Military test and training ranges operate with live-fire engagements to provide realism important to the maintenance of key tactical skills. Ordnance detonations during these operations typically produce minute residues of parent explosive chemical compounds. Occasional low-order detonations also disperse solid-phase energetic material onto the surface soil. Key questions arise regarding how these residues and the environmental conditions (e.g., weather and geostratigraphy) contribute to groundwater contamination.
The objective of this project was to develop an energetic material source release function that describes the mass transfer of solid-phase energetic material to a solute in soil-pore water. This source release function could be used in a solute transport model with linkages to time-dependent weather phenomena to assess the migration potential of residual energetic material.
This project analyzed the mass transfer process using laboratory measurements and numerical simulation methods. Phase I consisted of an initial series of experiments designed to determine the critical parameters affecting the mass transfer of energetic materials to pore water and the derivation of a mathematical function that incorporated the most significant factors. Phase II tests evaluated the effects of bed loading, bed depth, initial mass, flow, temperature, energetic material particle size, initial wetting phase, porous media saturation, pulsed water flow, and the first set of low-order detonation debris. Phase III tests evaluated repeatability, effect of bed location, energetic material particle size, and the second set of low-order detonation debris. To simulate the dissolution and transport experiments, a simple mass transfer formulation was added to T2TNT. The model contains two fitting parameters—the mass transfer coefficient from the solid phase to the water, k, and an exponent on the solid surface area function, X.
Summary results of the laboratory porous media column experiments are as follows:
- RDX effluent concentrations mostly near Cmax
- Water flow rate has minimal impact (impact on depletion phase only)
- Water pulse limits C/Cmax vs. continuous flow
- Temperature has major impact
- EM initial mass important in duration of peak effluent C/Cmax
- EM bed depth/load not important
- EM particle size influences depletion phase
- excess mass capacity stage – high C/Cmax ratio
- mass depletion stage – declining C/Cmax until exhaustion
Under most circumstances, the effluent concentrations were very high, near the maximum controlled by temperature-dependent water solubility. The average RDX concentration was about 31 mg/L (~75% C/Cmax) and TNT was about 43 mg/L (~33% C/Cmax). This result implies that rainfall can induce a rapid mass transfer of RDX and TNT to soil-pore water and additional work is needed to assess the impacts to groundwater.
The effect of bed location showed that, in general, surface deposits showed slightly sustained greater effluent concentrations for both RDX and TNT. However, as shown in the cumulative discharge profiles, as the mass became depleted, the effluent concentrations declined more rapidly. The cumulative discharge of RDX showed high values of 70 to 85%; however, TNT values were significantly lower, at 25 to 40%. The remaining mass in the column after test termination showed very small amounts. Thus, degradation losses must account for the difference.
The initial mass and total surface area of energetic material particles plays a key role in the effluent concentrations of RDX and TNT. With a similar mass, but a 5X increase in particle diameter, the effluent concentrations never reach peak concentrations as with the smaller particles. Increasing the mass (and the total surface area) with the larger particles creates an effluent profile similar to that with the smaller particles.
The low-order detonation debris contained both soil and energetic material. With similar initial-mass buried deposits, the low-order detonation debris showed significantly lower peak effluent concentrations and a slower decline in the mass depletion stages. These effects may be due to partitioning onto the soil solid phase or may be an effect of the detonation on the energetic material.
The modeling effort was successful in fitting a simple two-parameter mass transfer function. The experimental design for the laboratory tests was not structured for statistical analysis, which challenged the structured statistical analysis to develop correlations and significance. However, the statistical analysis supports a hypothesis that the variation of the surface area exponent, X, is correlated with the initial particle diameter—the greater the initial particle diameter, the greater the surface area exponent. This has importance principally in the mass depletion stage where small initial particles are exhausted in a narrow time window with a precipitous drop in effluent concentration compared to a much longer and slower decline for larger particles. This implies that particle size distributions for detonation debris on ranges may be needed for accurate estimates of mass transfer rates into soil over longer time domains that include mass depletion stages.
Statistical correlations for the mass transfer parameter, k, did not show any significant relationships with the experimental parameters. However, researchers observed that the fitted mass transfer coefficients were in a narrow range (for the experimental conditions employed) and may represent a general mass transfer rate that is not dependent on other external conditions. This would benefit applications of the mass transfer function by simplifying input parameters needed for analysis of the contamination potential of various range conditions.
Understanding the mass transfer rate from discrete, solid-phase energetic particles into the soil-pore water is critical to the impact analysis of these residues for groundwater contamination. This project provided the foundation for a new predictive ability to assess the migration potential of residual energetic materials on military test and training ranges.