Transport and fate of colloids and microbes in granular aqueous environments (original) (raw)
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Water Research, 2007
Because of heterogeneity among members of a bacteria population, deposition rates of bacteria may decrease upon the distance bacteria are transported in an aquifer. Such deposition rate decreases may result in retained bacteria concentrations, which decrease hyper-exponentially as a function of transport distance, and may therefore significantly affect the transport of colloids in aquifers. We investigated the occurrence of hyperexponential deposition of Escherichia coli, an important indicator for fecal contamination, and the causes for such behavior. In a series of column experiments with glass beads of various sizes, we found that attachment of E. coli decreased hyper-exponentially, or, on logarithmic scale in a bimodal way, as a function of the transported distance from the column inlet. From data fitting of the retained bacteria concentration profiles, the sticking efficiency of 40% of the E. coli population was high (a ¼ 1), while the sticking efficiency of 60% was low (a ¼ 0.01). From the E. coli total population, an E. coli subpopulation consisting of slow attachers could be isolated by means of column passage. In subsequent column experiments this subpopulation attached less than the E. coli total population, consisting of both slow and fast attachers. We concluded that the main driver for the observed dual mode deposition was heterogeneity among members of the bacteria population. Intrapopulation may result in some microbes traveling surprisingly high distances in the subsurface. Extending the colloid filtration theory with intra-population variability may provide a valuable framework for assessing the transport of bacteria in aquifers.
Journal of Contaminant Hydrology, 2002
In riverbank filtration, contaminant transport is affected by colloidal particles such as dissolved organic matter (DOM) and bacterial particles. In addition, the subsurface heterogeneity influences the behavior of contaminant transport in riverbank filtration. A mathematical model is developed to describe the contaminant transport in dual-porosity media in the presence of DOM and bacteria as mobile colloids. In the model development, a porous medium is divided into the mobile and immobile regions to consider the presence of ineffective micropores in physically heterogeneous riverbanks. We assume that the contaminant transport in the mobile region is controlled by the advection and dispersion while the contaminant transport in the immobile region occurs due to the molecular diffusion. The contaminant transfer between the mobile and immobile regions takes place by diffusive mass transfer. The mobile region is conceptualized as a four-phase system: two mobile colloidal phases, an aqueous phase, and a solid matrix. The complete set of governing equations is solved numerically with a fully implicit finite difference method. The model results show that in riverbank filtration, the contaminant can migrate further than expected due to the presence of DOM and bacteria. In addition, the contaminant mobility increases further in the presence of the immobile region in aquifers. A sensitivity analysis shows that in dual-porosity media, earlier breakthrough of the contaminant takes place as the volumetric fraction of the mobile region decreases. It is also demonstrated that as the contaminant mass transfer rate coefficient between the mobile and immobile regions increases, the contaminant concentration gradient between the two regions reverses at earlier pore volumes. The contaminant mass transfer coefficient between the mobile and immobile regions mainly controls the tailing effect of the contaminant breakthrough. The contaminant breakthrough curves are sensitive to changes in contaminant adsorption and desorption rate coefficients on DOM and bacteria. In situations where the contaminant is released in the presence of DOM and bacteria in
Intermittent filtration of bacteria and colloids in porous media
Water Resources Research, 2005
1] Intermittent filtration through porous media used for water and wastewater treatment can achieve high pathogen and colloid removal efficiencies. To predict the removal of bacteria, the effects of cyclic infiltration and draining events (transient unsaturated flow) were investigated. Using physical micromodels, we visualized the intermittent transport of bacteria and other colloids in unsaturated porous media. Column experiments provided quantitative measurements of the phenomena observed at the pore scale. Tagged Escherichia coli and a conservative tracer (NaI) were introduced in an initial pulse into a 1.5 m sand column. Subsequent hydraulic flushes without tagged bacteria or tracer were repeated every 4 hours for the next 4 days, during which outflow concentrations were monitored. Breakthrough behavior between colloids and dissolved tracer differed significantly, reflecting the differences in transport processes. Advancement of the wetting front remobilized bacteria which were held in thin water films, attached to the airwater interface (AWI), or entrapped in stagnant pore water between gas bubbles. In contrast, the tracer was only remobilized by diffusion from immobile to mobile water. Remobilization led to successive concentration peaks of bacteria and tracer in the effluent but with significant temporal differences. Observations at the pore-scale indicated that the colloids were essentially irreversibly attached to the solid-water interface, which explained to some extent the high removal efficiency of microbes in the porous media. Straining, cluster filtration, cell lysis, protozoa grazing, and bacteriophage parasitism could also contribute to the removal efficiency of bacteria.
Environmental science & Technology, 1991
A filtration model commonly used to describe removal of colloids during packed-bed filtration in water treatment applications was modified for describing downgradient transport of bacteria in sandy, aquifer sediments. The modified model was applied to the results of a small-scale (7 m), natural-gradient tracer test and to observations of an indigenous bacterial population moving downgradient within a plume of organically contaminated groundwater in Cape Cod, MA. The model reasonably accounted for concentration histories of labeled bacteria appearing at samplers downgradient from the injection well in the tracer experiment and for the observed 0.25-mu-m increase in average cell length for an unlabeled, indigenous bacterial population, 0.6 km downgradient from the source of the plume. Several uncertainties were apparent in applying filtration theory to problems involving transport of bacteria in groundwater. However, adsorption (attachment) appeared to be a major control of the extent of bacterial movement downgradient, which could be described, in part, by filtration theory. Estimates of the collision efficiency factor, which represents the physicochemical factors that determine adsorption of the bacteria onto the grain surfaces, ranged from 5.4 x 10(-3) to 9.7 x 10(-3).
Colloid-facilitated groundwater contaminant transport
Water Resources Research, 1993
•tation Colloidal particles or dissolved organic matter (DOM) can act as carriers to enhance the transport of contaminants in groundwater by reducing retardation effects. When either of these materials is present, the system can be treated as consisting of three phases: an aqueous phase, a carrier phase, and the stationary solid matrix phase. The contaminant may be present in either or all of these phases. In the work reported, a mathematical model was developed to describe the transport and fate of the contaminant and carrier material in a porous medium. The model is based on mass balance equations describing the transport and fate of the contaminant and carrier in a three-phase medium. Colloid/ contaminant and colloid/matrix mass transfer mechanisms are represented by first-order kinetics. Equilibrium partitioning of DOM acting as a carrier of the contaminant introduces a significant simplification in the model formulation. For a constant DOM concentration a much smaller retardation coefficient can be obtained in the three-phase system than the coefficient obtained in a conventional advective/dispersive transport equation for a two-phase system. The modified retardation coefficient reflects the presence of the mobile carrier by incorporating both the sorption of the contaminant and capture of the carrier on the solid matrix. Numerical solutions for the model were obtained by using a finite difference scheme to provide estimates of contaminant and carrier concentrations. Significant sensitivities to model parameters, particularly the rate constants of carrier capture and sorption were discovered. The numerical results of the DOM carder effect matched favorably with experimental data reported in the literature. INTRODUCTION The role of colloids in groundwater contaminant transport has recently attracted attention due to its importance in water management and remediation efforts. It has been reported that mobile subsurface colloids can carry groundwater contaminants adsorbed onto their surfaces [McCarthy and Zachara, 1989]. In liquid waste disposal projects, groundwater quality can be endangered by suspended clay and silt particles migrating from the formation adjacent to the well bore. Hydrophobic organic contaminants leaking from containers can be adsorbed on particles in the clay liners under landfills. These contaminated clay particles can be mobilized due to contact with a rising water table and then transported to water wells by groundwater flow. Piping of earth structures such as compacted soils also introduces fines into groundwater, facilitating the contaminant transport. Keeb, [1989] noted that pumping often mobilizes colloidal particles from the aquifer, thus complicating remediations. Keely has reported that retardation of contaminants in groundwater can be short-circuited by particle-facilitated transport. Clay particles in drilling muds can also migrate into the aquifer formation during we!l-drilling operations and carry contaminants on them [Corapcioglu, 1988; Corapcioglu and Abboud, 1990]. Therefore groundwater contaminants such as organics or dissolved metals adhered to colloid surfaces by sorption, ion exchange, or other means may migrate to far greater distances than predicted by using the conventional advective/dispersive transport equation with normal retardation values. The significance of colloid-facilitated subsurface contaminant migration has been reviewed by McCarthy and Zachara [1989]. McCarthy and Zachara noted that since the 'composition of mobilized colloidal particles is similar to that Copyright 1993 by the American Geophysical Union. Paper number 93WR00404. 0043-1397/93/93 WR-00404505.00 of immobile soil particles in groundwater reservoirs, colloids can adsorb contaminants in a similar fashion and maintain them in the mobile phase. Detachment of immobile colloids depends on the chemistry and hydraulic conditions prevailing in the aquifer. McCarthy and Zachara [1989] further noted that failure to account for the role of colloids in facilitating contaminant transport can lead to serious underestimation of the distances the groundwater contaminants can migrate. At two separate sites at Los Alamos, New Mexico, plutonium and americium were detected at distances much further than predicted by conventional techniques. Colloid-controlled transport of radionuclides in groundwater has also been reported by yon Gunten et al. [1988]. Vinten et al. [1983] presented data showing that pesticides with a very high distribution coefficient are not immobile in the soil. Under favorable conditions, 18% of the applied DDT was transported on solids in sewage effluent to a depth greater than 9 cm in a sandy loam soil column. Vinten et al. also reported the transport of 50% of the applied Li-montmori!lonite particles suspended in soil water. They concluded that the extent of facilitated pesticide transport depends on the clay or organic matter mobilized from the topsoil, colloid mobility and capture, the mass partition coefficient between the pesticide and colloids, and the kinetics of desorption of pesticide from the mobile colloid surfaces. Kan and Tomson [1990] obtained similar results and showed that dissolved organic matter (DOM) can increase the migration of highly hydrophobic substances, such as DDT, by a factor of a thousand or more. Magee et al. [ 1991] demonstrated that the retardation factor of phenanthrene in a quarry sand column was reduced by an average factor of !.8 in the presence of soil DOM. Their analysis indicated that distribution coefficients between phenanthrene, and DOM and soil matrix, as well as DOM concentration are the critical factors affecting the phenanthrene migration. Puls and Powell [1992] took other parameters such as flow rate, p H, ionic strength, electrolyte composition, and colloid size 2215 ' .
Physical and chemical factors influencing transport of microorganisms through porous media
Applied and Environmental Microbiology, 1991
Resting-cell suspensions of bacteria isolated from groundwater were added as a pulse to the tops of columns of clean quartz sand. An artificial groundwater solution (AGW) was pumped through the columns, and bacterial breakthrough curves were established and compared to test the effects of ionic strength of the AGW, cell size (by using strains of similar cell surface hydrophobicity but different size), mineral grain size, and presence of heterogeneities within the porous media on transport of the bacteria. The proportion of cells recovered in the effluent ranged from nearly 90% for AGW of a higher ionic strength (I = 0.0089 versus 0.00089 m), small cells (0.75-micron-diameter spheres versus 0.75 by 1.8-micron rods), and coarse-grained sand (1.0 versus 0.33 mm) to less than 1% for AGW of lower ionic strength, large cells, and fine-grained sand. Differences in the widths of peaks (an indicator of dispersion) were significant only for the cell size treatment. For treatments containing h...