Acta Musei Moraviae

Radioactivity of sedimentary rocks of the Ostrava Formation in the Czech part of the Upper Silesian Basin and possible environmental risks

The paper provides detailed information about natural radioactivity of sedimentary rocks of the Ostrava Formation in the Czech part of the Upper Silesian Basin. The Ostrava Formation consists of Carboniferous (Namurian) coal-bearing strata of paralic development. Lithologically, the formation is very heterogeneous, with alternating cyclically arranged marine, brackish and terrigenous sediments. These are predominantly siliciclastics with a psammitic, aleuritic or pelitic structure (predominantly fine- to medium-grained sandstones, arkose sandstone, siltstone and claystone), often with an increased content of organic matter, concentrating into coal seams. The coal seams mainly comprise high rank (bituminous) coal. Conglomerates are less abundant in the Ostrava Formation. Potassium, uranium and thorium contents were measured using a laboratory gamma-ray spectrometer in 226 rock samples. It is evident from the calculated values of mass activity of ²²⁶Ra equivalent (a_m) that the natural radioactivity of conglomerates and psammites without a visible organic matter (on average 72 and 144 Bq.kg^-1, respectively) is lower as the natural radioactivity of the average Earth crust (about 180 Bq.kg^-1). Slightly higher average am values were found in siltstones and claystones with a low organic matter content (226 Bq.kg^-1), coaly siltstones and coaly claystones (240 Bq.kg^-1), coaly sandstones and arkose sandstones (232 Bq.kg^-1), and in coal (202 Bq.kg^-1). These values pose no health risk. However, high contents of uranium in coal and coaly sediments (on average around 8 ppm U) and consequently very high concentrations of uranium predicted in fly ash derived from the coal can be harmful to the environment and to human health.