Radon and helium in soil gases at Caٌadas caldera, Tenerife, Canary Islands, Spain

The spatial distribution of soil radon was investigated at Cañadas caldera, Tenerife, in two surveys carried out in the summers of 1992 and 1995 by using α-particle-sensitive cellulose nitrate films (Track-Etch) and emanometry, respectively. Soil helium was studied at several transects crossing different structural features of the area. Radon concentration measured by Track-Etch ranged from 1.0 to 1990 pCi/l while that measured by emanometry ranged from 0.1 to 618 pCi/l. Soil helium concentration varied from 5250 to 15 560 ppb with an average value of 6197 ppb. The spatial distribution of soil radon correlates quite closely with structural features (fractures, emission centers, etc.), where the main geothermal manifestations (fumaroles, steam ground and high subsurface temperature and gas contents) also occur. Areas showing high soil Rn concentrations occur at the summit of Teide, Roques de Garcı́a, caldera rim and south and east off side of the caldera. High soil helium anomalies correlate well spatially with those of radon, especially at the summit of Teide where relatively high 3He/4He isotopic ratios occur, suggesting a deep contribution for these emanations. Data indicate that radon and helium are supplied mostly from a deep source, with a minor contribution from U- and Th-rich shallow rocks and soils.