The Numerical Simulation Of Nitrogen Migration And Transformation In Heterogeneous Vadose Zone

Nitrogen pollution of groundwater has become a popular problem in the world. The soiltexture, structure and thickness decide the unsaturated zone pollution prevention ability andgroundwater contamination possibility and degree. The study based on lab experiment and tookthe heterogeneous unsaturated zone composed by sand and loam lenses as object, usingHYDRUS-2D to study the process of moisture migration and nitrogen migration, transformationand accumulation in the sandbox. Then simulated results were compared with the test data toidentification and authentication the model. Finally, a solute transport model of unsaturatedzone profile of Beijing plain area was established using the calibrated parameters.What’s more,the process of nitrification and denitrification in20years were simulated.The main resultswere as follows:(1). Mean square error between simulated results and experiment results of soil moistureand the concentration of NH4-N,NO2-N and NO3-N are0.01、0.01、0.014、0.02, which showedthat the simulation result were accurate and reliable. The model can reflect changes of flowfield with time and space in vadose zone from a mathematical perspective.(2). The phenomenon of flow around exists in heterogeneous vadose zone. It can bespeculated that under natural conditions, heterogeneous vadose zone contribute to the rapidpenetration of water and pollutants by a preferential flow and cause pollution of deeperunsaturated zone and groundwater pollution.(3). The simulated results of ammonia intermittent pollution showed that: heterogeneousvadose zone media has obvious affects on nitrogen migration and distribution.Nitrateaccumulate easier on the upper soil layer than on the lower soil layer，but ammonia and nitritehave opposite results。ammonia and nitrite also are likely to accumulate on the surface of theclay lenses above and blow near ground surface.Nitrogen transport and cumulativedistribution changes with the position of the lens.(4). The study identified and validated the migration and transformation parameters ofnitrogen in the model established based on laboratory test. Then it applied the parameters to thenumerical model of a typical vadose zone profile in Beijing.The prediciton results of the modelwere reasonable and accurate,so that the conclusions can be used in fieldscale.The forecast results and parameters have very important significance in studing nitrogenmigration and transformation prcess.