Soil H2 degassing studies: a useful geochemical tool for monitoring Cumbre Vieja volcano, La Palma, Canary Islands

Hydrogen (H₂) is one of the most abundant trace gases in volcano-hydrothermal systems and plays a key role in redox reactions within hydrothermal reservoirs. Although H₂ can be biologically produced in soils by nitrogen-fixing and fertilizing bacteria, soils are currently regarded as net sinks of molecular hydrogen. Due to its physical and chemical properties, H₂ generated in the crust migrates rapidly and easily escapes into the atmosphere, making it a highly sensitive geochemical tracer of deep magmatic and geothermal processes. Since 2001, systematic surface geochemical studies have been conducted along the Cumbre Vieja volcano (La Palma, Canary Islands) to monitor diffuse hydrogen emissions. The lack of visible surface manifestations of volcanic degassing at Cumbre Vieja, such as fumaroles or hot springs, highlights the importance of diffuse gas studies as a fundamental tool for continuous volcanic monitoring. Soil H₂ concentrations were measured using a gas microchromatograph (Agilent 490 micro-GC) in samples collected at approximately 40 cm depth at around 600 points during each study. The soil H₂ concentration data were used to estimate the H₂ flux at each point. Spatial distribution maps were generated using sequential Gaussian simulation (sGs) to quantify the diffuse H₂ emissions across the volcanic edifice. The analysis of the H₂ emission time series reveals significant increases coinciding with seismic swarms recorded between 2017 and 2021, with a peak flux of 36 kg·d^-1 observed in June 2017, approximately four months before the beginning of seismic activity. During the eruptive phase, sharp peaks in H₂ emissions (up to 30 kg·d^-1) closely followed increases in volcanic tremor. In contrast, estimations obtained in the post eruptive period (Jan 2022-Dec 2025) show H₂ emissions values ranging from 1 to 19 kg·d^-1. This work summarizes the continuous effort to characterize the hydrogen degassing behavior within an active volcanic system and has provided valuable insights into volcanic dynamics and potential precursory signals relevant for hazard assessment and risk mitigation.