Constructed
Wetlands
Description
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.
Applicability
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.
Web
Links
http://www.frtr.gov/matrix2/section4/4-43.html
http://www.itrcweb.org/Documents/WTLND-1.pdf
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,
http://www.serdp-estcp.org/content/download/3224/54416/file/ER-199520-CP.pdf
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.