Selenium (Se) and nitrogen (N) compounds represent constituents of environmental concern at coal mine operations in the cordillera of western Canada. Se is released to the environment via the weathering of exposed rock surfaces associated with mine waste materials (e.g., waste rock, pit walls, coarse coal reject (CCR) and tailings). Conversely, mine-related loadings of nitrogen compounds [nitrate (NO3), nitrite (NO2) and ammonia (NH3)] are primarily governed by the leaching of blasting residues associated with nitrogen-based explosives.
There is evidence to indicate that, in some environments, NO3 can affect both the oxidation and attenuation of Se. There are several biogeochemical mechanisms by which NO3 may be linked to Se leaching, mobility and attenuation, including; 1) concurrent microbially-mediated reduction of NO3 and Se (common attenuation mechanism); 2) inhibition of Se reduction by NO3 (maintenance of Se mobility); and 3) Se mobilization via the direct oxidation of Se-bearing minerals (e.g., pyrite) by NO3.
In order to provide more insight into the links between NO3 and Se in coal mine waste environments in western Canada, a two-phased assessment was conducted: 1) a literature review of information relevant to the interactions of NO3 with Se; and 2) compilation and assessment of drainage chemistry data for mines in northeast and southeast British Columbia. Mine waste facility drainage chemistry was compiled from eight mine sites, comprising a dataset of 833 samples (mostly seeps and sedimentation ponds). Se behaviour was also considered in the context of waste facility characteristics and mining practices, including explosive use, degree of saturation, and waste type (e.g., CCR versus waste rock).
The roles of NO3 as an inhibitor to Se reduction, as well as an oxidant of both pyrite and Se, are supported by laboratory and field-based studies of agriculture-impacted systems. The inhibition of selenate reduction by the presence of NO3 has been demonstrated to occur at NO3-Nconcentrations as low as 1 mg/L in laboratory settings, while other studies have found simultaneous reduction of selenate and NO3. The oxidation of reduced Se and sulfur by NO3 has been observed in groundwater systems hosting Se in Cretaceous shales. This is supported by the thermodynamics of sulfur and Se oxidation which indicate reduced sulfur (e.g., iron sulfide or pyrite) and reduced Se (e.g., Se0 and Se2-) can be oxidized through denitrification pathways.
The importance of NO3 in affecting Se release and mobility from mine wastes is strongly dependent on the potential for suboxic conditions to develop within the interior of waste facilities. Specifically, both the reduction of selenate as well as Se oxidation by NO3 are inhibited by the presence of atmospheric O2. Data for redox indicators of mine waste seepage (NO2, NH3, Mn, Fe, NO2:NO3 and NH3:NO3) and pore gas profiles examined as part of this study suggest that oxygen depletion is a common feature to coal waste rock environments, the scale and magnitude of which is likely highly variable. Given the likelihood of suboxia on either micro and macro scales, and the availability of NO3-N (most values > 10 mg/L), NO3 has the potential to affect both Se remobilization and attenuation.
With regards to concurrent NO3 and Se attenuation, there is considerable evidence that anaerobic environments within some mine wastes facilitate the removal of both Se and NO3 from solution through microbially-mediated reduction reactions, producing water compositions with relatively low NO3 concentrations and low Se:SO4 ratios. This is particularly evident for CCR and saturated pit backfill facilities, which are more likely to develop suboxia in comparison to subaerial waste rock dumps. Nitrogen redox couples (NO2:NO3 and NH3:NO3) and Mn concentrations show inverse relationships with Se concentration and Se:SO4 ratios in some waste rock drainages, consistent with Se mobility being reduced in low redox potential environments.
Waste facilities with relatively low Se:SO4 ratios show weaker correlations of Se with major ion indicators of waste rock weathering (SO4, Ca and Mg) compared to waste facilities with elevated Se:SO4 ratios. Facilities with elevated Se:SO4 ratios generally also show strong correlations between Se and NO3. The fact that Se does not correlate well with SO4 or other major ions in drainages with low Se:SO4 ratios is indicative of non-conservative Se behaviour (Se attenuation), and specifically suggests that Se is being attenuated by processes which do not affect SO4, Ca or Mg. In these facilities, Se maintains its correlation with NO3, suggesting that the primary Se attenuation process also affects NO3. The only processes that have the potential to attenuate both Se and NO3 relate to anaerobic reduction mechanisms (i.e., selenate reduction and denitrification).
In terms of NO3 as an inhibitor of Se reduction, it is possible that elevated NO3 concentrations inhibit Se attenuation in anaerobic zones of waste rock facilities. However, this mechanism cannot be defined with certainty as there are no direct measurements of the degree of anoxia in most waste rock facilities. Possible indicators of inhibitory behaviour include the observation of strong Se correlations with major ions (SO4, Ca and Mg) in waste facilities characterized by elevated Se:SO4 ratios and NO3 concentrations. This may indicate that within these facilities Se is relatively mobile and exhibits similar (conservative) behavior as other weathering products. In Gates Formation facilities, there is little evidence of Se attenuation as inferred from Se:SO4 ratios. Such observations may reflect the inhibition of Se reduction by NO3, as Gates Formation waste rock facilities generally produce a higher range of NO3 concentrations (NO3-N of 15 mg/L to 133 mg/L) in comparison to Gething and Mist Mountain Formation drainages.
The potential for Se remobilization via direct oxidation pathways is more difficult to identify. Such difficulties relate to the complexity in differentiating multiple oxidation pathways that share common reaction products, and the potential for auto-correlative effects relating to the flushing of soluble Se, N and S from freshly blasted rock surfaces. However, given the likelihood of suboxia on macro and/or micro-scales and the abundance of NO3 in waste rock environment, it is possible that Se/S oxidation is variably governed by NO3 reduction pathways. The relative importance of this secondary oxidation pathway is difficult to quantify.
Overall, results of the literature review and drainage chemistry analysis suggest that NO3 has the potential to influence Se behaviour in coal mine environments, although the scale and magnitude of such influences are uncertain. With regards to the inhibition of Se reduction, the presence of NO3 may be a rate limiting variable, and has relevance to both passive and active treatment systems. The role of NO3 as an oxidizer of reduced Se and S is less clear. However, the prerequisite conditions for this process to operate (suboxia combined with NO3 availability) are present for the majority of waste facilities examined.
