Non-equilibrium partitioning tracer transport in porous media: 2-D physical modelling and imaging using a partitioning fluorescent dye

This paper describes an investigation into non-equilibrium partitioning tracer transport and interaction with non-aqueous-phase liquid (NAPL) contaminated water-saturated porous media using a two-dimensional (2-D) physical modelling methodology. A fluorescent partitioning tracer is employed within a transparent porous model which when imaged by a CCD digital camera can provide full spatial tracer concentrations and tracer breakthrough curves. Quasi one-dimensional (1-D) benchmarking tests in models packed with various combinations of clean quartz sand and NAPL are described. These modelled residual NAPL saturations, S_n, of 0–15%. Results demonstrated that the fluorescent partitioning tracer was able to detect and quantify the presence of NAPL at low flow rates. At larger flow rates and/or higher NAPL saturations, the tracer increasingly underpredicted the NAPL volume as expected and this is attributed primarily to non-equilibrium partitioning. Despite little change in permeability, change in NAPL saturations from 4% to 8% resulted in significant NAPL saturation underestimates at the same flow rates implying coalescence of NAPL into wider separated but larger ganglia. A 2-D investigation of an idealised heterogeneous residual NAPL contaminated flow field indicated little permeability change in the NAPL contaminated zone and thus little flow bypassing, leading to reduced underpredictions of NAPL saturations than for equivalent quasi 1-D cases. This was attributed to increased ‘sampling’ of the NAPL by the tracer. The process is clearly visually identifiable from the experimental images. This rapid and relatively inexpensive experimental method is of value in laboratory studies of partitioning tracer behaviour in porous media; in particular, the ability to observe full field concentrations makes it valuable for the study of complex heterogeneous systems.     

土体固结与溶质运移三维耦合模型

考虑堆场上覆堆积体自重引起的土体固结与溶质运移的耦合效应已成为环境土工领域中的热点问题。在 Biot 固结理论和溶质运移理论相结合的基础上,以孔隙率作为耦合参量,将固结方程与运移方程相联系,考虑土体固结对溶质运移模型参数的影响,建立了溶质在固结变形土体中运移的三维耦合模型。采用多场耦合有限元软件 COMSOL Multiphysics 对模型进行了数值求解。数值模拟结果表明,土体固结变形对溶质运移过程具有较强和持久的阻滞作用,导致溶质运移深度以及污染羽范围的减小,在模拟运移20 a 时,运移距离减小率已高达27．75％,且随着时间的增加而逐渐增大;溶质在水平方向的迁移距离不可忽略,随着横向水力梯度的增大而增大。     

Predicting suitability of different scale-dependent dispersivities for reactive solute transport through stratified porous media

In this paper,the behavior of breakthrough curves(BTCs) for reactive solute transport through stratified porous media is investigated.A physical laboratory model for layered porous media was constructed,in which thin layer of gravel was sandwiched in between two thick layers of natural soil.Gravel layer and natural soil layers were hydraulically connected as single porous continuum.A constant source of tracer was connected through gravel layer and elucidated at different sampling points in the direction of flow.Flexible multiprocess non-equilibrium(MPNE) transport equation with scale-dependent dispersivity function was used to simulate experimental BTCs of reactive solute transport through layered porous media.The values of equilibrium sorption coefficient and other input parameters were obtained experimentally.The simulation of BTC was performed using MPNE model with scale-dependent dispersivity.The simulation of different scale-dependent dispersivities was then compared and it was found that for field scale of estimation of dispersivity,asymptotic and exponential dispersivity functions performed better.In continuation to the comparison of simulated BTCs obtained using different models,spatial moment analysis of each aforesaid scale-dependent dispersivity model was also done.Spatial moment analysis provides the information related to mean solute mass,rate of mass travel,and mean plume dispersion.Linear and constant dispersivities showed higher variance as compared to asymptotic and exponential dispersion functions.This supports the field applicability of asymptotic and exponential dispersivity functions.The BTCs were also found to elucidate a nonzero concentration with time,which was clearly affected by physical non-equilibrium.In natural condition,such information is required in effective aquifer remediation process.     

Colloid transport in porous media: impact of hyper-saline solutions

The transport of colloids suspended in natural saline solutions with a wide range of ionic strengths, up to that of Dead Sea brines (10^0.9 M) was explored. Migration of microspheres through saturated sand columns of different sizes was studied in laboratory experiments and simulated with mathematical models.Colloid transport was found to be related to the solution salinity as expected. The relative concentration of colloids at the columns outlet decreased (after 2-3 pore volumes) as the solution ionic strength increased until a critical value was reached (ionic strength > 10^−1.8 M) and then remained constant above this level of salinity.The colloids were found to be mobile even in the extremely saline brines of the Dead Sea. At such high ionic strength no energetic barrier to colloid attachment was presumed to exist and colloid deposition was expected to be a favorable process. However, even at these salinity levels, colloid attachment was not complete and the transport of ∼30% of the colloids through the 30-cm long columns was detected.To further explore the deposition of colloids on sand surfaces in Dead Sea brines, transport was studied using 7-cm long columns through which hundreds of pore volumes were introduced. The resulting breakthrough curves exhibited a bimodal shape whereby the relative concentration (C/C₀) of colloids at the outlet rose to a value of 0.8, and it remained relatively constant (for the ∼18 pore volumes during which the colloid suspension was flushed through the column) and then the relative concentration increased to a value of one. The bimodal nature of the breakthrough suggests different rates of colloid attachment. Colloid transport processes were successfully modeled using the limited entrapment model, which assumes that the colloid attachment rate is dependent on the concentration of the attached colloids. Application of this model provided confirmation of the colloid aggregation and their accelerated attachment during transport through soil in high salinity solution.     

Fluorescent dye labeled bacteriophages--a new tracer for the investigation of viral transport in porous media: 1. Introduction and characterization

A new method for the study of pathogen transport in porous media is presented. The method is based on conjugation of fluorescent dyes to target bacteriophages and application of the modified bacteriophages for tracer studies. We demonstrate that the relevant transport determining properties of Rhodamine and several fluorescein-labeled phages are practically identical to those of the native bacteriophages. The advantages of the proposed method relative to direct enumeration of bacteriophages by plaque forming unit method, turbidity, fluorescent microspheres, and other alternative tracers are discussed. Notable advantages include simple quantitation by optical methods, unbiased signals even when virus aggregates are formed, and the ability to decouple inactivation kinetics from transport phenomena. Additionally, the signal reflects the removal and transport of the studied microorganism and not a surrogate.     

Fluorescent dye labeled bacteriophages--a new tracer for the investigation of viral transport in porous media: 2. Studies of deep-bed filtration

Viral transport in deep-bed sand filters was studied by a new method that enables rapid and simple quantitation of labeled viruses. The residence time distribution (RTD) of viruses in the bed was compared to the RTD of a fluorescein dye under conditions that simulate a filter run. The characteristics of the RTD curves for the free dye and the labeled bacteriophages followed very different trends during the filter run. While the retention time of free dye was practically independent of the filtration stage, the average retention time of the labeled bacteriophage depended in a non-linear way on filtration time. Average virus retention time as well as virus-removal efficiency were minimal at the ripening stage, increased during the operational stage and then decreased again towards the turbidity breakthrough stage. This complex trend reflects two opposing mechanisms that dominate the behavior of the filter. During the ripening stage the accumulation of the kaolin-alum material in the filter increases the adsorption surface area and retards virus mobility. After sufficient kaolin-alum deposit is accumulated in the filter, aging and densification of the alum deposit induces size exclusion phenomenon giving faster apparent mobility of viruses in the filter bed.     

Reactive and non-reactive transport modelling in partially water saturated argillaceous porous media around the ventilation experiment, Mont Terri

At Mont Terri Underground Research Laboratory (URL) Switzerland, a specific experiment has been performed in a tunnel, in order to investigate the hydro-mechano-chemical (HMC) perturbations induced in the argillaceous formation by forced ventilation. This experiment has been selected in the international project DECOVALEX to be used for process model development and validation. The numerical simulation of the geochemical response to the ventilation experiment (VE) is the object of the present paper, focusing on the transport of chloride as a conservative species and sulphate as a reactive species. Utilising the validated hydro-mechanical (HM) results from earlier steps of the DECOVALEX task, reactive and non-reactive transport models, incorporating the current understanding of the geochemistry at the site, were successfully constructed for the whole experimental period. The associated parametric and process uncertainty analyses clearly demonstrate that the basic HM understanding must be sound. However, to demonstrate this degree of robustness, the explicit inclusion of process representations of water desaturation, liquid vaporisation, species exclusion porosity, and redox processes, is required.     

重力对饱和多孔介质中颗粒输运特性的影响

利用室内土柱试验研究重力对饱和多孔介质中颗粒输运特性的影响,包括渗流方向自上向下和自下向上两种不同的试验系列,每个系列中进行了5种不同渗流速度的试验。研究结果表明,渗流方向自上向下时的流出液中颗粒的浓度峰值要大于自下向上时的浓度峰值,并且前者的浓度峰值出现的也快。同一渗流方向条件下,渗流速度越大,流出液中颗粒的浓度峰值越大。其次,渗流自上向下时,随着渗流速度的增加,弥散度的值随之减小,但在渗流自下向上时,渗流速度的变化对弥散度的影响很小。另外,同一渗流方向中,随渗流速度增大,沉积系数逐渐减小,回收率逐渐增大,并且沉积系数在渗流自下向上时较渗流自上向下时的要大。可见,重力和渗流速度是影响饱和多孔介质中颗粒输运的重要因素,渗流速度越小,重力的作用越明显。     

考虑加速效应的多孔介质中颗粒三维迁移模型研究

基于已有的颗粒一维迁移模型,建立一个考虑加速效应的颗粒三维迁移模型。通过Laplace变换和Fourier变换,给出点源和面源形式下的颗粒瞬时注入和周期形式注入问题的解析表达式,分析了点源瞬时注入情况下时间、距离、沉积系数、弥散系数等的影响机理。研究结果表明：随着时间增大,迁移颗粒浓度峰值逐渐减小,并且浓度峰值所对应的x坐标值逐渐增大。其次,浓度等值线在x-y平面上呈椭圆形状,在x方向上靠近颗粒注入口的等值线排列较密,远离注入口的等值线排列较疏。随着时间增大,低浓度等值线的范围逐渐向四周扩大,高浓度的等值线的范围逐渐缩小。另外,沉积系数越大,浓度等值线的范围越小。然而,随着x方向的弥散系数增大,等值线在x方向上逐渐向两侧拉长,而等值线在y方向上的范围逐渐缩小。     

Mass transfer model of nanoparticle-facilitated contaminant transport in saturated porous media

A one-dimensional model has been evaluated for transport of hydrophobic contaminants, such as polycyclic aromatic hydrocarbon (PAH) compounds, facilitated by synthetic amphiphilic polyurethane (APU) nanoparticles in porous media. APU particles synthesized from poly(ethylene glycol)-modified urethane acrylate (PMUA) precursor chains have been shown to enhance the desorption rate and mobility of phenanthrene (PHEN) in soil. A reversible process governed by attachment and detachment rates was considered to describe the PMUA binding in soil in addition to PMUA transport through advection and dispersion. Ultimately, an irreversible second-order PMUA attachment rate in which the fractional soil saturation capacity with PMUA was a rate control was found to be adequate to describe the retention of PMUA particles. A γ-distributed site model (GS) was used to describe the spectrum of physical/chemical constraints for PHEN transfer from solid to aqueous phases. Instantaneous equilibrium was assumed for PMUA-PHEN interactions. The coupled model for PMUA and PHEN behavior successfully described the enhanced elution profile of PHEN by PMUA. Sensitivity analysis was performed to analyze the significance of model parameters on model predictions. The adjustable parameter α in the γ-distribution shapes the contaminant desorption distribution profile as well as elution and breakthrough curves. Model simulations show the use of PMUA can be also expected to improve the release rate of PHEN in soils with higher organic carbon content. The percentage removal of PHEN mass over time is shown to be influenced by the concentration of PMUA added and this information can be used to optimize cost and time require to accomplish a desired remediation goal.     

Degradation of azo dye by an UV/H2O2 advanced oxidation process using an amalgam lamp

The discharge of dyes into water is an ecological problem that can be alleviated by advanced oxidation processes (AOPs), such as UV/H₂O₂ treatments. Searching for more efficient light sources is a way to improve AOPs’ efficiency. This work tested the efficiency of an amalgam lamp on the degradation of an azo dye, studying the effect of dye and H₂O₂ concentrations and pH, and the influence of some salts on the decolouration rate of methyl orange. Actinometry showed that the amalgam lamp system was able to provide a high incident photon irradiance (6.30·10^?5 mol/cm² s). The amalgam lamp‐driven AOP was able to decolourize the dye at pseudo‐first‐order rates of 0.654–4.008 1/min, with increasing rates at low dye concentration and low pH and at high H₂O₂ concentrations until a maximum value is reached. The results show that the amalgam lamp can be an alternative light source for fast dye degradation by AOPs.     

Characteristics of two types of stabilized nano zero-valent iron and transport in porous media

Nano-scale zero-valent iron (NZVI) has been shown to be suitable for remediating contaminated aquifers. However, they usually aggregate rapidly and result in a very limited migration distance that inhibits their usefulness. This study employed poly acrylic acid (PAA) and carboxymethyl cellulose (CMC) to synthesize two types of stabilized styles of NZVI with finer sizes (namely PNZVI and CNZVI). The mobility of stabilized NZVI was also demonstrated on the basis of transport in porous media. The results show that the PNZVI has a uniform particle size of 12 nm. However, tens of CNZVI particles with diameters of 1-3 nm were packed into secondary particles. Both the PNZVI and the CNZVI exhibited amorphous structures, and the stabilizer was bound to particle surfaces in the form of bidentate bridging via the carboxylic group, which could provide both electrostatic and steric repulsion to prevent particle aggregation. This study also proposes presumed stabilized configurations of PNZVI and CNZVI to reasonably illustrate their different dispersed suspension types. On the basis of the breakthrough curves and mass recovery, this study observed that the mobility of PNZVI in classic Ca²⁺ concentration of groundwater was superior to CNZVI. Nonetheless, the mobility of CNZVI would be decreased less significantly than PNZVI when encountering high Ca²⁺ concentrations (40 mM). Presumably, increasing the pore flow velocity would enhance the mobility of stabilized NZVI. Overall, the results of this study indicate that PNZVI has the potential to become an effective reactive material for in situ groundwater remediation.     

The role of starvation on Escherichia coli adhesion and transport in saturated porous media

The influence of bacterial starvation on cell transport has been examined using two Escherichia coli isolates: one from human (HU) and one from dairy cattle (DC). To better understand the fate of starved bacteria, experiments were conducted in a packed bed column using cells that had been incubating at room temperature without nutrients for 6, 12, and 18h, as well as cells, which had not been starved (referred to as time zero). Complimentary cell characterization techniques were conducted to evaluate the hydrophobicity, mobility, size, and surface charge density of the cells at the conditions considered. It was observed that non-starved HU cells were more adhesive than starved HU cells. This behavior is attributed to the relatively high hydrophobicity of the starved cells, which resulted from greater extracellular polymeric substance (EPS) presence. Non-starved DC cells were also the most adhesive whereas 18h starved cells were the least adhesive, although cell characterization results did not correlate to transport trends like HU cells. For both isolates, the cells after 6h of starvation showed high levels of sugar relative to protein in the EPS. Additionally, following 6h of starvation, the cells did not follow expected transport trends as anticipated from the cellular characterization. Our results suggest transport behavior of environmental E. coli isolates differs in terms of isolate host and starvation conditions. Possible mechanisms responsible for this are changes in key cell surface characteristics and synthesis of starvation-induced proteins. This study highlights the importance of consistency in bacterial preparation for experimental studies and has considerable implications for future evaluation and prediction of E. coli fate in subsurface environments.     

Interactions between laponite and microbial biofilms in porous media: implications for colloid transport and biofilm stability

Quartz sand columns and sand-filled microscope flow cells were used to investigate the transport characteristics of the clay colloid laponite, and a biofilm-forming bacterium, Pseudomonas aeruginosa SG81. Separate experiments were performed with each particle to determine their individual transport characteristics in clean sand columns. In a second set of experiments, bacterial biofilms were formed prior to introduction of the clay colloids. In the independent transport experiments, bacteria and laponite each conformed to known physicochemical principles. A sodium chloride concentration of 7 x 10(-2) M caused complete retention of the laponite within the sand columns. P. aeruginosa SG81 was generally less influenced by ionic strength effects; it showed relatively low mobility at all ionic strengths tested and some (albeit reduced) mobility when introduced to the columns in 1M NaCl, the highest concentration tested, but nevertheless showed reproducible trends. Under conditions favourable to laponite retention and biofilm stability (7 x 10(-2) MNaCl), laponite suspensions were able to remobilise a portion of the attached bacterial biomass. At low ionic strength, the profile of laponite elution was also altered in the presence of a P. aeruginosa biofilm. These observations suggest that while a reduction in ionic strength has a dominant influence on the mobilisation of biological and inorganic colloids, the presence of laponite and biomass can have a distinct influence on the mobility of both types of colloids. Since these events are likely to occur in subsurface environments, our results suggest that colloid-biofilm interactions will have implications for colloid-bound contaminant transport and the remobilisation of pathogens.     

Influence of tetracycline resistance on the transport of manure-derived Escherichia coli in saturated porous media

In this research, tetracycline resistant (tet^R) and tetracycline susceptible (tet^S) Escherichia coli isolates were retrieved from dairy manure and the influence of tetracycline resistance on the transport of E. coli in saturated porous media was investigated through laboratory column transport experiments. Experimental results showed that tet^RE. coli strains had higher mobility than the tet^S strains in saturated porous media. Measurements of cell surface properties suggested that tet^RE. coli strains exhibited lower zeta potentials than the tet^S strains. Because the surface of clean quartz sands is negatively charged, the repulsive electrostatic double layer (EDL) interaction between the tet^R cells and the surface of sands was stronger and thus facilitated the transport of the tet^R cells. Although no difference was observed in surface acidity, cell size, lipopolysaccharides (LPS) sugar content and cell-bound protein levels between the tet^R and tet^S strains, they displayed distinct outer membrane protein (OMP) profiles. It was likely that the difference in OMPs, some potentially related to drug efflux pumps, between the tet^R and tet^S strains led to alteration in cell surface properties which in turn affected cell transport in saturated porous media. Findings from this research suggested that manure-derived tet^RE. coli could spread more widely in the groundwater system and pose serious public health risks.     

层状饱和介质中瑞利波传播特性薄层分析方法

层状饱和半无限体中瑞利波传播特性研究对工程物理勘探及土性参数反演很重要,传统的层传递矩阵或刚度矩阵分析方法需在复数域搜索瑞利波频率方程根,不仅耗时,收敛性也差。根据研究波长范围,在底部半无限体某一深度设置刚性边界,将层状介质离散成薄层,建立研究平面应变状态下层状饱和介质中瑞利波传播特性薄层法分析模型。依据该模型,频率方程根搜索问题可转换成矩阵特征值问题。根据瑞利波沿深度衰减特性,从一组计算的特征值中筛选出与瑞利波模态对应特征值,由特征值及相应特征向量得到各模态频率特性以及孔隙压力、骨架位移随深度变化。通过与饱和半无限体瑞利波解析解比较,验证该模型可行性,由此研究几种层状介质中瑞利波各阶模态的频率特性。分析方法对开展动力本构模型更复杂的层状介质中瑞利波传播特性研究也具有指导意义。     

Experimental modelling of buoyancy-driven flows in buildings using a fine-bubble technique

A new technique has been developed for demonstration and experimental modelling of buoyancy-driven ventilation airflows in buildings by using electrolytically generated fine hydrogen bubbles. Experiments for displacement natural ventilation in a single-zone building induced by two types of buoyancy sources, i.e. a point source and a line source, showed that the ventilation and stratification phenomena are successfully modelled by the fine-bubble technique. The experimental results for stratification interfacial positions are in good agreement with both the experimental data and theoretical predictions available in the literature. An analysis has also been carried out on similarities between the natural ventilation flows due to temperature difference and that due to concentration difference.     

Aggregation and transport of nano-TiO2 in saturated porous media: effects of pH, surfactants and flow velocity

Transport of manufactured nano-TiO₂ in saturated porous media was investigated as a function of morphology characteristics, pH of solutions, flow velocity, and the presence of anionic and non-ionic surfactants in different concentrations. Surfactants enhanced the transport of nano-TiO₂ in saturated porous media while a pH approaching the point of zero charge of nano-TiO₂ limited their transport. The deposition process, a retention mechanism of nano-TiO₂ in saturated porous media was impacted by surfactant and pH. In Dispersion 1 systems (pH 7), the size of the nano-TiO₂ aggregates was directly related to the presence of surfactants. The presence of non-ionic surfactant (Triton X-100) induced a size reduction of nano-TiO₂ aggregates that was dependent on the critical micelle concentration. In Dispersion 2 systems (pH 9), the stability provided by the pH had a significant effect on the size of nano-TiO₂ aggregates; the addition of surfactants did impact the size of the nano-TiO₂ aggregates but in less significance as compared to Dispersion 1 systems. The electrostatic and steric repulsion forces in connection with the size of nano-TiO₂ aggregates and flow velocity impacted the single-collector efficiency and attachment efficiency which dictated the maximum transport distance of nano-TiO₂ for the Dispersion 1 and Dispersion 2 systems. By doubling the flow velocity at pH 9, the No Surfactant, 50% CMC Triton X-100, 100% CMC Triton X-100 and 100% CMC SDBS dispersion systems allowed nano-TiO₂ to attain maximum transport distances of 0.898, 2.17, 2.29 and 1.12 m, respectively. Secondary energy minima played a critical role in the deposition mechanisms of nano-TiO₂. Nano-TiO₂ deposited in the secondary energy wells may be released because of changes in solution chemistry. The deposition of nano-TiO₂ in primary and secondary energy minima, the reversibility of their deposition should be characterized to analyze the transport of nanoparticles in porous media. This is necessary to assess the risk of nanoparticles to the environment and public health.     

The effects of biofilm on the transport of stabilized zerovalent iron nanoparticles in saturated porous media

The transport of stabilized zerovalent iron nanoparticles (nZVI) has recently been the topic of extensive research due to its proven potential as an in situ remediation tool. However, these studies have ignored the effects of biofilms—complex aggregations of bacterial cells and excreted extracellular polymeric substances present in nearly all aquatic systems—on the transport of these particles. This study examines the effects of Pseudomonas aeruginosa (PAO1) biofilm, at a cell concentration similar to that reported for saturated aquifers, on the transport of commercially available, poly (acrylic acid) stabilized nZVI (pnZVI) in 14 cm long columns packed with saturated glass beads at salt concentrations of 1 and 25 mM NaCl. Compared to retention on uncoated columns, in the presence of biofilm the retention of pnZVI increased at higher ionic strength, while ionic strength played no role in retention of these nanoparticles in the absence of biofilm. The Tufenkji-Elimelech correlation equation predicts lower retention of pnZVI on biofilm coated columns compared to uncoated columns due to a lower Hamaker constant, and DLVO energy considerations predict the most favorable attachment to uncoated porous media at higher ionic strength. A steric (polymer-mediated) model that considers the combined influence of steric effects of polymers and DLVO interactions is shown to adequately describe particle retention in columns.