Article pubs.acs.org/est
Reservoirs of Selenium in Coal Waste Rock: Elk Valley, British Columbia, Canada M. Jim Hendry,*,† Ashis Biswas,† Joseph Essilfie-Dughan,† Ning Chen,†,‡ Stephen J. Day,§ and S. Lee Barbour∥ †
Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada Canadian Light Source, Incorporated, University of Saskatchewan, 101 Perimeter Road, Saskatoon, Saskatchewan S7N 0X4, Canada § SRK Consulting (Canada), Incorporated, 22nd Floor, 1066 West Hastings Street, Vancouver, British Columbia V6E 3X2, Canada ∥ Department of Civil and Geological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, Saskatchewan S7N 5A9, Canada ‡
S Supporting Information *
ABSTRACT: Selenium (Se) reservoirs in coal waste rock from the Elk Valley, southeastern British Columbia, the location of Canada’s major steelmaking coal mines, were characterized and quantified by analyzing samples collected from the parent rock, freshly blasted waste rock (less than 10 days old), and aged waste rock (deposited between 1982 and 2012). Se is present throughout the waste rock dumps at a mean digestible (SeD) concentration of 3.12 mg/kg. Microprobe analyses show that Se is associated with the primary minerals sphalerite, pyrite, barite, and chalcopyrite and secondary Fe oxyhydroxides. Selenium K-edge X-ray absorption near-edge spectroscopy analyses indicate that, on average, 21% of Se is present as selenide (Se2−) in pyrite and sphalerite, 19% of Se is present as selenite (Se4+) in barite, 21% of Se is present as exchangeable Fe oxyhydroxide and clay-adsorbed Se4+, and 39% of Se is present as organoselenium associated with coaly matter. The dominant source minerals for aqueous-phase Se are pyrite and sphalerite. Secondary Fe oxyhydroxide sequesters, on average, 37% of Se released by pyrite oxidation. Measured long-term Se fluxes from a rock drain at the base of a waste dump suggest that at least 20% of Se2−-bearing sulfides were oxidized and released from that dump over the past 30 year period; however, the Se mass lost was not evident in SeD analyses.
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for toxic bioaccumulation of 2.0 μg/L established by the U.S. EPA.12 The Elk Valley, located in the East Kootenay coal district of southeastern British Columbia (Figure 1), is a major coalproducing region in Canada.13 The Elk River flows through the valley and into the United States about 110 km downstream of the town of Sparwood. Se concentrations in the Elk River have increased with time. In the mid-1980s, Se concentrations at surface water quality monitoring sites were below the current provincial aquatic life guidelines of 2.0 μg/L.9 Se concentrations at these sites began to increase in the mid-1980s, and by the late-1990s, the average concentrations exceeded the provincial guidelines. Since 2008, Se concentrations have exceeded 2.0 μg/L near Sparwood.14 Dessouki and Ryan14 and Kennedy et al.15 attribute the source of Se in the Elk River to surface coal mining in the valley. Further, Wellen and Carey8 identify a relationship between mining practices, stream flows, and Se concentrations in the Elk Valley. Lindberg et al.16 also show
INTRODUCTION Non-metal selenium (Se) occurs naturally in most rocks and sediments at low (near parts per million) concentrations.1 In natural waters (groundwaters and surface waters), Se is usually present as a very minor constituent at concentrations ranging from 6 mg/kg, sample-to-sample variability at the scale of measurement within the dumps was generally low. The variability in SeD through the waste rock is reflected in the high mean relative error of duplicate sample analyses (n = 33) of 16.7% (standard deviation = 22.8%). The mean SeD concentration for all waste rock samples (n = 260) was 3.12 ± 1.43 mg/kg. Because SeD data exhibit only a slightly skewed Gaussian distribution (reflected by minimal difference to the median value of 3.06 mg/kg; see Table S2 of the Supporting Information), mean values are discussed below. With the exception of samples from FRO, SeD concentrations were not significantly different between samples from individual dumps or between samples of the Mist Mountain Formation (source rock) and samples from individual dumps (see Table S3 of the Supporting Information). Further, results were not significantly different with respect to the age of the waste rock (see Tables S2 and S3 of the Supporting Information). The lack of statistically lower SeD concentrations in older dumps (i.e., 1982−1990) versus the Mist Mountain Formation samples (parent rock) or fresh waste rock implies that measurement of SeD cannot be used to detect the loss of Se from primary minerals via weathering and leaching over the current life of the dumps. The concentration range within the dumps was consistent with total Se values from Mist Mountain Formation samples reported by other studies, including the study by Ryan D
DOI: 10.1021/acs.est.5b01246 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology and Dittrick.27 They report mean Se concentrations in rock between seams ranging from 1.1 mg/kg for sandstones to 3.2 mg/kg for mudstones with coal stringers and the greatest Se concentrations in the hanging- and foot-wall samples (mean Se concentrations of 4.2 mg/kg). The similarity of our data set to that of Ryan and Dittrick27 suggests that the variability in concentrations in the dumps is the result of natural variability in the host rock. Our data also suggest that, despite the large numbers of SeD analyses in some age groupings (see Table S2 of the Supporting Information), quantifying or monitoring the leaching of Se from these dumps using these SeD concentration data would be difficult. Rock drains, either engineered or formed during the natural segregation of the rock when it is end-dumped, underlie many of the waste rock dumps in the Elk Valley. These drains collect water that migrates through the overlying waste rock and conducts it to adjacent surface waters. Routine monitoring of chemistry, including Se concentrations, and water discharge rates from a rock drain at the base of the LCO-WLC dump from 2003 to 2012 was conducted by Teck Resources, Ltd. These data were used to estimate the Se flux from this dump. Data yielded an average annual flux from the dump of 710 kg/a (see Figure S1 of the Supporting Information). The total Se release from the dump was estimated to be 2.1 × 104 kg by applying this annual flux over the life of the dump (30 years). This estimate of the total Se mass released represented
