Characterizing the geochemical reactions in overburden waste pile : Synerude Mine Site, Fort McMurray, Alberta, Canada
Potential environmental risks of constructing waste piles from sulphide-bearing salinesodic overburden include acid-mine drainage and salinization of surface soils. Acid-mine drainage may result from the production of sulphuric acid during the oxidation of sulphide minerals. Acid production during the oxidation of sulphide minerals may also increase the concentrations of S0₄ (from sulphuric acid), Ca (from dissolution of carbonates), and Na (from cation exchange with saline-sodic overburden) in surface soils. To identify and quantify these potential environmental risks, in situ S0₄ and Ca production rates were calculated using two different methods. Sulphide mineral oxidation rates were calculated using simple one-dimensional analytical modelling (assuming diffusive gas transport) of in situ pore-gas 0₂ concentrations. Ca loading from carbonate mineral dissolution (resulting in C0₂ production) was also calculated using the simple one-dimensional modelling of measured pore-gas C0₂ concentrations. Mass balance calculations using solid sample chemistry (total S, soluble ion and TIC concentrations) were also used to quantify the rate of S0₄ and Ca production rates. Geochemical and geotechnical parameters controlling acid production and salt loadings were measured by installing gas probes to a depth of 25 m (n = 34) for in situ pore-gas 0₂, C0₂, CH₄ and N₂ concentrations, and δ¹³Cc₀₂ values), diviner tubes to depths of 1.6 m (n = 3; for shallow moisture contents), a neutron access tube to 25 m (for deep moisture contents), and a thermistor string to 20m (for temperatures). Pore-gas 0₂, C0₂, CH₄ and N₂ concentrations were measured using a field-portable gas chromatograph. Depth profiles of solid sulphur (samples stored in anaerobic chambers) and carbon concentrations and forms were measured and used for acid-base accounting. Pore-gas chemistry showed that 0₂ concentrations decreased from atmospheric to less than 13% at 5 m depth. C0₂ concentrations increased from atmospheric (0.04%) to less than 4% at the same depth The δ¹³Cc₀₂ results suggested that at South Bison Hill C0₂ was derived primarily from an inorganic carbon source. The moisture content data indicated higher moisture contents between 0.5 and 2.5 m depth than at greater depths in the profile. The pore-gas chemistry in conjunction with solids chemistry suggested the presence of an oxidation zone (acid producing) in the top 3m of the profile. However, the results suggested that the oxidation zone was limited to the top 3 m of the profile by the presence of a saturated shale zone. Acid-base accounting results suggested that the acid-producing potential of the shale only slightly exceeded the neutralization potential (NNP = -6.5). The rates of 0₂ and C0₂ flux through the reclamation cover were estimated to be 0.07 moles 0₂/m²/day and 0.03 moles C0₂/m²/day. The corresponding S0₄²⁻ production and CaC0₃ dissolution rates were estimated to be 1.3 and 4.2 g/m²/day respectively. These results were in keeping with rates estimated from mass balance calculations. The results show that a current moisture content conditions, the waste pile was not at risk for acidification or salinization.
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