Abstract
The relationship between petrological and hydrogeochemical characteristics of Neogene marine deposits in the Horonobe area, northern Hokkaido, Japan, was investigated in order to clarify the water-rock interaction associated with changes in groundwater pH. Whole-rocks chemical compositions, mineral compositions, cation exchange capacity, the exchangeable cation content of the rocks, and the chemical compositions of ground and surface waters were determined at various borehole sites at depths of up to 700 m. Leaching of Na from the marine deposits occurred in the shallow meteoric water infiltration zone, caused mainly by cation exchange between Na+ in smectite and cations in the groundwater. Groundwater pH in the shallow zone is inferred to be controlled primarily by cation exchange between Na+ in smectite and H+ in groundwater, where the pH is elevated by the infiltration of meteoric water. This interpretation is in close agreement with other hydrogeochemical models of similar deposits.
