Technical Report NTB 90-27

Data are presented on suspended particles and colloids in groundwaters from the Osamu Utsumi mine and the Morro do Ferro analogue study sites. Cross-flow ultrafiltration with membranes of different pore sizes (450 nm to 1.5 nm) was used to prepare colloid concentrates and ultrafiltrates for analyses of major and trace elements and U- and Th-isotopic compositions. Additional characterisation of colloidal and particulate material was performed by ESCA, SEM and X-ray diffraction. The results obtained indicate the presence of low concentrations ( 450 nm) in the waters show the same trends as the colloids with respect to U, Th and REE associations, but elemental concentrations were typically higher by a factor of 1'000 or more. Calculated association ratios (in mI·g^-1) are in the order of 10⁴ to 10⁵ for U, 10⁵ to 10⁶ for the REEs and 10⁶ to 10⁷ for Th. In waters with low pH and high sulphate content, these ratios are considerably lower. Due to the low concentrations of suspended particles in groundwaters from the Osamu Utsumi U-mine (< 0.5 mg/l), these particles carry only a minor amount of U and REEs (< 10 % of the total concentration of the unfiltered groundwater), but a significant, usually predominant, fraction of Th (30 – 70 %). The suspended particle load in groundwaters from the Morro do Ferro environment is typically higher than in those from the mine by a factor of 5 to 10. This suggests that U and the REEs could be transported predominantly by particulate matter. However, there is chemical and mineralogical evidence that these particles, composed mainly of Fe(III) oxyhydroxides and humic-like compounds, have a low capacity for migration.