Oxygen influx and geochemistry of percolate water from reactive mine waste rock underlying a sloping channelled soil cover

An ideal engineered soil cover can mitigate acid rock drainage (ARD) by limiting water and gaseous O₂ ingress into an underlying waste rock pile. However, the barrier layer in the soil cover almost invariably tends to develop cracks or fractures after placement. These cracks may change water flow and O₂ transport in the soil cover and decrease performance in the long run. The present study employed a 10-cm-wide sand-filled channel installed in a soil barrier layer (silty clay) to model the aggregate of cracks or fractures that may be present in the cover. The soil cover had a slope of 20%. Oxygen transport through the soil cover and oxidation of the underlying waste rock were investigated and compared to a controlled column test with bare waste rock (without soil cover). Moreover, gaseous O₂ transport in the soil cover with channel and its sensitivity to channel location as well as the influence of the saturated hydraulic conductivity of the channel material were modeled using the commercial software VADOSE/W. The results indicted that the waste rock underlying the soil cover with channel had a lower oxidation rate than the waste rock without cover because of reduced O₂ ingress and water flushing in the soil cover with channel, which meant a partial soil cover might still be effective to some extent in reducing ARD generation. Gaseous O₂ ingress into the covered waste rock was more sensitive to the channel location than to the saturated hydraulic conductivity of the material filling the channel. Aqueous equilibrium speciation modeling and scanning electron microscopy with energy dispersive X-ray analysis indicated that secondary minerals formed as a result of the oxidation of the waste rock included gypsum and goethite in the covered waste rock and schwertmannite and other Fe oxides in the uncovered waste rock. The findings of the study provided insight into the effect of channel flow on O₂ transport and oxidation of the covered waste rock, which may help to improve soil cover design and construction to minimise the generation of preferential flow in the barrier layer.     

砾石覆盖紫色土坡耕地水文过程

紫色土中砾石分布广泛,地表常为砾石覆盖,砾石覆盖对土壤水文过程有着重要影响。试验小区(2 m×1 m)为坡度23°的坡耕地, 试验降雨强度为(53.9±2.8)mm/h、 (90.8±6.1)mm/h和(134.3±14.9)mm/h, 砾石覆盖度为0%,11%,20%,33%和42%。通过原位人工模拟降雨试验,定量研究了不同降雨强度下砾石覆盖对降雨入渗、地表产流及壤中流产流的影响。结果表明:砾石覆盖对入渗过程影响显著,稳定入渗速率及稳定入渗系数与砾石覆盖度呈正相关关系,3种降雨强度下,稳定入渗系数分别为47.70%~86.59%,30.61%~82.83%、17.76%~77.44%,42%砾石覆盖度小区的稳定入渗速率分别是裸露小区的1.95~4.94倍;地表砾石覆盖延迟地表产流、减少地表径流量,地表产流时间随着砾石覆盖度的提高呈增加趋势,地表径流速率及地表径流系数随砾石覆盖度的增加而降低,相对地表径流系数与地表砾石覆盖度呈指数负相关关系;地表砾石覆盖促进壤中流的发生、增加壤中流量,壤中流产流时间随着砾石覆盖度的增加逐渐缩短,壤中流径流速率及壤中流径流系数随地表砾石覆盖度的增加而提高,相对壤中流径流系数与地表砾石覆盖度呈指数正相关关系。     

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.     

层状盐岩双井流场浓度场试验及数值计算

为了研究层状盐岩小间距双井水溶造腔中流场浓度场特征,进行了相关的模型试验和数值计算。基于量纲分析法搭建了模拟建腔期腔内流场的相似模型试验平台,利用染色法研究了建腔期腔内流场运移规律,并采用时均法测量腔内浓度。试验发现：流量越大,浮羽流极限高度越高,但腔内浓度变化速率减小,排卤口浓度变低;提管高度的改变实际上是改变了淡水相对于腔体的初始空间位置;夹层赋存状态变化实际是改变夹层在溶腔中的空间位置,处于不同区域的夹层会改变腔体流场的运动趋势和作用范围。利用Fluent软件进行3D数值计算,结果验证了室内试验流场的普遍规律,得出同样的分层结果;但在速度分布上,发现流场底部并非无宏观流动,而是自然对流扩散和沉降作用占主导。     

岩体裂隙中渗流场有限元随机模拟分析

采用有限元方法数值,模拟了开度随机分布裂隙中的渗流场问题。根据给定的裂隙开度均值和标准差,随机分布生成有限元模型中的单元裂隙开度,模型中的材料参数和单元属性用ANSYS中的APDL参数化语言赋值。根据有限元随机模拟断面的流量分布和稳态渗流问题的达西定律,计算在不同裂隙开度标准差条件下的等效导水系数,研究等效导水系数与裂隙粗糙度之间的关系。数值模拟结果表明,与光滑平直的裂隙相对比,在裂隙平均开度为常数、开度随机正态分布的条件下,表面粗糙裂隙的等效导水系数略有降低;对于裂隙平均开度为0.5 mm的裂隙模型,裂隙开度标准差从0.01 mm变化到 0.2 mm,其等效导水系数变化小于5 %。传统的渗流控制方程无法精确描述地下水在粗糙裂隙中的流动特征,因为在压力水头的定义中忽略了速度项。     

The state of the art of numerical modeling of thermohydrologic flow in fractured rock masses

The state of the art of numerical modeling of thermohydrologic flow in fractured rock masses is reviewed and a comparative study is made of several models which have been developed in nuclear waste isolation, geothermal energy, ground water hydrology, petroleum engineering, and other geologic fields. The general review is followed by individual summaries of each model and the main characteristics of its governing equations, numerical solutions, computer codes, validations, and applications. This work was supported by the Assistant Secretary for Nuclear Energy, Office of Waste Isolation of the U. S. Department of Energy under Contract Number DE-AC03-76SF00098. Funding for this project is administered by the Office of Nuclear Waste Isolation at Battelle Memorial Institute.     

A review on soil cover in waste and contaminant containment: design, monitoring, and modeling

Soil cover is a widely-used but relatively new method for solid waste containment. Standard while site-specific procedures for cover design, monitoring, and evaluation are needed to insure reliable cover performance. This paper presents a review of soil cover types, design principles and procedures, cover monitoring, and long-term performance modeling. Cover types and cover design are introduced with the general concepts and discussed on their specific applicabilities in different circumstances. Detailed discussion is given on unsaturated flow system properties and their field measurements, including methods, apparatuses/equipments and their advantages and disadvantages. Several unsaturated flow simulators are discussed and compared with regards to their simulation capacities for critical parameters closely related to soil cover performance such as runoff, infiltration and evaporation. Finally, research subjects are suggested for future work for better soil cover monitoring and modeling.     

Application of geogrids in waste dump stability: a numerical modeling approach

Geosynthetic is widely used to reinforce the weak rock mass, mine waste dump, soil slopes road cut slopes, etc. The present paper discusses the effect of geogrids on the stability of mine waste dump. The stability of mine waste dump has been done by Fast Langrage Analysis of Continua (FLAC) slope software, which is based on finite difference method. Reinforcement by geogrids mainly depends on the tensile strength, aperture size of geogrids, and particle size distribution of dump rock mass. Different permutations and combinations of spacing between two geogrid sheets have been taken into consideration to study the stability of mine waste dump. The factor of safety is calculated to quantify the effect of geogrids on waste dump slope. It has been observed from numerical modeling that the maximum slope angle is 45° at a height of 10 m. The scope of increasing slope angle from 45 to 60° is evaluated using geogrids. It has been found from the study that the factor of safety increases as the spacing between geogrids decreases. Maximum strain is also plotted of each case to identify the slip circle. The positions of geogrids modify the probable slip circle or failure plane of mine waste dump. Using ten geogrids at a spacing of 1 m, the slope angle can be increased up to 60° with factor of safety of 1.4.     

基于包气带水分运移数值模型的黄河三角洲蒸发量研究

蒸发是重要的水循环过程,在野外试验和室内分析的基础上,建立了黄河三角洲地下水浅埋区观测试验场土壤水分运移数值模拟模型,模型中对上边界条件处理采用了考虑气象因素、地表覆盖条件和表层土壤含水量的Penman Wilson公式。利用模型对试验点1999年、2000年、2002年的蒸发量进行了数值模拟计算,计算并分析了裸地、棉花地、芦苇地不同地表植被条件下,地表蒸发量(包括棵间裸地蒸发量和植物蒸腾量)的变化规律。从时间上看,黄河三角洲7、8月是蒸发量较高的时期, 1、2月是蒸发量最低时期,一般日蒸发量在0~8 mm/d。影响地面蒸发的主要因素除气象因素外,还有植被类型和覆盖程度、地下水位埋深等因素。植被覆盖程度越高、地下水位埋深越浅,则蒸发量越大,从而导致裸地蒸发量相对较小,农田年蒸发量稍大,而芦苇地蒸发量最大。研究表明,土壤水分运移模型是估算各种复杂条件下蒸发量的有效工具。     

Analysis of flow behavior in a landfill with cover soil of low hydraulic conductivity

 This paper presents the results of field tests of hydrologic parameters in a landfill and the results of numerical simulation to find the efficiency of the pumping method to reduce leachate levels in the landfill. The field hydraulic conductivity and storativity of waste and buried cover soils in the landfill are measured by pumping and slug tests. The hydrologic condition inside the landfill is first calibrated using the drawdown-time curve obtained from the pumping test, and the flow behavior of leachate during pumping in the landfill, when various layers of waste and buried cover soil exist, is analyzed through three-dimensional numerical simulation of flow. The results of the field investigation show that the buried cover soil of low hydraulic conductivity forms an impermeable layer preventing the downward flow of leachate and upward flow of landfill gas. The hydraulic conductivities of the pumping test and slug tests were quite close on the same order of magnitude. It was also possible to match the drawdown-time data of the field tests with those of the model using input data close to the hydrologic property obtained from the field tests. The numerical flow analysis showed that pumping was possible up to 120 tons/day for a single well without a drain, while the pumping rate could be increased to 300 tons/day for the same well with the drain. From the vertical section of the flow vector with a horizontal drain, the barrier role of buried cover soil is identified, which was proposed by examining the water contents of the disposed cover soil and waste in the field. Received: 15 May 1998 · Accepted: 4 January 1999     

Centrifugal and numerical modeling of embankment widening over soft soils treated by pile-supported geosynthetic-reinforced soil wall

Acta Geotechnica - More and more expressways built on soft soil foundations need to widen their embankments because they cannot fulfill the demand for the traffic volume. Geosynthetic-reinforced...     

均质岩体中隧道围岩破坏过程的试验与数值模拟

采用模型试验与离散元模拟的方法,对均质岩体中隧道开挖后围岩的变形破坏过程进行了研究,并对围岩变形破坏过程中围岩应力及地表位移的变化规律进行了分析,模型试验结果与数值模拟结果取得了较好的一致性。研究结果表明：隧道开挖后,拱顶处围岩变形明显并出现裂缝,围岩破坏从拱顶开始,进而呈渐进式向上发展,最终形成稳定的塌落拱;开挖后,隧道围岩径向应力减小,隧道周边一定范围内围岩切向应力减小,且随隧道变形破坏的发展,围岩切向应力减小的区域逐渐扩大;隧道塌方后,拱底垂直应力增加,开挖结束至塌方开始期间地表位移增量最大,塌方期间地表位移增量最小。     

基于颗粒流数值试验的岩土材料内尺度比研究

内尺度比是材料的最小结构和材料尺寸的比。在细观条件下,岩土材料可以看成是由不同尺度矿物颗粒组成的颗粒集聚体,含有不同尺度的天然缺陷。这些带有尺度特点的材料结构、构造特征无疑会影响岩土材料性质测试的结果和准确性,即岩土材料具有和金属材料一样的内尺度比。利用颗粒流软件fish语言编写程序虚拟实现了岩土材料单轴压缩试验,设计了0.05、0.10、0.20、0.40、0.60、0.80、1.00、1.20、1.40、1.60、1.80、2.00 mm等12种粒径数值试件,分别进行了单轴压缩试验。根据试件的破坏形态、应力-应变曲线,分析了内尺度比对测试结果的影响,发现:粒径小于0.40 mm之后,计算时间急剧增加,计算效率急剧减小;内尺度比小于0.01之后,材料的数值试验结果趋于稳定。这说明岩土材料也存在内尺度比,岩土体的物理力学参数的尺寸效应问题就是材料内尺度问题的一种表现形式。     

岩土流变数值计算中一些问题的探讨

针对黏弹性组合模型中的两类参数,即拉压模量和拉压黏性系数以及剪切模量和剪切黏性系数在数值反分析和正分析应用中存在的概念模糊,以及转换不合理等一些问题,首先分析了这两类参数概念上的差异,接着从理论上建立了拉压模量和拉压黏性系数（常体积模量假设下广义Kelvin模型）到剪切模量和剪切黏性系数之间的转换关系,并通过一个算例,从正分析和参数反演两个角度,对这些问题进行了探讨,给出了产生这些问题的内在原因。最后,提出了解决这些问题的可行性措施,并指出这一解决措施也同样适用于其它假设情况。     

On persistent primary variables for numerical modeling of gas migration in a nuclear waste repository

Numerical simulation of gas migration driven by compressible two-phase partially miscible flow in porous media is of major importance for safety assessment of deep geological repositories for long-lived high-level nuclear waste. We present modeling of compositional liquid and gas flow for numerical simulations of hydrogen migration in deep geological radioactive waste repository based on persistent primary variables. Two-phase flow is considered, with incompressible liquid and compressible gas, which includes capillary effects, gas dissolution, and diffusivity. After discussing briefly the existing approaches to deal with phase appearance and disappearance problem, including a persistent set of variables already considered in a previous paper (Bourgeat et al., Comput Geosci 13(1):29–42, 2009), we focus on a new variant of the primary variables: dissolved hydrogen mass concentration and liquid pressure. This choice leads to a unique and consistent formulation in liquid saturated and unsaturated regions, which is well adapted to heterogeneous media. We use this new set of variable for numerical simulations and show computational evidences of its adequacy to simulate gas phase appearance and disappearance in different but typical situations for gas migration in an underground radioactive waste repository.     

Interaction of diamond mine waste and surface water in the Canadian Arctic

Factors controlling the chemical composition of water interacting with finely-crushed kimberlite have been investigated by sampling pore waters from processed kimberlite fines stored in a containment facility. Discharge water from the diamond recovery plant and surface water from the containment facility, which acts as plant intake water, were also sampled. All waters sampled are pH-neutral, enriched in SO₄, Mg, Ca, and K, and low in Fe. Pore-water samples, representing the most concentrated waters, are characterized by the highest SO₄ (up to 4080 mg l⁻¹), Mg (up to 870 mg l⁻¹), and Ca (up to 473 mg l⁻¹). The water discharged from the processing plant has higher concentrations of all major dissolved constituents than the intake water. The dominant minerals present in the processed fines and the kimberlite ore are serpentine and olivine, with small amounts of Ca sulphate and Fe sulphide restricted to mud xenoclasts. Reaction and inverse modeling suggest that much of the water-rock interaction takes place within the plant and involves the dissolution of chrysotile and Ca sulphate, and precipitation of silica and Mg carbonate. Evapoconcentration also appears to be a significant process affecting pore water composition in the containment facility. The reaction proposed to be occurring during ore processing involves the dissolution of CO_2(g) and may represent an opportunity to sequester atmospheric CO₂ through mineral carbonation.     

Mineralogical and geochemical investigations of silicate-rich mine waste from a kyanite mine in central Virginia: implications for mine waste recycling

A kyanite mine in central Virginia produces a silicate-rich waste stream which accumulates at a rate of 450,000–600,000 tons per year. An estimated 27 million tons of this waste stream has accumulated over the past 60 years. Grain size distribution varies between 1.000 and 0.053 mm, and is commonly bimodal with modes typically being 0.425 and 0.250 mm and uniformity coefficients vary from 2.000 to 2.333. Hydraulic conductivity values vary from 0.017 to 0.047 cm/s. Mineralogy of the waste stream consists of quartz, muscovite, kyanite and hematite. Muscovite grains have distinct chemical compositions with significant Na₂O content (1.12–2.66 wt%), TiO₂ content (0.63–1.68 wt% TiO₂) and Fe content, expressed as Fe₂O₃ (up to 1.37 wt%). Major element compositions of samples were dominated by SiO₂ (87.894–90.997 wt%), Al₂O₃ (6.759–7.741 wt%), Fe₂O₃ (1.136–1.283 wt%), and K₂O (0.369–0.606 wt%) with other components being <1.000 wt%. Elements of environmental concern (V, Cr, Ni, Cu, Zn, As, Ag, Sn, Sb, Ba, Hg, Tl, and Pb) were detected; however, the concentrations of all elements except Ni were below that of the kyanite quartzites in the region from which the waste is derived. Both major and trace element compositions indicate minimal variation in composition. The waste stream has potential for recycling. Muscovite is suitable for recycling as a paint pigment or other industrial applications. Muscovite and hematite are commonly intergrown and are interpreted to be material where much of the elements of environmental concern are concentrated. Reprocessing of the waste stream to separate muscovite from other components may enable the waste stream to be used as constructed wetland media for Virginia and nearby states. Recycling of this mine waste may have a positive impact on the local economy of Buckingham County and aid in mitigation of wetland loss.     

Speciation and solubility relationships of Al, Cu and Fe in solutions associated with sulfuric acid leached mine waste rock

 Solutions from oxidized waste rock originating from an acid-leached waste dump were studied. The dissolution data suggest that after the majority of the soluble solid phases are removed, remaining solid phases continued to buffer the solutions in the acidic pH range. Incorporating the solution data into MINTEQA2 identified controls on the solubility of Al, Cu and Fe at pH values from about 2.5 to slightly over 5. Sulfate appears to play a significant role in the formation of solubility controlling solid phases for Al and Cu. This is not the case for Fe, and is suggested that Fe and Cu solubility may be controlled by cupric ferrite at low pH values. Received: 5 April 1998 · Accepted: 27 July 1998     

Estimation of actual evapotranspiration by numerical modeling of water flow in the unsaturated zone: a case study in Buenos Aires, Argentina

A method is presented to estimate actual evapotranspiration (ET_A) from potential evapotranspiration (ET_P) by numerical modeling of water flow in the unsaturated zone. Water flow is described by the Richards equation with a sink term representing the root water uptake. Evaporation is included in the model as a Neumann boundary condition at the soil surface. The Richards equation is solved in a one-dimensional domain using a mixed finite element method. The values of ET_A are obtained by applying a water stress factor to ET_P to account for soil moisture changes during the simulation period. The proposed numerical model is used to estimate ET_A in an experimental plot located in a flatland area in Buenos Aires (Argentina). Numerical results show that the proposed model is a useful tool for evaluating evapotranspiration under different scenarios.