Location: Middletown, Iowa
Title: Phytoremediation/wetlands Treatment at the Iowa Army Ammunition Plant
The Iowa Army Ammunition Plant (IAAP) is located in Middletown, Iowa. IAAP is a 20,000 acre site (31.25 mi2), of which 8,000 acres are used for both munitions production and testing. The IAAP is on the Comprehensive Environmental Response and Compensation Liability Act (CERCLA) National Priority List. From 1943 to 1975, the U.S. Army generated large volumes of explosives-laden wastewater at its Line 1 and Line 800 production facilities at the IAAP. In 1948, a concrete impoundment was constructed near Line 1 to contain explosives production wastewater in a lagoon having an area of about 3.6 acres. The five-acre Line 800 Pink Water Lagoon Area is surrounded by an earthen berm and at one time was also used for sludge disposal (Figure 1). Preliminary studies investigating the potential of phytoremediation and active remediation at the site were conducted as a coordinated effort by the U.S. Army Corps of Engineers, Waterways Experiments Station, and the University of Iowa.
The lagoon basins and underlying groundwater have been contaminated with 2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) as well as many other compounds and heavy metals. The action levels were driven by the explosives contamination. Soil action levels were set at 47 mg/kg for TNT and 1.3 mg/kg for RDX. Action levels for aqueous concentrations were set at existing EPA health advisory lifetime levels-2 mg/L for both TNT and RDX.
A number of treatment technologies were considered for the site. Phytoremediation, the use of plants in site cleanup, was considered, but the levels of TNT and RDX were too high at the most concentrated locations for the plants to survive. Excavation was chosen as the method to deal with the most highly contaminated spots. Approximately 100,000 cubic yards of soil were excavated from the Line 1 (24,000 yd3) and Line 800 (76,000 yd3) areas in early 1997. After excavation, the threat to local groundwater supplies was expected to be greatly diminished, but the U.S. Army Corps of Engineers anticipated a level of residual contamination equal to or less than the action levels. As a safety margin, residual contamination would require additional treatment. Studies were then conducted to assess the potential for using phytoremediation with poplars as a final polishing step for soil and with wetland to treat residual contamination in surface waters and groundwaters.
At IAAP, a full-scale wetlands treatment system is currently in place, entering the third year of operation. The process of selecting the wetland treatment approach and getting the wetland approved and operational comprised many steps and much investigation.
Research performed at the University of Iowa (Thompson et al., 1998; Thompson et al., 1999) evaluated the potential for using poplar trees to remediate TNT and RDX contamination at the IAAP. This work determined the feasibility and impacts of terrestrial phytoremediation at the IAAP site. Goals of that work were to:
Research was also conducted on aquatic systems at Waterways Experiments Station in Vicksburg, Mississippi (Best et al., 1997; Larson, 1997; Best et al., 1998). These screening studies also investigated the potential of phytoremediation, looking at aquatic-based systems. The goals of the research were to:
In these lab studies, TNT was never detected in plant tissues and TNT degradation products were also below detection limits. RDX was found to accumulate in a number of the plants, reaching concentrations up to 1000 mg/kg D.W. (Best et al., 1997). RDX had also been detected in plant tissues from the IAAP site prior to the initial excavation in arrowhead roots (7 mg/kg D.W.) and in reed canary grass shoots (10 mg/kg D.W.) (Schneider et al., 1995). The detection of RDX in plant tissues was a concern going into implementation at the IAAP.
Following excavation of the Line 1 and Line 800 sites at the IAAP, the excavation pits were not backfilled, but were converted to constructed wetlands. The Line 800 site during and after the process is displayed in Figures 2 and 3. The wetlands were seen as a way to remediate the collected groundwater, provide ecological enhancement to the area, and avoid the costly process of backfilling the area from another location. Sediment/seed material from nearby Stump Lake was used in each wetland to introduce indigenous species. Following the addition of the sediment, the wetlands were allowed to fill with the infiltrating groundwater. The sediment removal from Stump Lake created enhanced fish habitat at that location also. The U.S. Fish and Wildlife Service stocked Stump Lake after the sediment was taken.
Desorption of RDX from the contaminated soils tainted the groundwater that filled the excavation pit/wetland resulting in a rebound effect in the RDX concentration. In January 1998, the Line 1 wetland RDX concentration reached 760-830 ppb. RDX concentration declined in the wetland over the following months, reaching 1.6 ppb in August 1998. The following December, RDX reached 22 ppb in the Line 1 wetland. However, the concentrations were non-detect ( < 0.25 ppb) the next summer. No water was released from the wetland that contained RDX above the 2 ppb action level. The seasonal swings in RDX concentration was also observed in the Line 800 impoundment wetland (Figure 4). The Line 800 wetland reached 190-760 ppb in January 1998. RDX was below detect during the following summer and rebounded to 1.9 ppb in November 1998.
The cause of the observed RDX concentration increase during the winter months has not been delineated at the site. Possible reasons for the increase include decreased plant activity and biomass, decreased rates of enzymatic reactions, decreased physical-chemical reaction rates, decreased photolytic reactions as incident sunlight decreases, and/or increased concentration in the infiltrating groundwater. Residual soil and sediment concentrations of RDX are greater than 1.3 mg/kg soil. This residual concentration, while not widespread, is capable of contaminating groundwater and surface water at greater than 2 mg/L, the USEPA Health Advisory Level (HAL).
Bioaccumulation of RDX in plant tissues was a concern at the site, as stated previously. Sampling of the plant material took place in June 1999. Duplicate testing was conducted by the environmental engineering laboratories at the University of Iowa in Iowa City (Just, 1999) and the testing laboratories at the U.S. Army Corps of Engineers Waterways Experiments Station, Vicksburg Mississippi (Larson, 1999). The testing revealed that RDX and TNT were both below detection limits in the plant tissues tested. Detection limits were determined to be lower than 1 mg/kg D.W. Plant species tested included reed canary grass, arrowhead water plantain, and coon tail. 1,3,5-trinitrobenzene, a photodegradation product, was detected in some plant tissues at < 30 mg/kg D.W. This suggests that other reactions are working to degrade the explosives, not just plant-associated reactions.
Summary
Natural systems can be used to engineer processes addressing a number of problems. The IAAP treatment wetlands are providing effective treatment at a Superfund site, and further treatment of the surface water and groundwater at the site is planned. Processes are being considered to collect and treat contaminated groundwater. The use of wetlands and plant-based systems has potential for many applications. A number of contaminated sites with differing contaminants and conditions are being tested as part of the EPA's Superfund Innovative Technology Evaluation (SITE) program and in other research endeavors. The applications cover a wide range including metals removal with hyperaccumulators, domestic waste treatment with subsurface wetlands, chlorinated solvents groundwater plumes being treated with poplar trees, and constructed wetlands for mine drainage. Advantages for these natural systems are lower cost, the public acceptability of such natural solutions, and ecological advantages of simultaneous remediation, restoration, and revegetation. However, concerns relating to such systems do exist. In many instances, the long-term effects of the introduced plant species are unknown. Species native to the region are preferred. The potential of the plant systems serving to move the contaminants from the subsurface to the edible tissues is a concern. The potential for contaminating the base of the food chain is always an issue and must be monitored. The use of transgenic (genetically altered) plants is also being investigated. There are many questions associated with using transgenic plants and microbes in the uncontrolled environment, as opposed to laboratory study. Through continued research, these and many other questions can hopefully be answered.
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