British Columbia Mine Reclamation Symposia

The biogeochemical behaviour of selenium in two lentic environments in the Elk River Valley, British.. Martin, Alan J.; Wallschläger, Dirk; London, Jacqueline; Wiramanaden, Cheryl I. E.; Pickering, Ingrid J.; Belzile, Nelson; Chen, Yu-Wei; Simpson, Stephanie 2008

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THE BIOGEOCHEMICAL BEHAVIOUR OF SELENIUM IN TWO LENTIC ENVIRONMENTS IN THE ELK RIVER VALLEY, BRITISH COLUMBIA 1  A.J. Martin, 2D. Wallschläger, 2J. London, 3C.I.E. Wiramanaden, 3I.J. Pickering, 4N. Belzile, 4 Y.W. Chen, & 1S. Simpson 1  Lorax Environmental Services Ltd., Vancouver, BC V6J 3H9, Canada 2 Trent University, Peterborough, ON K9J 7B8, Canada 3 University of Saskatchewan, Saskatoon, Saskatchewan, S7N 5B3, Canada 4 Laurentian University, Sudbury, ON, P3E 2C6, Canada  ABSTRACT The biogeochemical behaviour of selenium (Se) in two lentic environments (Goddard Marsh (GM) and Fording River Oxbow (FRO)) was assessed through detailed examination of Se speciation in bottom water, porewater and sediment components. The depositional environments at GM and FRO differ with regards to organic matter content, organic matter sources (as revealed by C:N ratios) and redox character. X-ray absorption near edge spectral (XANES) data suggest that elemental Se and organo-Se represent the dominant hosts for Se at GM and FRO. At both sites, the vertical distributions of dissolved Se species in porewater are closely linked to the profiles of redox-sensitive metabolites. Porewater profiles indicate that the sediments at GM and FRO are serving as diffusive sinks for Se through in situ adsorption/precipitation of Se in suboxic horizons. Although the sediments at both sites serve as net sinks for dissolved Se, interfacial peaks in dissolved selenite (SeIV) and organo-Se demonstrate these species are recycled back into the water column. The conditions present at GM are more favourable for the recycling of reduced Se species. Such observations can be linked to subtle differences in redox conditions as illustrated by profiles of redox-sensitive species (dissolved NO3-, Fe, Mn, SO42- and ΣH2S). These differences have important implications to both the recycling of reduced Se species into the water column and Se uptake by aquatic biota. Implications with regards to Se management, bioremediation and biologically availability (food chain transport) are discussed. INTRODUCTION The fine-grained organic-rich substrates typical to lentic systems (e.g., wetlands, ponds, lakes) serve as optimum media for the microbially-mediated transformations of selenate (SeVI) to reduced forms, including selenite (SeIV), elemental selenium (Se0) and organic species (Masscheleleyn and Patrick 1993; Zhang and Moore 1996; Simmons and Wallschläger 2005). Accordingly, understanding and quantifying mechanisms involved in Se cycling within lentic environments is required to assess the long-term fate of Se and risks to biological receptors. Studies to date conducted through the Elk Valley Selenium Task Force (EVSTF), including assessments of fish, waterbirds, waterfowl and amphibians (McDonald and Strosher 2000; Minnow 2004; Golder 2005), have advanced our understanding of the effects of Se on biological receptors in both lentic and lotic environments. However, there remains a dearth of information with respect to the biogeochemical mechanisms controlling the speciation, accumulation and remobilization of Se within lentic environments in the region.  To expand our current understanding of Se behaviour in lentic systems, studies were conducted in two lentic environments in the Elk River Valley of southeastern B.C. This study represents a collaborative effort between the EVSTF, Lorax Environmental Services Ltd., Trent University (Dr. Dirk Wallschläger), University of Saskatoon (Drs. Cheryl Wiramanaden and Ingrid Pickering) and Laurentian University (Dr. Nelson Belzile). The approach focused on the collection of high vertical-resolution profiles of Se species in sediment, bottom water and porewater, with the primary objective being to delineate the biogeochemical processes governing Se behaviour. The results have both local and global relevance to our understanding of Se behaviour in aquatic systems. The preliminary work presented herein, prepared for the EVSTF, has not been fully reviewed nor endorsed by its membership. FIELD AND ANALYTICAL METHODS Environmental Setting Field surveys at Goddard Marsh (GM) (Elkview Coal Operations) and Fording River Oxbow (FRO) (Fording River Operations) were conducted between August 21-23 and September 4-7, 2007. These lentic zones were selected for study based on previous work at these sites, ecological significance, and proximity to mine-related inputs. GM is located immediately downstream of a sediment-pond discharge from Elkview Mine, and comprises a dense cattail (Typha latifolia) marsh with limited areas of open water. Water depths range from ~0.5 to 1.0 m. Sediments at GM are organic rich and fine-grained. FRO is located adjacent to the Fording River ~9 km downstream of the Fording Mine. FRO extends for several hundred metres and comprises narrow channels and open ponds which are hydraulically connected to the Fording River. Water depths at FRO range from 0.5 to 1.5 m. The system is replete in organic matter and hosts fine-grained sediments. Field Methods Duplicate sediment cores were collected by hand from GM and FRO using 8 cm diameter butyrate tubing. Cores were extruded and sliced at intervals ranging from 1 cm in thickness near the sediment-water interface to 5 cm in thickness at deeper sediment depths. Sediment sub-samples were placed in polyethylene bags and frozen prior to transport. The post-depositional behaviour of Se and exchange with the overlying water column was assessed by sampling of the porewaters and bottom waters using dialysis arrays (peepers), as described in Martin et al. (2002, 2003). The peepers afford 7 mm-resolution profiling of dissolved constituents (0.45 μm pore size) from ~20 cm above the benthic boundary to a sub-interface depth of ~30 cm. Dissolved metal samples were acidified to pH <2 with ultrapure nitric acid while samples for nutrients and sulfate analysis were frozen. For hydrogen sulfide analysis, a 2.0 mL sample was taken and spiked with 50 µL of 1 M zinc acetate. Samples for Se speciation analysis were frozen with dry ice immediately upon collection. Analytical Methods Total Se in porewaters was determined by inductively-coupled-plasma dynamic-reaction-cell massspectrometry (ICP-DRC-MS). Inorganic Se species in porewater were determined by anion-exchange  chromatography coupled to ICP-DRC-MS (AEC-ICP-DRC-MS), similar to Wallschläger and Roehl (2001). Dissolved organic selenium was converted to selenite (SeIV) by selective UV-photo-oxidation, and then determined indirectly (by subtraction of the sample’s native selenite (SeIV) concentration) by hydride generation-atomic fluorescence spectrometry (HG-AFS) (Chen et al., 2005). Determinations of dissolved trace element concentrations were performed using inductively-coupled plasma mass spectrophotometry (ICP-MS) at the Pacific Centre for Isotopic and Geochemical Research, University of British Columbia. Sulfate and nitrate concentrations in porewaters were measured by ion chromatography and total sulfide (ΣH2S = S2-, HS- and H2S) was measured spectrophotometrically. Total carbon and sulphur concentrations in sediments were determined by combustion/gas chromatography at the University of British Columbia. Carbonate carbon was determined by coulometry. Organic carbon was determined by subtracting carbonate carbon values from the total value. Trace elements were analyzed by inductively-coupled plasma optical emission (ICP-OES) and mass spectrometry (ICP-MS) using solutions prepared by fusing sub-samples in lithium metaborate (LiBO2), followed by dissolution of the quenched glass in 10% nitric acid (HNO3). X-ray absorption near edge spectra (XANES) data were collected using the synchrotron at the Canadian Light Source Saskatoon, SK. XANES probes the absorption characteristics of a particular electron shell using tunable synchrotron light. The geometry of the resulting spectra is valence dependent so it is possible to determine the specific elemental oxidation states present (i.e., Se-2, Se0, Se+4, Se+6) (Pickering et al., 1995). As well, XANES spectra can be used to obtain semi-quantitative determinations of the relative abundance of each oxidation state. To quantify the relative contribution of various Se forms, a XANES library of known Se compounds was compared to the sample spectra. RESULTS AND DISCUSSION Sediments Contrasts between the depositional environments at GM and FRO are illustrated by their carbon (C), nitrogen (N) and sulfur (S) content (Figure 1). The greater organic carbon content at GM (25 to 30 wt.%) in comparison to FRO (5 to 7 wt.%) likely relates to differences in the source(s) of organic matter. Specifically, the C-Org:N ratio in GM sediments (mean = 38) is closer to the C:N signature of terrestrial organic matter (45 to 50:1), while the C-Org:N ratio at FRO (mean = 20) is similar to organic matter produced by plankton decomposition (12:1) (Wetzel 1975). The higher C:N at GM indicates that the organic matter content at this site is composed largely of decomposing wetland vascular plants (e.g., Typha sp.). Conversely, the lower C:N contents at FRO imply a greater portion of the organic matter originates from in situ sources such as algal production. These differences likely have relevance to Se accumulation in sediments. Total-Se in sediments at GM range from 7 to 71 mg/kg dry wt. (mean = 37 mg/kg) while deposits at FRO range from 2 to 19 mg/kg dry wt. (mean = 10 mg/kg) (Figure 2). XANES spectra suggest that elemental Se, organo-Se (possibly seleno-methionine) and selenite (sorbed SeIV), are the dominant hosts for Se at  both GM and FRO (Figure 2). Of these, elemental phases and organo-Se contribute most to the total sediment inventory, which account for on average 35% and 50%, respectively, of the total at both sites. 0  0  0  10  10  10  20  20  20  GM-1 GM-2  GM-1 GM-2  GM-1 GM-2  30  30  0  5  10  15  20  25  30  30  0  35  10  20  30  40  50  0.0  0  0  10  10  10  20  20  0.6  30  20  Org-C (wt.%)  30  1.0  1.2  1.4  1.2  1.4  FRO-1 FRO-2  FRO-1 FRO-2  30  0.8  20  FRO-1 FRO-2  10  0.4  Total S (wt. %)  0  0  0.2  Org C:N wt. ratio  Org-C (wt.%)  30  0  10  20  30  Org C:N wt. ratio  40  50  0.0  0.2  0.4  0.6  0.8  1.0  Total S  Figure 1. Sediment profiles of organic carbon (Org-C), ratio of organic carbon to nitrogen (Org C:N) and total sulfur in duplicate cores collected at GM and FRO. The higher concentrations of elemental Se (Se0) at GM may reflect greater contributions of Se0 in minerelated sediment inputs and possibly higher rates of in situ precipitation of secondary Se0. Using concentrations of polycyclic aromatic hydrocarbons (PAH data not shown) as a proxy for coal content (Woo et al. 1978), the PAH data indicate that the deposits at GM host greater concentrations of coal fines than FRO. XANES spectra on coal-bearing fines collected upstream of GM show that the Se in these materials are dominantly represented by Se0 (50% of Se inventory). Se is also predicted to accumulate in GM sediments through the in situ precipitation of secondary Se0. The microbially-mediated process of selenate (SeVI) reduction to elemental Se (Se0) is well documented (Oremland et al., 1990; Tokunaga et al., 1996; Guo et al., 1999), and is tightly linked to redox conditions. The anaerobic conditions present in both the lower water column and porewaters at GM likely favour the accumulation of Se0. The higher organic-Se content at GM likely relates in part to the decay of wetland plants (Zhang and Moore, 1997). The Se values observed at both sites are significantly greater than concentrations reported for various lithologies of coal-bearing strata in the region (Ryan and Dittrick 2001; Lussier et al., 2003), which show Se values ranging from ~2 to 5 mg/kg dry wt. in coal and from ~0.8 to 8 mg/kg dry wt. in wasterock  (mean ≅ 4 mg/kg). The elevated concentrations in GM and FRO sediments demonstrate that these sites are serving as a preferential repository for Se. Likely mechanisms include: 1) in situ adsorption/precipitation of Se as secondary (authigenic) phases; and 2) the accumulation of Se-rich organics associated with the decay of wetland plants. The porewater data (presented below) confirm that in situ reduction contributes to the Se sediment inventory.  Sediment Depth (cm)  0  10 GM-1 GM-2 20  Selenite: GM-1 Organo Se: GM-1 Elemental Se: GM-1  30  0  10  20  30  40  50  60  70  80  0  5  10  15  20  25  30  [Selenium] mg/kg  [Selenium] mg/kg 0  10  20 Selenite: FRO-1 Organo Se: FRO-1 Elemental Se: FRO-1  FRO-1 FRO-2 30  0  2  4  6  8 10 12 14 16 18 20  [Selenium] mg/kg  0  2  4  6  8  10  [Selenium] mg/kg  Figure 2. (Left) Sediment profiles of total Se in duplicate cores collected at GM and FRO. (Right) Se species as determined by XANES showing profiles of selenite (SeVI) , organo-Se and elemental Se (Se0). Sedimentary Redox Conditions Redox reactions have been shown to be the most important biogeochemical processes controlling Se speciation, precipitation/dissolution, sorption/de-sorption, methylation and volatilization (Masscheleyn and Patrick 1993). Accordingly, understanding sediment redox conditions is key to understanding the post-depositional behaviour of Se. Redox conditions in submerged sediments are driven by the oxidation (decomposition) of organic matter. The remineralization of organic matter occurs through microbiallymediated reactions which liberate energy from the oxidation of organic molecules. In the oxidation process, microbial assemblages will utilize electron acceptors in order of their free energy yield (ΔG). In the presence of dissolved oxygen, aerobic bacteria will utilize O2 as a terminal electron acceptor since this redox reaction affords the greatest ΔG. However, where the rate of O2 consumption exceeds the rate of  re-supply, O2 will become depleted and other secondary oxidants will be utilized. These, in order of their free energy yield, include NO3-, FeIII-oxides, MnIV-oxides, SO42- and CO2. The profiles of these redoxsensitive species, and their products, provide the information necessary to elucidate redox chemistry. Profiles of dissolved NO3-, Mn, Fe, SO42- and ΣH2S clearly illustrate the redox conditions at GM and FRO (Figure 3). At FRO, the decrease in NO3- concentration immediately below the sediment-water interface reflects NO3- reduction within 3 cm of the benthic boundary. Concentrations of dissolved Mn at FRO remain uniform across the sediment-water interface, below which concentrations dramatically increase to a porewater maximum at ~ 3 cm depth. In this scenario, the reductive dissolution of MnIV-oxides(s) to MnII(aq) results in the release of dissolved Mn to porewater. Dissolved Fe shows a similar profile to Mn. The increase in concentration of dissolved Fe below a depth of 3 cm reflects the addition of dissolved Fe to porewater from the reductive dissolution of FeIII-oxides. Dissolved ΣH2S at FRO appears in porewater below the zone of Fe and Mn release at horizons consistent with a reduction in SO42- concentration (Figure 3). H2S is a direct product of sulfate reduction, and its presence in the uppermost 5 cm of the deposit is indicative of reducing sedimentary conditions. In summary, the profiles for FRO show a vertical redox gradient from aerobic (oxic) conditions at the sediment-water interface to strongly reducing conditions within 5 cm of the benthic boundary. At GM, the profiles of redox-sensitive metabolites are similar in appearance to those at FRO (Figure 3). However, there are important distinctions. NO3- concentrations at GM, for example, exhibit a decrease above the sediment-water interface and demonstrate that NO3- reduction is occurring at or slightly above the benthic boundary. Similarly, the presence of abundant dissolved Fe and Mn above the sediment-water interface at GM is indicative of suboxic conditions which extend into the lowermost portion of the water column (Figure 3). Therefore, unlike FRO which exhibits aerobic (oxic) conditions at the benthic boundary, an interfacial aerobic zone at GM is absent. At GM, the higher degree of water stagnation imposed by dense emergent vegetation, in conjunction with the high sediment-oxygen demand, permit the development of suboxia above the sediment-water interface. In contrast, less-sluggish flows and lack of emergent vegetation at FRO maintain aerobic conditions at the benthic boundary. These differences have important implications to both the recycling of reduced Se species into the water column and likely Se uptake by aquatic biota (discussed below). Selenium Speciation in Water The dissolved Se inventory in the bottom waters at FRO is dominated by selenate (SeVI), with selenite (SeIV) at ~2% and organic species comprising relatively-small proportions (Figure 4). At GM, selenite (SeIV) accounts for ~15% of the total water column concentration. The suboxic conditions of the lower water column at GM are predicted to be more favourable to the persistence of remobilized selenite (SeIV). Profiles of dissolved Se (total), selenate (SeVI), selenite (SeIV) and organo-Se are closely linked to the profiles of redox-sensitive species (Figure 4). At GM, dissolved Se (total) decreases above the benthic boundary from ~15 μg/L in bottom waters to porewater minima of ≤1.5 μg/L (Figure 4). A similar pattern is apparent at FRO, although the decline in dissolved Se (total) concentration occurs below the sediment-water interface as opposed to in bottom waters (Figure 4). The decrease in dissolved Se (total)  at both sites reflects the removal of selenate (SeVI) from solution, where the zones of selenate (SeVI) consumption coincide with peaks in selenite (SeIV) (Figure 4). The location of the selenite (SeIV) peaks suggest that they reflect a direct result of selanate (SeO4-2) reduction in suboxic porewaters. Conversely, the selenite (SeIV) peaks may represent the result of remobilization from the reductive dissolution of Sebearing Fe-Mn oxyhydroxides (Belzile et al., 2000). In either case, the reduction mechanism involved is predicted to represent a microbially-mediated process that is strongly tied to redox conditions. Specifically, at GM the selenite (SeIV) peak occurs exactly at the benthic boundary, while at FRO, the peak occurs 1.5 cm below the sediment-water interface. Such observations can be linked to the subtle differences in redox zonation between the two sites. Profiles of dissolved organo-Se across the sedimentwater interface are similar to selenite (SeIV) (Figure 4). The formation of organo-Se is likely linked to the decomposition of organic matter in the near-surface sediments (Belzile et al., 2000). Dissolved [ΣH2S] μg/L  Distance from Sediment-Water Interface (cm)  Dissolved [Fe] mg/L 0  2  4  6  8  10  12 0  GM  20  30  40  50  GM  GM  0  SO4  Mn  -10  -20 Fe  H2S  -30  -40 0.0  0.5  1.0  1.5  2.0 0.0  Dissolved [Nitrate] mg/L  0.2  0.4  0.6  0.8  1.0 0  Dissolved [Mn] mg/L  0  10  20  30  40  50  50 100 150 200 250 Dissolved [Sulphate] mg/L  Dissolved [ΣH2S] μg/L  Dissolved [Fe] mg/L Distance from Sediment-Water Interface (cm)  10  10  60  0  5  10  15  20  20 FRO  FRO  FRO  10 0 H2S  -10 Fe  -20 -30  SO4  Mn -40 0  5  10 15 20 25 30 35  Dissolved [Nitrate] mg/L  0  1  2  3  Dissolved [Mn] mg/L  4  0  50  100  150  200  Dissolved [Sulphate] mg/L  Figure 3. Profiles of redox-sensitive parameters (dissolved nitrate, manganese, iron, sulfate and hydrogen sulfide) across the sediment-water interface at GM and FRO.  GODDARD MARSH Distance from Sediment-Water Interface (cm)  10  0  -10  -20 Se(IV) Se(VI) Se (Organic) Se (Total)  -30  Se(IV) Se (Organic)  -40 0  2  4  6  8  10 12 14 16 18 20 0  Dissolved [Selenium] μg/L  1  2  3  4  5  Dissolved [Selenium] μg/L  FORDING RIVER OXBOW Distance from Sediment-Water Interface (cm)  20  10  0  -10 Se(IV) Se (Organic)  -20 Se(IV) Se(VI) Se (Organic) Se (Total)  -30  -40 0  5  10  15  20  25  Dissolved [Selenium] μg/L  30  0  1  2  3  4  5  Dissolved [Selenium] μg/L  Figure 4. Profiles of dissolved Se (Total), SeIV, SeVI and Se-organic across the sediment-water interface at GM and FRO. Plots on right shows SeIV and Se-organic profiles at expanded scale. Below the selenite (SeIV) porewater peaks, concentrations of both selenite (SeIV) and selenate (SeVI) drop to very low levels. These drops in concentration reflect the near quantitative removal of Se from porewaters, and show that the sediments at both GM and FRO are serving as pronounced sinks for dissolved Se. Specifically, the removal of dissolved Se from solution sustains a concentration gradient between the water column and sediments that supports the diffusion-controlled transport of Se into the sediments. Possible removal mechanisms include selenite (SeIV) adsorption to mineral phases (e.g., clays, Fe oxides), precipitation of elemental Se (Se0), sequestration into algal/bacterial assemblages as organo-  selenides, and co-precipitation with secondary sulfide minerals (e.g., pyrite). XANES spectral data suggest that Se removal in elemental forms represents a dominant accumulation pathway for dissolved Se (Figure 2), as has been shown in other ponded systems (Tokunaga et al., 1996). Although the sediments at both sites serve as net sinks for Se, the interfacial peaks in selenite (SeIV) and organo-Se demonstrate that these species are recycled back into the water column (Figure 4). At GM, presence of suboxic bottom waters allows a greater proportion of the remobilized Se to escape to the water column. At FRO, the more aerobic interfacial conditions will serve to attenuate the flux of reduced Se species through oxidation/re-precipitation. Dissolved Selenium Fluxes Across Sediment-Water Interface Fluxes of the various Se species across the sediment-water interface were calculated based on Fick's First Law as described in Martin et al. (2003) (Table 1). Using the flux values for dissolved Se (total) as a measure of the net Se flux, the results show that the sediments at both GM and FRO are serving as net sinks for Se, with removal rates ranging from ~11 to 14 mg/m2/year. The flux for dissolved Se (total) at GM likely underestimates Se removal rates at GM, since it is predicted that some Se removal occurs in the suboxic bottom waters at this site. The flux calculations also show a greater degree of diffusive transport of reduced Se species back into the water column at GM in comparison to FRO. At GM, the sum of the upward fluxes of selenite (SeIV) and organo-Se are comparable in magnitude to the downward flux of selenate (SeVI). Conversely, the upward fluxes of selenite (SeIV) and organo-Se at FRO are much smaller by comparison. It can be concluded that the more anaerobic redox zonation at GM favours the recycling of reduced species in two ways: 1) selenite (SeIV) and organo-Se are remobilized closer to the sediment-water interface (shorter diffusive path length) and 2) the suboxic conditions in the lower water column at GM limit the attenuating effects of oxidation/re-precipitation. Table 1. Diffusive flux estimates for Se species across the sediment-water interface at GM and FRO. Positive values show diffusion into the sediments and negative indicate diffusion into the water column. Location FRO FRO FRO FRO GM GM GM GM  Se Species Dissolved Se (total) Se (IV) Se (VI) Se (org) Dissolved Se (total) Se (IV) Se (VI) Se (org)  Flux (mg/m2/y) 13.61 -0.39 14.28 -0.71 11.41 -2.77 7.43 -4.50  Implications for Selenium Management The data presented here provide further support that the recycling of reduced Se species is strongly dependent on the redox environment. Redox conditions are driven by the rate of organic matter oxidation, which in turn is strongly governed by organic matter supply and nutrient availability. Redox conditions in aquatic settings can be greatly influenced by mining activities through increased loadings of phosphorus (P) and nitrogen (N) associated with deforestation, sewage, and blasting chemicals (ANFO) (Martin and Pedersen 2004). Given the indirect relationship between Se cycling and nutrient loadings, current mine practices should ensure loadings of N and P to sensitive habitats are minimized. The data demonstrate that the sediments at GM and FRO are serving as sinks for dissolved Se in the water column. In this regard, these sites can be viewed as passive bioremediation systems that represent biogeochemical analogues to other forms of active anaerobic treatment systems (Adams 1998). The passive nature Se removal occurring at FRO and GM likely relates to several mechanisms including Se uptake by emergents, Se uptake by autotrophic algae/bacteria, in situ adsorption/precipitation of reduced species in suboxic zones and volatilization of methylated species. The porewater data for GM and FRO were used to estimate the potential importance of bioremediation associated with the in situ removal of reduced Se species (diffusion-controlled transport of Se across the sediment-water interface). Given a hypothetical pond surface area of 50 ha and a flux of 13.6 mg/m2/year yields a removal rate of ~7 kg/year. To put this value in the context of a mine effluent, this loading is equivalent to a waste stream with a Se concentration of 30 ppb and a flow rate of 7.5 L/s. This magnitude of removal is insufficient to mitigate site-wide loadings from coal mines in the region. However, bioremediation in this form may provide benefit for problematic waste streams that can be isolated and directed to certain pond environments (re-configured sediment ponds, tailings ponds, etc.). There is also the strong likelihood that the rates of removal observed in GM and FRO could be increased in response to the addition of phosphorus fertilizer, which would promote increased growth of autotrophic algae/bacteria and increased rates of redox reactions. Se bioremediation also has potential application to pit lakes, where the objective of fertilization would be to promote anaerobic conditions in pit bottom waters and the removal of Se from solution as reduced species. The application of any form of bioremediation would require careful consideration to the potential risks associated with increased accumulation of Se in aquatic organisms. Specifically, those conditions which favour the removal of selenium from the water column (reducing depositional environments) also favour the increased recycling of organic-Se. Implications for Selenium Bioavailability and Food Chain Transfer In Minnow (2004), GM exhibited increased Se uptake in benthic detrivores in comparison to FRO. Such observations are supported by the Se speciation data collected in this study. At GM, the shallower depth of remobilization of reduced Se species supports greater fluxes of organo-Se to the water column. Further, the presence of suboxic conditions adjacent to the sediment-water interface at GM is more  favourable for the accumulation of organo-Se in the water column. These conditions are likely more favourable for selenium accumulation in sediment-detrital food chains. AKNOWLEDGEMENTS The authors wish to acknowledge the funding provided by Elk Valley Coal (EVC) through the auspices of the Elk Valley Selenium Task Force. The success of the field campaigns was greatly facilitated through logistical support provided through EVC by Ron Jones, Greg Sword (Fording River Operations), Lany Amos (Elkview Operations) and Marc Meyer (Elkview Operations). On-site field support was additionally provided at Elkview by Lany Amos and Jody Frenette (MOE), and at Fording River by Michelle Jelinski, Larry Poch and Suzanne Adrain. 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