Selenate bioreduction in a large in situ field trial.

Removing selenium (Se) from mine effluent is a common challenge. A long-term, in situ experiment was conducted to bioremediate large volumes (up to 7500 mc d-1) of Se(VI)-contaminated water (mean 87 µg L-1) by injecting the water into a saturated waste rock fill (SRF) at a coal mining operation in Elk Valley, British Columbia, Canada. To stimulate/maintain biofilm growth in the SRF, labile organic carbon (methanol) and nutrients were added to the water prior to its injection. A conservative tracer (Br-) was also added to track the migration of injected water across the SRF, identify wells with minimal dilution and used to quantify the extent of bioreduction. The evolution of the Se species through the SRF was monitored in time and space for 201 d. Selenium concentrations of <3.8 µg L-1 were attained in monitoring wells located 38 m from the injection wells after 114 to 141 d of operation. Concentrations of Se species in water samples from complementary long-term (351-498 d) column experiments using influent Se(VI) concentrations of 1.0 mg L-1 were consistent with the results of the in situ experiment. Solid samples collected at the completion of the column experiments confirmed the presence of indigenous Se-reducing bacteria and that the sequestered Se was present as insoluble Se(0), likely in Se-S ring compounds. Based on the success of this ongoing bioremediation experiment, this technology is being applied at other mine sites.

Quantifying denitrification in a field-scale bioremediation experiment.

Nitrate (NO3-) in mine waste rock derived from undetonated NH4NO3 can contaminate receiving waters. An in-situ bioremediation experiment was conducted at a coal mining operation in Elk Valley, British Columbia, Canada to remediate NO3- from large volumes of mine water. Over the test period (201 d), 5000 to 7500 m3 d-1 of NO3--rich (mean concentration 22 mg N L-1) mine water was injected into saturated waste rock along with methanol, nutrients, and a conservative tracer (Br-). Complete denitrification (<0.5 mg N L-1) was recorded in monitoring wells located 38 m from the injection wells after 114 to 141 d of operation. Plots of δ15N- and δ18O-NO3- versus NO3--N concentrations for monitoring wells yielded isotopic enrichment factors (ε) for δ15N- and δ18O-NO3- of -25.7 and -13.2 ‰ for high C/C0 NO3- concentrations (>10.5 mg N L-1) and -5.5 and -3.6 ‰ for lower C/C0 values. The fraction of NO3- denitrified (Dp) calculated using bi-linear ε values for δ15N- and δ18O reproduced the Dp determined independently using a conservative tracer indicating that stable isotope tracers of the NO3- reducing processes in bioremediation are invaluable to determine Dp. Based on the success of this ongoing bioremediation experiment, the technology is being applied at other sites.

Ebullition enhances chemical mass transport across the tailings-water interface of oil sands pit lakes.

Base Mine Lake (BML) was the first commercial-scale demonstration oil sands pit lake established in northern Alberta, Canada. Recent studies indicate that ebullition enhances internal mass loading of dissolved constituents during settlement and dewatering of methanogenic fluid fine tailings (FFT) below the overlying water cap. Here, we describe results of integrated field measurements and numerical modelling to (i) determine potential for ebullition and enhanced mixing within BML, and (ii) assess impacts on chemical mass transport across the tailings-water interface. We observed sharp increases in [CH4(aq)] with depth from <0.1 mg L-1 immediately above the interface to >60 mg L-1 over the upper 1.5 to 3.0 m of FTT. Thermodynamic modelling revealed that maximum [CH4(aq)] values represent 60 to 80% of theoretical saturation, and corresponding total dissolved gas pressures approach or exceed fluid pressures. These findings supported integration of enhanced mixing into one-dimensional (1-D) advective-dispersive transport models, which substantially improved upon previous simulations of conservative tracer (i.e., Cl-) profiles and chemical mass fluxes. The models revealed a positive relationship between CH4(aq) saturation and enhanced mixing, showing that ebullition enhances internal mass loading. This information has potential to inform ongoing assessments of pit lake performance and support improved closure and reclamation planning at oil sands mines.

The geochemistry and hydrology of coal waste rock dumps: A systematic global review.

Coal has been a major global resource for at least the past 250 years. The major waste product of coal mining is waste rock, which is stored in dumps of various sizes. Although the adverse effects of coal waste rock dumps on ecosystems and human health are widely recognised, there is little information on their internal hydrological and geochemical processes in the peer-reviewed literature. Coal and conventional waste rock dumps share many similarities, but coal waste rock dumps differ in structure, organic matter content, and size, which can affect the timing and rate of aqueous chemical release. In this global systematic review, we identify limited links to climate setting and dump construction, and inconsistent reporting of sampling and monitoring approaches, as limitations to the generalisation of findings. Furthermore, sources of aqueous constituents of interest (COIs) are not routinely or adequately identified, which can lead to incorrect assumptions regarding COI availability and geochemical mobility. Water flow regimes within dumps are dominated by matrix and/or preferential flow, depending on dump texture; these flow mechanisms exert a primary control on patterns of aqueous COI release. The inability to successfully transfer COI release rates from laboratory or field scale trials to operational scale dumps is primarily due to limitations of testing methods and fundamental characteristics of scale. Prediction of future release rates is hampered by a lack of long-term studies that fully characterise geochemistry (e.g., source and COI production rates) as well as dump hydrology (e.g., water balance, water migration). Five critical elements to include in best practice investigations are climate setting, dump physical characteristics, geochemical processes, water regime, and environmental load over time, as aqueous release of COIs from coal waste rock dumps occurs over decades to centuries. Key considerations are identified for each of these elements to guide best practice.

Simulating nitrate release from an unsaturated coal waste rock dump.

Knowledge of the controls affecting the release of contaminants from waste rock dumps is critical for developing strategies to mitigate downstream impacts on water quality. In this study, a three-dimensional model of a large coal waste rock dump constructed in the Elk Valley, British Columbia, Canada was developed to capture the impact of construction history (1981-2012) and solute transport on nitrate (NO3-) release over a 100-year timeframe. The model consisted of 21, one-dimensional finite element models that represented the temporal evolution of the dump. Nitrate, derived from undetonated blast products, was assumed to be present at the time of waste rock placement and was simulated as a conservative species. The simulated pattern of NO3- release to the surface water receptor occurred approximately 8 years before its measured arrival. This time lag is attributed to displacement of the water within a basal alluvial aquifer by dump effluent. The simulated patterns of historic releases corrected for the 8-year time lag, compare favourably with monitoring data and suggest the dominant hydrogeological and geochemical mechanisms are captured in the model. The model indicated the flushing of NO3- from the dump should be complete by about 2042 with a peak effluent concentration of NO3- in 2008. The addition of reclamation covers to the model resulted in an immediate decrease in the annual NO3- loading rate but extended the time frame for NO3- release from the dump relative to the no cover case. The model also showed that the timing of cover placement had little impact on NO3- release relative to the no cover case due to long duration of waste rock placement (~30 years) over a relatively large footprint.

Assessing the fate of explosives derived nitrate in mine waste rock dumps using the stable isotopes of oxygen and nitrogen.

Ammonium nitrate (NH4NO3) mixed with fuel oil is a common blasting agent used to fragment rock into workable size fractions at mines throughout the world. The decomposition and oxidation of undetonated explosives can result in high NO3- concentrations in waters emanating from waste rock dumps. We used the stable isotopic composition of NO3- (δ15N- and δ18O-NO3-) to define and quantify the controls on NO3- composition in waste rock dumps by studying water-unsaturated and saturated conditions at nine coal waste rock dumps located in the Elk Valley, British Columbia, Canada. Estimates of the extent of nitrification of NH4NO3 in oxic zones in the dumps, initial NO3- concentrations prior to denitrification, and the extent of NO3- removal by denitrification in sub-oxic to anoxic zones are provided. δ15N data from unsaturated waste rock dumps confirm NO3- is derived from blasting. δ15N- and δ18O-NO3- data show extensive denitrification can occur in saturated waste rock and in localized zones of elevated water saturation and low oxygen concentrations in unsaturated waste rock. At the mine dump scale, the extent of denitrification in the unsaturated waste rock was inferred from water samples collected from underlying rock drains.

Nitrate release from waste rock dumps in the Elk Valley, British Columbia, Canada.

The origin, distribution and leaching of nitrate (NO3-) from coal waste rock dumps in the Elk Valley, British Columbia, Canada were defined using chemical and NO3- isotope analyses (δ15N- and δ18O-NO3-) of solids samples of pre- and post-blast waste rock and from thick (up to 180m) unsaturated waste rock dump profiles constructed between 1982 and 2012 as well as water samples collected from a rock drain located at the base of one dump and effluent from humidity cell (HC) and leach pad (LP) tests on waste rock. δ15N- and δ18O-NO3- values and NO3- concentrations of waste rock and rock drain waters confirmed the source of NO3- in the waste rock to be explosives and that limited to no denitrification occurs in the dump. The average mass of N released during blasting was estimated to be about 3-6% of the N in the explosives. NO3- concentrations in the fresh-blast waste rock and recently placed waste rock used for the HC and LP experiments were highly variable, ranging from below detection to 241mg/kg. The mean and median concentrations of these samples ranged from 10-30mg/kg. In this range of concentrations, the initial aqueous concentration of fresh-blasted waste rock could range from approximately 200-600mg NO3--N/L. Flushing of NO3- from the HCs, LPs and a deep field profile was simulated using a scale dependent leaching efficiency (f) where f ranged from 5-15% for HCs, to 35-80% for the LPs, to 80-90% for the field profile. Our findings show aqueous phase NO3- from blasting residuals is present at highly variable initial concentrations in waste rock and the majority of this NO3- (>75%) should be flushed by recharging water during displacement of the first stored water volume.

Fate and Transport of Shale-derived, Biogenic Methane.

Natural gas extraction from unconventional shale gas reservoirs is the subject of considerable public debate, with a key concern being the impact of leaking fugitive natural gases on shallow potable groundwater resources. Baseline data regarding the distribution, fate, and transport of these gases and their isotopes through natural formations prior to development are lacking. Here, we define the migration and fate of CH4 and δ13C-CH4 from an early-generation bacterial gas play in the Cretaceous of the Williston Basin, Canada to the water table. Our results show the CH4 is generated at depth and diffuses as a conservative species through the overlying shale. We also show that the diffusive fractionation of δ13C-CH4 (following glaciation) can complicate fugitive gas interpretations. The sensitivity of the δ13C-CH4 profile to glacial timing suggests it may be a valuable tracer for characterizing the timing of geologic changes that control transport of CH4 (and other solutes) and distinguishing between CH4 that rapidly migrates upward through a well annulus or other conduit and CH4 that diffuses upwards naturally. Results of this study were used to provide recommendations for designing baseline investigations.

Measuring Concentrations of Dissolved Methane and Ethane and the 13 C of Methane in Shale and Till.

Baseline characterization of concentrations and isotopic values of dissolved natural gases is needed to identify contamination caused by the leakage of fugitive gases from oil and gas activities. Methods to collect and analyze baseline concentration-depth profiles of dissolved CH4 and C2 H6 and δ13 C-CH4 in shales and Quaternary clayey tills were assessed at two sites in the Williston Basin, Canada. Core and cuttings samples were stored in Isojars® in a low O2 headspace prior to analysis. Measurements and multiphase diffusion modeling show that the gas concentrations in core samples yield well-defined and reproducible depth profiles after 31-d equilibration. No measurable oxidative loss or production during core sample storage was observed. Concentrations from cuttings and mud gas logging (including IsoTubes® ) were much lower than from cores, but correlated well. Simulations suggest the lower concentrations from cuttings can be attributed to drilling time, and therefore their use to define gas concentration profiles may have inherent limitations. Calculations based on mud gas logging show the method can provide estimates of core concentrations if operational parameters for the mud gas capture cylinder are quantified. The δ13 C-CH4 measured from mud gas, IsoTubes® , cuttings, and core samples are consistent, exhibiting slight variations that should not alter the implications of the results in identifying the sources of the gases. This study shows core and mud gas techniques and, to a lesser extent, cuttings, can generate high-resolution depth profiles of dissolved hydrocarbon gas concentrations and their isotopes.

Characterization of physical mass transport through oil sands fluid fine tailings in an end pit lake: a multi-tracer study.

Soft tailings pose substantial challenges for mine reclamation due to their high void ratios and low shear strengths, particularly for conventional terrestrial reclamation practices. Oil sands mine operators have proposed the development of end pit lakes to contain the soft tailings, called fluid fine tailings (FFT), generated when bitumen is removed from oil sands ore. End pit lakes would be constructed within mined-out pits with FFT placed below the lake water. However, the feasibility of isolating the underlying FFT has yet to be fully evaluated. Chemical constituents of interest may move from the FFT into the lake water via two key processes: (1) advective-diffusive mass transport with upward pore water flow caused by settling of the FFT; and (2) mixing created by wind events or unstable density profiles through the lake water and upper portion of the FFT. In 2013 and 2014, temperature and stable isotopes of water profiles were measured through the FFT and lake water in the first end pit lake developed by Syncrude Canada Ltd. Numerical modelling was undertaken to simulate these profiles to identify the key mechanisms controlling conservative mass transport in the FFT. Shallow mixing of the upper 1.1 m of FFT with lake water was required to explain the observed temperature and isotopic profiles. Following mixing, the re-establishment of both the temperature and isotope profiles required an upward advective flux of approximately 1.5 m/year, consistent with average FFT settling rates measured at the study site. These findings provide important insight on the ability to sequester soft tailings in an end pit lake, and offer a foundation for future research on the development of end pit lakes as an oil sands reclamation strategy.

Defining near-surface groundwater flow regimes in the semi-arid glaciated plains of North America.

The dominant transport mechanisms controlling the migration of contaminants in geologic media are advection and molecular diffusion. To date, defining which transport mechanism dominates in saturated, non-lithified sediments has been difficult. Here, we illustrate the value of using detailed profiles of the conservative stable isotope values of water (δ(2)H and δ(18)O) to identify the dominant processes of contaminant transport (i.e. diffusion- or advection-dominated transport) in near-surface, non-lithified, saturated sediments of the Interior Plains of North America (IPNA). The approach presented uses detailed δ(18)O analyses of glacial till, glaciolacustrine clay, and fluvial sand core samples taken to depths of 11-50 m below ground at 22 sites across the IPNA to show whether transport in the fractured and oxidized sediments is dominated by advection or diffusion. Diffusion is by far the dominant transport mechanism in fine-textured lacustrine and glacial till sediments, but lateral advection dominates transport in sand-rich sediments and some oxidized, fine-textured lacustrine and glacial till sediments. The approach presented has a number of applications, including identifying dominant transport mechanisms in geomedia and potential protective barriers for underlying aquifers or surface waters, constraining groundwater transport models, and selecting optimum locations for monitoring wells. These findings should be applicable to most glaciated regions of the world that are composed of similar hydrogeologic units (i.e. low K clay till layers overlain by higher K coarse-textured aquifers or weathered clay till layers) and may also be applicable to non-glaciated regions exhibiting similar hydrogeologic characteristics.

Reservoirs of Selenium in Coal Waste Rock: Elk Valley, British Columbia, Canada.

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 (Se(2-)) in pyrite and sphalerite, 19% of Se is present as selenite (Se(4+)) in barite, 21% of Se is present as exchangeable Fe oxyhydroxide and clay-adsorbed Se(4+), 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 Se(2-)-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.

Measuring solid percentage of oil sands mature fine tailings using the dual probe heat pulse method.

The reclamation of mature fine tailings (MFT) is a critical challenge for the oil sands industry in western Canada, and a nonradioactive, automated, and inexpensive method to monitor the MFT solidification is needed. The objective of this paper is to evaluate the feasibility of a dual-probe heat pulse (DPHP) method to measure MFT solid percentage. Dual-probe heat pulse measurements were performed on three MFT samples, each at various solid percentages. A linear relationship ( = 0.9495 + 0.0558) was established between the DPHP-measured solid percentage () and that of oven-dry method (). Six additional MFT samples were collected and measured to validate the DPHP method. The specific heats of the six MFT solids were measured independently using a modulated differential scanning calorimetry method, and the sensitivity of DPHP-measured MFT solid percentage to the specific heat of MFT solids was evaluated. The result shows that the DPHP method can be used to accurately measure MFT solid percentages, and the accuracy can be further improved if the specific heat of the MFT solids is measured independently.

In situ experiment to determine advective-diffusive controls on solute transport in a clay-rich aquitard.

Solute transport in clay-rich aquitards is characterized as molecular diffusion- or advection-dominated based on the Péclet number (P(e)). However, few field-based measurements of the coefficient of molecular diffusion (D(e)) exist, and none with a range of advection- or diffusion-dominated conditions in the same aquitard. In this long-term field experiment, standing water in a recovering well was spiked with deuterium ((2)H), then water-level recovery and δ(2)H values were monitored as the well returned to static conditions over 1054 days. After a second (2)H spike, water levels and δ(2)H values were monitored to day 1644 while under near static conditions. Modeling of the second spike was used to define the D(e) of (2)H as (3-4)× 10(-10)m(2)s(-1) for an accessible porosity of 0.31. Reservoir concentrations from the initial spike were modeled to define the transition from advection- to diffusion-dominated transport. This occurred after 200 days, consistent with a transition in P(e) from <1 to >1 when the length term is taken as the radial extent of the tracer plume (normalized concentration <0.05). This study verifies plume extent as the characteristic length term in the calculation of P(e) and demonstrates the transition from advection- to diffusion-dominated transport as the value of P(e) decreases below unity.

A comparision of laboratory and field based determinations of molecular diffusion coefficients in a low permeability geologic medium.

Molecular diffusion is the dominant transport mechanism for contaminants in many saturated clay-rich aquitards. The effective coefficient of diffusion (Da) is traditionally determined by conducting laboratory tests on cm-scale core samples that may not be representative of the bulk geologic formation. Here we conducted the first long-term field based in situ diffusion experimentto compare the effect of experimental scale (5 x 10(-5) m3 in the diffusion cells and (5-20) x 10(-2) m3 in the in situ experiments) on De values for clay-rich aquitards. Using a conservative tracer (deuterium), our testing shows De values estimated from in situ testing ((2.5-3.5) x 10(-10) m2 s(-1)) are similar but lower than the average De values measured in the laboratory (4 x 10(-10) m2 s(-1)). The difference was attributed to greater porosity values in the laboratory samples resulting from core barrel extrusion and sample swelling. With representative core sampling and care, laboratory-based diffusion testing remains a viable method to assess solute transport mechanisms in clay aquitards.