Characterization of blowholes as radon and thoron sources in the volcanic region of La Garrotxa, Spain V. Moreno a, * , J. Bach b , C. Baixeras a , Ll. Font a a Grup de Fı ´sica de les Radiacions, Departament de Fı ´sica, Edifici Cc, Universitat Auto `noma de Barcelona, 08193 Bellaterra, Barcelona, Spain b Unitat de Geodina `mica Externa i d’Hidrogeologia, Departament de Geologia, Edifici Cs, Universitat Auto `noma de Barcelona, 08193 Bellaterra, Barcelona, Spain article info Article history: Received 17 October 2008 Received in revised form 27 August 2009 Accepted 7 October 2009 Keywords: Radon Blowhole Volcanic materials Radon time variations abstract In the volcanic region of La Garrotxa there are natural holes in the soil from which a flow of air can reach the outdoor/indoor environment. In a previous study, these holes (called blowholes) have been found to have a big influence on indoor radon levels in a couple of houses. This finding triggered the interest to characterize these blowholes as radon/thoron sources. In this work, a total of 26 holes have been identified and characterized by geological field observations and by measuring their dimensions, air speed, temperature, humidity, and radon and thoron concentrations. Radon has been measured with passive (Makrofol) and active detectors (monitors PRASSI and RAD 7, being the latter used also to measure thoron). One of the indoor blowholes has been studied in detail and important temporal variations have been found, showing that radon levels are clearly correlated with the soil-outdoor temperature difference. There is not a significant radiological risk for the inhabitants and workers of the studied homes and workplaces with indoor blowholes. However, taking into account that in warm seasons buildings with blowholes in pyroclastic materials from volcanoes or in volcanic scoria under a compact lava flow may have high indoor radon levels (up to 1.5 kBq m 3 ) we conclude that volcanic regions with similar geological features and with presence of blowholes should be considered as areas with a potential radon/thoron radiological risk. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction The studied region, located in the North-East of Catalonia, Spain, is a volcanic region known as La Garrotxa, with its main city Olot. In this region there are Quaternary volcanic materials over a Tertiary substratum, in a framework of a low volcanicity rift model (Martı ´ et al., 1992) and also some faults, as the Amer Fault, with evidences of recent activity (Fleta et al., 2001). A specific characteristic of this region is the presence of blowholes, natural holes in the volcanic soil that have the property of blowing fresh air in warm days. Traditionally they have been very appreciated by Olot inhabitants who take advantage of this property (Mallarach, 1998). In a preliminary radon survey in the city of Olot we obtained higher radon levels in buildings over volcanic materials. Two of them had high radon levels (>400 Bq m 3 ) and blowholes in their basements (Baixeras et al., 2005). In a bigger survey in Olot and in four villages close to the Amer Fault we confirmed the results of the preliminary survey and we also observed an influence of the soil type on radon seasonal variations. Dwellings having blowholes presented the most important variations with summer radon concentrations up to 40 times higher than winter values (Moreno et al., 2008). This finding triggered the interest on studying blow- holes as radon sources in more detail. On the other hand, the Spanish regulation of protection against ionising radiation (Real Decreto 783/2001, 2001) established the need of performing studies in workplaces to determine if there exists a significant increment of the exposure to natural radioactivity to the workers or the public that might be relevant from the Radiological Protection point of view. The goals of the present work are: i) to identify the maximum number of blowholes, ii) to study the geological characteristics of the blowholes and to determine relevant parameters for the air movement, iii) to measure radon and thoron concentrations in the blowholes and iv) to analyse the possible radiological risk in workplaces and dwellings with blowholes. 2. Materials and methods Blowholes have been localized by means of a bibliographical research in local publications (Dane ´ s, 1977; Mallarach, 1998) and by searching directly in the field. The characterization of blowholes * Corresponding author. Fax: þ34 93 581 2155. E-mail address: victoria.moreno@uab.cat (V. Moreno). Contents lists available at ScienceDirect Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas 1350-4487/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2009.10.008 Radiation Measurements 44 (2009) 929–933