Geochemical study of groundwater in and around the Iwate volcano, Japan: supply of the magmatic components

Thirty-three water samples (spring: 1, groundwater from shallow wells: 25, thermal water from deep wells: 7) were collected at various depth in and around the Iwate volcano located in the NE Japan. Water samples were measured in chemical (major components) and isotopic compositions (hydrogen, oxygen, carbon and helium isotope ratios). Based on chemical and isotopic features of groundwater and thermal water, we discuss the groundwater flow system and supply process of magmatic components. The western part of Iwate is developing an erosional valley and has several volcanic peaks and an active fumarolic area. The eastern part of Iwate forms a comparatively new volcanic body with less-active fumarolic area at a summit. An active fault is located in the southwest of the volcano. In the east flank, a biggest spring called Oide spring is located with a flow rate of 50000 t/d. The chemical and isotopic compositions of Oide spring water show very interesting compared with those of the groundwater around Oide spring. The features of the Oide spring water are, (1) low δ D and δ ¹⁸O ratio, (2) high HCO₃^- content, and (3) high ³He/⁴He ratio (3.9Ra). The low δ D and δ ¹⁸O ratio indicate that Oide spring water is derived from high elevation whose recharged area fully covered the summit area (Kazahaya et al., 2000). The high HCO₃^- content and high ³He/⁴He ratio are suggested to result from the magmatic components related to the volcano. For a thermal water of 810m depth located near Oide spring, the features are as follows, (1) much lower δ D and δ ¹⁸O than the Oide spring, (2) Na-Cl, HCO₃ type water, and (3) low ³He/⁴He ratio (0.1Ra). The thermal water in the east flank is suggested to have an older age and accumulated radiogenic ⁴He. In the southwest flank, some thermal waters are collected from drill holes of about 1000m depth located along a fault. The chemical and isotopic features of these thermal waters are, (1) shifted δ D and δ ¹⁸O, (2) high Cl, HCO₃, SO₄ content, and (3) high ³He/⁴He ratio (2.5-3.3Ra). As contrasted to the water in east flank, the magmatic components are enriched in the thermal waters in the southwest flank, which may be supplied via an active fault. The groundwater system of the Iwate volcano is summarized as follows. (1) East flank: There is a biggest shallow aquifer in which the water recharged from summit area of volcano. The largest amount of magmatic component is supplied to the shallow water system, whereas the thermal waters in this area which form deep aquifer system contained less magmatic components. (2) Southwest flank: The thermal waters containing high concentrations of magmatic component are found along the active fault. The magmatic components are likely supplied via fault to the thermal waters. Based on the above results, further results on the residence time of water and the fluxes of magmatic component into the aquifers will be presented.