[en] Waste water from uranium mines in the Grants, New Mexico region must be treated to remove trace elements before discharge. During the treatment process waters are retained in three consecutive algae ponds. The system was found to be effective in lowering the concentrations of uranium, selenium and molybdenum. The uranium had the greatest apparent depletion, present in the mine's effluents at 4.6 and 5.8 ppm and reduced to 0.8 ppm in the final discharge. The mechanism of depletion has not yet been defined but settling of particulates and biological accumulation are considered to be the active processes. Large populations of microorganisms were found in the waters pumped from the uranium mines and passing through the pond system. Of particular interest was the presence of sulphate-reducing bacteria, believed to be Desulfovibrio and/or Desulfotomaculum