Role of physical heterogeneity in the interpretation of small‐scale laboratory and field observations of bacteria, microbial‐sized microsphere, and bromide transport through aquifer sediments

Abstract

The effect of physical variability upon the relative transport behavior of microbial-sized microspheres, indigenous bacteria, and bromide was examined in field and flow-through column studies for a layered, but relatively well sorted, sandy glacioftuvial aquifer. These investigations involved repacked, sieved, and undisturbed aquifer sediments. In the field, peak abundance of labeled bacteria traveling laterally with groundwater flow 6 m downgradient from point of injection was coincident with the retarded peak of carboxylated microspheres (retardation factor, RF = 1.7) at the 8.8 m depth, but preceded the bromide peak and the retarded microsphere peak (RF = 1.5) at the 9.0 m depth. At the 9.5 m depth, the bacterial peak was coincident with both the bromide and the microsphere peaks. Although sorption appeared to be a predominant mechanism responsible for immobilization of microbial-sized microspheres in the aquifer, straining apl•ared to be primarily responsible for their removal in 0.6-m-long columns of repacked, unsieved aquifer sediments. The manner in which the columns were packed also affected optimal size for microsphere transport, which in one experiment was near the size of the small (•-2/•m) groundwater protozoa (flagellates). These data suggest that variability in aquifer sediment structure can be important in interpretation of both small-scale field and laboratory experiments examining microbial transport behavior. of Technol., Cambridge, 1988.

A brief review of bacterial transport in natural porous media

“…Thus, .exclusion is expected in microbial transport experiments in such'media, where the microbe size is roughly two orders of magnitude smaller than the media grain size. The effect is more pronounced at larger observation scales, as has been reported in many recent works [Wood and Ehrlich 1978 (although it was concluded that the aquifer was actually sorbing the bromide tracer); Pyle 1979; Engfield and Bengtsson 1988; Harvey et al 1989Harvey et al , 1993Powelson et al 1993 (using viruses); Shonnard et al 19941. …”

Processes in microbial transport in the natural subsurface

“…Behavior of bacteria and similar-sized microspheres was observed to be significantly different. Harvey and Garabedian [51] applied filtration theory to quantitatively model transport of bacteria in a subsequent naturalgradient experiment at the same site, and estimated a collision efficiency of 5 Â 10 À3 to 1 Â 10 À2 at a travel distance of 6.9 m. Harvey et al [52] demonstrated the importance of physical heterogeneity in controlling field-scale bacterial transport. Observations of micro-sphere, bromide, and bacterial transport 6 m downgradient of the injection point varied significantly in three sampling ports separated by a total vertical distance of 0.7 m. Bales et al [5] observed bacterial breakthrough at a distance of 11 m, but their experiment focused on effects of pH on phage (virus) attenuation and remobilization.…”

“…Harvey et al [52] reported dramatically different character of bacterial breakthrough at three adjacent elevations in a single multi-level sampler, separated by only tens of centimeters. At the South Oyster site, significant effort was invested in characterization of the spatial structure of physical aquifer properties using geological, hydrologic, and geophysical techniques [19,58,59].…”

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

“…Transverse mixing across layer interfaces is, in some ways, conceptually similar to exchanges among multiple mobile zones. Harvey et al [1993] speculated that the effects of such transverse mixing were negligible for their experimental conditions, finding that the composite breakthrough in a fully penetrating well could be reproduced by simply adding the contribution of three separate layers ignoring communication among them. The data from the field experiments described in Harvey et al [1993] are not precise 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 (what we will call a structured heterogeneity) 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?…”