Measurements of dissolved hydrogen and other biologically active solutes in groundwater from a shallow petroleum hydrocarbon-contaminated aquifer indicate that the distribution of microbial terminal electron-accepting processes (TEAPs), such as methanogenesis, sulfate reduction, and ferric iron (Fe (III)) reduction, is highly dynamic in both time and space. Delivery of sulfate to methanogenic zones by infiltrating recharge or lateral transport can result in a TEAP shift from methanogenesis to sulfate reduction. Conversely, lack of recharge and consumption of available sulfate can result in a shift from sulfate reduction to methanogenesis. Temporal shifts between sulfate and Fe (III) reduction were also observed. Time lags associated with TEAP shifts ranged from less than 10 days to about 3 months. The relation between TEAP and biodegradation rates of a variety of organic compounds indicate that biodegradation rates of petroleum hydrocarbons probably vary temporally and spatially in a contaminated aquifer.