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

La Palma Island, spanning 708.32 km², resides at the north-western edge of the Canary Archipelago and is among the youngest within this group. In the last 123,000 years, volcanic activity has been exclusive to Cumbre Vieja in the island's southern region. The last volcanic activity (Tajogaite eruption) took place at the west flank of Cumbre Vieja from 19 September to 13 December 2021. It was a fissure and powerful strombolian eruption with a magnitude VEI = 3 (Bonadonna et al., 2022). Due to the absence of visible geothermal manifestations, recent decades have witnessed a burgeoning interest in studying diffuse degassing as a vital tool for volcano monitoring. With the aim of strengthening the geochemical monitoring of Cumbre Vieja volcanic activity, we have conducted on a weekly basis from October 2017 to the present, two distinct studies for volcano monitoring. Firstly, we monitored physical-chemical parameters and the chemical/isotopic composition and dissolved gases in the groundwater of two galleries (Peña Horeb and Trasvase Oeste) and three water wells (Las Salinas, Charco Verde, and San Isidro) before, during, and after the Tajogaite Volcano eruption at Cumbre Vieja from September 19 to December 13, 2021. We observed significant temporal variations in pH, EC, ion content, pCO₂, and δ¹³C-CO₂, correlating with interactions between deep volcanic fluids and groundwater. These changes showed good temporal agreement with the eruption and seismic swarms leading up to it. Simultaneously, we established a network of 21 closed static chambers to measure soil CO₂ effluxes. Before the eruption (October 2017 to December 2020), the recorded soil CO₂ efflux averaged 7.30 g·m^-2·d^-1 across Cumbre Vieja (7.48, 7.35, and 7.11 g·m^-2·d^-1 in the north, east, and west, respectively). Post-eruption (March 2022 to present), it averaged 7.51 g·m^-2·d^-1 (8.09, 7.58, and 6.99 g·m^-2·d^-1). The absence of data during this crucial period impedes direct comparisons and assessment of volcanic activity using this specific monitoring tool.

These methods underscore an approach to bolster volcanic surveillance on La Palma Island. Our study emphasizes the importance of monitoring the chemical and isotopic composition of groundwaters linked to active volcanic systems. Such evaluations offer critical insights into magmatic gas input within aquifers, despite limitations encountered during eruptive phases, as witnessed in the case of Cumbre Vieja in 2021.

Bonadonna et al. (2022). J.  Geophys. Res: Solid Earth, 127, e2022JB025302.