Natural and artificial gamma-emitting radionuclides in volcanic soils of the Western Canary Islands

Terrestrial gamma absorbed dose rates as well as spatial distribution of gamma-emitting radionuclides 40K, 226Ra and 232Th and 137Cs in soils, are provided in detail for the first time in the Western Canary Islands. The distribution of terrestrial gamma absorbed dose rates and the activity concentrations of 40K, 226Ra and 232Th are discussed with respect to the main lithologies and geological features of each island. The average terrestrial absorbed dose rate (71.4 nGy.h-1) reported here was slightly higher to previously reported values for the Eastern islands of this archipelago (43 nGy.h-1) and close to the world's average (59 nGy.h-1). Relatively high 137Cs activity concentrations, up to 100 Bq.kg- 1, were measured in some of the collected samples. To understand the origin of these activities, we compared the results from two different field surveys performed in 1991 and in 2013. The activity concentrations were found to be rather similar despite the 22 years gap between the measurements. Complementary radiometric data from aerosol samples collected at this site show that there are additional contributions of 137Cs to this site produced by the deposition of dust particles brought by Saharan dust storms. In terms of radiological risk, the gamma absorbed doses reported (external exposure) pose no risk to the local population. However, the contribution of the soil 137Cs activity concentrations to the gamma absorbed dose were as high as 50% in some few locations.