Evaluation of toxicity, bioavailability and speciation of lead, zinc and cadmium in mine/mill wastewaters

The toxicity of common compounds of lead, cadmium and zinc was evaluated in waters similar to that found in the world's largest lead producing area in Missouri. Static, acute toxicity tests were performed using fathead minnows (Pimephales promelas) and water fleas (Daphnia magna) respectively. Test organisms were subjected to varying amounts of sulfide, carbonate, chloride and sulfate salts of lead, zinc and cadmium mixed in hard, alkaline waters typical to this region. Median lethal concentrations were calculated using nominal versus measured metal concentrations. Measured metal concentrations included four different metal fractionation (extraction/filtration) techniques at different pH levels which included "dissolved", "available", "easily dissolved" and "total" metals. Most consistent correlations between mortality and metal concentrations were found with the "Total" or "Nominal" values. The "Easily Dissolved" metals which corresponded to acid soluble criteria did not effectively represent toxicity and bioavailability of metals. A metal speciation model, MINTEQA2, was used to predict the concentrations of the dissolved metal species at the pH levels commonly seen in the toxicity test vessels. MINTEQ model results suggested that analysis of metals in aqueous environment is better understood when examining speciation characteristics and would likewise be a better mechanism to develop site-specific water quality criteria for metals.