Arsenic distribution during formation and capping of an oxidised sulphidic minesoil, Macraes mine, New Zealand     

D. Craw  P. O. Koons  D. A. Chappell 《Journal of Geochemical Exploration》2002,76(1)

A minesoil has developed over 5 years oxidative exposure on sulphide concentrate tailings (ca. 1 wt.% As) at the Macraes mesothermal gold mine, New Zealand. The minesoil has a dry crust which has formed due to evaporative drying. This dry crust is enriched in arsenic (ca. 5 wt.% As) as scorodite (FeAsO₄·2H₂O) because of upward mobility of dissolved arsenic during drying. Similar enrichment of arsenic has occurred along the walls of desiccation cracks which extend over 1 m into the minesoil. Capping of the tailings and minesoil with wet tailings (pH=8) results in dissolution of scorodite and remobilization of arsenic on the millimetre scale. Experimental capping of the minesoil with wet calcium carbonate remobilized some arsenic from scorodite on the centimetre scale, but much original arsenic enrichment was preserved after 400 days. A layer of gypsum (CaSO₄·2H₂O) and iron oxyhydroxide cementation developed at the interface between the minesoil and the experimental calcium carbonate cap, restricting water flow. This layer was ca. 1 mm thick after 400 days. Theoretical comparison between advection and diffusion in the minesoil suggests that diffusion is an important mechanism for chemical mobility on the 1–50-year time scale. However, advection can be important in secondary porosity of the dry crust of the minesoil and water penetrates this zone at a rate of 1.5 mm/day.  相似文献   

Forward modeling corona growth in a partially eclogitized leucogabbro, Bourbon Island, North-East Greenland     

C. Sartini-Rideout  J.A. Gilotti  C.T. Foster Jr. 《Lithos》2007,95(3-4):279-297

Coronitic textures are common in partially eclogitized igneous bodies, such as gabbros, leucogabbros, and anorthosites, east of the Germania Land Deformation Zone in North-East Greenland. Coronas formed by prograde metamorphic processes that transformed the gabbroic bodies to eclogite facies, and record frozen stages of the prograde metamorphic evolution of these rocks. A metaleucogabbro-norite body on Bourbon Island in Jøkelbugt is characterized by three concentric areas: a coronitic core, a mottled inner rim with areas of completely eclogitized material surrounded by a matrix of coronitic metaleucogabbro, and an outer rim of strongly foliated and completely retrogressed amphibolite. The Bourbon body preserves four stages of the prograde metamorphic history: Stage I, Stage II, Stage III, and Eclogite Stage. Stage I coronas are found only in the core of the body, which is the least reacted part of the leucogabbro-norite and the closest to the protolith, and is characterized by the corona sequence Pl_rim/Grt + Kfs + Amp/Grt + Amp/Cpx_rim. The typical corona sequence for Stage II is Pl_rim/Grt + Pl + Zo/Cpx_rim/Amp_rim. Stage III samples show a Pl_rim + Ky + Scp/Grt + Pl + Qtz/Qtz + Pl sequence, with the relict clinopyroxene being replaced in part by microcrystalline aggregates of Cpx + Amp + Pl. The Eclogite Stage shows relict Pl completely replaced by Grt, and the relict Cpx completely replaced by aggregates of Omp + Pl + Kfs + Amp. We tested open-system grain boundary diffusion (OSGBD) theories to model the prograde Stage I symplectitic coronas. The observed ratio of the thickness of the different layers is Pl_rim:Grt + Kfs + Amp:Grt + Amp:Cpx_rim equal to 3:1.3:0.95:0.5. These ratios are very close to the modeled ones of 2.7:1.1:1:0.5. Furthermore, subtle textural changes within the Grt + Kfs + Amp corona were also reproduced by the model. The model gave us insight into the conditions of the metamorphic system in which the coronas formed. The sequence Pl_rim/Grt + Kfs + Amp/Grt + Amp/Cpx_rim formed by diffusion driven reactions in an open system involving gain of Fe, K, and Na, and loss of Ca and Mg at the original clinopyroxene–plagioclase boundary. Relative mobilities of the different components within the corona layers were L_MgMg > L_AlAl > L_SiSi > L_CaCa > L_KK > L_FeFe > L_NaNa. Fluid circulation was active to some degree during the transformation to eclogite. The differences between core, inner rim, and the two domains within the inner rim of the metaleucogabbro-norite can be explained by different degrees of fluid circulation in different portions of the rock. The presence of phases containing Cl and P, such as scapolite, in completely eclogitized samples supports the presence of fluid circulation in the system. Another possible explanation for the mottled appearance of the inner rim is protolith heterogeneity.  相似文献   

Interpretation of carbon dioxide diffusion behavior in coals   总被引：3，自引：1，他引：3  

Nikolai Siemons  Karl-Heinz A.A. Wolf  Johannes Bruining 《International Journal of Coal Geology》2007,72(3-4):315-324

Storage of carbon dioxide in geological formations is for many countries one of the options to reduce greenhouse gas emissions and thus to satisfy the Kyoto agreements. The CO₂ storage in unminable coal seams has the advantage that it stores CO₂ emissions from industrial processes and can be used to enhance coalbed methane recovery (CO₂-ECBM). For this purpose, the storage capacity of coal is an important reservoir parameter. While the amount of CO₂ sorption data on various natural coals has increased in recent years, only few measurements have been performed to estimate the rate of CO₂ sorption under reservoir conditions. An understanding of gas transport is crucial for processes associated with CO₂ injection, storage and enhanced coalbed methane (ECBM) production.A volumetric experimental set-up has been used to determine the rate of sorption of carbon dioxide in coal particles at various pressures and various grain size fractions. The pressure history during each pressure step was measured. The measurements are interpreted in terms of temperature relaxation and transport/sorption processes within the coal particles. The characteristic times of sorption increase with increasing pressure. No clear dependence of the characteristic time with respect to the particle size was found. At low pressures (below 1 MPa) fast gas diffusion is the prevailing mechanism for sorption, whereas at higher pressures, the slow diffusion process controls the gas uptake by the coal.  相似文献   

Is there a significant diffusive “salt-pump mechanism” for the transport of organic contaminants from the marine environment into fresh water aquifers? A discussion contribution     

M. E. Bridge  D. R. Lloyd 《Environmental Geology》2005,49(2):207-213

We analyse recent claims of an additional risk of diffusive intrusion by marine organic contamination (OC) into coastal aquifers, and discuss the activity-driven “salt-pump mechanism” which is claimed to accelerate this diffusion. The reported accelerations of transport of OC out of saltwater are far too large to be due to activity effects on simple diffusion. There probably is a small acceleration of diffusion out of saltwater, but there is no direct experimental evidence for this. Possible experimental artefacts are suggested. Larger effects on diffusion in the literature are shown to be not relevant to this problem. Diffusion of suspended OC, present in the experiments, is too slow to be significant; however, this suspended material is shown to have serious implications for the proposed interpretation of the results. Until these results are better understood, no conclusions about any possible environmental implications can be drawn from them.

D. R. LloydEmail: Phone: +353-1-6081726Fax: +353-1-6712826

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Review of the use of Péclet numbers to determine the relative importance of advection and diffusion in low permeability environments   总被引：1，自引：0，他引：1  

Marijke Huysmans  Alain Dassargues 《Hydrogeology Journal》2005,13(5-6):895-904

In low permeability environments, transport by advection is often neglected based on a Péclet number criterion. Such a criterion usually states that if the Péclet number (Pe) is much smaller than 1, diffusion dominates over advection and transport may be modeled considering diffusion only. Unfortunately, up to 10 different Péclet number definitions exist and for a particular case these different definitions lead to very diverse Péclet number values, differing several orders of magnitude from each other. In this paper, the different Péclet number definitions are therefore evaluated on their ability to determine the relative importance of transport by advection and by diffusion in low permeability environments. This is done by comparing the results of the analytical solution for pure diffusion with the analytical solution for diffusion, advection and dispersion for a large number of different input parameter values. The relation between the different Péclet numbers and the difference between the calculated concentration considering diffusion only and the calculated concentration considering both diffusion and advection is studied. These calculations show that some Péclet number definitions are not well suited to decide whether advection may be neglected in low permeability media.

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Resumen Frecuentemente se descuida el transporte por avección en ambientes de baja permeabilidad sobre la base del criterio del número de Péclet. Dicho criterio usualmente sostiene que si el número de Péclet (Pe) es mucho menor a 1, la difusión domina sobre la avección y es posible modelar el transporte considerando la difusión únicamente. Desafortunadamente existen 10 definiciones diferentes del número de Péclet y para un caso en particular estas diferentes definiciones conducen a valores de número de Péclet muy diversos los cuales difieren entre sí en varias magnitudes. Por lo tanto, en este artículo se evalúan las diferentes definiciones en base a su habilidad para determinar la importancia relativa del transporte por avección y difusión en ambientes de baja permeabilidad. Esto se lleva a cabo mediante la comparación de los resultados de la solución analítica para difusión pura con los de la solución analítica por difusión, avección y dispersión para un número amplio de diferentes valores como parámetro. Se ha estudiado la relación entre los diferentes números de Péclet y las diferencias entre la concentración calculada considerando difusión únicamente y considerando tanto difusión como avección. Estos cálculos muestran que algunas de las definiciones de los números de Péclet no son muy apropiadas par decidir si se puede descuidar la avección en medios de baja permeabilidad.

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Résumé Dans les milieux à faible perméabilité, le transport par advection est souvent négligeable lorsquon se base sur le critère du nombre de Péclet. Un tel critère suggère habituellement que pour un nombre de Péclet beaucoup plus petit que 1, la diffusion domine sur ladvection et que le transport peut être modélisé par diffusion seulement. Malheureusement, il existe jusquà environ dix définitions du nombre de Péclet et pour un cas spécifique, ces différentes définitions mènent à des valeurs du nombre de Péclet très différentes, et qui varient entre elles de plusieurs ordres de grandeur. Dans cet article, les différentes définitions du nombre de Péclet sont évaluées en fonction de leur habileté à déterminer limportance relative du transport par advection et par diffusion dans les milieux à faible perméabilité. Ceci est fait en comparant les résultats de la solution analytique pour la diffusion pure à la solution analytique pour la diffusion, ladvection et la dispersion, et ce, pour un grand nombre de valeurs différentes comme paramètre dentrée. La relation entre les différents nombres de Péclet et la différence entre la concentration calculée en considérant la diffusion seulement et la concentration calculée considérant simultanément la diffusion et ladvection est étudiée. Ces calculs prouvent que certaines définitions de nombre de Péclet ne sont pas appropriées afin de décider si ladvection peut être négligée dans les milieux à faible perméabilité.

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Determining the optimal soil temperature scheme for atmospheric modelling applications     

M.?J.?BestEmail author  P.?M.?Cox  D.?Warrilow 《Boundary-Layer Meteorology》2005,114(1):111-142

The treatment of the land surface can have a significant impact on the performance of atmospheric models, influencing the surface energy balance and near surface atmospheric variables. In numerical weather prediction models it is especially important to reproduce the observed diurnal cycle in screen-level temperature, which requires an accurate representation of the surface temperature, and therefore an accurate and computationally efficient representation of soil heat storage and transfer. We present a technique for analysing the accuracy of numerical soil temperature schemes, and a methodology for choosing the optimal layer thicknesses for schemes with a given number of layers. Furthermore, the analysis suggests that first generation land surface schemes, which typically used a layer-average surface temperature, may be more accurate in this respect than the latest land surface schemes, which tend to use a skin surface temperature boundary condition.The British Crowns right to retain a non-exclusive royalty-free license in and to any copyright is acknowledged.  相似文献   

Parameterization and Micrometeorological Measurement of Air–Sea Gas Transfer     

C. W. Fairaill  J. E. Hare  J. B. Edson  W. McGillis 《Boundary-Layer Meteorology》2000,96(1-2):63-106

Because of the combination of smallconcentrations and/or small fluxes, the determinationof air–sea gas fluxes presents unusual measurementdifficulties. Direct measurements (i.e., eddycorrelation) of the fluxes are rarely attempted. Inthe last decade, there has been an intense scientificeffort to improve measurement techniques and to placebulk parameterizations of gas transfer on firmertheoretical grounds. Oceanic tracer experiments,near-surface mean concentration profiles, eddyaccumulation, and direct eddy covariance methods haveall been used. Theoretical efforts have focusedprimarily in the realm of characterizing the transferproperties of the oceanic molecular sublayer. Recentmajor field efforts organized by the U.S.A. (GASEX-98) andthe European Union (ASGAMAGE) have yielded atmospheric-derivedresults much closer to those from oceanographicmethods. In this paper, we review the physical basisof a bulk-to-bulk gas transfer parameterization thatis generalized for solubility and Schmidt number. Wealso discuss various aspects of recent sensor andtechnique developments used for direct measurementsand demonstrate experimental progress with resultsfrom ASGAMAGE and GASEX-98. It is clear that sensornoise, sensitivity, and cross talk with other speciesand even ship motion corrections still need improvement foraccurate measurements of trace gas exchange over theocean. Significant work remains to resolve issuesassociated with the effects of waves, bubbles, andsurface films.  相似文献   

    

Yu Xing  Dai Jin  Jiang Weimei  Fan Peng 《大气科学进展》2000,17(4):617-635

It is essential to learn the temporal and spatial concentration distributions and variations of seeding agents in cloud seeding of precipitation enhancement. A three–dimensional puff trajectory model incorporating a mesoscale nonhydrostatic model has been formulated, and is applied to simulating the transporting and diffusive characteristics of multiple line sources of seeding agents within super–cooled stratus. Several important factors are taken into consideration that affect the diffusion of seeding materials such as effects of topography and vertical wind shear, temporal and spatial variation of seeding parameters and wet deposition. The particles of seeding agents are assumed to be almost inert, they have no interaction with the particles of the cloud or precipitation except that they are washed out by precipitation. The model validity is demonstrated by the analyses and comparisons of model results, and checked by the sensitivity experiments of diffusive coefficients and atmospheric stratification. The advantage of this model includes not only its exact reflection of heterogeneity and unsteadiness of background fields, but also its good simulation of transport and diffusion of multiple line sources. The horizontal diffusion rate and the horizontal transport distance have been proposed that they usually were difficult to obtain in other models. In this simulation the horizontal diffusion rate is 0.82 m s⁻¹ for average of one hour, and the horizontal average transport distance reaches 65 km after 1 h, which are closely related to the background fields.  相似文献   

Iron Diffusion in Single-Crystal Diopside     

F. Azough  R. Freer 《Physics and Chemistry of Minerals》2000,27(10):732-740

 Iron tracer diffusion experiments in diopside have been performed using natural and synthetic single crystals of diopside, and stable iron tracers enriched in ⁵⁴Fe, at temperatures in the range 950–1100 °C, total pressure 1 atm, for times up to 29 days. Iron isotope diffusion profiles were determined with an ion microprobe. For experiments performed at log pO₂ = −13, in directions parallel to the c axis and the b axis of two natural, low iron (Fe ∼ 1.8 at %) diopsides, the data obey a single Arrhenius relationship of the form D = 6.22_−5.9 ^+49.6×10⁻¹⁵ exp(−161.5 ± 35.0 kJ mol⁻¹/RT) m² s⁻¹. A single datum for iron diffusion in iron-free, single-crystal diopside at 1050 °C, is approximately 1 order of magnitude slower than in the natural crystals. The pO₂ dependence of iron diffusion in natural crystals at 1050 °C (power exponent = 0.229 ± 0.036) indicates a vacancy mechanism; this is consistent with the results of unpublished atomistic simulation studies. There is no evidence of anisotropy for iron diffusion in diopside. Received: 16 March 1999 / Accepted: 10 April 2000  相似文献   

城市热岛成因及其对污染物扩散影响的数值模拟   总被引：5，自引：4，他引：1       下载免费PDF全文

张凯杨  许春晓  崔桂香  张兆顺  王志石 《气象与环境学报》2007,23(3):10-14

采用大涡模拟方法,通过数值求解可压缩流体的动力学方程和欧拉型的浓度扩散方程,建立了不同的地表模型,将城市地表差异等因素作为地面热通量的边界条件引入,得到了简单区域上的温度场和流场的三维结构,证实了城乡地表差异是引起热岛现象的根本原因之一,成功再现了城市热岛现象,并研究了城市热岛效应的存在对污染物扩散的影响。  相似文献