A growing concern exists that the accumulation of unexploded or unconsumed energetic compound residues in soils on military testing and training ranges represents a threat to human health and the environment and that the ultimate fate and transport of these compounds can be influenced by land management practices. One such practice may be prescribed or controlled burning, which is used on military training ranges for a variety of purposes including safety clearance prior to detection and demolition of unexploded ordnance (UXO), wildfire avoidance, and plant and wildlife management.
During testing of explosive devices, unexploded or unconsumed energetic compounds including RDX, HMX, and TNT frequently remain as residues in the surface soils. These residues eventually may be transported to surface and ground waters and create a health risk. Currently, there is no protocol for routinely removing explosive residues from surface soils. If procedures were instituted to reduce the levels of energetic compounds in surface soils (the source of groundwater contamination), future impacts to groundwater resources could be significantly reduced or eliminated.
The objective of this project was to determine the impact that prescribed burning had on the fate and transport of residual energetic compounds in surface soils on test and training ranges. This included:
Variables such as burn intensity, soil depth, and vegetation density were measured to quantify the conditions necessary to optimize reduction of energetic residuals by prescribed burning. The project involved the following three key tasks: (1) conducting a laboratory study designed to determine the rate at which soil-associated energetics decomposed with respect to temperature; (2) defining the relationships between energetic residuals and vegetation on ranges; and (3) burning of test plots to determine the impact burning had on energetic residuals in surface soils and runoff.
Based on this study, it is clear that controlled burning can heat surficial soils to the point that thermal destruction of TNT and RDX will occur. This is supported by laboratory data demonstrating temperatures necessary for destruction, field measurements of temperature during the controlled burn and analysis of soils before and after the burn. It is also clear that TNT, RDX, and other energetics were found in and on plant tissue prior to the burn and that these were destroyed in the burn. Numerous lessons were learned about the problems associated with monitoring an experiment of this kind; energetics are heterogeneously distributed in soils and their concentrations change over time, making before and after burn soil sampling results of limited value. Much of the test monitoring relied on soils placed in various containers immediately before and then analyzed immediately after the burn; this was in anticipation of the problem of soil heterogeneity. Unfortunately, many of these containers had bottoms and were discovered not to be representative; columns appeared to have worked better. Fuel was added to the site, as is common practice to insure burn across sparsely vegetated areas. It appears that the use of too much fuel may actually insulate the ground surface and reduce soil temperatures and the destruction of energetics. In the end, the experiments were successful and given the first time nature of this work, the problems encountered were not surprising.
Prescribed burning on ranges has potential for destroying a significant amount of explosives residual in surface soils and in and on plant tissues. Incorporating a well-planned approach that takes into account specific considerations of each range could be an effective tool for minimizing the impacts of explosives residuals on groundwater and local surface water bodies. It is anticipated that Fire Ecology Range Management (FERM) would be a component of a multi-component program for managing explosives residuals resulting from testing, training, and EOD operations on ranges. While this SERDP demonstration has accomplished the objective of providing preliminary evidence for the potential for using FERM for this intended purpose, additional work is needed to better understand plant uptake of explosive residuals, to quantify the thermal destruction of explosive residuals under natural settings, and to improve the design of the burn to increase the transfer of heat to the soil profile.