| 抄録 |
Coarse alluvial deposits are increasingly important as water reservoirs, especially in arid and semi-arid regions. Coarse alluvial deposits consist mainly of poorly sorted sand and gravel, and the geologic heterogeneity is generally large and trending as a result of depositional and post-depositional processes. Geostatistical approaches in groundwater reservoir modeling are various, but are often based on the assumption of stationarity. This assumption is not necessarily valid in coarse alluvial deposits, and a standard approach in trending heterogeneity is to separate the target variable (e.g., hydraulic conductivity) into a global trend component and a residual component. However, the trend component has rarely been determined because of scarcity and uncertainty in measurements, especially in deep parts of deposits. This dissertation primarily covers a focused recharge zone around and beneath a losing river flowing on coarse alluvial deposits. The zone is just one part of a basin, and is thus limited in area. However, the complex system of groundwater flow and solute transport is directly reflected in the large and trending heterogeneity. First, surface water infiltration from the riverbed induces downward groundwater flows. Next, lateral flow components are gradually added, and the magnitudes are changed depending on geologic heterogeneity. In particular, connected voids in gravel deposits give rise to preferential groundwater flows. If the depth dependence of permeability exhibits, for example, a coarsening-upward trend, the groundwater flows accumulate in the shallower parts. As a result of the complex flow system, solute concentration and water temperature are characterized specifically in space and time. Thus, detailed research in a limited zone will reveal the trending heterogeneity in coarse alluvial deposits, and will allow assessment of its importance in groundwater modeling. Targeting the Toyohira River alluvial fan (Sapporo, Hokkaido, Japan), this dissertation presents well data analysis, flow measurements, permeability modeling, and stochastic simulation.First, vertical hydraulic gradient in the fan is mapped to elucidate the three-dimensional groundwater flow system in the fan, and to determine the focused recharge zone along the Toyohira River. The wide range of uncertainty in available well data is addressed by combining a filtering process, kriging interpolation, and cross-validation. Consequently, the reasonably accurate maps show that there are downward groundwater flows in the fan and a negative peak in vertical hydraulic gradient along the losing river. A synoptic survey of discharge is performed to determine losing section in the Toyohira River, and to estimate the rate of seepage loss. The synoptic survey of the Toyohira River proves especially difficulty due to the unsteady turbulent flow and roughness of the gravel bed. This research develops survey methodology employing a high-accuracy current meter and a detailed arrangement of verticals. As a result, the relative uncertainty in discharge measurements is only 2-4%, and the distinct losing section is found in the mid-fan. The seepage rate is estimated to be about 1 m3/s. Depth dependence of permeability in the fan deposits is also examined to detect the global trend in the vertical direction. For this purpose, this study uses the index “matrix packing level” for relatively undisturbed cores of gravel deposits, and finds good correlation between slug test results and core properties. The vertical profiles of estimated hydraulic conductivity are analyzed by using moving average and linear regression methods. Hence, in the apex and mid-fan, an exponential function of permeability is formulated, of which the exponent decay is two- to three orders of magnitude larger than that in consolidated rock. No vertical trend in permeability is found in the fan-toe, indicating that the vertical trend in the upper fan is formed by depositional processes with high-energy flows. Stochastic simulation of trending heterogeneity is also performed by using the exponent model derived from the field observations. Sequential Gaussian simulation is applied to produce 100 realizations of hydraulic conductivity in each study case. The variations in hydraulic heads, river leakage, and groundwater temperature are calculated in each realization by using a high-resolution two-dimensional cross-sectional model. The calculated results are compared with measurements in observation wells, and optimal realizations that satisfy error estimators are extracted in each study case. Consequently, modeling of trending heterogeneity is indispensable for describing groundwater flow and heat transport in the focused recharge zone, because several optimal realizations are obtained in the case of trending heterogeneity and none in the case of stationary heterogeneity. Compared with whether there is trending or stationary heterogeneity, other uncertain factors (river conditions, thermal diffusivity, the range of variograms of residuals, and temperature-dependent effects) less significantly affect the optimal solutions. On the other hand, a block-averaged model of trending heterogeneity also yields optimal realizations that are slightly inferior to those of a high-resolution model in terms of agreement between calculations and observations. However, the number of optimal realizations is larger than that of a high-resolution model, indicating the applicability of block averaging to a larger basin model. A current avenue of the research is to expand the two-dimensional high-resolution model to a three-dimensional basin model, not only for the Toyohira River alluvial fan but also for other coarse alluvial aquifers. Lastly, this dissertation discusses several problems to be addressed with the advanced model in order to determine hydrogeologic sequences, diffuse recharge, urbanization effects, and upscaling procedure.
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