The purpose of this paper is to clarify the mechanism of adsorption and leaching of mercury to the soil, containg allophane clay which exists in most of all volcanic ash soils in Japan.
Mixing experiment between the soils sampled and 500 ml mercury solutions which indicate pH 2 to pH 8 with the concentration of 1 μ g/ml and infiltration experiment between the soils sampled and 500 ml mercury solutions which indicate pH 7 with the concentration of 1 μ g/ml are carried out. These results, are summarized as follows:
In mixing experiment, the mercury in its solutions with pH 4 to pH 8 is adsorbed by the soil more than 99% and adsorption quantity decreases from 96% for the solutions with pH 3 to 51% for those with pH 2.
It is already reported there are two groups in the allophane, silanol groups (= Si-OH) and (Al-OH) groups. The adsorption of mercury occurs by cation exchange reaction between divalant mercury ion and two hydrogens of silanol group.
Mercury is adsorbed more than 99% in the infiltration experiments, too. The mercury concentration of infiltrated water is in the range of 0.03 ppb to 1.5 ppb. The amount of leached mercury is very small compared to the amount adsorbed in the soil particle.

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Considering that hydrogeology essentially comes under Quaternary research, the author emphasizes that aquifers should be arranged in time scale series by means of compiling geomorphic history of the concerned area, taking a serious view of the relations between geomorphic surfaces and geologies (aquifers).
Based on this principle, the author compiled the hydrogeology of the lower course plain of the Tokachi River.
1. Along the lower course plain of the Tokachi River, the author found 9 terracegroups as well as a buried terrace (connected with one of the terrace-groups) and a buried valley (connected with one of the terrace-groups) by means of the electric restivity method, test borings, topographic and geological surveys.
2. The author found 2 aquifers of Miocene,2 aquifers of Pliocene,3 aquifers of Pleistocene and 1 aquifer of Alluvium. Among them, the“ Sunagawa” formation of Pliocene is the most excellent aquifer in this area.
3. The sedimentary basin of the Pliocene series has been separated into 2 parts at Moiwa town, effected by the NNE-SSW directional anticline structure of the pre-Pliocene series; and the distribution of the above-mentioned“ Sunagawa” formations restricted only under the upper side alluvial plain from Moiwa town.
4. Aquifers of Pleistocene are diluvial terrace gravels, diluvial buried terrace gravels and diluvial buried valley gravels. The first contains groundwater of superior quality but small in quantity; however, the second and the third contain groundwater of inferior quality and abundant quantity.
5. It may be possible to obtain superior groundwater resembling the surface water of the Tokachi River at the natural-levee along the river.

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One of the major problems facing ground-water hydrologists is to clarify the mechanism between ground-water flow and land subsidence.
A generalized equation governing the movement of water in saturated deformable porous media is derived, in the present study, from the equations of mass conservation, the equations of motion, and the equations of state. The equations of mass conservation are expressed in the Eulerian specification of the flow field. The equation for groundwater is approximated by a generalized Darcy's equation. The equation of motion for porous media is expressed by an equilibrium equation. The stress-strains are approximated by the well-known expressions of generalized Hooke's law for an isotropic elastic body.
It is assumed that densities of porous media do not change, that soil displacements occur only in the vertical direction, that the total stresses do not change, that the advective rate of change is neglected by comparison with the local rate of change, and that ground-water is an ideal liquid with small compressibility.

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