Environmental Behavior and Fate of Explosives in Groundwater from the Milan Army Ammunition Plant in Aquatic and Wetland Plants

The present study was performed to elucidate the environmental behavior and fate of TNT and RDX in aquatic and wetland plants collected from a field-scale wetland demonstration deployed at Milan Army Ammunition Plant for removal of explosives from groundwater. The study had three objectives: (1) To establish the physiological capacity of plants to absorb and transport TNT or RDX from explosives-contaminated groundwater in the absence of substrates and their sorptive activities; (2) To quantify partitioning of TNT and RDX between plant portions; and (3) To establish the short-term chemical fate of TNT and RDX in plant tissues of these species. Substrates in which these plants were rooted at the Milan field site (sediment, gravel) were also incubated without plants to investigate sorptive activities, and to evaluate microbial/chemical transformation of TNT and RDX that may affect the explosives availability to plants. Hydroponic batch incubations of plant or substrate treatments with "C-TNT or C-RDX were used to evaluate explosives fate. The study surveyed seven plant species and two substrates in sequential, independent incubations of 7 and 13 days with TNT and RDX, respectively. Radiolabel distribution in intact plants was followed using autoradiography and radio-analytic imaging. Parent compounds and degradation products were determined through chemical (HPLC) analyses of plant tissue extracts, aqueous phases and substrate extracts. The fate of radiolabel in plants and substrates was followed using thin layer chromatography and radioanalytic imaging. While growth of most plants except parrot-feather was low in groundwater amended to contain 1.6 to 3.4 mg TNT L'\ TNT disappeared completely from groundwater incubated with plants in 7 days. Highest specific removal rates were found in submersed plants in elodea (0.05 mg TNT g FW d"') and in emergent plants in parrot-feather, sweet-flag, and reed canary grass (0.006 mg TNT g total FW d"). TNT declined less with substrates, and least in unplanted controls. Radiolabel was present in all plants after incubation. In the submersed species radioactivity was concentrated in physiologically active roots and shoots, and in emergent species in roots. Mineralization to C02 was very low, and evolution into volatile organic compounds was negligible. TNT residues were extremely bw or below chemical detection in plant tissues. Radioactive degradation products accumulated at the sites of uptake and transport was limited. TNT degradation took place via reduction of a single nitro-group. At least five other unknown metabolites were found. In RDX incubations growth of submersed plants was normal, but growth of emergent plants was reduced in groundwater amended to contain 1.5 mg RDX L'\ RDX disappeared less rapidly than TNT from the incubated groundwater. Highest specific RDX removal rates were found in submersed plants in elodea (0.004 mg RDX g FW" cf), and in emergent plants in reed canary grass (0.001 mg RDX g total FW d"'). Radiolabel was present in all plants after incubation. Mineralization to C02 was tow, but relatively higher than in the TNT incubation. Evolution into volatile organic compounds was negligible. Radioactive degradation products accumulated at physiologically active sites, and transport to leaves was substantial, ranging from 23% of total plant radioactivity in sweet-flag to 81% in parrot-feather. RDX residues were low in most plants, or below detection in the below-ground portions of two emergent species. The RDX residues ranged from 0.3 ug g FW" in pondweed to 8.6 ug g FW in parrot-feather shoots. RDX degradation into at least five unknown compounds was shown to occur. No detectable residues of either explosive were found in substrates. The promise of phytoremediation in constructed wetlands as a technology for removal of explosives from groundwater is supported by several results of this study. 1) The rapid decrease in TNT and relatively slower decrease in RDX in the presence of certain aquatic or wetland plants under viable environmental conditions, 2) The relatively rapid metabolism of the parent compounds inside the plants, and 3) Low explosives residues in plant tissues and substrates. However, it must be realized that metabolic pathways of degradation of TNT and RDX in plants are still unknown, and that certain explosives degradation products may exert other biological and toxicoiogical activities. Decreases in TNT and RDX levels in water with plants may also be due partly to chemical binding between explosives transformation products and organic matter. The generation of plant-specific dissolved organic matter and leachates, may also play a role in stimulating micrcbial activity and result in degradation of explosives.