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Dynamic Variations of Nitrogen in Groundwater under Influence of Seasonal Hydrological Condition in Typical Area of Jianghan Plain

Shen Shuai;Ma Teng;Du Yao;Deng Yamin;Yu Haotian;Luo Kewen;School of Environmental Studies,China University of Geosciences;State Key Laboratory of Biogeology and Environmental Geology,China University of Geosciences;Geological Survey,China University of Geosciences;  
The pollution of NO_3~--N and NH_3-N in groundwater is very serious in the eastern of Jianghan plain,which is characterized by strong interactions between groundwater and surface water and obvious variations of seasonal hydrological condition.Sediment samples along vertical profiles of three boreholes and groundwater samples with different depths in different seasons were sampled to analyze the nitrogen speciation coupled with supporting chemistry in a typical field site.The results show that NO_3~--N with concentrations from 25.8to 119.48mg/kg is a dominant speciation of the total exchangeable nitrogen in sediments.The NH_3-N concentrations have a good positive correlation with TOC and TN contents in sediments,implying NH_3-N was mainly influenced by organic matter.The concentration and speciation of nitrogen in the groundwater deeper than 2m show a significant response to the fluctuation of groundwater level.During the monsoon season,groundwater level increased,enhancing the reducing potential of aquifer media.As a result,NO_3~--N was not liable to be produced and was more likely to be con-sumed by denitrification.At the same time,with the mineralization enhanced,NH_3-N was gradually becoming the main speciation of dissolved inorganic nitrogen.After the monsoon season,groundwater level declined,enhancing the oxidizing potential of aquifer media.As a result,nitrification was enhanced and denitrification was limited,and the concentration of NO_3~--N increased.Anthropogenic activities near the surface had a significant effect on the concentration and speciation of nitrogen in the groundwater shallower than 2 m.In summary,under the coupled influence of natural and anthropogenic factors,seasonal hydrological change resulted in the variation of redox potential,which was the main trigger for seasonal variation of nitrogen concentration and speciation in the groundwater.
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