Flow of Groundwater on Muikamachi Basin in Niigata Prefecture*

Abstract

Muika-machi basin is located on middle river basin of Uono river. On this area, great amount of snow fall in winter. Inhabitants (about 50 thousands population) sprinkle groundwater for melting snow on the road and roofs. Some damage has been done to the area as a result of lowering of hydraulic head. The author investigated distribution of location of aquifers, permeability coefficient, pumping volume of groundwater, hydraulic head, etc. to clarify the groundwater flow system on this area. The summary of the results is shown below. (1) In Muika-machi town which is located on the center of Muika-machi basin, confined aquifers are ten-odd meter deep, about 10 meter thick (the AG layer) and about 30 meters deep,50-60 meters thick (the BG1 layer). These aquifers show good continuity. The BG1 layer continues to unconfined aquifer on the suburbs of Muika-machi. (2) The pumping volume in a snowfall season is 76% of the volume in a year (48 million m3). The pumping volume from the BG1 layer is94% of the all. (3) In a non-snowfall season, groundwater which is pumped up in Muika-machi town is mainly supplied from the direction of the upstream of Uono river. In snowfall season, pumping volume of Muika-machi town increase largely. Then groundwater which is supplied from the direction of the upstream of Uono river to the BG1 layer increase a few times. But the amount of increase from this direction is insufficiency. Through examinations it is considerd that this shortage is supplied by a large amount of leakage from the AG layer.