A RESEARCH ON THE SOLUTE TRANSFER BY UNSATURATED FLOW IN POROUS MEDIUM

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This research aims to make clear the characteristics of solute transfer in unsaturated soil and to model this phenomena. The resarch deals with two topics, i.e., (1) modelling of a advection-dispersion processes and (2) ion exchange characteristics between soil and soil water.To investigate the first topic, we conducted a series of miscible displacement experiments under saturated-unsaturated condition, using several sized of glass beads (average diameter. : 0.25mm, 0.5mm.0.75mm) as porous medium. It was revealed from the experiments in unsaturated condition that a dispersion coefficient increases with the average pore water velocity except in the vicinity of saturation. And it starts to decrease for the value of saturation. We comprehended this phenomena with concept of Mixing length usually used in the field of turbulence. To apply this concept, the variability of pore water velocity is estimated based on both soil water characteristic curve and unsaturated hydraulic conductivity. Th … More e mixing length obtained in the analysis is 8 times of averaga grain diameter for unsaturated condition and 1.8 times for saturated condition. To discuss the phenomena in general.we did another set of experiment using rather large size of diameter (average diameter : 3.7mm, 6.8mm, 9.1mm). The mixing lengths are rather large compared to fine glass beads. It may be reduced to the turbulence mixing occurring within each pore. The dispersion coefficient in transverse direction is also examined. The mixing lengths are 1/10 times of grain diameter which are unrealistic. We must wait for future work in this respect.The ion exchange in soil water are examined both by the field measurements and in situ experiments. The geology of the experimental hillslope is a weathered Granite. Rain water, stream water and soil water at 4 depths for 3 location on the hillslope are sampled each 1 hour during four storms. Ion concentration of each sample is measured by ionchromatograpy. From the results, Na^+ found to decrease in the period of direct runoff while Ca^<2+> tends to increase. This phenomena is considered that Na^+ rich base water is the result of cation exchange in Granite layr and Ca^<2+> rich water dominated at the peak of hydrograph is originated from surface layr. To make sure this idea, in situ experiments are conducted using different concentration of NaCl and CaCl_2 solutions as the displacing water. The results are numerically simulated based on ADE with ion exchange term. In the analysis Gapon's formula is used as law of mass action. The numerical simulation could reproduce similar behavior of the experimental result. Less