The Miyakonojo Basin is one of major agricultural and stockbreeding areas in Japan. Groundwater sampling from different type of aquifers was conducted over the area to understand nitrate concentration distribution and to reveal the process controlling nitrate concentration change along the groundwater flow. In shallow groundwater, high NO₃^－ concentrations were found especially in the western side of the basin. In the middle depth and deep groundwater, high NO₃^－ concentration areas were found in the western and southern part of the basin. In general, nitrate concentration decreased along the groundwater flow from upstream to downstream, suggesting the occurrence of natural nitrate attenuation process in this area. Considering the results of redox parameters and nitrogen isotope ratios in nitrate, it is suggested that dilution process is major nitrate attenuation controlling factor in the midstream of groundwater flow, while denitrification process is dominating in the downstream of middle and deep groundwater.

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[in Japanese]

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In the conventional finite element method (FEM) for groundwater flow analysis, hydraulic head at each node is set as an unknown, and the Darcy velocity of each element is calculated from the nodal data separately. Therefore, discontinuity of the Darcy velocity at some element boundaries can occur. Several methods have been proposed to solve this problem. For example, a method, namely FE-CK, that divides each element into sub-elements depending on the preceding calculation by FEM can be used to meet the continuity of velocity at each elementʼs boundary. Another method, namely HM, which utilizes hybrid finite elements for independently interpolating hydraulic head and the Darcy velocity is also considered to be applicable. Previous studies on validity of such correctional methods have been conducted mostly in 2-D problems. In this paper, we investigate application of the FE-CK to 3-D problems, and propose a path-line analysis method which uses velocity space obtainded by the FE CK and the zeroth order Raviart-Thomas interpolation (RT0). Benchmark calculation with the modelincluding irregular-shaped elements showed the HM caused numerical error in cross-sectional flow rate, whereas the FE-CK gave good agreement with theoretical value. Path-line analysis of the both methods were found to be affected by irregularity in element shapes.

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