The Effects of Alkalinity and Hardness on Uranium Speciation and Toxicity to Daphnia pulex

The Province of Saskatchewan in western Canada is a global player when it comes to uranium (U) production. There are several active and decommissioned uranium mines in the northern region of the province, and new deposits continue to be discovered. Increases in U in the aquatic ecosystems surrounding uranium mining operations can lead to toxic effects on aquatic organisms. However, aquatic organisms only take up U when it is in its bioavailable form. Bioavailability of uranium is affected by such water quality variables as pH, dissolved organic carbon, and hardness and alkalinity. Hardness and alkalinity usually vary together and have often been examined in other studies in this fashion. The aim of this study was to investigate the effect alkalinity has on the toxicity of U to Daphnia pulex when hardness is kept constant and vice versa. The acute lethality effects after a 96 h exposure period were determined. The toxicity of U to D. pulex was highly dependent on hardness. At an alkalinity of 20 mg/L as CaCO₃, the 96 h LC₅₀ value for U increased from 170.1 to 4050 μg/L when the hardness increased from 20 to 180 mg/L as CaCO₃. A similar hardness effect on U toxicity was seen at higher alkalinity values. Alkalinity also had a significant effect on U toxicity as the 96 h LC₅₀ at a hardness of 20 mg/L as CaCO₃ increased from 170.1 to 1048.7 μg/L when alkalinity increased from 20 to 180 mg/L as CaCO₃. U speciation modeling using the chemical equilibrium program Visual MINTEQ revealed significant changes in U species present at different alkalinities, suggesting a possible explanation for the decrease in U toxicity.