Clumn Holdup Formula of Soil Solute Transport

The shortcomings of the present two formulae for describing column holdup are analyzed and deductions are ade to find a new formula,The column holdup,Hw,described by the new formula is dimensional,and related to soil soulte transport kinesis and column physical properties,Compared with the other two column holdups,Hw is feasible to describe dimensional column holdup during solute transport process,The relationships between Hw and retardation factor,R,in different solute transport boundary conditions are established.     

利用溶质锋信息估计溶质迁移参数方法分析

获取土壤溶质迁移参数是利用数学模型预测土壤溶质迁移过程的前提。本文根据对流弥散方程（Convective-dispersive equation，CDE）的一级近似解，建立了土壤溶质迁移过程中溶质锋随时间变化的函数关系，并以时域反射仪（Time domain reflectometry，TDR）为手段、Cl-为对象，通过与土壤溶质穿透曲线（Breakthrough curve，BTC）拟合法相比较，研究了利用该函数关系确定溶质迁移参数的可行性。研究结果表明，两种方法估计的R比较接近，而溶质锋信息法估计的水动力弥散系数D小于穿透曲线拟合法。TDR探测的溶质锋滞后于理论溶质锋，是导致利用溶质锋信息估计水动力弥散系数D偏小的原因之一。将溶质锋信息法估计的迁移参数代入CDE计算的土壤溶质穿透曲线与实测土壤溶质穿透曲线比较发现，风沙土中计算的穿透曲线整体滞后于实测穿透曲线，但两者的穿透过程基本一致，蝼土中计算的初始、完全穿透时间与试验穿透曲线一致，但穿透过程有所差异，说明溶质锋信息法估计的迁移参数具有一定的可应用性，在估计水动力弥散系数的精度方面有待提高。     

An analytical solution for advance of solute front in soils

Based on the assumption that solute transport in a semi-infinite soil column or in a field soil profile can be described by the boundary-layer method, an analytical solution is presented for the advance of a solute front with time. The traditional convection-dispersion equation (CDE) subjected to two boundary conditions: 1) at the soil surface (or inlet boundary) and 2) at the solute front, was solved using a Laplace transformation. A comparison of resident concentrations using a boundary-layer method and an exact solution (in a semi-infinite-domain) showed that both were in good agreement within the range between the two boundaries. This led to a new method for estimating solute transport parameters in soils, requiring only observation of advance of the solute front with time. This may be corroborated visually using a tracer solution with marking-dye or measured utilizing time domain reflectometry (TDR). This method is applicable to both laboratory soil columns and field soils. Thus, it could be a step forward for modeling solute transport in field soils and for better understanding of the transport processes in soils.     

SIMULATION MODEL OF WATER AND SOLUTE TRANSPORT IN TWO TYPES OF ROCKWOOL

The aim of this study is to present a two dimensional model for describing the movement of water and solute transport in two types of rockwool slabs – Floriculture, a high density type; and Expert, which has a low density. Water movement is described by the Richards equation, which is implicitly solved using the control volume finite element method. The governing convection dispersion equation describing the solute transport is explicitly solved. Numerical solutions are obtained for the distribution of water and also for the concentration of solute. The model is validated by comparing the results of water transport developed by this model with numerical results obtained by CFD that are validated by experimental results. A composite substrate is tested and validated, and is shown to give good conditions for development of plants.     

土壤溶质锋运移的解析解

为了推求土壤溶质锋运移与时间的关系 ,假设土壤溶质运移发生在溶质输入内边界至溶质锋之间 ,应用拉普拉斯变换方法求解输入内边界到溶质锋边界的对流—弥散方程 ( CDE)。溶质锋浓度解与半无限精确解的比较表明 ,在内边界至溶质锋边界内具有良好的一致性。溶质锋运移解的一个重要应用是估计实验室和田间条件的溶质运移参数。这个新的参数估计方法要求土壤中溶质锋随时间运移数据。如果应用有色示踪剂 ,溶质锋运移可以目测 ;如果应用其他示踪剂 ,可以通过 TDR或其它仪器测量示踪剂通量或体积浓度 ,确定溶质锋的深度。这个新的方法简单易行、节省时间 ,而且能够应用到实验和田间条件。     

土壤溶质运移参数估计图解方法

基于土壤中溶质运移的对流 弥散方程 (CDE)提出了溶质运移参数估计的图解方法。根据土柱溶质运移“穿透曲线”(BTC)数据计算通量浓度随时间变化 (dc/dt)。应用绘图软件 ,例如MicrosoftExcel,绘制出t1 5dc/dt和dc/dt相对于时间的变化曲线。这两条曲线都是单峰曲线 ,并被用于估计CDE中延迟因子(R)和弥散系数 (D0 )。设计算例和穿透实验数据被用于校正图解法估计参数的精度和稳定性。参数估计的稳定性由估计参数对它们平均值的标准差评价 ;参数的估计精度与CXTFIT法和等斜率法比较。结果表明图解法具有较高的精度和较好的稳定性 ,特别是对于运移参数R的估值。图解法是一个确定性方法 ,它具备确定性方法中估计参数唯一性特点 ,克服了统计方法中需要给定参数初值的缺点。因此 ,这个方法是土壤溶质运移参数估计的可选方法之一。     

A transfer-function method for analysing breakthrough data in the time domain of the transport process

A transfer‐function method is proposed to determine transport parameters from solute breakthrough data. The method is based on the assumptions that a linear process governs the transport of solute through soil and that the soil is homogeneous. It needs breakthrough data at two different vertical locations from a pulse input of solute to the soil. The method predicts the response by convoluting the input with the transfer function in the time domain. Solute breakthrough data were measured in unsaturated soil columns by time‐domain reflectometry (TDR). An experimental soil column was placed over a supporting column filled with sandy soil. A constant hanging water table, maintained in the lower column, created suction in the upper column and maintained unsaturated conditions. A solution of calcium chloride (CaCl₂) was spread over the soil in the upper column during steady flow of water in the column. Resident concentrations of solute in terms of electrical conductivity were measured at two depths by TDR sensors. We analysed breakthrough curves of CaCl₂ in 81 experiments to determine the transport parameters in coarse sand, sandy loam soil and clay loam soil by the transfer‐function method. The transport parameters obtained were compared with those determined by the widely used deterministic equilibrium model of the CXTFIT program. The transfer‐function method provided a better fit between the measured and estimated breakthrough curves in almost all cases and resulted in stable values of the parameters. The method is robust against small errors in measurements. It is a mathematically sound and efficient method for analysing breakthrough data.     

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2种:一种是由土壤介质的非均质性所驱动的优势流;另一种是由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有取土壤原状土、实验室内的土柱出流试验及染色示踪试验.尽管优势流的模拟非常困难,但人们仍提出了许多模型来描述它.今后优势流研究的重点应放在进行大量的综合性野外试验,以获取足够的数据资料来确切地刻划优势流;发展新的观测技术以便在时间和空间上监测优势流;建立新的模型来模拟优势流,从而进一步解决与土壤和地下水污染有关的环境问题.     

饱和非均质土壤中溶质大尺度运移的两区模型模拟

目前,用于模拟土壤中溶质运移过程的两区模型(TRM)的研究均集中在实验室的短土柱上,涉及的尺度较小。本研究分别应用两区模型(TRM)、对流-弥散方程(CDE)和分数微分对流-弥散方程(FADE)对1 250 cm长一维非均质土柱中NaCl的运移过程进行模拟,并分析了TRM模型参数的变化特征。结果表明:实验土柱中存在一定的不动水体,与CDE和FADE相比,TRM能更好地描述土柱中不同位置处溶质穿透曲线的提前穿透和拖尾特征,表明TRM对较大尺度条件下非均质土壤中溶质运移的模拟具有更高的精度;应用TRM研究长土柱中溶质的运移问题依然存在弥散系数的尺度效应问题,但TRM的弥散尺度效应小于CDE;TRM中的可动水体含量可以由土壤的有效孔隙率与总孔隙率的比值来确定;而质量交换系数则与对流时间(x/vm)之间存在幂函数的相关关系。     

二维虚拟土壤中溶质迁移行为的数值模拟研究

利用Voronoi图逐级碎裂方法,在二维正方网格上构造出不同等级的虚拟土壤来仿真具有丰富孔隙结构的真实土壤,并借助于随机行走模型,数值模拟了溶质粒子在该虚拟土壤中的迁移行为。结果表明,溶质粒子表现出反常扩散行为。对有偏倚的随机行走模型,其均方位移与时间呈正比关系∝tK,即扩散系数D=K-1,长时间的K值更大,溶质粒子扩散更快;首次穿越时间满足正态对数分布,说明溶质粒子迁移是一阶随机过程;由连续时间随机行走理论,发现溶质粒子扩散非费克现象明显。同时发现不同的土壤孔隙结构及随机行走类型所对应的拟合参数不同,即它们也影响溶质粒子的迁移行为。该研究有利于人们从理论上定性理解和预测水及溶质在有大孔隙土壤中的迁移,保护地下水环境。     

非反应性物质在土壤中的迁移及其参数确定

采用一维对流－水动力弥散方程研究了非反应性化学物质在土壤中的迁移行为。通过实验研究了土壤团聚体大小和孔隙水流速对非反应物质在土壤中迁移的影响。结果表明,对流－水动力弥散方程能较好地预报氚（＾３Ｈ２Ｏ）的穿透英线和迁移行为,随着团聚体由小变大,＾３Ｈ２Ｏ在其土柱中的穿透曲线的溶质出流提前,而淋洗结果推迟,峰值变小,最佳拟合Ｄ值由小变大。流速增加,Ｄ值增大,扩散现象加剧,Ｄ和流速呈正比。这为进一步研究     

土壤水流通量对饱水带溶质运移参数的影响

The transport processes of solutes in two soil columns filled with undistrubed soil material collected from an unsaturated sandy aquifer formation in Belgium subjected to a variable upper boundary condition were identified from breakthrough curves measured by means of time domain reflectometry(TDR),Solute breakthrough was measured with 3 TDR probes inserted into each soil column at three different depths at a 10 minutes time interval.In addition,soil water content and pressure head were measured at 3 different depths.Analyteical solute transport models were used to estimate the solute disperison coefficient and average pore-water velocity from the observed breakthrough curves,the results showed that the analytical solutions were suitable in fitting the observed solute transport,The dispersion coefficient was found to be a function of the soil depth and average proe-water velocity,imposed by the soil water flux.the mobile moistrue content on the other hand was not correlated with the average pore-water velocity and the dispersion coefficient.     

塿土水分入渗、再分布过程反应性溶质(NH_4~ )运移特征研究

利用土柱切割法研究（土娄）土水分入渗、再分布过程反应性溶质（NH4 ）运移特征。在水分入渗初期,土壤剖面溶质分布曲线为斜线,随入渗时间延长,土壤剖面溶质分布曲线上部逐渐出现垂直于横坐标的垂直线段。与水分入渗、再分布相比,溶质（NH4 ）入渗、再分布明显滞后;水分入渗深度增加,水溶质入渗距离比增加,溶质入渗滞后程度增大。入渗液浓度增大,溶质入渗滞后程度及溶质入渗阻滞因子减小,相应的土壤吸持溶质的反应速率常数增大,表观活化自由能减小。不同再分布时间的土壤剖面溶质分布曲线具有交点（溶质再分布等浓度点）,且该交点随入渗液浓度增大而加深。水分再分布过程中,溶质再分布等浓度点的土壤溶质吸持量基本不变,而其上部的土壤吸持量减小,下部的土壤吸持量增大。     

红壤中非吸附性离子水平运移特征试验研究

以南方第四纪红壤为研究对象.以KBr溶液为示踪剂,采用水平土柱入渗法研究不同土壤前期含水量和入渗液浓度条件下非吸附性离子溴在红壤中的水平运移特征.结果表明,浓度梯度和水势梯度分别在水平入渗的初期和后期阶段起主导作用;Br~-整体水平运移速率与入渗液浓度之间基本存在正相关关系,土壤前期含水量的不同,入渗液浓度对溴离子运移速率的影响程度各异;土壤前期含水量的提高促进了Br~-的运移,同时也揭示了当浓度梯度一定时,水势梯度的变化直接影响湿润锋及Br~-的运移.     

(土娄)土水分入渗、再分布过程反应性溶质(NH4+)运移特征研究

利用土柱切割法研究Lou土水分入渗、再分布过程反应性溶质（NH4^ )运移特征。在水分入渗初期，土壤剖面溶质分布曲线为斜线，随入渗时间延长，土壤剖面溶质分布曲线上部逐渐出现垂直于横坐标的垂直线段。与水分入渗、再分布相比，溶质（NH4^ )入渗、再分布明显滞后；水分入渗深度增加，水溶质入渗距离比增加，溶质入渗滞后程度增大。入渗液浓度增大，溶质入渗滞后程度及溶质入渗阻滞因子减小，相应的土壤吸持溶质的反应速率常数增大，表观活化自由能减小。不同再分布时间的土壤剖面溶质分布曲线具有交点（溶质再分布等浓度点），且该交点随入渗液浓度增大而加深。水分再分布过程中，溶质再分布等浓度点的土壤溶质吸持量基本不变，而其上部的土壤吸持量减小，下部的土壤吸持量增大。     

Simulation of water flow and solute transport in free-drainage lysimeters and field soils with heterogeneous structures

Lysimeters are valuable for studying the fate and transport of chemicals in soil. Large‐scale field lysimeters are used to assess pesticide behaviour and radionuclide transport, and are assumed to represent natural field conditions better than laboratory columns. Field lysimeters are usually characterized by a free‐draining lower boundary. As a result, the hydraulic gradient is disrupted, and leachate cannot be collected until the bottom of the lysimeter becomes saturated. We compared heterogeneously structured, free‐drainage lysimeters and field soils with respect to water flow and solute transport. Numerical simulations were carried out in a two‐dimensional heterogeneous sandy soil under unsaturated water flow conditions with the CHAIN_2D code. Three different soil structures (isotropic, horizontal, and vertical) were generated, and Miller–Miller similitude was used to scale the hydraulic properties of the soil. The results showed that ponding occurs at the bottom of the lysimeter for the three soil structures and that it occurred faster and was more pronounced with the vertical structure (preferential flow effect). Breakthrough curves of a conservative solute (bromide) showed that solutes are moving faster in the field than in the lysimeters. Fewer differences between lysimeters and field soils were found with the horizontal soil structure than with the isotropic and vertical structures.     

考虑尺度效应的瞬时输入溶质运移模型及解析解

为探索土壤环境中尺度效应对溶质运移的影响,建立了瞬时输入条件下考虑尺度效应的溶质运移模型。通过Laplace变换和复变函数理论得到了模型的解析解,并利用解析解分析了弥散尺度效应对溶质运移过程的影响。结果表明：随着土壤弥散尺度效应的增强,土壤中溶质浓度分布范围越广,浓度峰运移的距离越大,但浓度峰值越小;随着入口弥散系数（D0）的增加,溶质运移的范围更大,溶质浓度峰值越小,但浓度峰运移的距离几乎没有变化;随着入口孔隙水流速度（v0）的增加,溶质浓度峰运移的距离越大,溶质的运移锋面越远,而溶质浓度峰值及溶质浓度分布范围几乎没有变化。用一维8 m 长土柱中的溶质运移试验资料对所推解析解进行验证,模拟结果与试验结果吻合较好,决定系数可达0.95以上。结果表明所推得的解析解可用来模拟预测较大尺度上溶质运移过程,为土壤环境治理等实际工程提供理论依据。     

Solute transport in aggregated porous media: Comparing model independent and dependent parameter estimation

In many studies on solute transport soil column experiments arc used to obtain the transport characteristics for Convection-Dispersion-Models. Early breakthrough of the solute pulse has been attributed to a non-equilibrium in phase exchange. It is a standard procedure to determine several model parameter values from such breakthrough curves (BTC). This investigation is focused on the physical significance of simultaneously fitted parameter values used in the convection and diffusion-controlled mass transfer model (mobile — immobile phase concept). Saturated column experiments were conducted with solid phases consisting of porous and solid glass beads. One set of model parameter values was obtained from the breakthrough curves by simultaneous optimization and a second set was determined by independent measurements of individual parameter values. Both sets of parameter values described the BTCs equally well but deviated substantially from each other. These discrepancies were analysed in terms of local parameter sensitivities.     

土壤溶质迁移的指数函数模型

边界层方法是描述土壤溶质迁移的简单方法,通过边界层距离与时间的关系可以估计溶质迁移参数。基于边界层方法,研究了土壤溶质迁移的数学模拟及相应参数估计问题。假定土壤溶质浓度剖面为指数函数,得到了描述溶质浓度分布的指数函数模型。各参数对边界层距离的影响分析表明,应选取较小的孔隙水流速度、短历时推求土壤溶质迁移参数;对不同模型预测土壤溶质分布进行比较,结果表明,在短距离处指数型解与精确解的误差比其它都要小。误差分析表明了指数函数模型的有效性和实用性。