Natural series nuclide and rare-earth element studies of groundwaters from two radioactive deposits

Data are presented on natural series radionuclides and rare-earth elements (REE) in pre-filtered (<450 nm) near-surface and deeper groundwaters from the Osamu Utsumi uranium mine and the Morro do Ferro thorium-REE-deposit, both situated in the Pocos de Caldas plateau in the state of Minas Gerais (MG), Brazil. In the Osamu Utsumi mine, very high concentrations of uranium (up to 10 mg/l) were measured in near-surface waters. In groundwaters (60-120 m), concentrations of this element are typically between 3 and 10 μg/l. The reduced concentrations of uranium in the aquifer rock of the Morro do Ferro are reflected in the considerably lower concentrations of this element in waters from this environment. The concentrations of 232Th in groundwaters from both sites are very low and generally <0.1 μg/l, however, they can be higher by a factor of 100 or more in surficial waters rich in humic compounds (Morro de Ferro) or in sulfate (Osamu Utsumi mine). Pronounced radioactive disequilibria between 234U and 238U were measured in groundwaters, which are characterized by an excess of 234U due to preferential leaching of this isotope and/or recoil effects. An increase of the 234U:238U activity ratios with sampling depth has been measured. Due to extreme low solubility of thorium and its tendency for sorption on suspended particles, the 230Th:234U ratios in groundwaters are also very low (<0.001). Rare-earth element concentrations in groundwaters from both sites are typically in the range between 1 - 50 μg/l for the LREE (La, Ce, Nd) and 0.001 - 0.1 for the HREE (Ho-Lu), but can be much higher in surficial waters of high complexation capacities, as observed for thorium. 49 refs, 16 figs, 10 tabs