HYDROGEOCHEMICAL CHARACTERIZATION AND BASELINE WATER QUALITY: INTERPRETATION AND MODELING THAT CONSIDER MINERAL DISSOLUTION AND HYDROLOGIC DYNAMICS

Improvements in geologic, hydrogeologic, and geochemical characterization methods and the increased use of such approaches at newer hardrock mines provide important hints about operational environmental conditions. Depending on the depth of the mineral resource and weathering conditions, baseline surface water and groundwater quality can also improve the understanding of potential environmental effects and mine water treatment requirements. In mined areas with buried ore deposits, baseline streamwater quality will show little change seasonally, while areas affected by mine drainage will have characteristic concentration spikes associated with hydrologic events. In areas with surface-exposed ore deposits, baseline water quality can show similar seasonal variability. As noted by Nordstrom in numerous publications, weathering of metal sulfide minerals produces hydrated metal sulfate salts that can store sulfate, metals, and acidity and dissolve readily when contacted by snowmelt, rain, infiltrating precipitation, or rising groundwater. Such metal sulfate salts have been found on the walls of open pits and underground workings and seasonally in the upper parts of waste rock and tailings impoundments. If mined materials used in short- or long-term leach tests are properly weathered before testing begins, a concentration spike in the “first flush” of leachate produced is often evident. Mine water quality predictions and mitigation measure design are based on geochemical, hydrogeologic, meteorologic, and baseline water quality characterization results. However, with leach test results in particular, “steady state” concentrations rather than those that reflect temporal chemical dynamics are often used as inputs to mine hydrogeochemical models. Because of differences in grain sizes between materials in leach tests and waste deposits, and the effects on the kinetics of dissolution and resulting concentrations, paired field and laboratory leach testing is recommended. Use of inputs that reflect observed ranges of laboratory/field characterization results and that link with hydrologic conditions will produce more reliable outputs. Examples from the Yanacocha Mine, Peru; Pebble Project, Alaska; Buckhorn Mine, Washington State; and a cobalt mine in Idaho will be used to demonstrate the issues.