TRANSPORT OF DISSOLVED AND PARTICULATE EXPLOSIVES COMPOUNDS IN SATURATED FLOW COLUMN EXPERIMENTS

Contamination of surface soils with explosives residues (ER) is a common phenomenon on military operational ranges. However, at some sites (e.g. Camp Edwards, MA), plumes of munitions constituents in groundwater have also been observed. The exact mechanism of ER transport through the vadose zone to underlying aquifers is poorly understood. To date, most research and modeling work was based on the assumption that ER transport occurs exclusively in the dissolved phase. The failure of such models to predict the extent and concentration of ER in groundwater suggests other forms of transport may be occurring. This study presents results from experiments designed to investigate the potential for ER particulates to be transported through the vadose zone, especially during infiltration of storm flow, and then to dissolve in underlying aquifers.

A series of column experiments were conducted in repacked soils utilizing both dissolved and particulate explosives compounds. In separate experiments, latex microspheres (0.5, 1,0 and 2.0 um) and fine grained (<250 um) 2,6- Dinitrotoluene (DNT) particles were applied to columns, both in suspension and mixed with the first centimeter (cm) of soil to mimic near-surface soil contamination. The columns were pre-scored in one cm sections for easy post-experiment segmentation enabling high resolution analyses of the spatial distribution of explosives particulates/microspheres entrained in the porous media. A novel combination of detection methods, including chemical staining, flow cytometry, and quantitative microscopic analysis, was used to analyze column effluent and soil samples. In each of the microsphere experiments, the 0.5 µm size particles accounted for the majority of particles detected in the effluent. However, in the suspended flow experiment the 1.0 µm spheres were also transported and comprised almost 30% of the spheres in effluent, compared to < 5% in the surface application experiment. Concentrations differences in filtered and unfiltered samples from both DNT experiments, indicates that particulate transport accounts for a measurable fraction of the DNT detected in the effluent. Our preliminary results suggest that mobilization of colloid-sized ER plays a significant role in the transport of explosive contaminants under steady-state flow conditions.