Uranium mining in relation to toxicological impacts on inland waters.

Protection of tropical rivers from metal pollution requires that mining wastewaters be biologically tested for aquatic toxicity before release from the site into natural ecosystems occurs, and that a 'safe' dilution which incorporates a minimum 10-fold safety factor applied to the lowest NOEC threshold value be utilized. Application of these test methods to wastewaters from an operating uranium mine has shown that pre-release toxicity testing provides accurate information on the toxicity of metal-containing wastewaters with a high degree of confidence. Field validation of the laboratory results was obtained when wastewaters which were field diluted through a release into a billabong gave similar results to laboratory-diluted wastewaters. No one species is always the most sensitive to exposure to complex wastewaters. Changes with time in wastewater chemistry, toxicity, and in the physiological capacity of specific organisms to survive in a contaminated environment (tolerance), can result in different species having varying sensitivities over time to exposure to complex wastewaters collected from the same location. As a result of the remote likelihood of finding the 'most sensitive species', it is necessary to test the toxicity of complex wastewaters to a battery of organisms, representing different trophic levels of the ecosystem, under physical conditions representative of the specific environment needing protection. Use of a natural billabong as a 'biological filter' for releasing mine wastewaters did not result in toxicity mitigation and prevented controlled dilution from occurring during periods of high creek flow.