Nitrate Nitrogen Traceability in Coastal Watersheds Based on IsoSource Model
Nitrate nitrogen excess is known to result in decreased water quality,negative impacts on watershed ecosystems and human production and livelihood.In the southeastern coastal region,the issue of nitrate nitrogen pollution is of particular concern.Thus,a comprehensive investigation of nitrogen pollution in coastal watersheds is urgently needed.Such an investigation would elucidate the distribution pattern of nitrogen pollution,identify the main pollution sources,and quantify the contribution of these sources,all with the goal of protecting the watershed environment and promoting the sustainable development of coastal cities.This study focuses on the Baixi Basin,located in Southeast China,and uses the IsoSource mass conservation model to quantitatively analyze the distribution patterns and source of nitrogen pollution by combining long time series water quality monitoring data and dual isotopes of nitrate(δ15N and δ180)from 2016 to 2020.The results demonstrate that:① total nitrogen in the watershed primarily exists in the form of dissolved inorganic nitrogen,with an overall high trend observed in 2019.Nitrated nitrogen is identified as the primarily nitrogen pollution pattern in both the Shuiche national controlled section and the Tuaohu municipal control section,which are located in the main stream and tributaries,respectively.The total nitrogen concentration ranges from 1.46 mg/L to 1.83 mg/L in the Shuiche section and 2.12 mg/L to 2.36 mg/L in the Tuaohu section.② Nitrate nitrogen pollution in the watershed is mainly caused by the discharge of sewage and manure from villages along the watershed,which contributes 44％,followed by soil organic nitrogen,atmospheric deposition,and chemical synthetic fertilizer at 25％,16％,and 15％,respectively.③ Several strategies are proposed for mitigating nitrogen pollution in the watershed,including reducing discharges,improving crop nitrogen uptake rates,improving breeding strategies,and improving sewage treatment facilities.In addition,strict water resource management system should be fully implemented,and all resource development and utilization activities in the reservoir area should be further regulated to reduce the discharge of land-based pollutants and protect drinking water sources.
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