Ronald W. Harvey

US Geological Survey

Boulder, Colorado

The importance of subsurface bacterial transport in the spread of waterborne diseases has long been recognized. Indeed, the first indirect evidence of subsurface bacterial transport occurred in 1854, when it was observed that a cholera epidemic in central London was the result of bacterial contamination of a public well.¹ Injection and recovery tests involving the direct addition of bacteria to freshwater aquifers were conducted as early as the 1890s.²,³ The purpose of early additions of bacteria was to provide a convenient tracer for following the movement of groundwater through karstic, or fractured-rock, terrain. Indicator bacteria, such as coliforms, have been added to the subsurface since the 1930s⁴ to investigate the transport potential of pathogenic bacteria near water-supply wells. Only within the last 10 yr have specific populations of bacteria been added to aquifers to enhance the degradation of subsurface contaminants. However, many current mechanistic studies of subsurface bacterial transport are being carried out to learn more about the feasibility of using nonindigenous or waste-adapted bacteria in the cleanup of subsurface contamination.

Subsurface bacterial transport is potentially important for engineered remediation and natural attenuation at various contaminated sites at Department of Energy (DOE) facilities, especially those subject to subsurface contamination with organic chemicals. The role of bacterial transport at such sites can be complex and can involve various effects, such as the following:

• Seeding of contaminated zones with bacteria that are genetically engineered for or have adapted to a particular contaminant.

• Facilitated transport of hydrophobic or surface-active contaminants by mobile bacteria, which might enhance the spread of these chemicals.

• Cotransport of bacteria with dissolved organic contaminants, which results in longer contact between the contaminants and the populations that affect their degradation.

• Increased dissemination of genetic information, particularly if introduced bacteria carry genes that confer more-efficient breakdown of organic contaminants.

Little is known about the last 2 mechanisms, and more information is needed about all 4. More research on the role of bacterial transport in the ecology of subsurface microbial communities that ultimately degrade contaminants is also needed.

Many of the field experiments in which labeled bacteria were injected directly into an aquifer with a conservative tracer (nonreactive solute) involved rather homogeneous sandy deposits.^5-7 In well-sorted sandy deposits, the bacteria injected into the aquifer are assumed to follow the same flow paths as the conservative tracer. Therefore, the conservative tracer can be used to predict what points in the acquirer need to be sampled in order to capture breakthrough of labeled bacteria. Also, the concentration histories of the conservative tracer provide important information for the hydrologic portion of a predictive transport model and allow for a reasonable estimate of bacterial retardation.⁸