A constructed wetland uses natural geochemical and biological processes in a wetland ecosystem to treat metals, explosives, and other contaminants in groundwater. Usually, the constructed wetland has three primary components: an impermeable layer (generally clay), a gravel layer that provides a substrate (i.e., an area that provides nutrients and support) for the root zone, and an above-surface vegetation zone. The impermeable layer prevents infiltration of wastes down into lower aquifers. The gravel layer and root zone is where water flows and bioremediation and denitrification take place. The above ground vegetative layer contains the plant material. Both aerobic and anaerobic systems (i.e., systems with and without oxygen) exist within the wetland, and these can be divided into separate cells. Groundwater is either pumped or allowed to naturally flow through the wetland. The anaerobic cell uses plants in concert with natural microbes to degrade the contaminant. The aerobic cell further improves water quality through continued exposure to the plants and the movement of water between cell compartments. Straw, manure or compost is used, with little or no soil, in wetlands constructed primarily for the removal of metals. For wetlands constructed to treat explosives-contaminated water, certain plant species are used to support degradation. The process of using plants to break down contaminants is also referred to as phytoremediation.
The process filters some materials and degrades others. The technology incorporates the principal components of wetland ecosystems that promote degradation and control of contaminants by plants: degradation by microbial activity and increased sorption, filtering, and precipitation. The technology can be adapted to treatment needs by selecting a design, such as surface or subsurface-flow, single or multiple cells, and parallel or series flow. Constructed wetlands are sometimes built as part of a treatment train that may include processes in series such as settling ponds, oil/water separators, and physical/chemical treatment methods.
Removal mechanisms can act uniquely, sequentially, or simultaneously on each contaminant group or species. Volatile organic compounds (VOCs) in contaminated groundwater are primarily removed through the physical mechanism of diffusion-volatilization. However, mechanisms such as adsorption to suspended matter, photochemical oxidation, and biological degradation may also play a role. Major physical removal mechanisms in wetlands include settling, sedimentation, and volatilization. Gravitational settling is responsible for most of the removal of suspended solids.
Limitations and Concerns
The long-term effectiveness of constructed wetlands to contain or treat some contaminants is not well known. Wetland aging may contribute to a decrease in contaminant removal rates over time.
Constructed wetlands, like other biological methods, are limited by the ability of the biota to withstand exposure to their environment. Natural systems must establish themselves in order for this method to be successful. Weather events, wildlife, and contaminant concentrations may be problematic in establishing the systems. For example at one demonstration, a hailstorm decimated one of the few plants able to reestablish itself; a tadpole infestation severely defoliated the plants within two months of planting; and there was difficulty encountered in reestablishing plant growth because photo-degradation of explosives in the contaminated groundwater colored the water a dark red, which in turn inhibited photosynthesis.
High contaminant concentrations with low permissible effluent concentrations require long retention times, hence large wetland areas.
In cases where metals are the key contaminant, constructed wetlands do not destroy the metals; they restrict their mobility through sorption.
During operation of the constructed wetland, wildlife may be adversely affected by the presence of metals that have accumulated in plants.
After the pumping of contaminated water ceases, the artificial wetland ecosystem changes. This could severely affect the plant and animal life that comes to depend on the wetland, and it may leave a waste byproduct contaminated with metals and other contaminants. This residue or sludge may have to be disposed or capped.
The outlet of the monitored wetland should be carefully monitored. Underlying aquifers must also be monitored to assure that the impermeable base has not leaked.
When developing a constructed wetland, exotic and invasive species should not be used, and a plan should be prepared to remove these species if they appear.
Constructed wetlands have most commonly been used in wastewater treatment to control organic matter, nitrogen, and phosphorus. The wetland process is also used for controlling trace metals and other toxic materials in groundwater. Most experimental work is geared towards groundwater contaminated with explosives such as trinitrotoluene (TNT), Royal Demolition Explosive (RDX), and High-Melt Explosive (HMX), as well as perchlorate and nitrates.
Technology Development Status
Constructed wetlands have been commercially used to control and degrade municipal and industrial wastewater. For more exotic wastes such as explosives, it is being field tested.
Other Resources and Demonstrations
See description of Phytoremediation.
Also see http://www.itrcweb.org/Documents/WTLND-2.pdf for related document on mitigated wetlands.
See The Use of Constructed Wetlands to Phytoremediate Explosives-Contaminated Groundwater at the Milan Army Ammunition Plant, Milan, Tennessee, July 1999,
for a description of a demonstration at the Milan Plant.
See http://clu-in.org/characterization/technologies/exp.cfm#86 for a technical description of explosives in different media and the use of some analytical techniques.