Environ. Sci. Technol. 2003, 37, 496-501
Characterizing Geochemical Reactions in Unsaturated Mine Waste-Rock Piles Using Gaseous O2, CO2, 12CO2, and 13CO2 T Y L E R K . B I R K H A M , † M . J I M H E N D R Y , * ,† L. I. WASSENAAR,‡ CARL A. MENDOZA,§ AND E U N G S E O K L E E †,# Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan, Canada S7N 5E2, National Water Research Institute, Environment Canada, 11 Innovation Boulevard, Saskatoon, Saskatchewan, Canada S7N 3H5, and Department of Earth and Atmospheric Sciences, 126 Earth Sciences Building, University of Alberta, Edmonton, Alberta, Canada T6G 2E3
In situ determinations of geochemical reaction rates in mine waste-rock piles remain a challenge. Depth-profiles of field O2 and CO2 pore-gas concentrations, δ13CCO2 values, and moisture contents were used to characterize and quantify geochemical reaction rates in two waste-rock piles at the Key Lake Uranium Mine in northern Saskatchewan, Canada. Traditionally, the presence of O2 concentrations less than atmospheric in waste-rock piles has been attributed to mineral oxidation. This study showed that the interpretation of O2 and CO2 concentration profiles alone could not be used to identify the depths of dominant geochemical reactions in the piles and could lead to erroneous estimates of reaction rates. Modeling of the δ13CCO2 depth profiles clearly showed that the gas concentration profiles present in the piles were the result of the oxidation of organic matter present below the piles, a mechanism not previously reported in the literature. Based on these findings, the rates of reactions in the organic zone were determined. The oxidation of organic matter at the base of waste-rock piles should be considered in future mine-waste pore-gas studies, in addition to sulfide oxidation and carbonate buffering.
Introduction Acid mine drainage is a common environmental problem in the mining industry (1) and is primarily attributed to the oxidation of sulfide minerals in waste-rock or tailing piles. The oxidation of sulfide minerals by the consumption of molecular O2 in unsaturated mine waste-rock piles can result in the production of sulfuric acid that lowers the pH of associated pore water, thereby increasing the solubility of many elements in infiltrating waters. Dissolved ions and metals may be transported to lakes, rivers, and groundwater * Corresponding author phone: (306)966-5720; fax: (306)966-8593; e-mail:
[email protected]. † University of Saskatchewan. ‡ National Water Research Institute, Environment Canada. § University of Alberta. # Present address: Department of Geological Sciences, The Ohio State University, Columbus, OH 43210. 496
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and adversely impact water quality and biota in the receiving environment. Acid production may be buffered, however, by the dissolution of carbonate minerals but results in an increase in gaseous CO2 within the waste rock. The mining industry has generally used pore-gas O2 concentration-depth profiles to characterize and quantify sulfide mineral oxidation within waste-rock piles (2-7). Conversely, most studies in natural subsuface environments typically focus on pore-gas CO2 concentration-depth profiles to characterize and quantify geochemical reactions (cf., refs 8-12). Additionally, as CO2 may have multiple sources (1315), its stable carbon isotope composition (δ13CCO2) has been used to identify the source(s) of CO2 gas and identify geochemical reactions (cf., refs 9, 12, 16, and 17). Here we used depth profiles of O2 and CO2 concentrations and δ13CCO2 values to characterize and quantify geochemical reactions in two unsaturated waste-rock piles at the Key Lake uranium mine located in northern Saskatchewan, Canada (57° 12′ latitude, 105° 35′ longitude). Of particular interest here is our modeling of 12CO2 and 13CO2 concentration-depth profiles as it incorporated three potential sources of CO2 having distinct δ13C isotope signatures (atmospheric, carbonate minerals, and organic matter). Previously, Thorstenson et al. (16) and Cerling (17) modeled the production and redistribution of 12CO2 and 13CO2 in unsaturated zones using a two-source model, as summarized by Amundson et al. (18).
Description of Waste-Rock Piles The uranium ore at the Key Lake Mine was deposited within a shear zone that intersects the unconformity between Archean and Early Proterozoic basement gneisses and the overlying Middle Proterozoic Athabasca Group sandstone (19). Two ore bodies, the Gaertner and Deilmann, were mined from 1982 to 1987 and 1984 to 1997 using open-pit methods (Figure 1). Excavation of the two uranium ore bodies resulted in the construction of three large surficial waste-rock piles: the Deilmann South waste-rock pile (DSWR), Deilmann North waste-rock pile (DNWR), and Gaertner (GWR) piles (Figure 1). This study focused on one location on GWR and one on DSWR (ie., GWR-3 and DSWR-1; Figure 1). These sites were selected for investigation because they were located centrally on the piles, thereby minimizing edge effects. Both DSWR and GWR are composed of sand and sandstone overburden and waste rock. DSWR has a total volume of about 20 million m3 and a maximum height of about 28 m above ground surface. GWR has a total volume of about 5.4 million m3 with a maximum height of about 20 m. Most of DSWR was deposited directly upon organic-rich lake-bottom sediments that were dewatered at the onset of mining. GWR was constructed on top of a natural forest soil. Carbon and sulfur forms and concentrations in the wasterock materials, natural forest soil, and the underlying lacustrine deposit are reported in ref 20. Mean organic C concentrations were greater in the natural forest soils (1.4 wt %) and lake-bottom sediment (0.4 wt %) than in the waste rock (e0.01 wt %; detection limit). Mean inorganic C concentrations were 0.3 wt % in lake-bottom sediment and forest soils and 30 m (Figure 3e). Closer to the surface, δ13CCO2 measurements made between 1997 and 1999 were more positive than those at depth, typically ranging between -16‰ and -21‰. Samples collected in 2002 yielded slightly more negative values (approximately -20‰). Kinetic cell testing by Lee et al. (20) supported our conclusion that organic oxidation was the dominant reaction influencing the pore-gas chemistry at GWR-3 and DSWR-1. They reported that the lake sediments and natural surface soils from Key Lake yielded O2 consumption rates and CO2 production rates up to more than an order of magnitude greater than pyrite oxidation/carbonate buffering in wasterock samples. Our finding that oxidation of organic matter beneath the waste-rock piles dominated the pore-gas chemistry showed that significant changes in O2 and CO2 concentrations (relative to the atmosphere) within mine waste piles may not be the result of sulfide mineral oxidation/carbonate buffering. Because the predominant oxidation reactions in this study occurred in the underlying organic material, and thereby masking any minor reactions in the waste rock, the only O2 consumption rates and CO2 production rates we could reliably model were those from the organic zone. These gas consumption and production values ranged between 0.05 and 0.15 µg O2/g soil/day and 0.04 and 0.15 µg CO2/g soil/ day, respectively. The modeled ratio of molar CO2 production to O2 consumption rates in the organic zones at GWR-3 and DSWR-1 typically ranged from 0.58 to 0.68. These values were similar to ratios reported by Lee et al. (20) for microbial respiration in lake sediments (mean ) 0.5) and natural surface material (mean ) 0.7) collected from the Key Lake site but were significantly greater than ratios for pyrite oxidation/ carbonate buffering in waste-rock samples (mean ) 0.2). The application of O2 and CO2 pore-gas concentrations, along with δ13CCO2 values, provides a powerful tool to quantify and identify the predominant oxidation reactions in unsaturated mine waste materials. The results of this study show that the design of mine waste piles may be improved by removing underlying organic materials prior to pile construction. This design change would help postconstruction studies to better differentiate between geochemical processes occurring in the natural, underlying material and those in the waste material.
Acknowledgments Funding for this work was provided by research grants from Cameco Corporation Ltd. and Cogema Ltd. (M.J.H.), the Natural Science and Engineering Research Council of Canada (NSERC) (M.J.H., and C.A.M.), and Environment Canada (L.I.W.). T.K.B. was supported in part by a NSERC Industrial PGS, and E.S.L. was supported in part by NSERC. Techincal assistance was provided by Pat Landine, Trevor Hamm, Ray Kirkland, Sheldon Gibb, Zane Itterman, and Geoff Koehler.
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Received for review February 11, 2002. Revised manuscript received November 18, 2002. Accepted November 18, 2002. ES020587C
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