To physically represent oxygen reaeration in contaminant fate and transport modeling Fick's Second Law of Diffusion is numerically solved to obtain a mass flux of oxygen entering the groundwater. This mass flux of oxygen results from the passive diffusion of oxygen from the ground surface. Using the computed mass flux, the diffusive recharge option in the BIOPLUME II fate and transport model is used to introduce the oxygen to the contaminant plume. This method was tested on a hypothetical plume and using field data from Hill AFB. The oxygen supplied by the diffusive recharge had an effect on the biodegradation of the hypothetical plumes at all contaminant concentrations. Significant effects were observed throughout the entire 1 mg/L plume and at the perimeter of all the plumes (maximum concentrations of 2, 5, and 9 mg/L). Modeling of the field site at Hill using this method was not as successful. The lack of agreement between the modeled and observed contaminant values could be a result of the poor source definition at the site and anaerobic degradation processes that are suspected to be contributing to the degradation of the plume.