The addition of oxygen in a fractured bedrock aquifer was evaluated to determine its effectiveness in accelerating the in situ bioremediation of petroleum hydrocarbons. The source of oxygen selected to provide the enhancement for the pilot-scale study was Oxygen Release Compound (ORC®) produced by Regenesis Bioremediation Products. ORC is a patented formulation of magnesium peroxide (MgO2) that slowly releases molecular oxygen into the aquifer when hydrated. The hydrated product becomes magnesium hydroxide (Mg(OH)2). The oxygen release is dependent upon the level of the contaminant flux at the point of treatment. This allows ORC to release oxygen at a relative constant rate over an extended period of time. The subsequent increase in dissolved oxygen creates aerobic conditions that promote in situ bioremediation of the hydrocarbon plume by the naturally present microorganisms. Although ORC has been used successfully for numerous enhanced in situ bioremediation systems installed in unconsolidated aquifers, less data is available for highly contaminated bedrock aquifers. Two phases of ORC were tested in this project. First, ORC was injected into five wells in a small portion of the plume (approximately 25 feet). Second an ORC barrier, consisting of 28 injection wells was extended across the plume to cover an approximately 150-foot treatment zone. Based on an estimated residual concentration of 26 mg/L total BTEX compounds in the groundwater, 21 pounds of molecular oxygen (in the form of ORC) was also injected into three soil borings in the source area. The results of both phases indicated that these applications were successful at enhancing the aerobic conditions at the site and ultimately reducing the offsite flux of petroleum hydrocarbons. However, several limitations were realized in the fractured bedrock environment, which are not normally encountered in unconsolidated deposits. First, despite extensive site characterization, the true interconnectivity of fractures remains unknown. Thus, accessibility to all contamination is believed to be limited. Second, in some fracture zones, the hydraulic conductivity is higher than expected, causing increased velocities which limited residence time and microbial degradation. Third, it is expected that a large percentage of flow occurs through preferential pathways (i.e., a small percentage of the contaminated mass). As a result, the available matrix for microbial growth is reduced. Finally, due to lack of an anticipated extensive fracture network, infiltration is reduced.