Global Approaches to Environmental Management on Military Training Ranges. Tracey Temple
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.
1.3.2 Nitroaromatic explosives
Nitroaromatic explosives such as TNT and DNAN have been used in high explosive fills and propellants for many years. TNT in particular is often used in combination with RDX and is therefore commonly detected in surface soils around impact areas [11, 27, 45, 46]. Similarly to RDX, deposition on military training ranges is most likely from second-order detonations, open-burning and UXO [17]. The nitroaromatic 2,4-dinitrotoluene (2,4-DNT) is found in smokeless powders and as a plasticizer in propellants and, along with 2,6-dinitrotoluene (2,6-DNT), is a by-product of TNT manufacture and a common TNT biodegradation product. Both 2,4-DNT and 2,6-DNT are frequently detected on training ranges [3, 36].
The impact of TNT on the environment can be influenced by its joint deposition with RDX in formulations. TNT is more soluble in water than RDX and HMX (130 mg l−1 at 20 °C), but in Comp B formulations TNT dissolution is limited by RDX, which is slower to dissolve and protects TNT deeper inside the particle [27]. Unlike RDX, very little TNT has been detected in groundwater at military training ranges though it is commonly detected in surface soils [3, 47]. However, TNT degradation products such as 2-amino-4,6-dinitrotoluene (2A-DNT) and 4-amino-2,6-dinitrotoluene (4A-DNT) are frequently detected suggesting that TNT transport is limited by biodegradation [36]. In addition, TNT and its degradation products are more likely to sorb to soil compared to RDX, and the amino groups on 2A-DNT and 4A-DNT may covalently bind to functional groups in the soil, immobilising the contaminants [41, 42].
In recent decades, attention has turned to the development of IHE fills resulting in renewed use of DNAN as replacement for TNT in melt-cast formulations (up to 40%) [48, 49]. In the past, DNAN has not been found in large quantities on training ranges, mainly due to its limited use in ordnance. However, as IM are increasingly adopted by militaries worldwide, DNAN may present a future environmental issue. Similar to TNT and RDX, very little DNAN has been found to be deposited in high order detonations with only a few milligrams per tested round (60 and 81 mm mortars), amounting to less than 0.005% of the original mass [50]. Low order or blow-in-place detonations are much more likely to result in significant deposition, with up to 20% of the initial mass recovered after simulated detonations. This can be as much as 34 g for the 60 mm mortar, and 49 g for the 81 mm mortar [50].
Unlike Comp B residues, IHE containing DNAN (such as IMX-101 and IMX-104) dissolve throughout their volumes as DNAN and the other major constituents are soluble in water (DNAN: 276 mg l−1) and therefore dissolve more quickly [51]. Dissolution rates are dependent on average rainfall, as well as the size of the residue particles, making it difficult to accurately predict the rate of dissolution in a given environment [38].
Research into DNAN has shown that it undergoes rapid aerobic biodegradation with complete mineralisation to nitrites possible within 100 h in culture [52]. In soil environments, DNAN has been shown to be degraded by both anaerobic and aerobic processes [53]. However, under aerobic conditions very little chemical transformation was observed, with DNAN more likely to be absorbed to the soil — particularly soils with high organic content [54–57]. Under anaerobic conditions DNAN is reductively bio-transformed to 2-amino-4-nitroanisole (2-ANAN) and 2,4-diaminoanisole (DAAN). In soil column studies 2-ANAN has been shown to be the most common degradation product, with trace dinitrophenol (DNP) also detected [57]. DNP has been identified as a common photodegradation product of DNAN, and is more likely to be produced in solid IMX-104 residues on the surface, or in surface waters [58].
DNAN has an oral LD50 of 199 mg kg−1 and a LD of 300 mg kg−1 in rats, suggesting that DNAN would also be toxic to humans [59, 60]. The common degradation products 2-ANAN, DNP and DAAN are associated with slightly lower toxicity compared to DNAN in standard toxicity models [53]. DNAN and its degradation products exhibit significant toxicity in aquatic environments with a half effective concentration (EC50) of 60.3 mg l−1 as determined by Microtox [61]. Whilst limited to model species, the repeated evidence of toxicity suggests that DNAN presents a significant risk to freshwater aquatic species such as micro-organisms, amphibians and fish [62, 63].
1.3.3 Propellants
Nitroglycerin (NG), NQ, 2,4-DNT and 2,6-DNT have been detected at firing points on many US training ranges [9]. Studies have also found high concentrations of perchlorates [36]. The presence of NC on firing ranges has not been extensively investigated as it tends to be deposited as discrete fibres that are difficult to quantify [9]. An added complication is the tendency for other explosives