Effects of transverse mixing on transport of bacteria through heterogeneous porous media

Abstract

Abstract. In this paper, we examine two questions: (1) can the effects of transverse mixing of bacteria in a system constructed to have a permeability discontinuity in the direction parallel to the flow be measured; and (2) if the effects are measurable, can they be calculated using a transverse dispersion coefficient estimated from experiments using a conservative tracer? Pulses of chloride and bacteria were transported downward through heterogeneous columns constructed with a tubule of coarse, quartz sand surrounding an annulus of fine, quartz sand. Pulses of each were also transported through homogeneous columns of the two sands. Doubly peaked breakthrough curves resulted from the columns containing two distinct sand sizes. Modeling of the breakthrough curves was performed taking into account advection, dispersion, deposition, entrainment, and pore-size exclusion. The results revealed that transverse mixing does occur during transport of bacteria through heterogeneous material and that this mixing can be estimated using a conservative tracer. IntroductionProcesses affecting the transport of bacteria through aquifers have been the focus of increased scientific investigation recently. As with the transport of dissolved constituents through soils and rocks, the transport of bacteria by ground water is conditioned strongly by heterogeneities in the medium. In many aquifers, flow is mainly parallel to the dip of hydrostratigraphic units that define major heterogeneities.

Noninvasive Quantitative Measurement of Bacterial Growth in Porous Media under Unsaturated-Flow Conditions

“…A number of studies in the literature indicate that under unsaturated conditions, bacteria may associate with air-water interfaces and are not necessarily attached to solid surfaces (see, e.g., references 25 and 41). Furthermore, it is well documented that physical heterogeneities in porous media locally modify flow, and these perturbations influence microbial growth and transport behavior (19,20,36). In our experiments, microbial growth itself created the heterogeneity in a previously homogeneous porous medium, thereby perturbing flow and promoting spatial heterogeneity in the microbial colony.…”

Role of chemotaxis in the transport of bacteria through saturated porous media

“…Morley et al [45] investigated dispersion of non-motile bacteria transverse to flow in laboratory columns. The columns were packed to create a vein of coarse-grain sand in the center surrounded by an annulus of fine-grain sand.…”

Colloid particle size‐dependent dispersivity

“…The velocity enhancement of colloid particles, which is often observed in studies of colloid and biocolloid transport in fractures and micromodels, is commonly attributed to colloid exclusion from the lower-velocity regions [James and Chrysikopoulos, 2003;Auset and Keller, 2004]. However, in porous media, in addition to colloid exclusion from the lower-velocity regions [Scheibe and Wood, 2003], colloid particles experience a substantial reduction of the effective porosity [Morley et al, 1998], as illustrated in Figure 5. Essentially, the volume of void-space accessible by colloids or colloid effective porosity decreases as the particle size of the colloids increases, because colloids do not enter pore spaces with opening smaller than d p .…”

“…Early breakthrough of colloids and biocolloids as compared to that of conservative tracers has been observed in several studies [Bales et al, 1989;Toran and Palumbo, 1992;Powelson et al, 1993;Grindrod et al, 1996;Dong et al, 2002;Keller et al, 2004;Vasiliadou and Chrysikopoulos, 2011;Sinton et al, 2012;Syngouna and Chrysikopoulos, 2013;Chrysikopoulos and Syngouna, 2014]. Colloid early breakthrough can be attributed to effective porosity reduction (colloids cannot penetrate smaller pores due to their inability to fit into them), preferential flow paths through high-conductivity regions, and exclusion from the lower-velocity regions [Chrysikopoulos and Abdel-Salam, 1997;Dong et al, 2002;Ginn, 2002;Ahfir et al, 2009], which can also be viewed as a reduction of the effective porosity of the porous medium [Morley et al, 1998]. The finite size of a colloid particle excludes it from the slowest moving portion of the parabolic velocity profile within a fracture or a pore throat, thus the effective particle velocity is increased, while the overall particle dispersion is reduced compared to Taylor dispersion, but with a tendency to increase with increasing particle size over a certain range of particle diameters Chrysikopoulos, 2000, 2003].…”