Radon in soil gas in contactmetamorphosed hornfelses of Blovice Proterozoic (SW Bohemia, Czech Republic)

 

Ivan Barnet, Petra Pacherová, Josef Procházka

Geoscience Research Reports 42, 2009 (GRR for 2008), pages 140–142

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Abstract

Soil gas radon in contact hornfelses was measured at the contact aureole of the Central Bohemian Plutonic Complex in Blovice Proterozoic area (Klatovy, SW Bohemia). At most of 11 test sites the mean concentrations from 15 probes at each test site varied between 20-30 kBq.m-3, whilst the maximum values have reached nearly triple of mean values. The gamma-spectrometric measurements have detected slightly increased U concentrations (5-13 ppm) on two tectonically affected test sites and areas with intercalations of black shales and silicites (locality Struhadlo and Prádlo). The rest of test sites exhibits the U concentrations close to 3 ppm. The presence of locally confined maximums (in the scale of meters) cause also the occurence of indoor radon values exceeding the action level 400 Bq.m-3 in the area mentioned above. The negative regression between the distance from the contact with granitoids and soil gas radon concentrations was surprisingly observed, which might be explained more by the primary lithological and radiometric character of hornfelses' Proterozoic precursor rocks than by the thermal and pressure effects of granitoid body. The new data enable to change the classification of radon index on hornfelses' bedrock from low to medium in future mapping projects of the Czech Radon Programme and to intensify the indoor radon measurements in the confined area.