Dissolution of nitroglycerin from small arms propellants and their residues

Propellant residues are scattered onto the soil surface when guns are fired. Here, we studied fired residues and unfired propellants from 9-mm pistols, 5.56-mm rifles, and 7.62-mm and 0.50-caliber machine guns. These propellants are composed of nitrocellulose and nitroglycerin (NG) and although nitrocellulose is insoluble, NG is soluble and toxic. Therefore, data are needed on how quickly NG dissolves from propellant residues in order to determine the flux of NG to soil and from there to groundwater. We measured the NG dissolution rate using batch tests and laboratory drip tests. The latter mimic field conditions on training ranges, where NG is dissolved from fired residues wetted by precipitation. To eliminate the confounding effects of soil biochemical and geochemical processes, we studied only propellant-water interactions. Because the surfaces of propellants and residues may play an important role in dissolution of the energetic constituents, we photographed these using both light and electron microscopy. Our results show that about 3% of the NG in the fired residues and 20% in the unfired grains dissolve over month time scales. Diffusion continues to transport NG to the propellant surface but at a very slow rate. Unfortunately, the distribution of NG and how it moves within the nitrocellulose matrix is poorly understood, hampering our ability to derive a physically based dissolution model that can predict NG loss from a variety of propellant types. Nevertheless, the slow dissolution rate of NG, coupled with its short half-life in most soils, suggests that it should rarely reach groundwater.