Numerical simulation of nitrogen transportation and transformation in fully coupled unsaturated-saturated zones
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Abstract:
The Shandong Peninsula has become one of the most serious nitrate pollution region in China.Excess fertilization in the surface is a main reason of groundwater nitrate pollution.Because of the complex transportation and transformation processes of nitrogen in the unsaturated zone and saturated zone,numerical models can be served as useful tool to understand the influences of fertilization on groundwater pollution.The studies of fully coupled unsaturatedsaturated model with reactions is lacked at present. A three-dimensional fully coupled unsaturated-saturated numerical model of groundwater dynamic and solute transport was established.The reaction processes of nitrogen mineralization,nitrification and denitrification were added in the model,and COMSOL was used to solve the model.The model was applied to the Xiaogu River area of Shandong Peninsula,and verified by the monitoring data of groundwater level and nitrogen concentrations.Several scenarios of different pumping irrigation amounts,fertilizing amounts and crop types were simulated to analyze their influences on distribution of nitrogen in groundwater.The long-term enrichment process of nitrogen in groundwater was also predicted by the model. The results showed that the fully coupled unsaturated-saturated model could accurately simulate the groundwater dynamics and solute transportationtransformation process.Under normal irrigation and fertilization conditions,the water level changes were closely related to seasonal changes and agriculture activities,and the concentrations of nitrate and DON in groundwater were closely related to the fertilization time.When the pumping irrigation amount decreased 10% and 20%,the water level increased 0.04 m and 0.06 m on average,and the nitrate mass concentration decreased 2.17 mg/L and 2.98 mg/L,and the ammonia nitrogen mass concentration decreased 0.02 mg/L and 0.03 mg/L,respectively.When the fertilization increased 25% and decreased 25%,nitrate mass concentration in groundwater increased 2.60 mg/L and decreased 3.16 mg/L,and the ammonia nitrogen mass concentration increased 0.02 mg/L and decreased 0.02 mg/L,respectively.There was no linear relationship between the increase of nitrate mass concentration and the increase of the fertilization.When the summer cornwinter wheat rotation farmland was changed to vegetable farmland,the nitrate mass concentration increased 23.93 mg/L,and the ammonia nitrogen mass concentration increased 0.29 mg/L and a large amount of DON remained in the soil layer.Under long term normal fertilization conditions,the nitrate mass concentration gradually increased,the average increase in 5 years was 13.64 mg/L.On the contrary,under long term no fertilization conditions,the nitrate mass concentration decreased 33.48 mg/L in 5 years.Without the input of agricultural pollution sources,the nitrate mass concentrations of each region were lower than that of normal fertilization scenarios,especially in the southeast region. It was showed that the pumping irrigation and fertilization influence the groundwater nitrate pollution,which choud provide valuable scientific reference for groundwater resource managements and water pollution control.In future studies,the model can be applied to different regions and more farmland management measures can be simulated to show their influences on groundwater nitrogen pollution.
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