Soil Gas Measurements of Radon, Thoron and Carbon Dioxide: An Approach To Identifying Permeability In Geothermal Areas of Colombia

This work presents the results of progress in the investigation of 222Rn, 220Rn and CO2 in soil gas in two geothermal areas studied in Colombia. The first corresponds to the Cerro Machín volcano (CMV), an active stratovolcano known for the high discharge temperature of bicarbonated thermal springs (up to 96 °C) and the presence of fumaroles. The second area is the San Diego maar (SDM), a volcano produced by phreatomagmatic eruptions and with hydrothermal manifestations recorded on the surface. The 222Rn, 220Rn and CO2 have important characteristics that contribute to geothermal exploration, the determination of their concentration in soil air, among other aspects, provides information on soil permeability and structural control of rising fluids in geothermic areas. Measurements of radon isotopes in the CMV geothermal area recorded maximum values of 144,000 Bq/m3 and 15,100 Bq/m3 for 222Rn and 220Rn, respectively, and anomalous values of soil CO2 flux up to approximately 19,990 g/m2d. The results have established that in the area delimited by the pyroclastic ring (whose interior houses domes with fumarolic activity) the sector with the greatest permeability in the geothermal area is identified. With respect to the geothermal area of the SDM, higher magnitude radon emissions are recorded, related to the presence of 238U anomalies, obtaining values greater than 1,000,000 Bq/m3 for the two radon isotopes and CO2 flux values up to 250.2 g/m2d. The spatial distribution of the diffuse gases in the soil shows that the permeability of the geothermal area predominates in the NE sector, where the San Diego lineament is present, which enables a favorable structural environment for the discharge of hydrothermal fluids.