Transport of Microorganisms in the Terrestrial Subsurface: In Situ and Laboratory Methods

Although the use of microbial tracers has been instrumental in developing a better understanding of groundwater movement in many types of aquifers (1, 17, 34, 141, 149, 189, 190, 239), the importance of studying the transport behavior of the microorganisms themselves is now very apparent. This is due largely to an increasing dependence on limited and fragile groundwater resources. In particular, widespread contamination of shallow drinking-water aquifers by microbial pathogens and chemical wastes has led to increased interest in the factors that control subsurface microbial transport. The movement of nonindigenous bacteria, viruses, and protozoa through aquifers has been a public health concern in the United States (see chapter 71), where inadvertent contamination of water supply wells by microbial pathogens contributes significantly to the total number of waterborne disease outbreaks (139). Deliberate additions of nonindigenous populations of bacteria to organically contaminated aquifer sediments are now conducted to enhance in situ bioremediation rates in laboratoryand fieldscale bioaugmentation experiments (191, 194, 210, 237, 238) and to increase oil recovery from less transmissive zones by selective bacterial plugging of more permeable strata (see chapter 70). The success of bioaugmentation can depend as much on the ability of the introduced microorganisms to reach the contaminants as on in situ survival and factors affecting contaminant bioavailability (204a) such as its pore-scale distribution (71) and diffusion limitations (72). Transport properties of introduced bacteria in groundwater can play major roles in the transmission of some waterborne diseases (21), in the success of microbially enhanced oil recovery processes (39, 94, 138), in the mobility of surface-active or hydrophobic groundwater contaminants (38, 96, 115, 132), in pore clogging (14, 226), and in the potential subsurface dissemination of genetically engineered bacteria for biorestoration (220). However, many of the factors controlling subsurface microbial transport are still poorly understood. To better study the controls of microbial transport, it has been necessary to improve available methodology for investigating microbial transport behavior in the laboratory and in the field. The many physical, chemical, and biological factors that affect microbial transport through aquifer materials (75) have necessitated refinement of techniques, allowing observations under more controlled experimental conditions. In this chapter, laboratory and field techniques for studying microbial transport behavior in aquifer materials and model porous media are described and discussed.