Underwater unexploded ordnance (UXO) casings eventually breach and release nitrogenous energetic compounds (NEC) through mechanical stress, corrosion, and low-order remedial detonations. These compounds can remain intact in the sediment, dissolve into the overlying waters, or bind to particles and be resuspended into the overlying waters. Over time, various chemical, biological, and physical processes (e.g., sequestration, metabolism, and photolysis) change the NEC to other chemical forms that have different transport and toxicity properties in various ecosystems. Although SERDP has sponsored much research on NEC transformation in terrestrial and groundwater systems, very little information is available on rates of attenuation or transport of energetics in coastal aquatic systems.
The objective of this project was to determine and predict the fate of NEC released from munitions in coastal waters and submerged sediments.
The NEC examined included the military explosive compounds 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). Laboratory and mesocosm experimentation, as well as field surveys and experimentation, were conducted to provide rate measurements and assess typical environmental conditions found at Department of Defense (DoD) coastal sites impacted by NEC.
In general, photolytic transformation rates of TNT > microbial metabolism > abiotic chemical hydrolysis in the coastal water column. In sediments, microbial metabolism is likely to be the most important transformation process. Natural microbial assemblages were found to mineralize TNT at rates similar to that seen for other aromatic carbon compounds in coastal environments (up to 145 µg C kg-1d-1 in estuarine sediments). Incorporation rates of TNT carbon into bacterial macromolecules were typically an order of magnitude more rapid than those for mineralization. Both the rate and efficiency of incorporation of TNT carbon into microbial biomass appeared to decrease when migrating from freshwater to marine environments.
Evidence of rapid attenuation rates and a better understanding of NEC degradation mechanisms may reduce DoD’s cost of environmental compliance and increase the likelihood that current coastal ranges will be available for future military training. (Project Completed – 2008)