H23B-88 1330h
Subsurface caldera and hydrothermal system of Kusatsu-Shirane Volcano, Japan, based on the gravimetric analysis
The microgravity survey with GPS surveying was carried out in the vicinity of Kusatsu-Shirane summit, between 2001-2003. We measured gravity values at 233 observation points. From these new gravity data and the previous data (36 points), Bouguer gravity anomaly map was compiled with the appropriate estimation of the basement density. The estimated density was 2.29g/cm3 by F-H relation, and the correlation coefficient beween F and H was 0.998. On the other hand, the surface density was estimated to be 2.03g/cm3, coefficient 0.997. The low anomaly at the summit can be interpreted by the caldera of 2km in diameter, the center located Mizugama lava-dome near the epicenter of volcanic earthqakes and tremors. After 2-D gravity analysis, it was found that the west part of the Shirane Pyroclastic Cone is composed of high density materials, but the east part has a subsurface caldera with the depth of 300m. This estimated depth corresponds with depths of volcanic earthqakes and tremors, which might be caused by the hydorthermal acitvity. The subsurface structure may explain the volcanic earthquake sytem, the demagnetizing system and the hydrothermal system.
H23B-89 1330h
A Hydrochemical Study on Seepage From Masyu Caldera Lake, Northern Japan
The Masyu caldera lake (212 m deep, 19.6 km$^{2}$ in area and water surface at an elevation of 352 m a.m.s.l.), Hokkaido, Japan is a seepage lake with no surface outflow and inflow. Early studies on the lake indicated that, on the basis of the water balance method, there is major discharge of lake water of around 65,000 km$^{3}$ day$^{-1}$ to the surrounding volcanic aquifers. In July 2002, about 80 water samples were taken from rivers, wells, springs, and hot springs as wells as from the lake. They were analyzed for $\delta$D, $\delta^{18}$O, $\delta^{13}$C, and water chemistry to determine the extent and degree of lake water contribution to river and spring discharges and downgradient groundwater field. The water sample from the Masyu caldera lake (-7.47 per mil $\delta^{18}$O; -54.6 per mil $\delta$D) is markedly enriched in heavy isotopes and falls on a line with a slope of about 5.0, which is characteristic of open-water bodies subject to evaporation. The mixing proportion of lake water was calculated by applying a two end-member mixing model. The lake-water contribution is found to be significant for the southern flank springs in the Nishibetsu-Gawa headwaters, contributing 26-29% of the spring discharges. Taking into account their daily discharge of 170,000m$^{3}$, it is calculated lake water of 45,000 to 50,000 m$^{3}$ day$^{-1}$ is lost through these springs. In contrast, the Kaminoko-Ike spring on the northern flank (15,000m$^{3}$day$^{-1}$), water of which long has been considered to be of the Masyu lake water origin, isotopically showed little or no evidence of lake water. Neither did river waters originating around the Masyu caldera. The analyses of stable carbon isotope, high isotopic values of -9 per mil $\delta^{13}$C for the springs in the Nishibetsu-Gawa headwaters and a low value of -21 per mil $\delta^{13}$C for the Kaminoko-Ike spring, are consistent with these estimations. An overall estimate is that 15,000 to 20,000 m$^{3}$ day$^{-1}$ of leakage is still missing. The hydraulic connection of thicker and excellent aquifers in the western flank to the lake is likely to account for this. The isotopic analyses show some hot springs on the western flank contain substantial proportions of lake water, indicating leakage from the lake is of significance for the formation of hydrothermal system in the western flank.
H23B-90 1330h
Groundwater in the Small Shield-Volcano Island of Ta'u, American Samoa
Hydrogeologic understanding of the shield-volcano island of Ta'u, American Samoa has a limitation typical of small, remote, oceanic islands: a lack of basic hydrogeologic data. But the need for hydrologic guidance for managing groundwater resources has become more pressing as wells succumb to high salinity and threats from contamination. Despite the lack of data, a useful interim groundwater model can be developed by (1) adopting hydrologic concepts from similar but better-studied oceanic islands, (2) using a range of plausible values for parameters having large uncertainty, and (3) testing multiple scenarios where several hydrogeologic conditions are possible. Ta'u exhibits many hydrogeologic features found in Hawaiian shield volcanoes, such as a principal aquifer formed by a thick accumulation of thin high-permeability basaltic lava flows, and a caldera and rift zones that probably have numerous low-permeability dikes. Ta'u probably receives high volumes of groundwater recharge, but the freshwater lens is thin throughout most of the island because Ta'u is small, has little or no coastal confining units, and is formed by lava flows having high hydraulic conductivity. Wells penetrating the thinnest part of the freshwater lens near the coast are most susceptible to high salinity, but locating wells farther inland in attempt to reach a thicker part of the lens may not be practical given the steep topography of Ta'u and the high cost of deep wells. By drawing on hydrogeologic knowledge of Hawaiian shield-volcano islands, a numerical model of groundwater on Ta'u was developed that was useful to demonstrate the challenges of developing groundwater on the island. The model was also used to assess the possibility of contamination of proposed and existing wells.