Groundwater Flow System of Unzen Volcano, Japan

Unzen volcano (peak 1486 m) is developed on the western part of Beppu-Shimabara Graben (20 km NS wide and 200 km EW long) located at Kyushu island, SW Japan. We have been studied groundwater system of the volcano using geochemical and hydrological technique in order to estimate flux of magmatic volatiles through the groundwater. We have collected over 150 sample waters from springs, rivers, and wells, and they are analyzed for major chemistry and stable isotope ratios. Over 50 pore waters were extracted from 100-1200m-deep drilled cores at the eastern flank of the volcano by a centrifugal separator. The results are summarized as follows: 1) Flow rates of springs and rivers indicate that most of the groundwater recharged at Unzen volcano flew down the slope directed to the east, which is restricted by graben structure. 2) All the groundwaters and spring waters collected inside the graben area are isotopically homogeneous, i. e., -48~-45 permil for hydrogen isotope ratio, indicating that the groundwater is well mixed during flowing. 3) In spite of the isotopic homogeneity, the groundwaters are chemically different from each other. In particular, bicarbonate concentration ranged from 20 to 180 mg/l, and it is inconsistent with the isotopic results. There are some active faults parallel to the graben, and bicarbonate anomalies are found close to the faults. Therefore, the chemical variation is likely to be made due to the addition of deep-seated CO2 ascending through the faults. 4) Linear relation between 1/DIC and carbon isotope ratio of DIC indicates that the DIC in groundwater is explained by simple mixing with two source, magmatic and organic matters. Combining the flow rate data, DIC concentrations and carbon isotope ratios, we estimated the magmatic CO2 flux as 30 t/d through the fault system. 5) Pore waters at 100-300m deep have similar isotopic composition to the present shallow groundwater and river waters, suggesting that those pore waters occupy a part of the shallowest aquifer. On the other hand, pore waters collected from the drilled core at greater than 500m show isotopic discontinuity in the vertical variation, indicating that stagnant aquifers formed at deeper levels.