Bioplume III is a two-dimensional, finite difference model for simulating the biodegradation of hydrocarbons in groundwater. The Bioplume III model simulates both aerobic and anaerobic biodegradation processes in addition to advection, dispersion, sorption and ion exchange. Bioplume III simulates the biodegradation of organic contaminants using a number of aerobic and anaerobic electron acceptors: oxygen, nitrate, iron (III), sulfate, and carbon dioxide. Bioplume III is based on the U. S. Geologic Survey (USGS) Method of Characteristics Model (MOC) dated July 1989 (Konikow and Bredehoeft). The Bioplume III code was developed primarily to model the natural attenuation of organic contaminants in groundwater due to the processes of advection, dispersion, sorption and biodegradation. For the case where iron (III) is used as an electron acceptor, the model simulates the production and transport of iron (II) or ferrous iron.