Effects of Wildfire and Prescribed Burning on ... - ACS Publications

Chapter 19

Downloaded by UNIV OF ROCHESTER on June 14, 2013 | http://pubs.acs.org Publication Date (Web): November 21, 2011 | doi: 10.1021/bk-2011-1069.ch019

Effects of Wildfire and Prescribed Burning on Distributed Particles of Composition-B Explosive on Training Ranges Richard A. Price* and Michelle Bourne U.S. Army Engineer Research and Development Center (ERDC), 3909 Halls Ferry Road, Vicksburg, MS 39180 *[email protected]

Residual ordnance compounds may exist on artillery training areas after low-order detonations. Particles of Composition-B (Comp-B) explosives distributed on training ranges could potentially be a source of RDX, TNT, HMX and their degradation products in various migration pathways such as leaching, surface runoff and biological exposure. Several studies have been conducted to identify toxicity and potential risks of munitions compounds to human and ecological receptors. However, little research has been conducted to quantify the effects of natural processes on the persistence of these and other materials used in military training activities. One such process is the occurrence of incidental or controlled burning of vegetation on training lands that theoretically could provide a remedial effect on residual Comp-B explosive on surface soils. Battelle Memorial Institute (under SERDP Work Unit CP-1305) evaluated effects of fire on subsurface concentrations of RDX and TNT in soil and found that under normal burn conditions on training ranges, thermal degradation did not occur a couple centimeters below the soil surface. This study evaluated the effects of fire on the fate of surface-distributed particulate Comp–B explosive that would result from low-order detonations on training ranges. Initial tests were conducted in a wind tunnel by placing pre-weighed 1 to 2-g particles of Comp-B explosive on the soil surface of vegetated test cells measuring 1.2 x 4.8 m. Vegetation was then

Not subject to U.S. Copyright. Published 2011 by American Chemical Society

try of Explosives and Propellant Compounds in Soils and Marine Systems: Distributed Source Characterization and Remedial Technolo ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


ignited and burned under various vegetation moisture and wind speed conditions. Loss of Comp-B particles was determined both by weight loss of recovered particles and chemical analysis of surface soil. Soil surface temperatures were measured at the soil surface and generally peaked at less than 176 deg C. Most Comp-B particles were easily ignited in the wind tunnel burns, and where complete ignition of particles occurred, chemical analysis confirmed residual Comp-B parent and degradation compounds were less than 3% of original mass. Plot-scale studies were conducted on unconfined 12 x 12 m plots following procedures describe above and confirmed wind tunnel results. Field evaluations were conducted during controlled burns at Forts McCoy, Pickett, Stewart, and Camp Shelby,and resulted in reductions ranging from 79-to 100%. Variations in vegetative biomass, moisture conditions, wind speeds, and other factors affected burn temperature and duration, ultimately controlling exposure of Comp-B particles to sufficient heat/spark for ignition. However, these tests verified that controlled or incidental burns can significantly remove residual Comp-B from training ranges, minimizing potential adverse impacts these materials can pose to the environment.

Introduction The distribution and transport of munitions compounds on training lands for live-fire artillery training is a concern. Unexploded ordnance (UXO) or low-order detonations (LOD) can result in particles of explosive materials, particularly Composition-B explosive (Comp-B), being distributed within training area ecosystems (1–3). The primary components of Comp-B, 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) may potentially migrate into groundwater and surface water. Dissolution rates have been described (4, 5) for Comp-B, and aquatic toxicity studies (6, 7) have shown aquatic toxicity and bioaccumulation of Comp-B constituents to aquatic organisms. While research has shown this potential to exist, widespread occurrences and adverse impacts on US Army training lands are not well documeneted. Simmers (8) found that while evidence of considerable UXO existed on an artillery training range at Fort McCoy, Wisconsin, sampling and analysis of sediments and surface waters did not detect any explosive residues. Several studies have evaluated plant uptake of RDX and TNT (9–13) demonstrating biotransformation of TNT in plant tissue and significant accumulation of RDX into leafy tissues. There are a number of factors potentially responsible for significant migration of Comp-B constituents in training areas that have not been thoroughly evaluated. Wildfires are natural events that once were considered destructive to ecological habitats. We now know that many native plant ecosystems require occasional wildfires to sustain native plant communities while inhibiting the 364



establishment of adventive vegetation. Keane (14) summarizes the effects of large fires in various regional ecosystems where most benefit from occasional fires as both a possible tool for the efficient restoration of fire-dominated ecosystems and an effective treatment for reducing fuel hazards. Many resource agencies, including US Army Natural Resource Offices, have pro-active prescribed fire programs to manage natural diversity on public lands. Fires that occur on an artillery impact area (AIA), as a result of munitions detonation, are generally allowed to burn and are simply contained within the boundaries of the AIA. The lack of interference has unintentionally resulted in high-quality native plant ecosystems on many U.S. Army training ranges, leading to establishment of critical habitat for many threatened and endangered species that appear to coexist with training activities. Fort Bragg, NC and Fort McCoy, WI are good examples of places where the existence of native plant ecosystems in artillery impact areas are sustained by incidental fires resulting from training activities. However, the occurrence of distributed UXO and their explosive components in training land soils serve as potential sources for contaminant migration into ground and surface waters, as well as potential impacts in foodwebs. A study conducted by Simmers et al. (15) and numerous field evaluations (16–19) observed that frequent burning of the training range vegetation resulted in high quality native flora, and authors concluded that migration of explosive compounds to surface waters or wetland and aquatic sediments did not occur at active artillery training ranges where UXO was present and wildfires frequently occurred. It is theorized, that residual sources of Comp-B in artillery impact areas exist mainly as particle forms of Comp-B distributed on the soil surface during low order detonations and that these particles are subsequently ignited during the occurrence of wildfire. The significance of this theory is that the continual introduction of residual Comp-B from training activities is mitigated by the thermal effects of wildfire. Comp-B is currently the most widely used explosive in the U.S. Army arsenal. These high explosive munitions are used in mortars, howitzers, and tanks on U.S. Army AIA. Comp-B is composed of approximately 59.9% RDX, 39.9% TNT, and 1% paraffin wax. In many cases, the percentages may vary and 1 to 7 % of HMX may exist as a byproduct of RDX production. The release of these compounds and their degradation products into the environment can have adverse effects on water quality, biota, and human health once these compounds migrate through groundwater, surface water and food chain pathways. A quantitative assessment of the fate of Comp-B during wildfire events was needed and is the basis of this study. This study evaluated the potential for thermal combustion of Comp-B particles and reduction in total mass of RDX, TNT and their transformation products to determine the efficacy of prescribed or incidental burning as an effective mitigation tool.

Materials and Methods Bench-scale, plot-scale, and pilot-scale studies were conducted to address both controlled and field conditions. Comp-B was obtained as Hexolite, reclaimed military grade B from a demil facility in Indiana and used throughout the study. 365



Comp-B was obtained in a 2.5 lb cylindrical shape. It was necessary to reduce the cylinder of Comp-B into smaller pieces for use in the needed studies. Pennington et al. (20) suggested that for modeling purposes, particle size should range from >12.5 mm up to the diameter of the ordnance round. This was based on studies that indicated the majority of distributed particles from LOD were >12.5 mm. The cylinders were determined to be brittle and capable of breakage by light impact. Each 2.5 lb cylinder was carefully tapped with a non-sparking hammer until cracks formed and large pieces were broken loose. These pieces were further reduced in size until all were less than 2.5 cm in diameter. Samples of the particles were submitted for chemical analysis by USEPA Method 8330 (21) to confirm the percentages of the parent compounds RDX, TNT and HMX.

Bench-Scale Tests Weighed particles of Comp-B explosive were placed in a pre-weighed pan of soil and covered with increasing weights of wheat straw and pines needles. The vegetation fuel was ignited with a hand torch and allowed to burn freely. Combustion of particles was noted, and residual Comp-B residues in the soil following burning were determined by chemical analysis of the soil. Additional tests were conducted to determine effects of weathering on combustion of Comp-B when exposed to heat and flame. Comp-B particles were placed in an outdoor test facility and exposed to climatic conditions for a full year. These particles were compared to unweathered particles in a specially designed propane combustion column equipped with temperature probes to determine effects of temperature on melting and combustion of Comp-B particles.

Wind Tunnel Tests These tests were conducted at the ERDC Big Black Test Site (BBTS) near Vicksburg, MS in a wind tunnel designed for this purpose. The purpose of these tests was to determine fate of Comp-B particles on a vegetated soil surface under different wind speeds and soil types. Soils were collected from near Camp Shelby, MS, Camp Bullis, TX, and Vicksburg, MS representing a sandy clay loam, clay, and loam classifications, respectively. Soils were placed in aluminum soil boxes each measuring 1.2 x 4.9 m and were seeded with Schizachyrium scoparium, a native perennial grass distributed over most of North America. After maturation and dormancy of the vegetated soils, 0.5, 1 and 2 g particles of Comp-B were placed at the soil surface, and the vegetation was burned under wind speeds of 1 and 4 mph and two plant moisture conditions (9 and 20%) to produce different burning characteristics (rate of spread, heat per unit area, fireline heat intensity). Temperature was monitored at the soil surface where Comp-B particles were placed and the fate of the particles (combustion, migration into the soil surface) was determined. Residual Comp-B was determined by weight loss of recoverable particles and chemical analysis of surface soil after each burn. 366



Plot-Scale Tests Three replicate plots measuring 12 x 12 m were established at the BBTS on indigenous vegetation (primarily Paspalum notatum). After vegetation became dormant, pre-weighed particles (approximately 0.5 g each) of Comp-B was randomly placed at eight sample points within the plots. Each sample point was marked with a 40.6 cm length of 1.3 cm diameter steel rebar. Three replicate particles were placed in three 5.08 cm diameter x 2.54 cm high stainless steel rings driven into the soil flush with the soil surface. Temperature probes were placed at each of the eight sample locations, and soil surface temperatures were measured and recorded on a data logger. In order to measure flame height, two 2.4 meter lengths of 1.3 cm steel rebar were placed 1.8 m apart. A 15 cm diameter cotton string was tied between the two rebar at heights of 30 cm up to 150 cm. The plots were burned following established techniques (22) for prescribed burning to control excess vegetative fuel. Burning occurred when climatic and fuel moisture conditions suitable for prescribed burning were present as determined by field measurements and as provided by the National Oceanic and Atmospheric Administration’s (NOAA) Fire Weather Website. Following burning of Paspalum notatum, a second test was performed on pine straw purchased from a local nursery and placed on the plots at a rate of 1360Kg/ha to replicate a natural evergreen forest floor. After allowing for a 4 month period of weathering, the process above was repeated.

Field Tests ERDC personnel coordinated with personnel at U.S. Army installations at Forts McCoy, WI, Pickett, VA, Stewart, GA, and Camp Shelby, MS to schedule field evaluations during prescribed burning at each installation. Since climatic conditions are critical in planning the execution of a prescribed fire, 1-3 day notices were the best lead-time available for ERDC personnel to mobilize to the field site. Particles of pre-weighed Comp-B were placed at various locations within the target prescribed fire area. Eight sample points were marked with a steel rebar, and three pre-weighed Comp-B particles were placed at each point. Temperature probes were placed in selected locations when time allowed. Flame height was determined as described in the previous section. Personnel from each installation executed the prescribed fire following established protocols at each installation. Following the conclusion of each prescribed fire, ERDC personnel entered the burn zone and recovered any unburned particles, exposed soil and documented observations. Project Note: In the initiation of the field tests, changes in the Department of Transportation (DOT) regulations severely restricting the transport of explosive materials made it necessary to investigate a substitute energetic material not classed as an explosive for DOT purposes. Several military propellants were investigated and compared to Comp-B for effects of temperature and flame exposure on combustion response. Laboratory tests determined that M10 propellant exhibited the same burning response to temperature and exposure to 367


fire, and was used as a surrogate for Comp-B in field tests at Fort Stewart, Fort Pickett and Camp Shelby.

Results and Discussion


Bench-Scale Tests Chemical analysis by USEPA Method 8330 verified the Comp-B used in these tests contained HMX, RDX and TNT at 6, 57, and 36%, of the total mass, respectively, with approximately 1% desensitizing wax. Both aged (1-year) and un-aged Comp-B began to melt at 200 deg F and combusted at 340 deg F. This is consistent with Material Safety Data Sheets (MSDS) for Comp-B which provide a melting point for TNT and the wax at 174-176 deg F and RDX at 374 - 392 deg F. The MSDS provides a boiling point of 464 deg F, a point which TNT explodes. In our testing, boiling of Comp-B initiated around 320 – 330 deg F at which point no solid Comp-B remained and combustion quickly followed. These observations are exhibited in Figures 1 and 2, with the aged Comp-B on the left. When solid Comp-B was exposed directly to flames, combustion was immediate at normal air temperatures.

Figure 1. Effects of temperature on melting combustion of aged and un-aged Comp-B. (see color insert)

Figure 2. (Left) Initial melting, (Middle) Liquid state, (Right) Combustion. (see color insert) 368



Wind Tunnel Tests Figures 3 and 4 show wind tunnel results comparing temperature profiles to loss of solid Comp-B under two windspeeds and moisture conditions. Temperature probe (T/C) numbers for the temperature plots correspond to the pole numbers in the % reduction graph. As these figures show, there is no clear correlation to peak temperature on the complete combustion of Comp-B. Length of elevated temperatures indicates greater potential exposure to burning vegetation and embers that quickly result in combustion of Comp-B. Increasing wind speed decreases heat exposure time but can increase temperature depending on the amount and characteristics of vegetative fuel. As shown in bench-scale tests, exposure to elevated temperatures (340°F) results in combustion of Comp-B. In wind tunnel tests, these temperatures were often not reached at the soil surface and direct exposure to flame and embers from burning vegetation was necessary for combustion of Comp-B. As shown in Figure 5, burn patterns may be affected by various factors, resulting in incomplete burns and lack of Comp-B combustion.

Figure 3. Temperature profiles and effects on Comp-B combustion (see color insert)

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Figure 4. Temperature profiles and effects on Comp-B combustion. (see color insert)

Figure 5. Complete burn (left) and incomplete burn (right) with Comp-B particle (circle). (see color insert)

Soil analysis indicated that parent compounds of RDX, TNT and HMX were present in soil where exposure of Comp-B particles to heat only resulted in slight melting of the particle (Table 1). Where combustion of Comp-B was 100%, residual concentrations of Comp-B parent compunds RDX, TNT, HMX and degradation products DNX, MNX and 2-A-DNT were also present. Migration of TNT to soil tends to be higher where combustion fails to occur as it has a lower melting point, while RDX is the prevalent constituent following combustion.

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Table 1. Comp-B constituents in soil following burning, mg kg-1. HMX

DNX

MNX

RDX

TNT

2-A-DNT

1.2 %

1.5

Effects of Wildfire and Prescribed Burning on ... - ACS Publications

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