Biodegradation in contaminated aquifers: Influence of microbial ecology and iron bioavailability

Groundwater aquifers have been contaminated with a wide variety of hazardous organic chemicals as a result of both primitive waste disposal practices and accidental releases. Natural bioattenuation processes are recognized to have a profound influence on the fate of hazardous contaminants in the subsurface. However, these processes are typically neglected in the formulation of risk assessments and remediation plans due to our inability to predict contaminant biodegradation rates. The objective of this research was to study factors controlling in-situ contaminant biodegradation rates.;The distribution and activity of bacteria, as well as the concentration of nutrients, substrates and electron acceptors, and pH, will influence contaminant fate and transport. In the first chapter, data from two aquifers contaminated with petroleum hydrocarbons were analyzed to evaluate the presence and significance of spatial heterogeneity. Results show that microbial processes and geochemistry varied at the two aquifers. There was little variability in microbial populations in closely spaced samples (2--3 cm) while there was substantial variability in microbial and geochemical parameters at 7.5 cm and 0.3 m intervals. Variations in microbial and geochemical parameters in thin layers suggests that techniques utilizing blended sediment for the measurement of in situ biodegradation rates and vertically averaged contaminant concentrations obtained from traditional monitoring may provide a misleading description of the dominant microbial processes.;Predator-prey analysis of microbial populations reported in Chapter 1 suggested that protozoan grazing may be controlling bacterial populations. In the second chapter, the influence of protozoan grazing on the biodegradation of model substrates was evaluated. Tests were conducted with sediment known to contain protozoa. Under aerobic conditions tests were conducted with a sediment extract while under anaerobic conditions tests were conducted in 1-L macrocosms with 1000 g of sediment. The results showed that protozoan grazing on bacterial populations can be significant under aerobic conditions utilizing benzene, toluene, ethylbenzene, xylenes (BTEX) as substrate. However, under anaerobic conditions with benzaldehyde as the substrate, there was not a significant effect of protozoa on benzaldehyde degradation.;One factor that undoubtedly controls the rate and extent of biodegradation is the availability of electron acceptors. In Chapter 3, the availability of Fe(III) for microbial reduction was studied in BTEX contaminated sediment. The results showed that as Fe(III) was consumed or became unavailable there was a shift from iron reduction to fermentation. Fe(III) bioavailability was limited by sorption and/or precipitation of Fe(II) on the mineral surfaces. Selective chemical extractions did not provide a useful indicator of total amount of bioavailable Fe(III) and more Fe(III) was reduced than was extracted with 0.5 N HCl. The results of this research emphasize the unique nature of each contaminated aquifer with respect to dominant microbial processes. These processes can vary both horizontally across an aquifer and at short intervals. In addition, traditional aquifer monitoring techniques that utilize wells that provide vertically averaged samples and neglect the solid phase are unlikely to provide sufficient data to understand active bioattenuation processes in sufficient detail for predictive models.