Objective:
Groundwater contamination due to explosives such as TNT,
RDX, and HMX is a problem at many Army ammunition plants.
This ESTCP project evaluated the ability of selected aquatic
macrophytes (aquatic plants) in constructed wetlands to clean
up explosives-contaminated groundwater through a phytoremediation
process. A field demonstration of two types of artificial
wetlands was conducted at Milan Army Ammunition Plant (MAAP)
in Tennessee. The first type was a two-celled lagoon-based
wetland containing submergent plants to test the concept of
explosive degradation via nitroreductase enzyme produced by
the plants. The second type was a two-celled gravel-based
wetland planted with emergent wetland plants to test the concept
of explosive degradation via both microbial and plant enzyme
processes. A variety of submergent and emergent aquatic macrophytes
were screened for their ability to remediate the contaminated
water.
Results:
Each wetland received 5 gpm contaminated groundwater containing
3,250 ppb of nitrobodies (including TNT, RDX, HMX, TNB, 2AA-DNT,
4A-DNT) from June to November 1996, and 9,200 ppb from November
1996 to August 1997. While both systems could remove explosives,
the gravel-based system was clearly superior. The gravel-based
wetland remediated groundwater contaminated with a variety
of explosives, including both TNT and RDX, and was able to
reduce TNT concentrations to less than 2 ppb and total nitrobody
concentrations to less than 50 ppb during all but the coldest
months. During winter operations, the gravel-based system
had difficulty meeting the total nitrobody reduction goals
due to reduced microbial activity and a decrease in treatment
efficiencies at low water temperatures. In contrast, the lagoon-based
wetland was unable to satisfactorily remove RDX and HMX, or
meet the total nitrobody removal goals, and was only able
to meet the TNT reduction goal of 2 ppb during the initial
stages of the demonstration.
Benefits:
A gravel-based system can be economically resized to overcome
the winter performance issues. Assuming a 95% system availability
and 30-year life, the total cost for treating 200 gpm of MAAP
groundwater with a gravel-based system was estimated at $1.78
(surface water discharge) or $2.06 (groundwater reinjection
discharge) per thousand gallons of groundwater. This includes
capital costs estimated at $3.465 million for surface water
discharge and $4.125 million for groundwater reinjection discharge.
Implications:
Wetland phytoremediation is an environmentally friendly technology
that is widely accepted, relatively self-sustaining, and cost-effective
to maintain. In addition, unlike granular activated carbon
(GAC), this technology does not produce secondary waste streams.
The plants appear to be metabolizing the explosives over time,
thus keeping the level of explosives or metabolites to a low
level in plant tissues. As a result, the explosive concentrations
in the plants should not pose any adverse environmental effects. (Project Completed - 1999)
References:
- ESTCP Cost
and Performance Report. July 1999 (PDF File Format)
- Sikora F.J., R.A. Almond, L.L. Behrends, J.J. Hoagland
and D.A. Kelly. U.S. Army Environmental Center. Demonstration
Results of Phytoremediation of Explosives Contaminated Groundwater
Using Constructed Wetlands at the Milan Army Ammunition
Plant, Milan Tennessee. SFIM-AEC-ET-CR-97059. DTIC
ADA372286, ADA372287 and ADA372288 (Volumes 1-3). December
1998.
ESTCP Point of Contact:
Program Manager, Cleanup
ESTCP Program Office
901 North Stuart Street, Suite 303
Arlington, VA 22203
Telephone: (703) 696-2118
Fax: (703) 696-2114
E-mail: Cleanup@estcp.org
