|We are currently conducting laboratory and field research at the DOE’s NABIR Field Research Center at Oak Ridge National Laboratory. In this project, which was initiated in October 2000, we are testing the hypothesis that indigenous microorganisms with the capability to reduce U and 99Tc are present at the site and that their activity can be stimulated by the addition of exogenous electron donors. We have conducted an extensive series of field push-pull tests in six wells to determine the effect of electron donor additions on concentrations of nitrate, U, and 99Tc. Injected test solutions consisted of amending 200 L of site groundwater that contained 2000 mg/L NO3-, 1.4 mg/L U, and 31,000 pCi/L 99Tc with Br- tracer, sodium bicarbonate, and selected electron donors (acetate, ethanol, and glucose). Injected test solutions in control wells were identical except that no exogenous electron donors were added. In the first series of tests (first exposure of indigenous microorganisms to exogenous electron donor), the addition of electron donor stimulated denitrifying activity and resulted in the complete utilization of injected donor ~ 500 hours (Fig. 1). Peak NO2- concentrations reached ~ 300 mg/L within ~ 250 hours following injection and then declined to zero. No NO2- production was observed in the absence of added electron donor. However, in our first series of field tests we were unable to stimulate U or 99Tc reduction, which we attribute to low microbial activity or biomass and electron donor limitations. During a push-pull test, electron donor concentrations decrease due to (1) dilution of the injected test solution by regional groundwater flow and (2) microbial utilization. Thus, it is necessary to devise an electron donor addition strategy that achieves sufficiently high donor concentrations in the immediate vicinity of the test well for a sufficient amount of time to remove all injected NO3- so that U and 99Tc reduction may proceed. The required time can be reduced by pre-stimulating microbial activity before additions of U and 99Tc. To test this hypothesis we conducted a series of electron donor additions into our test wells over an approximately 9-week period to develop an active denitrifying microbial population. From this second series of tests (in progress) we have obtained clear evidence that the addition of glucose, ethanol, or acetate stimulated denitrification and 99Tc reduction (U results are not yet available), which we believe to represent the first in situ stimulation of 99Tc reduction in the subsurface (Fig. 2). Although peak NO2- concentrations were smaller in these tests, rates of NO3- removal were increased and technetium concentrations are decreasing relative to the Br tracer.