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

摘要

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-l,3,5-trinitro-l,3,5- triazine (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 RDX- degraders. As the aquifer temperature decreased from 15 to 10 and eventually to S 5 c, the concentration of nitroso substituted metabolites and untreated RDX increased in the effluent stream. The estimated first-order biodegradation rate coefficient k for RDX at 15 5 C was 0.155 1/hr (+/-0.0l9, n = 3). This rate coefficient decreased by about 37 percent to 0.098 1/hr (+/-0.0l7, n = 3) at 10 5 C, and by another 38 percent to 0.061 l/hr (+/-0. 016, n = 3) at 5 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, DeltaEh = -550 to-700 mV), 23-46 percent of initial radiocarbon was mineralized to (14)CO2 compared with <5 percent in control columns, where DeltaEh 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.