3:00 PM - 3:15 PM
[SVC26-18] Measurement of gaseous elemental mercury emissions from soils for evaluation of subsurface permeability of volcanoes
Keywords:Gaseous Elemental Mercury, lateral eruption, phreatic eruption, permeability, Kusatsu-Shirane volcano
Sudden lateral eruptions that occur outside the active crater on volcanoes are essential problems for disaster prevention. Since many tourists can easily visit outside the crater, it is important to assess the possibility of lateral eruptions in advance. When magmatic gases containing mercury rise from depth to shallow part of volcanoes, they selectively pass through areas with high permeability, such as fracture zone and falul. Because fracture zones have a high potential for future lateral eruption, measurements of the spatial distribution of mercury emitted from the ground surface on volcanoes may provide risk information. However, the mechanism of mercury release near the ground surface is not yet understood enough to be applicable to permeability assessments. For example, mercury emission rates vary markedly over the course of a day. In this study, we have developed a method to prevent the contamination of small amounts of mercury in the surrounding atmosphere, and conducted several continuous measurements of mercury emission rate from soil, near surface and underground temperatures at Kusatsu-Shirane volcano.
Our precise measurements show that the logarithm of the mercury release rate φ, ln(φ) showed an elliptic relationship to the inverse of the near-surface temperature Tc (Tc-1). This relationship is the same for all our other measurements, although it was not observed in previous studies. We found φ varied with Tc with a time delay. Therefore, with reference to Tc measured Δt (seconds) before the measurement of mercury emission φ, ln(φ) has a linear relationship with Tc-1. The linear relationship between lnφ and Tc-1 is generally referred to as the Arrhenius equation, which is consistent with previous studies. A possible mechanism for this time delay is that subsurface temperature changes, which lag behind the surface temperature change, control the mercury emissions. To estimate the depth that dominate mercury emission, we measure the soil thermal diffusivity based on subsurface temperatures at multiple depths. Our results show that φ varies with temperature 20 mm below the ground surface. The activation energy Ea calculated based on the slope of the Alenius plot is estimated to be 60±20 kJ/mol, which is consistent with the mercury evaporation enthalpy of 59.1 kJ/mol.
The activation energy Ea estimated in this study suggests that mercury emission from the surface is dominated by mercury evaporation at a depth of about 20 mm below the surface. At a depth of 20 mm, the mercury emission caused by photoreduction induced by solar radiation does not occur, but the daily temperature change is significant. By simultaneously measuring the subsurface temperature at a depth of 20 mm, the amount of mercury emission at a given temperature at each measurement site can be calculated. Conducting mercury and subsurface temperature at many sites on volcanoes, it is possible to show the details of the temporal and spatial variation of the mercury emission rate in volcanoes. This study contributes to the estimation of shallow permeable structures in volcanoes and to the monitoring of subsurface temperature.
Our precise measurements show that the logarithm of the mercury release rate φ, ln(φ) showed an elliptic relationship to the inverse of the near-surface temperature Tc (Tc-1). This relationship is the same for all our other measurements, although it was not observed in previous studies. We found φ varied with Tc with a time delay. Therefore, with reference to Tc measured Δt (seconds) before the measurement of mercury emission φ, ln(φ) has a linear relationship with Tc-1. The linear relationship between lnφ and Tc-1 is generally referred to as the Arrhenius equation, which is consistent with previous studies. A possible mechanism for this time delay is that subsurface temperature changes, which lag behind the surface temperature change, control the mercury emissions. To estimate the depth that dominate mercury emission, we measure the soil thermal diffusivity based on subsurface temperatures at multiple depths. Our results show that φ varies with temperature 20 mm below the ground surface. The activation energy Ea calculated based on the slope of the Alenius plot is estimated to be 60±20 kJ/mol, which is consistent with the mercury evaporation enthalpy of 59.1 kJ/mol.
The activation energy Ea estimated in this study suggests that mercury emission from the surface is dominated by mercury evaporation at a depth of about 20 mm below the surface. At a depth of 20 mm, the mercury emission caused by photoreduction induced by solar radiation does not occur, but the daily temperature change is significant. By simultaneously measuring the subsurface temperature at a depth of 20 mm, the amount of mercury emission at a given temperature at each measurement site can be calculated. Conducting mercury and subsurface temperature at many sites on volcanoes, it is possible to show the details of the temporal and spatial variation of the mercury emission rate in volcanoes. This study contributes to the estimation of shallow permeable structures in volcanoes and to the monitoring of subsurface temperature.