THE EFFECT OF BICARBONATE GENERATED FROM MICROBIAL OXIDATION OF ACETATE DURING BIOSTIMULATION OF URANIUM REDUCTION AT THE OLD RIFLE UMTRA SITE

The Old Rifle site in Rifle, CO was a uranium- and vanadium-milling site active from 1924-1958. Since 2002, acetate has been added to groundwater to stimulate bacterial growth and promote reductive immobilization of uranium. While biostimulation successfully decreases groundwater uranium concentrations to levels below the drinking water standard (0.13µM) during iron-reduction, prolonged removal of uranium is less successful once sulfate reduction becomes the predominant metabolic process. The effect has previously been interpreted to indicate that sulfate-reducing organisms are less efficient at uranium reduction. During August-September of 2010, an injection experiment was conducted to test the effects of flushing sorbed uranium from aquifer sediments with a concentrated bicarbonate solution coupled with acetate-based biostimulation. One half of the test plot was perfused with 50mM NaHCO₃with 85 ppm of deuterated water as a conservative tracer; this was followed by addition of 5mM Na-acetate with 2mM of NaBr across the experimental plot. Breakthrough of the bicarbonate/D₂O solution was monitored in quasi-real time using a Los Gatos Research, Inc. (LGR) water isotope analyzer, which enabled separation of the distinct effects induced by the bicarbonate and acetate solutions. The bicarbonate led to an initial release of uranium from the sediments, increasing dissolved uranium concentrations 5 to 12-fold (0.2 to 2.3µM) depending upon position and distance from the injection location. Injection of acetate created reducing conditions in the aquifer, replicating previous biostimulation experiments at the site. The characteristic rebound in uranium concentrations was observed when sulfate reduction became predominant in the region of the aquifer receiving only acetate. In contrast, uranium concentrations remained well below pre-injection levels in the section of the experimental plot flushed with bicarbonate after sulfate reduction became dominant. This indicates that the rebound observed during sulfate reduction is primarily caused by an increase in alkalinity accompanying the nearly complete microbial consumption of acetate during sulfate reduction rather than a lack of uranium-reducing capacity by sulfate reducing bacteria.