Consideration on Wide Area Land Subsidence of the Muikamachi Basin by Groundwater Use in Winter

Abstract

The densely populated area ofminami-Uonuma City, Niigata, central Japan, has been exposed to exceptionally rapid subsidence which is highly related to groundwater pumping for snowmanagement. In spite that the rate of subsidence is potentially hazardous to surface properties and infrastructures, the severe snowfall in this area definitely requires such snow-melting facilities that use groundwater. Thus, it is necessary to establish and promote a sustainable regional water use system that can prevent such rapid subsidence. This paper examines this issue in terms of geological background, fluctuation of the groundwater level in the aquifer, and consolidation behavior of clay layers.

The studied area is in a narrow valley in between the basementmountains and the Uonuma Hills, both of which were uplifted rapidly since the Late Pleistocene. Because of this landform, with a wet climate, river discharge and clastic supply have been basically high enough to fill the valleymostly with gravels for about the last 0.4million years. The complex lithology of the valley fill with large amount of gravels and laterally discontinuousmuddy sediments explains vertical hydraulic continuity in the groundwater circulation within the basin.

This situation formed a uniquemechanism of the land subsidence inminami Uonuma City. Thick permeable coarse-grained layers lacking basin-wide vertical separation by clay layers permit the pressure propagation from the shallow aquifers to the deeper ones. This causes the consolidation of clay layers at any level of the valley fill deposits, even if the groundwater use is concentrated to the shallow aquifers. This situation also forces each clay layer to generate rapid subsidence under the double-sided drained conditions of both the base and upper contacts with the surrounding coarse-grained layers.

The 17 years record of fluctuation of the groundwater level indicates that the lowering of the groundwater level in winter less than 8m has not caused significant residual subsidence. This value should be critical for sustainable use of the groundwater in this particular area.