Chemical changes in a receiving stream due to oxygenation and dilution of mine tailings seep waters

Chemical speciation and downstream transport of trace metals following mixing of mine tailings seep water with high quality surface water in High Ore Creek were determined. Changes in specific conductance on mixing of these waters did not reflect dilution factors and thus indicated that some chemical reactions were occurring very rapidly. The major chemical reaction was the oxidation of ferrous to ferric iron which, on hydrolysis, formed a suspended colloidal phase which was subsequently precipitated. The presence of these colloidal hydrous oxides facilitated the rapid removal of copper, lead and arsenic from solution. Manganese and zinc remained predominately in solution throughout their downstream transport. Concentration gradients were similar to that observed for sulphate ions (about a factor or two lower than would have been expected on the basis of dilution). Iron and arsenic concentrations decreased by a factor of more than 500, and manganese and zinc by at least a factor of 15. The decrease in major cation (sodium, potassium, calcium, magnesium) concentrations downstream from the mine tailings area was primarily due to dilution.