ACID DRAINAGE AND METAL ION TRANSPORT AT HOLDEN MINE, LAKE CHELAN

The abandoned Holden Mine produces acidic, metal-laden waters which feed directly into Railroad Creek, one of the tributaries for Lake Chelan. This large, recreation-oriented lake is the primary water supply for the City of Chelan and surrounding communities. The acid mine drainage (AMD) is produced by the oxidation of sulfide minerals that occur unexploited within the Holden deposit, in waste rock at the mine mill, and as millimeter- sized grains in the mill tailings. Field-based (conductivity, pH, temperature) and analytical (XRD, ICP-MS, SEM/EDAX) measurements show that metals such as As, Cd, Cu, Fe, Pb, and Zn are liberated from oxidized minerals into acidified rain and surface waters where they begin transportation to Lake Chelan 18 km distant. Some metal ions precipitate into secondary minerals in the tailings, but most are transported to Railroad Creek where they precipitate out into orange-red colored oxides/hydroxides and form a thick and hard cement known as ferricrete. Metal ions may eventually be transported, either in solution or as particulates, into Lake Chelan. Based on our data and geochemical modeling we show that under ambient moderate- flow conditions, goethite [FeO(OH)] of the ferricrete is stable, and has the ability to retain other, more toxic elements within its structure. It would be important when planning and implementing remedial actions at Holden Mine to avoid causing rapid changes in stream pH, in order to reduce the potential for a sudden release of metals into the stream that could pose a threat to stream biota.