The Investigation Of The Changhe River Basin’s Shallow Groundwater And Surface Water For Nitrate Sources

Nitrogen is a key cause of water quality deterioration in the water environment.Since the 1950s,due to the increase of industrial and agricultural sewage discharge,anthropogenic nitrogen input has increased exponentially,leading to the increase of nitrogen load in the global water environment.Nitrate(NO3-),as the primary source of nitrogen in the aquatic environment,has become an important and growing pollutant due to its stability,high solubility and mobility.At the same time,it also causes a series of water environment problems,such as eutrophication,algae blooms,greenhouse gas emissions and water quality deterioration.Environmental problems caused by the increasing nitrate content in water environment have attracted global attention.The identification of nitrate sources and conversion is essential to protect water and eliminate nitrate pollution in river basins.In this study,the Changhe River basin of Jincheng City,Shanxi Province was taken as the research object.Samples were collected from the source of the Changhe River to the Qinkou of Qillong River in dry season,normal season and wet season respectively.Based on the spatial and temporal distribution characteristics of δD and δ18O of surface water and shallow groundwater,the recharge relationship between the two rivers in different periods was explored.On the basis of elucidating the composition characteristics of dissolved inorganic nitrogen(DIN)in the water environment of different periods in the long river basin,the spatiotemporal variation of DIN concentration and its influencing factors were clarified.Combined with water chemistry,stable isotope of nitrogen and oxygen,and SIAR model,the main nitrogen conversion modes were identified,and the main sources of nitrate in the water environment in different periods were determined and quantified.The effects of mixed land use on the temporal and spatial distribution of nitrate sources were also studied.The main research contents and conclusions arc as follows:(1)The surface water and shallow groundwater in the study area mainly come from atmospheric precipitation.The hydrogen and oxygen isotopes of both water show a characteristic of "enrichment during dry season compared to high season",and both dry and high season are affected by varying degrees of evaporation and water rock interaction.with δ18O was used as a tracer to further explore the relationship between surface water and shallow groundwater supply in the study area.During the two water periods from the source of Changhe River to the Chengzhuang Mine Sewage Plant,surface water supplied groundwater;The shallow groundwater and surface water in the section from the bottom of the Shuanghe River to the bottom of the Chuan River have an alternating replenishment phenomenon in seasons;From Getao Reservoir to Qilong Qinkou,shallow groundwater recharges surface water at different stages.(2)The DIN composition of surface water and shallow groundwater in the study area in different periods is dominated by NO3--N.The proportion of NO3--N in surface water presents a trend of flat water period(79.13%)>dry water period(75.81%)>high water period(53.35%),while the average proportion of NO3--N in shallow groundwater in three periods is as high as 97.89%,and there is no obvious seasonal difference.The NO3--N concentration of water in each period is surface water<shallow groundwater.(3)Nitrate in the study area mainly comes from the mixed pollution of chemical fertilizer,soil nitrogen,dung and sewage.The contribution rate of each nitrate source to the water body in different periods is MS>SN>CF>NP,and the contribution rate of soil nitrogen and fertilizer has increased significantly in the wet season due to agricultural activities and the increase of fertilizer application.Nitrogen migration and transformation in the basin are mainly nitrification in the water environment,while denitrification and assimilation have no significant impact on nitrate isotope composition in the study area.(4)The results of land use and nitrate traceability at each sampling point in the study area show that,in terms of time,compared with the dry season,with the increase of agricultural activities and fertilizer use,the contribution rate of soil nitrogen and fertilizer increased significantly in the wet season,so land use has a significant impact on the distribution of nitrate pollution sources in the study area;In space,according to the correlation between LUI and NO3source proportion contribution,the spatial impact of land use on the distribution of nitrate pollution sources in the Changhe River Basin is not significant,which is mainly due to the interference of a large number of external nitrogen sources,resulting in the mixing of nitrate from different sources.