The key objective of this project was to assess the formation of microscale munition constituents (MC) residues during detonations and weathering of larger detonation residues, and to determine the dissolution and transport properties of these microscale residues as they relate to their fate on testing and training ranges.

The objectives of the project were achieved by a variety of laboratory experiments and some limited field sampling. Composition B (Comp B) was used as the model MC for this research, given its widespread use and that it contains the widely detected explosive compounds TNT and RDX (and HMX present as a co-product from RDX manufacture). Residues from controlled detonations of Comp B (both standard and tagged) were collected and characterized, with special attention given to the size distribution of microscale particles <250 μm. The dissolution of Comp B residues ranging in size from 20 μm to 1 mm to was examined in batch experiments under continuous flow conditions, and dissolution of microscale Comp B was also visualized using spectral confocal microscopy using glass micromodels. Combined transport and dissolution experiments with mm-sized and microscale residues was examined in vertical sand columns under unsaturated flow conditions. The production of microscale particulates from cm-sized Comp B chunks under realistic simulated precipitation was evaluated. Surface runoff was collected over a 1.5 years from an east coast DoD facility and analyzed for dissolved and particulate explosives.

Glass micromodel experiments. This work discovered that TNT, RDX, and HMX autofluoresce under 405 nm laser illumination, and utilized this property to visualize and quantify the dissolution of microscale Comp B residues in saturated glass bead micromodels. The results demonstrated that within a given Comp B particles, TNT dissolved preferentially over RDX/HMX, and the mass ratio of RDX/HMX to TNT initially increased by >5.3 times. The particles dissolved in a stepwise fashion, with >72% of particle volume reduction in <36 min. Moreover, the results suggested that the particle shape factor was relatively stable, and the particles retained their highly irregular shape throughout the dissolution processes. This is the first work to demonstrate application of spectral confocal microscopy for visualizing and quantifying the behavior of energetic residues at the pore-scale.

Batch dissolution experiments. The dissolution of microscale Comp B particles (<250 μm) was compared to dissolution from macroscopic particles (>0.5 mm), and dissolution of detonation soot was also examined. The measured mass transfer coefficients for the microscale particles were one to two orders of magnitude greater than the macroscopic particles. When normalized to particle surface area, mass transfer coefficients of microscale and macroscale particles were similar, indicating that the bulk dissolution processes were similar throughout the examined size range. However, an inverse relationship was observed between the particle diameter and the RDX:TNT mass transfer rate coefficient ratio for dry-attritted particles, which suggests that RDX may be more readily dissolved (relative to TNT) in microscale particles compared to macroscale particles. Aqueous weathering of larger Composition B residues generated particles that possessed mass transfer coefficients that were on the order of 5- to 20-fold higher than dry attritted particles of all sizes, even when normalized to particle surface area. These aqueous weathered particles also possessed a four-fold lower absolute zeta-potential than dry-attritted particles, which is indicative that they were less hydrophobic (and hence, more wettable) than dry-attritted particles. The increased wettability of these particles provides a plausible explanation for the observed enhanced dissolution.

Unsaturated sand column experiments. Under a continuous application of artificial rainwater, greater dissolved effluent concentrations of TNT and RDX (5- and 10-fold, respectively) were observed for sand columns amended with microscale residues than for the columns amended with the mm-sized residues. The increased dissolution could not be completely explained by simple mass transfer differences based on particle size. Elution of particulate Comp B from the columns, based on the difference between total and dissolved explosives concentrations in column effluent, indicated higher and more frequent detections of particulate explosives in the columns amended with microscale Comp B than the columns amended with mm-sized Comp B. Examination of the vertical profiles of explosives in sand indicated that particulate residues had migrated into the sand, with a greater particulate mass observed in the columns which had received the microscale Comp B compared to those which received the mm-sized Comp B. This particulate transport increases the effective contact time between residues and infiltrating rainwater, leading to overall increases in the dissolved mass contaminant flux.

Residue aging experiments. Initial experiments, in which artificial rainwater was applied dripwise to single chunks of Composition B detonation residues from multiple heights, confirmed that microscale particles were produced during precipitation-driven aging, with 30% of the explosive mass collected detected as particulate Composition B (e.g., particles >0.45 μm in diameter). Follow-on experiments, during which multiple cm-sized residue chunks were subjected to realistic simulated precipitation, demonstrated an initial large pulse of particulate Composition B, followed by sustained production of microscale particles that represented 15% to 20% of recovered explosives.

Field surface runoff sampling. Surface runoff sampling was performed at the active explosives testing area of an east coast DoD facility. Sampling over the course of 1.5 years detected low concentrations of dissolved perchlorate, RDX, and HMX, but no confirmed detections of particulate explosives. Sporadic detections of the RDX breakdown products MNX and TNX were also observed. No clear temporal correlation between range activities and the occurrence of the target compounds was detected

This research has provide data that can be used to allow the mass loading of explosive residues at active training ranges to be more accurately estimated, as the impact of microscale MC migration and dissolution was quantified. Preliminary survey data of surface runoff from an active explosive testing area has also indicated that more research into this pathway of explosive compound migration would aid in refining site-specific fate and transport models used for range management.