However, most contaminated DOE sites are characterized by substantial heterogeneity that can involve fracture-flow or adjacent strata that vary in hydraulic conductivity by a factor of several powers of 10. In heterogeneous aquifers dominated by preferential flow paths, bacteria can be excluded on the basis of size and porosity from substantial fractions that are accessible to the conservative tracer. Injection and recovery investigations involving fractured-rock aquifers suggest that transport of bacteria in fracture-flow environments can be much faster than that of the conservative tracer.9 That phenomenon has also been demonstrated in the laboratory10 for highly stratified formations in which the major conductive zone is next to a layer characterized by porosity too fine for access by microorganisms.

On the larger scales that are typically involved in natural attenuation or engineered restoration of contaminated aquifers, even the relatively homogeneous sites look heterogeneous, at least for the purposes of predictive modeling of bacterial transport. It is evident that the simple deterministic models that are typically used to describe small-scale (<<10 m) bacterial transport in saturated porous media are not adequate for larger-scale application at most, if not all, contaminated DOE sites. That is, in part, because of the aforementioned geohydrologic complexities that often become manifest at larger scales. Bacterial transport models are needed that can account for physical variability in aquifer structure, perhaps in a manner consistent with the application of stochastic theory to describe mathematically the large-scale (>100 m) movement of conservative solutes in sandy aquifer sediments.11

Reconciling Discrepancies Between Field And Laboratory Results

Much of the detailed information about bacterial transport behavior in saturated granular media derives from flow-through column experiments involving repacked subsurface material. A number of the more-recent studies on this subject were sponsored by DOE.7,12-14 Typically, filtration theory is used to describe the removal of unattached bacteria being advected downgradient through granular media. In the filtration model, a bacterium's affinity for attachment to the grain surfaces that it comes into contact with is quantified with a collision efficiency (a) factor,5 whose value can vary from 0 to 1.15 Recent findings from flow-through column experiments involving bacteria7,12-14 and bacteriophage16 transport through representative granular media under reasonable chemical and hydrologic conditions suggest little microbial mobility and collision efficiencies of 10-2 to 100. In contrast, recent field observations of bacteria5,17 and bacteriophage18 transport through aquifer sediments suggest substantial mobility with corresponding collision efficiencies of 10-4 to 10-2.

The discrepancy between field and laboratory results can be explained, in part, by the destruction of pore structure when subsurface soil is repacked into columns. It has been demonstrated that, when intact subsurface soils are repacked, bacterial transport is greatly diminished because of the destruction of preferential flow paths.19 It has also been shown that in situ transport of bacteria through undisturbed aquifer sediments can be different from that in core material taken from the same location.20 A major advantage of flow-through columns is that they offer a much greater degree of control over experimental conditions. However, the apparent discrepancies between field and laboratory results, and the growing recognition that processes that control subsurface microbial transport behavior can be interrelated and operate on spatial and temporal scales that are not conducive for laboratory study, emphasize the need for more in situ transport studies. Accurate assessment of the potential role of bacterial transport in the restoration of contaminated groundwater at many DOE sites might require injection-and-recovery experiments on scales that are larger than those of previous studies. However, because much of the emphasis over the last 2 decades has been on flow-through column experiments, development of methods for conducting large field-scale investigations of subsurface bacterial transport has lagged.8

Biologic Factors

Most of the previous research on the controls of subsurface bacterial transport has focused on physical and chemical factors. Immobilization at stationary surfaces is a major determinant of the extent to which bacteria move within aquifers. However, many of the other important determinants of a bacterium's fate and transport in aquifers are biologic,21 and the biologic controls of subsurface bacterial transport have been largely ignored. Although protozoa are known to be common features of both shallow22 and deep23 groundwater habitats, their role

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