Implications of groundwater weathered profile interactions to the mobilization of radionuclides

This study reports the nature and extent of open-system interaction between groundwater and a weathered profile developed in the high grade thorium and REE ore body in Morro do Ferro, Poços de Caldas plateau. The radioelement mobility in the shallow oxidizing environment was considered by using chemical data in conjunction with 234U/238U, 226Ra/230Th, 230Th/234U, 228Th/232Th, 228Ra/232Th and 230Th/228Ra activity ratios (AR’s) for borehole spoil and groundwater samples. ging groundwater from the studied borehole has low salinity values, with total dissolved solids content of 14.7 mg/l and total ionic strength of 0.00018. The ratio of the weight of dissolved radioelement per unit volume of solution to the weight of radioelement in solid phase per unit weight of solid phase showed that the radioelement solubility in the studied waters varied according to the following order: radium>uranium>thorium. 38U AR’s less than 1 were measured in solid phase and can justify the enhancement of 234U in solution. 226Ra/230Th AR’s greater than 1 and 230Th/234U AR’s less than 1 were evaluated between 20 and 27 m in depth, where a 2.1-m thick magnetite dike was intersected. These ratios could be justified by deposition of U and Ra associated with Fe-Mn oxides and kaolinite, where mineral saturation indices evaluated from the available data confirm this possibility. Covariations among disequilibria involving 228Th/228 Ra and 228Ra/232Th AR’s showed the preferential removal of 228Ra relative to Th isotopes, 228Th and 232Th. The recent deposition of radium within the timescale of at least the last 35 years also is suggested.