VARIOUS ORIGINS OF CIRCUMNEUTRAL MINE DRAINAGE AND PROBLEMS WITH CLASSIFICATION

Although mine drainage can be classified broadly into circumneutral and acidic types, this taxonomy reflects a static perception that mine drainage is either one type or the other. Because acidic waters are caused predominantly by pyrite oxidation and circumneutral waters are caused predominantly by carbonate dissolution, this classification suggests inappropriately that either pyrite oxidation predominates over carbonate dissolutionor vice versa. However, circumneutral water can turn acidic without encountering pyrite and acidic water can be neutralized without encountering carbonates. A more realistic conception is to think of mine-water chemistry like groundwater chemistry, i.e. as a continuum over the whole range of pH from hyperacidic (<1) to hyperalkaline (>12) and as a mixture of different waters. Circumneutral water can become acidic from aqueous Fe(II) oxidation, or aqueous As(III) oxidation, or elemental S oxidation, and/or from mixing with acidic waters. Acidic waters can be neutralized from carbonate dissolution, or feldspar dissolution, or mafic-silicate dissolution, or zeolite dissolution, and/or from mixing with neutral or higher pH waters. Examples from the Giant Mine (NWT, Canada), Red River Valley (NM, USA), and the Forest Queen Mine (CO, USA) demonstrate some of these points. Temporal changes in water chemistry can be substantial, causing mine water discharging from a portal to oscillate between neutral and acidic. These changes can be caused by different waters mixing in different proportions, by mine collapses changing the underground flowpaths, and by construction on the surface that can affect sources of recharge. Mine waters that are (or have gone to) neutral or higher pH values tend to mobilize anions such as F, As, Sb, Se, V, Mo, and Cr(VI). Examples include mines in the Kola Peninsula, Russia (high-F), the Schwaz and Brixlegg Mines, Austria (high-Sb), the Endako Mine, BC, Canada (high-Mo), and the Telluride tailings pond, CO, USA (high-Cr). Metals such as Zn, Cd, Ni, Co are often mobile in neutral waters, especially if the iron concentrations are low. Hence, a wide range of potential contaminants are possible in neutral mine waters.