[SVC42-P07] Compilation of existing materials to reveal mass and heat budgets as associating with activities of Izu-Oshima volcano
Keywords:Izu-Oshima volcano, groundwater, permeability, heat flux
Introduction
Revealing conservative quantities of mass and heat is essential need to understand volcanic activity. Here we compile and analyze existing materials relating to subsurface fluid and heat flows and physical properties of the host medium, to construct a conceptual model of Izu-Oshima volcano.
Hydrological Setting
The elevation of the water table was measured in many wells in the island. In the low-elevation areas, the water tables are almost at the sea level (e.g. Takahashi et al., 1987; 1988). In a deep well about 1 km from Mt. Mihara in the southwestern caldera, the water table is 36 m ASL (Nakada et al. 1999). The annual precipitation at Izu-Oshima is as much as about 3 m/yr. Nevertheless, surface waters such as ponds and rivers rarely appear except for shortly after squalls. Since springs are found at higher elevations than the sea level, there must be local impermeable layers hosting local perched waters. However, the total of the discharge rate from these springs is thought to be much smaller than the recharge rate. Therefore, most of the infiltrated water should reach the basal groundwater.
Permeability
The permeability is an important parameter constraining subsurface groundwater flows, development of hydrothermal systems and so on. In southeastern foot areas, the permeabilities are estimated on the basis of pumping tests as from 6.3×10-9 to 6.9×10-11 m2 (Ministry of Agriculture, Forestry and Fisheries of Japan 1980; 1986). At northwestern foot, 5.7-7.8×10-9 m2 of the permeability was estimated from tidal response of the water table elevation in a borehole (Koizumi et al. 1998). These probably reflects local horizontal permeability around boreholes. In order to constrain the permeability of the volcanic edifice, Onizawa et al. (2009) simulated a steady groundwater flow due to topographic relief on the Izu-Oshima island, so as to reproduce the observed water table elevation (c.a. 0 m at coastal areas and 36 m at the summit area). When we assume all of the precipitation infiltrate subsurface, the best results was around 3×10-11 m2 and 3×10-13 m2 for horizontal and vertical permeabilities, respectively. At the northern caldera rim, the groundwater level is as high as about 200 m above sea level, implying local low permeability at the area.
Heat Flux
Temperature profiles and thermal conductivities from deep wells provide information of a conductive heat flux. Honda et al. (1979) inferred 203 mW/m2 of the conductive heat flux by using data at the depth of 500-700 m in the northwestern coastal area. At the southwestern caldera rim, steep temperature increase in the lower 500-m interval below the water table was observed (Nakada et al., 1999). Within the basement layer (below 250 BSL), the temperature gradient is estimated 0.29 K/m. By applying the thermal conductivity obtained from the well at the northwestern coastal area, the heat flux is estimated as to be 266 mW/m2(6.35 HFU). These values are about three and four times of the averaged crustal heat flux, respectively. When we apply these heat flux to whole the island having 91 km2 of the area, the total heat flux is estimated as to be 18-24 MW.
Volatiles discharged from magma and those heat energy transports such background conditions. In the presentation, we will show fundamental information summarised above, as relating to knowledges about volatile transports.
Revealing conservative quantities of mass and heat is essential need to understand volcanic activity. Here we compile and analyze existing materials relating to subsurface fluid and heat flows and physical properties of the host medium, to construct a conceptual model of Izu-Oshima volcano.
Hydrological Setting
The elevation of the water table was measured in many wells in the island. In the low-elevation areas, the water tables are almost at the sea level (e.g. Takahashi et al., 1987; 1988). In a deep well about 1 km from Mt. Mihara in the southwestern caldera, the water table is 36 m ASL (Nakada et al. 1999). The annual precipitation at Izu-Oshima is as much as about 3 m/yr. Nevertheless, surface waters such as ponds and rivers rarely appear except for shortly after squalls. Since springs are found at higher elevations than the sea level, there must be local impermeable layers hosting local perched waters. However, the total of the discharge rate from these springs is thought to be much smaller than the recharge rate. Therefore, most of the infiltrated water should reach the basal groundwater.
Permeability
The permeability is an important parameter constraining subsurface groundwater flows, development of hydrothermal systems and so on. In southeastern foot areas, the permeabilities are estimated on the basis of pumping tests as from 6.3×10-9 to 6.9×10-11 m2 (Ministry of Agriculture, Forestry and Fisheries of Japan 1980; 1986). At northwestern foot, 5.7-7.8×10-9 m2 of the permeability was estimated from tidal response of the water table elevation in a borehole (Koizumi et al. 1998). These probably reflects local horizontal permeability around boreholes. In order to constrain the permeability of the volcanic edifice, Onizawa et al. (2009) simulated a steady groundwater flow due to topographic relief on the Izu-Oshima island, so as to reproduce the observed water table elevation (c.a. 0 m at coastal areas and 36 m at the summit area). When we assume all of the precipitation infiltrate subsurface, the best results was around 3×10-11 m2 and 3×10-13 m2 for horizontal and vertical permeabilities, respectively. At the northern caldera rim, the groundwater level is as high as about 200 m above sea level, implying local low permeability at the area.
Heat Flux
Temperature profiles and thermal conductivities from deep wells provide information of a conductive heat flux. Honda et al. (1979) inferred 203 mW/m2 of the conductive heat flux by using data at the depth of 500-700 m in the northwestern coastal area. At the southwestern caldera rim, steep temperature increase in the lower 500-m interval below the water table was observed (Nakada et al., 1999). Within the basement layer (below 250 BSL), the temperature gradient is estimated 0.29 K/m. By applying the thermal conductivity obtained from the well at the northwestern coastal area, the heat flux is estimated as to be 266 mW/m2(6.35 HFU). These values are about three and four times of the averaged crustal heat flux, respectively. When we apply these heat flux to whole the island having 91 km2 of the area, the total heat flux is estimated as to be 18-24 MW.
Volatiles discharged from magma and those heat energy transports such background conditions. In the presentation, we will show fundamental information summarised above, as relating to knowledges about volatile transports.