Biologically Active Zone Enhancement (BAZE) Supplemental Study: Mass Balance of RDX Biotransformation and Influence of Aquifer Temperature on RDX Biodegradation in Groundwater

A series of column studies with site-specific aquifer material from the former Nebraska Ordnance Plant were performed to evaluate the influence of aquifer temperature on in situ hexahydro-1,3,5-trinitro-1,3,5triazine (RDX) biodegradation, and to assess the ultimate fate of RDX in groundwater under biologically induced reductive conditions. In treatment columns RDX-contaminated water was amended with acetate as readily available carbon source, and in control columns no electron donor was used. The results of the temperature study demonstrated clear indications of adverse effects of lower aquifer temperature on biological activity of RDXdegraders. As the aquifer temperature decreased from 15 to 10 and eventually to 5 °C, the concentration of nitrososubstituted metabolites and untreated RDX increased in the effluent stream. The estimated first-order biodegradation rate coefficient k for RDX at 15 °C was 0.155 1/hr (±0.019, n = 3). This rate coefficient decreased by about 37 percent to 0.098 1/hr (±0.017, n = 3) at 10 °C, and by another 38 percent to 0.061 1/hr (±0.016, n = 3) at 5 °C. An activation energy of 63.54 kJ/mol RDX was estimated from these reaction rate coefficients at three different aquifer temperatures. Results of the radiolabel study demonstrated that the ultimate fate of RDX under in situ reductive conditions is highly dependent on redox conditions in the aquifer. In treatment columns (redox change, ∆Eh = -550 to -700 mV), 23-46 percent of initial radiocarbon was mineralized to CO2 compared with <5 percent in control columns, where ∆Eh ranged between 70 and -70 mV. The dissolved fraction of initial radiocarbon in treatment columns was estimated between 46 and 64 percent. No or very low levels of nitroso-substituted RDX transformation products were identified in dissolved fraction from treatment columns. In control columns dissolved fraction accounted for about 86 percent of initial C and was composed of mainly untreated RDX. DISCLAIMER: The contents of this report are not to be used for advertising, publication, or promotional purposes. Citation of trade names does not constitute an official endorsement or approval of the use of such commercial products. All product names and trademarks cited are the property of their respective owners. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents.