考虑固结效应的溶质传输研究

传统的孔隙介质水动力学采用对流-扩散方程,研究溶质在流体中的迁移。在这个过程中,孔隙介质被认为是不变形的,因而是一个稳态问题。针对二维情况下孔隙介质变形对溶质传输的影响,给出了考虑孔隙介质固结效应的溶质传输方程,并且探讨了该类问题的求解方法。     

滨海地区分层含水层中陆源溶质运移过程

为更好地保护沿海水环境和水生态功能，亟需探究陆源溶质在滨海分层含水层中的迁移机制。构建二维分层含水层数值模型，采用地下水软件SUTRA-MS模拟分析潮汐作用下陆源溶质在分层含水层中的运移特征。结果表明：(1)与均质含水层相比，弱透水层的存在会延长溶质在含水层中的滞留时间、增大溶质最大扩散面积、削弱溶质在水平方向和垂直方向的扩散程度；(2)弱透水层能够降低陆源溶质向海水排放的最大通量，并且延长溶质排放的持续时间；(3)溶质扩散面积、弱透水层中溶质残余量、水平和垂向扩散程度等对弱透水层水力传导系数和厚度的敏感性优于弱透水层深度。     

二维分数阶对流-弥散方程的数值解

对二维时间分数阶对流-弥散方程和二维空间分数阶对流-弥散方程分别建立了差分格式,实现了对其的数值求解。针对理想算例进行计算求解,分析了时间和空间分数阶阶数取不同值时的扩散变化规律,验证了各自所描述的时间相关性与空间相关性。同时与传统的二维整数阶对流-弥散方程的求解结果作了对比。当时间和空间分数阶阶数α与γ分别取整数时,二维时间分数阶对流-弥散方程和二维空间分数阶对流-弥散方程都与传统二维整数阶对流-弥散方程的计算结果相同,说明提出的对二维分数阶对流-弥散方程的数值求解方法是可行的。其结果对地下水溶质运移的进一步研究提供了有效的手段。     

反应性溶质在不同质地饱和土柱中运移的数值模拟

盐渍土壤中的物理化学作用对溶质运移具有重要影响.吸附和离子交换作用是土壤中常见的反应.利用室内土柱出流实验对这两种作用下的单组分和多组分溶质运移进行了探讨,用CXTFIT软件模拟了只考虑对流-弥散的常规溶质运移;用水文地球化学模拟软件PHREEQC进行了耦合吸附和离子交换反应的模拟.结果表明,土壤质地对单组分溶质的运移具有重要影响,而在多组分溶质运移中,组分之间的相互作用对溶质运移具有更为重要的影响,并且耦合物理化学作用的模拟精度更高.     

死端孔隙对溶质运移影响的实验研究

拖尾分布是溶质运移规律研究的热点和难点。死端孔隙、透镜体等空间结构形成的非均质性以及密度变化产生的指状流会导致溶质运移拖尾分布。论文选择保守性溶质氯化钠作为示踪剂,采用砂柱实验系统地研究了以死端孔隙导致的非均质性以及由不同浓度和温度形成的变密度条件对溶质运移拖尾分布的影响。实验结果表明:死端孔隙的存在会延迟溶质运移穿透曲线出峰时间,降低峰值及出现拖尾分布,随着死端孔隙介质体积或数量的增加这一现象越明显;随着溶液浓度的增加拖尾现象越明显,且温度的升高会加剧这一现象。对比死端孔隙、温度和浓度三者对溶质运移拖尾分布的影响,其中影响最大的是死端孔隙,其次是浓度和温度。     

模拟岩溶含水系统演化的耦合网络模型

提出一个数学模型以研究碳酸盐岩含水层由其原始状态到成熟岩溶含水系统的发展过程。模型由３个模块组成：（ａ）二维连续空隙水渗流模块,表示裂隙系统（“分散流”系统）,即岩溶空管道之间水性差而贮水性强的那部分岩体;（ｂ）非连续管道网络模块,模拟冉溶道内发生的层流、紊流运动和溶质运移过程;（ｃ）碳酸盐岩溶蚀模块。管道－裂隙系统模型的模拟结果表明,模拟那种复杂的、具有多种地下水补给方式的岩溶含水盆地的演化是可     

地下水溶质迁移模拟研究进展

在地下水的相关研究中,农药和石油等地下水污染、土地盐碱化、海水入侵等诸多实际问题主要的研究方法都涉及地下水溶质迁移模拟. 相比地下水水流模拟的相对完善,对溶质迁移的模拟比较薄弱且迁移过程本身复杂性较高,目前地下水溶质迁移的研究工作还处在全面发展的阶段. 文中阐述了反映地下水溶质迁移机理和过程的数学模型,综述了溶质迁移模拟在地下水污染物防治、土地盐碱化、海水入侵、石油和放射性废物扩散等问题的诸多应用,归类了目前溶质迁移模拟所使用的对流迁移、对流-弥散模拟等主要数值方法,并对这些方法的优缺点和应用实例做了总结. 最后,分析了目前溶质迁移模拟中存在的不足,展望了未来在参数确定、裂隙介质运移机理和多相介质条件下运移模拟可能取得的突破.     

多组分离子交换吸附反应-运移的土柱试验及模拟

利用室内一维饱和垂直土柱对保守性溶质运移和反应性溶质运移分别进行了试验,并用水文地球化学模拟软件PHREEQC模拟了保守性溶质Cl-的运移以及K+、Ca2+、Na+、Mg2+4种主要阳离子的反应运移过程,分析了土壤中发生的离子交换吸附和盐分运移过程,进一步模拟分析了不同入流溶液种类、次序以及有无置换反应情况下出流溶液的差别。结果表明:这些条件的不同均会对出流溶液的种类和浓度产生明显影响,表明水文地球化学反应对地下水中污染物的迁移会产生很大影响,在相关领域的研究中不宜忽略。     

山东烟台夹河中、下游地区海水入侵三维水质数值模拟研究

建立了三维变密度对流弥散水质数学模型来研究山东省烟台夹河中、下游地区咸淡水界面的运移规律。以四面体为基本离散单元 ,推导出三维海水入侵变密度水质模型求解的数值方法 ,其中水流方程求解时运用了迦辽金有限单元法。溶质运移方程求解时运用了欧拉拉格朗日混合方法 ,将对流项与弥散项分离 ,用传统迦辽金有限元方法求解弥散项 ;采用自适应MOC MMOC法求解对流项 ,以消除人工过量和数值弥散。根据地下水的潮汐效应观测信息 ,确定了含水系统的海底延伸边界 ;利用该地区地下水水头及水质长观资料识别了模型的水文地质参数 ,探讨了夹河地区海水入侵的原因 :认为夹河下游地区滨海地带地下水过量开采是造成烟台地区海水入侵的主要原因。此外 ,海水随潮定期地倒灌进入夹河 ,通过局部岩性天窗侵入淡水含水层加剧了沿夹河河床两侧地下水的咸化。同时还预测了几种情况下地下水的水质演化趋势 ,为防止和减轻夹河地区海水入侵提供合理、科学的依据。     

潍河下游地区海咸水入侵动态三维数值模拟分析

文中以潍河下游的莱洲湾南岸地区为研究区 ,阐述了海咸水分布规律。针对海咸水运移过程中的水动力学特征和化学动力学特征 ,推导了水动力化学动力耦合的盐分运移对流弥散方程 ,水动力过程考虑了浓度变化对水流运动的影响 ,化学动力学过程考虑了含水介质表面吸附 (解吸 )作用对盐分运移的影响。采用改进的非线性特征有限元法 ,求解高浓度溶质运移三维模型。计算结果表明 ,该模型更真实地刻画了海咸水入侵系统。通过对海咸水入侵规律进行模拟 ,指出海咸水入侵的根本原因是过量开采地下淡水资源 ,要防止海咸水入侵 ,必须科学开采地下淡水资源。     

Three-dimensional benchmark for variable-density flow and transport simulation: matching semi-analytic stability modes for steady unstable convection in an inclined porous box

This benchmark for three-dimensional (3D) numerical simulators of variable-density groundwater flow and solute or energy transport consists of matching simulation results with the semi-analytical solution for the transition from one steady-state convective mode to another in a porous box. Previous experimental and analytical studies of natural convective flow in an inclined porous layer have shown that there are a variety of convective modes possible depending on system parameters, geometry and inclination. In particular, there is a well-defined transition from the helicoidal mode consisting of downslope longitudinal rolls superimposed upon an upslope unicellular roll to a mode consisting of purely an upslope unicellular roll. Three-dimensional benchmarks for variable-density simulators are currently (2009) lacking and comparison of simulation results with this transition locus provides an unambiguous means to test the ability of such simulators to represent steady-state unstable 3D variable-density physics.     

A general polynomial solution to convection–dispersion equation using boundary layer theory

A number of models have been established to simulate the behaviour of solute transport due to chemical pollution, both in croplands and groundwater systems. An approximate polynomial solution to convection–dispersion equation (CDE) based on boundary layer theory has been verified for the use to describe solute transport in semi-infinite systems such as soil column. However, previous studies have only proposed low order polynomial solutions such as parabolic and cubic polynomials. This paper presents a general polynomial boundary layer solution to CDE. Comparison with exact solution suggests the prediction accuracy of the boundary layer solution varies with the order of polynomial expression and soil transport parameters. The results show that prediction accuracy increases with increasing order up to parabolic or cubic polynomial function and with no distinct relationship between accuracy and order for higher order polynomials (\(n\geqslant 3\)). Comparison of two critical solute transport parameters (i.e., dispersion coefficient and retardation factor), estimated by the boundary layer solution and obtained by CXTFIT curve-fitting, shows a good agreement. The study shows that the general solution can determine the appropriate orders of polynomials for approximate CDE solutions that best describe solute concentration profiles and optimal solute transport parameters. Furthermore, the general polynomial solution to CDE provides a simple approach to solute transport problems, a criterion for choosing the right orders of polynomials for soils with different transport parameters. It is also a potential approach for estimating solute transport parameters of soils in the field.     

The potential for free and mixed convection in sedimentary basins

 Free thermal convection and mixed convection are considered as potential mechanisms for mass and heat transport in sedimentary basins. Mixed convection occurs when horizontal flows (forced convection) are superimposed on thermally driven flows. In cross section, mixed convection is characterized by convection cells that migrate laterally in the direction of forced convective flow. Two-dimensional finite-element simulations of variable-density groundwater flow and heat transport in a horizontal porous layer were performed to determine critical mean Rayleigh numbers for the onset of free convection, using both isothermal and semi-conductive boundaries. Additional simulations imposed a varying lateral fluid flux on the free-convection pattern. Results from these experiments indicate that forced convection becomes dominant, completely eliminating buoyancy-driven circulation, when the total forced-convection fluid flux exceeds the total flux possible due to free convection. Calculations of the thermal rock alteration index (RAI=q·∇T) delineate the patterns of potential diagenesis produced by fluid movement through temperature gradients. Free convection produces a distinct pattern of alternating positive and negative RAIs, whereas mixed convection produces a simpler layering of positive and negative values and in general less diagenetic alteration. Received, January 1999/Revised, June 1999/Accepted, July 1999     

土柱变密度溶质运移试验推求水平及垂直方向弥散度研究

基于溶质运移对流弥散理论的变密度海水入侵模型广泛用于海水入侵研究,而水动力弥散系数是影响模型模拟效果的关键性参量之一。利用传统土柱溶质运移试验结合旁侧抽水,采用数值反演法成功获取了水平及垂直两方向弥散度。相对于传统方法,在不增加试验复杂度的前提下,同时推求了不同方向的弥散度,提高了试验效率,节约了试验成本,可广泛用于测定水动力弥散系数等参数。     

基于FEFLOW的三维土壤-地下水耦合铬污染数值模拟研究

`土壤-地下水耦合数值模拟是定量刻画水流和溶质运移的主要手段。现有大范围场地尺度的研究受到数据采集难度及模拟计算量的限制,多是将土壤和地下水分成两个系统,这种方式不利于模型之间的计算反馈,易出现计算误差,因此将土壤和地下水作为整体系统研究具有重要意义。为精确刻画实际场地土壤-地下水系统中污染物迁移规律,揭示变饱和反应溶质迁移模型的参数敏感性,以某铬污染场地为研究对象,基于现场试验及前人研究所获数据,采用Galerkin有限元法建立三维土壤-地下水模型,定量描述六价铬在土壤-地下水中的迁移规律。在此基础上,通过改变补给条件,研究潜水面在土壤-地下水系统中的波动。并讨论阻滞系数和反应常数对溶质运移的影响。结果表明:在土壤中,污染物最大水平迁移距离为场地东南侧300 m;地下水中污染晕最大分布面积约为1.632 km2;垂向上土壤中的六价铬仅需15.6 h即可下渗至潜水面,第6天贯穿含水层。当潜水面随着补给量变化而波动时,地下水中六价铬会随水流进入土壤,影响土壤中污染分布。对溶质运移参数的讨论显示,当反应常数由0增大至10?6 s-1时,迁移出场区边界时地下水中污染物浓度约减少2000 mg/L,较难迁移至涟水河。基于FEFLOW的数值模型,能够解决各系统之间交互性差的问题,提供较为精确的模拟结果。     

Transverse Dispersion of a Kinetically Sorbing Solute

A recursion formulation for the transverse spreading of a solute is developed, and under conditions of steady flow in a stratified aquifer, the transport of a linearly sorbing solute undergoing nonequilibrium sorption is studied. The effect of spatial variability in the velocity field and the sorption kinetics are modeled to see the combined effect of the two processes on the spreading of the solute injected at a point in the aquifer. The main result of this work is a transport model based on a discrete formulation that includes local dispersion and leads to nonasymptotic behavior in the spreading of the plume in a direction normal to the mean flow velocity.     

The use of a mixed scheme: mixed hybrid finite elements method/finite volumes (MHFE/FV), for the modeling of contaminants transport in unsaturated porous mediums

This work is intended for the development of a numerical method to simulate flows and solute transport in multiphasical porous medium taking into consideration the interaction of solid/solute. More precisely, the studied problem is modeled by a coupled system composed of an elliptical equation (for the flow) and an equation convection–diffusion–reaction (for the transfer). Numerical simulations were realistic for two-dimensional problems confirming the stability and efficiency of the combined scheme in the characterization of a pollutant transport through an unsaturated zone of an industrial site.     

Classification and delineation of groundwater–lake interactions in the Nebraska Sand Hills (USA) using electrical resistivity patterns

Lake?Cgroundwater interactions exhibit a complex three-dimensional (3D) structure that is seldom studied. The utility of waterborne electrical resistivity (ER) surveys is explored for characterization of 3D groundwater flow and solute transport patterns for three lakes in the Nebraska Sand Hills, USA. Waterborne ER surveys, using contrasts between lake and groundwater solutes as natural tracers, are useful for inferring 3D patterns of groundwater flow and solute transport as well as classifying groundwater?Clake interactions. Three unique groundwater flow systems are interpreted under each lake from dense networks of two-dimensional (2D) waterborne ER surveys. A lateral transition from high to low ER values beneath the saline Wilson Lake expresses its flow-through regime, where groundwater salinity indicates changes from groundwater inflow to outflow. Alkali Lake ER profiles reveal a prevalent ER increase with depth over the lakebed area that is characteristic of groundwater discharge lakes. ER profiles beneath Gimlet Lake are the most resistive and indicate pockets of high ER related to fresh groundwater discharge into the lake, supporting a flow-through regime with a short flushing time. These ER patterns correctly classify groundwater?Clake interactions and provide high spatial resolution of mixing patterns for systems with varying water salinity.     

Free convective controls on sequestration of salts into low-permeability strata: insights from sand tank laboratory experiments and numerical modelling

Using sand tank experiments and numerical models, local-scale solute-transport processes associated with free convection in both the region surrounding as well as within discrete low-permeability strata are explored. Different permeability geometries and contrasts between high- and low-permeability regions are tested. Results show that two free convective processes occur at different spatial and temporal scales. In the high-permeability region, salinisation was rapid and occurred predominantly by free convective flow around the low-permeability blocks (interlayer convection). A free convection flow field also became concurrently established within the low-permeability lenses (intralayer convection). It was found that upward vertical flow created by the large-scale interlayer free-convective flow field in the high-permeability region retards salinisation of the lenses as buoyant freshwater displacements oppose the downward penetration of dissolved salts. Salinisation of the low-permeability structures eventually takes place from below as saltwater is dragged upwards. This bottom up convective salinisation process of low permeability strata has not been reported in previous literature. These results demonstrate that variable-density sequestration of solutes driven by a source resident above the low-permeability layer is a complicated function of the geometry of the permeability distribution and the permeability contrast between low- and high-permeability regions.     

Numerical model of contaminant transport through conduit-porous matrix system

A mathematical model for determining solute concentrations at a point within a cylindrically symmetrical conduit-porous matrix system is described. Both convection and diffusion are considered within the conduit subsystem, whereas only diffusion is considered within the porous matrix subsystem. The two subsystems are coupled through continuity conditions imposed at their interface boundary. The transport equations are solved utilizing an alternating-direction implicit-method technique. The solute concentration profiles resulting from this model are then sketched and discussed.