Subsurface spatial heterogeneity and its effects on biodegradation rates and hydrocarbon plume behavior

Numerous field studies have demonstrated the intrinsic potential of subsurface microorganisms to degrade subsurface pollutants and affect the remediation of contaminated aquifers. The perception that the presence of microbes in the subsurface equates to compound biodegradation may be misleading, however, given the wide number of variables that influence biodegradation rates including the spatial variation in microbial populations and heterogeneity in solid and liquid-phase geochemistry. Thus, it is frequently difficult to duplicate field conditions in the laboratory and to be confident that the rates obtained in the laboratory are applicable to a field setting. The purpose of this research was to improve our ability to predict the rate and extent of contaminant biodegradation in the subsurface.; In the first component of this study, laboratory microcosms, field data, and split spoons were used to determine if in-situ benzene, toluene, ethylbenzene, and xylene (BTEX) biodegradation corresponded to field data. Results implied that geochemical data showing depletion of electron acceptors is not sufficient to demonstrate BTEX biodegradation.; The second component of this study used several computer simulations to examine the in-situ biodegradation of benzene and MTBE. Comparison of results to observed data suggest that although benzene and MTBE biodegradation may be approximated by simulation, the model output may severely over- or under-estimate downgradient compound concentrations, thereby, decreasing the effectiveness of the model in risk assessment.; Components three and four examined the use of a new type of in-situ column/sampler for the determination of in-situ biodegradation rates. Column results were compared to microcosm rates with some positive correlation. Microbial and geochemical studies were also performed on four split spoon samples, three outside of the in-situ columns and one inside a live column. Results showed substantial horizontal heterogeneity in microbial populations and sediment characteristics, although the parameters could not always be correlated with each other.; Overall, the results of this research highlight the unpredictable nature of the subsurface and the challenge of predicting compound biodegradation in a heterogeneous system.