Open burn (OB)/open detonation (OD) activities are necessary to destroy unserviceable, unstable, or unusable munitions and explosives. Munitions must be demilitarized or destroyed depending on their lifespan and other requirements. There are commercial demilitarization options available, but most of these cannot sustain the volume and sometimes the specific requirements for munitions destruction. Therefore, commercial demilitarization can be costly and inflexible.
The Department of Defense (DoD) operates about a hundred OB/OD areas. These areas are usually located at fixed locations on installations. These locations may be limited to one type of operation (i.e., burning of propellants during training activities), or they may be used for multiple operations (i.e., to destroy many types of explosives, pyrotechnics, and propellants). Due to the relatively small area that OB/OD areas cover, there is a high probability of explosives contamination in the soil.
Currently there is no in situ or remote alternative for management of soils on OB/OD areas. Addressing the problem of OB/OD areas acting as source zones for mobile contaminants using existing, ex situ remediation technologies will far exceed resources available for facility or range management and is currently not practiced.
The objective of this demonstration was to evaluate and develop a management technology to control active OD area contaminant mobility and promote contaminant degradation that is low cost and minimally resource intensive. The technology will reduce munitions constituents (MC) migrating into the environment by adding lime to the soil for alkaline hydrolysis of explosives and hydroxide metals stabilization.
The technology is based on the alkaline hydrolysis reaction of nitroaromatic and nitramine compounds at high pH (>10.5). The reaction occurs in the soil pore water where the explosive residues are rapidly degraded into smaller molecular weight compounds or byproducts. These end products, including formate and nitrite, are readily degraded by indigenous soil bacteria using both anaerobic and aerobic degradation pathways.
Topical application of hydrated lime, which is mixed into the surface layers of the soil (about 6 inches) has been demonstrated in the confined space of a hand grenade range (HGR) as part of ESTCP project ER-200216. The increased pH of the soil destroys energetic compounds even through the continuous loading of training activities on the range. In addition, the lime amendment provides hydroxides to the soil that can react with soluble metals and stabilize them within the soil matrix.
The field demonstration was conducted on the Aberdeen Proving Ground (APG) OD site. It involved adding hydrated lime to the OD area to transform explosive residues and stabilize metals at the site to prevent off-site migration. Aberdeen Test Center (ATC) tills the OD site approximately once a year to prevent vegetation growth and brush fires. Lime addition was coordinated with the discing to manage explosives in the shallow surface soil layer. Lime was dispersed on the site and mixed with the deeper soils by adding it to the hole dug for the waste munitions before the detonations. The detonations dispersed the lime along with the crater ejecta. Effective dispersion was monitored by surface soil sampling after the detonation fallout had settled. Once surface soil sampling was complete, additional lime was placed in the bottom of the crater prior to pushing the dispersed soil back into the crater. The mechanical movement of the soil back into the crater served to further mix the dispersed lime into the soil. The end result was a reactive zone of elevated pH that spans the depth of the detonation crater. This dispersion method would be the typical application technique for sites that are dudded (contain unexploded ordnance [UXO]) or not regularly tilled as a standard maintenance practice.
A laboratory treatability study determined the amount of lime to be added to the initial 9-acre OD site, the detonation pit, and the backfill operation. A lime loading rate of 0.5% to the APG soil was determined to be optimal to raise the soil pH to the required level of 11.5 for alkaline hydrolysis. A laboratory column study, also using the APG soil, was performed to study the depth of pH change that could be expected if the limed soil was overcovered by unlimed soil (as in ejecta from a detonation) compared to limed soil covering unlimed soil. When the lime amendment was well-mixed and covered the untreated soil, there was an increase in soil pH of <1 standard unit (SU) over the untreated control soil (study average). There was an insignificant change in leachate pH from Day 1 to Day 9 showing that, while the increase was stable, the lime transport, as indicated by pH change, was minimal.
The objectives of the field study dealing with explosives in soil pore water, groundwater, and source zone soil were all deemed successful. There was >90% reduction in RDX in soil pore water compared to the baseline, and the concentration of RDX was <2 parts per billion (ppb). Additional explosive compounds in the pore water were also reduced below baseline levels (to non-detect concentrations). The concentrations of all explosives compounds in the groundwater were also reduced to non-detect values. Soil explosives concentrations were less than baseline values even though the site experienced continued loading of explosives constituents. The pH changes in the soil were maintained >10.5 in the source area but decreased to <9.0 outside the source area. When comparing the metals (total and dissolved concentrations) in groundwater and soil pore water to baseline values, aluminum (Al) values were generally slightly higher following the lime treatment. This is hypothesized to be due to the high clay content of the APG soil and the subsequent high natural concentration of Al in the soil.
The technology had no, or minimal, impact on the range downtime and no health risk for personnel following standard health and safety guidelines. In an evaluation of the potential ecological effects of the liming, there was found to be minimal impact. The impact of the detonations and earth-moving activities were great enough that they masked any potential contribution from the lime.
This in situ lime technology does not involve the use of any toxic or hazardous chemicals. The only chemical used as the amendment is hydrated lime [Ca(OH)2], which is not regulated for addition to soil. Potential regulatory concerns associated with the use of the lime amendments on OD ranges include the potential for runoff with elevated pH. The elevated pH may be detrimental to biota or surface water quality. However, in neither ESTCP project ER-200912 (HGR) nor ESTCP project ER-200742 (OD site) was the pH of surface water runoff affected for more than a few feet downstream of the source zone.
The results from this study will improve OB/OD area design and operations and may result in sustainable management practices. The application of the technology could supersede the need for intensive characterization and result in relatively short-term degradation of explosives (hexahydro-1,3,5-trinitro-1,3,5-triazine [RDX], 1,3,5,7-tetranitro-1,3,5,7-tetrazocane [HMX], 2,4,6-trinitrotoluene [TNT], and associated compounds) in the soil, while stabilizing many munitions associated metals.