Detailed surface geothermal exploration by means of diffuse CO2 efflux, radon measurements and radon/thoron ratio in Jedey, La Palma, Canary Islands

Soil diffuse CO₂ efflux and soil radon (²²²Rn) and thoron (²²⁰Rn) gases activities measurements may be useful geochemical indicators of subsurface volcano-hydrothermal processes in geographical areas where visible gas emissions are nearly absent. Both radon (²²²Rn) and thoron (²²⁰Rn) are radioactive isotopes derived from the natural decay of uranium (²³⁸U) and thorium (²³²Th) respectively, present in the mineralogical composition of rocks. The main difference between these two isotopes is their half-life time. While ²²²Rn presents a half-life of 3.8 days, ²²⁰Rn has a shorter half-life of 55 seconds. Therefore, high ²²²Rn surface activity is considered to be associated with deep magmatic sources of gas while high ²²⁰Rn activity is associated with shallow soil gas sources.

A total of 968 sampling sites in an area of 25 Km² have been considered as part of a detailed surface geochemical study at the central-western part of La Palma and southwards from the 2021 volcanic eruption lava flow of Tajogaite Volcano. Both diffuse soil CO₂ efflux and radon and thoron activities discrete measurements were executed during field surveys between 2023 and 2024.

The diffuse CO₂ efflux measurements were determined, based on the non-stationary static accumulation chamber technique, using CO₂ sensors contained in a portable flux-meter, and the radon and thoron activities were evaluated using a SARAD radon monitor connected to a stainless steel probe inserted at 40 cm depth. Soil gas samples were also collected and analyzed in the laboratory to obtain the chemical and carbon isotopic composition profile.

Data analysis and treatment showed CO₂ efflux values up to 106 g*m^-2/day, ²²²Rn values up to 27000 Bq/m³ and ²²²Rn/²²⁰Rn ratio up to a maximum of 49. Both ²²²Rn versus ²²²Rn/²²⁰Rn ratio plotted together enabled to identify areas with a higher contribution of deeper sourced gas,which might indicate potential zones of interest of geothermal resources.

Furthermore, spatial distribution maps of these variables showed that the main CO₂ and radon gases anomalies are located along the coastline of the studied area, coincident with anomalous magmatic-hydrothermal origin CO₂ diffuse degassing areas. The magmatic-hydrothermal CO₂ might have acted as a carrier gas controlling the migration and transport of the radon trace gas towards the surface.

In conclusion, surface geochemical surveys might be useful for geothermal resources exploration studies, providing a reasonable definition of potential geothermal system boundaries and permitting an efficient and cost-effective posterior subsurface exploration phase.