PNNL

Abstract

Uranium mining and processing near Monument Valley, Arizona resulted in the formation of a large nitrate plume in a shallow alluvial aquifer. The results of prior field characterization studies indicate that the nitrate plume is undergoing a slow rate of attenuation via denitrification, and the results of bench-scale studies suggest that denitrification rates can potentially be increased by an order of magnitude with the addition of ethanol as a carbon substrate. The objective of the study was to investigate the potential of ethanol amendment for enhancing the natural denitrification occurring in the alluvial aquifer. Pilot tests were conducted using the single well, push-pull method and a natural-gradient test. The results showed that the concentration of nitrate decreased, while the concentration of nitrous oxide (a product of denitrification) increased. In addition, changes in aqueous concentrations of sulfate, iron, and manganese indicate the ethanol amendment effected a change in prevailing redox conditions. The results of compound-specific stable isotope analysis for nitrogen indicated that the nitrate concentration reductions were biologically mediated. Continued monitoring after completion of the pilot tests has shown that nitrate concentrations in the injection zone have remained at levels three orders of magnitude lower than the initial values, indicating that the impacts of the pilot tests have been sustained for several months.