A novel two-dimensional model for colloid transport in physically and geochemically heterogeneous porous media

A two-dimensional model for colloid transport in geochemically and physically heterogeneous porous media is presented. The model considers patchwise geochemical heterogeneity, which is suitable to describe the chemical variability of many surficial aquifers with ferric oxyhydroxide-coated porous matrix, as well as spatial variability of hydraulic conductivity, which results in heterogeneous flow field. The model is comprised of a transient fluid flow equation, a transient colloid transport equation, and an equation for the dynamics of colloid deposition and release. Numerical simulations were carried out with the model to investigate the colloid transport behavior in layered and randomly heterogeneous porous media. Results demonstrate that physical and geochemical heterogeneities markedly affect the colloid transport behavior. Layered physical or geochemical heterogeneity can result in distinct preferential flow paths of colloidal particles. Furthermore, the combined effect of layered physical and geochemical heterogeneity may result in enhanced or reduced preferential flow of colloids. Random distribution of physical heterogeneity (hydraulic conductivity) results in a random flow field and an irregularly distributed colloid concentration profile in the porous medium. Contrary to random physical heterogeneity, the effect of random patchwise geochemical heterogeneity on colloid transport behavior is not significant. It is mostly the mean value of geochemical heterogeneity rather than its distribution that governs the colloid transport behavior.     

Colloid transport in unsaturated porous media: the role of water content and ionic strength on particle straining

Packed column and mathematical modeling studies were conducted to explore the influence of water saturation, pore-water ionic strength, and grain size on the transport of latex microspheres (1.1 microm) in porous media. Experiments were carried out under chemically unfavorable conditions for colloid attachment to both solid-water interfaces (SWI) and air-water interfaces (AWI) using negatively charged and hydrophilic colloids and modifying the solution chemistry with a bicarbonate buffer to pH 10. Interaction energy calculations and complementary batch experiments were conducted and demonstrated that partitioning of colloids to the SWI and AWI was insignificant across the range of the ionic strengths considered. The breakthrough curve and final deposition profile were measured in each experiment indicating colloid retention was highly dependent on the suspension ionic strength, water content, and sand grain size. In contrast to conventional filtration theory, most colloids were found deposited close to the column inlet, and hyper-exponential deposition profiles were observed. A mathematical model, accounting for time- and depth-dependent straining, produced a reasonably good fit for both the breakthrough curves and final deposition profiles. Experimental and modeling results suggest that straining--the retention of colloids in low velocity regions of porous media such as grain junctions--was the primary mechanism of colloid retention under both saturated and unsaturated conditions. The extent of stagnant regions of flow within the pore structure is enhanced with decreasing water content, leading to a greater amount of retention. Ionic strength also contributes to straining, because the number of colloids that are held in the secondary energy minimum increases with ionic strength. These weakly associated colloids are prone to be translated to stagnation regions formed at grain-grain junctions, the solid-water-air triple point, and dead-end pores and then becoming trapped.     

Transport and deposition of functionalized CdTe nanoparticles in saturated porous media

Comprehensive understanding of the transport and deposition of engineered nanoparticles (NPs) in subsurface is required to assess their potential negative impact on the environment. We studied the deposition behavior of functionalized quantum dot (QD) NPs (CdTe) in different types of sands (Accusand, ultrapure quartz, and iron-coated sand) at various solution ionic strengths (IS). The observed transport behavior in ultrapure quartz and iron-coated sand was consistent with conventional colloid deposition theories. However, our results from the Accusand column showed that deposition was minimal at the lowest IS (1mM) and increased significantly as the IS increased. The effluent breakthrough occurred with a delay, followed by a rapid rise to the maximum normalized concentration of unity. Negligible deposition in the column packed with ultrapure quartz sand (100mM) and Accusand (1mM) rules out the effect of straining and suggests the importance of surface charge heterogeneity in QD deposition in Accusand at higher IS. Data analyses further show that only a small fraction of sand surface area contributed in QD deposition even at the highest IS (100mM) tested. The observed delay in breakthrough curves of QDs was attributed to the fast diffusive mass transfer rate of QDs from bulk solution to the sand surface and QD mass transfer on the solid phase. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis were used to examine the morphology and elemental composition of sand grains. It was observed that there were regions on the sand covered with layers of clay particles. EDX spectra collected from these regions revealed that Si and Al were the major elements suggesting that the clay particles were kaolinite. Additional batch experiments using gold NPs and SEM analysis were performed and it was observed that the gold NPs were only deposited on clay particles originally on the Accusand surface. After removing the clays from the sand surface, we observed negligible QD deposition even at 100mM IS. We proposed that nanoscale charge heterogeneities on clay particles on Accusand surface played a key role in QD deposition. It was shown that the value of solution IS determined the extent to which the local heterogeneities participated in particle deposition.     

铁盐改性砂制备及其吸附Zn~(2+)的性能研究

通过改变石英砂表面的物理化学性质,提高石英砂的吸附效率,考察其对废水中的Zn~(2+)去除效果.以石英砂为载体,分别用反复高温加热法和反复碱性沉积法制备了三氯化铁改性砂、硝酸铁改性砂,测定2种方法制备的铁盐改性砂的表面含铁量、铁盐的酸稳定性及比表面积,并比较2种铁盐改性砂对Zn~(2+)的吸附效果.结果表明,三氯化铁改性砂、硝酸铁改性砂的比表面积分别为2.468、4.247 m~2/g,比石英砂比表面积分别提高6.910、12.612倍;在pH为中性条件下,石英砂对Zn~(2+)去除率为43%左右,三氯化铁改性砂对Zn~(2+)去除率达到70%左右,硝酸铁改性砂对Zn~(2+)去除率达到85%左右,表明铁盐改性砂对Zn~(2+)去除能力比石英砂有很大提高;铁盐改性砂对Zn~(2+)的吸附有一定容量,表面的活性中心越多,吸附能力越大;铁盐改性砂对Zn~(2+)的去除率随着pH的升高而增加,当pH>8.5时,Zn~(2+)去除率可达90%左右.     

Anomalous transport of colloids and solutes in a shear zone

Transport experiments with colloids and radionuclides in a shear zone were conducted during the Colloid and Radionuclide Retardation experiment (CRR) at Nagra's Grimsel Test Site. Breakthrough curves of bentonite colloids and uranine, a non-sorbing solute, were measured in an asymmetric dipole flow field. The colloid breakthrough is earlier than that of uranine. Both breakthrough curves show anomalously long late time tails and the slope of the late time tails for the colloids is slightly higher. Anomalous late time tails are commonly associated with matrix diffusion processes; the diffusive interaction of solutes transported in open channels with the adjacent porous rock matrix or zones of stagnant water. The breakthrough curves for different colloid size classes are very similar and show no signs of fractionation due to their (size-dependent) diffusivity. It is proposed that tailing of the colloids is mainly caused by the structure of the flow field and that for the colloid transport, matrix diffusion is of minor importance. This has consequences for the interpretation of the uranine breakthrough. Comparisons of experimental results with numerical studies and with the evaluation of the colloid breakthrough with continuous time random theory imply that the tailing in the conservative solute breakthrough in this shear zone is not only caused by matrix diffusion. Part of the tailing can be attributed to advective transport in fracture networks and advection in low velocity regions. Models based on the advection-dispersion equation and matrix diffusion do not properly describe the temporal and spatial evolution of colloid and solute transport in such systems with a consistent set of parameters.     

Analysis of colloid and tracer breakthrough curves

We consider the dispersion and elution of colloids and dissolved nonsorbing tracers within saturated heterogeneous porous media. Since flow path geometry in natural systems is often ill-characterized macroscopic (mean) flow rates and dispersion tensors are utilized in order to account for the sub-model scale microscopic fluctuations in media structure (and the consequent hydrodynamic profile). Even for tracer migration and dispersal this issue is far from settled.Here we consider how colloid and tracer migration phenomena can be treated consistently. Theoretical calculations for model flow geometries yield two quantitative predictions for the transport of free (not yet captured) colloids with reference to a non-sorbing dissolved tracer within the same medium: the average migration velocity of the free colloids is higher than that of the tracer; and that the ratio of the equivalent hydrodynamic dispersion rates of colloids and tracer is dependent only upon properties of the colloids and the porous medium, it is independent of pathlengths and fluid flux, once length scales are large enough.The first of these is well known, since even in simple flow paths free colloids must stay more centre stream. The second, if validated suggests how solute and colloid dispersion may be dealt with consistently in macroscopic migration models. This is crucial since dispersion is usually ill-characterized and unaddressed by the experimental literature. In this paper we present evidence based upon an existing Drigg field injection test for the validity of these predictions.We show that starting from experimental data the fitted dispersion rates of both colloids and non-sorbing tracers increase with the measured elution rates (obeying slightly different rules for tracers and colloids); and that the ratio of colloid and nonsorbing tracer elution rates, and the ratio of colloid and nonsorbing tracer dispersion rates may be dependent upon properties of the colloids and the medium (not the flow regime).It is important to realize that even for unretarded species, an earlier peak in the breakthrough curve does not necessarily correspond to a faster mean elution rate, or vice versa. But rather that a colloid may elute faster but disperse less than an equivalent tracer. Hence its peak may be retarded compared to that of the tracer, even assuming no retardation. Hence one must consider a combination of mean elution rate and mean dispersion rate, and not rely on “peak times” to corroborate chromatographic effects. The importance of this lies in the fact that these processes are not independent and yet upscale differently. Thus realistic estimates of effective colloid dispersion rates should be upscaled in a way consistent with that adopted for tracers within the same system.     

Influence of soluble copper on the electrokinetic properties and transport of copper oxychloride-based fungicide particles

This article describes the influence of dissolved copper on the electrokinetic properties and transport of a copper oxychloride-based fungicide (COF) in porous media. The Zeta potential (ζ) of COF particles increases (viz. becomes less negative) with increasing concentration of Cu(2+) in the bulk solution. ζ decreases for COF when the electrolyte (NaNO(3)) concentration is raised from 1 to 10mM. This can be ascribed to ion correlation of Cu(2+) in the electrical double layer (EDL). COF transport tests in quartz sand columns showed the addition of Cu(2+) to the bulk solution to result in increased retention of the metal. Modelling particle deposition dynamics provided results consistent with kinetic attachment. Based on the effect of soluble Cu on colloid mobility, the transport of particulate and soluble forms of copper is coupled via the chemistry of pore water and colloid interactions. Mutual effects between cations and colloids should thus be considered when determining the environmental fate of particulate and soluble forms of copper in soil and groundwater (especially at copper-contaminated sites).     

Transport of a non-volatile contaminant in unsaturated porous media in the presence of colloids

Stable colloidal particles can travel long distances in subsurface environments and carry particle-reactive contaminants with them to locations further than predicted by the conventional advective-dispersive transport equation. When such carriers exist in a saturated porous medium, the system can be idealized as consisting of three phases: an aqueous phase, a carrier phase, and a stationary solid matrix phase. However, when colloids are present in an unsaturated porous medium, the system representation should include one more phase, i.e. the air phase. In the work reported, a mathematical model was developed to describe the transport and fate of the colloidal particles and a non-volatile contaminant in unsaturated porous media. The model is based on mass balance equations in a four-phase porous medium. Colloid mass transfer mechanisms among aqueous, solid matrix, and air phases, and contaminant mass transfer between aqueous and colloid phases are represented by kinetic expressions. Governing equations are non-dimensionalized and solved to investigate colloid and contaminant transport in an unsaturated porous medium. A sensitivity analysis of the transport model was utilized to assess the effects of several parameters on model behavior. The colloid transport model matches successfully with experimental data of Wan and Wilson. The presence of air-water interface retards the colloid transport significantly counterbalancing the facilitating effect of colloids. However, the retardation of contaminant transport by colloids is highly dependent on the properties of the contaminant and the colloidal surface.     

Transport of reactive colloids and contaminants in groundwater: effect of nonlinear kinetic interactions

Transport of reactive colloids in groundwater may enhance the transport of contaminants in groundwater. Often, the interpretation of results of transport experiments is not a simple task as both reactions of colloids with the solid matrix and reactions of contaminants with the solid matrix and mobile and immobile colloids may be time dependent and nonlinear. Further colloid transport properties may differ from solute transport properties. In this paper, a one-dimensional model for coupled and contaminant in a porous medium (COLTRAP) is presented together with simulation results. Calculated breakthrough curves (BTC's) during contamination and decontamination show systematically the effect of nonlinear and kinetic interactions on contaminant transport in the presence of reactive colloids, and the effect of colloid transport properties that differ from solute transport properties. It is shown that in case of linear kinetic reactions, the rate of exchange of mobile and immobile colloids have a large impact on the shape of BTC's even if the solid matrix is saturated with respect to colloids. BTC's during the contamination and decontamination phase have identical shapes in this case. Moreover, the slow reactions of contaminants and colloids may lead to unretarded breakthrough of contaminants. Independent of reaction rates, nonlinear reactions lead to BTC's that are steeper during contamination than in the linear case. A characteristic aspect of nonlinear sorption is that shapes of BTC's differ during the contamination and decontamination phase. It has been observed that shapes of some of the simulated adsorption and desorption curves are similar as shapes found in experiments reported in literature. This stresses the importance of incorporating both kinetics and nonlinearity in models for coupled colloid and contaminant transport and the capability of COLTRAP to interpret experimental results. Finally, to figure out whether nonlinear processes play a role, it is very important to consider both contamination and decontamination in transport experiments.     

Virus transport in physically and geochemically heterogeneous subsurface porous media

A two-dimensional model for virus transport in physically and geochemically heterogeneous subsurface porous media is presented. The model involves solution of the advection-dispersion equation, which additionally considers virus inactivation in the solution, as well as virus removal at the solid matrix surface due to attachment (deposition), release, and inactivation. Two surface inactivation models for the fate of attached inactive viruses and their subsequent role on virus attachment and release were considered. Geochemical heterogeneity, portrayed as patches of positively charged metal oxyhydroxide coatings on collector grain surfaces, and physical heterogeneity, portrayed as spatial variability of hydraulic conductivity, were incorporated in the model. Both layered and randomly (log-normally) distributed physical and geochemical heterogeneities were considered. The upstream weighted multiple cell balance method was employed to numerically solve the governing equations of groundwater flow and virus transport. Model predictions show that the presence of subsurface layered geochemical and physical heterogeneity results in preferential flow paths and thus significantly affect virus mobility. Random distributions of physical and geochemical heterogeneity have also notable influence on the virus transport behavior. While the solution inactivation rate was found to significantly influence the virus transport behavior, surface inactivation under realistic field conditions has probably a negligible influence on the overall virus transport. It was further demonstrated that large virus release rates result in extended periods of virus breakthrough over significant distances downstream from the injection sites. This behavior suggests that simpler models that account for virus adsorption through a retardation factor may yield a misleading assessment of virus transport in "hydrogeologically sensitive" subsurface environments.     

Physical versus chemical effects on bacterial and bromide transport as determined from on site sediment column pulse experiments

Twenty-eight bacterial and Br transport experiments were performed in the field to determine the effects of physical and chemical heterogeneity of the aquifer sediment. The experiments were performed using groundwater from two field locations to examine the effects of groundwater chemistry on transport. Groundwater was extracted from multilevel samplers and pumped through 7-cm-long columns of intact sediment or repacked sieved and coated or uncoated sediment from the underlying aquifer. Two bacterial strains, Comamonas sp. DA001 and Paenibacillus polymyxa FER-02, were injected along with Br into the influent end of columns to examine the effect of cell morphology and cell surface properties on bacterial transport. The effects of column sediment grain size and mineral coatings coupled with groundwater geochemistry were also investigated. Significant irreversible attachment of DA001 was observed in the Fe oxyhydroxide-coated columns, but only in the suboxic groundwater where the concentrations of dissolved organic carbon (DOC) were ca. 1 ppm. In the oxic groundwater where DOC was ca. 8 ppm, little attachment of DA001 to the Fe oxyhydroxide-coated columns was observed. This indicates that DOC can significantly reduce bacterial attachment due electrostatic interactions. The larger and more negatively charged FER-02 displayed increasing attachment with decreasing grain size regardless of DOC concentration, and modeling of FER-02 attachment revealed that the presence of Fe and Al coatings on the sediment also promoted attachment. Finally, the presence of Al coatings and Al containing minerals appeared to significantly retard the Br tracer regardless of the concentration of DOC. These findings suggest that DOC in shallow oxic groundwater aquifers can significantly enhance the transport of bacteria by reducing attachment to Fe, Mn and Al oxyhydroxides. This effect appears to be profound for weakly and strongly charged hydrophilic bacteria and may contribute to differences in observations between laboratory experiments versus field-scale investigations particularly if the groundwater pH remains subneutral and Fe oxyhydroxide phases exist. These observation validate the novel approach taken in the experiments outlined here of performing laboratory-scale experiments on site to facilitate the use of fresh groundwater and thus be more representative of in situ groundwater conditions.     

Strontium migration in a crystalline medium: effects of the presence of bentonite colloids

The effects of bentonite colloids on strontium migration in fractured crystalline medium were investigated. We analyzed first the transport behaviour of bentonite colloids alone at different flow rates; then we compared the transport behaviour of strontium as solute and of strontium previously adsorbed onto stable bentonite colloids at a water velocity of approximately 7.1·10(-6)m/s-224m/yr. Experiments with bentonite colloids alone showed that - at the lowest water flow rate used in our experiments (7.1·10(-6)m/s) - approximately 70% of the initially injected colloids were retained in the fracture. Nevertheless, the mobile colloidal fraction, moved through the fracture without retardation, at any flow rate. Bentonite colloids deposited over the fracture surface were identified during post-mortem analyses. The breakthrough curve of strontium as a solute, presented a retardation factor, R(f)~6, in agreement with its sorption onto the granite fracture surface. The breakthrough curve of strontium in the presence of bentonite colloids was much more complex, suggesting additional contributions of colloids to strontium transport. A very small fraction of strontium adsorbed on mobile colloids moved un-retarded (R(f)=1) and this fraction was much lower than the expected, considering the quantity of strontium initially adsorbed onto colloids (90%). This behaviour suggests the hypothesis of strontium sorption reversibility from colloids. On the other hand, bentonite colloids retained within the granite fracture played a major role, contributing to a slower strontium transport in comparison with strontium as a solute. This was shown by a clear peak in the breakthrough curve corresponding to a retardation factor of approximately 20.     

Migration of colloids in discretely fractured porous media: effect of colloidal matrix diffusion

Numerical simulations of colloid transport in discretely fractured porous media were performed to investigate the importance of matrix diffusion of colloids as well as the filtration and remobilization of colloidal particles in both the fractures and porous matrix. To achieve this objective a finite element numerical code entitled COLDIFF was developed. The processes that COLDIFF takes into account include advective-dispersive transport of colloids, filtration and remobilization of colloidal particles in both fractures and porous matrix, and diffusive interactions of colloids between the fractures and porous matrix. Three sets of simulations were conducted to examine the importance of parameters and processes controlling colloid migration. First, a sensitivity analysis was performed using a porous block containing a single fracture to determine the relative importance of various phenomenological coefficients on colloid transport. The primary result of the analysis showed that the porosity of the matrix and the process of colloid filtration in fractures play important roles in controlling colloid migration. Second, simulations were performed to replicate and examine the results of a laboratory column study using a fractured shale saprolite. Results of this analysis showed that the filtration of colloidal particles in the porous matrix can greatly affect the tailing of colloid concentrations after the colloid source was removed. Finally, field-scale simulations were performed to examine the effect of matrix porosity, fracture filtration and fracture remobilization on long-term colloid concentration and migration distance. The field scale simulations indicated that matrix diffusion and fracture filtration can significantly reduce colloid migration distance. Results of all three analyses indicated that in environments where porosity is relatively high and colloidal particles are small enough to diffuse out of fractures, the characteristics of the porous matrix that affect colloid transport become more important than those of the fracture network. Because the properties of the fracture network tend to have greater uncertainty due to difficulties in their measurement relative to those of the porous matrix, prediction uncertainties associated with colloid transport in discretely fractured porous media may be reduced.     

Functional models for colloid retention in porous media at the triple line

Spectral confocal microscope visualizations of microsphere movement in unsaturated porous media showed that attachment at the Air Water Solid (AWS) interface was an important retention mechanism. These visualizations can aid in resolving the functional form of retention rates of colloids at the AWS interface. In this study, soil adsorption isotherm equations were adapted by replacing the chemical concentration in the water as independent variable by the cumulative colloids passing by. In order of increasing number of fitted parameters, the functions tested were the Langmuir adsorption isotherm, the Logistic distribution, and the Weibull distribution. The functions were fitted against colloid concentrations obtained from time series of images acquired with a spectral confocal microscope for three experiments performed where either plain or carboxylated polystyrene latex microspheres were pulsed in a small flow chamber filled with cleaned quartz sand. Both moving and retained colloids were quantified over time. In fitting the models to the data, the agreement improved with increasing number of model parameters. The Weibull distribution gave overall the best fit. The logistic distribution did not fit the initial retention of microspheres well but otherwise the fit was good. The Langmuir isotherm only fitted the longest time series well. The results can be explained that initially when colloids are first introduced the rate of retention is low. Once colloids are at the AWS interface they act as anchor point for other colloids to attach and thereby increasing the retention rate as clusters form. Once the available attachment sites diminish, the retention rate decreases.     

Humic colloid-borne migration of uranium in sand columns

Column experiments were carried out to investigate the influence of humic colloids on subsurface uranium migration. The columns were packed with well-characterized aeolian quartz sand and equilibrated with groundwater rich in humic colloids (dissolved organic carbon (DOC): 30 mg dm(-3)). U migration was studied under an Ar/1% CO2 gas atmosphere as a function of the migration time, which was controlled by the flow velocity or the column length. In addition, the contact time of U with groundwater prior to introduction into a column was varied. U(VI) was found to be the dominant oxidation state in the spiked groundwater. The breakthrough curves indicate that U was transported as a humic colloid-borne species with a velocity up to 5% faster than the mean groundwater flow. The fraction of humic colloid-borne species increases with increasing prior contact time and also with decreasing migration time. The migration behavior was attributed to a kinetically controlled association/dissociation of U onto and from humic colloids and also a subsequent sorption of U onto the sediment surface. The column experiments provide an insight into humic colloid-mediated U migration in subsurface aquifers.     

Numerical modeling of humic colloid borne americium (III) migration in column experiments using the transport/speciation code K1D and the KICAM model

The humic colloid borne Am(III) transport was investigated in column experiments for Gorleben groundwater/sand systems. It was found that the interaction of Am with humic colloids is kinetically controlled, which strongly influences the migration behavior of Am(III). These kinetic effects have to be taken into account for transport/speciation modeling. The kinetically controlled availability model (KICAM) was developed to describe actinide sorption and transport in laboratory batch and column experiments. Application of the KICAM requires a chemical transport/speciation code, which simultaneously models both kinetically controlled processes and equilibrium reactions. Therefore, the code K1D was developed as a flexible research code that allows the inclusion of kinetic data in addition to transport features and chemical equilibrium. This paper presents the verification of K1D and its application to model column experiments investigating unimpeded humic colloid borne Am migration. Parmeters for reactive transport simulations were determined for a Gorleben groundwater system of high humic colloid concentration (GoHy 2227). A single set of parameters was used to model a series of column experiments. Model results correspond well to experimental data for the unretarded humic borne Am breakthrough.     

Modeling colloid-facilitated transport of multi-species contaminants in unsaturated porous media

Colloid-facilitated transport has been recognized as a potentially important and overlooked contaminant transport process. In particular, it has been observed that conventional two phase sorption models are often unable to explain transport of highly sorbing compounds in the subsurface appropriately in the presence of colloids. In this study a one-dimensional model for colloid-facilitated transport of chemicals in unsaturated porous media is developed. The model has parts for simulating coupled flow, and colloid transport and dissolved and colloidal contaminant transport. Richards' equation is solved to model unsaturated flow, and the effect of colloid entrapment and release on porosity and hydraulic conductivity of the porous media is incorporated into the model. Both random sequential adsorption and Langmuir approaches have been implemented in the model in order to incorporate the effect of surface jamming. The concept of entrapment of colloids into the air-water interface is used for taking into account the effect of retardation caused due to existence of the air phase. A non-equilibrium sorption approach with options of linear and Langmuir sorption assumptions are implemented that can represent the competition and site saturation effects on sorption of multiple compounds both to the solid matrix and to the colloidal particles. Several demonstration calculations are performed and the conditions in which the non-equilibrium model can be approximated by an equilibrium model are also studied.     

Contaminant transport in groundwater in the presence of colloids and bacteria: Model development and verification

Colloids and bacteria (microorganisms) naturally exist in groundwater aquifers and can significantly impact contaminant migration rates. A conceptual model is first developed to account for the different physiochemical and biological processes, reaction kinetics, and different transport mechanisms of the combined system (contaminant–colloids–bacteria). All three constituents are assumed to be reactive with the reactions taking place between each constituent and the porous medium and also among the different constituents. A general linear kinetic reaction model is assumed for all reactive processes considered. The mathematical model is represented by fourteen coupled partial differential equations describing mass balance and reaction processes. Two of these equations describe colloid movement and reactions with the porous medium, four equations describe bacterial movement and reactions with colloids and the porous medium, and the remaining eight equations describe contaminant movement and its reactions with bacteria, colloids, and the porous medium. The mass balance equations are numerically solved for two-dimensional groundwater systems using a third-order, total variance-diminishing scheme (TVD) for the advection terms. Due to the complex coupling of the equations, they are solved iteratively each time step until a convergence criterion is met. The model is tested against experimental data and the results are favorable.     

Influence of the colloid type on the transfer of 60Co and 85Sr in silica sand column under varying physicochemical conditions

The influence of two types of colloids (natural organic matter, NOM), a colloid with high affinity for radionuclides (RN(s)), and hydrophilic synthetic latex (SHL), a colloid with low affinity for RN(s) on the transfer of (60)Co and (85)Sr in a silica sand column was studied under different physicochemical conditions: pH (4.9), ionic strength (10(-3) M and 10(-2) M), concentration of colloids (100 mg l(-1), 10 mg l(-1)), flow velocity (12.4 cm h(-1) and 3.7 cm h(-1)), water saturation of the column (100% and 70%). In the absence of colloids, the transfer of (60)Co and (85)Sr was retarded compared to the transfer of the conservative tracer. In the presence of colloids and according to the specific physicochemical conditions, an acceleration or retardation of (60)Co and (85)Sr transfer was observed compared to their transfer in the absence of colloids. Our results evidenced that any colloids even with low reactivity could significantly modify the RN transfer. However, the extent to which the transfer was influenced differs according to the colloid type; the NOM exhibiting higher impact than SHL. Batch experiments helped in interpreting of the interactions between the colloids, RN(s) and solid phase observed in column.