Soils and groundwaters face an influx of contaminants, many of which are known to be hazardous to the public's health. Several remediation technologies have been developed to clean-up contaminated soils, but additional information on the behavior of organic chemicals in the subsurface environment is needed for effective remediation. The purpose of this study was to investigate the relationship between sorption and biodegradation on the ultimate fate of subsurface contaminants. Sorption and biodegradation of phenanthrene, a hydrophobic polycyclic aromatic hydrocarbon, were modeled using soil and water microcosms. The soils were prepared in the laboratory from a parent soil to obtain sorbents that were similar in all respects except for soil organic matter content. Biodegradation was evaluated by production of CO2 and disappearance of phenanthrene from the sorbed and aqueous phases. The sorption of phenanthrene to each soil occurred in two distinct stages. The majority of phenanthrene mass sorbed during the first few days. After the initial rapid sorption period, the sorption rate continually decreased. The time required for complete sorption varied according to soil organic matter content, with soils having higher organic matter contents requiring more time for complete adsorption. Desorption of phenanthrene from soils was hysteretic, although it appeared that phenanthrene would completely desorb. Phenanthrene desorption rates decreased as the chemical exposure time increased. The biodegradation of phenanthrene also occurred with a fast and slow phase. The slowly degradable fraction increased with soil organic matter content. The data suggest that the biodegradation of phenanthrene is desorption controlled. / Master of Science