Weathering processes in uranium tailings and the migration of contaminants

The degree to which weathering affects the tailings and the subsequent release and mobilization of the contaminants depends on a number of important factors: (a) Characterization of the tailings: The mineralogy and associated gangue minerals, the leach process, particle size, type and nature of the sulphide minerals, tailings treatment (i.e. dewatering, neutralization), the chemical nature of slurry water (pH, Eh, ions). (b) Oxidation reactions involved in the conversion of sulphides to sulphuric acid, whether by chemical or bacterial oxidation. (c) The role of bacteria present, whether iron oxidizing or sulphate reducing, and the relative population of the microorganisms. (d) Stability of Ba(Ra)So4 precipitation in the tailings with respect to the effect of weathering and bacteria, particularly that of the sulphate reducing microorganisms. (e) The influences of hydrology and geology relating to the siting of the tailings; dam construction; method of tailings deposition; amount of groundwater in and out of the tailings; amount of process water remaining with the solids which affect the quantity and quality of the seepage; rainfall, atmospheric temperature, seasonal flooding, dissolved O2 or other gases; porosity and permeability; geochemical processes of sorption, desorption, precipitation and redissolution; capillary forces, electrolyte concentrations and salt formation. (f) Migration of salts upward by capillary action; (g) Static leach tests on tailings can provide valuable information as to some of the chemistry occurring within the tailings impoundments. However, more valid information can be gained by long term controlled weathering tests in lysimeters where a number of parameters can be monitored and assessed simultaneously. 68 refs, 18 figs, 15 tabs