Sustainable use of high explosives compounds such as TNT and RDX on training ranges is an important goal from both a conservation and tactical standpoint. Although both explosives are regulated environmental contaminants, the use of these materials is critical in enabling most effective training of the U.S. military. Use of containment systems that prevent the transport of contaminants towards ground and surface waters can be logistically difficult, so technologies that completely degrade the chemical to environmentally benign end products are more attractive. Contaminant- destructive technologies, furthermore, reduce costs associated with eventual range closure.
Hand grenade ranges (HGR) are facilities that have been largely ignored from an environmental perspective in the past, but can present considerable problems. Explosive residues have been detected in HGR soils at levels from the low parts per billion (µg/kg) up to percent levels. RDX has been detected in leachate waters below active ranges and in surface waters leaving HGR impact areas. Further, the metal casing materials, particularly iron and zinc, can leach and contaminate surface and groundwater.
This article describes testing of lime treatment to destroy residual contaminants at HGRs. The lime increases the soil pH above 10.5, allowing for a chemical reaction called alkaline hydrolysis to destroy the explosive residuals. This approach effectively transformed both the legacy and ongoing energetic contamination, reducing transport of these residues into the groundwater while stabilizing the metal component of the munitions. The treatment was tested at an active, fixed-position HGR in the southeastern U.S. The range is used to train soldiers to use fragmentation grenades, which typically contain Composition B (60 percent RDX, 39 percent TNT, 1 percent wax binder) within a steel shell casing,.
During the field demonstration, the “boom count”—the number of hand grenades thrown per bay—averaged 13,750 grenades annually. Range maintenance during the field demonstration consisted of application of commercially available hydrated lime onto top soil twice a year. In addition, the bay impact area was re- graded to smooth out divots when deemed appropriate by the range managers. After approval from explosive ordnance disposal personnel, soil, groundwater and surface water samples were taken from both the limed and control ranges prior to and following each lime application event and throughout the field demonstration.
Lime application and sampling procedures required about four hours each, causing no impact to range training. Weather data were collected daily as the temperature and rainfall directly affect the percent moisture in the HGR soil and, in turn, the efficiency of the alkaline hydrolysis reaction responsible for transforming the explosives and stabilizing the metals.
An application of one percent lime was needed to elevate the HGR soil above the desired pH of 10.5. This equated to approximately 1-T of lime, which was added to the training bay and mixed to a depth of 6- in. Several techniques were used to apply the lime in the bays, from simply opening bags on the range by hand and raking to a uniform color distribution to using a drop-seed spreader. Mixing was accomplished by using a garden rotor tiller, a small disc, a cultivator, or a rake. Application of the lime in the HGR bays requires only Level D personal protective equipment, modified by the addition of a particulate respiratory mask and, possibly, the substitution of goggles as protective eyewear.
The results indicated that application of hydrated lime reduced the migration potential of munitions constituents from the HGRs. RDX concentrations in the soil were reduced by more than 90 percent compared to the untreated control, despite the continuous loading from training exercises. In addition, the metals were stabilized in the soil, with reduction in the concentrations of both iron and zinc leaving the range via surface water and leachate.
There was concern over the possibility of negative impacts on groundwater and receiving surface waters from the lime treatment of the soil. The impact area of the bay had to be maintained at a pH over 10.5 in order to accomplish the alkaline hydrolysis transformation; however, the water leaving the area was required to have a pH less than 9.5. It was established during the study that the pore water pH for the limed bay averaged slightly less than for the control bay, indicating the hydroxide ion was completely neutralized before the leachate from the bay could impact the groundwater. The pH of the surface water runoff collected from the limed bay remained at approximately 6.3. The surface soil pH in the offsite area collecting this water averaged 7.4, approximately 1 pH unit above the control bay. Therefore, there was no evidence of impact from the lime outside the treated area.
The cost of the technology was approximately $400 per lime application in 2007. The cost is based on using equipment already available and used for regular road and range maintenance. The time investment is measured in hours and no specialized equipment, operators, or training are required. Factors that may affect the frequency at which lime will need to be reapplied and, therefore, the cost are the natural buffering capacity of the soil being treated; the weather conditions on the range, and the frequency and type of range maintenance operations.
If the natural buffering capacity of the soil being treated is very high, the initial application rate of the lime could also be very high. A soil with low natural buffering capacity will require very little lime to reach the target pH, but may also require more frequent reapplication rates. The soil pH should be monitored to establish the need for reapplication of the lime.
Weather conditions will also affect the application and reapplication rate of the hydrated lime. The alkaline hydrolysis reaction requires some soil moisture; 20 percent to 30 percent of soil capacity is optimum. A dry climate will require fewer lime applications to maintain the target treatment pH. In a wet climate, the application rate may need to be more frequent.
Range management also affects the use of hydrated lime. The normal addition of top soil to fill divots or re-grade berms dilutes the lime and lowers the soil pH. When the soil pH drops under 10.5, alkaline hydrolysis slows and there is a reduction in the rate of explosive transformation. Additional liming should be considered along with normal range operations.
Although this technology has been demonstrated to be effective at reducing or eliminating munitions explosives on a specific type of range, it is not appropriate for all ranges. The Department of Defense (DOD) performs live fire training in a wide range of environments. Variations in soil type, climate, frequency of range use, and type of munitions used across the vista of DOD training facilities negate a simple answer to this complex environmental engineering problem.