Multiple isotopes reveal the driving mechanism of high NO3- level and key processes of nitrogen cycling in the lower reaches of Yellow River.

The continuous increase of nitrate (NO3-) level in rivers is a hot issue in the world. However, the driving mechanism of high NO3- level in large rivers is still lacking, which has limited the use of river water and increased the cost of water treatment. In this study, multiple isotopes and source resolution models are applied to identify the driving mechanism of high NO3- level and key processes of nitrogen cycling in the lower reaches of the Yellow River (LRYR). The major sources of NO3- were sewage and manure (SAM) in the low-flow season and soil nitrogen (SN) and chemical fertilizer (CF) in the high-flow season. Nitrification was the most key process of nitrogen cycling in the LRYR. However, in the biological removal processes, denitrification may not occur significantly. The temporal variation of contributions of NO3- sources were estimated by a source resolution model in the LRYR. The proportional contributions of SAM and CF to NO3- in the low-flow and high-flow season were 32.5%-52.3%, 44.2%-46.2% and 36.0%-40.8%, 54.9%-56.9%, respectively. The driving mechanisms of high NO3- level were unreasonable sewage discharge, intensity rainfall runoff, nitrification and lack of nitrate removal capacity. To control the NO3- concentration, targeted measures should be implemented to improve the capacity of sewage and wastewater treatment, increase the utilization efficiency of nitrogen fertilizer and construct ecological engineering. This study deepens the understanding of the driving mechanism of high nitrate level and provides a vital reference for nitrogen pollution control in rivers to other area of the world.

Spatial patterns in water quality and source apportionment in a typical cascade development river southwestern China using PMF modeling and multivariate statistical techniques.

River cascade development is one of the human activities that have the most significant impact on the water environment. However, the mechanism of cascade development affecting river hydrochemical components still needs to be further studied. In this study, water quality index(WQI), positive matrix factorization(PMF) model and multivariate statistical techniques were used to identify the mechanism of cascade development affecting river hydrochemical components in an typical cascade development Rivers, Lancang River, China. The results showed that the water quality of Lancang River is relatively good due to less affected by human activity. The spatial variation of river hydrochemistry is affected by the development of cascade reservoirs, and shows three patterns: irregular variation (pH and DO), fluctuating decreasing (Na+, Cl-, SO42- and HCO3-) and multi-peak variation (TN, TDN, NO3--N and NH4+-N). It's worth noting that the concentration of the most hydrochemical parameters is higher in the upper reaches (less human activities) than that in the middle and lower reaches of river due to the retention effect of the reservoir on the chemical composition. The PMF model outputs revealed that the rock weathering and internal source, sewage and soil nitrogen, and chemical fertilizer were primary material sources of Lancang River. Compared with the natural channel zone (41.0%), the interaction of water-rock has more influence on chemical component in the reservoir area (56.3%), while the contribution of fertilizer (11.2%) to the river hydrochemistry is less. The sites of downstream of the reservoir dam were affected by the retention of the reservoir and the disturbance of the bottom drainage, which leads to the weakening of the influence of the sewage (44.7%) on the river material and the increase of the contribution of fertilizer (25.0%). These results could provide valuable information in controlling the eutrophication of cascade reservoirs and the scientific construction of river cascade reservoirs.

Research Advances in the Analysis of Nitrate Pollution Sources in a Freshwater Environment Using δ15N-NO3- and δ18O-NO3.

Nitrate is usually the main pollution factor in the river water and groundwater environment because it has the characteristics of stable properties, high solubility and easy migration. In order to ensure the safety of water supply and effectively control nitrate pollution, it is very important to accurately identify the pollution sources of nitrate in freshwater environment. At present, as the most accurate source analysis method, isotope technology is widely used to identify the pollution sources of nitrate in water environment. However, the complexity of nitrate pollution sources and nitrogen migration and transformation in the water environment, coupled with the isotopic fractionation, has changed the nitrogen and oxygen isotopic values of nitrate in the initial water body, resulting in certain limitations in the application of this technology. This review systematically summarized the typical δ15N and δ18O-NO3- ranges of NO3- sources, described the progress in the application of isotope technique to identify nitrate pollution sources in water environment, analyzed the application of isotope technique in identifying the migration and transformation of nitrogen in water environment, and introduced the method of quantitative source apportionment. Lastly, we discussed the deficiency of isotope technique in nitrate pollution source identification and described the future development direction of the pollution source apportionment of nitrate in water environment.