The focus of our research is to determine whether the availability of major nutrients (C, N, P) control the abundance, distribution, and activity of heterotrophic microorganisms in subsurface sediments. Specifically we have developed and are testing a microcosm system using subsurface sediment samples from a DOE site. We are using this system to conduct a preliminary nutrient enrichment experiments. The flow-through microcosm system simulates conditions in natural aquifers and can be regulated with respect to water flow rate, temperature, oxygen concentration, and nutrient concentration. Furthermore, the system cam be run aseptically, excluding exogenous bacteria from test cores. In our preliminary nutrient enrichment experiment, we collected deep vadose sediment from the Savannah River Site and pumped sterilized well water from sampling area through microcosms that were filled with this sediment. Experimental treatments included the flow-through of this water amen with different combinations of nutrients (+N+P, +C, +N+P+C) or no additional nutrients. We found that asepsis was maintained for the entire 30 day experiment. We also found that microbial abundance, biomass, and activity increased in several microcosms in response to the flow through of the well water, which was rich in nitrate. This response, however, was variable among both microcosms containing intactmore » sediments and microcosms containing intact sediment core sections. No significant treatment responses were observed, but each microcosm that had an increase in microbial growth also immobilized N from the well water. We infer that water and N availability, and water table fluctuations strongly influence microbial growth and activity in the deep vadose.« less