British Columbia Mine Reclamation Symposium

In-situ immobilization of selenium within the saturated zones of backfilled pits at coal-mine operations Bianchin, Mario; Martin, Alan J.; Adams, Jack 2013

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IN-SITU IMMOBILIZATION OF SELENIUM WITHIN THE SATURATED ZONES OF BACKFILLED PITS AT COAL-MINE OPERATIONS   Mario Bianchin, Ph.D.1  Alan Martin, M.Sc., 1 Jack Adams, Ph.D.3    1Lorax Environmental Services Ltd., 2289 Burrard Street, Vancouver, B.C. V6J 3H9 2Inotec Environmental Innovations, 85 S Ft. Douglas Blvd. Salt Lake City, UT 84112 ABSTRACT The removal of selenium (Se) from water using passive technologies at mine sites is challenging as a result of associated high flow rates. However, the chemical and hydraulic conditions of backfilled pits offer a potential means to bioremediate large volumes of water passively.  Specifically, the oxidation demand associated carbonaceous waste materials in conjunction with long water residence times within backfilled pits can produce the suboxic conditions required to support the removal of Se from solution through a suite of microbially-mediated processes.  Evidence for Se removal is provided from a study of a backfilled pit at a coal mine in Northern Canada.  Redox conditions within the saturated backfill are mildly-suboxic, as inferred from low levels of oxygen, nitrate and the presence of dissolved Mn and Fe.  Results show pronounced removal of dissolved Se within the saturated backfill, with concentrations decreasing from 40 ?g/L (upgradient of saturated zone) to <1 ?g/L within the saturated waste rock zone. The reduction in Se concentration was accompanied by a shift in Se speciation from dominantly selenate (SeVI) to selenite (SeIV) along the flow path. Nitrate-N values are also reduced to values below detection (<0.05 mg/L) in the saturated backfill.  The data suggest that this relatively small backfilled pit achieves a Se removal rate ranging from 0.03 to 0.3 mg/day/??????????3  with a hydraulic residence time of 0.3 to 3 years.  Collectively, the data imply that the saturated backfilled zones are serving as an effective site for the bioremediation of Se under mildly suboxic conditions.  KEY WORDS Selenium removal rate, bioremediation, passive treatment INTRODUCTION The bioremediation of selenium (Se) from mine waters using passive systems (e.g., subsurface-flow biological reactors and permeable reactive barriers) has received considerable attention owing to the prevalence of elevated Se concentrations in coal-mine drainages (CH2MHill, 2013).     Selenium is naturally abundant in rocks with sedimentary facies hosting carbonaceous shales and shales having particularly high levels; natural ranges documented are 206-280 mg kg-1 and 1?675 mg kg-1, respectively (Fern?ndez-Martinez and Charlet, 2009).  These rock types typically make up a fraction of the waste rock material at coal mines and constitute a major source of selenium to groundwater and surface water receptors. Toxicity is manifested as reproductive impairment due to maternal transfer resulting in juvenile abnormality or embryo death (Chapman et al., 2009). In coal mine wastes, Se may be associated with both reduced sulphur species (e.g., pyrite) and as discrete reduced Se compounds (e.g., elemental Se and selenides).  The release of Selenium from source rocks is hypothesized through the following reactions: Oxidation of selenide to selenite ????(?) + 3?2 +  3?+ +  5?? ? ???32? +  ??(??)3   (1) Oxidation of selenite to selenate ???32? + ?2 ? ???42?        (2) Both selenite and selenate have an affinity to absorb onto ferric oxyhydroxides; however, the selenate molecule has a lower sorptive affinity rendering it more mobile (Su and Suarez, 2000, Lo and Chen, 1997, Zhang and Sparks, 1990).   The backfilling of wasterock into the voids of open pits offers considerable potential for Se management with respect to both remobilization and attenuation.  From a remobilization perspective, the backfilling of waste rock below the water table in open pits will act to shut down oxidation mechanisms responsible for the release of Se to solution.  In saturated settings, the oxidation of these reduced species is effectively curtailed, thereby reducing the potential for Se remobilization.  This relates to the tendency of oxygen to become depleted in saturated pore spaces, and more importantly, to the diffusion-controlled replacement of dissolved oxygen to reaction sites, which is very slow.  In terms of attenuation, the presence of suboxic conditions is a pre-requisite for effective Se attenuation in saturated settings, whereby the removal of Se can occur via: 1) adsorption of selenite; 2) precipitation of elemental Se; 3) precipitation of Se as inorganic/organic selenides; and 4) co-precipitation of Se with reduced sulphur.  Suboxia is also required to reduce nitrate, the presence of which can inhibit Se reduction and selenite sorption (Dhillon and Dhillon, 2000).  Since backfilled-pit environments have the potential to develop suboxia, these systems offer significant potential with regards to Se attenuation. This paper summarizes the findings of a field-based hydrogeologic program that assessed the nature of Se behaviour in the saturated backfilled portion of a relatively small pit located in the Rocky Mountain Foothill region of northeastern British Columbia, Canada.  The results have broad-scale application to the management of and treatment Se at coal mines and other mining environments that are characterized by Se enrichment (e.g., phosphate mines, metal mines).  SITE DESCRIPTION AND METHODS The investigation was conducted at an active coal mining operation located in the Rocky Mountain Foothills in the northeastern portion of British Columbia, Canada.  The Rocky Mountain Foothills are part of the continental-scale Foreland Fold and Thrust Belt forming the eastern margin of the North American Cordillera.  Elevations range from 1,830 m to 2,130 m while precipitation ranges from 500 to 1,000 mm annually with more than 60 % falling as snow.   The hydrogeology of the region consists of a shallow water table (unconfined) system within the overlying surficial (quaternary-aged) sediments where present, and a bedrock system in fractured sedimentary rocks with hydraulic conductivity ranging from 10-10 ms-1 for a low-permeability shale to 10-3 ms-1 for a vuggy limestone.  The foothills are underlain by a succession of marine and non-marine sedimentary rocks (B.C. Environment, 1994).  This section provides a description of the pit shell physiology, characteristics of the backfilled (waste rock) material, the design of the porewater/groundwater monitoring network, methods for collecting and analyzing porewater samples.    Backfilled Pit Physiology The coal reserve associated with the study site is situated on a small synclinal structure minimizing the strip ratio which resulted in the development of an elongated shallow open pit approximately ~1000 m long, ~150 to 200 m wide and ~50 m deep.  Approximately 1.6 million tonnes of pulverized coal injection (PCI) grade coal was excavated from the pit using typical surface mining techniques relying on blasting and shovel methods.  Mining of the pit was completed by 2006 and backfilling of the pit occurred in 2011. A total of 4.8 Mm3 of waste rock was backfilled into the pit, and of this approximately 0.2 Mm3 is saturated.  The maximum depth of the waste rock in pit is on the order of 50 m (mean depth of about 28 m) with saturated thickness of approximately 22 m.  The pit shell configuration and the placement of backfilled waste rock is shown in Figure 1.  Table 1 summarizes the dimensions of the pit shell, as well as total and saturated backfilled waste rock material. Monitoring Network In November 2011, eleven (11) monitoring wells were installed with seven (7) of the wells completed as two monitoring well nests (MW11-01 and MW11-02).  A dual rotary (Barber) rig was used to advance the boreholes through waste rock and bedrock and to install the monitoring wells.  All monitoring wells consist of 2-inch schedule 80 polyvinylchloride (PVC).  Table 2 summarizes the details of the monitoring well installations.  Water quality profiling within the saturated portion of the in-pit dump was accomplished with the use of nested monitoring wells with 5-foot long screens (1.5 m) (Elci et al. 2001).  The location of these wells with respect to the in-pit dump is shown on Figure 1. The monitoring wells were placed along the centerline of the pit shell bottom as determined from a 2011 Lidar survey. MW11-01 and MW11-02 consist of four (4) and three (3) nested wells, respectively, installed within the saturated backfilled waste rock.  MW11-04 is screened across the bedrock/wasterock interface upgradient of the saturated zone, and it is assumed that groundwater within this well is representative of water flowing along the bottom of the backfilled pit and representative of non-attenuated source waste rock water.  MW11-03 and MW11-06 were installed within bedrock adjacent to the pit to characterize background water quality and water levels.       Table 1. Summary of Backfilled Pit Characteristics Description Dimension Units Size Pit Shell Length  Width Depth  Area Volume (m) (m) (m) (m2) (Mm3) 1100 125-205 ?50 171,792 4,709,000 Waste Rock Area Volume (m2) (Mm3) 168,871 4,761,000 Saturated Waste Rock Depth (mean) Area Volume (m) (m2) (m3) 18.81-20.32 22,7281-23,9732 197,0001-232,0002 Notes: 1 winter low groundwater level; 2 freshet high groundwater level    Figure 1. Study site map showing the physiology of the backfilled pit, instrumentation and water table surface map. Table 2. Summary of monitoring well installations. Monitoring Well Screened interval Instrumentation Upgradient Well MW11-04 Waste rock Levelogger? Saturated Backfill MW11-02B Waste rock/bedrock interface - MW11-02C Waste rock, mid pit HOBO? Levelogger? MW11-01A Waste rock/bedrock interface HOBO? MW11-01B Waste rock, lower pit Levelogger? MW11-01C Waste rock, water table HOBO? MW11-01D Waste rock, water table HOBO? Levelogger? Peripheral Bedrock MW11-02A Bedrock HOBO? Levelogger? MW11-03 Pit wall (southwest) HOBO? MW11-06 Pit wall (southeast) Levelogger? Notes: HOBO sensors provide hourly fluid conductivity data. Porewater Monitoring and Sampling Select monitoring wells were installed with Solnist Leveloggers? water level sensors and onset? HOBO? conductivity loggers for continuous (hourly) monitoring from November 2011 to March 2013.  Table 2 summarizes the distribution of instrumentation among the wells.  Manual measurements of water level and fluid conductivity were conducted during each groundwater sampling round.  Three (3) rounds of sampling were conducted coinciding with anticipated variations in groundwater level associated with a freshet-dominated hydrologic system.  As such, sampling occurred as follows: December 2011 coincident with fall low water levels; July 2012 representing freshet high water levels; and March 2013 representing the stream base flow and lowest water levels.  All groundwater wells went through the process of development, purging and sampling.  Wells were developed by the drillers using compressed air and air lift system.  To minimize artifacts of sampling associated sample quality variability and error, monitoring wells were purged and sampled using the Low-Flow technique (Varljen et al., 2006; Elci et al. 2001).  All monitoring wells with the exception MW11-03 were sampled using the QED bladder pump.  MW11-03 was sampled using a Waterra inertial pump.      Porewater Sample Analysis Groundwater samples were collected for bulk chemistry, anions, dissolved metals, Se speciation and microbial analyses.  Samples for the general aqueous analyses were sent to ALS Laboratories of Burnaby, BC.  Se speciation in water was completed at the Trent Water Quality Centre, Trent University similar to the method by Petrov et al. (2012).  The ion chromatography-separation method for Se species followed that of Wallschl?ger and Roehl (2001).  Samples for screening of Se-reducing microbes were sent to Department of Metallurgical Engineering, University of Utah.  Microbes from collected samples were analyzed by classical solid media (plating) and liquid culture techniques modified to enumerate both total microbes and Se reducers present.  RESULTS Backfilled Pit Hydrogeology Continuous water level readings from MW11-01D along with continuous fluid conductance readings from MW11-01D and MW11-01A are presented in Figure 2 for the period December 2011 to May 2013.  Water level records show that freshet leads to an increase of 2 m in saturated thickness within the waste rock pile.  Porewater conductance at the MW11-01D is approximately 300 ?S/cm lower than observed at MW11-01A (800 uS/cm), likely an effect of recharge events through the waste rock.  The effect of recharge, particularly during freshet, appears to reduce the fluid conductance within the saturated backfilled. However, the effect is more pronounced at the bottom of the pit (i.e., at MW11-01D).  These data indicate there could a preferential flow path for water at the interface of the waste rock and pit bottom.     Figure 2.  Water levels readings measured at MW11-01.  Continuous readings are shown for MW11-01D which is screened across the water table.  Manual measurements are also shown for MW11-01A,-01B, -01C and -01D. 02004006008001000120011271127.511281128.511291129.514-Sep-11 23-Dec-11 01-Apr-12 10-Jul-12 18-Oct-12 26-Jan-13 06-May-13 14-Aug-13Fluid Conductance (?S/cm) Water Elevation (masl) Date (dd/mm/yr) MW11-01D MW11-01AMW11-01B MW11-01CMW1-01D Logger Data MW11-01A ConductanceMW11-01D ConductanceThe water table surface map shown in Figure 1 was generated using the manual water measurements observed during March 2013.  In general, groundwater flow follows a south-easterly direction with deviations in flow patterns at a smaller scale due to influence of the pits.  Within the saturated backfilled waste rock, water is predicted to flow relatively rapidly from the upper unsaturated zone to the downstream zone of saturation (~20 m in thickness).  The water level within the lower saturated section of the pit is controlled by the southern pit wall which acts as decant point. This remnant bedrock wall produces the ?bath tub? conditions within the lower portion of the in-pit dump.  The greatest gradient, ranging from 10% to 20%, occurs between MW11-04 and MW11-01 due to the topographic relief between these two monitoring wells (see Table 3).  The horizontal groundwater gradient between MW11-01 and MW11-02 is negligible, however; the variation in elevation between the in-pit water level and southern pit wall at MW11-03 is 1.4 % and 2.5% for winter and freshet conditions, respectively.   The groundwater heads within the bedrock at MW11-02A and -02B are 0.25 m lower than that within wasterock at MW11-02C, indicating groundwater flow is downward (data not presented).  The gradient at MW11-02 was calculated to be 1.6%.  The water elevation in MW11-03 is higher than MW11-01 and MW11-02 indicating an upward gradient in the southern pit wall on the order of 1%.   Conceptually, water flow through the pit consists of rainfall infiltrating downward through the waste rock and collecting along the pit bottom.  Ponded water then follows along the pit floor to lower elevation where it pools in the lower portion of the pit within the 20 m-thick saturated zone.  Groundwater exits the pit in two ways: 1) spillage over the bedrock ridge at the southeastern of the pit (see Section B-B? in Figure 1); and 2) downward flow through the underlying bedrock. Groundwater velocities along these flows paths are predicted to be markedly different as the hydraulic conductivity of bedrock making up the pit floor is likely three 3 to 4 orders of magnitude lower than that of the waste rock material.  An estimate of waste rock hydraulic conductivity based on literature values for gravel yields a range of 10-4 to 10-5 m s-1 whereas, the hydraulic conductivity of the siltstone is predicted to be on the order of 10-7 to 10-10 m s-1 (Freeze and Cherry, 1979).  The groundwater flux through the lower pit floor (area = 24,000 m2 see Table 1) is estimated to be 3.3 m3/day (~0.04 L/s).  Overflow at the decant point is dependent on the porosity of the waste rock material, an uncertainty in flow estimates.  It is generally accepted that the porosity of end-dumped waste rock material could vary considerably thus varying the hydraulic conductivity.  Estimates of the horizontal flux through the pit assumed a porosity that ranges from 25% to 40% and a hydraulic conductivity of 10-4 to 10-5 ms-1.   During winter, the horizontal flux component range from 43 to 430 m3day-1, whereas during freshet, the flux ranges from 80 to 800 m3day-1.   Accordingly, the hydraulic residence time of the saturated backfilled pit ranges from 0.5 to 3 years during winter, and from 0.3 to 2 years during freshet (See Table 3). Pit Water Quality  In general, water quality observed during the first round is artificially elevated as a result of drilling-related impacts.  The disruption to the ambient geochemical environment by mechanical forces and injection of air during drilling can result in sampling artifacts associated with parameters associated with sulfides (e.g., Se), as well as redox-sensitive parameters.  Indeed, well development was insufficient to remove all artifacts of drilling for the first round of sampling.  As a result of drilling artifacts in the first round of sampling, the following discussion considers only the results of the 2nd and 3rd sampling rounds (May 2012 and March 2013).  Redox reactions are integral to the biogeochemical processes controlling the attenuation of Se (Masscheleyn et al. 1993; Martin et al., 2011).  Redox-sensitive parameters include dissolved oxygen (DO), dissolved nitrate (??3?), dissolved manganese (Mn), dissolved iron (Fe), sulfate (??42?), and hydrogen sulfide (?H2S = S2-, HS- and H2S).  Inspection of the data in Table 4 for DO, nitrate, dissolved Fe, dissolved Mn show evidence of mild suboxia within the saturated portions of the backfilled pit.  Dissolved oxygen concentrations were consistently elevated in the upgradient groundwater (MW11-04) ranging from 2 to 6.3 mg/L but for the remainder of the wells concentrations were generally below 0.7 mg/L.  The presence of dissolved iron in the saturated backfill groundwater would mean that DO is less than observed. Likewise, nitrate levels at MW11-04 was observed at 1 mg/L and at less than detection limits for the peripheral and bedrock wells.  Dissolved Mn values for wells screened in backfill (e.g., MW11-04, MW11-02C, MW11-01-A/B/C) show values ranging from 0.3 to 0.9 mg/L. Such values are likely indicative of Mn(IV)-oxide reduction. Similarly, the presence of slightly elevated dissolved Fe is indicative of suboxia.     Table 3. Hydrogeological parameters and estimates of flow through backfilled pit.  Well Pair Gradient by Well Pairs (%) Lithology K5 Average Width of Pit Saturated Thickness @           MW-11-01 Saturated Waste Rock Volume6 Porosity5 Saturated Waste Rock Pore Volume GW Q Pit Residence Time MW11-04: MW11-02C MW11-02C : MW11-01C MW11-1C:     Seep  (m s-1) (m) (m) (Mm3) (%) (Mm3) (m3/day) (years) Baseflow1             MW11-04 19            MW11-02C  -0.06           MW11-01C   1.4 Wasterock4 1E-05 200 17.77 0.20 25 0.05 42.7 3 Spill Point - Seep3   1.4 Wasterock4 1E-04 200 17.77 0.20 40 0.08 426.7 0.5 Freshet2             MW11-04 11            MW11-02C  -0.05           MW11-01C   2.5 Wasterock4 1E-05 200 18.75 0.20 25 0.05 80.4 2 Spill Point - Seep3   2.5 Wasterock4 1E-04 200 18.75 0.20 40 0.08 803.8 0.3 Notes: 1 water levels observed on 8-Mar-13 during low winter low flow period 2 water levels observed on 3-Jun-13 during mid-freshet flow period 3 elevation of top of pit wall at MW11-03; active flow out of pit is assumed to occur as spill over remnant southern pit wall. 4 waste rock assumed to have consistency of gravel.  5 range of hydraulic conductivity and porosity values expected for gravel. Data from Freeze & Cherry 1979. 6 value based on estimate derived from 1m resolution maps of original ground, mined out pit and waste rock surface (Lidar surveys).     Table 4. Porewater chemistry for general parameters and redox indicators for upgradient pit water, saturated backfill and peripheral bedrock. Data represent geometric means samples collected in December 2011, May 2012 and March 2013. Parameter1 pH Conductivity Temp. DO ORP Alkalinity2 DOC ??3? ??42?  ?2?3  Mn-D Fe-D Units (pH) (uS/cm) (?C) (mg/L) (mV) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) Upgradient Pit Water MW11-04 7.1 1524 7.3 4.8 14 289 2.4 1.47 652 <0.001 0.0704 0.5370 Saturated Backfill MW11-02-B 7.1 1888 7.1 1.3 411 286 3.7 0.447 893 <0.001 0.8268 0.1671 MW11-02-C 7.1 1725 7.0 0.6 459 308 2.2 0.208 785 <0.001 0.4295 0.1180 MW11-01A 7.0 1327 5.7 1.4 90 329 3.3 0.116 443 <0.001 0.2577 0.0999 MW11-01B 6.8 1636 5.6 0.2 459 345 4.3 0.200 781 <0.001 0.6321 0.1274 MW11-01-C 6.9 1664 6.6 40.3 4-96 426 1.8 0.082 715 <0.001 0.4828 0.5342 Peripheral Bedrock MW11-02-A 7.1 1700 7.7 0.9 4-29 325 8.7 0.102 691 0.004 3.3168 0.8552 MW11-01-D 7.1 1151 7.5 0.4 4-90 379 4.8 0.085 282 0.002 0.6112 0.2858 MW11-03 7.2 581 3.2 0.3 4-118 338 3.9 <0.005 10 0.003 0.3944 4.5303 MW11-06 7.1 1080 4.3 1.1 4-88 565 4.7 0.096 25 0.005 0.0485 0.1278 Notes: 1Where concentration was below the method detection limit, concentration was set equal to detection limit value for statistical calculations 2Alkalinity = total (CaCO3) 3 H2S as S 4Arithmetic mean shown due to negative values in dataset Conversely, H2S concentrations do not reveal evidence of ??42? reduction as levels are below the limits of analytical detection (<0.002 mg/L) in all samples with the exception of the peripheral bedrock wells.  Dissolved organic carbon (DOC) is present within the porewater of the backfilled pit, although at approximately half the concentrations observed within the peripheral wells.  Under the current redox regime, sulphate represents a suitable proxy for waste rock drainage, and concentrations of this parameter can be used to further assess pit hydraulics.  Figure 3 summarizes the sulphate levels during the three rounds of sampling.  The sulphate levels in those wells screened across the wasterock-pitfloor interface, namely, MW11-01A and MW11-04, are considerably lower during the May 2012 sampling round under freshet recharge conditions.  These fluctuations correlate well with the variations observed in the fluid conductance data in Figure 2 indicating that preferential flow paths along the pit floor may exist.  Further, elevated sulphate levels in bedrock at MW11-02 (A/B) support the interpretation of water levels at this location which indicate a downward vertical gradient as background sulfate concentrations are typically less than 100 mg/L, as observed at MW11-03 and MW11-06.   MW11-04MW11-02CMW11-02BMW11-01AMW11-01BMW11-01CMW11-01DMW11-02AMW11-03MW11-06Concentration (mg/L)02004006008001000201120122013Upgradient Pit Water In-Pit Saturated WaterPeripheral Bedrock Figure 3. Sulfate concentrations measured in water for December 2011, May 2012 and March 2013 sampling rounds for upgradient pit water, saturated backfill and peripheral bedrock. Selenium Total Se, SeIV and SeVI concentrations measured during for each sampling round are summarized in Figure 4 while the mean concentrations 2nd and 3rd sampling rounds are shown in Figure 5.  The highest Se levels are observed upgradient of the saturated zone (MW11-04), while the lowest values are observed in the peripheral bedrock wells. The Se levels in the in-pit saturated water are consistently below 1 ?g/L.   Inspection of the data shows a predominance of SeVI at MW11-04, comprising on average 95% of the total Se inventory. The reduced form SeIV is present, albeit at low levels (less than 2%).  In contrast, the in-pit saturated water shows a predominance of SeIV, on average making up 74% of the total Se inventory.  SeIV is also the dominant species within the peripheral bedrock making up 71% of the inventory.  The in-pit saturated water contains less than 1% of the Se-T observed at MW11-04 suggesting that 99% of the Se-T entering the in-pit saturated zone is lost by attenuation through mechanisms which may include adsorption of SeIV, precipitation of elemental Se, or precipitation of metal/organic selenides.  This conclusion is supported in the shift in dominant Se species from SeVI in the source water to SeIV within the in-pit saturated water. The reduction in Se concentrations within the backfilled pit cannot be attributed to dilutionary processes, as revealed by the persistence of elevated SO4 levels and inferred groundwater flow paths which do not indicate significant recharge of more dilute groundwater sources. In this regard, the backfilled pit can be viewed as a passive bioreactor, where suboxic redox processes promote denitrifcation and immobilization of Se.          2011     2012     2013                  2011      2012      2013        2011       2012       2013             2011          2012          2013             2011         2012         2013             2011        2012        2013      2011 2012 2013        2011  2012  2013          2011   2012   2013            2011    2012    2013Concentration (?g/L)0.0010.010.11101001000Total Se Se IV Se VI MW11-01DMW11-02AMW11-03 MW11-06MW11-01CMW11-04MW11-02C MW11-02B MW11-01BMW11-01AUpgradient Pit WaterIn-Pit Saturated Water Peripheral BedrockFigure 4. Summary of total selenium and selenium species concentrations in groundwater for upgradient pit water, saturated backfill and peripheral bedrock. All values in ?g/L. MW11-04MW11-02CMW11-02BMW11-01AMW11-01BMW11-01CMW11-01DMW11-02AMW11-03MW11-06Concentration (?g/L)0.010.1110100Total SeSe IV Se VIIn-Pit Saturated Water Peripheral BedrockUpgradient Pit Water Figure 5. Geometric mean Se-T, SeVI and SeIV concentrations in groundwater of May 2012 and March 2013 sampling rounds for upgradient pit water, saturated backfill and peripheral bedrock. Microbial Communities Results of total microbial and Se-reducer counts in groundwater samples are summarized in Figure 6.  Total microbial counts provide an indication of microbe population viability at time of sampling.  The   microbial counts over the three sampling periods demonstrate variability in both saturated backfilled and peripheral groundwater wells.  Se-reducers were recorded in only two samples in December 2011 (MW11-02C-FD) and March 2013 (MW11-04-FD).  Although specific counts of Se-reducers were low and varied between sampling events, the potential for selenium reduction in the saturated backfilled is better characterized taken into account the results of the selenium containing media microbial analysis.  Se precipitation was observed following two weeks of incubation in samples from MW11-02C-FD and MW11-04.   Following a month of incubation, Se reduction was also detected, albeit at much lower levels, in samples MW11-02B and MW-11-03.    Figure 6. Summary of microbial plates counts for upgradient pit water, saturated backfill and peripheral bedrock. Estimates of Selenium Removal Rates Estimates a Se removal rates are not well constrained due to uncertainty in the upper concentration end member and flow through rate (i.e., water residence time).  Nonetheless, in-situ Se removal rates were calculated based on the observed difference in total dissolved Se (Se-T) concentration between MW11-04 (upgradient pit water) and saturated backfilled porewater during both low (winter) and high (freshet) flow periods.  The parameters for the Se removal rate estimates are summarized in Table 5.  The mean Se-T concentration observed at MW11-04 for the May 2012 and March 2013 sampling rounds was 38 ?g/L, and the mean concentration within the saturated backfilled porewater was less than 0.1 ?g/L.  As   discussed previously, pit flow through rate varies due to uncertainly in waste rock porosity values; accordingly a range of possible flow rates were considered.  Overall, removal rates were calculated to range from 0.03 to 0.17 mg/day/m3 of waste rock (winter low flow) and 0.06 to 0.33 mg/day/m3 of waste rock (freshet flow) (Table 5).  Table 5. Summary of Se removal rates Waste Rock Volume Porosity Flow Rate Residence Time [Se-T] In [Se-T] Out Se Loading Rate Se Removal Rate (m3) (%) (m3/day) (years) (?g/L) (?g/L) (mg/d) (mg/d/m3) 197000 25 42.7 3 38 0.1 1618 0.03 232000 40 426.7 0.5 38 0.1 16172 0.17 197000 25 80.4 2 38 0.1 3047 0.06 232000 40 803.8 0.3 38 0.1 30464 0.33 CONCLUSIONS This paper represents the first known study of its kind in northeastern B.C. to rigorously examine Se behaviour in the saturated zone of a backfilled pit.  The monitoring methods employed have specifically allowed delineation of the transport characteristics and bioremediation potential of Se within suboxic zones of the waste rock backfill.  The results provide direct evidence of in situ Se reduction under mildly suboxic conditions, commensurate with nitrate reduction.  Overall, the data suggest that a significant degree of Se attenuation is occurring within the backfilled pit under relatively short residence times.  These results have direct application to coal mines globally that employ pit backfill as part of water/waste management planning. The data also suggest that for larger backfilled systems characterized by longer groundwater retention time, a higher magnitude of Se attenuation could be achieved. In this regard, this form of in situ bioremediation represents a possible means to treat large water volumes not accommodated by traditionally forms of passive treatment (e.g., wetlands, permeable reactive barriers). REFERENCES Benner, S.G., Blowes, D.W. and Ptacek, C.J. 1997. A full-scale porous reactive wall for prevention of acid mine drainage. Ground Water Monitoring and Remediation 17:99-107. Blowes, D.W., Ptacek, C.J., Benner, S.G., McRae, C.W.T., Bennett, T.A. and Puls, R.W. 2000. Treatment of inorganic contaminants using permeable reactive barriers. Journal of Contaminant Hydrology 45: 123-137. B.C. Environment, 1994. Ground Water Resources of British Columbia.  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Selenium environmental cycling and bioavability: a structural chemist point of view. Reviews in Environmental Science and Bio/Technology 8:81-110. Lo, S.L., and Chen, T.Y. 1997. Adsorption of SeVI and SeIV on an iron-coated sand from water. Chemosphere 35:919-930. Martin A.J., Simpson S., Fawcett S., Wiramanaden C.I.E., Pickering I.J., Belzile N., Chen Y.-W., London J., Wallschl?ger  D. (2011) Biogeochemical mechanisms of selenium exchange between water and sediments in two contrasting lentic environments Environmental Science and Technology. 45:2605-2612. Masscheleyn, P.H. and Patrick, W.H. 1993. Biogeochemical processes affecting selenium cycling in wetlands. Environmental Toxicology Chemistry Journal 12:2235-2243. Petrov, P.K., Charters, J.W. and Wallschl?ger, D. 2012.  Identification and determination of selenosulfate and selenocyanate in flue gas desulfurization waters, Environmental Science and Technology 46, 1716-1723 Su, C.M. and Suarez, D.L. 2000. Selenate and selenite sorption on iron oxides: an infrared and electrophoretic study. Soil Science Society of America Journal 64:101-111. Varljen, M.D., Barcelona, M.J., Oberiener, J. and Kaminski, D. 2006. Numerical Simulations to Assess the Monitoring Zone Achieved during Low-Flow Purging and Sampling. Ground Water Monitoring & Remediation 26, 1:44-52.   Wallshl?ger, D. and Roehl, R. 2001.  Determination of inorganic selenium speciation in waters by ion chromatorgraphy-inductively coupled plasma-mass spectrometry using eluant elimination with a membrane suppressor. Journal of Analytical Atomic Spectrometry 16:922-925. Zhang, P.C. and Sparks, D.L. 1990. Kinetics of selenate and selenite adsorption desorption at the goethite water interface. Environmental Science and Technology 24: 1848-1856.  


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