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.

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.

Characterization and environmental implications of selenate co-precipitation with barite

This study investigated the sequestration of dissolved selenate (SeO₄^2-) via co-precipitation in barite for a range of SeO₄^2- concentrations (0-~8650 mg/L), as well as its release at near neutral pH conditions (pH = ~5.5-6.5). Solid precipitates were characterized via X-ray diffraction and subsequent Rietveld refinements, Raman spectroscopy, Brunauer-Emmett-Teller surface area analyses, scanning electron microscopy, electron probe microanalyses (EPMA), inductively coupled plasma optical emission spectroscopy (ICP-OES), and X-ray absorption spectroscopy (XAS). ICP-OES results suggested barite efficiently removed >99% of SeO₄^2- from the test solutions during all co-precipitation experiments. EPMA results showed the SeO₄^2- was sequestered from the aqueous phase via co-precipitation with barite. XAS analyses indicated the SeO₄^2- tetrahedron is incorporated into the barite structure by substituting for sulfate (SO₄^2-) and bonding to Ba²⁺ atoms through bidentate mononuclear and bidentate binuclear complexes. Dissolution data showed the release of SeO₄^2- sequestered in barite to the aqueous phase is unlikely due to the low solubility and stability of the barite phase. As such, co-precipitation of SeO₄^2- with barite could be effective for removing SeO₄^2- from waters affected by mining and metallurgical operations.

A Critical Review of State-of-the-Art and Emerging Approaches to Identify Fracking-Derived Gases and Associated Contaminants in Aquifers

High-volume, hydraulic fracturing (HVHF) is widely applied for natural gas and oil production from shales, coals, or tight sandstone formations in the United States, Canada, and Australia, and is being widely considered by other countries with similar unconventional energy resources. Secure retention of fluids (natural gas, saline formation waters, oil, HVHF fluids) during and after well stimulation is important to prevent unintended environmental contamination, and release of greenhouse gases to the atmosphere. Here, we critically review state-of-the-art techniques and promising new approaches for identifying oil and gas production from unconventional reservoirs to resolve whether they are the source of fugitive methane and associated contaminants into shallow aquifers. We highlight future research needs and propose a phased program, from generic baseline to highly specific analyses, to inform HVHF and unconventional oil and gas production and impact assessment studies. These approaches may also be applied to broader subsurface exploration and development issues (e.g., groundwater resources), or new frontiers of low-carbon energy alternatives (e.g., subsurface H₂ storage, nuclear waste isolation, geologic CO₂ sequestration).

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.

Estimates of water and solute release from a coal waste rock dump in the Elk Valley, British Columbia, Canada

Long term (1999 to 2014) flow and water quality data from a rock drain located at the base of a coal waste rock dump constructed in the Elk Valley, British Columbia was used to characterize the release of three solutes (NO₃^-, Cl^- and SO₄^2-) from the dump and obtain whole dump estimates of net percolation (NP). The concentrations of dump derived solutes in the rock drain water were diluted by snowmelt waters from the adjacent natural watershed during the spring freshet and reached a maximum concentration during the winter baseflow period. Historical peak baseflow concentrations of conservative ions (NO₃^- and Cl^-) increased until 2006/07 after which they decreased. This decrease was attributed to completion of the flushing of the first pore volume of water stored within the dump. The baseflow SO₄^2- concentrations increased proportionally with NO₃^- and Cl^- to 2007, but then continued to slowly increase as NO₃^- and Cl^- concentrations decreased. This was attributed to ongoing production of SO₄^2- due to oxidation of sulfide minerals within the dump. Based on partitioning of the annual volume of water discharged from the rock drain to waste rock effluent (NP) and water entering the rock drain laterally from the natural watershed, the mean NP values were estimated to be 446±50mm/a (area normalized net percolation/year) for the dump and 172±71mm/a for the natural watershed. The difference was attributed to greater rates of recharge in the dump from summer precipitation compared to the natural watershed where rainfall interception and enhanced evapotranspiration will increase water losses. These estimates included water moving through subsurface pathways. However, given the limitations in quantifying these flows the estimated NP rates for both the natural watershed and the waste rock dump are considered to be low, and could be much higher (e.g. ~450mm/a and ~800mm/a).

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.

Geochemical and mineralogical characterization of sulfur and iron in coal waste rock, Elk Valley, British Columbia, Canada

Exposure of coal waste rock to atmospheric oxygen can result in the oxidation of sulfide minerals and the release of sulfate (SO₄^2-) and associated trace elements (e.g., Se, As, Cd, and Zn) to groundwaters and surface waters. Similarly, reduced iron minerals such as siderite, ankerite, and the sulfide, pyrite, present in the waste rock can also undergo oxidation, resulting in the formation of iron oxyhydroxides that can adsorb trace elements released from the oxidation of the sulfide minerals. Characterization and quantification of the distribution of sulfide and iron minerals, their oxidation products, as well as leaching rates are critical to assessing present-day and future impacts of SO₄^2- and associated trace elements on receiving waters. Synchrotron-based X-ray absorption near edge spectroscopic analysis of coal waste rock samples from the Elk Valley, British Columbia showed Fe present as pyrite (mean 6.0%), siderite (mean 44.3%), goethite (mean 35.4%), and lepidocrocite (mean 14.3%) with S present as sulfide (mean 26.9%), organic S (mean 58.7%), and SO₄^2- (mean 14.4%). Squeezed porewater samples from dump solids yielded mean concentrations of 0.28mg/L Fe and 1246mg/L SO₄^2-. Geochemical modeling showed the porewaters in the dumps to be supersaturated with respect to Fe oxyhydroxides and undersaturated with respect to gypsum, consistent with solids analyses. Coupling Fe and S mineralogical data with long-term water quality and quantity measurements from the base of one dump suggest about 10% of the sulfides (which represent 2% of total S) in the dump were oxidized over the past 30years. The S from these oxidized sulfides was released to the receiving surface water as SO₄^2- and the majority of the Fe precipitated as secondary Fe oxyhydroxides (only 3.0×10^-5% of the Fe was released to the receiving waters over the past 30years). Although the data suggest that the leaching of SO₄^2- from the waste rock dump could continue for about 300years, assuming no change in the rate of oxidation of sulfides, SO₄^2- is currently not a concern in receiving surface waters as the concentration levels are below regulatory limits.

Dissolved Selenium(VI) Removal by Zero-Valent Iron under Oxic Conditions: Influence of Sulfate and Nitrate

Dissolved Se(VI) removal by three commercially available zero-valent irons (ZVIs) was examined in oxic batch experiments under circumneutral pH conditions in the presence and absence of NO₃ ^- and SO₄ ^2-. Environmentally relevant Se(VI) (1 mg L^-1), NO₃ ^- ([NO₃-N] = 15 mg L^-1), and SO₄ ^2- (1800 mg L^-1) were employed to simulate mining-impacted waters. Ninety percent of Se(VI) removal was achieved within 4-8 h in the absence of SO₄ ^2- and NO₃ ^-. A similar Se(VI) removal rate was observed after 10-32 h in the presence of NO₃ ^-. Dissolved Se(VI) removal rates exhibited the highest decrease in the presence of SO₄ ^2-; 90% of Se(VI) removal was measured after 50-191 h for SO₄ ^2- and after 150-194 h for SO₄ ^2- plus NO₃ ^- depending on the ZVI tested. Despite differences in removal rates among batches and ZVI materials, Se(VI) removal consistently followed first-order reaction kinetics. Scanning electron microscopy, Raman spectroscopy, and X-ray diffraction analyses of reacted solids showed that Fe(0) present in ZVI undergoes oxidation to magnetite [Fe₃O₄], wüstite [FeO], lepidocrocite [γ-FeOOH], and goethite [α-FeOOH] over time. X-ray absorption near-edge structure spectroscopy indicated that Se(VI) was reduced to Se(IV) and Se(0) during removal. These results demonstrate that ZVI can be effectively used to control Se(VI) concentrations in mining-impacted waters.

Geochemistry of arsenic in low sulfide-high carbonate coal waste rock, Elk Valley, British Columbia, Canada

This study investigated the geochemistry of arsenic (As) in low sulfide-high carbonate coal waste rock of the Elk Valley, British Columbia, Canada. Its abundance and mineralogical associations in waste rock of different placement periods were determined in addition to its mobilization into porewater and rock-drain effluent. The mean (5.34mg/kg; 95% confidence interval: 4.95-5.73mg/kg) As concentration in the waste rock was typical of sedimentary rock. Electron microprobe and As K-edge X-ray absorption near-edge spectroscopic analyses showed the As is predominantly associated with primary pyrites in both source and freshly blasted waste rock. However, in aged waste rock the As is associated with both primary pyrites and secondary Fe oxyhydroxides. Oxidation of pyrite in waste rock dumps was reflected by the presence of high concentrations of SO₄^2- in porewater and oxidation rims of Fe oxyhydroxides around pyrite grains. Acid released from pyrite oxidation to Fe oxyhydroxides is neutralized by carbonate mineral dissolution that buffers the pH in the waste rock to circumneutral values. Adsorption of As onto secondary Fe oxyhydroxides provides an internal geochemical control on As release during pyrite oxidation and porewater flushing from the dump, resulting in the low As concentrations observed in porewater (median: 9.91μg/L) and rock-drain effluent (median: 0.31μg/L). Secondary Fe oxyhydroxides act as a long-term sink for As under present day hydrologic settings in waste rock dumps in the Elk Valley.

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.

An evaluation of materials and methods for vapour measurement of the isotopic composition of pore water in deep, unsaturated zones

The development of in situ vapour sampling methods to measure δ(2)H and δ(18)O in pore water of deep, unsaturated soil profiles, including mine tailings and waste rock, is required to improve our ability to track water migration through these deposits. To develop appropriate field sampling methods, a laboratory study was first undertaken to evaluate potential materials and sampling methods to collect and analyse vapour samples from unsaturated mine waste. Field methods were developed based on these findings and tested at two mine sites using either on-site analyses with a portable isotope laser spectrometer or sample collection and storage prior to laboratory analyses. The field sites included a series of deep (>50 m) multiport profiles within a coal waste rock dump and open wells installed in a sand tailings dyke at an oil sands mine. Laboratory results show that memory effects in sample bags and tubing require 3-5 pore volumes of vapour flushing prior to sample collection and sample storage times are limited to 24 h. Field sampling highlighted a number of challenges including the need to correct for sample humidity and in situ temperature. Best results were obtained when a portable laser spectrometer was used to measure vapour samples in situ.

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.

Fe(ii)(aq)uptake of Mg(ii)–Al(iii)/Fe(iii)–SO4/CO3HTLCs under alkaline conditions: adsorption and solid state transformation mechanisms

Abiotic reduction of Mg(ii)–Al(iii)/Fe(iii)–SO₄/CO₃hydrotalcites (HTLCs) was investigated under three anoxic abiotic reaction conditions: (1) a target pH of 8 and 10 mM Fe(ii)_(aq), (2) a target pH of 8 and 0.5 mM Fe(ii)_(aq), and (3) a target pH of 10 and 0.5 mM Fe(ii)_(aq).

Mineralogical controls on aluminum and magnesium in uranium mill tailings: Key Lake, Saskatchewan, Canada

The mineralogy and evolution of Al and Mg in U mill tailings are poorly understood. Elemental analyses (ICP-MS) of both solid and aqueous phases show that precipitation of large masses of secondary Al and Mg mineral phases occurs throughout the raffinate neutralization process (pH 1-11) at the Key Lake U mill, Saskatchewan, Canada. Data from a suite of analytical methods (ICP-MS, EMPA, laboratory- and synchrotron-based XRD, ATR-IR, Raman, TEM, EDX, ED) and equilibrium thermodynamic modeling showed that nanoparticle-sized, spongy, porous, Mg-Al hydrotalcite is the dominant mineralogical control on Al and Mg in the neutralized raffinate (pH ≥ 6.7). The presence of this secondary Mg-Al hydrotalcite in mineral samples of both fresh and 15-year-old tailings indicates that the Mg-Al hydrotalcite is geochemically stable, even after >16 years in the oxic tailings body. Data shows an association between the Mg-Al hydrotalcite and both As and Ni and point to this Mg-Al hydrotalcite exerting a mineralogical control on the solubility of these contaminants.

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.

Effects of adsorbed arsenate on the rate of transformation of 2-line ferrihydrite at pH 10

2-Line ferrihydrite, a form of iron in uranium mine tailings, is a dominant adsorbent for elements of concern (EOC), such as arsenic. As ferrihydrite is unstable under oxic conditions and can undergo dissolution and subsequent transformation to hematite and goethite over time, the impact of transformation on the long-term stability of EOC within tailings is of importance from an environmental standpoint. Here, studies were undertaken to assess the rate of 2-line ferrihydrite transformation at varying As/Fe ratios (0.500-0.010) to simulate tailings conditions at the Deilmann Tailings Management Facility of Cameco Corporation, Canada. Kinetics were evaluated under relevant physical (~1 °C) and chemical conditions (pH ~10). As the As/Fe ratio increased from 0.010 to 0.018, the rate of ferrihydrite transformation decreased by 2 orders of magnitude. No transformation of ferrihydrite was observed at higher As/Fe ratios (0.050, 0.100, and 0.500). Arsenic was found to retard ferrihydrite dissolution and transformation as well as goethite formation.

Correcting for methane interferences on δ2H and δ18O measurements in pore water using H2Oliquid-H2Ovapor equilibration laser spectroscopy

Cavity ring-down spectroscopy (CRDS) is a new and evolving technology that shows great promise for isotopic δ(18)O and δ(2)H analyses of pore water from equilibrated headspace H(2)O vapor from environmental and geologic cores. We show that naturally occurring levels of CH(4) can seriously interfere with CRDS spectra, leading to erroneous δ(18)O and δ(2)H results for water. We created a new CRDS correction algorithm to account for CH(4) concentrations typically observed in subsurface and anaerobic environments, such as ground waters or lake bottom sediments. We subsequently applied the correction method to a series of geologic cores that contain CH(4). The correction overcomes the spectral interference and provides accurate pore water δ(18)O and δ(2)H values with acceptable precision levels as well as accurate concentrations of CH(4).

Transport and bacterial interactions of three bacterial strains in saturated column experiments

The impact of bacteria-solid and bacteria-bacteria interactions on the transport of Klebsiella oxytoca, Burkholderia cepacia G4PR1, and Pseudomonas sp. #5 was investigated in saturated sand column experiments (L = 114 mm; ø = 33 mm) under constant water velocities (∼ 5 cm · h(-1)). Bacterial strains were injected into the columns as pulses either individually, simultaneously, or successively. A one-dimensional mathematical model for advective-dispersive transport and for irreversible and reversible bacterial kinetic sorption was used to analyze the bacterial breakthrough curves. Different sorption parameters were obtained for each strain in each of the three experimental setups. In the presence of other bacteria, sorption parameters for B. cepacia G4PR1 remained similar to results from individual experiments, indicating the presence of other bacteria generally had a lesser influence on its migration than for the other bacteria. K. oxytoca is more competitive for the sorption sites when simultaneously injected with the other bacteria. Ps. sp. #5 generally yielded the greatest detachment rates and the least affinity to attach to the sand, indicative of its mobility in groundwater systems. The results of this study clearly indicate both bacteria-solid and bacteria-bacteria interactions influence the migration of bacteria. A more complete understanding of such interactions is necessary to determine potential migration in groundwater systems.

Molybdenum speciation in uranium mine tailings using X-ray absorption spectroscopy

Uranium (U) mill tailings in northern Saskatchewan, Canada, contain elevated concentrations of molybdenum (Mo). The potential for long-term (>10,000 years) mobilization of Mo from the tailings management facilities to regional groundwater systems is an environmental concern. To assist in characterizing long-term stability, X-ray absorption spectroscopy was used to define the chemical (redox and molecular) speciation of Mo in tailings samples from the Deilmann Tailings Management Facility (DTMF) at the Key Lake operations of Cameco Corporation. Comparison of Mo K near-edge X-ray absorption spectra of tailings samples and reference compounds of known oxidation states indicates Mo exists mainly as molybdate (+6 oxidation state). Principal component analysis of tailings samples spectra followed by linear combination fitting using spectra of reference compounds indicates that various proportions of NiMoO(4) and CaMoO(4) complexes, as well as molybdate adsorbed onto ferrihydrite, are the Mo species present in the U mine tailings. Tailings samples with low Fe/Mo (<708) and high Ni/Mo (>113) molar ratios are dominated by NiMoO(4), whereas those with high Fe/Mo (>708) and low Ni/Mo (<113) molar ratios are dominated by molybdate adsorbed onto ferrihydrite. This suggests that the speciation of Mo in the tailings is dependent in part on the chemistry of the original ore.

Transformation of two-line ferrihydrite to goethite and hematite as a function of pH and temperature

Under oxic aqueous conditions, two-line ferrihydrite gradually transforms to more thermodynamically stable and more crystalline phases, such as goethite and hematite. This temperature- and pH-dependent transformation can play an important role in the sequestration of metals and metalloids adsorbed onto ferrihydrite. A comprehensive assessment of the crystallization of two-line ferrihydrite with respect to temperature (25, 50, 75, and 100 °C) and pH (2, 7, and 10) as a function of reaction time (minutes to months) was conducted via batch experiments. Pure and transformed phases were characterized by X-ray diffraction (XRD), X-ray absorption near-edge spectroscopy (XANES), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The rate of transformation of two-line ferrihydrite to hematite increased with increasing temperature at all pHs studied and followed first-order reaction kinetics. XRD and XANES showed simultaneous formation of goethite and hematite at 50 and 75 °C at pH 10, with hematite being the dominant product at all pHs and temperatures. With extended reaction time, hematite increased while goethite decreased, and goethite reaches a minimum after 7 days. Observations suggest two-line ferrihydrite transforms to hematite via a two-stage crystallization process, with goethite being intermediary. The findings of this study can be used to estimate rates of crystallization of pure two-line ferrihydrite over the broad range of temperatures and pH found in nature.

Controls and rates of acid production in commercial-scale sulfur blocks

Acidic drainage (pH 0.4-1.0) from oxidizing elemental sulfur (S(0)) blocks is an environmental concern in regions where S(0) is stockpiled. In this study, the locations, controls, and rates of H(2)SO(4) production in commercial-scale S(0) blocks ( approximately 1-2 x 10(6) m(3)) in northern Alberta, Canada, were estimated. In situ modeling of O(2) concentrations ([O(2)]) suggest that 70 to >97% of the annual H(2)SO(4) production occurs in the upper 1 m of the blocks where temperatures increase to >15 degrees C during the summer. Laboratory experiments show that S(0) oxidation rates are sensitive to temperature (Q(10) = 4.3) and dependent on the activity of autotrophic S(0)-oxidizing microbes. The annual efflux of SO(4) in drainage water from a S(0) block (5.5 x 10(5) kg) was within the estimated range of SO(4) production within the block (2.7 x 10(5) to 1.2 x 10(6) kg), suggesting that H(2)SO(4) production and removal rates were approximately equal during the study period. The low mean relative humidity within the block (68%; SD = 17%; n = 21) was attributed to osmotic suction from elevated H(2)SO(4) concentrations and suggests a mean in situ pH of approximately -2.1. The low pH of drainage waters was attributed to the mixing of fresh infiltrating water and low-pH in situ water. Heat generation during S(0) oxidation was an important factor in maintaining elevated temperatures (mean, 11.1 degrees C) within the block. The implications of this research are relevant globally because construction methods and the physical properties of S(0) blocks are similar worldwide.

Inferring heterogeneity in aquitards using high-resolution deltaD and delta18O profiles

Vertical depth profiles of pore water isotopes (deltaD and delta18O) in clay-rich aquitards have been used to show that solute transport is dominated by molecular diffusion, to define the timing of geologic events, and to estimate vertical hydraulic conductivity. The interpretation of the isotopic profiles in these studies was based on pore water samples collected from piezometers installed in nests (typically 4 to 15 piezometers) over depths of 10 to 80 m. Data from piezometer nests generally have poor vertical resolution (meters), raising questions about their capacity to reveal the impact of finer scale heterogeneities such as permeable sand bodies or fractured till zones on solute transport. Here, we used high-resolution (30-cm) depth profiles of deltaD and delta18O from two continuously cored boreholes in a till aquitard to provide new insights into the effects of sand bodies on solute transport. High-resolution core-derived profiles indicate that such heterogeneities can cause major deviations from one-dimensional diffusion profiles. Further, comparison of piezometer-measured values with best-fit diffusion trends shows subtle deviations, suggesting the presence of heterogeneities that should not be ignored. High-resolution profiles also more clearly defined the contact between the highly fractured oxidized zone and the underlying unoxidized zone than the piezometers.

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.

Migration of colloids through nonfractured clay-rich aquitards

This study examined characteristics and controls on diffusive transport of colloids through nonfractured, clay-rich glacial till. The range in molecular weight (M(w) of the colloids tested (five polymers and three natural dissolved organic carbons) was 910-15 450 Da. Hydrodynamic diameters increased from 1.45 to 6.05 nm, and aqueous diffusion coefficients decreased from 2.6 x 10(-10) to 6.3 x 10(11) m2 s(-1) with increasing M(w). All colloids were subjected to diffusion testing using undisturbed core samples placed in double reservoir diffusion cells. All colloids decreased in concentration with time in the spiked reservoirs. Concentrations in the receiving reservoirs increased for only the four smallest colloids. The lack of breakthroughs for larger colloids was attributed to straining. Transport modeling using data from colloids exhibiting breakthrough shows effective diffusion coefficients and tortuosity factors decrease from 1.5 x 10(-10) to 6.5 x 10(-11) m2 s(-1) and from 0.6 to 0.3, respectively, with increasing M(w). The effective porosities are slightly less than total porosity (0.31). Our data suggest diffusive transport through clay-rich aquitards is limited to colloids with mean diameters < 2-2.2 nm and that measuring the diameter of dissolved organic carbon from nonfractured clay-rich aquitards may be an effective method to estimate effective pore throat diameters of these media.

Coupling centrifuge modeling and laser ablation inductively coupled plasma mass spectrometry to determine contaminant retardation in clays

Quantifying the retardation (Rd) of reactive solutes as they migrate through low-permeability clay-rich media is difficult, thus motivating this study to assess the viability of combining centrifuge modeling and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) techniques. An influent solution containing Cl-, trace metals, and lanthanide species flowed at 1.0 mL x h(-1) through an undisturbed clay-rich core sample (33 mm diameter x 50 mm long) mounted in a UFA Beckman centrifuge operating at 3000 rpm (N factor = 876 g). During the 87 day experiment the hydraulic conductivity of the core was 3.4 x 10(-10) m x s(-1). Effluent breakthrough data indicate the Rd of Tl to be 10; incomplete breakthrough (non-steady-state) data for 145Nd and 171Yb suggest Rd values of >>75 and >>85, respectively. At the completion of the transport experiment, longitudinal sections of the core solid were analyzed for 145Nd and 171Yb using a Cetac laser ablation system coupled with an ICP-MS. The longitudinal core sections yielded Rd values of >10000 for 145Nd and 171Yb. This study demonstrates coupling these techniques can provide Rd values for a wide range of reactive solutes with relatively rapid testing of small-scale, low hydraulic conductivity core samples.

High resolution pore water delta2H and delta18O measurements by H2O(liquid)-H2O(vapor) equilibration laser spectroscopy

A new H2O(liquid)-H2O(vapor) pore water equilibration and laser spectroscopy method provides a fast way to obtain accurate high resolution deltaD and delta18O profiles from single core samples from saturated and unsaturated geologic media. The precision and accuracy of the H2O(liquid)-H2O(vapor) equilibration method was comparable to or better than conventional IRMS-based methods, and it can be conducted on geologic cores that contain volumetric water contents as low as 5%. Significant advantages of the H2O(liquid)-H2O(vapor) pore water equilibration method and laser isotopic analysis method include dual hydrogen- and oxygen-isotope assays on single small core samples, low consumable and instrumentation costs, and the potential for field-based hydrogeologic profiling. A single core is sufficient to obtain detailed vertical isotopic depth profiles in geologic, soil, and lacustrine pore water, dramatically reducing the cost of obtaining pore water by conventional wells or physical water extraction methods. In addition, other inherent problems like contamination of wells by leakage and drilling fluids can be eliminated.

Long-term reactive solute transport in an aquitard using a centrifuge model

Characterizing and predicting reactive solute transport in low hydraulic conductivity (K) clay-rich media is challenging because the very long transport time for solutes renders conventional column tests impractical. In this study, a centrifugation technique was developed to assess the transport of a simple aqueous solution (NaCl) by accelerating flow by centrifugal force through low K (1.1 x 10(-11) m/s) core samples. Duplicate cores (52-mm length x 33-mm diameter) were centrifuged at 330 xg for 90 d to model the migration of saline pore water (0.5 M NaCl) under in situ conditions through an approximately 17-m-thick clay prototype over approximately 24,000 years. A PHREEQC one-dimensional reactive solute transport code simulated effluent breakthrough of the NaCl during centrifugation, with best-fit cation exchange coefficients similar to batch tests. The calibrated code was used to predict solute profile development over the long term in the prototype or simulated field-scale conditions. Chromatographic separation of solutes due to ion exchange was evident over several meters in the simulated prototype and the field profile. The applicability of centrifugation methods to predict transport of more complex suites of reactive solutes over the long term is yet to be verified.

Clay aquitards as archives of holocene paleoclimate: delta18O and thermal profiling

Paleoclimatic conditions in the Holocene were reconstructed from a detailed vertical profile of pore water delta(18)O and a series of downhole thermal profiles at a thick, hydrogeologically simple, aquitard research site in the Northern Great Plains of Saskatchewan. Reconstructions were obtained using the theory of one-dimensional diffusive transport and an empirical Bayesian inversion technique. Inversion of the delta(18)O profile shows that input signal consists of a sudden increase of +6 per thousand (corresponding to a temperature increase of about 18 degrees C) at about 12,000 years before present (BP), after which no measurable change in delta(18)O is apparent to present day. This research shows, at this location, that there is no evidence of large amplitude temperature shifts in the Holocene and supports the commonly used assumption in ground water studies of constant atmospheric delta(18)O values throughout the Holocene. Inversion of the temperature profiles suggests the ground surface temperature increased primarily in the last half of the 20th century, with a peak temperature (about 3 degrees C) circa 1990. For both profiles, the ability to resolve historical variability decays rapidly with time. For the temperature profiles, the decay in resolution precluded the development of reliable estimates of climatic conditions prior to about 1950 and, in the case of the delta(18)O profile, it prevented the precise definition of climate changes (e.g., Hypsithermal and Little Ice Age) in the Holocene.

Complexation of aqueous elements by DOC in a clay aquitard

The extent of partitioning of several elements (Cu, Mn, Mo, Ni, Sr, U, and Zn) on dissolved organic carbon (DOC) was investigated in pore water samples collected from a clay-rich aquitard. High DOC concentrations in the aquitard, ranging from 21 to 143 mg C/L, and natural aqueous metal concentrations higher than in most ground water environments facilitated complexation studies at this site. Analyses were conducted using on-line coupling of asymmetrical flow field-flow fractionation with ultraviolet, total organic carbon, and inductively coupled plasma-mass spectrometry detectors. Of the elements investigated, only U and Zn were complexed with all DOC samples, ranging from 2.2 to 60 microg U/g DOC (0.4% to 3% of the total U in the pore water) and 0.04 to 0.5 microg Zn/g DOC (0.1% to 0.9% of the total Zn in the pore water), respectively. Laboratory experiments conducted over a range in pH (1.3 to 9.7) and geochemical modeling supported the measured complexation of U and Zn on the DOC. The in situ association constant, K(d), for U decreased with depth from 76 mL/g C for pore water samples at 2.2 m below ground (BG) to 24 mL/g C at 9.7 m BG. The decrease was attributed to a decrease in aromaticity of the DOC with depth. Zn K(d)constants ranged from 2 to 12 mL/g C and exhibited no trend with depth. Results of the current study suggest minor masses of U and Zn (less than or equal to 4% of total) complex with this DOC under in situ pH conditions. Our data suggest that competitive complexation by other ligands may limit the importance of DOC-facilitated transport of the elements studied in water of similar chemical composition.

Dynamics and stable isotope composition of gaseous and dissolved oxygen

The vadose zone and ground water environments are a sink for atmospheric O(2). The pathways and rates of O(2) consumption are primarily related to the availability and rate of oxidation of key reductants (e.g., organics, sulfides), through a combination of biological or abiotic reactions. The range in delta(18)O of O(2) in the subsurface is large, from +20 per thousand to +39 per thousand (Vienna Standard Mean Ocean Water) in the vadose zone and from +12 per thousand to +46 per thousand in ground water. The aggregated O(2) isotope fractionation by consumption (alpha(k)) was found to range from 0.970 to 1.300 and 0.980 to 1.030 in vadose zones and aquifers, respectively. These data suggest the delta(18)O patterns in both unsaturated zones and aquifers can be attributed to microbially mediated reactions (alpha(k)= range from 0.975 to 1.000), but there are apparently other inverse isotope fractionating processes (alpha(k) > 1.000). Circumstantial evidence suggested O(2) processed during the sulfide oxidation and precipitation of Fe-oxyhydroxides process (or other unidentified processes) could be the cause of the significant (18)O depletions. Overall, delta(18)O data from vadose zones and ground water revealed that isotope fractionation by consumption of gaseous and dissolved O(2) in the subsurface and ground water environments is more complicated than what has classically been attributed solely to geomicrobial respiration. Given the questions and inexplicable data arising from this study, further detailed research on O(2) consuming processes in the Earth's subsurface and ground water is warranted.

Quantifying uranium complexation by groundwater dissolved organic carbon using asymmetrical flow field-flow fractionation

The long-term mobility of actinides in groundwaters is important for siting nuclear waste facilities and managing waste-rock piles at uranium mines. Dissolved organic carbon (DOC) may influence the mobility of uranium, but few field-based studies have been undertaken to examine this in typical groundwaters. In addition, few techniques are available to isolate DOC and directly quantify the metals complexed to it. Determination of U-organic matter association constants from analysis of field-collected samples compliments laboratory measurements, and these constants are needed for accurate transport calculations. The partitioning of U to DOC in a clay-rich aquitard was investigated in 10 groundwater samples collected between 2 and 30 m depths at one test site. A positive correlation was observed between the DOC (4-132 mg/L) and U concentrations (20-603 microg/L). The association of U and DOC was examined directly using on-line coupling of Asymmetrical Flow Field-Flow Fractionation (AsFlFFF) with UV absorbance (UVA) and inductively coupled plasma-mass spectrometer (ICP-MS) detectors. This method has the advantages of utilizing very small sample volumes (20-50 microL) as well as giving molecular weight information on U-organic matter complexes. AsFlFFF-UVA results showed that 47-98% of the DOC (4-136 mg C/L) was recovered in the AsFlFFF analysis, of which 25-64% occurred in the resolvable peak. This peak corresponded to a weight-average molecular weight of about 900-1400 Daltons (Da). In all cases, AsFlFFF-ICP-MS suggested that<or=2% of the U, likely present as U(VI), was complexed with the DOC. This result was in good agreement with the U speciation modeling performed on the sample taken from the 2.3 m depth, which predicted approximately 3% DOC-complexed U. This good agreement suggests that the AsFlFFF-ICP-MS method may be very useful for determining U-organic matter association in small volume samples. Because the pH (7.0-8.1) and carbonate concentrations of these waters are typical of many groundwaters, these data suggested that facilitated transport of U by DOC may be limited in its importance in many groundwater systems.

Characterizing dissolved organic carbon using asymmetrical flow field-flow fractionation with on-line UV and DOC detection

A method of characterizing dissolved organic carbon (DOC) by asymmetrical flow field-flow fractionation with on-line UV and DOC detection is described and applied to standards and natural water samples. Poly(styrenesulfonate) polymer standards, Suwannee River humic standards, and naturally occurring surface water and groundwater DOC were analyzed using this coupled detection technique. Molecular weight determinations in the samples and standards were 6-30% lower with DOC analysis than UV analysis. This difference was attributed to the insensitivity of the latter technique to nonaromatic carbon and suggests the molecular weight determined with the DOC detector is a more accurate representation of the actual molecular weight of the DOC. A normalized intensity comparison (NIC) method was proposed to distinguish differences in the relative amounts of aromatic and aliphatic carbon in DOC by comparing the two detector responses. The NIC method was applied to yield an average aromatic content of the bulk DOC and to detail the aromatic content over a range of molecular weights in a single DOC fraction.

Characterization of aqueous lead removal by phosphatic clay: equilibrium and kinetic studies

Immobilization of heavy metals from contaminated environments is an emerging field of interest from both resource conservation and environmental remediation points of view. This study investigated the feasibility of using phosphatic clay, a waste by-product of the phosphate mining industry, as an effective sorbent for Pb from aqueous effluents. The major parameters controlling aqueous Pb removal, viz. initial metal ion concentrations, solution pH, sorbent amounts, ionic strength and presence of both inorganic and organic ligands were evaluated using batch experiments. Results demonstrated that aqueous Pb removal efficiency of phosphatic clay is controlled mainly by dissolution of phosphatic clay associated fluoroapatite [Ca(10)(PO(4))(5)CaCO(3)(F,Cl,OH)(2)], followed by subsequent precipitation of geochemically stable pyromorphite [Pb(10)(PO(4))(6)(F,Cl,OH)(2)], which was confirmed by both X-ray diffraction (XRD) and scanning electron microscopic (SEM) analysis. Lead removal efficiency of phosphatic clay increased with increasing pH, sorbent amount and decreasing ionic strength. It also depends on the nature of complexing ligands. Formation of insoluble calcium oxalate and lead oxalate in the presence of oxalic acid explained high uptake of Pb by phosphatic clay from aqueous solution. However, Pb sorption kinetics onto phosphatic clay were biphasic, with initially fast reactions followed by slow and continuous Pb removal reactions. The slow reactions may include surface sorption, co-precipitation and diffusion. The exceptional capability of phosphatic clay to remove aqueous Pb demonstrated its potential as a cost effective way to remediate Pb-contaminated water, soils and sediments.

Decadal geochemical and isotopic trends for nitrate in a transboundary aquifer and implications for agricultural beneficial management practices

Nitrate contamination of aquifers is a global agricultural problem. Agricultural beneficial management practices (BMPs) are often promoted as a means to reduce nitrate contamination in aquifers through producer optimized management of inorganic fertilizer and animal manure inputs. In this study, decadal trends (1991-2004) in nitrate concentrations in conjunction with 3H/3He groundwater ages and nitrate stable isotopes (delta15N, delta18O) were examined to determine whether BMPs aimed at reducing aquifer-scale nitrate contamination in the transboundary Abbotsford-Sumas aquifer were effective. A general trend of increasing nitrate concentrations in young groundwater (< approximately 5 yr) suggested that voluntary BMPs were not having a positive impact in achieving groundwater quality targets. While the stable isotope data showed that animal manure was and still is the prevalent source of nitrate in the aquifer, a recent decrease in delta15N in nitrate suggests a BMP driven shift away from animal wastes toward inorganic fertilizers. The coupling of long-term monitoring of nitrate concentrations, nitrate isotopes, and 3H/3He age dating proved to be invaluable, and they should be considered in future assessments of the impact of BMPs on nutrients in groundwaters. The findings reveal that BMPs should be better linked to groundwater nutrient monitoring programs in order to more quickly identify BMP deficiencies, and to dynamically adjust nutrient loadings to help achieve water quality objectives.

Characterizing and quantilying controls on arsenic solubility over a pH range of 1-11 in a uranium mill-scale experiment

A mill-scale hydrometallurgical experiment (2700 m3 of effluent treated/day) was conducted for three months at the Rabbit Lake uranium mine site located in northern Saskatchewan, Canada, to determine the controls on the solubility of dissolved arsenic over a pH range of 1-11 and to develop a thermodynamic database for the dominant mineralogical controls on arsenic in the mill and the resulting mill tailings. The arsenic concentrations in the mill ranged from 526 mg/L at pH 1.0 (initial) to 1.34 mg/L at pH 10.8 (final discharge). Geochemical modeling of the chemistry data shows that arsenic solubility is controlled by the formation of scorodite (FeAsO4-2H2O) from pH 2.4 to pH 3.1, with 99.8% of dissolved arsenic precipitated as scorodite. Model results show that scorodite is unstable (releasing arsenic back in to solution) above pH 3.1 and arsenic adsorption to the surface of 2-line ferrihydrite is the dominant controlling factor in the solubility of arsenic from pH 3.2 to pH 11.0, with 99.8% of dissolved arsenic removed from solution via this mechanism. Finally, model results show -0.2% of the total dissolved arsenic adsorbs to the surface of amorphous aluminum hydroxide from pH 5.0 to pH 8.0. Minor alterations to the thermodynamic properties of arsenite and arsenate adsorption to 2-line ferrihydrite allowed the fit between measured mill-scale and modeled concentrations for the pH range of 3.2-11.0 to be optimized.

Transport and geochemical controls on the distribution of solutes and stable isotopes in a thick clay-rich till aquitard, Canada

A comprehensive understanding of the transport and geochemical processes controlling solutes in clay-rich aquitard confining units is needed to accurately predict the long-term migration of contaminants into the subsurface. To this end, the geochemical and stable isotopic composition of porewaters in the upper 22 m of a thick, unoxidized and nonfractured clay-rich, till aquitard (Sutherland Group) was examined in detail. The aquitard is overlain by about 8 m of oxidized and fractured till (Floral Fm). Concentrations of TDS, SO4(2-), HCO3-, Cl-, Na+, Mg2+, Ca2+ and porewater deuterium were greater in the Floral Fm and decreased with depth through the aquitard. The elevated and seasonably variable solute concentrations in the oxidized Floral Fm were attributed to geochemical weathering and dynamic water movement through fractures. Good fits between measured delta2H, TDS, SO4(2-), Cl- and HCO3- profiles through the aquitard and simulated solute transport profiles were obtained by diffusion (without advection) with transport times of 4-6 ka. The deficiency of geochemical reactions affecting HCO3- and SO4(2-) in the aquitard was supported by delta13C(DIC) and delta34S(SO4) analyses. Geochemical and isotope mass balance modelling (NETPATH) indicated that diffusive mixing with minor calcite dissolution and ion exchange could account for the distribution of Na+, Ca2+ and Mg2+ in the aquitard. Results of this study further suggested that microbiological activity in the aquitard was limited. With minor exceptions, the solute and isotopic profiles, their transport and the controlling geochemical reactions in the Sutherland are similar to those determined at another clay-rich till aquitard, 160 km south of this site, suggesting that geochemical and biological processes in some clay-rich aquitards may have a minimal effect on the migration of dissolved constituents.

Use of O2 consumption and CO2 production in kinetic cells to delineate pyrite oxidation-carbonate buffering and microbial respiration in unsaturated media

Identifying zones of sulphide oxidation and carbonate buffering is important in the development of a management plan for mine waste-rock piles. In this study, we used a kinetic cell technique to measure rates of O2 consumption and CO2 production in low sulphide (<0.12 wt.% S), low inorganic carbon (<0.20 wt.% C(inorganic)), gneissic waste rock and associated organic-rich lake sediment (0.7 wt.% C(organic)), and forest soil (1.4 wt.% C(organic)) collected from the Key Lake uranium mine in Saskatchewan, Canada. Solid chemistry, stable carbon isotope, pore water sulphate concentration data, and stoichiometric considerations indicated that O2 consumption and CO2 production were constrained by microbial respiration in the lake sediment and forest soil and by pyrite oxidation-carbonate buffering in the gneissic waste rock. Mean ratios of molar CO2 production to O2 consumption rates were 0.5 for lake sediment, 0.7 for forest soil, and 0.2 for gneissic waste rock. The different O2/CO2 ratios suggested that O2-CO2 monitoring may provide a practical tool for identifying the zones of microbial respiration and pyrite oxidation-carbonate buffering in mine waste-rock piles. Rates of O2 consumption and CO2 production were about one order of magnitude greater in lake sediment than in gneissic waste rock, indicating that microbial respiration would exert a control on the distribution of O2 and CO2 gas in waste-rock piles constructed upon the dewatered lake sediments.

Microbial respiration and diffusive transport of O2, 16O2, and 18O15O in unsaturated soils and geologic sediments

Molecular oxygen (O2) in unsaturated geologic sediments plays an important role in soil respiration, biodegradation of organic contaminants, metal oxidation, and global oxygen and carbon cycling, yet little is known about oxygen isotope fractionation during the consumption and transport of O2 in unsaturated zones. We used a laboratory kinetic cell technique to quantify isotope fractionation due to respiration and a numerical model to quantify both consumptive and diffusive fractionation of O2 isotopes at a field site comprised of unsaturated lacustrine sandy materials. The combined use of laboratory-based kinetic cell experiments and field-based isotope transport modeling provided an effective tool to characterize microbial respiration in unsaturated media. Based on results from the closed-system kinetic cells, O2 consumption and isotope fractionation were attributed to the alternative cyanide-resistant respiration pathway. At the field site, the modeled depth profiles for O2 and delta18O matched the measured in situ data and confirmed that the consumption of O2 was via the alternative respiration pathway. If the cyanide-resistant respiration pathway is indeed widespread in soils, its high oxygen isotope enrichment factor could help to explain the discrepancy between the predicted present-day Dole effect (+20.8/1000) and the observed Dole effect (+23.5/1000). Thus, further soil O2 isotope studies are needed to better characterize and model the fractionation of oxygen isotopes during subsurface respiration and the potential impact on the isotopic content of atmospheric O2.

Application of harmonic analysis of water levels to determine vertical hydraulic conductivities in clay-rich aquitards

A harmonic analysis method was used to determine vertical hydraulic conductivities (Kv) in geologic media between vertically separated piezometers using water level measurements. In this method, each water level time series was filtered and then decomposed using harmonic analysis into a sum of trigonometric components. The phase and amplitude of each harmonic function were calculated. These data were used to estimate Kv values between vertically separated data sets assuming one-dimensional transient flow. The method was applied to water level data collected from nested piezometers at two thick clay-rich till aquitards in Saskatchewan, Canada. At one site, routine water levels were measured in 12 piezometers (installed between 1 and 29 m below ground surface) since installation (1995). At the other site, water levels were measured in seven piezometers (installed between 4 and 53 m below ground surface) since installation (1998-1999). The Kv calculated using harmonic analysis decreased with depth below the water table at both sites, approaching matrix estimates of hydraulic conductivity between 10 and 11 m and between 21 and 43 m below ground surface. These depths reflected the depth of extensive vertical fracturing at the sites and showed that the depth of fracturing may be site specific.

Mineralogical characterization of arsenic in uranium mine tailings precipitated from iron-rich hydrometallurgical solutions

Arsenic-rich uranium mine tailings from the Rabbit Lake in-pit tailings management facility (RLITMF) in northern Saskatchewan, Canada, were investigated to determine the mineralogy and long-term stability of secondary arsenic precipitates formed from iron-rich hydrometallurgical solutions. Total arsenic and iron concentrations in six iron-rich samples of the mine tailings ranged from 56 to 6,000 microg/g and from 12 600 to 30 200 microg/g, respectively (Fe/As molar ratios of 5.3-303). On the basis of stability field diagrams generated from pH, Eh, and temperature measurements on tailings samples (mean values of 9.79, +162 mV, and 2.8 degrees C, respectively), it was concluded that arsenic and iron in the tailings were stable as As5+ and Fe3+. Synchrotron-based X-ray absorption spectroscopic studies of tailings samples, fresh mill precipitates, and reference compounds showed that the arsenic in iron-rich areas of the tailings existed as the stable As5+ and was adsorbed to 2-line ferrihydrite through inner-sphere bidentate linkages. Furthermore, under the conditions in the RLITMF, the 2-line ferrihydrite did not undergo any measurable conversion to more crystalline goethite or hematite, even in tailings discharged to the RLITMF 10 yr prior to sampling.

Characterizing geochemical reactions in unsaturated mine waste-rock piles using gaseous O2, CO2, 12CO2, and 13CO2

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, delta13C(CO2) 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 delta13C(CO2) 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.

An assessment of a mesocosm approach to the study of microbial respiration in a sandy unsaturated zone

Microbial respiration rates were determined through a 3.2 m thick, sandy unsaturated zone in a 2.4 m diameter x 4.6 m high mesocosm. The mesocosm was maintained under near constant temperature (18 degrees to 23 degrees C) and reached steady moisture content conditions after several hundred days. Soil-gas CO2 concentrations in the mesocosm ranged from 0.09% to 3.31% and increased with depth. Respiration rates within the mesocosm were quantified over a 342-day period using measured CO2 concentrations and a transient, one-dimensional finite-element model. Microbial respiration rates were 2 x 10(-1) micrograms C.g-1.d-1 throughout most of the system, but decreased to 10(-4) to 10(-3) micrograms C.g-1.d-1 within the capillary fringe. Microbial respiration rates were also determined in minicosms (500 g sample mass) over a range in temperatures (4 degrees to 30 degrees C) and volumetric moisture contents (0.044 to 0.37). The functional dependence of CO2 production on temperature and soil-moisture content was similar for the two scales of laboratory observation. Respiration rates in the minicosms, for temperatures and moisture contents in the mesocosm, were up to an order of magnitude greater than those determined for the mesocosm. The higher respiration rates in the minicosms, compared to the mesocosm, were attributed to greater disturbance of the samples and to shorter acclimation time in the minicosms. Extrapolating the laboratory respiration rates to field conditions yielded rates that were two to three orders of magnitude greater than rates previously determined in situ for C-horizon material. Results show that in situ microbial reaction rates determined using disturbed samples in minicosms and mesocosms yielded respiration rates that greatly exceeded field conditions. Mesocosms can, however, provide a useful environment for conducting process-related research in unsaturated environments.

An automated technique for measuring deltaD and delta18O values of porewater by direct CO2 and H2 equilibration

The stable-oxygen and -hydrogen isotopic values (deltaD, delta18O) of porewater in geologic media are commonly determined on water obtained by extraction techniques such as centrifugation, mechanical squeezing, vacuum heating and cryogenic microdistillation, and azeotropic distillation. Each of these techniques may cause isotopic fractionation as part the extraction process and each is laborious. Here we demonstrate a new approach to obtain automated, high-precision deltaD and delta18O measurements of porewater in geologic sediments by direct H2- and CO2-porewater equilibration using a modified commercial CO2-water equilibrator. This technique provides an important and cost-effective improvement over current extraction methods, because many samples can be rapidly analyzed with minimal handling, thereby reducing errors and potential for isotopic fractionation. The precision and accuracy of direct H2- and CO2-porewater equilibration is comparable to or better than current porewater extraction methods. Finally, the direct equilibration technique allows investigators to obtain high-resolution (cm scale) porewater deltaD and delta18O profiles using cores from individual boreholes, eliminating the need for costly piezometers or conventional porewater extractions.