Study On Permeation And Migration Law Of Ammonia Nitrogen In The Aquitard

In recent years, it has been gradually recognized that the clay layers as impermeable layers in the groundwater system actually include some weak impermeable layers(or aquitards). When leakage recharge occurs, the adjacent aquifers exchange a large amount of water. Shortage of water resources and deteriorating water quality of shallaw groundwater cause large-scale exploitation of deep groundwater. In the present, over-exploitation of groundwater development has been in a serious condition, the deep confined water level sharp decline. When the confined water level is lower than the groundwater level, the phreatic water will penetrates through the aquitard to supply the confined water. The pollutants in the unconfined aquifer will seeps into the confined aquifer, resulting in deep groundwater pollution. Therefore, the permeability of aquitard and the migration law of contaminants in the aquitard have become impartant research issues in groundwater pollution control and management.On the basis of consulting a large number of domestic and foreign literature, this paper combines theory study and simulation experiment in laboratory to analyse the transfer forms, the attenuation rule, the migration and transformation of ammonia nitrogen, the most common groundwater pollutant, as permeating through the aquitard, as well as the influence of different pH, water hardness and redox conditions with this process. Based on the study, mathematics model is applied to simulate transportation of ammonia nitrogen. This study provides theoretical support for the reveal of nitrogen pollution rules in the deep groundwater and the exploration on restoration of groundwater pollution.In this paper, experimental research can be divided into three parts:the static adsorption and dynamic adsorption experiments, the infiltration and migration experiments in the high-pressure soil column.Static and dynamic adsorption experiments focus on the static and dynamic adsorption processes and the factors of the ammonia nitrogen in the clay (layer). adsorption characteristics in the tested clay of ammonia nitrogen is summarized, and the nitrogen adsorption kinetic equation and dynamic adsorption model is constructed. The results can be summarized as follows:(1) Clay can strongly adsorb ammonia nitrogen, it generally tends to the equilibrium in about 120min, and the equilibrium adsorption rate is more than 70%. According to the experimental data, ammonia nitrogen adsorption kinetics equation is established. The goodness of fit between the fitting curve of the equation and the measured curve is high. In addition, each parameter in the equation shows clear physical meaning. Compared with the classical kinetic equation, the new equation showed some advantages. (2) The adsorption of clay for ammonia nitrogen is consistent with Langmuir and Freundlich adsorption isotherms, it shall be the monolayer adsorption. But when the ammonia nitrogen concentration is low(c<15mg/L), nitrogen adsorption isotherm is more in line with Henry. Clay adsorption of ammonia nitrogen relies mainly on molecular attraction and the ionic bond strength. (3) Aquitard shows strong blocking ability on ammonia nitrogen, because of the strong adsorption ability of clay, as well as the special aquitard pore structure. The permeation of ammonia nitrogen accompanies with the process of bound water transforming to free water, so that contact of ammonia nitrogen and clay particles is more directly and fully. On the basis of experimental data, dynamic adsorption model is established using Origin software fitting model. Model formula can be expressed as: The regression equation fitted using origin software shows high precision, and can well reflects the dynamic adsorption process of ammonia nitrogen solution in trial clay layer.The infiltration and migration experiments in the high-pressure soil column mainly analyzes the aquitard permeability characteristics, the transfer forms, attenuation rule, migration and transformation of ammonia nitrogen as permeating through the aquitard, as well as the influence of different pH, water hardness and redox conditions with this process. The results can be summarized as follows:(1) The process of groundwater permeating through the aquitard is mainly to overcome the compressive resistance of bound water and transform bound water into free water, the permeability is weak. Under the condition of exerting great external pressure, segmental bound water in the micro-pore channel of clay is transformed into free water, osmotic water flow tends to steady-going, the permeability law conforms to Darcy's law. The aquitard permeability in this study is 6.27×10-7cm/s. (2) After penetrating through the aquitard, ammonia nitrogen removal rate is up to 98%. Aquitard pollutant removal mechanism is different from the upper water layer, the ammonia nitrogen removal mainly depends on adsorption. Therefore, in the weak permeable layer, ammonia nitrogen is not easily transformed into other forms of nitrogen, NH4+ is the chief form. (3) pH conditions of infiltration solution have a certain influence on the removal of nitrogen pollution in the clay layers. Nitrification is inhibited due to the reduction environment of aquitard and the buffer action of solid material in clay, pH major influences the adsorption of ammonia nitrogen. (4) Redox condition mainly affects the occurrence of nitrification. Because of weak biological action in the anoxic aquitard, solution redox conditions show little effect on the ammonia nitrogen migration. (5) The strong ammonia nitrogen adsorption is a result of common action of surface adsorption and ion exchange adsorption, and the surface adsorption is the dominant factor. Ion exchange is an unstable reversible reaction, so it can not remove ammonia nitrogen completely. water hardness or other exchangeable ions would affect the breakthrough time and adsorption stability of ammonia nitrogen in aquitard. (6) Based on the experimental data, parameters of model can be calculated. Solute transportation model is established to simulate concentration change of ammonia nitrogen by mathematical model transform. The simulation result fits the experimental result well, it can predict the ammonia nitrogen change in the aquitard to a certain extent.