Geochemical transport modelling of drainage from experimental mine tailings cells covered by capillary barriers     

J. Molson  M. Aubertin  B. Bussière  M. Benzaazoua 《Applied Geochemistry》2008

The use of covers with capillary barrier effects (CCBEs) for reducing acid mine drainage (AMD) from sulphidic mine tailings is simulated using the MIN3P finite volume model for coupled groundwater flow, O₂ diffusion and multi-component reactive transport. The model is applied to simulate five pilot-scale in situ test cells containing reactive tailings from the Manitou mine site, Val d’Or, Que., Canada. Four of the cells were constructed with CCBEs over the tailings, while the fifth tailings cell was left uncovered. Observed and simulated discharge from the base of each cell showed that the capillary barrier covers significantly reduced sulphide oxidation and AMD. Compared to acidic discharge from the uncovered cell, discharge from the four CCBE-covered cells had neutral pH levels and 1–7 orders of magnitude lower concentrations of SO₄, Fe, Zn, Cu and Al. The simulations showed that the moisture retaining layer of the CCBEs reduced AMD by inhibiting O₂ diffusion into the underlying reactive wastes. Provided the moisture-retention layer of the CCBE remains close to saturation, its thickness had a relatively minor effect. Under such near-saturated conditions, O₂ availability is limited by its diffusion rate through the bulk porous medium and not by the diffusion rate through the oxidized grain shells. The model is providing important new insights for comparing design alternatives for reducing or controlling AMD.  相似文献   

Modeling the diffusion of Na in compacted water-saturated Na-bentonite as a function of pore water ionic strength     

Ian C. Bourg  Garrison Sposito  Alain C.M. Bourg 《Applied Geochemistry》2008,23(12):3635-3641

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The presence of anomalous trace element levels in present day Jamaican soils and the geochemistry of Late-Miocene or Pliocene phosphorites     

Robert G. Garrett  Anthony R.D. Porter  Patricia A. Hunt  Gerald C. Lalor 《Applied Geochemistry》2008

High levels of Cd and Zn in Jamaican soils observed in geochemical surveys are related to the presence of phosphorites of possible Late-Miocene or Pliocene age. The trace element and REE geochemistry of the phosphorites, together with SEM studies, indicate a guano origin for the phosphorites. No specific host minerals for Cd could be identified in the fossiliferous phosphorite which is characterized by uniquely high levels of Cd, Zn, Ag, Be, U and Y. However, in the soil Cd is present in lithiophorite and a complex history of pedological development is preserved in the aluminous–goethite present in the soil. The unique guano signature is preserved in the soil despite the fact that guanos themselves have either not been observed or have been destroyed by continuing karst and soil development. The phosphorite geochemical signature can be traced in the data of a 1988 island-wide soil geochemical survey, identifying areas where the Palaeo-environment that supported bird ‘rookeries’ existed in the Late-Miocene or Pliocene.  相似文献   

Geochemical characterization of surface waters and groundwater resources in the Managua area (Nicaragua,Central America)     

F. Parello  A. Aiuppa  H. Calderon  F. Calvi  D. Cellura  V. Martinez  M. Militello  K. Vammen  D. Vinti 《Applied Geochemistry》2008

This paper reports new geochemical data on dissolved major and minor constituents in surface waters and ground waters collected in the Managua region (Nicaragua), and provides a preliminary characterization of the hydrogeochemical processes governing the natural water evolution in this area. The peculiar geological features of the study site, an active tectonic region (Nicaragua Depression) characterized by active volcanism and thermalism, combined with significant anthropogenic pressure, contribute to a complex evolution of water chemistry, which results from the simultaneous action of several geochemical processes such as evaporation, rock leaching, mixing with saline brines of natural or anthropogenic origin. The effect of active thermalism on both surface waters (e.g., saline volcanic lakes) and groundwaters, as a result of mixing with variable proportions of hyper-saline geothermal Na–Cl brines (e.g., Momotombo geothermal plant), accounts for the high salinities and high concentrations of many environmentally-relevant trace elements (As, B, Fe and Mn) in the waters. At the same time the active extensional tectonics of the Managua area favour the interaction with acidic, reduced thermal fluids, followed by extensive leaching of the host rock and the groundwater release of toxic metals (e.g., Ni, Cu). The significant pollution in the area, deriving principally from urban and industrial waste-water, probably also contributes to the aquatic cycling of many trace elements, which attain concentrations above the WHO recommended limits for the elements Ni (∼40 μg/l) and Cu (∼10 μg/l) limiting the potential utilisation of Lake Xolotlan for nearby Managua.  相似文献   

Thermal history of the unsaturated zone at Yucca Mountain,Nevada, USA     

Joseph F. Whelan  Leonid A. Neymark  Richard J. Moscati  Brian D. Marshall  Edwin Roedder 《Applied Geochemistry》2008

Secondary calcite, silica and minor amounts of fluorite deposited in fractures and cavities record the chemistry, temperatures, and timing of past fluid movement in the unsaturated zone at Yucca Mountain, Nevada, the proposed site of a high-level radioactive waste repository. The distribution and geochemistry of these deposits are consistent with low-temperature precipitation from meteoric waters that infiltrated at the surface and percolated down through the unsaturated zone. However, the discovery of fluid inclusions in calcite with homogenization temperatures (T_h) up to ∼80 °C was construed by some scientists as strong evidence for hydrothermal deposition. This paper reports the results of investigations to test the hypothesis of hydrothermal deposition and to determine the temperature and timing of secondary mineral deposition. Mineral precipitation temperatures in the unsaturated zone are estimated from calcite- and fluorite-hosted fluid inclusions and calcite δ¹⁸O values, and depositional timing is constrained by the ²⁰⁷Pb/²³⁵U ages of chalcedony or opal in the deposits. Fluid inclusion T_h from 50 samples of calcite and four samples of fluorite range from ∼35 to ∼90 °C. Calcite δ¹⁸O values range from ∼0 to ∼22‰ (SMOW) but most fall between 12 and 20‰. The highest T_h and the lowest δ¹⁸O values are found in the older calcite. Calcite T_h and δ¹⁸O values indicate that most calcite precipitated from water with δ¹⁸O values between −13 and −7‰, similar to modern meteoric waters.  相似文献   

Reply to the comment on “Some numerical considerations in the geochemical analysis of distal microtephra”, by Nicholas J.G. Pearce,Charles A. Bendall and John A. Westgate     

A.M. Pollard  S.P.E. BlockleyC.S. Lane 《Applied Geochemistry》2008

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Diagenetic processes on metals in hypersaline mudflat sediments from a subtropical saltmarsh (SE Gulf of California): Postdepositional mobility and geochemical fractions     

M.F. Soto-Jiménez  F. Páez-Osuna 《Applied Geochemistry》2008

To understand the biogeochemical cycles of trace metals (Cd, Cu, Fe, Mn, Ni and Zn) in a hypersaline subtropical marsh, geochemical studies of both interstitial and solid phases were conducted on sediment cores from Chiricahueto marsh, SE Gulf of California. The sequential extraction procedure proposed by Tessier was used to estimate the percentages of the metals present in each geochemical phase of the sediment. Metal concentrations in the solid phase were found to be enriched in the upper layers and mainly associated with reactive fractions such as organic matter, Fe–Mn oxyhydroxides and carbonates (46–74% of Ni, Mn and Cd, and 11–19% of Cu and Zn). Principal factor analysis (PFA) and Spearman correlation analysis revealed a strong positive association of metals and their reactive phases with OC (the diagenetic component), and a negative or non-association with the mud content, Al, Fe and Li (the lithogenic component). Diagenetically released metals are mainly mobilized within hypersaline sediments by buoyancy transport (>90% of total flux) in response to an extreme salinity gradient by input of fresh groundwater (3–6 psu cm⁻¹). The molecular diffusion due to the gradient of metals in porewater (maximum and higher levels at 5–7 and below 20 cm depth, respectively) is significantly less important to the advective transport. Most of the metals mobilized by diffusion–advection processes are re-precipitated in the sediments by authigenic minerals, only <10% of most metals are extruded out to the overlying water column. Authigenic accumulation rates were estimated as 1.42–7.09 mg m⁻² a⁻¹ for Cd; 58.8–378 for Cu; 6922–17,985 for Fe; 38.2–345 for Mn; 20.8–263 for Ni; and 282–2956 mg m⁻² a⁻¹ for Zn. The Mn–Fe oxyhydroxides (40–85% of reactive metals) in the upper oxic–suboxic layers (<5 cm below surface) and sulfide minerals (75–97%) in anoxic sediment layers (7–18 cm) constitute the main scavengers for metals.  相似文献   

The arsenic source term for an in-pit uranium mine tailings facility and its long-term impact on the regional groundwater     

Brett J. Moldovan  M. Jim Hendry  Glenn A. Harrington 《Applied Geochemistry》2008

Detailed field sampling and analyses and laboratory-based diffusion-cell experiments were used in conjunction with 3-D reactive transport modeling (MODFLOW and MT3D99) to quantify the fate and long-term (10 ka) transport of As in the Rabbit Lake In-pit Tailings Management Facility (RLITMF), northern Saskatchewan, Canada. The RLITMF (300 m × 425 m × 90 m thick) was engineered to ensure solute transport within the RLITMF is dominated by diffusion. Concentrations of As in the tailings pore fluids ranged from 0.24 to 140 mg/L (n = 43). Arsenic speciation analyses indicate 90% of this arsenic exists as As⁵⁺. This observation is supported by pH–Eh measurements of pore fluids (n = 135). Geochemical analyses yielded a strong inverse correlation between the Fe/As molar ratio in the tailings solids and the corresponding concentration of dissolved As, which is attributed to the adsorption of As to secondary 2-line ferrihydrite present in the tailings. Diffusion-cell testing yielded values for the effective diffusion coefficient, sorption coefficient, and effective porosity of As in the tailings of 4.5 × 10⁻¹⁰ m²/s, 2–4 cm³/g and 0.36, respectively. Reactive transport simulations using the field and laboratory data show adsorption of As to the tailings and diffusive transport of dissolved As in the tailings should reduce the source term concentration of As to between 40% and 70% of the initial concentrations over the 10 ka simulation period. Based on these simulations, the As concentrations in the regional groundwater, 50 m down gradient of the tailings facility, should be maintained at background concentrations of 0.001 mg/L over the 10 ka period. These findings suggest the engineered in-pit disposal of U mine tailings can provide long-term protection for the local groundwater regime from As contamination.  相似文献   

High-level radioactive waste disposal in Sweden: Hydrogeochemical characterisation and modelling of two potential sites     

M. Gascoyne 《Applied Geochemistry》2008

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Geochemistry and mineralogy of arsenic in (natural) anaerobic groundwaters     

J.A. Saunders  M.-K. Lee  M. Shamsudduha  P. Dhakal  A. Uddin  M.T. Chowdury  K.M. Ahmed 《Applied Geochemistry》2008

Here new data from field bioremediation experiments and geochemical modeling are reported to illustrate the principal geochemical behavior of As in anaerobic groundwaters. In the field bioremediation experiments, groundwater in Holocene alluvial aquifers in Bangladesh was amended with labile water-soluble organic C (molasses) and MgSO₄ to stimulate metabolism of indigenous SO₄-reducing bacteria (SRB). In the USA, the groundwater was contaminated by Zn, Cd and SO₄, and contained <10 μg/L As under oxidized conditions, and a mixture of sucrose and methanol were injected to stimulate SRB metabolism. In Bangladesh, groundwater was under moderately reducing conditions and contained ∼10 mg/L Fe and ∼100 μg/L As. In the USA experiment, groundwater rapidly became anaerobic, and dissolved Fe and As increased dramatically (As > 1000 μg/L) under geochemical conditions consistent with bacterial Fe-reducing conditions. With time, groundwater became more reducing and biogenic SO₄ reduction began, and Cd and Zn were virtually completely removed due to precipitation of sphalerite (ZnS) and other metal sulfide mineral(s). Following precipitation of chalcophile elements Zn and Cd, the concentrations of Fe and As both began to decrease in groundwater, presumably due to formation of As-bearing FeS/FeS₂. By the end of the six-month experiment, dissolved As had returned to below background levels. In the initial Bangladesh experiment, As decreased to virtually zero once biogenic SO₄ reduction commenced but increased to pre-experiment level once SO₄ reduction ended. In the ongoing experiment, both SO₄ and Fe(II) were amended to groundwater to evaluate if FeS/FeS₂ formation causes longer-lived As removal. Because As-bearing pyrite is the common product of SRB metabolism in Holocene alluvial aquifers in both the USA and Southeast Asia, it was endeavored to derive thermodynamic data for arsenian pyrite to better predict geochemical processes in naturally reducing groundwaters. Including the new data for arsenian pyrite into Geochemist’s Workbench, its stability field completely dominates in reducing Eh–pH space and “displaces” other As-sulfides (orpiment, realgar) that have been implied to be important in previous modeling exercises and reported in rare field conditions.  相似文献