Elution of Nitrate at the NABIR Field Research Center, Oak Ridge Reservation, Oak Ridge, TN

As part of a bioremediation project for the in situ bioreduction of uranium at the Department of Energy Natural and Accelerated Bioremediation Research (NABIR) Field Research Center (FRC) in Oak Ridge, TN, aquifer and groundwater conditioning is required before conducting the remediation experiment. One step includes flushing of the aquifer with pH-adjusted fresh water in order to remove extremely high concentrations of nitrate, calcium, and aluminum that would interfere with in situ bioreduction. The elution of nitrate from the test zone was used as an inverse tracer to discern contaminant transport pathways and model parameters. Concentration time series data augmented pressure tests, a bromide tracer study, and electromagnetic borehole flowmeter (EBF) measurements. The aquifer at the FRC is a fractured shale with strike of about 1.5 degrees north of west, and dip of about 30 degrees to the southwest, as inferred from area observations and EBF logging. A network of injection and extraction wells are aligned along strike, while a separate network of observation wells with multiple screen intervals (MLS wells) are oriented along dip at the midpoint of the injection/extraction well network. Flow generally occurs along strike in fractures associated with bedding planes, however other lesser fracture networks provide communication between the major fracture sets. Previous data have indicated a high hydraulic conductivity zone, approximately 10-50 cm thick located at a depth of about 12 m along the centerline of the injection/extraction well network. Above the major flow zone, the matrix weathers to saprolite, decreasing hydraulic conductivity. The elution tracer test was conducted by injecting clean, acidified tap water in the farthest upgradient injection well, and extracting at half the injection rate from the farthest downgradient extraction well. This flow ratio was chosen to produce flushing focused on a small cell of the aquifer which will later be used as an in situ bioreactor. Nitrate concentrations were measured frequently over 100 hours. Concentration time series indicate that flushing occurred predominantly in the previously identified high conductivity zone at 12 m depth. The strongest, most rapid nitrate concentration decrease was observed at a similar depth in each MLS well, which is consistent with the depth at which injection and extraction occurred. A decrease in downdip, nitrate concentration extended to 15m depth, whereas at the centerline and updip, little nitrate concentration reduction occurred below 13.75 m depth. This asymmetry is consistent with the competing contributions of bedding plane fractures and weathering depths. In the future, time series crosshole seismic and ground penetrating radar collected by Lawrence Berkeley National Laboratory, and borehole electrical conductivity measurements collected by Oak Ridge National Laboratory will be integrated with the concentration, pressure, and EBF data to obtain a comprehensive interpretation of this test.