Metal Leaching in Mine-Waste Materials and Two Schemes for Classification of Potential Environmental Effects of Mine-Waste Piles

Surface waste material was collected from 19 metallic mining-related dumps at 10 sites in the Boulder River water‖[' shed study area. Leach and acidity analyses of waste material provided a foundation for the developmentof two classification schemes for evaluating the potential environmental effect of mine-waste piles. Mineralogical and geochemical analyses of a vertical core of tailings at a mill site showed a distinction in the amount of deposit-related elements between surface and buried waste material. Mineralogy of bulk surface samples was determined primarily by X-ray diffraction. Anglesite and jarosite were present in about 80 percent of the waste piles. Sphalerite and arsenopyrite, both common primary minerals in the mineral deposits, were not present in the mine-waste samples. Goslarite (a zinc sulfate) and scorodite (iron arsenate) were identified in the field. Total-digestion inductively coupled plasma-atomic emission spectroscopy (ICP-AES) elemental and X-ray diffraction analyses of a core from the Buckeye flotation tailings revealed that most of the pile contained oxidized tailings composed of secondary minerals produced during weathering and sparse to no original sulfide minerals; the bottom of the core contained a layer of primary sulfide minerals. Comparison of concentrations in the sulfide and oxidized layers in this core demonstrated that significant leaching of deposit-related trace elements had occurred in the oxidized layer, with some elements migrating downward. The surface samples were subjected to two leaching techniques to determine the concentration of water-soluble, deposit-related trace elements in the dump materials. Net acid production, an indication of the ability of a material to produce acid when oxidized over a long period of time, was also determined. The sum of the concentrations of deposit-related elements (Ag+As+Cd+Cu+Pb+Zn) in these leachates, when plotted against either the leachate pH or the net acid-production value, provided two schemes for classifying these mine wastes. Metal-mining wastes in the study area had classifications ranging from low potential to high potential for causing environmental degradation. The leach and acidity tests showed that material with net acid potential greater than 20 pounds CaCO 3 per ton equivalent will release high concentrations of iron, arsenic, cadmium, copper, lead, and zinc, and produce leachates with low pH. The two classification schemes derived from the chemical and waste-pile-size data provide a means to compare the potential of the waste piles in a watershed to cause environmental degradation. These methods are meant to provide numerical means of evaluating and comparing wastes, but do not incorporate such factors as proximity to ground or surface water, presence of surface water, or draining adit water flowing over or infiltrating into the piles. These factors must be considered by land managers along with the classification results.