Modeling preferential water flow and solute transport in unsaturated soil using the active region model

Preferential flow and solute transport are common processes in the unsaturated soil, in which distributions of soil water content and solute concentrations are often characterized as fractal patterns. An active region model (ARM) was recently proposed to describe the preferential flow and transport patterns. In this study, ARM governing equations were derived to model the preferential soil water flow and solute transport processes. To evaluate the ARM equations, dye infiltration experiments were conducted, in which distributions of soil water content and Cl⁻ concentration were measured. Predicted results using the ARM and the mobile–immobile region model (MIM) were compared with the measured distributions of soil water content and Cl⁻ concentration. Although both the ARM and the MIM are two-region models, they are fundamentally different in terms of treatments of the flow region. The models were evaluated based on the modeling efficiency (ME). The MIM provided relatively poor prediction results of the preferential flow and transport with negative ME values or positive ME values less than 0.4. On the contrary, predicted distributions of soil water content and Cl⁻ concentration using the ARM agreed reasonably well with the experimental data, with ME values higher than 0.8. The results indicated that the ARM successfully captured the macroscopic behavior of preferential flow and solute transport in the unsaturated soil.     

封闭气泡对一维积水入渗影响的试验研究

非饱和土体中的部分气体极容易在入渗过程中被封闭在孔隙或裂隙中,形成互不连通的与外界隔绝的封闭气泡。封闭气泡的存在阻碍了水相的流动,使土体的渗透性能降低,从而对土体的入渗速率产生不可忽视的影响。在分析前人研究成果的基础上,结合室内一维积水入渗试验结果,初步探讨了封闭气泡的形成原因,及其对土体入渗性能的影响机制,对研究非饱和土中水气二相流规律有重要指导意义。试验结果表明：积水入渗时,土体中存在封闭气泡,封闭气泡的含量与土性、干密度和初始含水率等有关。干密度越大,封闭气泡占据孔隙空间越大,阻渗作用越明显,干密度相同时,黏性土中封闭气泡对入渗的阻碍作用较砂性土明显;初始含水率较小时,封闭气泡含量相对较大。由于封闭气泡的存在,入渗稳定时的传输区土体含水率只有土样饱和含水率的80 %左右。     

Modeling the 3D crack network and anisotropic permeability of saturated cracked soil

Cracks on a natural soil surface provide preferential pathways for water infiltration and contaminant solute transport. Before the mechanical property and permeability of cracked soil can be studied, a crack network model must first be established. Based on statistical analysis of crack geometry from field observations, a new method of representing a 3D crack network was developed. A horizontal plane of a crack network was derived using the Voronoi diagram. Each vertical crack was idealized as an inverted triangular prism. The 3D permeability tensor was determined by modeling the water flow through the crack network. A computer program was developed to generate a 3D crack network automatically and compute the permeability tensor of cracked soil at different depths. The model was verified by comparing the measured permeability and computed permeability of cracked soil. The results showed that the computed permeability was consistent with the measured permeability.     

非均质土壤饱和稳定流中盐分运移的传递函数模拟

对室内人工构造的两种非均质土柱,以传递函数模型作为模拟手段,研究了稳定流场中饱和非均质土壤盐分优先运移的随机特征。首先计算了模型参数μ和σ2,获得了氯离子在非均质土壤中迁移时间的概率密度函数,然后应用传递函数模型对土柱中氯离子的出流浓度动态进行了随机模拟,并对非均质土壤中氯离子的均值和中值迁移时间及相应的运移体积与可动体积进行了分析和讨论,还依据质量守衡原理获得了土壤溶液中氯离子平均驻留浓度的变化。     

多孔介质中非均匀流动特性的染色示踪试验研究

通过土壤染色剂进行的4组试验,对不同介质结构条件下的水流和溶质非均匀运动规律,非均匀流动变异信息分布特征关系以及全局性非均匀流动示踪方法进行了研究。结果表明,即使在相对比较均匀的介质条件下,流动也表现出明显的非均匀特性;对数正态分布能够较好的反映水流的运动分布模式,相比水流运动,溶质的运动和分布规律明显不同,表现出更多的不确定性和变异性。     

基于室内土柱穿透实验的优先流定量评价

土壤优先流对包气带水流和污染物运移的影响已引起广泛关注,但大多数研究还停留在定性描述上,缺乏对优先流定量评价的标准。基于室内土柱实验获取的穿透曲线,分析了饱和条件下土壤溶质优先运移的一般规律,采用解析法模拟软件CX-TFIT拟合了溶质优先运移的参数,同时结合时间矩方法计算了定量评价优先流的指标PFSP。研究结果表明:时间矩方法中的一阶标准矩μ′1、二阶中间矩μ2以及偏态系数S可以定量刻画穿透曲线的特征,从而指示优先流的程度;PFSP能很好地刻画优先流作用的大小。该研究对优先流的定量评价具有重要指导意义。     

土壤中优势流的几个基本问题研究

优势流是指土壤在整个入流边界上接受补给,但只通过少部分土体的快速运移,优势流是一种普遍存在的现象,而不是一种特例,它受许多因素的控制,如土壤中的大空隙,土壤结构,土壤质地,土壤水分含量,土壤初始水分含量,水和溶质的施加速率及溶质的施加方法等,优势流的产生机理主要有两种,一种是由土壤介质的非均质所驱动的优势流;另一种是湿润锋的不稳定性所驱动的优势流,目前优势流的监测方法主要取土壤原状土,实验室内的土     

Rapid runoff via shallow throughflow and deeper preferential flow in a boreal catchment underlain by frozen silt (Alaska, USA)

In high-latitude catchments where permafrost is present, runoff dynamics are complicated by seasonal active-layer thaw, which may cause a change in the dominant flowpaths as water increasingly contacts mineral soils of low hydraulic conductivity. A 2-year study, conducted in an upland catchment in Alaska (USA) underlain by frozen, well-sorted eolian silt, examined changes in infiltration and runoff with thaw. It was hypothesized that rapid runoff would be maintained by flow through shallow soils during the early summer and deeper preferential flow later in the summer. Seasonal changes in soil moisture, infiltration, and runoff magnitude, location, and chemistry suggest that transport is rapid, even when soils are thawed to their maximum extent. Between June and September, a shift occurred in the location of runoff, consistent with subsurface preferential flow in steep and wet areas. Uranium isotopes suggest that late summer runoff erodes permafrost, indicating that substantial rapid flow may occur along the frozen boundary. Together, throughflow and deep preferential flow may limit upland boreal catchment water and solute storage, and subsequently biogeochemical cycling on seasonal to annual timescales. Deep preferential flow may be important for stream incision, network drainage development, and the release of ancient carbon to ecosystems.     

孔隙尺度多孔介质流体流动与溶质运移高性能模拟

深入探究孔隙尺度下的流体流动特性和溶质运移规律对石油开采、农田养分管理、地下水污染修复有着重要意义。以人工构建的多孔介质结构和同步辐射X射线显微CT扫描的土壤团聚体(分辨率3.7μm)为研究对象,在空间节点数多达64 000 000的情况下,基于格子Boltzmann模型和GPU并行技术计算得到多孔介质流体运动和溶质运移过程的关键参数,并据此探究多孔介质空间异质性对水力学特性的影响。通过对3组不同结构的多孔介质比较发现,结构复杂程度最高的土壤样品和不规则堆叠的圆球结构的渗透率在100 mD(即10^-13m^2)量级,远低于规则堆叠的圆球结构(>20 000 mD);土壤的迂曲度为1.40~1.60,明显高于规则堆叠的圆球结构。研究结果表明,渗透率大的样品具有较小的迂曲度,这与结构的空间异质性有较强的关系;土壤的渗透率和迂曲度呈现各向异性;在水力梯度一定的前提下,渗透率较大的样品,纵向弥散系数也较大;同时,结构的异质性也会影响溶质的穿透曲线。本研究提出的模拟方法可在土壤结构中进行高效的水流运动和溶质运移模拟,可用于土壤多孔介质在孔隙尺度下的水力学特性研究。     

孔隙尺度多孔介质流体流动与溶质运移高性能模拟

深入探究孔隙尺度下的流体流动特性和溶质运移规律对石油开采、农田养分管理、地下水污染修复有着重要意义。以人工构建的多孔介质结构和同步辐射X射线显微CT扫描的土壤团聚体（分辨率3.7 μm）为研究对象，在空间节点数多达64 000 000的情况下，基于格子Boltzmann模型和GPU并行技术计算得到多孔介质流体运动和溶质运移过程的关键参数，并据此探究多孔介质空间异质性对水力学特性的影响。通过对3组不同结构的多孔介质比较发现，结构复杂程度最高的土壤样品和不规则堆叠的圆球结构的渗透率在100 mD（即10^-13m²）量级，远低于规则堆叠的圆球结构（>20 000 mD）；土壤的迂曲度为1.40~1.60，明显高于规则堆叠的圆球结构。研究结果表明，渗透率大的样品具有较小的迂曲度，这与结构的空间异质性有较强的关系；土壤的渗透率和迂曲度呈现各向异性；在水力梯度一定的前提下，渗透率较大的样品，纵向弥散系数也较大；同时，结构的异质性也会影响溶质的穿透曲线。本研究提出的模拟方法可在土壤结构中进行高效的水流运动和溶质运移模拟，可用于土壤多孔介质在孔隙尺度下的水力学特性研究。     

Solute transport as affected by surface land characteristics in gravely calcareous arid soils

Gravely calcareous soils cover approximately most of arid lands (in percent); however, the solute transport behavior in these soils remains a current issue. This research aimed at estimating and correlating the solute transport parameters in gravely calcareous soils as being affected by different land uses through the knowledge of the soil morphological, physical, and chemical properties. Four different land use sites were selected: irrigated trees and bare, range, and alluvial sediment lands. Solute transport parameters of soil pore water velocity (V), dispersion coefficient (D), and retardation factor (R) were estimated using bromide breakthrough curve tests for surface soil columns. In addition, field Brilliant Blue FCF dye tracing experiment was conducted to determine the maximum dimensional movements. Soil morphological analysis was able to explain the heterogeneity in the solute transport parameters. Conductive solute transport mechanism with V of 17.99 m/day was favored in a high continuous pore system observed under tree lands. Presence of high gravel and CaCO₃ contents under range lands increased pore system tortuosity and thus increased D magnitude up to 1,339.88 cm²/day. Existence of thin surface crusts at both bare soils and alluvial sediments had considerably restricted V down to 1.46 m/day. Dye staining technique aided the explanation of the existing variations by providing visual evidence on the preferential flow paths and patterns governing the solute transport mechanism at each site.     

裂隙网络管道模型弥散试验

为了探求不同裂隙几何参数对裂隙网络溶质运移的影响,基于离散裂隙网络思想和优势流、沟槽流理论,建立裂隙管道网络概念模型,搭建不同管径、不同连通方式的管道网络试验装置,进行渗流和溶质运移实验。运用应用广泛的模拟软件CHEMFLO-2000建立等效多孔介质模型,拟合不同几何参数下等效弥散度,定量刻画不同管道网络几何参数对溶质运移的影响,讨论了不同管径、连通方式等与等效弥散度之间的关系。通过进一步分析得出:在连通方式相同的情况下,不同管径的管道网络等效弥散度存在差异,但是差异不大;溶质在小管径中的穿透时间短于大管径,穿透曲线缓和程度相差不大;管道网络连通方式越复杂,其等效弥散度越大、对溶质运移的影响越大、穿透曲线越缓和、路径越长,等效弥散度越大;用这种等效弥散度的方法表征管道网络对溶质运移的影响,与多孔介质弥散度具有相似性;管道数目、管道面数目与等效弥散度成正相关关系,且等效弥散度随尺度的增大而增加。     

基于地中渗透仪的入渗补给方式分析

文章通过在某均衡试验场地中的渗透仪上开展土壤水流穿透试验,分析灌溉水对地下水补给方式。试验结果表明:淹灌条件下,粉细砂扰动土中的穿透曲线为典型的单峰对称型,表明粉细砂中的土壤水通过活塞式入渗补给地下水;而亚粘土中的穿透曲线显示多峰、优先穿透、拖尾等现象,表明亚粘土中的土壤水以优势流方式补给地下水。通过对均衡场降水入渗补给的长观资料分析发现:在自然降雨条件下亚粘土中优势流明显,粉细砂中主要以活塞流为主。     

长江三峡花岗岩林地坡面降雨渗流与水土流失关系研究

在大量的野外观测试验的基础上,根据土壤水分的形态和能态,对长江三峡花岗岩林地坡面降雨渗流机制进行了分析研究,研究发现在试验区当降雨强度大于表层土壤入渗率,即地表产生积水的情况下,长江三峡花岗岩林地坡面优先流在土壤含水量未达到最大毛管持水量时开始产流,早于土壤渗流产流的时间;在降雨强度小于表层土壤入渗率,即地表不产生积水的情况下,土壤渗流产流早于优先流产流;从壤中流产流过程来看,在地表不产生积水的条件下,土壤渗流先于优先流产生;在地表产生积水而土壤水分含量未达到最大毛管持水量的条件下,优先流先于土壤渗流产生。在两种情况下,优先流的峰值都先于土壤渗流。     

土石混合介质中非反应性阴离子运移试验研究

为探讨化学物质在土石混合介质中的运移过程和机理,采用饱和稳态流下的Cl-1混合置换试验,测定水流和溶质运移过程,分析土石混合介质的溶质穿透曲线特征及碎石组成和含量对运移过程的影响。选用CXTFIT2.1的平衡和物理非平衡对流弥散模型,对参数弥散系数D和滞留因子R进行反求。结果显示:不同土石比的D变异较大:0.258~22.31 cm2/h。R的波动范围为0.6~1.54;碎石含量影响土石介质的溶质运移过程表现为平均孔隙流速、弥散系数、弥散度均与土石比成负指数的幂函数关系。对碎石粒径与溶质运移参数进行相关分析发现,小粒径的碎石含量增加,则孔隙流速和弥散系数有减少的趋势,而大于10 mm的碎石有利于溶质的运移。通过土石介质的非反应性阴离子的混合置换试验研究,可以为非均一介质中化学物质运移提供参考。     

Translation of field tracer-test results into bounding predictions of matrix diffusion in the shallow subsurface at Idaho National Laboratory, USA

Matrix-diffusion parameters deduced from an infiltration tracer test at Idaho National Laboratory (INL), USA, are combined with other site information in an analysis involving two dimensionless lumped parameters to assess the effects of matrix diffusion on contaminant transport at the INL over longer distance and time scales than were evaluated in the test. Matrix diffusion was interrogated in the test by comparing, in three different observation wells, the breakthrough curves of two simultaneously injected nonsorbing solutes that have different diffusion coefficients. The matrix-diffusion parameters deduced from the different breakthrough curves were in good agreement, suggesting that the parameters may be broadly applicable at the INL. With this in mind, the uncertainties in the individual parameters that make up the two lumped parameters were estimated, and the resulting ranges of parameter values were used to assess matrix diffusion over larger scales. Assessments of the effects of flow transients, spatial heterogeneity in transport parameters, and sorption on solute transport in the shallow subsurface flow system were also conducted. The methods presented here should be generally applicable to other settings for making bounding assessments of the effects of matrix diffusion while honoring the information obtained from tracer tests and other supporting data.     

Effect of the addition of granitic powder to an acidic soil from Galicia (NW Spain) in comparison with lime

The impact of diffuse pollution, agricultural land use and climate change on the long-term response of subsurface–surface water quality is not well understood, but is a prerequisite for evaluation of water management options. The goal of this study is to model geochemical evolution of water chemistry from the infiltration through soil into the unsaturated zone, transport through bedrocks and granular aquifers to a river in order to identify zones of steep concentration gradients and high dynamics under transient flow conditions. A numerical model was constructed comprising a 2-D 1,500 m × 150 m vertical cross-section of typical sedimentary rock formations, a glacio-fluvial quaternary gravel aquifer in the valley and soil layers. The model coupled saturated/un-saturated flow and reactive transport under steady state and transient conditions. Geochemical interactions, include intra-aqueous kinetic reactions of oxygen with dissolved organic matter, as well as kinetics of carbonate dissolution/precipitation. This model section was chosen to provide insight in to the principal processes and time scales affecting water chemistry along different flow paths. The numerical simulator MIN3P was used, a finite volume program for variably saturated subsurface flow and multi-component reactive transport. The results show that subsurface water residence times range from approximately 2 to 2,000 years. Different zones are to be expected with respect to the development of mineral equilibria; namely, purely atmospherically influenced, as well as open and closed system carbonate dissolution. Short-term responses to daily averaged changes in precipitation, however, are only visible to some extent in the shallower and near-river parts of flow system and solute loads. This can most likely be explained by directional changes in flow paths, indicating that equilibrium geochemical condition predominate at the hillslope scale, i.e. water quality depends on transport pathways rather than on kinetic effects. The extent of reducing conditions is controlled by the presence of organic-rich layers (i.e. peat deposits), the dissolution kinetics of aquifer organic matter and the subsequent mixing with oxygenated water by hydrodynamic dispersion.     

Preferential water and solute transport through sandy soil containing artificial macropores

Macropores resulting from soil pedogenesis and biological activity play important roles in soil water and chemical transport. Numerous studies have examined individual macropores and the effects of their size on solute transport, but few have assessed the effects of macropore continuity and of neighboring macropores. This paper describes a laboratory investigation of the effects of macropores, with varying degrees and types of continuity, on the transport and distribution of solutes in a sandy soil from the northern Loess Plateau, China. Breakthrough curves were obtained from 60 cm tall, 2-D columns containing standardized artificial macropores using an input solution of 1,190 mg/L KBr and 100 mg/L FD&C Blue #1 under a constant hydraulic head of 8 cm. The types of macropore were: open at both the surface and bottom of the soil column (O–O); open at the surface, closed at the bottom (O–C); and closed at the surface, open at the bottom (C–O). Columns with no macropores served as a control. In the O–O columns the solution reached the bottom 10–50 times faster than in any other treatment, bypassing most of the soil matrix. The presence of an O–C macropore resulted in weak retardation and much deeper penetration of the bromide and FD&C Blue #1 solution than in the control columns. However, the C–O macropore had little effect on either breakthrough curves or solute distributions. In further experiments that considered neighboring macropores effects, an inclined macropore strongly affected solute concentrations in the profile around a nearby vertical macropore. It was concluded that the length, type and position of single macropores, and the presence of neighboring macropores, all affect soil water flow and solute infiltration parameters in a sandy loam soil.     

Influence of Hydraulic Conductivity and Wellbore Design in the Fate and Transport of Nitrate in Multi-aquifer Systems

Nitrate concentrations in multi-aquifer systems are heavily affected by the presence of wellbores (active or abandoned) that are screened in several aquifers. The spatial variability of hydraulic conductivity in the confining layers has also an important impact on the concentrations. A synthetic three-dimensional flow and transport exercise was carried in a multi-aquifer system consisting of two aquifers separated by an aquitard in which 100 vertical wellbores had been drilled. To model the wellbores and the flow and transport connection between aquifers that they may induce, we assign a high vertical hydraulic conductivity and a low effective porosity to the cell blocks including the wells. With these parameters, a solute will travel quickly from one aquifer to the other without being stored in the well itself. The wellbores will act as preferential pathways, and the solute will move quickly between aquifers according to the hydrodynamic conditions. Not considering these preferential pathways could induce erroneous interpretations of the solute distribution in an aquifer. We also noted that when there are vertical wellbores that connect aquifers in a multi-aquifer system, low conductivity in the aquitard enhances the flow of solute through the wellbores. Time-varying pumping rates induce important fluctuations in nitrate concentrations; therefore, any estimate of the water quality of the aquifer will depend on the moment when the data has been recorded. Consequently, concentration maps obtained by interpolation of point samples are seldom a good indicator of the chemical status of groundwater bodies; alternatively, we recommend complementing the usual interpolated maps with numerical models to gain a true understanding of the spatial distribution of the solute concentration.     

基于COMSOL Multiphysics的非饱和裂隙土降雨入渗特性研究

基于COMSOL Multiphysics软件对非饱和裂隙土降雨入渗特性进行数值模拟研究。通过将裂隙和基质分别离散成有限单元,建立了能充分模拟土中裂隙流、基质流以及裂隙-基质流量交换的离散裂隙-孔隙介质模型。结合"空气单元"的概念,对裂隙土的上边界进行模拟。该方法不仅能描述降雨初期雨水沿裂隙优先入渗的现象,还能描述当降雨量大于裂隙土入渗量时雨水沿地表流走的现象。通过对地表以下2 m深度内低渗含裂隙土体进行模拟,分析了裂隙的几何特征、基质的水力特性、前期水分条件以及降雨强度对非饱和裂隙土降雨入渗过程的影响。结果表明,在非饱和裂隙土中,存在两个主要的渗流过程:一是水沿裂隙优先流动;二是水不断从裂隙吸入基质中,基质吸收水的作用抑制了裂隙中优势流的发展。与裂隙的几何特征相比,基质的水力特性对非饱和裂隙土渗流的影响较大。增大基质的饱和渗透系数可能使由裂隙流主导的渗流过程转变为由基质流主导的渗流过程,而基质的非饱和特性与裂隙土的初始含水率改变了土体的储水能力,从而加速或延缓了降雨入渗至某一深度的时间。降雨强度对土体入渗速率和入渗量均有影响,当超过裂隙土的入渗能力时,多余积水沿地表流走,断面入渗率随...