Groundwater and soil CO2 efflux weekly monitoring network for the surveillance of Cumbre Vieja volcano, Canary Islands

La Palma Island (708.32 km²) is located at the north-western end of the Canary Archipelago and is one of the youngest of the archipelago. In the last 123 ka, volcanic activity has taken place exclusively at Cumbre Vieja, the most active basaltic volcano in the Canaries, which is located at the southern part of the island. Since no visible geothermal manifestations occur at the surface environment of this volcano, during the last 20 years there has been considerable interest in the study of diffuse degassing as a powerful tool in the volcano monitoring program. In this study we have used two different geochemical approaches for volcano monitoring from October 2017 to November 2019. First, we have developed a network of 21 closed static chambers to determine soil CO₂ effluxes. Additionally, we have monitored physical-chemical parameters (temperature, pH, electrical conductivity -EC-) and chemical/isotopic composition and dissolved gases in the water of two galleries (Peña Horeb and Trasvase Oeste) and one water well (Las Salinas). Soil CO₂ effluxes for the alkaline traps showed an average value of 7.4 g·m^-2·d^-1 for the entire Cumbre Vieja volcano. The gas sampled on the head space of the traps can be considered as CO₂-enriched air, showing an average value of 1,942 ppmV of CO₂. Regarding the CO₂ isotopic composition (δ¹³C-CO₂), most of the stations exhibited CO₂ composed by different mixing degrees between atmospheric and biogenic CO₂ with slight contributions of deep-seated CO₂, with an average value of -19.3‰. The results of the physical-chemical parameters measured in waters showed mean temperature values of 23.7ºC, 19.6ºC and 22.1ºC, 7.40, 6.27 and 6.60 for the pH and 1,710 µS·cm^-1, 411 µS·cm^-1 and 41,100 µS·cm^-1 for the EC, for Peña Horeb, Trasvase Oeste and Las Salinas, respectively. The δ¹³C-CO₂ composition of the dissolved gas has a mean value of -7.8‰, -10.2‰ and -3.8‰ vs. VPDB for Peña Horeb, Trasvase Oeste and Salinas, respectively. The highest values of CO₂ efflux coincided with the stations showing highest CO₂ concentration values located at the southern end of Cumbre Vieja, where the most recent volcanic eruption took place, and also on the northwest flank. This is in accordance with the results obtained for Las Salinas well, located in the south of the island, which show a high concentration of dissolved CO₂ and δ¹³C-CO₂ values with a strong deep-seated CO₂ contribution. This study represents an interesting contribution to detect early warning signals of future unrest episodes at Cumbre Vieja.