The United States Army has estimated that over 1.2 million tons of soil across its various facilities is currently contaminated due to explosives . This contamination can be found in a variety of locations, from formerly used munitions processing plants to active ammunition storage facilities to active ranges and artillery impact zones where training is ongoing. Outside the United States, the problem can be found not only amongst currently military facilities, but also in the remnants of minefields and battlefields from past and current conflicts. Due to the nature of ongoing conflicts, the remote location of training centers, and the danger of unexploded ordnance (UXO), site access for remediation of such contaminants is not always feasible or cost effective. However, the United States military and government is now armed with the knowledge that TNT and other explosives are known Group C carcinogens dangerous to both humans and the environment, thus justifying the necessity to remediate these contaminated sites before they create a greater hazard .
Past methods for disposal of military explosive and munitions waste have included dumping at sea, dumping in specified landfills, and incineration . Not only are these methods harmful to the environment, they are neither cost effective nor sustainable for a long term clean environment plan. In current range operations at demolitions sites, the range requirement for military units is to replace any craters created using explosives by simply filling in the hole with surrounding site soil. Over time, this simply churns the soil and transports the contamination closer to the groundwater table. The majority of TNT and RDX contamination found on training ranges today is lying on and near the surface (less than 1 foot underground) and is the primary source of soil, ground, and surface water contamination .
There is currently no best practice approved by the military for the remediation of the contaminated soil at its facilities and training sites; current practices rely on transporting the contaminated soil off site to be treated, which can unsurprisingly be a costly venture (and thus why proper remediation on military facilities are underutilized.) In order for on-site remediation to be an effective practice, a method must fit several criteria, including (1) be easily applied over a large area, (2) be easily applied without the need for special equipment or operators, (3) be cost effective, and (4) be able to be applied as part of normal site operations. Although military training sites often encompass hundreds of square miles, there is very limited space from which to conduct explosives training due to other environmental restrictions such as noise levels and endangered animal habitats. Typically there is only one demolitions range and one hand grenade range per military post, meaning that there is a concentration of contaminates on these sites compared to the rest of the military post. In addition, only having limited numbers of facilities means limitation to closing sites such as these for prolonged periods that would disrupt training required for deployments worldwide. Ideally, a remediation solution would be simple enough to be applied by the land user, such as an Army Combat Engineer unit that utilizes its own heavy equipment to apply a solution, and therefore not only remediates the soil but also gains invaluable equipment training in the process. Additionally, this process can be applied as part of everyday range operations, making costs more effective and predictable, and ensuring that the solution is sustainable and easily replicated across military facilities throughout the country.
Within our investigations, we begin with a historical look into military facilities around the world, followed by a brief introduction of the chemistry of typical military explosives. Following this, a discussion on the environmental effects of military explosives on humans and the surrounding environment will be addressed. Lastly, current and prospective remediation processes will be compared.
A HIBefore any investigations can be made regarding the importance or urgency of remediating explosive-contaminated sites, it is important to contextualize the number, and age, of said sites both within the United States and across the globe. Doing so will reveal the necessity in which remediation of these sites need to be undertaken. Our investigations begins in 1863 when German chemist Joseph Wilbrand first discovered 2,4,6-trinitrotoluene (TNT), one of the three main “energetic compounds” used for military application . It was not until around 20 years after Wilbrand’s initial discovery that TNT’s highly explosive properties began to be both recognized and utilized . The first major global use of TNT was during the onslaught of World War I. It wasn’t until the second World War that (1) the use of TNT around the world spiked to previously unprecedented levels and (2) the use of other energetic compounds such as hexa-hydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro- 1,3,5,7-tetrazocine (HMX) began to be utilized for military applications . (For more information regarding the chemical composition of TNT, RDX, and HMX, please visit the section titled “A Brief Introduction to the Chemistry of Common Military Explosives”.) Even today, TNT, RDX and HMX are still common energetic compounds for military use .
There are five main ways that energetic compounds in military applications have and are currently entering soil and groundwater environments :
The aforementioned modes of contamination take on various types and concentrations; however all contribute to the contamination that is the focus of our investigations. For our historical investigation of the five military uses of energetic compounds listed above, we focus on two areas: (1) the typical longevity of firing ranges and training facilities around the world and (2) the volume of such facilities throughout the last century.
Military production facilities, training complexes, and firing ranges within the United States have been active for many decades. For instance, the United States Pantex Ordnance Plant near Amarillo, Texas is currently a nuclear weapons production facility, but the plant has been in operation since 1942, where it first began production of artillery shells and bombs for US World War II efforts . The 16,000 acres of the plant, as well as surrounding areas of Amarillo, Texas, have been exposed to TNT, RDX, and HMX contaminated soils and groundwater for over 70 years. Given this, contamination near the Pantex plant has reached a depth of 10 meters below the ground surface, with reports of localized contamination reaching 85 meters . Contaminants have also been reported beyond the facility and are at a potential risk of contaminating the Ogallala aquifer (Figure 1), a 174,000 square mile aquifer that is a major drinking and agricultural water source for central United States .
Figure 1. A map of the continental United States showing the Ogallala aquifer in blue (modernfarmer.com)
A similar situation is for other US army bases. Joint Base Lewis-McChord (JBLM) (formerly Camp Lewis) in Washington state is one of the world’s largest military complexes with an almost 100 year history, and has been actively involved in all major wars since its inception . Currently, JBLM has 115 live fire ranges . Perhaps more unsettling than this is that JBLM also has 16 housing communities on the base, which, in regards to groundwater contamination caused by the firing ranges, are dangerously near-by .
We see similar trends in the age of military firing ranges beyond the United States as well. For instance, Cedar Spring Range and Training Area in Chatham, Ontario, Canada has been active since 1912, and Winona Range and Training Area in Grimsby, Ontario, Canada has been active for 77 years . England’s Defense Training Estate North (East) has been established since before World War I, with the nation’s other 15 major range and training facilities and 104 minor facilities following similar historic trends . We see similarly-aged firing ranges in all major nations across Europe and beyond .
With military facilities across the globe having been used for many decades, the contamination effects of TNT, RDX, and HMX on surrounding soil and groundwater sources have been nothing but exacerbated. The health effects of these energetic compounds have been heightened by the years of accumulated use at these military sites. (For more information regarding the environmental impacts of these three energetic compounds, please visit the section titled “Health and Environmental Concerns of TNT, RDX, and HMX”.)
Another important investigation regarding a historical look at military use of energetic compounds is the volume of production and use. TNT production peaked during times of war, namely World War II . According to Lewis et al., 2003, “By 1945, TNT production capacity in the US was as high as 65 tons of TNT per production line per day…while German production was 2.36x104 tons per month.” Similarly, in consequence of the Vietnam War, Vietnam was faced with a sharp increase of energetic compounds in its underground environments. Lewis et al., 2003 notes that “In 2002 The Viet Nam [sic] Ministry of Defense estimated UXO [unexploded ordinates, which are explosives or munitions that were fired, but did not explode because of technical malfunctions] and land-mine-affected land to comprise ‘approximately 7-8% of the country.’ Between 15-20% of UXO and mines from the war are believed to remain. Official sources estimate from 350,000-800,000 tons of war-era ordinance in the soil.”
Laos experienced more than 2 million tons of explosive air raids during 1964-1973 . Iraq, one of “the most energetics-contaminated countries in the world” has approximately 1,730km2 of energetic compound-contaminated ground . Lewis et al., 2003 goes onto say that in Iraq, “Affected sites…affect 1.6 million people. Contamination includes 20 million mines, numerous UXO sites, and many abandoned munitions sites. Over 50 million cluster bomblets were dropped on Iraqi soil.”
The majority of military energetic compound-contaminated sites within the United States and Canada are on testing and troop training facilities . Regardless, the estimated number of possible US UXO-contaminated sites is 10 million acres, not to mention the estimated 1.2 million tons of soil within the United States that have been contaminated with explosives (non-UXO) [5,7].
Investigating the volume of energetic compound soil contamination around the globe could continue endlessly – the Armenian-Azerbaijan conflict from 1988-1994, the Cold War (contamination due more to explosive manufacturing rather than actual use), the Indonesian and Korean Wars, etc. However the same conclusion would be determined: the volume of military-contaminated soil around the world has reached such enormous levels to not simply warrant, but rather require large-scale remediation. When this volume of soil is also put into context with the typically decades-old military facilities, the need for remediation ought to not simply be a military concern, but also an environmental and public health issue that warrants immediate attention.
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.
Figure 2. The chemical structures of TNT (a), RDX (b), and HMX (c) 
TNT’s simple and low cost production and overall chemical stability make it the ideal excitable compound for military applications . Despite TNT’s historic “popularity”, RDX has now become the more prefered excitable compound within the United States . 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 . 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]
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.
There are three main environmental concerns caused by military applications of energetic compounds according to Hansen et al., 2003:
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:
Figure 3. Potential routes military applications of energetic compounds can end up in the natural environment 
In Figure 3, the storage and waste disposal routes can prove the highest concentrations of contamination . “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 . However the effects of contamination caused by this thin layer, especially in the presence of surface water, can be profound . Contrary to the typical military fire range contamination, UXO contamination can span to deeper depths, and in greater concentrations . 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 . 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 . 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]
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 . For RDX, the EPA has determined a residential SSL value of 5.6 mg/kg and an industrial SSL value of 24 mg/kg . 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.
With a comprehensive background of the current state of excitable compound contamination within various military facilities, our focus turns towards remediation processes. Current remediation will be investigated first, followed by more promising in-situ processes that are currently in the experimental phase of implementation.
Since soil remediation for military facilities began, there have been three main remediation processes that have been utilized: incineration, composting, and soil slurry. While these methods prove effective in remediating energetic compound-contaminated soils, there are several large drawbacks to these methods (namely cost and feasibility). Such drawbacks call the need for improved remediation processes, such as lime treatment and land farming. However it is necessary to investigate current remediation practices in order to appreciate the need for improvements.
Incineration of military-contaminated soils has been one of the most effective and most common remediation techniques . The process offers a high level of accuracy and quality control . However, this method requires excavating the contaminated soil, transporting it to an incinerator, and the physical incineration - all of which can be very expensive and inefficient for large scale contamination sites . In the United States alone, the average cost of incineration is $30-40/ton, with some areas, such as Detroit, experiencing rates as high as $150/ton . With over 1.2 million tons of explosive-contaminated US military soil, high incineration costs are a driving determent to large scale remediation. Additionally, by incinerating the contaminated soil, more negative environmental effects (aside from the transportation of the soil to the incinerator) are incurred when the soil is incinerated, which can release harmful substances into the air if not properly performed and monitored. Lastly, after the soil is incinerated, the ashes of the soil need to be properly disposed of in a landfill - a process with a set of its own environmental factors. Reuse of the contaminated soil is impossible in this method, meaning additional resources are needed to resupply a military site with soil, which would begin the costly remediation process once again.
According to Lewis et al., 2003, composting “was the first biological treatment process to be tested, approved, and selected for use in remediating military sites.” Composting as a form of remediation involves mixing the contaminated soil with organic materials such as straw, wood chips, manure, or vegetable waste, as well as potentially a bulking agent for aeration [1,5]. Composting increases the amount of bacterial activity within the soil, allowing for both aerobic and anaerobic processes to occur [1,5]. These processes increase the degradation rates of energetic compounds in the soil. Similar to incineration, the contaminated soil used in the composting remediation method is transported off site. Once the soil is transported to an appropriate composting facility, it typically undergoes either static pile or windrow composting . Static pile, as the name implies, combines all the contaminated soil and organic additives into a large pile that remains untempered save for a motorized blower used for ventilation . Figures 4 and 5 depict a typical static pile composting system.
Figure 4. A schematic of static pile composting (fao.org)
Figure 5. A static pile in use, complete with ventilation system (o2compost.com)
Alternatively, windrow composting involves creating elongated piles of a soil-organic matter mixture that are periodically turned and hydrated. Windrow composting is a cheaper method than traditional static piles since a mechanized ventilation is not required for windrow. Figure 6 shows a typical field of windrows, while Figure 7 shows typical equipment used to turn the windrows.
Figure 6. Typical windrow field (labroots.com)
Figure 7. Typial windrow-turning equipment (dirtmaker.com)
While effective at minimizing levels of TNT, RDX, and HMX within various types of contaminated soils, composting as a form of remediation can be a slow process, difficult to use on large-scale remediation projects, as well as requiring large amounts of organic material per pile [1,5]. However, unlike incineration, soil treated via composting can be reused. Remediation composting is a cheaper alternative to incineration, but only in larger quantities; Lewis et al., 2003 notes that “Costs have been estimated at $206-$766 per ton for windrow composting, an estimated 40 to 50 percent savings over incineration at the 1,200-30,000 ton scale.”
The final of the three common remediation practices for military-contaminated soil is creating a soil slurry. The bioslurry process is “designed to optimize mass transfer of nutrients and electron acceptors by using mechanical mixing and aeration” . It involves mixing the contaminated soil with water and organic materials to form a watery slurry that is contained within an constructed lagoon pool. The pool is initially mixed, and then it is “[I]ncubated to allow anaerobic conditions to develop through metabolism of the organic amendment by the indigenous microorganisms. The highly reducing environment is intended to promote the complete reduction of TNT” .
Figure 8 depicts a lagoon pool with an agitator for the slurry.
Figure 8. A typical agitator in a slurry lagoon pool 
As with both incineration and composting, using slurry as a form of remediation requires the soil to be removed from its original location and treated. Original to the slurry method though, there are significant overhead costs for constructing the lagoon and installing the agitator equipment . According to Lewis et al., 2003, “Costs for treating 5000 cubic yards of soil [via the slurry method] have been estimated at $147 per cubic yard ($200-$600 per ton)” which is quite evenly comparable to the compost method. The slurry method is also very sensitive to the temperature fluctuations during the incubator process . However the time required for remediation through the slurry process is much lower than the composting method. Lastly, remediated soil via the slurry method is often not reused and instead either dumped into ditches or properly disposed of according to regulations .
While the three aforementioned soil remediation practices have a range of benefits and drawbacks, the largest issues with all three methods are that (1) the contaminated soil must be transported away from the military facility to be treated (and in some cases, no longer usable) and (2) the unrealistic use of these practices on large scale remediation practices. In regards to the latter, our earlier discussions both in terms of the history of military facilities and the health impacts of energetic compounds point to the fact that remediation of these facilities must be undertaken on a large-scale immediately. The call for such needs have been answered in the more recently-discovered in-situ remediation techniques discussed in the following sections. While these new practices are not without their own flaws, they are certainly a step towards a more effective remediation process.
In order to conduct soil remediation on large or even simply active sites, methods other than removing the soil for treatment must be given consideration for practicality purposes. In-situ methods fit this need, and may possibly even present a long-term sustainable solution for sites where it is presumed that contamination will continue for at least the near future. Currently three in-situ methods have been, and continue to be, researched and have shown some degree of success in breaking down contaminates caused by explosives. The three methods are lime treatment, land farming and phytoremediation.
Using lime to break down the contaminants of explosives to safer compounds is accomplished through a process called alkaline hydrolysis. The concept itself is not new, as German J.V. Janowsky first reported on the transformation of TNT in basic solutions in 1891 . Alkaline hydrolysis occurs within the pore water of a soil, where a hydroxide ion is attached to the contaminant in question. During this process, alkaline hydrolysis decomposes nitroaromatic and nitramine compounds into (in)organic salts, soluble organic compounds, and various gases, as shown in Figure 9 . Once these byproducts form a covalent bond with the surrounding soil, they are considered to be safe for release into the environment . Due to the reaction happening in the pore water, it is necessary for the soil to maintain enough moisture content (which the US Army Corps of Engineers’ Engineer Research and Development Center research shows is 25 to 30 percent ) for this chemical reaction to take place throughout the contaminated site .
Figure 9. TNT decomposition using alkaline hydrolysis 
The advantages of using lime as an in-situ treatment option is that lime is a relatively cheap substance that can be easily applied over large areas through typical spreading techniques that are already used in industries such as agriculture. One study conducted by researchers at a US Army hand grenade range estimated that the cost to treat soil for an entire year was only $21-$60 per meter cubed, which includes the cost of personal protective and monitoring equipment . In addition, since there is no need to remove the soil to mix in the lime, the hazard caused by disturbing any UXO hidden in the soil is greatly reduced. By placing the lime at various locations in a predefined pattern, the lime treatment also makes it possible for ranges (or similarly used sites) to remain active throughout the remediation process. This was the case for research conducted by Martin et al., who treated one bay of a four bay hand grenade range in the southeastern United States, with the other three bays remaining open . The schematic of the lime application in this case study is seen in Figure 10. Similar remediation processed can be made for demolitions ranges and artillery impact areas.
Figure 10. Site plan of lime application case study in southeastern United States performed by Martin et al. 
As far as effectiveness, lime treatment has been shown to reduce the concentration of RDX contaminants by 75%, the concentration in soil pore pressure by 75%, and the concentration in surface water runoff by 98% . This percentage is lower than several of the other methods, especially ex-situ methods such as incineration which can be carefully controlled and monitored. Since the lime process is in-situ and open to the environment, several factors such as moisture content, soil type, and land use/explosive contamination concentration all play a role in determining the process’ effectiveness per site. The alkaline hydrolysis process is also more effective in higher pH level soils, with 10 being the lower boundary of effectiveness, and high levels of remediation seen at a pH of 11 and higher.
The disadvantages to using lime to treat soil contaminated with TNT and RDX is that the high pH levels require achieving maximum effectiveness of the alkaline hydrolysis process. Having pH levels greater than 9.5 is considered to be detrimental to the groundwater, and more research must be done to ensure that the high pH levels on the soil surface during the lime application process are not translating to high pH levels at lower soil depths. During another research project conducted through the US Army Corps of Engineers, hydrated lime dosages of 1, 3, and 5 percent of the soil mass on the alkaline hydrolysis of explosive contaminants were tested. At the 3 percent dosage, the pH of the soil rose to over 10, while the 5 percent dosage achieved a soil pH over 11 (the initial pH of the soil prior to the lime application was averaged at 4.54) .
Another factor that is unavoidable due to the nature of the alkaline process is the requirement for a minimum moisture content in order for the process to correctly occur. While the moisture allows for the process to break down the contaminants, too much moisture could also carry any contaminants not exposed to the lime down to the groundwater table faster, thus making the situation a much greater problem than before. With careful design and a thorough site investigation of the TNT and RDX concentrations (in order to correctly determine the amount of lime and moisture content required to drive the alkaline hydrolysis process while avoiding groundwater contamination), the lime treatment is currently one of the most promising of the in-situ remediation methods.
Another in-situ technique that holds potential is the concept of land farming. In this method, water and a carbon source (usually molasses, but other sources such as birch bark have been investigated), are tilled into the soil in order to break down contaminants . Figure 11 shows a schematic of land farming. The advantages to land farming are similar to those of other in-situ techniques in that (1) the soil remains in-situ, (2) tilling is a well known and easily applied agriculture technique that does not require special equipment or operators, and (3) the cost for the carbon source materials is relatively cheap. A 2000 study listed the price for molasses at only 5 cents per gallon . Similarly, the birch bark used in one land farming study was taken directly from the site and ground up for the soil and thus proved enormous cost savings . In a remediation study by Gerth et al. conducted using land farming techniques, results showed that molasses was a very effective treatment for TNT contamination, with birch bark also reducing the TNT concentrations, but not to as low levels as was seen with molasses. By tilling through the land farming practice, there is the potential for the carbon source to neutralize the contamination up to several feet below the ground surface, which is an aspect the lime treatment does not currently address.
Figure 11. Example of Land Farming Operations (env.nm.gov)
Land farming is not a remediation method that is appropriate for a wide range of contaminated sites. Since the land must be treated like other agricultural sites that require constant tilling, the site must be clear of any other vegetation or debris in order to mix the carbon source into the soil. Constant tilling also means a greater chance for the contamination leachate to reach the groundwater; one study conducted at a Louisiana Army Ammunition Plant suggested that land farming should be conducted in a constructed cell with a liner to mitigate this issue. However doing this would make the process of land farming no longer in-situ, which is currently one of the large benefits to this process . In addition, the constant tilling does not lend this process well to sites that must remain active during the remediation process. This is especially true when using a carbon source like birch bark, which has a long period before reaching its maximum effectiveness and therefore will render portions of ranges unusable for extended periods of time. In the 2003 study conducted by Gerth et al. in Germany, the use of molasses took 20 days to reach a TNT transformation level of 97%, while it took 86 days for rotting birchwood to reach a TNT transformation level of 50% in lab conditions, and a full 90 days to reduce 80% in-situ in a full scale test . Another land farming study conducted by Brandon Clark and Raj Boopathy at an ammunition plant in Minden, Louisiana, United States, in 2007 showed it took 182 days for molasses to reduce TNT contamination by 82% .
Constant tilling in the land farming technique also increases the cost of this method, as the number of operator and equipment hours is much larger here than those for lime treatment (which is simply dropped in place and left to activate.)
Another disadvantage to land farming is the potential hazards that tilling land with possible unexploded ordnance (UXO) poses. On active military ranges in the United States, the procedures and necessary precautions are in place to prevent UXOs from being left in place. However, the same cannot be said for sites worldwide, especially those that that were once active battlefields and have never been subject to any supervision or maintenance operations. In these cases, in order for land farming to be a safe option, extensive work would have to be performed by both Explosive Ordnance (personnel specially trained in the disarming and removal of explosives) and geotechnical engineers using technology such as ground penetrating radar in order to develop a comprehensive site investigation and minimize any threats before moving heavy equipment onto the site.
Another in-situ remediation process is known as phytoremediation. This method is similar to land farming in that both processes remediate the underlying ground in similar “agricultural-like” fashions. In phytoremediation, rather than use a carbon source that is tilled into the ground, the process requires the use of plants or plant matter to trap the contaminants. Several researchers have looked into methods relating to phytoremediation, and the process is quite broad with several methods of implementation. The first is the planting and growing of actual plants designed to capture the TNT or RDX remnants within their structure through their roots system. One such plant is a genetically modified tobacco plant, and once the plant has become mature it is removed from the field and incinerated . A second method of phytoremediation involves spreading sludge over the contaminated soil. The sludge comprised of enzymes from a spinach extract that can break down the contaminant and transform the nitrogen bonds into byproducts deemed less harmful to the environment and people . A third method, which has only been accomplished in small scale testing, is the construction of specially engineered wetlands with specific vegetation that can trap the contamination and also serve a dual purpose of cleaning any contaminated groundwater . This method has shown promise in low levels of contamination, but has yet to be tested in a full-scale scenario.
Due to the large variety of techniques that can be implemented using phytoremediation, the advantages and disadvantages become much more “technique-specific”. In general, an advantage to phytoremediation is that it is a very environmentally friendly remediation approach that renders the land aesthetically pleasing to the public. Additionally, like the other in-situ methods, phytoremediation has the ability to be utilized over large land areas using simple agricultural processes and equipment. Phytoremediation methods have so far proven to be effective in small scale testing, with the spinach extract reducing the concentration of TNT between 71% and 94% over a 30-day period . At locations that are no longer under military control and are easily accessible to the public, phytoremediation methods offer a solution that may be more accepted with less negative connotation than ex-situ methods where the presence of large excavation equipment and removal of thousands to millions of tons of soil may create the perception of a much worse situation.
Cost for the spinach extract research treatment was very reasonable at $6.88 per cubic meter, although this cost only accounts for material and the process of making the spinach extract enzyme treatment, and not the cost for equipment and operators to apply it .
Unlike the process the spreading lime on the ground surface of a contaminated site, farming special plants and/or developing engineered wetlands are processes that require much more time and effort. For the constructed wetland, the recommended time for adaptation alone is more than eight weeks . In addition, once a plant is actively being farmed or a wetland is constructed, the site obviously no longer can maintain its status as an active military training ground, and due to the large amount of capital invested that can be required for some remediation techniques, it can be argued that it will not be desirable for the site to ever return to an active status. This in turn means that phytoremediation may be best applied to sites that have been identified where there is no current or planned activity (military or commercial), and where the is no need to develop the site for at least one year (many years in the case of a constructed wetland). This also becomes important because plant matter can only treat a limited amount of soil, and the soil can only sustain a certain amount of plant life. Research by Lewis et al. in the Journal of Environmental Management indicates that TNT can be removed efficiently at 4 ppm, but at concentrations higher than 20 ppm, the efficiency of the method becomes greatly diminished . Additionally, because this process is heavily reliant on the plants and organic matter being used, it is very susceptible to the soil and site conditions present. Tobacco is not a plant that can be grown worldwide, thus limiting its application to certain regions, and engineered wetlands will run into similar constraints in trying to find plants that not only can trap contaminants and filter them out, but can also thrive naturally given the site conditions present.
Out of the three proposed in-situ remediation techniques listed above, phytoremediation is the one with the least number of published case studies. Many conclusions regarding the effectiveness of this process have been gathered in laboratory settings. However the aforementioned 2010 study conducted by Richardson et al. opens the path for similar in-situ case studies to be conducted.
Table 3 below compares the three aforementioned in-situ remediation processes. An “x” in the appropriate cell represents that that particular method accomplishes the criteria necessary for large scale implementation. A “x(-)” represents that while the method meets the criteria, it is not the best suited to do so and depending on the site characterization, may not apply well. While all three methods are arguably more effective than the three ex-situ methods described earlier, these methods are still not without their own drawbacks. The key in deciding on which remediation process to utilize is to investigate the specific contaminated site in question and assess which method fits with the site’s logistics (geographical, monetary, time, etc).
Table 3. Evaluation Criteria for In-Situ Explosive Remediation Methods.
In order to best remediate a site contaminated with explosives such as TNT, RDX, or HMX one must carefully analyze not only the type of contamination, but also the size and nature of the site itself. For active sites such as weapons ranges, in-situ remediation is the preferred method, as it allows for ongoing operations, especially those critical for the training of Soldiers, Airmen, and Marines. In-situ remediation is also sought after for sites where site access is not easily obtained, and therefore large-scale remediation efforts are not feasible. Solutions for these active sites must meet criteria in which they prove to be cost-effective, applied without the use of special equipment or specially trained operators, and be able to be integrated in routine range operations. The need for a cost effective solution is driven by the range sizes, which can range from several acres to many square miles. In addition, without the “buy-in” from those operating the ranges and those allocating the funding for range maintenance, any remediation solution on an active site will not remain sustainable. Currently the application of lime through alkaline hydrolysis and the use of biological methods through phytoremediation hold the best potential for in-situ methods. This is not to discount ex-situ methods which, when applied to certain sites, can be the effective methods. Locations such as abandoned munitions plants or other facilities where there is not an immediate need for the land and it is deemed appropriate to bring in equipment and personnel may allow for the most complete removal of the contaminants from the site. Incineration, composting, land farming, and soil slurry methods all have proven to be effective ways to break down the contaminants, although for the reasons above they are only best for special situations and sites where other contaminates are present that are not removed using in-situ methods.
Ultimately, while ongoing research continues to examine ways to remediate contaminated sites in the most effective ways possible, there may also be the opportunity for research into the design and development of ranges that can prevent contamination from becoming an issue. Using design methodology from sites with contamination such as landfills could help develop base containment systems using clay liners and geotextiles that can prevent contaminants from reaching the groundwater and outside environment. While this would be of no consolation to the 1.2 million tons of soil already in need of remediation, it could help to prevent the situation from worsening and ultimately the cost of constructing the initial range could prove to be cheaper than the cost to continually remediate it in the future.
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