Escape of volcanic gas into shallow groundwater systems at Unzen Volcano (Japan): Evidence from chemical and stable carbon isotope compositions of dissolved inorganic carbon

Chemical and stable carbon isotopic analyses of dissolved inorganic carbon (DIC) were carried out for groundwater samples collected from cold springs and shallow wells in the Unzen volcanic region in 1999 and 2000. All of the data sets plotted on the carbon isotope ratio (δ13C) vs 1/DIC diagram can be explained by mixing of volcanic CO2 with DIC equilibrated with soil CO2. Groundwater DIC showing a high mixing ratio of volcanic CO2 appears to have a tendency to distribute along two major faults near the activity center of the 1990-1995 eruption. This suggests that these faults are escape routes of volcanic CO2 diffused into the volcanic edifice. The total flux of the volcanic DIC discharged from the cold springs is shown to be one to two orders of magnitude lower than the roughly estimated flux of volcanic CO2 discharged from the summit during the eruptive period.