Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is a common soil contaminant at current and former military facilities, including many training and testing ranges. Because RDX is readily transported through soils to the subsurface, this nitramine explosive now also impacts groundwater and drinking water at numerous locations across the country. A significant issue with RDX contamination on ranges and at other military installations is that it often occurs over expansive areas, where in situ or ex situ treatment technologies are difficult to implement. One potential alternative for military ranges and other facilities is monitored natural attenuation (MNA), in which contaminant degradation by natural processes, including biodegradation, are evaluated. However, one limitation of this approach for RDX is the inability to accurately evaluate whether the nitramine is biodegrading under field conditions, as rates may be relatively slow. One potential technique to overcome this limitation is the use of compound-specific stable isotope analysis (CSIA), where biological contaminant destruction can be documented as changes in the ratio of stable isotopes of specific elements in a molecule; for RDX, ratios of ¹⁵N/¹⁴N and ¹³C/¹²C are relevant. The objective of this project is to validate a CSIA method to confirm and constrain rates of aerobic and anaerobic biodegradation of RDX at field sites. This technique can be utilized by DoD to provide critical data to support MNA as a remedy for treating this energetic in groundwater, and confirm the effectiveness of in situ enhanced bioremediation remedies. The stable isotopic composition of NO₃- and NO₂- was also measured when these anions co-occurred with RDX to evaluate whether these potential degradation products from RDX could be used to further demonstrate MNA in the field.