Abstract
The formation system of groundwater quality was examined by geochemical classification using major ion concentrations with groundwater temperature distributions, and geological and topographical conditions for the groundwaters in three areas of Niigata Prefecture. Groundwaters were classified based on the ratios of major ion concentrations.
Spatial distributions of groundwater temperature and groundwater quality types were related to each other. This suggested the influences of geopressured hydrothermal water and groundwater flow through alluvial fans. Geological and topographical conditions resulted in the geological oxidation-reduction environment which determined the concentration of SO42- and so on. Cation concentrations were influenced by ion exchange with rock. Na-Cl type waters were estimated to be originated from the fossil sea water of deep ground. Na-HCO3 type waters could be categorized into two types based on the relation between groundwater temperature and Na+/Ca2+ equivalent concentration ratio. One type was strongly influenced by alluvial groundwater for which the rate of ion exchange was controlled mainly by residence time. The other type was influenced by hydrothermal water for which the rate of ion exchange was controlled mainly by water temperature. The existence of Na-Cl type and Na-HCO3 type groundwaters suggested the existence of concealed faults.
