On the Groundwater of the Mukawa River Valley with Relation to its Buried Topography

Abstract

The writer carried out some investigation in order to estimate the amount of groundwater discharge through the section of Mukawa River valley near Kawanishi intake-dam, Mukawa Town in Hokkaido.
He surveyed the geology of the flood plain by means of electric resistivity methods using apparatus of the Electro-Technical Laboratory type and checked the result by test borings.
The Mukawa River valley is underlaid with a Tertiary formation (most of it is conglomerate), diluvial and alluvial deposits ; the latter two are composed of loose sand and gravel.The diluvial and alluvial deposits are very productive aquifers.
The writer found the existence of an underground river valley, with a maximum depth of some 30 meters and also found there a buried flat surface of the Tertiary formation (conglomerate) lying at the depth of 5-8 meters under the surface. This buried surface is considered to be an old buried terrace surface.
The deposits in the river valley are divided into two parts, the upper and the lower. The sand and gravel of the upper is rather coarser than that of the lower. The boundary of them lies 5-8 meters deep under the surface, the same position of the buried terrace surface. Apparently, the upper is an alluvial deposit, but the lower is a buried deposit in the underground river valley. It may be possible to think that the lower is a diluvial deposit.
From the groundwater contour map obtained from the observation of 18 wells, it was found that the groundwater generally furnished the river water in the survey area.
The writer calculated the gradient of the groundwater flow and evaluated the permiability coefficient by an aquifer test. The amount of groundwater discharge through the valley section “D” is estimated as 0.0156 m³/sec.