Study On Nitrogen Migration And Transformation In Groundwater Fluctuation Zone

Groundwater is a valuable water resource.However,the rapid develop of economy and society has made nitrogen pollution a serious problem in nowadays.The groundwater level shows obvious fluctuation characteristics due to the changeable natural conditions and various human activities.For example,the interface among vadose zone,saturated zone and capillary zone are constantly changing,which makes the hydrodynamic conditions,physical and chemical processes,biological effected.In doing so,several environmental factors may change with the fluctuation of water level,and thereby affect the migration and transformation process of nitrogen.Through field sampling,laboratory experiments,theoretical analysis and numerical simulation,this paper firstly analyzed the characteristics and equations of nitrogen adsorption/ desorption in different media,and then studied the distribution characteristics of nitrogen in vadose zone and saturated zone under groundwater level fluctuation scenario,After that,the mechanism of nitrogen migration and transformation in groundwater level fluctuation zone was revealed,and a prediction of the temporal and spatial distribution of nitrogen under different fluctuation scenarios was made in the final part of this dissertation.The main research contents and achievements are listed as follows(1)The kinetic experiments show that: within a certain range of time,the adsorption capacity of soil for ammonium nitrogen increases gradually as the adsorption time extends.The adsorption rate of three media for ammonium nitrogen is as follows: Coarse Sand >medium sand > fine sand,and the adsorption capacity of ammonium nitrogen is as follows:fine Sand > medium sand > coarse sand.The date shows a positive relation between the desorption time and the desorption amount of ammonium nitrogen,but a negative relationship between the desorption time and the growth rate of desorption amount.By and large,the desorption rates of ammonium nitrogen in three kinds of media were almost the same,and reached equilibrium after 6 h.The desorption amount of ammonium nitrogen was fine sand >medium sand > coarse sand.The thermodynamic experiments show that: the adsorption capacity of soil for ammonium nitrogen increases linearly with the increase of liquid ammonium nitrogen concentration,and the adsorption capacity of Fine Sand > medium sand >coarse sand,but the overall difference is not obvious;the desorption capacity of soil for ammonium nitrogen enlarges gradually as the desorption time increases,and also shows a linear growth trend.In general,the desorption rate of three kinds of media of sand for ammonium nitrogen increase.The comparision of different desorption capacity is described as follows:fine sand > medium sand > coarse sand.(2)Applying origin Pro to test the data gathered from kinetic experiments and thermodynamic experiments,and the results showed that the kinetic experiments' results were in better agreement with the nonlinear second-order kinetic model,indicating that the adsorption of ammonium nitrogen on three media was mainly chemical adsorption;the thermodynamic experiment results were in better agreement with Henry model,and the adsorption / desorption capacity of ammonium nitrogen on three media was: Fine Sand >medium Sand > coarse sand.(3)The soil column experiment of nitrogen migration and transformation in groundwater fluctuation zone was carried out in this session.The results showed that: during the rising stage of water level,the concentration of nitrate nitrogen decreased as a whole and the concentration of ammonium nitrogen increased as a whole;When the water level decreased,the concentration of nitrate nitrogen increased as a whole and the concentration of ammonium nitrogen decreased as a whole,but the concentration of ammonium nitrogen decreased as a whole.The fluctuation of groundwater level can significantly affect the migration and transformation of nitrogen in soil and groundwater,which is conducive to the vertical migration of nitrogen and the aggravation of nitrate pollution in groundwater.But on the other side,it could be beneficial to the removal of ammonium nitrogen in groundwater.Therefore,its impact on nitrate pollution in groundwater environment can not be ignored.(4)HYDRUS-1D is used to simulate the trace of nitrite nitrogen,nitrate nitrogen and nitrite nitrogen in three media as well as to establish the model of three nitrogen's movement in groundwater wave zone The results showed that each model had a high correlation,so HYDRUS-1D software could be used to simulate and predict the migration and transformation of nitrogen in soil column experiment.(5)Four scenarios of water level fluctuation were simulated? Besides,the migration and transformation of nitrogen with water level were predicted.The results show that the fluctuation of water level has a great influence on the variation range of nitrogen;the increase of water level fluctuation can effectively increase the fluctuation range of nitrogen concentration;the influence on the three media is different,the concentration fluctuation range of the three media is coarse sand > medium sand > fine sand.Moreover,the influence on the three solutes is also different,the fluctuation of nitrate nitrogen has the greatest influence,and the fluctuation of nitrite nitrogen and ammonium nitrogen has the greatest influence Smaller.In the three media,the fluctuation range of water level fluctuation is coarse sand > medium sand > fine sand;in the three media,the fluctuation range of water level fluctuation has the greatest influence on the concentration fluctuation of nitrate nitrogen,and has less influence on the fluctuation of nitrite nitrogen and ammonium nitrogen.In short,The difference is most obvious in fine sand,followed by medium sand,and in coarse sand The difference is not significant.The change of initial concentration had little effect on the fluctuation range of nitrogen,which was basically the same as that of fluctuation experiment,but the whole simulated concentration changed greatly.As the initial concentration increases,the concentrations of coarse sand and medium sand increase by the same multiple of the initial concentration,while the concentrations of fine sand increase by 1.8 times of the initial concentration;increasing the initial concentration has the greatest impact on the solute concentration of nitrate nitrogen,followed by ammonium nitrogen,and the smallest impact on nitrite nitrogen.When the initial concentration is reduced,the coarse sand and medium sand concentrations are reduced by the same multiple of the initial concentration,while the fine sand concentration is reduced to 1/6of the original;reducing the initial concentration has the greatest impact on the solute concentration of nitrate nitrogen,while the ammonium nitrogen and nitrite nitrogen have little impact.In conclusion,water level,as an important factor of hydrological mechanism,plays an important role in the groundwater system.In the process of nitrogen migration and transformation,the influence of groundwater level and fluctuation intensity on its migration and transformation can not be ignored.The change of water level will affect soil water content,water movement state,soil physical and chemical properties and microbial state,and then affect the migration and transformation of nitrogen in soil.If the water level is too high,the dissolved oxygen content and redox ability will be affected,the nitrification will be weakened and the denitrification will be enhanced;When the water level was too low,the dissolved oxygen content increased,the nitrification increased and the denitrification weakened.At the same time,the fluctuation intensity of water level also affected the nitrogen migration and transformation,and the fluctuation range of nitrogen concentration was significantly expanded with the increase of water level fluctuation intensity;The fluctuation range of nitrogen concentration decreased significantly with the decrease of nitrogen intensity.