Effects of the 2021 La Palma volcanic eruption on groundwater hydrochemistry: Geochemical modelling of endogenous CO2 release to surface reservoirs, water-rock interaction and influence of thermal and seawater

Abstract

Volcanic islands face unique challenges in protecting and managing their water resources due to their small size, limited freshwater availability, and vulnerability to natural hazards. The recent 2021 eruption of the Tajogaite volcano on La Palma Island in the Canary Islands, Spain, raised concerns regarding the potential impact on groundwater hydrochemistry. This work aimed to characterize and model the processes that lead to the measured hydrochemical impacts in the groundwater of La Palma as a consequence of the volcanic eruption. The study involved conducting three groundwater sampling campaigns during the eruption, and six after the eruption ceased. A total of 15 monitored points, including piezometers, wells, water galleries, and the main gully collector of the island, all relatively close (2 to15 km) to the erupted volcano, were sampled for the analysis of major solutes and SiO2. Unpublished hydrochemical data previous to the eruption were provided by the local water management authorities of La Palma (CIALP) and the Geological Survey of Spain (IGME). Statistical analyses were performed to assess the differences in groundwater composition before, during, and after the eruption, and a Principal Component Analysis (PCA) mixing model was calculated. Three compositional extreme waters were defined as end members in the system: (1) a high SiO2 computed thermal end member; (2) a low salinity computed fresh groundwater; (3) and seawater. The results of the mixing model showed two main events of maximum mixing ratios in the fresh groundwater reservoirs of La Palma after the eruption; the first one of seawater in July 2022, and the next one of thermal fluids in December 2022. This water mixing during and after the eruption, together with a volcanic CO2 input to the reservoirs, led to significant increases in the concentrations of Na, Ca, SiO2 and SO4 in fresh groundwater, as well as a drop in pH. The significance of these findings relies in improving our understanding of the effects of volcanic eruptions on groundwater, emphasizing the necessity for frequent monitoring and evaluation, given the scarcity and vulnerability of groundwater resources in volcanic islands.