Pressure gradient and chemical-isotopic characterization of diffuse gas degassing at Puerto Naos, La Palma, Canary Islands

The 2021 Tajogaite volcano eruption on La Palma created a persistent geohazard due to diffuse volcanic carbon dioxide (CO₂) emissions in the inhabited areas of Puerto Naos and La Bombilla. Elevated indoor and outdoor CO₂ concentrations have restricted access to these zones, highlighting the need for improved tools to characterize gas migration processes and support long-term risk management. This study assesses the risk associated with CO₂ migration by analyzing subsurface gas pressure gradients, proposed as an effective physical proxy to identify preferential advective gas flow pathways. Integrating this approach with geochemical monitoring can improve hazard maps and long-term risk management strategies.  

To improve the assessment and reduction of this persistent hazard, a pressure gradient investigation has been conducted in Puerto Naos and La Bombilla. The main aim was to delineate pressure gradient patterns to detect areas dominated by advective gas transport. For this purpose, ten field surveys were performed between November 2024 and October 2025, covering approximately 274 measuring sites, including paved (184-204) and unpaved (70) zones of Puerto Naos, and one survey with 32 sampling sites at La Bombilla (unpaved). Measurements were done by means of an own-developed device that records the pressure difference between the shallow subsurface (40 cm) and the atmosphere, allowing calculation of the pressure gradient (Pa·m⁻¹) following Natale et al. (2000). Surveys were integrated with simultaneous diffuse CO₂ efflux measurements at the unpaved zones to assess the relationship between pressure-driven flow and gas emission intensity. At both zones, soil gas samples were sampled at 40 cm depth to analyze the He, H₂ and CO₂ concentration and isotopic composition of d¹³C-CO₂.  

Results reveal significant spatio-temporal variability, with markedly higher-pressure gradients during periods of enhanced advection. Maximum gradients exceeded 700 Pa⋅m⁻¹ in paved areas of Puerto Naos, where two persistent anomalous zones were identified. Notably, these values significantly exceed the maximum gradients of approximately 319 Pa⋅m⁻¹ reported by Natale et al. (2000) at Izu-Oshima volcano, suggesting a more potent advective driving force in La Palma’s post-eruptive system, potentially exacerbated by the "sealing effect" of urban pavement. The anomalous zones correlate spatially with elevated CO₂ effluxes, confirming a coupling between pressure gradients and emission intensity, consistent with the physical principles observed in previous volcanic studies. Conversely, reduced degassing periods showed near-zero or negative gradients, indicating diffusion-dominated transport. Isotopic analysis confirms a volcanic-hydrothermal origin for the gas. 

These findings demonstrate that subsurface pressure gradients are a sensitive and reliable proxy for identifying active advective migration in volcanic urban environments. Integrating this physical approach with traditional geochemical monitoring significantly enhances hazard mapping and supports dynamic access management in populated regions affected by persistent degassing. 

 

REFERENCES 

NATALE G., HERNÁNDEZ P.A., MORI T. AND NOTSU K. (2000). Pressure gradient measurements in volcanic diffuse gas emanations. Geophysical Research Letters 27(24):3985-3988. DOI:10.1029/2000GL008540.