Many of the organic pollutants entering groundwater are potentially biodegradable in the subsurface. The potential has been demonstrated in aquifers contaminated by wood creosoting process wastes and gasoline. The persistence of many of these organic compounds in the subsurface indicated that some factors must be limiting biodegradation. The research described has been aimed at identifying the major processes that limit biodegradation in aquifers and developing a mathematical model (BIOPLUME) for simulating these processes. In order to identify the rate limiting processes for biodegradation, the equations describing microbial growth and decay and transport of oxygen and contaminants were developed and solved in one and two dimensions. The main purpose of the research was to develop the mathematical tools necessary to describe and simulate the process of oxygen limited biodegradation of organics in groundwater. The model was applied at two sites, the United Creosote Company, Inc. site in Conroe, TX and an aviation gasoline spill site in Traverse City, MI. Field work at the Conroe, TX site indicated that oxygen was limiting the microbial degradation of dissolved hydrocarbons present in the shallow aquifer. At the Traverse City, MI site, model predictions for the rates of mass loss closely matched calculated rates from the field data.