Straining, Attachment, and Detachment of Cryptosporidium Oocysts in Saturated Porous Media
TL;DR: Experimental and modeling studies were undertaken to examine the roles of attachment, detachment, and straining on oocyst transport and retention, and a more physically realistic description of the data was obtained by modeling attachment, detachments, andstraining.
Abstract: Accurate knowledge of the transport and deposition behavior for pathogenic Cryptosporidium parvum oocysts is needed to assess contamination and protect water resources. Experimental and modeling studies were undertaken to examine the roles of attachment, detachment, and straining on oocyst transport and retention. Saturated column studies were conducted using Ottawa aquifer sands (U.S. Silica, Ottawa, IL) with median grain sizes of 710, 360, and 150 microm. Decreasing the median sand size tended to produce lower effluent concentrations, greater oocyst retention in the sand near the column inlet, and breakthrough of oocysts at later times. Oocyst transport data also exhibited concentration tailing. Mathematical modeling of the oocyst transport data using fitted first-order attachment and detachment coefficients provided a satisfactory description of the observed effluent concentration curves, but a poor characterization of the oocyst spatial distribution. Modeling of these data using an irreversible straining term that is depth dependent provided a better description of the oocyst spatial distribution, but could not account for the observed effluent concentration tailing or late breakthrough times. A more physically realistic description of the data was obtained by modeling attachment, detachment, and straining. The percentage of total oocysts retained by straining was estimated from effluent mass balance considerations to be 68% for 710-microm sand, 79% for 360-microm sand, and 87% for 150-microm sand. Straining coefficients were then selected to achieve these percentages of total oocyst retention, and attachment and detachment coefficients were fitted to the effluent concentration curves. Dramatic differences in the predicted oocyst breakthrough curves were observed at greater transport distances for the various model formulations (inclusion or exclusion of straining). Justification for oocyst straining was provided by trends in the transport data, simulation results, pore size distribution information, and published literature.
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TL;DR: Observations suggest that the extent of colloid removal by straining is strongly coupled to solution chemistry.
342 citations
Cites background or methods from "Straining, Attachment, and Detachme..."
...In this case, only a small portion of the deposited colloids was recovered in the effluent and the majority was still retained in the sand....
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...Effluent concentration curves and hyperexponential deposition profiles were strongly dependent on the solution chemistry, the system hydrodynamics, and the colloid and collector grain size, with greater deposition occurring for increasing ionic strength, lower flow rates, and larger ratios of the…...
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...…that the pore structure can play an important role in colloid deposition under unfavorable attachment conditions (Cushing and Lawler, 1998; Bradford et al., 2002, 2003, 2004, 2005, 2006a–d; Li et al., 2004, 2006a, b; Tufenkji et al., 2004; Bradford and Bettahar, 2005, 2006; Foppen et al., 2005)....
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...Negatively charged latex microspheres (1.1 and 3 mm) and quartz sands (360, 240, and 150mm) were used in packed column studies that encompassed a range in suspension ionic strengths (6–106 mM) and Darcy water velocities (0.1–0.45 cm min 1)....
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TL;DR: A review of colloid transport and retention at the interface, collector, and pore scales can be found in this article, where the potential for colloid attachment in the presence of hydrodynamic forces and torques is determined from a balance of applied and adhesive torques.
Abstract: Our ability to accurately simulate the transport and retention of colloids in the vadose zone is currently limited by our lack of basic understanding of colloid retention processes that occur at the pore scale. This review discusses our current knowledge of physical and chemical mechanisms, factors, and models of colloid transport and retention at the interface, collector, and pore scales. The interface scale is well suited for studying the interaction energy and hydrodynamic forces and torques that act on colloids near interfaces. Solid surface roughness is reported to have a significant influence on both adhesive and applied hydrodynamic forces and torques, whereas non-Derjaguin–Landau–Verwey–Overbeek (DLVO) forces such as hydrophobic and capillary forces are likely to play a significant role in colloid interactions with the air–water interface. The flow field can be solved and mass transfer processes can be quantified at the collector scale. Here the potential for colloid attachment in the presence of hydrodynamic forces is determined from a balance of applied and adhesive torques. The fraction of the collector surface that contributes to attachment has been demonstrated to depend on both physical and chemical conditions. Processes of colloid mass transfer and retention can also be calculated at the pore scale. Differences in collector- and pore-scale studies occur as a result of the presence of small pore spaces that are associated with multiple interfaces and zones of relative flow stagnation. Here a variety of straining processes may occur in saturated and unsaturated systems, as well as colloid size exclusion. Our current knowledge of straining processes is still incomplete, but recent research indicates a strong coupling of hydrodynamics, solution chemistry, and colloid concentration on these processes, as well as a dependency on the size of the colloid, the solid grain, and the water content.
330 citations
Cites background from "Straining, Attachment, and Detachme..."
...Other researchers have suggested that deposition may occur as a result of physical factors that are not included in fi ltration theory, such as straining (deposition of colloids in small pore spaces such as those formed at grain–grain contacts) (Cushing and Lawler, 1998; Bradford et al., 2002, 2003, 2004, 2005, 2006a,b; Li et al., 2004; Tufenkji et al., 2004; Bradford and Bettahar, 2005; Foppen et al., 2005), soil surface roughness (Kretzschmar et al....
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...In contrast, under saturated conditions that are unfavorable for attachment (when repulsive electrostatic interactions exist between the colloids and the grain surfaces), retained colloids frequently exhibit a depth-dependent deposition rate that produces hyperexponential (a decreasing rate of deposition coeffi cient with distance) (Albinger et al., 1994; Baygents et al., 1998; Simoni et al., 1998; Bolster et al., 2000; DeFlaun et al., 1997; Zhang et al., 2001; Redman et al., 2001; Bradford et al., 2002; Li et al., 2004; Bradford and Bettahar, 2005) or nonmonotonic (a peak in retained colloids away from the injection source) (Tong et al....
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..., 2003; Tufenkji and Elimelech, 2005a) and at larger transport distances (Bolster et al., 2000; Bradford and Bettahar, 2005)....
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TL;DR: In this review, the major processes known to determine the underground transport of E. coli (attachment, straining and inactivation) are evaluated and sticking efficiencies determined from field experiments were lower than those determined under laboratory conditions.
267 citations
TL;DR: In this paper, the authors highlight recent experimental evidence that indicates that straining can play an important role in colloid deposition under unfavorable attachment conditions and may explain many of the reported limitations of filtration theory.
Abstract: [1] Considerable research suggests that colloid deposition in porous media is frequently not consistent with filtration theory predictions under unfavorable attachment conditions. Filtration theory was developed from an analysis of colloid attachment to the solid-water interface of a single spherical grain collector and therefore does not include the potential influence of pore structure, grain-grain junctions, and surface roughness on straining deposition. This work highlights recent experimental evidence that indicates that straining can play an important role in colloid deposition under unfavorable attachment conditions and may explain many of the reported limitations of filtration theory. This conclusion is based upon pore size distribution information, size exclusion, time- and concentration-dependent deposition behavior, colloid size distribution information, hyperexponential deposition profiles, the dependence of deposition on colloid and porous medium size, batch release rates, micromodel observations, and deposition at textural interfaces. The implications of straining in unsaturated and heterogeneous systems are also discussed, as well as the potential influence of system solution chemistry and hydrodynamics. The inability of attachment theory predictions to describe experimental colloid transport data under unfavorable conditions is demonstrated. Specific tests to identify the occurrence and/or absence of straining and attachment are proposed.
257 citations
Cites background from "Straining, Attachment, and Detachme..."
...…theory, such as straining [Cushing and Lawler, 1998; Bradford et al., 2002, 2003, 2004, 2005, 2006a; Li et al., 2004; Tufenkji et al., 2004; Bradford and Bettahar, 2005, 2006; Foppen et al., 2005], soil surface roughness [Kretzschmar et al., 1997; Redman et al., 2001], and hydrodynamic…...
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TL;DR: In this article, column transport experiments using latex microspheres as the colloids and quartz sand as the porous medium were conducted to investigate the response of colloid straining to changes in colloid diameter (dp) and sand grain diameter (dg).
Abstract: [1] Straining may influence the mobility of colloid-sized particles within groundwater aquifers as well as within granular filters that are used in wastewater treatment. We conducted column transport experiments using latex microspheres as the colloids and quartz sand as the porous medium to investigate the response of colloid straining to changes in colloid diameter (dp) and sand grain diameter (dg). For these experiments the negatively charged microspheres were suspended in deionized water, and the quartz sand was thoroughly cleaned to minimize physicochemical deposition (attachment), which permitted the determination of straining in an unambiguous way. The measurements of strained (immobile phase) and effluent (aqueous phase) colloid concentrations could be described with a transport model that accounted for an exponential decline in straining rates with increasing concentrations of strained colloids. Best fit values of the model coefficient that quantified clean bed straining rates (ko) were negligibly small for dp/dg < 0.008 and, above this threshold, varied linearly with dp/dg. Our findings suggest that accurate inferences on the mobility of colloid-sized particles will require consideration of the effects of straining when dp/dg exceeds 0.008.
223 citations
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TL;DR: Van Genuchten et al. as mentioned in this paper proposed a closed-form analytical expression for predicting the hydraulic conductivity of unsaturated soils based on the Mualem theory, which can be used to predict the unsaturated hydraulic flow and mass transport in unsaturated zone.
Abstract: A new and relatively simple equation for the soil-water content-pressure head curve, 8(h), is described in this paper. The particular form of the equation enables one to derive closedform analytical expressions for the relative hydraulic conductivity, Kr, when substituted in the predictive conductivity models of N.T. Burdine or Y. Mualem. The resulting expressions for Kr(h) contain three independent parameters which may be obtained by fitting the proposed soil-water retention model to experimental data. Results obtained with the closed-form analytical expressions based on the Mualem theory are compared with observed hydraulic conductivity data for five soils with a wide range of hydraulic properties. The unsaturated hydraulic conductivity is predicted well in four out of five cases. It is found that a reasonable description of the soil-water retention curve at low water contents is important for an accurate prediction of the unsaturated hydraulic conductivity. Additional Index Words: soil-water diffusivity, soil-water retention curve. van Genuchten, M. Th. 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44:892-898. T USE OF NUMERICAL MODELS for simulating fluid flow and mass transport in the unsaturated zone has become increasingly popular the last few years. Recent literature indeed demonstrates that much effort is put into the development of such models (Reeves and Duguid, 1975; Segol, 1976; Vauclin et al., 1979). Unfortunately, it appears that the ability to fully characterize the simulated system has not kept pace with the numerical and modeling expertise. Probably the single most important factor limiting the successful application of unsaturated flow theory to actual field problems is the lack of information regarding the parameters entering the governing transfer equations. Reliable estimates of the unsaturated hydraulic conductivity are especially difficult to obtain, partly because of its extensive variability in the field, and partly because measuring this parameter is time-consuming and expensive. Several investigators have, for these reasons, used models for calculating the unsaturated conductivity from the more easily measured soil-water retention curve. Very popular among these models has been the Millington-Quirk method (Millington and Quirk, 1961), various forms of which have been applied with some success in a number of studies (cf. Jackson et al., 1965; Jackson, 1972; Green and Corey, 1971; Bruce, 1972). Unfortunately, this method has the disadvantage of producing tabular results which, for example when applied to nonhomogeneous soils in multidimensional unsaturated flow models, are quite tedious to use. Closed-form analytical expressions for predicting 1 Contribution from the U. S. Salinity Laboratory, AR-SEA, USDA, Riverside, CA 92501. Received 29 June 1979. Approved 19 May I960. 'Soil Scientist, Dep. of Soil and Environmental Sciences, University of California, Riverside, CA 92521. The author is located at the U. S. Salinity Lab., 4500 Glenwood Dr., Riverside, CA 92502. the unsaturated hydraulic conductivity have also been developed. For example, Brooks and Corey (1964) and Jeppson (1974) each used an analytical expression for the conductivity based on the Burdine theory (Burdine, 1953). Brooks and Corey (1964, 1966) obtained fairly accurate predictions with their equations, even though a discontinuity is present in the slope of both the soil-water retention curve and the unsaturated hydraulic conductivity curve at some negative value of the pressure head (this point is often referred to as the bubbling pressure). Such a discontinuity sometimes prevents rapid convergence in numerical saturated-unsaturated flow problems. It also appears that predictions based on the Brooks and Corey equations are somewhat less accurate than those obtained with various forms of the (modified) Millington-Quirk method. Recently Mualem (1976a) derived a new model for predicting the hydraulic conductivity from knowledge of the soil-water retention curve and the conductivity at saturation. Mualem's derivation leads to a simple integral formula for the unsaturated hydraulic conductivity which enables one to derive closed-form analytical expressions, provided suitable equations for the soil-water retention curves are available. It is the purpose of this paper to derive such expressions using an equation for the soil-water retention curve which is both continuous and has a continuous slope. The resulting conductivity models generally contain three independent parameters which may be obtained by matching the proposed soil-water retention curve to experimental data. Results obtained with the closedform equations based on the Mualem theory will be compared with observed data for a few soils having widely varying hydraulic properties. THEORETICAL Equations Based on Mualem's Model The following equation was derived by Mualem (1976a) for predicting the relative hydraulic conductivity (Kr) from knowledge of the soil-water retention curve
22,781 citations
TL;DR: In this paper, a method is presented for developing probability density functions for parameters of soil moisture relationships of capillary head [h(θ)] and hydraulic conductivity [K(α), which are required for the assessment of water flow and solute transport in unsaturated media.
Abstract: A method is presented for developing probability density functions for parameters of soil moisture relationships of capillary head [h(θ)] and hydraulic conductivity [K(θ)]. These soil moisture parameters are required for the assessment of water flow and solute transport in unsaturated media. The method employs a statistical multiple regression equation proposed in the literature for estimating [h(θ)] or [K(θ)] relationships using the soil saturated water content and the percentages of sand and clay. In the absence of known statistical distributions for either [h(θ)] or [K(θ)] relationships, the method facilitates modeling by providing variability estimates that can be used to examine the uncertainty associated with water flow or solute transport in unsaturated media.
2,050 citations
TL;DR: In this paper, the authors present theories describing colloid mobilization, deposition, and transport, laboratory experiments in model systems designed to test these theories, and applications of these theories to colloid-facilitated transport experiments in natural groundwater systems.
1,145 citations
810 citations