Читать книгу Global Approaches to Environmental Management on Military Training Ranges - Tracey Temple - Страница 16

RDX and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) are nitramine explosives with similar chemical structures and properties commonly used in combination with TNT in formulations such as Comp B (40:60/TNT:RDX) and Octol (25:75/TNT:HMX) [5, 6]. In addition, military grade RDX may contain up to 10% HMX impurity as a side-product of the manufacturing process [3]. RDX is also used as a constituent in several IM fills such as PAX-21 (RDX, DNAN and ammonium perchlorate (AP)) and IMX-104 (RDX, DNAN and NTO) [7, 8]. RDX and HMX are common contaminants at military training ranges and have been identified in soils at demolition, burning and impact areas with higher concentrations found in areas where intense firing occurs towards a compact target area [3, 9].

Research into residue deposition from munitions containing RDX, e.g. mortars, artillery rounds and grenades containing Comp B, has shown that second-order detonations from blow-in-place or partial detonations are responsible for the majority of residues [10]. First order, live-fire detonations have been shown to consume on average 99.997% of the energetic materials [11–14], which may be an under-estimate in some cases due to the method of detonation, i.e. simulated live-fire [15, 16]. Similarly, for munitions with the IHE fill Pax-21 high order detonations resulted in deposition of less than 0.001% RDX [8].

On the other hand, low order blow-in-place detonations have been shown to deposit up to 50% of the Comp B fill from mortar (60 mm, 81 mm, 120 mm) and artillery (105 mm and 155 mm) rounds, with detectable concentrations of HMX [17]. Explosive residue from low order detonations was found to be deposited in large friable chunks of the original formulation (up to 5.5 g for Pax-21), the highest concentration of explosive on training ranges is usually found near these types of deposits [8, 12]. Although the number of low order detonations is thought to be less than 1%, this can be equivalent to several hundred rounds depending on the number fired per year, which over time can culminate in kilograms of explosive residue being deposited on training ranges [18].

UXO is also a likely source of contamination as the safest course of action is often to leave them in situ on training ranges, particularly if they are buried. The percentage of duds can be up to 3% and given that some training ranges have been in use for almost one hundred years, the amount of buried ordnance could be significant [19]. For example, estimates for some North American training areas suggest that there may be tens of UXO per acre (4047 m²) [18]. However, contamination from UXO is dependent on the rate of leaching of the explosive from the munition, which can take between 10 and 90 years depending on the acidity of the soil [18]. Therefore, UXO is unlikely to be a large source of current contamination, but may pose a contamination issue in the long term [14].

Depositions of RDX and HMX on the soil surface, particularly large chunks of explosive, may pose a risk to users of the site if the material is handled with bare hands or ingested. RDX is a suspected human carcinogen [20], and if ingested targets the central nervous system with the potential to cause seizures in humans and animals [21, 22]. RDX has a median lethal dose (LD₅₀) of 119 mg kg⁻¹ in rats [23], while HMX is much less toxic due poor bioavailability (not adsorbed by mammalian gastrointestinal tract) [24, 25]. The minimal risk level (MRL) for non-cancer health effects by oral RDX and HMX exposure is 0.006 mg kg⁻¹ day⁻¹, exposure above this level should be considered a danger to health [26]. The United States Environmental Protection Agency (USEPA) has established residential and industrial soil screening levels of 5.6 mg kg⁻¹ and 24 mg kg⁻¹, which can be used as a guideline for safe exposure limits [20].

RDX and HMX have low solubility in water (42 mg l⁻¹ and 4.5 mg l⁻¹ at 20 °C–25 °C, respectively) [27, 28], and therefore tend to accumulate in the top layers of soil where they are likely to be exposed to sunlight and may undergo photodegradation [3]. RDX is susceptible to photodegradation as a solid and in solution, though photodegradation is far slower in the solid (half-life of 2–3 months, compared to 2–3 days in solution) [29]. One of the main degradation products is nitrate (NO^3-), accounting for 20% of the photo-degraded RDX and contributing to pandemic nitrate groundwater contamination in North America [30–32]. HMX has also been shown to photo-degrade under laboratory conditions at an order of magnitude slower than RDX. However, in this case HMX was dissolved in an aqueous solution and exposed to consistent UV irradiation forcing degradation to occur faster than in the real environment [28, 33].

Although RDX and HMX have low solubility, their detection in groundwater at several military training ranges demonstrates that significant quantities of the explosive are solubilised [3, 9, 34–36]. The limiting factor for the rate of transport from an RDX/HMX source is the rate of dissolution from the formulation matrix, which increases with increasing rainfall [21, 37, 38]. Once solubilised, they tend to be transported at the infiltration rate due to their low sorption to soil [38, 39].

RDX degrades slowly in soil environments to form methanol, hydrazine and dimethylhydrazine via anaerobic transformation [40–42]. RDX has two proposed degradation routes as investigated in municipal anaerobic sludge. The first is initial reduction of the nitro groups to nitroso groups to form hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX), hexahydro-1,3-nitroso-5-dinitro-1,3,5-triazine (DNX) and hexahydro-1,3,5-nitroso-1,3,5-triazine (TNX) followed by ring cleavage. The second mechanism is initial ring cleavage to give methylenedinitramine and bis(hydroxylmethyl)nitramine followed by further degradation into carbon dioxide, methane and water. The nitroso derivatives of RDX have been detected in soil and water samples, suggesting that the reductive pathway may be preferred under environmental conditions, or that these compounds may be stable in solution for environmentally significant periods of time, i.e. long enough to impact a receptor [43–45]. HMX is more stable than RDX in the environment, and no HMX degradation products have been detected in environmental samples. However, in laboratory studies, the nitro groups of HMX can be reduced to nitroso groups under anaerobic conditions [42]. Of the two compounds, RDX tends to present more of a problem on training ranges due to its use in large quantities in munitions.