Research On The Effect Of Natural Organic Carbon On The Transport And Transformation Of Nitrogen In The Hyporheic Zone

In recent years,for the intensification of the human activities,the contamination of nitrogen in surface water and groundwater has been more serious.The hyporheic zone is water-saturated sediment layer,and it locates under the river bed and extends to streamside zone.It plays an important role in the river ecosystem.More and more attention has been paid to the linkages between C and N cycles now.Therefore,studying on the theory of transportation-transformation of nitrogen in the hyporheic zone and the effect of organic carbon on N cycles can provide scientific basis for the development of hyporheic zone,as well as the nitrogen pollution control and remediation in surface water and groundwater.On the basis of field investigation,sampling and testing analysis,this paper designed indoor soil-column experiments to simulate the interactions between surface water and groundwater through the hyporheic zone.In this process,the effects of different kinds of natural organic carbon on the transportation and transformation of nitrogen were studied.We used the hydrogeochemical simulation software PHREEQC to establish 1D reactive-transport models of nitrogen in the hyporheic zone,and the experiment results were fitted.In this paper,the main research results are as follows:1.The results of the simulation experiment of surface water and groundwater interactions show that:1)Natural organic carbon additions,as well as the process of groundwater recharge to surface water can promote the removal of nitrate nitrogen.The main mechanism of nitrate removal is denitrification,and it's also accompanied by the process of dissimilatory nitrate reduction to ammonium.This process may lead to the pollution of ammonia nitrogen in groundwater,requiring for further study to weaken it.2)The addition of natural organic carbon results in rapid decrease of pH and DO concentration and rapid increase of DOC concentration,in this way promotes denitrification and dissimilatory nitrate reduction to ammonium,and ultimately affects N cycle in the hyporheic zone.The effect is mainly influenced by DO,DOC,followed by pH,and being enhanced with the increase of organic carbon content.The removal rate of nitrate nitrogen in Litter leaf group reaches up to 70%,followed by mixed group of about 50%,and sludge group of about 20%.In the experiment,the most suitable reaction conditions are as follows: pH = 6.5 ~ 7.5,DO < 4 mg/L and DOC concentration is relatively high.3)The release efficiency of DOC in natural organic carbon sources gradually slows down over time.Eventually,sediment layers are no longer as carbon sources.Litter leaves present stronger release of DOC and consumption of pH,DO more quickly,and the effect of organic matter in sludge is slow and obviously lagging.The promotion effect of sludge sediment on the process of dissimilatory nitrate reduction is more intense than that of denitrification.4)In the process of groundwater recharge to surface water,mixed interactions between them reduce DO content,thereby promoting the removal of nitrate nitrogen in the hyporheic zone.The effect is influenced by NO3-and DO concentration,organic carbon quantity and quality(C/N)together: Under the condition of sufficient organic carbon,the interactions between surface water and groundwater can further promote the consumption of the DO and DOC release in the sediment layer again,and adjust pH to appropriate reaction condition.2.The simulation results of the reactive-transport of nitrate nitrogen show that:1)In the process of leaching,natural organic carbon affects soil properties in the hyporheic zone,such as organic carbon content,bulk density,porosity,and so on,leading to the changes of convection and diffusion.It mainly shows by the significant increase in the average pore water velocity(u)and hydrodynamic dispersion coefficient(D).2)The reactive-transport of nitrate nitrogen is obviously influenced by the addition of natural organic carbon.At the initial stage of the experiment,nitrate nitrogen content in the hyporheic zone is low,and the denitrification process is weak.As time goes on,the organic carbon sources release DOC constantly,moving downward with the flow,as well as NO3-of high concentration in surface water.As a result,the denitrification rate is strengthen rapidly in days 6-8.