Please cite this article as: B. Burghele, A. Ţenter, A. Cucoş, et al., The FIRST large-scale mapping of radon concentration in soil gas and water in Romania, Science of the Total Environment,

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In the aftermath of postvolcanic activity, dry gases, containing mainly CO 2 , bring along radioactive gases such as radon ( 222 Rn) and thoron ( 220 Rn) that migrate together to the surface. Tectonic faults facilitate the upward migration of these gases. The study described here is based on measurements of radon and thoron activity concentrations performed around the mofettes and mineral springs at Harghita Bai, Harghita Mts. (part of the Neogene Volcanic region of the Eastern Carpathians, Romania). The aim of the study was to identify the location and the direction of the fault system that controls the occurrence of the two mofettes, as well as the mineral springs in the resort by means of systematic measurements of soil radon and thoron activity concentrations. During fieldwork, the existence of the presumed fault system across two different profiles, normal to assumed fault directions, was examined. We sought to identify the local fault hypothetically traced across the two mofettes directed at N 50°W. We also considered the major fault system connecting mofettes and mineral springs, whose direction is N 5°E. For radon and thoron concentration determinations in soil gas, we used a main measurement method (a LUK3C scintillation detector with Lucas cells). Results showed normal distributions for the activity concentrations for both radon and thoron, with a maximum value in both cases. The concentrations for radon and thoron were in the range of several kBq m )3 to tens of kBq m )3 . The distributions are consistent with the hypothesis that the fault lines cross at the point corresponding to maximum values of the radon and thoron concentrations. From the results it is also clear that high precision thoron concentration determinations provided a more accurate location for this type of geological environment than for radon alone, because of its shorter halflife and its shorter migration length.