Testing and training ranges are important for maintaining readiness of the Army and other military Services. Contamination by high explosives such as 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) at these ranges has been documented in the United States and in Canada and has the potential to impact military activities. The major contamination threat on ranges is the movement of contaminants through the soil profile causing contamination of groundwater by explosives residues and movement into the food chain through plant uptake. Promising in situ technologies for contaminated soils include phytoextraction, the use of plants to take up and remove contaminants from the soil, and phytostabilization, the use of both plants and soil amendments to prevent contaminants from moving out of the area. However, a better understanding is needed of the fate and transport of contaminants in vegetation and soil before effective treatments can be used. The objective of this project was to quantify phytoremediation, containment, and leaching of solid phase composition-B (comp-B)-derived TNT and RDX in soil cores vegetated by herbaceous plants under environmental conditions representative of the southern United States.
Loamy sand from Camp Shelby, Mississippi, was considered representative of field conditions on testing and training ranges and used in this research. Phytoremediation of comp-B-derived TNT and RDX was quantified in 0.5-m S. nutans (Indian grass)-vegetated organic matter and nutrient-poor soil over a 92-day period. The vegetation was allowed to establish in 0.5-m high soil cores before amendment with comp-B mixed with the same soil, and effects and fate of comp-B derived TNT and RDX were followed in plants, soil, and leachate under greenhouse conditions. Plant species identified as explosives-tolerant and metabolizing TNT and RDX were included in the tests. Effects, fate, and mass balances of comp-B-derived TNT and RDX were determined in vegetated and bare soil cores over a 3-month period; bioavailability was explored; and microbial composition of the upper soil layer was evaluated. Results formed the basis for estimating scaled-up phytoremediation in the field.
This project determined that phytoremediation can be successful in a given range of explosives contamination. Remediation in vegetated soils exceeded that in bare soils for TNT up to a comp-B level of 218 mg kg-1 and for RDX up to a comp-B level of 73 mg kg-1. The greatest annual remediation potential was 58.5 g TNT m-2 and 42.4 g RDX m-2 in vegetated soils and 54.5 g TNT m-2 and 51.0 g RDX m-2 in unvegetated soils. Sorption coefficients for Camp Shelby soil were three times greater for TNT than RDX and relatively low, indicating considerable potential for explosives leaching. Leaching was very low compared to loss of explosives due to other processes than plant uptake and leaching (i.e., (plant-assisted) bioremediation and possibly photochemical degradation). The microbial communities in the upper soil layers responded to comp-B exposure with decreased biomass production rather than distinct shifts in composition and were stimulated by vegetation up to a comp-B exposure of 146 mg kg-1 soil.
The results of this project are applicable to both routine and longer-term, sustainable military range operations and design. Indirectly, they will help remediation managers and risk assessors in the consideration, design, and implementation of plant-based remedial efforts and will likely lead to more detailed field remediation trials. Either phytoextraction or phytostabilization or a combination of both, ideally utilizing native species and existing training range vegetation management approaches, will be low-cost, self-sustaining, and not disruptive of range use.