Physical modeling of 2,4-DNT gaseous diffusion through unsaturated soil

Chemical detection of buried explosives devices (BEDs) through chemical sensing is influenced by factors affecting the transport of chemical components associated with the devices. Explosive-related chemicals, such as 2,4-dinitrotolune (DNT), are somewhat volatile and their overall transport is influenced by vapor-phase diffusion. Gaseous diffusion depends on environmental and soil conditions. The significance of this mechanism is greater for unsaturated soil, and increases as water content decreases. Other mechanisms, such as sorption and degradation, which affect the overall fate and transport, may be more significant under diffusion transport due to the higher residence time of ERCs in the soil system. Gaseous diffusion in soil was measured using a one-dimensional physical model (1-D column) to simulate the diffusion flux through soil under various environmental conditions. Samples are obtained from the column using solid phase microextraction (SPME) and analyzed with a gas chromatography. Results suggest that DNT overall diffusion is influenced by diffusive and retention processes, water content, source characteristics, and temperature. DNT effective gas phase diffusion in the soil decreases with increasing soil water content. Vapor transport retardation was more dominant at low water contents. Most of the retardation is associated to the partition of the vapor to the soil-water. DNT vapor flux is higher near the explosive source (mine) than at the soil surface. This flux also increases with higher soil water content and temperature. Results also suggest non-equilibrium transport attributed to mass transfer limitations and non-linear sorption.