Anthropogenic and natural radionuclides in soil of a tropical rainforest of Southern Costa Rica

Radiometric investigation in a rainforest of the Golfo Dulce region of southern Costa Rica has shown the presence of the anthropogenic radionuclides caesium-137, strontium-90 and the plutonium isotopes 238, 239 and 240 along with the naturally occurring radionuclides 7Be, 40K, 238U and 232Th, and respective progenies. 137Cs, 90Sr and Pu can be attributed to global fallout; no other sources could be identified. Soil cores were taken from five locations in 1996 and 2007. The mean inventory of 137Cs was determined to be 530 Bq/m2 (decay corrected 1 Jan 2007), the ratio 239+240Pu : 137Cs equals 4.5%, the 238Pu : 239+240Pu ratio, was 3.4%. All values are in accordance with expectations for global fallout in the region. Vertical migration rates of 137Cs were found to be 0.9-1.7 mm per year. Mean concentrations were 169 Bq/kg dry matter of 40K, 13 Bq/kg of 238U, 11 Bq/kg of 226Ra and 9 Bq/kg of 232Th, all of which are quite low values. There are very few radio-ecological data available for tropical ecosystems. Therefore, our findings may serve as baseline information for further studies, e.g. on vertical migration of reactive (sorbing) substances in soil, or for erosion studies (horizontal translocation) using caesium-137 or plutonium as tracers.