Long term geochemical evolution of U mine tailings from the Denison TMA-1 tailings management area, Elliot Lake, ON, Canada.

English Abstract:

Unexpected remobilization of ²²⁶Ra within uranium mine tailings disposal facilities after extended periods of stability is a complex phenomenon currently occurring within historic tailings sites. There are 11 legacy uranium mine tailings sites in the Elliot Lake mining camp in Ontario, Canada. At some sites ²²⁶Ra remobilization appears to have resulted from biogeochemical cycles during sulphate reduction, whereas at others, such as the Denison tailings management area, remobilization of ²²⁶Ra appears to have been caused by the dissolution of iron-oxy-hydroxides. We seek a better understanding of the long-term evolution of U-mine tailings through geochemical analysis and reactive transport modeling based on field sampling of the water-covered Denison tailings management area (TMA-1) in October 2014. We performed reconnaissance sonde probing of bottom water throughout the tailings pond and selected several areas with anomalous values of redox potential, pH and conductivity for sediment coring and pore water extraction. We will characterize core samples by SEM, XRD, XRF, MLA and solution ICP-MS analysis. Analytical results and observations will be used in conjunction with sequential extraction experiments performed by the CNSC, enabling a more thorough understanding of ²²⁶Ra behaviour within the tailings pond.

Pore waters showed conductivity values ranging from ~250 uS/cm to extreme values of 3100 uS/cm within areas having both neutral and low pH values with some areas having high redox potential. There is a distinct difference in pH and redox conditions between exposed and deeply submerged tailings environments. A pH of 2 was obtained from an area of exposed tailings whereas pH values of 7-8 were obtained where tailings are fully submerged. Redox potential ranged from 95 mV in neutral pH areas to ~530 mV closer to the location of the exposed tailings. Dissolved ferric and ferrous iron concentrations in tailings porewaters range from ~-2.0 and 0.5 mg/L respectively in reduced, water-covered tailings, reaching a high of 2300 mg/L and 6000 mg/L respectively in the exposed areas. Sediment cores display a zone of intense bleaching under a layer of thick, black organic material, thought to result from the dissolution of iron oxy-hydroxide phases and perhaps responsible for the remobilization of ²²⁶Ra.