Porous media and transport within them play technically important roles in many of our industries. However, classical mean field engineering descriptions used to model the complex interactions between the porous medium and the fluids and colloids within it are not completely satisfactory. The design capability of the engineering community would be greatly enhanced if these models could be more clearly linked to the mesoscopic details of the fluid/suspension/porous solid systems. This would allow cheaper, yet quicker, and more innovative design and optimization of systems involving fluid/suspension flow in porous media. Modern techniques for the explicit mesoscopic modelling of porous media, and fluid and colloid transport within them, have developed to a point where their combination in a single simulation tool can be contemplated. However, at present, no such tool exists. The aim of this study was to design and test a comprehensive simulation tool that could accurately model the transport phenomena of any given fluid and colloidal system within any given porous medium at a mesoscopic level. Lattice gas automata (LGA) modelling techniques for fluid and colloid transport, and the Joshi/Quiblier/Adler (JQA) statistical method for reconstructing porous media, were uniquely combined to achieve this. The results of simulations were compared to measurements obtained using an experimental apparatus. The objectives of the study were to: 1) determine a priori the permeability of porous media, and; 2) simulate deposition phenomena observed experimentally. The study showed that permeabilities predicted using the simulation tool were lower than those determined experimentally. Several causes for this were identified, all of which can be addressed in the short-term. Simulated changes in fluid velocity and particle concentration were found to alter the rate and pattern of deposition in a manner consistent with experimental results. Furthermore, the tool provided a rich description of fundamental physical phenomena at the pore scale level. These preliminary findings indicate that the combination of these models provide the basis for further development leading to a mesoscopic modelling tool capable of predicting fluid and colloid transport in porous media.

Depletion forces were used to separate an equinumber density binary dispersion of 1.5 and 0.82 µm polystyrene sulfate (PS) particles. Experiments consisted of injecting a pulse of a binary dispersion of PS particles into the inlet of a packed bed of 0.5 mm silica collector beads. Prior to injection, a carrier fluid of either KCl and KOH electrolyte or a silica nanoparticle dispersion was flowing through the column at steady state. When the carrier fluid was a dispersion of silica nanoparticles, the ratio of PS particles in the column outlet would change from 1:1 big to small particles to slightly over 2:1. This implies that more of the smaller 0.82 µm particles were being trapped on the surface of the collector beads due to depletion forces. Experiments with a single particle type (either 1.5 or 0.82 µm PS particle) were also done and correlated with the binary dispersion measurements. Potential energy profiles between a PS particle and a flat silica plate were calculated. The secondary energy barrier for the 1.5 µm particles was two times greater than for the 0.82 µm particles. Hence, the 0.82 µm particles were more likely to overcome the energy barrier and get trapped on the surface of the collector beads. Although the potential energy profiles were calculated at equilibrium, they can be used as a tool in finding the optimal conditions for separation. / Master of Science

<p>An improved understanding of colloid transport in fractured media is required to assess the potential for microorganisms to contaminate groundwater, to develop groundwater management/protection plans, to design remedial action strategies based on the application of microorganisms, and to quantify colloid-facilitated transport of many organic and inorganic contaminants. Although colloid transport has been investigated to an extent in porous media environments, this field is still in its infancy in fractured media environments.</p> <p>Colloid transport in fractured media involves a host of complex and interacting processes, including (among others): advection, hydrodynamic dispersion, attachment and detachment, straining, size/ charge exclusion, and gravitational settling. These processes are, in turn, influenced by the physicochemical properties of fractured media, the geochemical properties of groundwater, hydrodynamics, and the colloid properties. This research program focused on investigating the effects of aperture field variability, specific discharge, and ionic strength on colloid transport in saturated, variable-aperture, single fractures. An extensive literature review was first conducted, and a combination of physical model experiments and numerical simulations were then employed to achieve this goal.</p> <p> Three transparent fracture replicas were fabricated, and the light transmission method was employed to obtain a direct measurement of each of the three aperture fields. A systematic series of hydraulic and tracer tests was conducted on each of the three experimental fractures, and the cubic law, mass balance and frictional loss apertures were calculated. Additionally, the experimental breakthrough curves were fit to the one-dimensional advection-dispersion equation. The results clearly demonstrate that the mass balance aperture is the only appropriate 'equivalent aperture' for describing transport in a single variable-aperture fracture, and that the mass balance aperture is an excellent approximation ofthe arithmetic mean aperture.</p> <p>A 3^3 factorial experimental design was then implemented to numerically investigate the interactive effect of the arithmetic mean (μb), standard deviation (σb), and anisotropic ratio (AR=λ^b x/ λ^b,y where λ^b x and λ^b y is the correlation length of the aperture field along x- and y- direction respectively) of single fracture apertures on dispersion regimes (specifically Taylor dispersion and geometric dispersion) and dispersivity. The simulation results show that: (1) for a fixed hydraulic gradient: (a) the dominant dispersion regime is controlled by μb, and to a lesser degree, σb, and (b) geometric dispersion becomes more dominant as the coefficient of variation (CoV = σb/μb) increases; (2) for a fixed mean aperture, the dispersivity and the spread in dispersivity for varying ARs increase with the CoV; and (3) the AR has a significant effect on dispersivity only when the CoV is large (>0.2).</p> <p> Numerical simulations investigating colloid and solute transport in single parallel-plate fractures, conducted using the Random Walk Particle Tracking (RWPT) method, demonstrated that: 1) There exists a threshold value, δo , of the aspect ratio, δ (δ= 2rc/b, where rc and b represent the colloid radius and fracture aperture respectively), where the average transport velocities of colloids and solutes are similar. When δ> δo , the Taylor Aris assumption is satisfied, and tp (tp = tc/ts, where tc and ts represent colloid and solute retention times respectively) decreases as δ increases, as is well documented in the hydrodynamic chromatography literature. However, when δ < δo , the Taylor-Aris assumption is violated, and tp increases as δ increases. This has never been documented before, and it helps to explain some seemingly contradictory work in the literature. 2) The Taylor dispersion coefficient and its extension by James and Chrysikopoulos (2003) will overestimate the colloid dispersion coefficient significantly when the Taylor-Aris assumption is violated. Additionally, these simulations demonstrated that tp and DL,coll/DL,solute (where DL,coll and DL,solute represent the dispersion coefficients of colloids and solutes respectively) decrease with increasing CoV, and that the anisotropy ratio, AR, only plays a minor role on these two ratios compared to the CoV. These observations have never been documented before to the knowledge of these authors, and have important implications towards the interpretation of colloid transport in both porous and fractured media.</p> <p> A combination of physical experiments, numerical simulations and visualization techniques was employed to investigate the impact of aperture variability, specific discharge, and ionic strength on colloid transport processes. The mean colloid transport velocity and colloid dispersion coefficient were obtained by fitting the analytical solution of the one-dimensional advection-dispersion equation (ADE) to the measured breakthrough curves. Two significant observations were made from the colloid transport experiment images: (1) colloids migrate along preferential pathways, and bypass some aperture regions; and (2) the colloid plume is irregular in shape, and becomes more irregular with increasing specific discharge, indicating non-Fickian transport. It is therefore postulated that the dispersivity cannot be completely determined by the aperture field characteristics alone; it is also a function of specific discharge. The colloid recovery in all fractures was found to increase with increasing specific discharge. For each specific discharge, it was found that the colloid recoveries in F2 and F3 were similar, and were always larger than the recovery in F1. This is consistent with the fact that the arithmetic mean apertures of F2 and F3 were similar (μb,F2= 1.57 mm, /μb,F3= 1.75 mm), and larger than that of F1 (μb,F1 = 0.88 mm). This suggests that it is the transport step (the step in which the colloids are transported from the bulk fluid to the vicinity of the fracture wall), and not the attachment step, that plays the dominant role in the colloid sorption process. It was also found that the mean transport velocity and dispersion coefficient of colloids are larger than those of solutes in F3 (CoV = 0.29), but similar to those of solutes in F1 (CoV = 0.78) and F2 (CoV = 0.71). This confirms the numerical simulation results from this work indicating that tp and DL,coll/DL,solute decrease with increasing CoV. These findings have significant implications on the interpretation of colloid transport data.</p> / Thesis / Doctor of Philosophy (PhD)

Basic oxygen furnace (BOF) slag media were studied through a series of laboratory, modeling and field studies as a potential treatment material for use in on site wastewater disposal systems. Microsphere enumeration methodology was examined in a factorial experiment to evaluate the minimum density and minimum number of microspheres that should be counted to ensure accurate and precise estimations of concentration. The results suggest that to minimize variability at least 350 microspheres should be counted and a microsphere density of 25-40 microspheres field-1 is necessary. A review of existing methodologies for high-titer bacteriophage production was conducted and an amalgamation of existing methodologies was chosen that reliably achieves elevated concentration and ensures a purified suspension. A combination of batch and column studies was conducted to evaluate the removal of the bacteriophage, PRD-1, and virus-sized fluorescent microspheres by BOF media, and to delineate the relative contributions of the two principle attenuation processes, inactivation and attachment. In the batch studies, substantial removal of PRD-1 does not occur in the pH 7.6 and 9.5 suspensions, but at pH 11.4, removal of the virus was 2.1 log C/C0 day-1 for the first two days, followed by 0.124 log C/C0 day-1 over the subsequent 10 days. Two column studies were conducted after 60 and 300 days of saturation with artificial groundwater at a flow rate of 1 pore volume day-1 using two BOF mixtures. After 300 days of column saturation, microsphere concentrations approached input levels, indicating a removal of 0.1-0.2 log C/C0 and suggesting attachment processes were negligible. PRD-1 removal was more pronounced (1.0-1.5 log C/C0). The reduction of PRD-1 is likely the result of a combination of virus inactivation at elevated pH (10.6-11.4), and attachment processes. Geochemical factors controlling microsphere attachment were compared between the two sets of experiments after 60 and 300 days of column saturation. Differences in attachment efficiency may be due to higher influent DOC concentration in the second experiment, conversion of amorphous iron phases to more crystalline forms over time, reductive dissolution of preferable attachment sites on iron phases, or precipitation of calcite. Hydrus-1D, a one-dimensional numerical model, was used to quantify transport processes, inactivation and attachment/detachment, occurring in the column experiments by model inversion. Fitted microsphere breakthrough closely matched observed data, whereas PRD-1 breakthrough with realistic parameter values does not closely match the peaked nature of the observed curves. The model achieved improved fits for microsphere and PRD-1 breakthrough when both strongly- and weakly-binding sites are represented. A unique set of parameter estimates could not be determined because of overparameterization of the inverse modeling for the experimental systems. An alternative latrine incorporating BOF slag media was constructed in a periurban community located near São Paulo, Brazil. Pathogen indicator removal is approximately 4-5 orders of magnitude in less than one meter of vertical transport through the BOF slag media. In a control latrine, constructed with similar hydraulic characteristics and inert materials, comparable reductions in pathogenic indicators were observed over three meters of vertical transport.

Basic oxygen furnace (BOF) slag media were studied through a series of laboratory, modeling and field studies as a potential treatment material for use in on site wastewater disposal systems. Microsphere enumeration methodology was examined in a factorial experiment to evaluate the minimum density and minimum number of microspheres that should be counted to ensure accurate and precise estimations of concentration. The results suggest that to minimize variability at least 350 microspheres should be counted and a microsphere density of 25-40 microspheres field-1 is necessary. A review of existing methodologies for high-titer bacteriophage production was conducted and an amalgamation of existing methodologies was chosen that reliably achieves elevated concentration and ensures a purified suspension. A combination of batch and column studies was conducted to evaluate the removal of the bacteriophage, PRD-1, and virus-sized fluorescent microspheres by BOF media, and to delineate the relative contributions of the two principle attenuation processes, inactivation and attachment. In the batch studies, substantial removal of PRD-1 does not occur in the pH 7.6 and 9.5 suspensions, but at pH 11.4, removal of the virus was 2.1 log C/C0 day-1 for the first two days, followed by 0.124 log C/C0 day-1 over the subsequent 10 days. Two column studies were conducted after 60 and 300 days of saturation with artificial groundwater at a flow rate of 1 pore volume day-1 using two BOF mixtures. After 300 days of column saturation, microsphere concentrations approached input levels, indicating a removal of 0.1-0.2 log C/C0 and suggesting attachment processes were negligible. PRD-1 removal was more pronounced (1.0-1.5 log C/C0). The reduction of PRD-1 is likely the result of a combination of virus inactivation at elevated pH (10.6-11.4), and attachment processes. Geochemical factors controlling microsphere attachment were compared between the two sets of experiments after 60 and 300 days of column saturation. Differences in attachment efficiency may be due to higher influent DOC concentration in the second experiment, conversion of amorphous iron phases to more crystalline forms over time, reductive dissolution of preferable attachment sites on iron phases, or precipitation of calcite. Hydrus-1D, a one-dimensional numerical model, was used to quantify transport processes, inactivation and attachment/detachment, occurring in the column experiments by model inversion. Fitted microsphere breakthrough closely matched observed data, whereas PRD-1 breakthrough with realistic parameter values does not closely match the peaked nature of the observed curves. The model achieved improved fits for microsphere and PRD-1 breakthrough when both strongly- and weakly-binding sites are represented. A unique set of parameter estimates could not be determined because of overparameterization of the inverse modeling for the experimental systems. An alternative latrine incorporating BOF slag media was constructed in a periurban community located near São Paulo, Brazil. Pathogen indicator removal is approximately 4-5 orders of magnitude in less than one meter of vertical transport through the BOF slag media. In a control latrine, constructed with similar hydraulic characteristics and inert materials, comparable reductions in pathogenic indicators were observed over three meters of vertical transport.

In this thesis a study of the factors affecting virus and virus-sized particle transport in discrete fractured dolomite rocks is presented. Physical and chemical characteristics of two fractured rocks were determined, including fracture aperture distribution, rock matrix porosity, mineral composition, and surface charge. Hydraulic and transport tests were conducted in the fractures with a conservative solute (bromide) and carboxylate-modified latex (CML) microspheres of three sizes (20, 200, and 500 nm in diameter). The earlier arrival of larger microspheres as compared to bromide indicated the effects of pore-size exclusion and preferential flow paths in the fractures. The tailing of the bromide and the smaller microsphere (20 nm) in the breakthrough curves (BTC) indicated the diffusive mass transfer between the mobile water (flowing) and immobile water (stagnant water in the low aperture areas and porous rock matrix). The effects of ionic strength and cation type on the transport of viruses (bacteriophages MS2 and PR772) and virus-sized microspheres (20 and 200 nm) were determined from the transport tests in a fracture at three levels of ionic strength (3, 5, and 12 mM) and composition (containing Na+ and/or Ca2+ ions). Retention of the microspheres and bacteriophages increased with increasing ionic strength. The addition of divalent ions (Ca2+) influenced the retention to a greater extent than monovalent ions (Na+). The effects of the aperture distribution variability, matrix diffusion, and specific discharge on the solute and microsphere transport were determined from the transport tests conducted in two fractures. The higher variability in the aperture distribution contributed to higher solute dispersion, and flow channeling as evident from the breakthrough curves for individual spatially distributed outlets. A three-dimensional model simulation of the bromide transport with varying matrix porosity identified that the porous matrix influenced the solute transport. In the transport tests, retention of the microspheres decreased with increasing specific discharge in both fractures. The results of this research have helped in identifying the important factors and their effects on solute, virus, and virus-sized colloid transport in fractured dolomite rocks, which can be useful in determining the risk of pathogen contamination of water supplies in fractured dolomite rock aquifers.

In this thesis a study of the factors affecting virus and virus-sized particle transport in discrete fractured dolomite rocks is presented. Physical and chemical characteristics of two fractured rocks were determined, including fracture aperture distribution, rock matrix porosity, mineral composition, and surface charge. Hydraulic and transport tests were conducted in the fractures with a conservative solute (bromide) and carboxylate-modified latex (CML) microspheres of three sizes (20, 200, and 500 nm in diameter). The earlier arrival of larger microspheres as compared to bromide indicated the effects of pore-size exclusion and preferential flow paths in the fractures. The tailing of the bromide and the smaller microsphere (20 nm) in the breakthrough curves (BTC) indicated the diffusive mass transfer between the mobile water (flowing) and immobile water (stagnant water in the low aperture areas and porous rock matrix). The effects of ionic strength and cation type on the transport of viruses (bacteriophages MS2 and PR772) and virus-sized microspheres (20 and 200 nm) were determined from the transport tests in a fracture at three levels of ionic strength (3, 5, and 12 mM) and composition (containing Na+ and/or Ca2+ ions). Retention of the microspheres and bacteriophages increased with increasing ionic strength. The addition of divalent ions (Ca2+) influenced the retention to a greater extent than monovalent ions (Na+). The effects of the aperture distribution variability, matrix diffusion, and specific discharge on the solute and microsphere transport were determined from the transport tests conducted in two fractures. The higher variability in the aperture distribution contributed to higher solute dispersion, and flow channeling as evident from the breakthrough curves for individual spatially distributed outlets. A three-dimensional model simulation of the bromide transport with varying matrix porosity identified that the porous matrix influenced the solute transport. In the transport tests, retention of the microspheres decreased with increasing specific discharge in both fractures. The results of this research have helped in identifying the important factors and their effects on solute, virus, and virus-sized colloid transport in fractured dolomite rocks, which can be useful in determining the risk of pathogen contamination of water supplies in fractured dolomite rock aquifers.

Accurate prediction of colloid deposition rates in porous media is essential in several applications. These include natural filtration of pathogenic microorganisms such as bacteria, viruses, and protozoa, transport and fate of colloid-associated transport of contaminants, deep bed and river bank filtration for water treatment, fate and transport of engineered nanoparticles released into the environment, and bioremediation of contaminated sites. Colloid transport in porous media is a multi-scale problem, with length scales spanning from the sub-pore scale, where the particle-soil interaction forces control the deposition, up to the Darcy scale, where the macroscopic equations governing particle transport are formulated. Colloid retention at the Darcy scale is due to the lumped effect of processes occurring at the pore scale. This requires the incorporation of the micro-scale physics into macroscopic models for a better understanding of colloid deposition in porous media. That can be achieved through pore-scale modeling and the subsequent upscaling to the Darcy scale. Colloid Filtration Theory (CFT), the most commonly used approach to describe colloid attachment onto the soil grains in the subsurface, is found to accurately predict the deposition rates of micron-sized particles under favorable conditions for deposition. But, CFT has been found to over predict particle deposition rates at low flow velocity conditions, typical of groundwater flow, and for nanoscale particles. Also, CFT is found to be inapplicable at typical environmental conditions, where conditions become unfavorable for deposition, due to factors not considered in CFT such as deposition in the secondary minimum of the interaction energy profile, grain surface roughness, surface charge heterogeneity of grains and colloids, and deposition at grain-to-grain contacts. To the best of our knowledge, mechanistic-based models for predicting colloid deposition rates under unfavorable conditions do not exist. Currently, fitting the colloid breakthrough curve (BTC), obtained from the laboratory column-or field-scale experiments, to the advection-dispersion-deposition model is used to estimate the values of deposition rate coefficients. Because of their small size (less than 100 nm), nanoparticles, a sub-class of colloids, may interact with the porous medium in a different way as compared to the larger colloids, resulting in different retention mechanisms for nanoparticles and micron-sized particles. This emphasizes the need to study nanoparticles separately from larger, micrometer-sized colloids to better understand nanoparticle retention mechanisms. The work reported in this thesis contributes towards developing mathematical models to predict nanoparticle movement in porous media. A comprehensive mechanistic approach is employed by integrating pore-scale processes into Darcy-scale models through pore-network modeling to upscale nanoparticle transport in saturated porous media to the Darcy scale, and to develop correlation equations for the Darcy-scale deposition parameters in terms of various measurable parameters at Darcy scale. Further, a one-dimensional mathematical model to simulate the co-transport of viruses and colloids in partially saturated porous media is developed to understand the relative importance of various interactions on virus transport in porous media. Pore-network modeling offers a valuable upscaling tool to express the macroscopic behavior by accounting for the relevant physics at the underlying pore scale. This is done by idealizing the pore space as an interconnected network of pore elements of different sizes and variably connected to each other, and simulating flow and transport through the network of pores, with the relevant physics implemented on a pore to pore basis (Raoof, 2011). By comparing the results of pore-network modeling with an appropriate mathematical model describing the macro-scale behavior, a relationship between the properties at the macro scale and those at the pore scale can be obtained. A three dimensional multi-directional pore-network model, PoreFlow, developed by Raoof et al. (2010, 2013) is employed in this thesis, which represents the porous medium as an interconnected network of cylindrical pore throats and spherical pore bodies, to upscale nanoparticle transport from pore scale to the Darcy scale. The first step in this procedure is to obtain relationships between adsorbed mass and aqueous mass for a single pore. A mathematical model is developed to simulate nanoparticle transport in a saturated cylindrical pore by solving the full transport equation, considering various processes such as advection, diffusion, hydrodynamic wall effects, and nanoparticle-collector surface interactions. The pore space is divided into three different regions: bulk, diffusion and potential regions, based on the dominant processes acting in each of these regions. In both bulk and diffusion regions, nanoparticle transport is governed by advection and diffusion. However, in the diffusion region, the diffusion is significantly reduced due to hydrodynamic wall effects. Nanoparticle-collector interaction forces dominate the transport in the potential region where deposition occurs. A sensitivity analysis of the model indicates that nanoparticle transport and deposition in a pore is significantly affected by various pore-scale parameters such as the nanoparticle and collector surface potentials, ionic strength of the solution, flow velocity, pore radius, and nanoparticle radius. The model is found to be more sensitive to all parameters under favorable conditions. It is found that the secondary minimum plays an important role in the deposition of small as well as large nanoparticles, and its contribution is found to increase as the favorability of the surface for adsorption decreases. Correlation equations for average deposition rate coefficients of nanoparticles in a saturated cylindrical pore under unfavorable conditions are developed as a function of nine pore-scale parameters: the pore radius, nanoparticle radius, mean flow velocity, solution ionic strength, viscosity, temperature, solution dielectric constant, and nanoparticle and collector surface potentials. Advection-diffusion equations for nanoparticle transport are prescribed for the bulk and diffusion regions, while the interaction between the diffusion and potential regions is included as a boundary condition. This interaction is modeled as a first-order reversible kinetic adsorption. The expressions for the mass transfer rate coefficients between the diffusion and the potential regions are derived in terms of the interaction energy profile between the nanoparticle and the collector. The resulting equations are solved numerically for a range of values of pore-scale parameters. The nanoparticle concentration profile obtained for the cylindrical pore is averaged over a moving averaging volume within the pore in order to get the 1-D concentration field. The latter is fitted to the 1-D advection-dispersion equation with an equilibrium or kinetic adsorption model to determine the values of the average deposition rate coefficients. Pore-scale simulations are performed for three values of Péclet number, Pe = 0.05, 5 and 50. It is found that under unfavorable conditions, the nanoparticle deposition at pore scale is best described by an equilibrium model at low Péclet numbers (Pe = 0.05), and by a kinetic model at high Péclet numbers (Pe = 50). But, at an intermediate Pe (e.g., near Pe = 5), both equilibrium and kinetic models fit the 1-D concentration field. Correlation equations for the pore-averaged nanoparticle deposition rate coefficients under unfavorable conditions are derived by performing a multiple-linear regression analysis between the estimated deposition rate coefficients for a single pore and various pore-scale parameters. The correlation equations, which follow a power law relationship with nine pore-scale parameters, are found to be consistent with the column-scale and pore-scale experimental results, and qualitatively agree with CFT. Nanoparticle transport is upscaled from pore to the Darcy scale in saturated porous media by incorporating the correlations equations for the pore-averaged deposition rate coefficients of nanoparticles in a cylindrical pore into a multi-directional pore-network model, PoreFlow (Raoof et al., 2013). Pore-network model simulations are performed for a range of parameter values, and nanoparticle BTCs are obtained from the pore-network model. Those curves are then modeled using 1-D advection-dispersion equation with a two-site first-order reversible deposition, with terms accounting for both equilibrium and kinetic sorption. Kinetic sorption is found to become important as the favorability of the surface for deposition decreases. Correlation equations for the Darcy¬scale deposition rate coefficients under unfavorable conditions are developed as a function of various measurable Darcy-scale parameters, including: porosity, mean pore throat radius, mean pore water velocity, nanoparticle radius, ionic strength, dielectric constant, viscosity, temperature, and surface potentials on the nanoparticle and grain surface. The correlation equations are found to be consistent with the observed trends from the column experiments available in the literature, and are in agreement with CFT for all parameters, except for the mean pore water velocity and nanoparticle radius. The Darcy-scale correlation equations contain multipliers whose values for a given set of experimental conditions need to be determined by comparing the values of the deposition rate coefficients predicted by the correlation equations against the estimated values of Darcy-scale deposition parameters obtained by fitting the BTCs from column or field experiments with 1-D advection-dispersion-deposition model. They account for the effect of factors not considered in this study, such as the physical and chemical heterogeneity of the grain surface and nanoparticles, flow stagnation points, grain-to-grain contacts, etc. Colloids are abundant in the subsurface and have been observed to interact with a variety of contaminants, including viruses, thereby significantly influencing their transport. A mathematical model is developed to simulate the co-transport of viruses and colloids in partially saturated porous media under steady state flow conditions. The virus attachment to the mobile and immobile colloids is described using a linear reversible kinetic model. It is assumed that colloid transport is not affected by the presence of attached viruses on its surface, and hence, colloid transport is decoupled from virus transport. The governing equations are solved numerically using an alternating three-step operator splitting approach. The model is verified by fitting three sets of experimental data published in the literature: (1) Syngouna and Chrysikopoulos (2013) and (2) Walshe et al. (2010), both on the co-transport of viruses and clay colloids under saturated conditions, and (3) Syngouna and Chrysikopoulos (2015) for the co-transport of viruses and clay colloids under unsaturated conditions. The model results are found to be in good agreement with the observed BTCs under both saturated and unsaturated conditions. Then, the developed model was used to simulate the co-transport of viruses and colloids in porous media under unsaturated conditions, with the aim of understanding the relative importance of various processes on the co-transport of viruses and colloids. The virus retention in porous media in the presence of colloids is greater under unsaturated conditions as compared to the saturated conditions due to: (1) virus attachment to the air-water interface (AWI), and (2) co-deposition of colloids with attached viruses on its surface to the AWI. A sensitivity analysis of the model to various parameters showed that virus attachment to AWI is the most sensitive parameter affecting the BTCs of both free viruses and total mobile viruses, and has a significant effect on all parts of the BTC. The free and the total mobile virus BTCs are mainly influenced by parameters describing virus attachment to the AWI, virus interactions with mobile and immobile colloids, virus attachment to solid-water interface (SWI), and colloid interactions with SWI and AWI. The virus BTC is relatively insensitive to parameters describing the maximum adsorption capacity of the AWI for colloids, inlet colloid concentration, virus detachment rate coefficient from the SWI, maximum adsorption capacity of the AWI for viruses, and inlet virus concentration.

<p>To increase the understanding of contaminant transport, specifically biocolloid transport in fractured media, a series of experiments were conducted on single saturated fractures. Hydraulic and solute tracer tests were used to characterize three separate fractures: one natural fracture and two synthetic fractures. Zeta potentials are reported showing the high negative electric charge of the synthetic fractures relative to the natural fractures in the phosphate buffer solution (PBS) used during the biocolloid tracer tests.</p> <p><em>E. coli</em> RS2-GFP tracer tests were conducted on all three fractures at specific discharges of 5 m/d, 10 m/d and 30 m/d. Lower <em>E. coli</em> recovery was consistently observed in the natural fracture, due to 1) attachment because of the lower negative charge of the natural fracture relative to the synthetic fracture; and 2) the presence of dead end fractures within the fracture matrix. In the synthetic fractures, where surface charges were equal, in the larger, more variable fracture aperture, lower recoveries were found when compared to the smaller, less variable fracture aperture, which was not expected. This indicates that aperture variability plays a larger role than fracture aperture size in the retention of biocolloids in fractures.</p> <p>Differential transport was consistently observed in all three fractures, but was more prominent in the synthetic fractures. This indicates that charge exclusion plays a more dominant role in the differential transport of colloids than size exclusion, though size exclusion cannot be eliminated as a retention mechanism based on these experiments. Differential transport was also heavily influenced by specific discharge as the difference in arrival times between the bromide and <em>E. coli</em> increased in all three fractures as the specific discharge decreased.</p> <p>Visualization tests were completed on the synthetic fractures showing the location of multiple preferential flow paths, as well as areas with low flow.</p>