Analysis and modeling of watershed-scale nitrogen balance in relation to nitrogen pollution of unconfined groundwater in an alluvial fan

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Field measurements on discharge and water quality of spring water were carried out for accumulation of necessary data. Amounts and processes of effluent nitrogen load from the different pollution sources were also investigated. As the results of nitrogen balance analysis, it was clarified that more than half of the total nitrogen load of the basin came from dairy farming and massive nitrogen from live stock excreta was stored on/in the soil and intruded into the groundwater together with rainwater infiltration. The fortunate fact that despite the high pollutant inflow, the nitrate nitrogen concentration does not reach 10mg/L, the environmental benchmark value, was found to be explained by dilution by the high groundwater recharge rate from the river underflow and paddy field infiltration.Based on the above results, modeling of groundwater quality processes was conducted. The model consists of a groundwater hydrologic model as the basic structure and a pollutant process model (behavior … More equations). As the basic hydrologic model, a lumped system groundwater model based on Tank Model and a distributed system groundwater model were prepared. For the first approach, a lumped system groundwater quality model was developed by combining the L-Q type and dissolution type of pollutant behavior equations to the lumped groundwater model. Application of this model showed that the model calculation could represent the seasonal change in nitrate nitrogen concentration fairly well.Meanwhile, since the distributed property of water quality in the basin cannot be represented by the lumped model, the research proceeded to the next step for developing a distributed system groundwater quality model. A grid type groundwater flow model was employed as the base structure and the same two types of pollutant behavior equations were incorporated. Further, distribution of pollution sources in the basin was handled with the input data. The result of this model could represent the tendency of spatial movement of nitrogen load. Less