Formation Mechanism of High Sodium Bicarbonate Groundwater in Landslide Areas in the Kobe Group

Abstract

The tuffaceous-mudstone is softened by chemical weathering and causes to collapse or landslide, in the Kobe Group of the Sanda basin, Hyogo prefecture. Additionally some high sodium bicarbonate groundwaters occur in this area. Author describes formation mechanism of groundwater including sodium bicarbonate and considered the relation between chemical nature of water and landslides.
The Kobe Group investigated is composed mainly of tuffaceous-mudstone, sandstone and tuff. Among them the tuffaceous-mudstone regulates the chemical quality of groundwater. The tuffaceousmudstone contains both montmorillonite adsorbing exchangeable sodium ion and minor amount of calcium carbonate. Although calcium carbonate has very low solubility itself, in the case of coexistence with sodium-montmorillonite, dissolved calcium ion exchanges for sodium ion, then solubility equilibrium of calcium carbonate breaks. Consequently chemical nature of water becomes high sodium bicarbonate type and alkaline. The more exchangeable cation is abundant in exchangeable sodium, the more the solution becomes high concentration in bicarbonate.
In the Kobe Group, when the tuffaceous-mudstone which adsorbs exchangeable sodium ion highly reacts with rainwater or groundwater to generate high sodium bicarbonate water in high concentration, and as a results calcium carbonate in the tuffraceous-mudstone may solve effectively.
Under these conditions, because of existence of such high salt water and alkaline water, montmorillonite may be produced newly. Therefore tuffaceous-mudstone is softened with water and causes to collapse or landslide.