Effects of Reducing Conditions On the Fate and Transport of RDX.

Soil and groundwater investigations conducted at the Massachusetts Military Reservation show the impact of historic range activities on the development of groundwater contaminant plumes emanating from military ranges.  Several of the plumes, located on the southeastern side of the Reservation, contain elevated concentrations of RDX.  In most cases, these plumes show continuity from the source to the leading edge, indicating very little attenuation of RDX is occurring in the aquifer.  An interesting exception to this trend are the plumes consisting of RDX and perchlorate which intercept part of the aquifer that has been previously impacted by a fuel spill, where reducing conditions due to biological activity persist.  Whereas perchlorate shows no significant correlation with any groundwater physiocochemical parameter, RDX shows a significant correlation with oxidation-reduction potential and a significant negative correlation with specific conductance.  The distribution of RDX is more consistent upgradient of the oxygen depleted zone and implies that RDX (and HMX) are degrading in the aquifer in the vicinity of the fuel spill.  Possible explanations for this pattern include 1) lack of a consistent source, 2) smearing of the plume due to shifts in the apex of the aquifer, 3) biodegradation, 4) dispersion, and 5) attenuation of RDX due to reducing conditions in the aquifer.  The mechanism for RDX attenuation could be explained by the reduction of nitro aromatic compounds to primary amines by an oxidation-reduction reaction.