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Investigating Soil Remediation Techniques for Military Explosive and Weapons Contaminated Sites

 

IN-SITU REMEDIATION

 

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.

 

LIME TREATMENT

 

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 [18].  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 [13].  Once these byproducts form a covalent bond with the surrounding soil, they are considered to be safe for release into the environment [19].  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 [20].

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 Figure 9. TNT decomposition using alkaline hydrolysis [24]

 

 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 [13].  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 [13].  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.  

 

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Figure 10. Site plan of lime application case study in southeastern United States performed by Martin et al. [13] 

 

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% [13].  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) [3].

 

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.

 

LAND FARMING

 

 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 [1].  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 [21].  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 [22].  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.

 

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 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 [1].  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 [22].  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% [1].

 

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.

 

PHYTOREMEDIATION

 

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 [23].  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 [2].  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 [22].  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 [2].  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 [2].

 

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 [22].  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 [5].  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.

 

 

A SUMMARY OF IN-SITU REMEDIATION PROCESSES

 

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.

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