Geochemical study on groundwater and thermal water in and around Iwate volcano, Japan:
dispersion feature of magmatic volatiles through the groundwater flow system
Michiko Ohwada, Kohei Kazahaya, Jun’ichi Itoh, Masaaki Takahashi, Noritoshi Morikawa, Hiroshi A. Takahashi,
Akihiko Inamura, Atsuko Nakama, Hiroko Handa, Masaya Yasuhara and Hitoshi Tsukamoto
Research Center for Deep Geological Environment, Geological Survey of Japan, AIST
Corresponding author e-mail: ohwada.m@aist.go.jp

Iwate volcano, typical polygenetic volcano, located in the NE Japan is composed of two stratovolcanos, EastIwate and West Iwate. Groundwater flow system in and around this volcano is strongly restricted by the complex
structure of volcano made by several times collapses of volcanic body (Kazahaya et al., 1998). Understanding thegroundwater flow system is very important to evaluate the amounts and the influence range of the magmatic volatiles
from volcanic body. In this study, we show the feature of chemical and isotopic compositions of the groundwater andthermal water which have collected from variety of depth and discuss the groundwater flow system and the supply ofmagmatic volatiles to the groundwater and thermal water in and around Iwate volcano in conjunction with thesubsurface geological structure.
Two big springs called Kanazawa-Shimizu spring and Oide spring with flow rates reaching 50000 tons/day areplaced in the north and east flanks. The previous study of spring water shows that the shallow groundwaters in northand east flanks recharged at the summit area of Iwate volcano. The flow systems are restricted in north and east
directions which correspond to the places of younger collapses of volcanic body (Kazahaya et al., 1998). BothKanazawa-Shimizu and Oide springs have high concentration of major chemical components and high 3He/4He ratios(3.2Ra and 4.0Ra). The groundwaters in this north and east flanks also have relatively high concentration of major
chemical components and relatively higher 3He/4He ratios (1.1-4.1Ra) than the atmospheric value. In contrast, thethermal waters at several hundred meters depth boreholes have lower 3He/4He ratios (0.1-0.6Ra) in the same area.
These results suggest that the larger amount of magmatic volatiles at the summit area is supplied to the shallow
groundwater system and less amount of that have been injected into the deep aquifer system.
In the southwest flank, an active fault named Shizukuishi fault is placed. The thermal waters along this fault arecollected from boreholes with about 1000m depths and have high Cl- and HCO3- concentrations and high 3He/4He
ratios (2.5-3.0Ra). Additionally, the groundwaters in this area also have high HCO3- concentrations and 3He/4He ratios(1.7-5.3Ra). The recharge area of these groundwaters is placed at comparatively low altitude of the south flank ofvolcano and does not cover the fumarolic area, suggesting that the magmatic or mantle derived volatiles are likely
supplied through the fault system.
The vertical profile of radiogenic 4He concentration in W-E section of Iwate volcano which should be related toa residence time of groundwater indicates that the groundwater at shallow depth inside the Iwate volcanic body havevery low concentration of radiogenic 4He, whereas the thermal water in the Tertiary volcanics and pre-Tertiary
sediment have high concentration of radiogenic 4He. Accumulation of radiogenic 4He suggests that the age of thewaters is older. It is concluded that the groundwaters in the Tertiary volcanics and pre-Tertiary sediments have a longresidence time and is isolated from the shallow groundwater flow system.
Based on the above results, the groundwater flow system and supply of the magmatic volatiles in and around
Iwate volcano is summarized as follows. In the north and east flanks, large shallow groundwater systems exist wherewaters recharged at the summit fumarolic area of Iwate volcano, and have been supplied with the magmatic volatiles.
A deeper groundwater aquifer is isolated from the shallow groundwater flow system and slowly accumulates
magmatic volatiles. In the southwest flank, the magmatic volatiles are likely supplied via fault to the deep thermalwaters and the shallow groundwater. These results suggest that the difference in the subsurface geological structureleads the difference in the supply process of the magmatic volatiles to surrounding groundwater systems.