An investigation was carried out on the acid drainage (pH 3.96) of an old uranium mine (Quinta do Bispo mine, Portugal), containing activity concentrations of 61000±7300 mBq/L of 238U, 886±60 mBq/L of 226Ra and 504±27 mBq/L of 210Po, and relatively high mass concentrations of Ni, Al, Fe, Mn, and Zn. This mine water is treated with addition of BaCl2 and pH increase with addition of hydroxide, in order to co-precipitate radionuclides with barium while the overlying water is released into surface streams. This overlaying water still contained 8740±747 mBq/L of 238U, 250±22 mBq/L of 226Ra and 30±1 mBq/L of 210Po, with radionuclide activities remaining in the treated water ranging from 6% to 26% of initial concentrations. Stable elements present in mine water partly remained in treated water at 3% to 76% of their initial concentrations. The sludge from water treatment contained thus most of radionuclides and metals removed from mine water by co precipitation. This sludge, after sun drying and curing for months at the disposal pond, was leached with water and large fractions of contaminants could be re dissolved. Barium, used in the water treatment, was the only element in concentrations higher in treated water and sludge elutriates than in the original mine water. Results indicate that current treatment of uranium mine drainage with barium is reasonably effective in reducing concentrations of radioelements and stable metals, but treated mine water still contains significant amounts of contaminants. Disposal of sludge in uncovered landfills may also originate leachates and surface runoff toxic to aquatic fauna. Therefore, improved treatment of mine drainage is needed to remove contaminants to much lower levels and abate environmental contamination.
Keywords: uranium mines, acid mine drainage, radionuclides, mine water treatment