Host: The Association of Japanese Geographers
The purpose of this study is to understand the origin and geochemical process of a flowing confined groundwater (G1) in the Higashi-Hachimantai area, Iwate Prefecture Northeast Japan which is located on the northern foot of Mt. Iwate and the southeastern margin of Hachimantai volcanic groups. Spring, ground and river water samples were collected in and around study area, and major dissolved ions and stable isotopes of O, H and S of these samples were measured.
Major chemical compositions of the water samples were divided into two types on the boundary of Matsu River through the central part of study area. One is a Ca-HCO3 (Iwate) type water in Kanazawa district at the south part of the river and another is a Ca-SO4 (Hachimantai) type water in Kashiwadai district at the north part which located on the flowing well (Fig.1). Though chemical composition of G1 is Ca-SO4type it also contains relatively high concentrations of bicarbonate and chloride. On the basis of these chemical properties, it is assumed that G1 was formed by mixing of Ca-HCO3 type and Ca-SO4 type groundwaters. In order to estimate the mixing ratio of two types groundwater in G1 we calculated how the chemical compositions varied when G3 (Iwate type) and S2 (Hachimantai type) were mixed at the various rate. As a result of the calculation, the chemical composition of mixed water was similar to that of G1 when the ratio of G3 to S2 is 3:2.
The groundwater samples of Iwate and Hchimantai types are also different from each other in the d-excess value, which ranges from 14.6 to 16.4 in the former and from 17.7 to 21.0 in the latter. This means that the water samples of Kashiwadai district are recharged on the Hachimantai volcanic groups and those of Kanazawa district are recharged on the Mt. Iwate, because the d-excess of meteoric water in the Northeast Japan has higher value in the Japan Sea (west) side and it gradually declines toward the Pacific Ocean (east) side. Because the d-excess of G1 was 16.8 which is an intermediate value of both Iwate and Hachimantai types, it was estimated that G1 is formed by the mixing of groundwater of Iwate and Hachimantai types at the almost equal rate.
The sulfur isotope composition of dissolved sulfate in Iwate type groundwater was different from that in Hachimantai type one as well as major chemical composition and d-excess. The sulfur isotope compositions of two water samples in Iwate type were higher values of +10.0 and +12.3 per mil, while those of two water samples in Hachimantai type were lower values of -2.6 and -6.2 per mil. As the sulfur isotope composition of G1 is -1.6 per mil, the mixing ratio of groundwater of Iwate and Hachimantai types estimated by the sulfur isotope seems to be larger contribution of Hachimantai type groundwater than Iwate type one compared with the mixing ratio estimated by the major chemical composition and the d-excess.
As a result of above discussions, it is concluded that the flowing confined groundwater (G1) in the Kashiwadai district located on the north part of study area is mixed groundwater of Hachimantai type with that of Iwate type which flows from the Mt. Iwate into Kashiwadai district, passing through under the Matsu River.
