Identification and Quantification of Nitrate Transport in Agricultural Areas
Korea Environment Institute
As fertilizer is applied to agricultural area, it introduces a large amount of nitrate into aquifer and stream. Therefore it is necessary to identify the nitrate transport and to estimate its loads, in order to establish a proper water quality management. This study investigates the nitrate transport and estimates its loads in the HoengGaeRi area, South Korea, which causes severe eutrophication in the DoAm Lake, located downstream. It consists of four major components: estimation of pollution load to aquifer, development of data transforming interface, identification of nitrate transport, and estimation of pollution load to stream by flow separation. The SWAT model, the quasi-distributed watershed model, is employed to estimate the pollution load to aquifer by fertilizer application. According to its estimation and sensitivity analysis, the pollution load to aquifer is sensitive to the amount of fertilizer, the vegetation type and the infiltration rate. So as to import this estimated pollution load data into the MODFLOW and MT3D model, the fully distributed groundwater model, a GIS-based interface is developed to transform the output data from hydrologic response units of SWAT model to input data for finite-different grid of the MODFLOW and MT3D model. Nitrate transport in aquifer is described using the MODFLOW and MT3D model. The pollution load to stream is driven by two types of flow, which are direct flow affected by precipitation and base flow maintained all the year round. In order to consider both of nitrate transport mechanism and this temporal difference in water quality management, a tool is developed to estimate pollution load by base flow as a post-processor of the MODFLOW and MT3D model. Thus the pollution load by base flow is calculated using this tool and the pollution load by direct flow is calculated as the difference between total pollution load calculated by the SWAT model and the pollution load by base flow. This separation of pollution loads indicates the flow that has larger portion and has to be controlled. The approach suggested in this study shows the nitrate transport in aquifer from land surface to stream and estimates its loads to aquifer and stream. It would be useful to implement water quality management, especially when the agricultural area is located in riparian zone.