihe River Basin

As one of the main sources of drinking water and important irrigation water for agriculture, groundwater quality influences not only human health but also food security and ecological sustainability. This study attempted to lay the scientific basis for understanding nitrate pollution and groundwater quality evaluation. The study analyzed the composition of ions and concentration of nitrate in surface water and groundwater systems in Taizihe River Basin in the Northeast China. To assess nitrate pollution and identify the potential sources, surface water and groundwater samples were collected in Taizihe River Basin and the ions composition characteristics and nitrate isotopes in the groundwater samples analyzed. The results showed that nitrate was the dominant form of nitrogen pollution, accounting for 78.38% of total nitrogen concentration, with concentration range of 0.75-6.40 mg·L^-1 in surface water systems in Taizihe River Basin. The NO₂^--N was 0.78% of total nitrogen, with concentration range of 0-0.93 mg·L^-1 in surface water respectively. The concentration of NO₃^--N increased from upstream to midstream regions in the basin and the highest value (6.40 mg·L^-1) occurred at site S6, from where it declined gradually along the course of the river. However, NO₂^--N showed less change along the river. Because of extensive use of chemical fertilizers and intensive mineralization of organic nitrogen, the concentration of Cl^- and NH₄⁺ increased in the downstream regions. The concentration of NO₃^- in groundwater was generally higher than that in surface water systems. The average concentration of NO₃^- in groundwater was 20.26 mg·L^-1, suggesting that a severe NO₃^- pollution existed in the groundwater systems in Taizi River Basin. A wide range of studies has used isotropy to trace nitrate pollution sources in aquatic environment. Analysis of environmental isotopes in the study area showed that δ¹⁵N was in the range of -0.74‰-13.27‰ in surface water systems in Taizihe River Basin. Based on the results of nitrate isotope analysis, it was reasonable to conclude that NO₃^- in surface water systems in the upstream regions was mainly from mineralization of soil organic nitrogen. Agricultural fertilizers and livestock manure were the main pollution sources of NO₃^- in surface water systems in the middle and downstream regions of the basin. Higher δ¹⁵N values (5.7‰-17.5‰) were noted in groundwater system in the basin, indicating that NO₃^- in groundwater of Taizihe River Basin was mainly driven by human/animal feces and leakage of agricultural fertilizers.