Radial Flow Permeability Testing of an Argillaceous Limestone

Argillaceous Lindsay limestone is the geologic storage formation that will be encountered at the site for the construction of a deep ground repository in Ontario, Canada, for the storage of low to intermediate level nuclear waste. The permeability of the Lindsay limestone is a key parameter that will influence the long‐term movement of radionuclides from the repository to the geosphere. This paper describes the use of both steady‐state and transient radial flow laboratory tests to determine the permeability of this argillaceous limestone. The interpretation of the tests is carried out using both analytical results and computational models of flow problems that exhibit radial symmetry. The results obtained from this research investigation are compared with the data available in the literature for similar argillaceous limestones mainly found in the Lindsay (Cobourg) formation. The experiments give permeabilities in the range of 1.0 × 10^?22 to 1.68 × 10^?19 m² for radial flows that are oriented along bedding planes under zero axial stress. The factors influencing transient pulse tests in particular and the interpretation of the results are discussed.     

Estimating Flow Using Tracers and Hydraulics in Synthetic Heterogeneous Aquifers

Regional ground water flow is most usually estimated using Darcy's law, with hydraulic conductivities estimated from pumping tests, but can also be estimated using ground water residence times derived from radioactive tracers. The two methods agree reasonably well in relatively homogeneous aquifers but it is not clear which is likely to produce more reliable estimates of ground water flow rates in heterogeneous systems. The aim of this paper is to compare bias and uncertainty of tracer and hydraulic approaches to assess ground water flow in heterogeneous aquifers. Synthetic two-dimensional aquifers with different levels of heterogeneity (correlation lengths, variances) are used to simulate ground water flow, pumping tests, and transport of radioactive tracers. Results show that bias and uncertainty of flow rates increase with the variance of the hydraulic conductivity for both methods. The bias resulting from the nonlinearity of the concentration–time relationship can be reduced by choosing a tracer with a decay rate similar to the mean ground water residence time. The bias on flow rates estimated from pumping tests is reduced when performing long duration tests. The uncertainty on ground water flow is minimized when the sampling volume is large compared to the correlation length. For tracers, the uncertainty is related to the ratio of correlation length to the distance between sampling wells. For pumping tests, it is related to the ratio of correlation length to the pumping test's radius of influence. In regional systems, it may be easier to minimize this ratio for tracers than for pumping tests.     

Numerical Modeling of Unsaturated Flow in Wastewater Soil Absorption Systems

It is common practice in the United States to use wastewater soil absorption systems (WSAS) to treat domestic wastewater. WSAS are expected to provide efficient, long-term removal of wastewater contaminants prior to ground water recharge. Soil clogging at the infiltrative surface of WSAS occurs due to the accumulation of suspended solids, organic matter, and chemical precipitates during continued wastewater infiltration. This clogging zone (CZ) creates an impedance to flow, restricting the hydraulic conductivity and rate of infiltration. A certain degree of clogging may improve the treatment of wastewater by enhancing purification processes, in part because unsaturated flow is induced and residence times are significantly increased. However, if clogging becomes excessive, the wastewater pond height at the infiltrative surface can rise to a level where system failure occurs. The numerical model HYDRUS-2D is used to simulate unsaturated flow within WSAS to better understand the effect of CZs on unsaturated flow behavior and hydraulic retention times in sandy and silty soil. The simulations indicate that sand-based WSAS with mature CZs are characterized by a more widely distributed flow regime and longer hydraulic retention times. The impact of clogging on water flow within the silt is not as substantial. For sand, increasing the hydraulic resistance of the CZ by a factor of three to four requires an increase in the pond height by as much as a factor of five to achieve the same wastewater loading. Because the degree of CZ resistance directly influences the pond height within a system, understanding the influence of the CZ on flow regimes in WSAS is critical in optimizing system design to achieve the desired pollutant-treatment efficiency and to prolong system life.     

Modelling Feedback of Chemical Reactions on Flow Fields in Hydrothermal Systems

Coupled reactive transport models of hydrothermal systems provide new insights and deeper understanding of the processes occurring due to fluid flow, heat transfer, solute transport, and chemical reactions. Basic concepts of species transport (diffusion, dispersion, and advection) and chemical precipitation and dissolution reactions are discussed, and five end-member types of reactive transport environments are introduced. One of these reactive transport environments, named ‘reactions within thermal gradients’, is used to demonstrate how free thermal convection can lead to redeposition of minerals and, due to the feedback of reaction on the flow field, a change of the convection pattern. The direct consequence of changing the flow field is a significant variation of the temperature distribution within the modelled area. With the example it is shown how reactive transport simulation can be applied for the detailed study of fossil and recent hydrothermal systems.     

垂向和径向排水的潮汐水位振幅和位相变化研究

本文根据岩石力学和地下水动力学理论, 分析了潮汐力作用下含水层与隔水层之间垂向水流交换过程, 并提出了垂向排水条件下含水层潮汐孔压-引潮高振幅比和位相差公式。 结合Hsieh等关于潮汐力作用下井水位对含水层孔压(体应变)的振幅比和位相差响应公式, 进一步推导了垂向和径向水流条件下井水位-引潮高的振幅比和位相差公式。 在径向和垂向排水条件下, 应用M2和O1波潮汐水位-引潮高振幅比和位相差的数值关系, 可以识别影响潮汐水位振幅和位相的主要因素。     

Multiple Well Systems with Non‐Darcy Flow

Optimization of groundwater and other subsurface resources requires analysis of multiple‐well systems. The usual modeling approach is to apply a linear flow equation (e.g., Darcy's law in confined aquifers). In such conditions, the composite response of a system of wells can be determined by summating responses of the individual wells (the principle of superposition). However, if the flow velocity increases, the nonlinear losses become important in the near‐well region and the principle of superposition is no longer valid. This article presents an alternative method for applying analytical solutions of non‐Darcy flow for a single‐ to multiple‐well systems. The method focuses on the response of the central injection well located in an array of equally spaced wells, as it is the well that exhibits the highest pressure change within the system. This critical well can be represented as a single well situated in the center of a closed square domain, the width of which is equal to the well spacing. It is hypothesized that a single well situated in a circular region of the equivalent plan area adequately represents such a system. A test case is presented and compared with a finite‐difference solution for the original problem, assuming that the flow is governed by the nonlinear Forchheimer equation.     

Coupled Process Models of Fluid Flow and Heat Transfer in Hydrothermal Systems in Three Dimensions

Geothermal fields and hydrothermal mineral deposits are manifestations of the interaction between heat transfer and fluid flow in the Earth’s crust. Understanding the factors that drive fluid flow is essential for managing geothermal energy production and for understanding the genesis of hydrothermal mineral systems. We provide an overview of fluid flow drivers with a focus on flow driven by heat and hydraulic head. We show how numerical simulations can be used to compare the effect of different flow drivers on hydrothermal mineralisation. We explore the concepts of laminar flow in porous media (Darcy’s law) and the non-dimensional Rayleigh number (Ra) for free thermal convection in the context of fluid flow in hydrothermal systems in three dimensions. We compare models of free thermal convection to hydraulic head driven flow in relation to hydrothermal copper mineralisation at Mount Isa, Australia. Free thermal convection occurs if the permeability of the fault system results in Ra above the critical threshold, whereas a vertical head gradient results in an upward flow field.     

Refractive Flow and Treatment Systems: Conceptual, Analytical, and Numerical Modeling

Refractive flow and treatment (RFT) systems are designed for passive or low-maintenance in situ ground water remediation for rock or soil of low to moderate permeability. An RFT system captures and refracts contaminated ground water and conveys it to an in situ permeable treatment zone without the need for pumping. Flow to the treatment zone is through one or more high-permeability collection cells, and flow from the treatment zone back into the adjacent native media is through one or more high-permeability dispersal cells.
Conceptual, analytical, and numerical modeling demonstrates the potential for RFT systems to be successful. Analytical modeling shows that the most important factor for this success is that RFT system components be engineered to have comparatively high hydraulic conductivities. A numerical model, capable of representing site-specific conditions, is required for actual RFT system design.     

Online Preconcentration and Determination of Trace Levels Cadmium in Water Samples Using Flow Injection Systems Coupled with Flame AAS

A rapid and sensitive method for the determination of trace levels cadmium in water samples by flame atomic absorption spectrometry was developed. It is based on the online sorption of Cd(II) ions on a microcolumn packed with HCl treated bamboo charcoal. In a pH range of 5.0–7.5, Cd(II) ions were effectively retained on the microcolumn, which exhibited fast kinetics, permitting the use of high sample flow rates up to at least 12.8 mL/min without the loss of retention efficiency. The retained Cd(II) ions were quantitatively eluted with HCl (2.0 mol/L) for an online determination. With a preconcentration time of 80 s at a sample loading flow rate of 8.6 mL/min, a sensitivity enhancement factor of 63 was obtained compared with the slope of the linear portion of the calibration curves before and after preconcentration. The calibration graph using the preconcentration system for cadmium was linear with a correlation coefficient of 0.9997, at levels from 1–40 ng/mL. The precision (RSD) for 11 replicate measurements were 3.2% for the determination of 5 ng/mL Cd(II) and 1.8% for 20 ng/mL Cd(II), respectively, and the detection limit (3s) was 0.36 ng/mL. The accuracy was assessed through the determination of a certified reference material, and also through recovery experiments.     

广义散射层析成像反演

本文详细地给出了基于非均匀介质的体散射广义散射层析成像反演的基本理论.广义散射层析成像反演可以描述为波场的反向传播和对成像场进行局部波数域滤波的过程.在数值算例中,利用背景速度沿深度方向均匀变化的v（z）介质中的简单的方块作为速度异常体的模型,通过对该模型产生的低频的Born数据和声波的正演数据的测试,在对采集系统进行有限频率带宽和空间孔径的校正来进行局部成像矩阵谱的恢复中,可以看出模型中各点的谱在恢复后的质量无论从覆盖的面积范围还是幅值的均一性上都有着明显的提高;在对速度模型的重建中,广义散射层析成像反演能够很好地恢复速度模型的低频分量,即便是方块速度异常体相对于背景速度的平均速度扰动是23%也能很好地重建模型中的速度,且对于不同的背景速度模型基本上都能很好地恢复Marmousi速度模型的低频分量.所以该方法将基于Born模型的层析成像反演适应范围进行了一定程度的扩展.