Simulation of Water‐Table Aquifers Using Specified Saturated Thickness

Simulating groundwater flow in a water‐table (unconfined) aquifer can be difficult because the saturated thickness available for flow depends on model‐calculated hydraulic heads. It is often possible to realize substantial time savings and still obtain accurate head and flow solutions by specifying an approximate saturated thickness a priori, thus linearizing this aspect of the model. This specified‐thickness approximation often relies on the use of the “confined” option in numerical models, which has led to confusion and criticism of the method. This article reviews the theoretical basis for the specified‐thickness approximation, derives an error analysis for relatively ideal problems, and illustrates the utility of the approximation with a complex test problem. In the transient version of our complex test problem, the specified‐thickness approximation produced maximum errors in computed drawdown of about 4% of initial aquifer saturated thickness even when maximum drawdowns were nearly 20% of initial saturated thickness. In the final steady‐state version, the approximation produced maximum errors in computed drawdown of about 20% of initial aquifer saturated thickness (mean errors of about 5%) when maximum drawdowns were about 35% of initial saturated thickness. In early phases of model development, such as during initial model calibration efforts, the specified‐thickness approximation can be a very effective tool to facilitate convergence. The reduced execution time and increased stability obtained through the approximation can be especially useful when many model runs are required, such as during inverse model calibration, sensitivity and uncertainty analyses, multimodel analysis, and development of optimal resource management scenarios.     

压缩过程中饱和原状和饱和重塑黄土孔隙分布变化特征

为从微观孔隙分布角度上解释饱和原状黄土与饱和重塑黄土在相同应力状态下宏观变形的差异、在相近孔隙比下孔隙分布的差异以及所经历应力路径的差异,针对这二个目的,对泾阳崔师饱和原状黄土和饱和重塑黄土进行一维压缩和等向压缩试验并进行相同应力状态及相近孔隙比下的压汞试验。研究结果表明:泾阳崔师黄土孔隙分布呈三峰分布,重塑黄土孔隙分布呈两峰分布;相同应力状态下,两者粒间孔隙差异大而粒内孔隙基本相同,随着压力的增加,两者粒间孔隙逐渐相近,宏观变形的差异体现在粒间孔隙的差异;相近孔隙比下,二者孔隙分布差异不大但经历的应力路径差异很大。     

饱和黄土场地原位试验及液化势评价

目前,主要依靠室内动力试验对黄土液化势进行评价。由于黄土特殊的结构性,室内试验对其饱和的过程较为复杂,且与实际场地饱和黄土差异明显,导致室内黄土液化试验结果并不能代表现场饱和黄土的抗液化强度。本文选取兰州市西固区寺儿沟村某饱和黄土场地进行钻孔测试,现场实施了标准贯入试验、静力触探试验以及剪切波速测试。应用Robertson的土类指数分类图对该场地不同含水率黄土的土类进行了界定,确定了饱和黄土属于类砂土,有液化势。应用NCEER推荐方法,计算了3组原位试验数据的饱和黄土循环抗力比（CRR）,通过与1976年唐山地震和1999年集集地震液化土CRR对比,得出了饱和黄土抗液化强度很低的结论。     

Lattice Boltzmann Models for Flow and Transport in Saturated Karst

Flow and transport simulation in karst aquifers remains a significant challenge for the ground water modeling community. Darcy's law–based models cannot simulate the inertial flows characteristic of many karst aquifers. Eddies in these flows can strongly affect solute transport. The simple two-region conduit/matrix paradigm is inadequate for many purposes because it considers only a capacitance rather than a physical domain. Relatively new lattice Boltzmann methods (LBMs) are capable of solving inertial flows and associated solute transport in geometrically complex domains involving karst conduits and heterogeneous matrix rock. LBMs for flow and transport in heterogeneous porous media, which are needed to make the models applicable to large-scale problems, are still under development. Here we explore aspects of these future LBMs, present simple examples illustrating some of the processes that can be simulated, and compare the results with available analytical solutions. Simulations are contrived to mimic simple capacitance-based two-region models involving conduit (mobile) and matrix (immobile) regions and are compared against the analytical solution. There is a high correlation between LBM simulations and the analytical solution for two different mobile region fractions. In more realistic conduit/matrix simulation, the breakthrough curve showed classic features and the two-region model fit slightly better than the advection-dispersion equation (ADE). An LBM-based anisotropic dispersion solver is applied to simulate breakthrough curves from a heterogeneous porous medium, which fit the ADE solution. Finally, breakthrough from a karst-like system consisting of a conduit with inertial regime flow in a heterogeneous aquifer is compared with the advection-dispersion and two-region analytical solutions.     

Source Screening Module for Contaminant Transport Analysis Through Vadose and Saturated Zones

At complex sites there may be many potential sources of contaminants within the vadose zone. Screening‐level analyses are useful to identify which potential source areas should be the focus of detailed investigation and analysis. A source screening module (SSM) has been developed to support preliminary evaluation of the threat posed by vadose zone waste sites on groundwater quality. This tool implements analytical solutions to simulate contaminant transport through the unsaturated and saturated zones to predict time‐varying concentrations at potential groundwater receptors. The SSM integrates several transport processes in a single simulation that is implemented within a user‐friendly, Microsoft Excel? ‐ based interface.     

A Phenomenological Model for Particle Retention in Single,Saturated Fractures

Fractured aquifers are some of the most poorly characterized subsurface environments despite posing one of the highest risks to the protection of potable groundwater. This research was designed to improve the understanding of the factors affecting particle transport through fractures by developing a phenomenological model based on laboratory‐scale transport data. The model presented in this research employed data from over 70 particle tracer tests conducted in single, saturated, variable‐aperture fractures that were obtained from the natural environment and fractured in the laboratory or cast from epoxy in the laboratory. The particles employed were Escherichia coli RS2‐GFP and microspheres. The tracer experiments were conducted in natural (dolomitic limestone and granite) as well as epoxy replicas of the natural fractures. The multiple linear regression analysis revealed that the most important factors influencing particle retention in fractures are the ratio of the ionic strength of solution to collector charge, the ratio of particle to collector charge, and the ratio of advective to diffusive forces as described by the Peclet number. The model was able to reasonably (R² = 0.64) predict the fraction of particles retained; however, it is evident that some factors not accounted for in the model also contributed to retention. This research presents a novel approach to understanding particle transport in fractures, and illustrates the relative importance of various factors affecting the transport mechanisms. The utility of this model lies in the increased understanding of particle transport in fractures, which is extremely useful for directing future research.     

West Nile Virus and Drought

West Nile virus was first reported in Uganda in 1937. WNV is a zoonosis, with "spill-over" to humans, which also poses significant risks for wildlife, zoo and domestic animal populations. While it is not known how West Nile virus (WNV) entered the New World in 1999, anomalous weather conditions may have helped amplify this Flavivirus that circulates among urban mosquitoes, birds and mammals. We analyzed weather patterns coincident with a series of U.S. urban outbreaks of St. Louis encephalitis (SLE), (a disease with a similar life cycle), and four recent large outbreaks of WNV. Drought emerged as a common feature. As the potential risks from pesticides for disease control must be weighed against the health risks of the disease, an early warning system of conditions conducive to amplification of the enzootic cycle could help initiate timely preventive measures, and potentially limit chemical interventions.     

Monitoring in the Vadose and Saturated Zones Utilizing Fluoroplastic

A ground water monitoring program should include an investigation of all possible areas of concern. To be completely effective, the program should include soil sampling, soil analysis and water-quality examination of both the saturated and unsaturated zones. A well-tooled drill rig can take all the proper soil samples, perform all necessary tests and install a functional monitoring well. With the introduction of the fluoropolymer (Teflon(r)) sleeve lysimeter, a single monitoring well can be constructed to monitor both the saturated and unsaturated zones in one installation. The monitoring well screen and casing may also be completely constructed of fluoropolymer.
The sleeve lysimeter is designed with a threaded hollow inner diameter, allowing it to be attached between the joints of a casing string. This hollow I.D. acts as an extension of the casing; the lysimeter surrounds the casing. This creates an isolated vessel for sampling the vadose zone. Access to the screened monitoring well below is unaffected. Tests have shown that when properly installed, these porous fluoropolymer filter units can collect samples with no interaction between the filter and collected fluids.     

碎石桩加固饱和粉土地基的抗液化特性

除饱和砂土液化外,饱和粉土地震液化问题也是岩土地震工程中一个重要的研究课题。饱和粉土地基的地震液化及变形可以采用多种地基加固方法防治,碎石桩技术是常用方法之一。碎石桩复合地基的抗液化效应,主要是增加桩周土体的密度、利于桩体的排水以及由桩体分担地震水平剪应力（桩体减震作用）。但由于粉土的土质特性,粉土-碎石桩复合地基的抗液化特性与砂土有着明显的差异。本文结合目前国内外碎石桩复合地基抗液化研究的最新进展,对粉土-碎石桩的密实、排水减压和减震作用做了较详细的评述,最后提出了关于碎石柱复合地基抗液化特性需要进一步研究的问题。     

关于孔隙相互孤立饱和土体弹性模量的讨论

对孔隙相互孤立饱和土体的弹性力学模型的部分观点进行了讨论。按Carroll的力学响应理论重新推导了此类土体的弹性本构关系和体积模量表达式。认为在准静态(很低频率)的波动问题中孔隙流体与固体骨架的相对运动为零,土体渗透系数为零或流体粘性为无穷大的条件是等价的。在此条件下没有必要区分孔隙是否连通,它们遵循相同的规律,而且其土体弹性体积模量的结果与Biot-Geertsma理论的无排水准静态的弹性体积模量相同。     

基于重塑饱和砂土模型的现场液化试验方法

基于现场开展土体液化问题研究势必成为今后土动力学中的一个重要发展方向。目前人工激振下的现场液化试验方法还不够成熟,尚需进一步探索和发展。本文从试验设备组成、场地地震动激励、试坑布置、饱和砂土模型制备、数据测量与采集等5个方面论述该方法中的主要技术问题。研究表明:动力加载系统激励产生的地震动在0~7m/s2;系统工作频率13~15Hz,饱和砂土模型与基础边缘的距离在0.5~2.5m范围内,更适合进行液化试验;应用水沉法现场制备饱和砂土模型,要重点注意试坑防水和尺寸定位的问题;数据测量与采集中要充分考虑对现场液化问题认识不够这一因素的影响,需对数据测量与采集提出附加要求;试验实例初步表明,该方法可行,适合开展液化问题研究。     

On the Relation Between Saturated Conductivity and Capillary Retention Characteristics

Abstract. A simple closed-form expression relating saturated hydraulic conductivity to the van Genuchten capillary retention model parameters is derived. Application of this equation to an experimental data set shows reasonable agreement between measured and predicted saturated conductivity values. The proposed equation provides a consistent theoretical basis for estimating both saturated and unsaturated hydraulic conductivity from statistical pore structure models.