Hydrogeochemical temporal variations related to the recent volcanic eruption at Cumbre Vieja volcano, La Palma, Canary Islands

A recent volcanic eruption has occurred in Cumbre Vieja volcano located in the west of the La Palma Island (at the northwest of the Canary Islands) from September 19 to December 13, 2021. A total of 85 days of eruption makes it the longest volcanic event since historical data have been recorder on La Palma. This volcanic eruption is part of the last stage in the geological evolution of La Palma Island, the fifth in extension (706 km²) and the second in elevation (2,423 m a.s.l.) of the Canarian archipelago. Cumbre Vieja volcano, where the volcanic activity has taken place exclusively in the last 123 ka, forms the southern part of the island.  As a response to the occurrence of several seismic swarms and to strengthen the volcanic monitoring of Cumbre Vieja, a regular sampling of groundwater started in October 2017. Three sampling points have been selected, Las Salinas well and two horizontal galleries: Peña Horeb and Trasvase Oeste. Temperature (ºC), pH and electrical conductivity (EC, µS·cm ¹) were measured in situ. Water samples were taken to measure the chemical and isotopic composition of the groundwaters in the laboratory. The temperature values showed mean values of 22.1 ºC, 23.7 ºC and 19.6 ºC for Las Salinas, Peña Horeb and Trasvase Oeste, respectively. The mean pH values were 6.50 for Las Salinas, 7.33 for Peña Horeb and 6.81 for Trasvase Oeste, while the mean E.C. values were 41,566 µS·cm^-1, 1,684 µS·cm^-1 and 426 µS·cm^-1 for Las Salinas, Peña Horeb and Trasvase Oeste, respectively. The total alkalinity mean value of groundwater from Las Salinas well was 8.75 mEq·L^-1 HCO₃^-, while that from Peña Horeb was 18.8 mEq·L^-1 HCO₃^- and 4.4 mEq·L^-1 HCO₃^- for Trasvase Oeste. The isotopic composition of O and H (δ¹⁸O and δD) showed a meteoric origin, with mean values of 0.8‰ and 7‰ vs. VSMOW for Las Salinas, -4.0‰ and -15‰ vs. VSMOW for Peña Horeb and -4.5‰ and -17 ‰ vs. VSMOW for Trasvase Oeste. Significant changes were observed during the eruptive period, likely related to interaction with endogenous gases such as CO₂, H₂S and H₂. Regarding the isotopic composition of total dissolved carbon, expressed as δ¹³C-CO₂, the average values were -4.6‰, -8.1‰ and -10.8‰ for Las Salinas, Peña Horeb and Trasvase Oeste, respectively, what suggests an endogenous origin for the CO₂. Such deep-seated interaction seems stronger at Las Salinas. At this point, the isotopic composition of total dissolved carbon became more magmatic along the studied period, changing from an average of -3.79 in 2017-2018 to -5.17 from 2019 to 2021.Temporal variations were observed in the total alkalinity, δ¹³C-CO₂ andδ¹⁸O and δD during the eruption period on La Palma Island. These changes observed in both chemical and isotopic composition were related to interaction between deep volcanic fluids and the groundwaters. Groundwaters studies associated to volcanic aquifers can provide important information about the magmatic gas input in the aquifer, to model groundwater circulations edifice and to strengthen the volcano monitoring.