Hydrophobic organic contaminants released into the subsurface can become sorbed to the soil matrix. The sorbed compounds may slowly leach into the groundwater, providing a long-term source of contamination. Bioremediation has been cited as a potential clean-up strategy for residual organics in soils. At present, factors which control biological utilization and retention of organic contaminants in the subsurface are poorly understood. The purpose of this study was to investigate how the adsorption/desorption and biodegradation processes interact to control the distribution and biological fate of subsurface contaminants. Batch soil microcosms were used to evaluate the sorption of a chlorinated aromatic compound [2,4,6-trichlorophenol (TCP)] to mineral soils, organic soils, and dissolved macromolecules (humic acid). Microbial utilization of 2,4,6-TCP was then evaluated after addition of a bacterial culture previously acclimated to the substrate. Biodegradation was evaluated by production of ¹⁴CO₂ and disappearance of 2,4,6-TCP from the sorbed and solution phases. Size exclusion chromatography was used to distinguish between free 2,4,6-TCP in solution and 2,4,6-TCP bound to humic acid in solution. Sorption of 2,4,6-TCP to mineral surfaces was rapid but sorption to organic soils varied with contact time. Most 2,4,6-TCP sorption to organic soils occurred during the first few days of contact with the soil. However, the sorption rate gradually decreased with time and 2,4,6-TCP continued to sorb for weeks at a reduced rate. The amount of 2,4,6-TCP which sorbed at the slower rate was dependent upon the organic matter content of the soil. Desorption of 2,4,6-TCP from organic soils was similar to adsorption; however, it appeared that slow desorption took longer than slow adsorption. Humic acid in solution formed an additional phase to which 2,4,6-TCP sorbed. The sorption capacity of dissolved humic acid was a function of the humic acid concentration. Higher concentrations of humic acid had lower sorption capacities when normalized for total organic carbon content. The mineralization rate of 2,4,6-TCP by Pseudomonas aeruginosa was dependent upon the initial substrate concentration. Total mineralization of 2,4,6-TCP by P. aeruginosa decreased in the presence of organic soils. However, mineralization rates were elevated in organic soils compared to mineral soils after 5 days of incubation. This was attributed to an influx of substrate into solution from an initially sorbed state. Total mineralization of 2,4,6-TCP in humic-containing solutions was reduced by 5-10 % over humic-free solutions. The bioavailability of 2,4,6-TCP was greatly reduced in the sorbed state. Biodegradation of unbound 2,4,6-TCP in solution was very rapid while biodegradation of 2,4,6-TCP sorbed to organic soil or to dissolved humic acid was much reduced. Desorption of most 2,4,6-TCP from soil was rapid, thereby becoming available to acclimated bacteria. However, a small quantity of 2,4,6-TCP desorbed from organic soil and dissolved humic acid very slowly and became available for biodegradation at a rate limited by desorption. / Ph. D.