Inventories, profiles and deposition fluxes of 210Pbex in mineral and organic (peatlands) Southern Hemisphere soils

Abstract

Unsupported 210Pb (210Pbex) activity profiles have been determined in both mineral soils from Argentina (La Plata region) and organic soils from Brazil (Serra do Espinhaço Meridional). For the Argentine mineral soils, no significant differences in the 210Pbex inventories are found (ranging from 2920 ± 560 to 4570 ± 460 Bq/m2 ). The measured activity profiles are properly fitted by the one-dimensional transport model by effective diffusion and migration developed by He and Walling and the resulting parameters were analyzed considering the soil properties. It was observed that the migration rate is inversely proportional to the illite content, that the soils with higher amounts of smectite exhibit higher migration rates, and that the higher the pH, the higher the migration rate. These results agree with the higher Pb sorption capacity of smectite than illite, at identical conditions, and the Pb speciation with pH, that gives raise to different interactions between Pb and soil components. For organic soils, 210Pbex accumulates in superficial layers (5 cm depth) and the 210Pbex inventories (ranging from undetectable value to 5100 ± 200 Bq/m2 ) depend on the altitude of the sampling points. The activity values of 210Pbex and 137Cs (previously reported) are not significantly correlated, probably because the 137Cs fallout occurred more than 50 years ago, allowing the 137Cs profiles to develop. Finally, the deposit fluxes were determined for both regions studied and were incorporated into the global curve elaborated in previous works, contributing to a better definition of the general trend reported for the Southern Hemisphere.