Soil gas CO2, He, H2 and isotopic ratios for surface geothermal exploration in Puerto Naos, La Palma, Canary Islands.

Geochemical methods are extensively used in geothermal exploration and exploitation phases, played a major role in both the identification and utilization of resources. In regions where geothermal systems are concealed or located at significant depths, soil gas surveys become indispensable. These methods focus on detecting anomalous concentrations of hydrothermal gases within the soil atmosphere, providing key insights into subsurface geothermal activity. Previous studies in the western area of La Palma island (Canary Islands, Spain) identified the highest geothermal potential of the studied areas. Consequently, more detailed investigations were conducted in the zones with the most significant anomalies to better characterize their potential for economic exploitation. A detailed geochemical survey with an average measurement spacing of ~12 m was carried out in an area of 0.11 Km² at Puerto Naos. A total of 561 sites were sampled at 40 cm depth using a metallic probe. Gas samples were collected with 60 cc hypodermic syringes and stored in 10 cc glass vials for subsequent laboratory analyses. Spatial distribution maps of diffuse He, H₂, CH₄ and CO₂ emission and δ¹³C-CO₂ were constructed to study the presence of enhanced vertical permeability areas related to high temperature hydrothermal activity at depth. The main CO₂, H₂ and δ¹³C-CO₂ anomalies reveal two well-defined zones located in the southeast and west of the study area. In contrast, He highest values are observed in the northern and southern regions. These patterns may be attributed to secondary processes, including interactions with coastal water and the varying reactivity and mobility of the analyzed species. The spatial distribution of soil gases in Puerto Naos confirms a relative enrichment of and H₂, He, CH₄ and CO₂ in the soil gas atmosphere, suggesting a significant contribution from deep-seated sources. These studies aid in identifying permeable zones and potential upflow areas associated with geothermal system structures, thereby facilitating a more efficient subsequent phase of subsurface exploration.