A series of column studies, with aquifer material from the former Nebraska Ordinance Plant (NOP), were performed to explore the phenomenon of electron competition from ubiquitous inorganic electron acceptors (nitrate and sulfate) present in contaminated groundwater. Acetate was used as a source of readily biodegradable carbon in all of the treatment-column systems. Influent hexahydro-1, 3, 5-trinitro-1, 3, 5-triazine (RDX) concentrations (1-1.8 mg dm(-3)) were completely removed to below detection levels of 20 mug dm(-3) in all treatment-column systems without any nitroso-metabolites. In the control-column system (with no carbon amendment) significant levels (similar to30% of the inlet molar RDX) of nitroso-substituted RDX derivates were observed in the effluent stream. The estimated first-order biodegradation rate coefficient for RDX was highest (0.79 h(-1)) in the treatment-column system where acetate was the only amendment, about 52 times higher than the rate coefficient (0.015 h(-1)) obtained in the control-column system. The presence of sulfate (100 mg dm(-3)) in influent groundwater temporarily delayed the onset of RDX biotransformation without any adverse effects on overall RDX biotransformation. Coexistence of low (100 mg dm(-3)) nitrate levels in the influent feed water reduced the first-order biodegradation rate coefficient obtained in the absence of nitrate by about 80% to 0.16 h(-1). These nitrate levels, however, were low to halt the RDX biodegradation probably because the available carbon levels were high enough to exceed the demands for nitrate reduction. High levels of nitrate (500 mg dm(-3)) initially halted RDX removal, and significantly reduced the rate of RDX biotransformation by about 98% to 0.02 h(-1), thereby increasing the half-life from 0.9 h in the absence of nitrate to about 32 h, with noticeable levels of untreated RDX in the effluent stream. Contrary to the expectations, the presence of ammonium in conjunction with acetate resulted in a lower (0.09 h(-1)) biodegradation rate coefficient as compared with the one obtained in the absence of ammonium. (C) 2003 Society of Chemical Industry.