Geothermal prospecting by ground radon and radon/thoron ratio measurements at La Palma, Canary Islands

Geothermal energy has reached the front line on renewable resources assessment in recent years, especially in active volcanic areas such as the Canary Islands (Spain), and precisely in La Palma island, where a recent volcanic eruption occurred in 2021, representing a unique opportunity to carry out in situ exploration. Cost-effective geochemical surveys, like soil radon (²²²Rn) and thoron (²²⁰Rn) gases activities measurements, have demonstrated to provide relevant insights as part of surface geothermal exploration, helping to identify permeable areas and potential up-flow zones and, therefore, defining potential geothermal systems boundaries. Both radon (²²²Rn) and thoron (²²⁰Rn) are radioactive isotopes of radon gas and are derived from the natural decay of uranium (²³⁸U) and thorium (²³²Th) respectively, present in the mineralogical composition of, particularly, igneous rocks. However, ²²²Rn present a half-life of 3.8 days while ²²⁰Rn has a shorter half-life of 55 seconds. High ²²²Rn activity surface measurements are considered to be associated to deep magmatic sources of gas, providing additional value on defining high porosity and permeable zones. On the contrary, due to its ephemeral half-life, high ²²⁰Rn activity is associated to shallow soil gas sources.

A detailed and regular surface geochemical survey was carried out in an area of 25 Km² at the western side of La Palma island and southwards from the recent lava flow of Tajogaite Volcano. A total of 766 soil radon and thoron activities discrete measurements were performed (around 30 sample sites/Km²) using a SARAD radon monitor, model RTM-1688-2, connected to a stainless steel probe inserted at 40 cm depth. Data analysis and treatment showed an average ²²²Rn value of 1,056 Bq/m³, ranging from 0 to up to 27,000 Bq/m³, and an average ²²²Rn/²²⁰Rn ratio of 0.3, ranging from 0 to a maximum of 49. The spatial distribution maps has enabled to limit areas with higher values of these two variables,which might indicate zones of interest for further investigation. Higher soil ²²²Rn activity were concentrated along an specific segment of the coast line, which is coincident with the distribution of the well-known anomalous CO₂ active diffuse degassing of volcanic origin in Puerto Naos and La Bombilla, which may have played an important role in controlling the migration and transport of these trace gases towards the surface. Radon and thoron gases activities measurements have revealed to be a worthwhile and non-invasive technique for surface exploration in highly environmental-threatened areas, like La Palma, helping to provide the definition of permeable areas and potential up-flow zones of potential geothermal system structures and permitting an efficient posterior subsurface exploration phase.