HYDROGEOCHEMICAL PROCESSES GOVERNING THE ORIGIN, TRANSPORT, AND FATE OF TRACE ELEMENTS FROM MINE WASTES AND MINERALIZED ROCK

The formation of acid-rock drainage is a complex process that depends on petrology, mineralogy, structural geology, geomorphology, surface hydrology, hydrogeology, climatology, microbiology, chemistry, and mining and mineral processing history. Concentrations of metals, metalloids, acidity, alkalinity, chloride, fluoride, and sulfate found in receiving surface- and groundwaters are affected by all of these factors and their interactions. Generalizations can simplify our understanding of the origin, transport, and fate of contaminants released from mineralized areas. Waters of low pH (<4) solubilize metals and maintain constant element ratios indicative of the main mineral or group of minerals from which they dissolved, except iron and silica which precipitate during pyrite oxidation and acid dissolution of country rock. Relative proportions of minerals dissolved or precipitated can be estimated with mass-balance calculations if mineral and water compositions are known. Discharge of contaminated drainage waters from mines, waste rock, and tailings piles into receiving streams can be identified through synoptic sampling and discharges quantified with tracer-injection studies. Then, reactive-transport modelling is possible and evaluation of mine-site remediation can be much more cost-effective. Metal and metalloid concentrations are strongly affected by redox conditions and pH. Iron is the most reactive metal because it is rapidly oxidized by bacteria and archaea and ferric iron hydrolyzes and precipitates at low pH. Precipitation of hydrous ferric oxides (HFOs) creates colloidal particles that are not effectively filtered, giving unrealistically high supersaturation indices. When detection limits for Fe(III) determination and for redox-potential measurements are considered, the supersaturation disappears. Mineral precipitation near equilibrium solubility provides upper concentration limits for some elements such as aluminum hydroxysulfates for Al, hydrous ferric oxides for Fe(III), siderite for Fe(II), rhodochrosite for Mn(II), barite for Ba, fluorite for F, calcite and gypsum for Ca, and microcrystalline to amorphous silica for Si. Combining solubility constraints with mass fluxes is a powerful tool for quantifying processes governing trace-element mobility.