One of the most promising technologies in groundwater contaminant remediation is the active use of natural microbial activity to reduce aromatic hydrocarbons and other contaminants to simpler, non-toxic compounds. Biological treatment technologies which clean an aquifer without removing aquifer material fall into the broad category of in situ bioremediation, and have the potential to provide cost-effective remediation plans. Mathematical models used to simulate in situ bioremediation must deal with spatial variation in contaminant and electron acceptor concentration, microbial population, and media properties. Research has shown that the use of sequential electron acceptors significantly impacts biodegradation results. Aquifer conditions may switch between primary and secondary electron accepting conditions, further complicating the modeling process. This research examines the two-dimensional, sequential electron acceptor computer model SEAM2D, developed by Widdowson (1992), and extends the SEAM2D model by developing the equations and coding for the newly recognized solid phase, iron(Ill)-based contaminant reduction. Both a sensitivity investigation and field simulations are provided. The sensitivity investigation identifies which input parameters most significantly impact model results (i.e. changes in contaminant mass and concentration). The modeling simulations provide an illustration of model capabilities and documents procedures used in applying SEAM2D to a USGS study site in Laurel Bay, South Carolina. The Laurel Bay site and subsequent model simulations are unique in that the natural, sequential electron acceptor process of oxygen-iron(Ill) reduction is specifically monitored and modeled. / Master of Science