v-xEPA
United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-82-086 Nov. 1982
Project Summary
The Use of Wetlands for
Water Pollution Control
Emy Chan, Taras A. Bursztynsky, Norman Hantzsche, and Yoram J. Litwin
An investigation was made of the
use of wetlands as treatment mech-
anisms for urban stormwater runoff.
Application of municipal wastewaters
and polluted urban runoff to wetlands
may potentially provide low-cost
water quality protection for many
communities. Though the cost of
coventional treatment facilities may
be difficult to support, development of
wetlands for runoff treatment is easy
to justify because it meets many
community needs (recreation, wildlife
and fishery enhancement, recharge of
groundwater, and water quality reno-
vation, for example). This report
summarizes current kowledge about
the use of wetlands for treating urban
stormwater runoff.
Wetlands such as marshes, swamps
and artificial wetlands, have been
shown to remove selected pollutants
from urban stormwater runoff and
treated municipal wastewaters. Wet-
lands have produced reduction in
BOD, pathogens, and some hydro-
carbons, and they excel in nitrogen
removal. They have been reported to
act as sinks for trace metals, phos-
phorus, and suspended solids.
Physical/chemical pollutant re-
moval mechanisms in wetlands in-
clude sedimentation, coagulation,
chemical filtration, volatilization.
adsorption, and chelation. Vegetative
mechanisms include filtration, ad-
sorption through roots, stems, and
leaves, and chemical transformations
in the plants. Chemical transforma-
tions of some waterborne pollutants
also occur in the sediment layers or the
water column as a result of anaerobic
or aerobic conditions, the presence of
catalysts and reactive substances, and
microbial action.
Though individual plant species
have been studied for their pollutant
removal properties in a wetland, the
interaction of numerous plant and
animal species is not well understood.
Management of wetland vegetative
systems to optimize pollutant removal
requires further investigation.
Further research needs to be con-
ducted on long-term impacts to
wetlands, bioaccumulation of trace
metals, the interaction of individual
pollutant removal mechanisms in
various wetland systems, and manage-
ment techniques for wetlands used as
treatment systems.
This Project Summary was devel-
oped by EPA's Municipal Environ-
mental Research Laboratory. Cincin-
nati, OH, to announce key findings of
the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at bach).
Introduction
Strong evidence suggests that wet-
land and upland vegetative systems can
degrade and eliminate various water-
borne pollutants. But information on the
subject, is scattered over a wide range
of technical disciplines, and much of it
results from investigations undertaken
only within the last 5 to 10 years.
Consequently, technical guidance is
lacking on potentially useful manage-
ment practices for wetlands Before
policies are adopted for using the
natural treatment functions of vegeta-
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live systems, the current state of
knowledge on the subject should be
defined. This study was undertaken to
address this need. The object was to
summarize relevant findings on the use
of wetlands as treatment mechanisms
for urban stormwater runoff and to
guide users to more detailed sources of
information in the literature.
The technology and management tools
for controlling surface stormwater
runoff pollutants have advanced con-
siderably as a result of various EPA and
other research activities. But for a
variety of reasons (principally financial),
local control of nonpoint pollution
sources probably will not occur unless it
can be related to other enviromental
issues and public needs.
One way to achieve this goal is to tie
solutions to stormwater runoff to a
framework of other community needs
such as flood control, recreation en-
hancement, food and fiber production,
etc. This concept leads to active
consideration of wetlands and upland
vegetated areas for their potential in
removing pollutants from stormwater
runoff. These areas are highly valued for
their ecological, agricultural, and
hydrological significance. Preservation
and artificial creation of wetlands are of
great current interest because of their
importance as nesting, feeding, and
nursery areas for birds, fish, inverte-
brates, and wildlife, and as natural
runoff detention areas for flood storage
and water quality enhancement. Water
quality objectives may also be combined
with the agricultural open space and
landscapig needs of upland pastures
and cropland.
This study describes what is and is not
known about the use of wetlands as
treatment mechanisms for urban storm-
water runoff. The report is based on a
survey of scientific investigations and
basic literature sources related to
nutrient and pollutant cycling in wet-
land ecosystems. The survey includes a
review of fundamental research re-
garding plant morphology processes
and physical and hydrological relation-
ships, and the growing number of site-
specific investigations of pollution
impacts and treatment capabilities of
vegetative systems. Most investigative
work to date has been related to the
treatment of municipal wastewater by
vegetative systems. This report describes
the implications for the evaluation of
stormwater runoff treatment. Also
incorporated are recent findings from a
handful of studies concerned specifically
with stormwater runoff, including work
by the study team at the Palo Alto
Marsh/Flood Basin in California.
Literature and Practices
Wetland and upland vegetative sys-
tems have attracted attention as natural
sinks for containments and as potential
components of treatment systems for
wastewater and stormwater. The great-
est number of investigations and data in
the literature pertain to the treatment of
municipal wastewaters. Promising
results have fostered scientific interest
and a handful of investigations into the
effectiveness of vegetative systems for
the control of stormwater pollutants.
Wetlands Treatment of
Wastewater
Wetlands Treatment of
Municipal Wastewater
Wetlands occur in a wide range of
physical settings at the interface of
terrestrial and aquatic ecosystems.
Because of this position, some wetlands
have been subjected to inadvertent
municipal and industrial wastewater
discharges for many years. But only in
the past 10 to 15 years has attention
been focused on their planned use for
wastewater treatment. Promising results
have been obtained with experimental
applications in various natural wet-
lands, including:
northern peatlands
cattail marshes
southeastern swamplands
cypress domes
freshwater/tidal marshes
Consolidation of results indicates that
in nearly all instances, wetlands act to
renovate or improve water quality to
some extent. Pollutant removal efficien-
cies are extremely variable, and ques-
tions of treatment capacity and long-
term impacts on wetlands are un-
answered. Indiscriminant discharge of
wastewaters to wetland ecosystems is
not advised.
Artificial Wetlands for
Treatment of Municipal
Wastewater
Artificial wetlands for treating waste-
waters have been created for both
small- and large-scale applications in
Europe and the United States using
different types of vegetation and sub-
strates. These systems offer controlled
environments for testing and studying
vegetative treatment of wastewaters.
The resulting data establish hydrologic
and constituent balances and assess
pollutant removal capabilities for these
systems. Examples of artificial systems
include'
meadow-marsh-pond system (New
York)
ponds with reeds or rushes (Ger-
many and Holland)
peat filters (Minnesota)
marsh-pond system (California)
seepage wetland (Michigan)
water hyacinth ponds (Florida and
Texas)
Many researchers favor continued
work with artificial wetland systems
because of the high degree of control
and reliability. The environmental
enhancement they provide is an added
incentive.
Wetlands Treatment of
Stormwater
Field investigation and research on
using wetlands to treat stormwater
runoff have been extremely limited. The
few studies undertaken (a) exhibit great
dissimilarities in the type of wetland and
stormwater characteristics examined,
(b) contain a very slim data base from
which to draw conclusions, and (c)
encounter numerous complications in
determining hydrologic components.
Key investigations include studies of:
northern peatland (Minnesota)
cypress wetland (Florida)
brackish marsh (California)
high altitude meadows (Lake Tahoe)
wetland detention basins (Mary-
land)
In general, these studies revealed the
following:
(1) A wide disparity exists in the
ability of wetlands to remove
nonpoint source pollution, parti-
cularly with regard to nutrients;
(2) The greatest consistency in pollu-
tant reduction appears to be for
biological oxygen demand (BOD),
suspended solids, and heavy
metals;
(3) The nature of flow and seasonal
factors are major influences on
polluant removal capabilities
in certain wetlands.
Physical and Chemical
Removal Mechanisms in
Wetlands
Some investigations of the physical
and chemical removal mechanisms of
wetlands have been undertaken. Several
removal processes that occur in natural
waters are likely to occur in wetlands,
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but not many have been documented in
marsh environments. Studies of rivers,
lakes, and oceans have dominated
research. Nevertheless, a few general
conclusions can be drawn:
(1) A wide variety of physical/chemi-
cal pollutant removal mechanisms
occur in wetlands. Most common
are evaporation, sedimentation,
adsorption, filtration, chelation,
precipitation, decomposition, and
adsorption.
(2) Wetlands exhibit large variations
in type, climate, and ecosystem.
The interaction and relative im-
portance of physical/chemical
pollutant removal mechanisms
vary significantly among and
within wetlands.
(3) Studies of pollutant removal
mechanisms in wetlands have
generally been piecemeal. Suffi-
cient data have not been collected
to formulate a comprehensive
theory of pollutant transport and
fate within a wetland system.
Vegetative Treatment
Systems
Though the initial pollutant removal
mechanisms in wetlands are physical
and chemical processes, plants can
increase the overall capacity of a system
to retain or remove pollutants through
interactions with various anaerobic and
aerobic soil layers, water, and inter-
faces. In particular, plant root uptake of
pollutants from the sediments frees
more exchange sites in the sediments
for further pollutant interaction and
accumulation. The primary biochemi-
cal pollutant uptake and removal
processes in vegetative systems are:
(1) Uptake through the plant/soil
interface by means of below-
ground roots, rhizomes, holdfasts,
and buried shoots and leaves;
(2) Uptake through the plant/water
interface by means of submerged
roots, stems, shoots, and leaves;
(3) Translocation through the plant
vascular system from roots to
stems, shoots, leaves, and seeds
during growing season;
(4) Differential pollutant uptake such
as preferential storage of trace
contaminants in specific plant
parts and preferential uptake/
accumulation of certain trace
elements;
(5) Nonspecific pollutant uptake oc-
curring primarily as plants absorb
large quantities of nutrients from
water and sediments;
(6) Uptake and immobilization by
plant liner zones, where dead but
not decomposed plant litter se-
questers pollutants through chem-
ical interactions.
Nutrient Removal through
Wetlands
Wetland environments present ideal
conditions for nutrient cycling and
removal, particularly for ntirogen. The
aerated water column and aerobic
upper sediment layer promote nitrifica-
tion and the formation of insoluble
phosphorus-metal complexes. Reducing
(anaerobic) sediment conditions and the
interface between the aerobic and
anaerobic sediment layers promote
ammonification and denitrification.
Wetland vegetation can function as
nutrient pumps to take up nitirogen (in
the ammonium as well as nitrate form)
and phosphorus (in the orthophosphate
form). The highest observed nitrogen
removal potentials were 300 to 800
kg/ha for the above-ground parts of
cattails and reeds, and up to 1,290
kg/ha for the below-ground parts of
rushes and cordgrass. Nitrogen assi-
milated into wetland vegetation can be
translocated back to the roots and
stored during the plant dormancy season,
or it can be returned to the litter
component during senescence of above-
ground parts. The highest observed
phosphorus removal potentials were 30
to 80 kg/ha for the above-ground parts
of cattails, reeds, and sedges. Phos-
phorus assimilated into vegetation is
not translocated back to the roots, but
remains in the plants or plant litter.
Hydrologic variables are crucial (parti-
cularly during the high runoff season)
because particulate matter and organic
nitrogen and phosphorus from the litter
zone can be flushed. Plant uptake and
storage of phosphorus is highly variable
and is not a reliable mechanism for
phosphorus removal. But phosphorus-
metal interactions can form insoluble
complexes that can accumulate in
long-term sediment deposits.
Uptake and Removal of Trace
Elements
Wetland systems can function as
sinks for heavy metals and other trace
elements, either through vegetative
uptake and storage or through immobi-
lization in the sediment layers. The
observed heavy metal removal poten-
tials range from:
(1) 0.001 to 0.38 kg/ha of cadmium,
with highest levels effected by
Potamogeton crispus and Salicor-
nia pacifica;
(2) 0.007 to 1.58 kg/ha of copper,
with highest levels occurring in
Justicia americana and Salicornia
pacifica;
(3) 0.13 to 103.4 kg/ha of iron, with
highest levels occurring in Carex
stricta;
(4) 0.026 to 1.01 kg/ha of lead, with
the highest levels occurring in
Salicornia pacifica and Phalaris
arundinacea;
(5) 0.001 to 1.714kg/haofzinc,with
the highest levels occurring in
Phragmites communis, Carex
stricta. and Scirpus lacustris.
Though pollutant removal potentials
for floating aquatic vegetation are not
reported in kg/ha, observed uptakes
and concentrations in water hyacinth
(Eichhornia crassipes) are significant
for some heavy metals, particularly
cadmium, chromium, copper, lead,
nickel, gold, and strontium.
Hydrologic Practices
A clear knowledge of the hydro-
geology of an area is crucial for
understanding the wetland environ-
ment and assessing its potential for
assimilating waterborne pollutants. The
lack of adequate hydrologic information
has hampered numerous researchers in
quantifying and evaluating the pollutant
removal efficiencies of wetlands.
The relationships between hydrology
and ecosystem characteristics need to
be recognized when considering the
application of stormwaters and waste-
waters to wetlands. Factors such as
source of water, velocity, flowrate,
renewal rate, and frequency of inunda-
tion have a major bearing on chemical
and physical properties of the wetland
substrate. These properties in turn
influence the character and health of
the ecosystem as reflected by (a) species
composition and richness, (b) primary
productivity, (c) organic deposition and
flux, and (d) nutrient cycling. In general,
water movement through wetlands
tends to have a positive impact on the
ecosystem.
Hydrology controls pollutant removal
in wetlands through its influence on the
processes of sedimentation, aeration,
biological transformation, and soil
adsorption. Critical hydrologic factors
are:
velocity and flowrate
water depth and fluctuation
detention time
circulation and distribution patterns
U S GOVERNMENT PRINTING OFFICE: 1982 659-O17/O87O
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turbulence and wave action
seasonal and climatic influences
groundwater conditions
soil permeability and groundwater
movement
Various criteria and practices can be
identified for hydrologic management of
wetlands for improved wastewater and
stormwater treatment:
(1) Flow routing - Initial introduction
and subsequent distribution of
flow should attempt to maximize
effective contact between water
and wetland soils and vegetation.
(2) Water level maintenance - Mani-
pulation of water levels is a useful
means of enhancing pollutant
removal by vegetation and soil.
Regulation of levels must take
into account competing ecosystem
needs and the additional nuisance
problem of mosquitos.
(3) Inflow/outflow regulation - Pos-
sible techniques for regulation of
inflow/outflow containment of
the first flush of runoff or reten-
tion storage during spring runoff
until marsh communities are
functioning at higher uptake
rates.
(4) Seasonal application - Where
possible, seasonal applications of
wastewaters and stormwaters
might be used for specific treat-
ment or flushing purposes, taking
into consideration biological acti-
vity in the wetlands, availability of
dilution flows, and seasonal uses
and quality of downstream re-
ceiving waters.
(5) Infiltration - Maximum soil contact
should be emphasized, with at-
tention given to routing and/or
ponding wastewaters in areas of
highest soil permeability.
Conclusions
This study reviews numerous reports
of vegetative removals of waterborne
pollutants and several studies of
wetlands. Because of the nature of the
available literature, the conclusions
that can be drawn here reflect only an
initial interpretation of the reported
literature. Synthesis of this study
results into a theory of wetland pollu-
tant removal systems is beyond the
scope of this report, and perhaps it is
beyond the presently available informa-
tion. Conclusions drawn from this
literature survey may therefore have to
be revised as further research results
and operating information are collected.
The conclusions are as follows:
(1) Most wetlands studied were able
to receive treated municipal
wastewater and/or stormwater
runoff, remove certain pollutants,
and produce satisfactory plant
growth. Pollutants removed or
decreased included organic wastes
(as measured by BOD), nutrients,
suspended and volatile solids, and
trace metals.
(2) The application of hydraulic con-
trols and vegetation management
has the potential for improving
wetlands removal of pollutants.
(3) Wetlands remove waterborne
pollutants principally through
physical and chemical processes
that are substantially augmented
by biological processes associated
with wetland vegetation.
(4) Wetland system stress was re-
ported only in laboratory studies
and certain field studies below
municipal and industrial discharges
where plants were exposed to
excessive pollutant concentrations.
Abatement or reduction of pollu-
tant loadings usually led to re-
covery of wetland vegetation.
(5) Further research should be directed
at improving our understanding of
how wetland systems assimilate
pollutants after initial removal.
The full report was submitted in
fullment of Grant No. R-806357 by the
Association of Bay Area Governments
and RAMLIT Associates under the
sponsorship of the U.S. Environmental
Protection Agency.
Emy Chan and Taras A. Bursztynsky are with the Association of Bay Area
Governments. Berkeley. CA 94705; Norman Hantzsche and Yoram J. Litwin
are with RAMLIT Associates. Berkeley. CA 94705.
Richard Field is the EPA Project Officer (see below).
The complete report, entitled "The Use of Wetlands for Water Pollution Control."
(Order No. PB 83-107 466; Cost: $23.50, subject to change) will be available
only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Storm & Combined Sewer Section
Municipal Environmental Research LaboratoryCincinnati
U.S. Environmental Protection Agency
Edison, NJ 08837
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
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Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
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