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Impact of environmentally based chemical hardness on uranium speciation and toxicity in six aquatic species

Abstract of the journal article published in Environmental Toxicology and Chemistry (
Accepted article DOI: 10.1002/etc.2834

Goulet, R., Thompson, P.A., Serben, K.C. and Eickhoff, C.
Canadian Nuclear Safety Commission


Treated effluent discharge from uranium (U) mines and mills elevates the concentration of U, calcium (Ca), magnesium (Mg) and sulfate (SO42-) above natural levels in receiving waters. Many investigations on the effect of hardness on U toxicity were often experiments on the combined effect of changes in hardness, pH and alkalinity which do not represent water chemistry downstream of U mines and mills. Therefore, more toxicity studies with water chemistry encountered downstream of U mines and mills are necessary to support predictive assessments of impacts of U discharge to the environment. Acute and chronic U toxicity laboratory bioassays were realized with six freshwater species in waters of low alkalinity, circumneutral pH and a range of chemical hardness as found in field samples collected downstream of U mines and mills. In laboratory tested waters, speciation calculations suggested that free uranyl ion concentrations remained constant despite increasing chemical hardness. When hardness increased while pH remained circumneutral and alkalinity low, U toxicity decreased only to H. azteca and P. subcapitata. Also, Ca and Mg did not compete with U for the same uptake sites. Our study confirmed that the majority of studies concluding that hardness affected uranium toxicity were in fact studies where alkalinity and pH were the stronger influence. Our results thus confirmed that predicting impacts of U downstream of mines and mills should not consider chemical hardness.

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