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Investigating Soil Remediation Techniques for Military Explosive and Weapons Contaminated Sites

 

A BRIEF INTRODUCTION TO THE CHEMISTRY OF COMMON MILITARY EXPLOSIVES

 

Military explosives are made up of many different chemical compounds known collectively as excitable compounds because of their explosive capacity.  As mentioned at the start of the previous section, the three excitable compounds most commonly found in explosives are 2,4,6-trinitrotoluene (TNT), hexa-hydro-1,3,5- trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), with TNT being historically the most commonly used for military applications [1,5].  The chemical structure of TNT, RDX, and HMX are depicted in Figure 2.

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Figure 2. The chemical structures of TNT (a), RDX (b), and HMX (c) [7]

 

TNT’s simple and low cost production and overall chemical stability make it the ideal excitable compound for military applications [5].  Despite TNT’s historic “popularity”, RDX has now become the more prefered excitable compound within the United States [5].  As can be seen in Figure 2, RDX and HMX have very similar chemical structures, and as such, the two compounds both have approximately 1.5 times the explosive power of TNT [5].  RDX and HMX also have “less affinity for soil surfaces and [are] more mobile contaminant[s] than TNT…[with] greater mobility [than TNT].  Table 1 below lists the three aforementioned excitable compounds and their water solubility.  This data is critical when dealing with these compounds within soil and groundwater environments.

 

Table 1. TNT, RDX, and HMX Water Solubilities [adapted from 5]

 

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Having a thorough understanding of the chemical composition and properties of excitable compounds used in military explosives is crucial for analyzing contamination, and thus eventual remediation, processes.

 

HEALTH AND ENVIRONMENTAL CONCERNS OF TNT, RDX AND HMX

 

There are three main environmental concerns caused by military applications of energetic compounds according to Hansen et al., 2003:

 

(1)  Contamination migration via air or surface transport

(2)  Potential interaction between energetic compounds and surrounding ecological

receptors

(3)  Transportation of energetic compounds into surrounding soils and groundwater sources

 

For the sake of our investigations, we will focus mainly on the latter of the three paths.

In conjunction to the three possible contamination paths, Figure 3 lists potential ways energetic compounds can end up in the natural environment:

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Figure 3. Potential routes military applications of energetic compounds can end up in the natural environment [7]

 

In Figure 3, the storage and waste disposal routes can prove the highest concentrations of contamination [7].  “Military action or training” in regards to fire ranges (not UXO) predominantly affect only the first few centimeters (approx. the top 15 cm) of the ground surface [13].  However the effects of contamination caused by this thin layer, especially in the presence of surface water, can be profound [13].  Contrary to the typical military fire range contamination, UXO contamination can span to deeper depths, and in greater concentrations [14].  Corrosion of UXO will also increase the energetic compound concentrations in surrounding soil and groundwater. According to Clausen et al., 2006,

 

“In the case where UXO lie exposed on the ground surface, soil and atmospheric conditions can significantly impact corrosion processes. When an ordnance enters the soil, it compacts the soil adjacent to the round, resulting in compressed soil pore spaces, a decrease in soil permeability, and an increase in matric potential. Consequently, water in the compressed soil pores will take longer to drain than in the uncompressed surrounding soil. Therefore, water in the soil matrix immediately adjacent to UXO may be in contact with it longer than expected based on soil type.”

 

With such variable types and routes for military energetic compound contamination, the health and environmental effects of TNT, RDX, and HMX are of a major concern.  Energetic compounds negatively impact humans as well as plants, animals and soil-dwelling microorganisms.  According to Pichtel, 2012, “In humans, TNT is associated with abnormal liver function and anemia, and both TNT and RDX have been classified as potential human carcinogens...TNT was found to be mutagenic.”  Pitchel, 2012 goes on to mention that RDX leads to convulsions within mammals.  However, according to the Environmental Protection Agency (EPA), TNT, while it can be harmful to humans and animals, does not bioaccumulate in these species [15].  Alternatively, TNT is capable of being metabolized by garden, aquatic and wetland plants, as well as some species of trees - inhibiting plant growth [6,15].  TNT and other energetic compounds also affect the microbial levels in soils.  In a study conducted by Meyers et al., 2007, “With TNT contaminations as high as 6,435 mg TNT 1/kg soil and RDX up to 2,933 mg RDX 1/kg soil, the...soils were almost sterile with low microbial biomass...culturable bacterial population, and undetectable fungal population.”  Soil microbes and bacteria are critical to the soil’s agricultural applications, so a lack of these species due to the introduction of energetic compounds is detrimental.

 

Human exposure to TNT, RDX, and HMX can take on a variety of forms, but the most likely form is through contaminated soils and groundwater, especially those around military facilities [15].  Table 2 outlines major American and Canadian military facilities and their average concentration of various energetic compounds.  For our purposes, we will focus our investigation of Table 2 on the data for TNT, RDX and HMX only.

 

Table 2. Energetic compound concentrations on American and Canadian military facilities [adapted from 7]

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 The EPA has set many regulations on the health standards for various energetic compounds; we can compare these values to those found in Table 2.  The EPA determined a “residential soil screening level” (SSL) of 19 milligrams of TNT for every kilogram of soil and an “industrial” SSL of 79 mg/kg to be acceptable [15].  For RDX, the EPA has determined a residential SSL value of 5.6 mg/kg and an industrial SSL value of 24 mg/kg [16].  The EPA has not explicitly outlined SSL values for HMX, but given the compound’s chemical similarity to RDX, the two could very likely have similar levels of acceptability.  Using these EPA regulation values and Table 2, we see that the majority of the sites are below the EPA SSL levels, with a few exceptions (i.e. CFB Gagetown Training Area in New Brunswick, Canada, and Weldon Spring Ordnance Works in Missouri).  However just because a site does not contain levels of energetic compounds in its soil that exceed a specified EPA SSL does not mean that site does not still negatively impact its environment.  Nor does it mean that the site does not exceed the SSL levels in localized areas throughout the site, because as seen in several of the sites, compound concentrations can be given as site averages.

 

 Investigating the various modes of contamination (testing range, UXO etc), the human side-effects and environmental impacts of energetic compounds in soil and groundwater, as well as the compounds’ levels in major North American military sites and how they compare to EPA standards, it becomes quite evident that large-scale remediation across military sites is a pressing issue that warrants more attention.

 

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