Radon gas emissions during the 2021 Tajogaite eruption at Cumbre Vieja Volcano (La Palma, Canary Islands)

On 19 September 2021, the largest historic eruption of La Palma began, named the Tajogaite volcano. It was active for 85 days, ending on 13 December 2021. During eruptions, the exposure to natural pollutants rises above background levels due to gas emissions and particulate matter (ash and aerosols) into the atmosphere. Moreover, rock fracturing due to magma injection and seismic activity associated with the eruptive phenomena can increase the ground permeability, having a potential effect on radon (²²²Rn) emissions. During the eruptive and post-eruptive period of the Tajogaite volcano, ²²²Rn measurements were performed across the affected areas to assess the possible radiological impact of this volcanic episode on La Palma inhabitants.

During the first weeks of the eruption, 88 Solid State Nuclear Track (CR-39-SSNT) detectors were deployed at workplaces and dwellings, mainly located in the vicinity of the eruption, though several detectors were also placed in more distant areas for comparison. These detectors were exposed for ca. 90 days, from September 2021 to January 2022, though only 77 detectors could be retrieved as the rest were buried by the lavas. In addition, 3 portable RadonScout devices (SARAD GmbH) were used for continuous monitoring (1 h integration time) of radon and environmental parameters (air temperature, humidity and barometric pressure). They were installed inside 3 buildings located 2.8-5 km from the volcano.

Eighty percent of the CR-39-SSNT radon data were below the reference level of 300 Bq/m³ (Directive 2013/59/Euratom). Of the remaining detectors, a large percentage of radon levels were above 300 Bq/m³ in the Aridane valley, an area close to the volcano, and with a clear spatial pattern showing higher levels of ²²²Rn at shorter distances to the eruptive centre. Continuous monitoring of radon showed low ²²²Rn levels (< 300 Bq/m³) at the two sites furthest from the volcano, with fluctuations highly correlated with environmental variables. Several anomalies of ²²²Rn reaching up to 4000 Bq/m³ were detected during the eruptive period in the monitoring station located closer to the eruptive centre, unrelated to the observed environmental variables. These anomalies were synchronous with the occurrence of large explosive events and phreatomagmatic pulses during the eruption.

The computed effective dose due to the contribution of ²²²Rn during the 3 months of eruption was 0.3 mSv, which, extrapolated to the annual reference value, provides an estimated effective dose of 0.9 mSv/year. This value is 50% lower than the estimated worldwide annual average dose from natural and artificial radiation sources (2.4 mSv/year) (UNSCEAR 2000). Thus, radon levels during the Tajogaite eruption did not lead to a significant increase in exposure level to this radioactive gas. However, transient radon bursts have been recorded associated with several phases of the volcanic activity.

 Acknowledgments

This study was supported by the Spanish Ministry of Science and Innovation (BOE-A-2021-20262).

References:

Council Directive 2013/59/Euratom laying down basic safety standards for protection against the dangers arising from exposure to ionising radiation, and repealing Directives 89/618/Euratom, 90/641/Euratom, 96/29/Euratom, 97/43/Euratom and 2003/122/Euratom.

UNSCEAR (2000), Sources and effects of ionizing radiation. UNITED NATIONS, New York.