Chemical variations in the waters draining the Hi-Yu Mine

Abstract

There are two sources of contamination from the Hi-Yu mine: the 'mill seep' and the mine tailings. The mill seep has an acid pH (4.8) containing 20 parts per billion (ppb) Cd and 2400 ppb Zn. Water draining the mine tailings contains 300 ppb As and 30 ppb Sb. These concentrations are above the EPA and State of Alaska Maximum Contaminant Levels (MCL). Once the mill seep mixes with Moose Creek, the pH increases, leading to the precipitation of iron and manganese hydroxides with associated As, Cd, Sb, and Zn. Samples of the iron precipitate contain> 10,000 parts per million (ppm) As, 20 ppm Cd, 240 ppm Sb, and 2600 ppm Zn. Precipitation of metals, along with interaction of organic material and dilution, lowers metal contaminant levels below MCL levels within 0.8 km of the mixing point. Although technically a point source of pollution, the seep and tailings cause no significant increase in the metal content of Fairbanks Creek, 1.2 km downstream of the Hi- Yu mine. The mixing and dilution of waters, adsorption of metals onto Fe and Mn oxy-hydroxide phases, and the settling of particles make the discharge relatively benign, despite the unsightly iron hydroxide precipitate locally present.