Radon monitoring in a volcanic cave: El Viento Cave (Canary Islands, Spain)

Abstract

<p>Radon (<sup>222</sup>Rn, t <sub>1/2</sub> = 3.82 days) is by far the dominant radionuclide in indoor air and constitutes a health hazard in poorly ventilated environments, such as caves, mines or tunnels. In these contexts, radon gas can accumulate, reaching harmful concentrations due to the ionizing radiation from <sup>222</sup>Rn and its progeny. To minimize the exposure risk, a radon monitoring program is required to adopt mitigation measures for the radiological protection of workers, cavers and visitors. The Directive 2013/59/EURATOM sets the recommended occupational and public effective dose limits being 20 and 1 mSv/year, respectively.</p><p>El Viento Cave is a volcanic lava tube located in the northern flank of Pico-Viejo volcano, in the Icod Valley, (Tenerife, Canary Islands, Spain). It was formed during the early eruptions of the Pico Viejo volcano, 27,030 ± 430 years ago, from basaltic, plagioclase-rich pahoehoe lavas. The cave has an extraordinary complexity, with several sinuous tubes and branches in three superimposed and interconnected levels and is considered the 5<sup>th</sup> longest volcanic cavity on Earth (Carracedo and Troll, 2013). A 200 m long segment of this lava tube, named “El Sobrado Cave”, is enabled for touristic visits. Only in 2019 the cave received more than 28000 visitors.</p><p>Monthly radon profiles were obtained during one year (from 2020/10/01 to 2021/09/30) in the touristic section of the cave by using SSNTD (CR-39), installed approximately every 35 m. Besides, a RadonScout monitor (SARAD GmbH) was set up at about 100 m from the cave entrance, for continuous monitoring (integration time of 1 hour) of radon and environmental parameters (air temperature, relative humidity, and barometric pressure).</p><p><sup>222</sup>Rn levels inside the cave ranged from 0-5.000 Bq/m<sup>3</sup>, exhibiting seasonal, diurnal and semidiurnal fluctuations. Short-period radon variations (24 and 12 h frequencies) are related to air temperature and humidity. Long-period radon fluctuations (annual-seasonal) are correlated with rainfall, with lower radon levels in winter (rainy season) and higher in summer (dry season).</p><p>Annual mean effective dose due to <sup>222</sup>Rn gas exposure was estimated from the geometric mean of radon concentration during the studied period, assuming an average indoor occupancy of 10 working hours/week during 48 weeks/year for guides and a punctual visit of 1 hour for tourists. In these conditions, the resulting annual effective dose computed for guides is below 2mSv/year.</p><p><strong><em>