Column experiment for assessing microbial behavior around radioactive waste repositories,including migration of potentially radionuclide-accumulating bacteria

To assess microbial behavior at anticipated repositories of nitrate-containing radioactive waste such as TRU waste, we set up an anoxic single horizontal column filled with Pleistocene sand with indigenous microorganisms as model samples. The column was supplied with artificial groundwater containing nitrate and acetate for 9 weeks (Run 1) or nitrate-amended groundwater from the same Pleistocene stratum for 6 weeks (Run 2). Bacterial communities, including culturable denitrifiers, were established in the sand bed, resulting in acridine orange direct counts per pore water of 3 × 10⁸ cell mL⁻¹ in Run 1 and 5 × 10⁷ cell mL⁻¹ in Run 2 and nitrate-reducing activity per pore water of roughly 13 mg L⁻¹ d⁻¹ in Run 1 and 1–4 mg L⁻¹ d⁻¹ in Run 2. Eh and hydraulic conductivity declined in Run 1, indicating microbial activity capable of retarding radionuclide transport. However, the ratio of bacterial cell concentration found in the effluent water (free-living bacteria) to the total bacterial concentration in sand (R_mobile) exceeded 2%. This finding is relevant to the increase in radionuclide transport associated with free-living cells. As a tool for quantifying this influence, we introduced an index, K^d,att (distribution coefficient for microbes on sand particles), and calculated this value from the R_mobile value. By sensitivity analysis using a numerical simulation model (MINT), we then demonstrated that higher K^d,att values would suppress the detrimental effects of the free-living bacteria. Quantification of microbial influences can be made more realistic by obtaining K^d,att values in a column experiment and incorporating this index into radionuclide transport models.     

Weak coupling between sulfate reduction and the anaerobic oxidation of methane in methane-rich seafloor sediments during ex situ incubation

We investigated coupling between sulfate reduction (SR) and anaerobic oxidation of methane (AOM) by quantifying pore water geochemical profiles, determining rates of microbial processes, and examining microbial community structure at two sites within Mississippi Canyon lease block 118 (MC118) in the Northern Gulf of Mexico. Sediments from the northwest seep contained high concentrations of methane while sediments from the southwest seep contained methane, gaseous n-alkanes and liquid hydrocarbons and had abundant surficial accumulations of gas hydrate. Volumetric (21.5 μmol cm⁻³ day⁻¹) and integrated (1429 mmol m⁻² day⁻¹) rates of SR at MC118 in ex situ incubations are the highest reported thus far for seafloor environments. AOM rates were small in comparison, with volumetric rates ranging from 0.1 to 12.6 nmol cm⁻³ day⁻¹. Diffusion cannot adequately supply the sulfate required to support these high SR rates so additional mechanisms, possibly biological sulfide oxidation and/or downward advection, play important roles in supplying sulfate at these sites. The microbial communities at MC118 included sulfate-reducing bacteria phylogenetically associated with Desulfobacterium anilini, which is capable of complex hydrocarbon degradation. Despite low AOM rates, the majority of archaea identified were phylogenetically related to previously described methane oxidizing archaea. To evaluate whether weak coupling between SR and AOM occurs in habitats lacking the complex hydrocarbon milieu present at MC118, we compiled available SR and AOM rates and found that the global median ratio of SR to AOM was 10.7:1 rather than the expected 1:1. The global median integrated AOM rate was used to refine global estimates for AOM rates at cold seeps; these new estimates are only 5% of the previous estimate.     

Measuring the effective diffusion coefficient of dissolved hydrogen in saturated Boom Clay

Boom Clay is studied as a potential host formation for the disposal of high-and intermediate level long-lived radioactive waste in Belgium. In such a geological repository, generation of gases (mainly H₂ from anaerobic corrosion) will be unavoidable. In order to make a good evaluation of the balance between gas generation vs. gas dissipation for a particular waste form and/or disposal concept, good estimates for gas diffusion coefficients of dissolved gases are essential. In order to obtain an accurate diffusion coefficient for dissolved hydrogen in saturated Boom Clay, diffusion experiments were performed with a recently developed through-diffusion set-up for dissolved gases. Due to microbial activity in the test set-up, conversion of hydrogen into methane was observed within several experiments. A complex sterilisation procedure was therefore developed in order to eliminate microbiological disturbances. Only by a combination of heat sterilisation, gamma irradiation and the use of a microbial inhibitor, reliable, reproducible and accurate H₂(g) diffusion coefficients (measured at 21 °C) for samples oriented parallel (D_eff = 7.25 × 10⁻¹⁰ m²/s and D_eff = 5.51 × 10⁻¹⁰ m²/s) and perpendicular (D_eff = 2.64 × 10⁻¹⁰ m²/s) to the bedding plane were obtained.     

Physicochemical gradients,diffusive flux,and sediment oxygen demand within oil sands tailings materials from Alberta,Canada

The Athabasca Oil Sands contain one of the world's largest oil reserves consisting of approximately 168 billion barrels of currently recoverable bitumen. With 20% recoverable through open pit mining methods, this extraction process produces a considerable amount of fluid fine tailings (FFT) waste material, which must be deposited on site in tailings ponds. These ponds allow the waste sand, clay and residual bitumen to settle out of the water column, allowing for the water to be recycled for use again in the extraction process. It is vital to gain a better understanding of the processes contributing to the development of physicochemical gradients (pH, Eh, Oxygen etc…) that form in these tailings ponds over time, with the goal of remediation and subsequent construction of end-pit lake systems once oil extraction has ceased. To differentiate between the impacts of biotic and abiotic processes in fresh (newly processed material) and mature FFT (∼38 year old tailings) over a 52-week study, a specific experimental design was utilized in accordance with novel microsensor profiling techniques. The sulfide diffusive fluxes within mature biotic systems measured 37.6 μmol m⁻² day⁻¹ at the onset of the experiment, decreasing over time, as FeS mineralization progressed. In addition, DO fluxes also showed strong correlation to the physical affects of consolidation, and overall biological consumption of O₂ at the FFT-water interface. This holistic study comparing different tailings pond materials provides insight regarding biotransformation and physicochemical controls effecting sediment oxygen demand associated with reclaimed wetlands and end pit lake development.     

Solubility of Fe2(OH)3Cl (pure-iron end-member of hibbingite) in NaCl and Na2SO4 brines

Pure-iron end-member hibbingite, Fe₂(OH)₃Cl(s), may be important to geological repositories in salt formations, as it may be a dominant corrosion product of steel waste canisters in an anoxic environment in Na–Cl- and Na–Mg–Cl-dominated brines. In this study, the solubility of Fe₂(OH)₃Cl(s), the pure-iron end-member of hibbingite (Fe^II, Mg)₂(OH)₃Cl(s), and Fe(OH)₂(s) in 0.04 m to 6 m NaCl brines has been determined. For the reactionFe₂(OH)₃Cl(s) + 3H⁺ ? 3 H₂O + 2 Fe²⁺ + Cl^?,the solubility constant of Fe₂(OH)₃Cl(s) at infinite dilution and 25 °C has been found to be log₁₀ K = 17.12 ± 0.15 (95% confidence interval using F statistics for 36 data points and 3 parameters). For the reactionFe(OH)₂(s) + 2H⁺ ? 2 H₂O + Fe²⁺,the solubility constant of Fe(OH)₂ at infinite dilution and 25 °C has been found to be log₁₀ K = 12.95 ± 0.13 (95 % confidence interval using F statistics for 36 data points and 3 parameters). For the combined set of solubility data for Fe₂(OH)₃Cl(s) and Fe(OH)₂(s), the Na⁺–Fe²⁺ pair Pitzer interaction parameter θ_{Na⁺/Fe²⁺} has been found to be 0.08 ± 0.03 (95% confidence interval using F statistics for 36 data points and 3 parameters). In nearly saturated NaCl brine we observed evidence for the conversion of Fe(OH)₂(s) to Fe₂(OH)₃Cl(s). Additionally, when Fe₂(OH)₃Cl(s) was added to sodium sulfate brines, the formation of green rust(II) sulfate was observed, along with the generation of hydrogen gas. The results presented here provide insight into understanding and modeling the geochemistry and performance assessment of nuclear waste repositories in salt formations.     

Recarbonation of metamorphosed marls,Jordan

The Maqarin area, northern Jordan, hosts some unusual, hyperalkaline (pH= 12.5) groundwaters discharging from thermally metamorphosed bituminous marls which formed through spontaneous, in situ combustion of the bitumen. The groundwaters have evolved geochemically through hydration, recarbonation and sulphatization of high temperature minerals. Mineralogical relations of the carbonate phases were examined by XRD and cathodoluminescence in conjunction with a detailed investigation of stable isotope ratios by Nd-YAG laser microsampling. Carbon-13 contents trace the sequence of alteration reactions, involving high temperature decarbonation of host biomic marl, followed by in situ recarbonation of secondary calcium hydroxide and calcium-silicate-hydrates (CSH).Carbonation took place shortly after thermal metamorphism, when non-saturated conditions allowed an atmosphere rich in CO₂ from adjacent combustion zones to access reaction sites. Low δ¹⁸O_CaCO3 values suggest that the earliest phase of recarbonation took place by reaction with hydroxide at elevated temperatures while later phases formed at cooler temperatures. Variable¹⁴ activities show that soil CO₂ was a component of the later recarbonating atmosphere. Once saturated conditions prevailed in the alteration zone, recarbonation ended and alteration evolved to hydroxide and sulphate dissolution reactions. The recarbonation reactions are a field-scale analogue of recarbonation and¹⁴C attenuation in cementitious barriers for radioactive waste repositories.     

Oxidation of pyrite in low temperature acidic solutions: Rate laws and surface textures

Rate laws have been determined for the aqueous oxidation of pyrite by ferric ion, dissolved oxygen and hydrogen peroxide at 30°C in dilute, acidic chloride solutions. Fresh, smooth pyrite grain surfaces were prepared by cleaning prior to experiments. Initial specific surface areas were measured by the multipoint BET technique. Surface textures before and after oxidation were examined by SEM. The initial rate method was used to derive rate laws.The specific initial rates of oxidation (moles pyrite cm⁻² min⁻¹) are given by the following rate laws (concentrations in molar units): r_sp,Fe³⁺ = −10^−9.74M^0.5_Fe³+M^−0.5_H+ (pH 1–2)r_sp,_o2 = −10^−6.77M^0.5_O2 (pH 2–4)r_sp,_h2o2 = −10^−1.43M_H2O2 (pH 2−4)An activation energy of 56.9 ± 7.5 kJ mole⁻¹ was determined for the oxidation of pyrite by dissolved oxygen from 20–40°C. HPLC analyses indicated that only minor amounts of polythionates are detectable as products of oxidation by oxygen below pH 4; the major sulfur product is sulfate. Ferric ion and sulfate are the only detectable products of pyrite oxidation by hydrogen peroxide. Hydrogen peroxide is consumed by catalytic decomposition nearly as fast as it is by pyrite oxidation.SEM photomicrographs of cleaned pyrite surfaces indicate that prior to oxidation, substantial intergranular variations in surface texture exist. Reactive surface area is substantially different than total surface area. Oxidation is centered on reactive sites of high excess surface energy such as grain edges and corners, defects, solid and fluid inclusion pits, cleavages and fractures. These reactive sites are both inherited from mineral growth history and applied by grain preparation techniques. The geometry and variation of reactive sites suggests that the common assumption of a first-order, reproducible dependence of oxidation rates on surface area needs to be tested.     

Quantification of oxygen consumption and sulphate release rates for waste rock piles using kinetic cells: Cluff lake uranium mine,northern Saskatchewan,Canada

Oxidation rates of low sulphide (<0.5 wt.%) gneissic waste rock from the Cluff lake U mine, northern Saskatchewan, Canada were determined using 3 independent methods: O₂ consumption rates in kinetic cells, SO₄ measurements of kinetic cell effluent and humidity cell SO₄ release rates. The O₂ consumption measurements demonstrated that the oxidation of pyrite was strongly dependent on grain size and moderately dependent on water content, temperature and microbiology. Oxygen consumption rates were highest at water contents of 5–10 wt.% (12–25% saturation). Measured SO₄ release rates (3.1–91 mg SO₄ kg⁻¹ wk⁻¹) for the kinetic cells were comparable to rates calculated from the O₂ consumption values (6.9–70 mg SO₄ kg⁻¹ wk⁻¹). Sulphate release rates determined from humidity cells were generally higher than those obtained from the kinetic cells, ranging from 6 to 64 mg SO₄ kg⁻¹ wk⁻¹ for the coarsest and finest fraction, respectively. These differences were attributed to sample heterogeneity.     

Biogeochemical cycling in an organic-rich coastal marine basin. 6. Temporal and spatial variations in sulfate reduction rates

Rates of sulfate reduction were measured over a 3 year period in the anoxic nearshore sediments of Cape Lookout Bight, North Carolina, using both a tube incubation method and a ³⁵S-sulfate direct injection technique. The methods yielded similar depth-integrated rates over the upper 30 cm ranging from less than 10 mol SO⁼₄ · m⁻² · y⁻¹ in winter to greater than 50 mol SO⁼₄ · m⁻² · y⁻¹ in summer. There were also seasonal changes in the Arrhenius activation energies for the sulfate reduction rates indicating that the assumption that E_a is constant with temperature is not always valid. The time averaged annual turnover rate for all three years was 20.4 (±11.4) mol SO⁼₄ · m⁻² · y⁻¹. Surface rates ranged seasonally from less than 0.01 to over 3 mM SO⁼₄ · d⁻¹ between winter and summer, respectively. A subsurface rate maximum was observed to develop during the summer months which accounted for 28 percent of the annual depth integrated sulfate reduction rate. Subsurface rate maxima are the result of changes in the chemistry (substrate type and/or concentration) and the microbiology in the sediments. The possibility of the subsurface maximum being an artifact of the ³⁵S method is also discussed. However, the sulfate reduction rates compare well with previous measurements of the carbon sediment-water plus burial fluxes and with a depth integrated CO₂ production rate modelled from a ΣCO₂ concentration profile from the same site.     

Solubility of Ca6[Al(OH)6]2(CrO4)3·26H2O,the chromate analog of ettringite; 5–75°C

Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O, the chromate analog of the sulfate mineral ettringite, was synthesized and characterized by X-ray diffraction, Fourier transform infra-red spectroscopy, thermogravimetric analyses, energy dispersive X-ray spectrometry, and bulk chemical analyses. The solubility of the synthesized solid was measured in a series of dissolution and precipitation experiments conducted at 5–75°C and at initial pH values between 10.5 and 12.5. The ion activity product (IAP) for the reaction Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O⇌6Ca²⁺+2Al(OH)⁻₄+3CrO²⁻₄+4OH⁻+26H₂O varies with pH unless a CaCrO_4(aq) complex is included in the speciation model. The log K for the formation of this complex by the reaction Ca²⁺+CrO²⁻₄=CaCrO_4(aq) was obtained by minimizing the variance in the IAP for Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O. There is no significant trend in the formation constant with temperature and the average log K is 2.77±0.16 over the temperature range 5–75°C. The log solubility product (log K_SP) of Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O at 25°C is −41.46±0.30. The temperature dependence of the log K_SP is log K_SP=A−B/T+D log(T) where A=498.94±48.99, B=27,499±2257, and D=−181.11±16.74. The values of ΔG⁰_r,298 and ΔH⁰_r,298 for the dissolution reaction are 236.6±3.9 and 77.5±2.4 kJ mol⁻¹. the values of ΔC⁰_P,r,298 and ΔS⁰_r,298 are −1506±140 and −534±83 J mol⁻¹ K⁻¹. Using these values and published standard state partial molal quantities for constituent ions, ΔG⁰_f,298=−15,131±19 kJ mol⁻¹, ΔH⁰_f,298=−17,330±8.6 kJ mol⁻¹, ΔS⁰₂₉₈=2.19±0.10 kJ mol⁻¹ K⁻¹, and ΔC⁰_Pf,298=2.12±0.53 kJ mol⁻¹ K⁻¹, were calculated.     

The Contribution of Benthic Nutrient Regeneration to Primary Production in a Shallow Eutrophic Estuary,Weeks Bay,Alabama

Benthic oxygen, dinitrogen, and nutrient fluxes (NH₄⁺, NO₃⁻, and PO₄³⁻) were measured monthly during a 1-year period at two locations in Weeks Bay, a shallow (1.4 m) and eutrophic estuary in Alabama. Gross primary productivity (GPP), ecosystem respiration (R), and net ecosystem metabolism were determined from high-frequency dissolved oxygen measurements. Peak water column NO₃⁻ (55 μM) and chlorophyll a (138 μg/l) concentrations were measured during spring and fall, respectively. Sediments were a net source of NH₄⁺ (102 μmol m⁻² h⁻¹) and PO₄³⁻ (0.9 μmol m⁻² h⁻¹) but a sink for NO₃⁻ (−30 μmol m⁻² h⁻¹). Benthic N₂ fluxes indicated net N fixation (12 μmol N m⁻² h⁻¹). Sediment oxygen demand (0.55 g O₂ m⁻² day⁻¹) accounted for <10% of R (7.3 g O₂ m⁻² day⁻¹). Despite high GPP rates (4.7 g O₂ m⁻² day⁻¹), the estuary was net heterotrophic. Benthic regeneration supplied, on average, 7.5% and 4% of primary productivity N and P demands, respectively. These results contrast with the conventional view that benthic regeneration accounts for a large fraction of phytoplankton nutrient demand in shallow estuaries.     

Respiration and Calcification of <Emphasis Type="Italic">Crassostrea gigas</Emphasis>: Contribution of an Intertidal Invasive Species to Coastal Ecosystem CO<Subscript>2</Subscript> Fluxes

Respiration and calcification rates of the Pacific oyster Crassostrea gigas were measured in a laboratory experiment in the air and underwater, accounting for seasonal variations and individual size, to estimate the effects of this exotic species on annual carbon budgets in the Bay of Brest, France. Respiration and calcification rates changed significantly with season and size. Mean underwater respiration rates, deducted from changes in dissolved inorganic carbon (DIC), were 11.4 μmol DIC g⁻¹ ash-free dry weight (AFDW) h⁻¹ (standard deviation (SD), 4.6) and 32.3 μmol DIC g⁻¹ AFDW h⁻¹ (SD 4.1) for adults (80–110 mm shell length) and juveniles (30–60 mm), respectively. The mean daily contribution of C. gigas underwater respiration (with 14 h per day of immersion on average) to DIC averaged over the Bay of Brest population was 7.0 mmol DIC m⁻² day⁻¹ (SD 8.1). Mean aerial CO₂ respiration rate, estimated using an infrared gas analyzer, was 0.7 μmol CO₂ g⁻¹ AFDW h⁻¹ (SD 0.1) for adults and 1.1 μmol CO₂ g⁻¹ AFDW h⁻¹ (SD 0.2) for juveniles, corresponding to a mean daily contribution of 0.4 mmol CO₂ m⁻² day⁻¹ (SD 0.50) averaged over the Bay of Brest population (with 10 h per day of emersion on average). Mean CaCO₃ uptake rates for adults and juveniles were 4.5 μmol CaCO₃ g⁻¹ AFDW h⁻¹ (SD 1.7) and 46.9 μmol CaCO₃ g⁻¹ AFDW h⁻¹ (SD 29.2), respectively. The mean daily contribution of net calcification in the Bay of Brest C. gigas population to CO₂ fluxes during immersion was estimated to be 2.5 mmol CO₂ m⁻² day⁻¹ (SD 2.9). Total carbon release by this C. gigas population was 39 g C m⁻² year⁻¹ and reached 334 g C m⁻² year⁻¹ for densely colonized areas with relative contributions by underwater respiration, net calcification, and aerial respiration of 71%, 25%, and 4%, respectively. These observations emphasize the substantial influence of this invasive species on the carbon cycle, including biogenic carbonate production, in coastal ecosystems.     

Dissimilatory sulfate reduction in hypersaline coastal pans: Activity across a salinity gradient

The impact of salinity on the metabolic activity of sulfate-reducing bacteria in five highly saline to hypersaline coastal pans was studied using a radioactive tracer technique. We recorded sulfate reduction at in situ porewater salinities of up to 422. Furthermore, enumeration of sulfate reduction rates in whole core incubations conducted under in situ conditions suggested a high variability in the activity of sulfate-reducers. Average reduction rates (27-3685 nmol cm⁻³ d⁻¹) varied according to depth, season and site sampled. The highest reduction rates measured in the hypersaline pan were comparable to the highest reported rates from highly productive salt marsh and microbial mat ecosystems. Correspondingly, the depth-integrated rates (integrated to 12 cm) varied from 6 to 241 mmol m⁻² d⁻¹ and were also among the highest ever reported rates. The reduction rates decreased down-core and, surprisingly, were highest in the winter season when the lowest sediment temperatures were encountered.High salt concentrations did not inhibit sulfate reduction rates. Rather, higher rates were measured at pans with higher in situ salinities. In laboratory slurry incubation experiments, sediments from the saltpans were treated with increasing salt concentrations. Regression analysis suggested that the short term response of microbial consortia to up-shock was an increase in sulfate reduction activity up to salinities of 272-311 and 134-244, in hypersaline and highly saline pans, respectively. Beyond these salinities, the cells showed evidence of reduced activities.     

pH effect on Re(VII) and Se(IV) diffusion in compacted GMZ bentonite

In a high-level radioactive waste (HLW) repository, pH has an impact on the solubility, migration, and adsorption of radionuclides. Thus, understanding the effects of pH on the diffusion of radionuclides is essential for long-term disposal of HLW. In this work, the diffusion behaviors of Re(VII) and Se(IV) in compacted Gaomiaozi (GMZ) bentonite at different pH have been investigated by a through-diffusion method. The effective diffusion coefficient, i.e., D_e values of Re(VII) and Se(IV) were in the range of (1.0–2.4) × 10⁻¹¹ m²/s at pH 3.0–10.0 and (0.38–2.3) × 10⁻¹¹ m²/s at pH 3.0–9.0. In the case of Re(VII), the D_e values remained almost unchanged probably because ReO₄⁻ was the dominant species in the pH range of 3.0–10.0. In the case of Se(IV), whose predominant species were HSeO₃⁻ at pH < 9.0 and SeO₃²⁻ at pH ≥ 9.0, the D_e values decreased by a factor of 3–6 at pH 9.0, i.e., D_e (pH < 9.0)/D_e (pH 9.0) ≈ 3–6, implying that the species with a higher valence state had a stronger anion exclusion effect. The decrease in D_e values can be explained by the diffusion species of Se(IV). Additionally, the rock capacity factor α decreased with the increase of pH. HSeO₃⁻ was absorbed on GMZ bentonite with distribution coefficient K_d values in the range of (1.0–2.5) × 10⁻⁴ m³/kg at pH ≤ 8.0, whereas SeO₃²⁻ was negligibly sorbed at pH > 8.0.     

Experimental investigation on thermochemical sulfate reduction in the presence of 1-pentanethiol at 200 and 250 °C: Implications for in situ TSR processes occurring in some MVT deposits

Organic sulfur compounds are ubiquitous in natural oil and gas fields and moderate-low temperature sulfide ore deposits. Previous studies have shown that organic sulfur compounds are important in enhancing the rates of thermochemical sulfate reduction (TSR) reactions, but the details of these reaction mechanisms remain unclear. In order to assess the extent of sulfate reduction in the presence of labile sulfur species at temperature conditions near to those where TSR occurs in nature, we conducted a series of experiments using the fused silica capillary capsule (FCSS) method. The tested systems containing labile sulfur species are MgSO₄ + 1-pentanethiol (C₅H₁₁SH) + 1-octene (C₈H₁₆), MgSO₄ + 1-octene (C₈H₁₆), MgSO₄ + 1-pentanethiol (C₅H₁₁SH), 1-pentanethiol (C₅H₁₁SH)+H₂O, and MgSO₄ + 1-pentanethiol (C₅H₁₁SH) + ZnBr₂ systems. Our results show that: (1) intermediate oxidized carbon species (ethanol and acetic acid) are formed during TSR simulation experiments when 1-pentanethiol is present; (2) in the presence of ZnBr₂, 1-pentanethiol can be oxidized by sulfate to CO₂ at 200 °C, which is within the temperature range observed in natural TSR; and (3) the precipitation of sulfide minerals may significantly promote the rate of TSR, indicating that the rates of in situ TSR reactions in ore deposits could be much faster than previously thought. This may be important for understanding the possibility of in situ TSR as a mechanism for the precipitation of metal sulfides in some ore deposits. These findings provide important experimental evidence for understanding the role of organic sulfur compounds in TSR reactions and the pathway of TSR reactions initiated by organic sulfur compounds under natural conditions.     

The Diavik waste rock project: Water flow through mine waste rock in a permafrost terrain

A field experiment is being carried out at the Diavik diamond mine in northern Canada to investigate the influence of unsaturated flow behavior on the quality of drainage from mine waste rock piles in a region of continuous permafrost. This paper is part of a series describing processes affecting the weathering of waste rock and transport of reaction products at this site; here the focus is on unsaturated water flow and its role in mass loading. Two 15 m-high instrumented test piles have been built on 60 m by 50 m collection systems, each consisting of lysimeters and a large impermeable high-density polyethylene (HDPE) liner. Collection lysimeters are installed nearby to investigate infiltration in the upper 2 m of the waste rock. Porosity, water retention curves, and hydraulic conductivity functions are estimated from field measurements and for samples ranging in size from 200 cm³ to 16 m³. Net infiltration in 2007 is estimated to have been 37% of the rainfall for mean annual rainfall conditions. Early-season infiltration freezes and is remobilized as the waste rock thaws. Wetting fronts migrate at rates of 0.2–0.4 m d⁻¹ in response to common rainfall events and up to 5 m d⁻¹ in response to intense rainfall. Pore water and non-reactive solutes travel at rates of <10⁻² to 3 × 10⁻² m d⁻¹ in response to common rainfall events and up to 0.7 m d⁻¹ in response to intense rainfall. Time-varying SO₄ mass loading from the base of the test piles is dictated primarily by the flow behavior, rather than by changes in solute concentrations.     

An atomic force microscopy study of calcite dissolution in saline solutions: The role of magnesium ions

In situ Atomic Force Microscopy, AFM, experiments have been carried out using calcite cleavage surfaces in contact with solutions of MgSO₄, MgCl₂, Na₂SO₄ and NaCl in order to attempt to understand the role of Mg²⁺ during calcite dissolution. Although previous work has indicated that magnesium inhibits calcite dissolution, quantitative AFM analyses show that despite the fact that Mg²⁺ inhibits etch pit spreading, it increases the density and depth of etch pits nucleated on calcite surfaces and, subsequently, the overall dissolution rates: i.e., from 10^−11.75 mol cm⁻² s⁻¹ (in deionized water) up to 10^−10.54 mol cm⁻² s⁻¹ (in 2.8 M MgSO₄). Such an effect is concentration-dependent and it is most evident in concentrated solutions ([Mg²⁺] >> 50 mM). These results show that common soluble salts (especially Mg sulfates) may play a critical role in the chemical weathering of carbonate rocks in nature as well as in the decay of carbonate stone in buildings and statuary.     

Methane cycle in the Barents Sea

Biogeochemical cycle of methane in the Barents Sea was studied using isotope geochemistry to determine the rates of microbial methane oxidation. It was established that microbiological processes (glucose consumption, ¹⁴CO₂ assimilation, sulfate reduction, and slow methane oxidation) in oxidized surface and weakly reduced sediments are marked by only insignificant change in SO ₄ ^2? concentration and absence of notable growth of total alkalinity and N/NH₄ downward the sediment core. Microbial methane productivity was 0.111 × 10⁶ mol day^?1. Taking into account the volume of water column, microbial methane consumption therein can be as much as 1.8 × 10⁶ mol day^?1.     

Field tracer test for denitrification in a pyrite-bearing schist aquifer

A small-scale artificial tracer test performed on a schist aquifer in Brittany has helped clarify mechanisms and kinetics of in situ autotrophic denitrification. NO₃ was injected as a pulse simultaneously with a conservative tracer -Br⁻. During the test, which lasted 210 h, 73% of the injected Br⁻ was recovered, as against only 47% of the NO₃. The 26% difference in the recovery of the two injected species is interpreted as being the result of denitrification, in part due to the direct oxidation of pyrite present in the solid aquifer according to the reaction: 5FeS₂+14NO₃⁻+4H⁺→7N₂+10SO₄²⁻+5Fe²⁺+2H₂O, and in part due to subsequent iron oxidation according to the reaction: NO₃⁻+5Fe²⁺+6H⁺→1/2N₂+5Fe³⁺+3H₂O. Despite the potential increase in SO₄ and Fe resulting from denitrification through pyrite oxidation, the concentrations of these elements in the groundwater remain moderate due to the precipitation of minerals such as jarosite and/or natroalunite. Tracer transfer takes place in a heterogeneous medium which, according to the breakthrough curves, can be simplified to a dual-porosity aquifer comprising a high-permeability (fractures or large fissures) medium of low porosity from which only minor denitrification of circulating NO₃-bearing water was observed and a low-permeability (small fissures) medium of high porosity which induces a higher denitrification rate in the circulating NO₃-bearing water. The kinetics of the denitrification reaction are high compared with results obtained for other environments and can be described by a first-order model with a half life of 7.9 days for the low-porosity medium and only 2.1 days for the high-porosity medium.     

Tailings dam seepage at the rehabilitated Mary Kathleen uranium mine,Australia

This study reports on the seepage of metals, metalloids and radionuclides from the Mary Kathleen uranium mill tailings repository. Since rehabilitation in the 1980s, the capped tailings have developed a stratified hydrochemistry, with acid (pH 3.7), saline, metal-rich (Fe, Mn, Ni, U ± As, Pb, Zn), oxygenated (1.05 mg L⁻¹ DO), radioactive waters in the upper tailings pile and near-neutral pH (pH 7.57), metal-poor, reduced (0.08 mg L⁻¹ DO) waters at depth. Seepage (∼0.5 L s⁻¹) of acid (pH 5.5), metal-rich (Fe, Mn ± Ni, U, Zn), radioactive (U-235, U-238, Ra-226, Ra-228, Ac-227) waters occurs from the base of the tailings dam retaining wall into the former evaporation pond and local drainage system. Oxygenation of the seepage waters causes the precipitation of Fe and coprecipitation and adsorption of other metals (U, Y), metalloids (As), rare earth elements (Ce, La) and radionuclides (U-235, U-238). By contrast, alkalis and alkaline–earth elements (Ca, K, Mg, Na, Sr), Mn, sulfate and to some degree metals (U, Zn, Ni), rare earth elements (Ce, La) and radionuclides (U-235, U-238, Ra-226, Ra-228) remain in solution until pH neutralisation and evaporation lead to their precipitation in efflorescences and sulfate-rich evaporative sediments. While the release of contaminant loads from the waste repository through seepage is insignificant (e.g. ∼5 kg of U per year), surface waters downstream of the tailings impoundment possess TDS, U and SO₄ concentrations that exceed Australian water quality guideline values in livestock drinking water. Thus, in areas with a semi-arid climate, even insignificant load releases of contaminants from capped tailings repositories can still cause the deterioration of water quality in ephemeral creek systems.