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This is a review of physical, chemical, and biological processes governing microbial transport in the saturated subsurface. We begin with the conceptual models of the biophase that underlie mathematical descriptions of these processes and the physical processes that provide the framework for recent focus on less understood processes. Novel conceptual models of the interactions between cell surface structures and other surfaces are introduced, that are more realistic than the oft-relied upon DLVO theory of colloid stability. Biological processes reviewed include active adhesion/detachment (cell partitioning between aqueous and solid phase initiated by cell metabolism) and chemotaxis (motility in response to chemical gradients). We also discuss mathematical issues involved in upscaling results from the cell scale to the Darcy and field scales. Finally, recent studies at the Oyster, Virginia field site are discussed in terms of relating laboratory results to field scale problems of bioremediation and pathogen transport in the natural subsurface.

Abstract. Extended tailing of low bacterial concentrations following breakthrough at the Narrow Channel focus area was observed for 4 months. Bacterial attachment and detachment kinetics associated with breakthrough and extended tailing were determined by fitting a one-dimensional transport model to the field breakthrough-tailing data. Spatial variations in attachment rate coefficient (k f) were observed under forced gradient conditions (i.e., kf decreased as travel distance increased), possibly because of decreased bacterial adhesion with increased transport distance. When pore water velocity decreased by an order of magnitude at 9 days following injection, apparent bacterial attachment rate coefficients did not decrease with velocity as expected from filtration theory, but, instead, increased greatly for most of the wells. The coincidence of the increase in apparent attachment rate coefficient with the occurrence of protist blooms suggested that the loss of bacteria from the aqueous phase during the protist blooms was not governed by filtration but rather was governed by predation. Simulations were performed to examine the transport distances achieved with and without detachment, using attachment and detachment rate coefficients similar to those obtained in this field study. Simulations that included detachment showed that transport distances of bacteria may significantly increase because of detachment under the conditions examined. The relationship between the attachment rate and pore wa-2687 Introduction Bacterial transport in the subsurface is influenced by advec-

A geometric simulation method was used to develop a three-dimensional, highly detailed synthetic representation of point bar sediments in the Wabash River system. Geometric simulation methods, in comparison to well-known second-order stochastic methods, offer the advantage of being more closely related to depositional processes, which are often similarly conceptualized (i.e., described in terms of shapes of discrete bed forms, trends in grain size, and spatial relationships of defined geologic facies). Multiple scales of geometric variation were defined within a sedimentologically prescribed framework, and shapes of discrete geometric elements were established at each scale. The selected shapes were based on published field studies including sedimentological bed form studies and trench studies in active point bar sediments. The parameterization of the shapes allowed for random variability of the shape descriptors; discrete shapes were then generated and assimilated by computer. Hydraulic conductivity values were assigned to the discrete elements based on reports of observed variations in grain size and field measurements of hydraulic conductivity. The synthetic model, referred to as a numerical aquifer, is being used as the basis for extensive numerical experimentation to study the relationship between natural spatial structure and subsurface flow and transport. CriteriaGeological plausibility. The intended use of the numerical aquifer is as an assumed ground truth for the study of spatial structure and its impacts on flow and transport. Therefore it must be geologically plausible and arguably realistic. This requires that its construction represent variations in hydraulic conductivity in a manner consistent with relevant depositional processes, shapes and scales of depositing bed forms, field observations of stratification geometries and grain size variations, and actual measurements of hydraulic conductivity. 3259 3260 SCHEIBEAND FREYBERG: GEOMETRIC SIMULATION OF NATURAL POROUS MEDIA Cross sectionsof the numerical aquifer should give to the trained eye of a geologist the same qualitative visual impression as photographs and sketches of outcrops. Spatial resolution. It is widely accepted that variability at the pore scale can be adequately represented by scale-averaged parameters such as hydraulic conductivity and dispersivity at the scale of centimeters, corresponding to the laboratory scale of Dagan [1986]. At larger scales, however, it is not clear that similar constructs are well defined. Some investigators have defined scales, larger than the laboratory scale, below which variability can be neglected [e.g., Durlofsky, 1991; Dykaar and Kitanidis, 1992], but these depend on an assumed statistical homogeneity at small scales. Sedimentary deposits, however, are often highly structured at small scales and may not be well represented by statistically homogeneous models. The influence of small-scale sedimentary structure on flow in petroleum reservoirs is known to be significant [Weber, 1982[Weber, , 1986], but it...

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