The Topography and the Hydrogeology of the Kushiro Plain

Abstract

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 Kushiro Plain.
I. Summaries of the Hydrogeology of the Kushiro Plain
1) The Kushiro Plain is a down-warped depositional basin which consists of the Kushiro formation (diluvium) and alluvial deposits.
2) The Kushiro formation (composed of gravel, sand, mud and volcanic ash) is divided into 7 minor members, and the center of deposition in the plain was located in and around Lake Toro in the early Pleistocene Epoch.
3) In the alluvial age, however, the center of deposition in the plain has been located in the area 45 km from the present coast line.
4) The author found 2 marine terraces and 3 river terraces around the plain, and also found 3 buried terraces lying at a depth of 2070 meters and a buried valley with a maximum depth of 80 meters under the plain.
5) The deep artesian groundwater in the plain originates from the aquifers of the Kushiro formation and the buried terrace gravels.
6) Alluvial deposits consist of sand facies (coastal sand dunes and buried coastal sand dunes) and mud facies (most of the post-gracial deposits). The former has nice aquifers and the latter has only poor aquifers.
7) The deep artesian groundwater in the plain derives from the terrace area northwest of the plain. The flow of the ground water is almost at a standstill under the central part of the plain.
II. Special Features of the Hydrogeology of the Kushiro Plain
1) Particle Size Distribution
The author fully investigated considerable boring data located at the Kushiro Plain. He found that considerable sand facies and very little mud facies compared with standard alluvial plains of Japan are existing in the Kushiro Plain.
2) Thickness of Unit Stratum
The author found that the maximum thickness of the unit stratum of the whole facies does not exceed the limit of 20 m he found that the thickness of most of the unit stratum is below 6 m. He also found that the average thickness of the unit stratum does not have any relation to the particle size of each stratum.
3) Number of Strata
The ratio of the number of gravel-strata, sand-strata and mud-strata resembles the ratio of the particle size of each stratum.
4) Regional Difference of Specific Capacity
The author studied the regional difference of Specific Capacity using various boring data throughout the investigated area and he compiled “Specific Capacity Distribution Map” of the Kushiro Plain.