United states
Environmental Protection
Agency
Office of Water
4502F
Washington, DC 20460
843-fl-
February 1993
Natural Wetlands and
Urban Stormwater:
Potential Impacts and
Management
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NATURAL WETLANDS AND URBAN
STORMWATER: POTENTIAL
IMPACTS AND MANAGEMENT
February 1993
U.S. Environmental Protection Agency
Office of Wetlands, Oceans and Watersheds
Wetlands Division
Washington, DC
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February 1993
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CONTENTS
•-'''.'•'• • -, ' .- • • ••• "" • . . • • ' 'Page
Tables ..... ......... ...... .........
• • • • - - .-.. ............. . . ... ... : . ....................... vii
Acknowledgments ................ ......... .... ...... ix
. 1. Introduction ......... ........ . ...... . ........ ; . •
Background .......... ..... .........!..!!.."] ..... " " 3
Purpose ..... ..... ...... '. . . . . . . ....,; ',"', '.'.'.'.'.'.'..'.".'.I''' '.'••' ' " 4
2. Wetland Characteristics ...... ...... ....... ....... . . 7
Hydrology . .......... .........;....!!.!" ......... ......... 7
Water Quality/Benthic Processes . . . . ....... .^ ................... _v . .........
Biologic/Habitat Functions ..... ..... ..... " ' ..... ........... ..... *
Unresolved Issues ....... ..... .......... . . . . ." '.'', ; .]..!!.!!."] : '"'" {2
3. Stormwater Characteristics ............ ........ . ....... 13
Urban Activities That Affect Stonnwater Characteristics . . .......... 14
Chemical Characteristics ... . . . ..... ..... " " " ....... " ........ l«
Hydrologic Characteristics . . . . .......... ...........!.! . ...... . . ' ' ' ' j|
4. Regional Differences in Stormwater Characteristics and hi Wetland Types . ..... 19
Regional Differences in Stonnwater Characteristics ..... ........ ......... m
Regional Differences in Wetland Types '... . > . . . ..... . '.", '. . .' ......' ..... " ' 22
Relationship Between Regional Characteristics of Stonnwater and [Wedands! '. '. '. . . '. '. . . . . '/. '. 24
5. Potential Impacts of Urban Stonnwater Runoff on Natural Wetlands 20
Hydrologic Changes .'....• ......... " ' ...... " "" ...... ' ' •,<,
Water Quality Changes ........ ....... " " ' "..' ' " '-'-" ' ' • ' • . ......... ..... -*
Wetland Soil Changes . . ........... " ...... ....... " ...... ' * '' ' ' t\
Biologic/Habitat Impacts .... .......... .!!!.!'"""""""" ....... ••'".•• li
Regional Differences ...... ,. . . ....... . . . ! . M . ! ! . . i ! ] ! ! ' . " " *• " * 30
Impoundments . ..... .... ...... ..... ... ........ JQ
unresolved issues .......!.!!!!!!!!!!!""."!!! ......... ...... "49
Unresolved Impoundment Issues .. ........ ...... ..... ...'!.'!!!.'! ! ! ." .' ..... 50
6. Stonnwater Management Practices and Natural Wetlands . ...... , . . . . ......... 51
Federal and State Stonnwater Management Programs ........ ........ .........
Control of Adverse Impacts ....... . ........... ...'..'.'..'. ..... " ...... 51
Conclusions .................;. ..... .!!!!!" ..... ..... .'""'• ..... -i
Unresolved Issues ... ........ ........... . . . . I'.'.'.'.'.".'.'.'.'.'.',-'.'.', ] '. ' " 57
7i Summary '..'... ', ... ... • en
' ••••
j9
Impacts of Stonnwater Discharges .... ..'. ......... ..... '. . . . ""59
Management of Stpnnwater Discharges ......... '. '. '. . ; . .......... '. '.'.'. " ' ."'* " ^0
Literature Cited .................. ...........;. ...... ..... 61
Glossary .... ... ........ r ..... ...:.......... .......... ... «
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IV
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TABLES
Table
Page
1. Wetland Types g
2. Examples of Pollutant Characteristics; Found in Stonnwater Runoff From Various
Land Uses in the Great Lakes Region 14
3. Ranges in Pollutant Concentrations Found in Urban Runoff . . . .". ..... 16
4. Sources of Urban Runoff Pollutants >-vC.:« . . . . . . 16
5. Locations of Wetland Types in the United States .."'.. 25
6. Relationship of Wetland Type to Its Origin, Hydrology, Soils, and Vegetation 26
7. Wetlands Present in SCS Type RaMall Distribution Areas •-.-....;............. 28
8. Comparison of Stonnwater Runoff Quality '..•'.....• 32
9. Water Quality Changes fromSeveral Wetlands Receiving Stonnwater Runoff. . . . 33
10. Summary of Mean Soil and Sediment Chemistry Data as a Function of
Sampling Location, December 1978 ;.... 34
11. Distribution of Selected Constituents in Water, Sediments, and Groundwater
at the Silver Star Road Study Area ..
35
12. Median Values of Selected Constituents in the Water Colunm and Values
for One Sample of Bed Sediments at tiie Island Lake Wetland . 26
13. Mean Treatment Marsh Influent and Effluent Parameters for 1981 to 1984 and 1986 41
14. Selected State^Approaches Regarding tlhepischarge of Urban Stonnwater to Natural Wedands. ....... 53
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FIGURES
FiSure \ Page
1. Approximate geographic areas for SCS rainfall distributions ....................... 20
2. Hourly fraction of total rainfall within a 24-hour period for each rainfall
distribution type ,20
3. Major climatic regions of North America 21
4. Average number of days each year on which thunderstorms are observed
throughout the United States . , ... '. . . . . . 22
,5. Month-to-month variation of precipitation in the United States ......................... 23
6. Average annual runoff in the United States 23
7. Physiographic regions of the United States ...........-............;.. ... 24
8. Oxygen fluctuations in a shallow water impoundment in Minnesota . . . 43
9. Monthly distribution of fishes in Impoundment No. 12 and pond
water levels during 1979 .... 46
Ml
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viii
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February 1993
ACKNOWLEDGMENTS
Gratitude is expressed to die following persons, who have been actively
involved in the development, preparation, and review of the document.
Active participants from EPA include Dianne Fish ((Chief, Strategies and
Initiatives Section), Fran Eargle (Project Manager), Stephanie Krone
Firestone, Menchu Martinez, Jane Freeman, Aura Stouffer, and Kevin
Weiss. EPARegionalaiKiLabstaffincludePhilOshida,LindaStorm,and
Naomi Detenbeck. Special thanks arc extended to E>r. Rich Homer for his
review and comments. Other participants and contributors include Eric
Livingston, Earl Shaver, Tom Schueler, Dr. Arnold van der Valk,
Dr. Robert Kadlec, the Stonnwater Advisory Panel, and participants of the
January 1992 Wetlands and Stonnwater Workshop in Clearwater, Florida.
Contractual support was provided by Terra Tech, Inc. Staff Include Drew
Zacherle, Mary Beth Corrigan, John Hochheimer, and Colleen Charles.
IX
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1. INTRODUCTION
The U.S. Environmental Protection Agency (EPA) is in the process of
developing and implementing programs; to reduce pollutants in urban
runoff and stormwater discharges. The protection of natural drainage
systems, including wetlands, is an important pan of these efforts. The
need for a more complete understanding of the effects of stormwater
impacts on wetlands has been recognized (Newton, 1989; Stockdale,
A draft of this issue paper was prepared to focus discussion on these and
other related issues at an EPA-sponsored Wetlands and Stormwater Work-
shop held hi Clearwater, Florida, in January 1992. The purpose of the
workshop was to investigate and explore various issues, options, and
opinions related to the protection of natural wetlands that receive storm-
water and urban runoff. The focus of workshop" discussions was not on
methods for assessing or improving the capacity of wetlands to control
stormwater discharges, but on what is known and not known concerning
the impacts to natural wetlands from urban stormwater discharges and the
opportunities for protecting natural wetlands mat receive urban stormwa-
ter. The major themes discussed at the workshop include the following:
• Wetlands serve important water quality improvement functions
within the landscape, and these functions should be factored into
stormwater management strategies. - .
« Wedands.becauseofmeiruniqueiwsitionmthelandscape.natur-
alry receive stormwater. However, when considering diversion of
flows to a wetland (either from stormwater sources or non-storm-
water sources), it is important to consider that wetiands have a
limited capacity for handling increased flows or additional pollut-
ant loadings.
• There was a general recognition that wetiands in urban areas are
dramatically altered by uncontrolled runoff, either through natural
drainage to those systems or through direct discharge to wetlands.
Stormwater management techniques (best management practices,
or BMPs), specifically designed tp mitigate these impacts, may
offset some of the impacts of increased volumes and velocities of
runoff that cause changes to wetlands.
. • At least 19 potential impacts to wetlands (including changes to the
physical, chemical, and biological characteristics of wetlands)
.were identified by the workshop particpants as being associated
with the changes in the hydrology of the wetland system and
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February 1993
increases in pollutant loads, or modifications associated with some
stormwater management practices.
• There is a great deal of variability in site conditions, as well as
regional variability in stormwater characteristics, climate condi-
tions, urban development patterns, soil types, and wetland types
mat will make the development of nationally prescribed BMPs for
protecting wetlands from stormwater impacts difficult
• Past actions to control and manage stonnwater presented hard
choices: limiting or concentrating the areas of development to
accommodate upland stormwater management; discharging to a
wetland (with or without in-place BMPs); diverting stormwater
flows around the wetland; modifying the wetland to impound
surface water in wetlands to limit higher peak flows downstream;
or a combination of these methods. These management decisions
are difficult and are likely to require watershed-level planning.
• There was general recognition among the workshop participants
that national guidance is needed to provide a framework for
baseline protection of wetlands that receive stormwater. When
considering what is appropriate for national guidance,- it is impor-
tant to realize mat any such guidance must be flexible to address
a variety of site-specific factors, as well as regional and local
variability in conditions.
• There is a need to better integrate programs for stormwater man-
agement and wetlands at the Federal, State, and local levels.
Basinwide planning is needed to help mesh the sometimes con-
flicting goals of these programs and to address trade-off decisions
between pollution controls, habitat quality, and flood control ob-
jectives.
• There was general agreement mat in developing areas, a critical
step is to use best management practices to settle solids, regulate
flow, and remove harmful chemicals prior to discharging storm-
water into a wetland..
• In arid areas, State water quantity rights may preclude some
prerreatment options mat involve holding the water and releasing
it more slowly. Inaddition^itmaybcdifaculttoroutestormwater
through wetland areas if such an approach conflicts with water
rights downstream. • '
• Policies should reflect that urbanization can dramatically alter me
hydrology of a wetland system, and the discharges of stormwater
from urban areas may be an integral part of the flow patterns of a
wetland (particularly in arid regions). For example, strict restric-
tions on discharging stormwater to wetlands may create incentives
to route water around the wetland (depriving the wetland of an
important water source), which could diminish functions or con-
vert the wetland to upland.
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February 1993
The information developed at the workshop is being used by the Wetlands
Division of EPA's Office of Wetlands, Oceans and Watersheds (OWOW)
and the Office of Wastewater Enforcement and Compliance (OWEC) to
develop joint guidance to address urban stormwater discharges to natural
wetlands. This issue paper incorporates information generated at the
workshop as well as comments received on the draft issue paper.
Urbanization dramatically alters the natural faydrologic cycle> As urban
structures such as roads and buildings are built, the amount of impervious
area within a watershed increases. Increases in impervious area increase
the volume and rate of runoff, while decreasing groundwater recharge.
Urbanization also increases the type and amount of pollutants in surface
runoff:
Uncontrolled urban runoff can have adverse impacts on urban wetlands.
The dramatic increases in peak flow rates can cause erosion and channeli-
zation in the wetlands, which ultimately adversely impact the ability of
the wetland to support aquatic habitat. Reductions in groundwater re-
charge within a watershed can reduce dry weather flows in wetlands. The
hydrology of a wetiand is considered one of the most important factors in
establishing and maintaining specific types of wetlands and wetland
processes (Mitsch and Gosselink, 1986). Relatively little information has
been compiled on the adverse impacts of stormwater on natural wetlands
(Woodward-Clyde Consultants, 1991; Newton, 1989; Stockdate,J991).
Older approaches to stormwater management have f ocused on efficiently
collecting and conveying stormwater offsite. This approach can increase
downstream property damage and impacts on receiving waters. Newer
approaches to stormwater management seek to retain natural features of
drainage systems and provide onsite management to address water quality
and water quantity goals. This approach views stormwater as a resource
to be used to recharge groundwater and to supply fresh water to surface
waters, including wetlands. Properly managing stormwater can avoid
problems with erosion, flooding, and adverse impacts on natural drainage
features, including wetlands.
EffoitstodevelopStateandlocalstonnwaterinanagementprogramshave
been inconsistent nationwide. Stormwater management approaches have
been varied, andtheability of someapproachestoprotectreceivingwaters
is not well known. Some stormwater management controls, such as wet
ponds, are designed to preserve some of the features of predevelopment
hydraulic patterns and to provide some of the hydraulic and pollutant
removal features of natural receiving systems. The adverse effects and
benefits of siting these controls in or near wetlands are not well under-
stood. ' , - c '.-''•;_
The 1987 amendments to the Clean Water Act (CWA) contain two
provisions addressing the control of pollutants in urban runoff and storm-
water discharges. Section 402(p) of the CWA requires EPA to develop
phased requirements for discharges from municipal separate storm sewer
systems and stormwater discharges associated with industrial activity
under the National Pollutant Discharge Elimination System (NPDES)
permit program. NPDES permits for discharges from municipal separate
BACKGROUND
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February 1993
PURPOSE
storm sewer systems are to effectively prohibit nonstormwater discharges
to separate storm sewers and require municipalities to reduce die discharge
of pollutants in stormwater to the maximum extent practicable.
EPA issued NPDES permit application requirements for discharges from
municipal separate storm sewer systems serving a population of 100,000
or more on November 16,1990. The municipal component of the regu-
lations focuses on requiring affected municipalities to develop municipal
stormwater management programs to reduce pollutants in stormwater and
protect receiving waters.
The November 16, 1990, regulations also addressed which types of
facilities would be required to obtain NPDES permit coverage for storm-
water discharges associated with industrial activity and specified permit
application requirements for these discharges.
Section 319 of the CWAamendments requires States to identify waters that,
without further action to control nonpoint sources, cannot be expected to
attain the water quality standards or goals of the Act States were also to
submit i
The Wetlands and Stormwater Workshop was conducted to investigate the
status of the science regarding the impacts and potential for use of natural
wetlands for the storage and treatment of stormwater. To this end, EPA
formed a panel of wetland scientists, engineers, and environmental man-
agers to provide individual opinions and recommendations on related
issues, including:
• Status of the science regarding the treaniaent of urban stormwater,
• Chemical and physical characteristics of urban stormwater;
'-*.
• Hydrologic, chemical, and biological impacts of stoonwater dis-
charges to natural wetlands;
«, Wateishedmanagementpracticesrelatedtostormwaterdischarges
to natural wetlands;
• Regional and resource-related concerns associated with stormwa-
ter discharges to wetlands; and .
• Programmatic issues and opportunities for implementing sound
.practices.
The purposes of the workshop were to:
• Investigate the potential impacts on natural wetlands used for
urban stormwater control;
• Provide a forum for discussion of topics of concern;
• Form a general agreement as to the state of scientific information;
and
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February 1993
• Develop a sound scientific and technical base to derive govern-
ment policy concerning 'the use of natural wetlands for urban
stonnwatercontrol.
A draft of this issue paper was originally developed to provide a base for
discussion and to support deliberations at the Wetlands and Stonnwater
Workshop held in January 1992 in Clearwater, Florida, Chapter 2 of this
paper presents a summary of the characteristics and functions of natural
wetlands most likely to be impacted by stonnwater discharges. An
understanding of wetland functions is necessary to be able to predict and
measure impacts resulting from stonnwater discharges. The hydrologic
and chemical characteristics of urban stonnwater are summarized in
Chapter 3, with a focus on urban development activities that affect the
quantity and quality of stonnwater. Chapiter 4 presents a discussion of
regional differences in wetland types and stonnwater characteristics.
Such differences will influence the degree and character of potential
impacts on natural wetlands due to urban stonnwater discharges.
Chapter 5 presents a discussion of what is and is not known about changes
that can be caused in wetland systems by stonnwater, including hydrologic
changes, water quality changes, changes in the soil, and observed responses
in plants and animal communities. Chapter 6 presents an overview of
stonnwater management practices that include natural wetlands as a compo-.
nent Examples of practices currently being used by different States are
presented. The summary and conclusions are presentedin Chapter 7.
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2, WETLAND
CHARACTERISTICS
For the purpose of this papa-, wetlands are defined as "those areas that
are inundated or saturated by surface water or groundwater at a frequency
and duration sufficient to support, and that under normal circumstances
do support, a prevalence of vegetation typically adapted for life in satu-
rated soil conditions" (40 CFR 2303). This definition is used by EPA
and the U.S. Army Corps of Engineers (Corps) in irflpTeinenting section
404 of die dean Water Act. Table 1 briefly describes major freshwater
and coastal wetland systems.
1
Wetlands are subject to increased attention relative to receiving stonnwa-
ter runoff because of their inherent water storage and water quality
improvement capabilities. The role of wetlands as storage areas for
stormwater discharges was investigated by EPA (1985a) and Reinelt and
Homer (1990), while Richardson (1989) andEPA(1983) documented the
role of wetlands in water quality processes. The value of natural wetlands,
however, extends beyond their water storage and water quality functions
to include food chain support, erosion control, groundwater recharge/dis-
charge, and habitat functions. An understanding of .these functions is
necessary when contemplating the use of natural wetlands to store and
treat urban stormwater discharges in order to predict and measurepotential
impacts on wetland functions. The potential impacts of urban stormwater
on natural wetlands are discussed in Chapter 5.
Hydrology is probably the most important determinant for the estab-
lishment and maintenance of specific types of wetlands and wetland
processes (Mitsch and Gosselink, 1986). Precipitation, surface water
inflow and outflow, groundwater exchange, and evapotranspiration are the
major factors influencing the hydrology of most wetlands. The balance
of inflows and outflows of water through a wetland defines the water
budget and determines the amount of water stored within die wetland. A
wetland experiences natural water level fluctuations (WLFs) that are
closely associated with the wetland's morphology and the basin's hydro-
logic regime (Stockdale* 1991). WLFs are also determined by specific
factors including wetiand-to-watershed area ratios, level of watershed
development, outlet conditions, and soils (Reinelt and Homer, 1990).
Changes in activities within the watershed (e.g., urbanization) will affect
these natural WLFs.
HYDROLOGY
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February 1993
Table 1. Wetland Types
NONTTOAL FRESHWATER
Lacustrine - Associated with bodies of water greater than 2 m in depth, or less than 8 ha in area, or less
than 30 percent covered by emergent plants.
Riparian - Associated with flowing water systems. For example, bottomland wetlands are lowlands found
along streams and rivers, usually on alluvial floodplains that are periodically flooded. These are often flooded
and termed bottomland hardwood forests.
Palustrine - Do not have channelized flow and either are not associated with bodies of water or form the
headwaters of streams. These wetlands include the following:
Marsh - A frequently or continually inundated wetland generally characterized by emergent, soft-
stemmed herbaceous vegetation adapted to saturated soil conditions.
Swamp - Wetland dominated by woody vegetation.
Bog - A peat-accumulating wetland that has no significant inflows and outflows and supports acidophilic
mosses, especially sphagnum.
Fat - A peat-accumulating wetland that receives some drainage from surrounding mineral soil and usually
supports marshlike vegetation.
Wet prairie -'Similar to a marsh. ' . .
Wet meadow - Grassland with waterlogged soil near the surface but without standing water for most of
the year.
Pothole - Shallow marsh-like pond, particularly as found in the Dakotas.
Playa - Term used in southwest United States for marshlilce ponds similar to potholes, but with different
geologic origin.
COASTAL
Tidal salt marshes - Found throughout the world .along protected coastlines in the middle and high latitudes.
In the United States, these wetlands are often dominated by Spartina and Juncus grasses. Plants and
animals in these systems are adapted to'the stresses of salinity, periodic inundation, ami extremes m
temperature. -
Tidal freshwater marshes - Found inland from tidal salt marshes, but still experience tidal effects. These
marshes are an intermediate in the continuum from coastal salt marshes to freshwater marshes.
Mangrove wetlands- Found in subtropical and tropical regions. These wetlands are dominated by
salt-tokrant red mangrove or black mangrove trees.
SOURCE: Mhsch and Gossdink, 1986. : '" ~
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February ,1993
The time ofyear and the depth, frequency, and duration of inundation and
soil saturation (wetland hydroperiod) are key factors in determining the
impacts of water-level changes in wetlands (Stockdale, 1991). Within the
wetland, the wetland hydroperiod influences the biochemistry of the soils
and is a major factor in the natural selection of wetland biota (Reinelt and
Homer, 1990). The hydroperiod is unique to each type of wetland, and
its relative constancy ensures stability for that wetland. Mitsch and
Gosselink (1986) suggest characterizing hydroperiod by the ratio of flood
duration divided by flood frequency (i.e., the amount of time a wetland is
exposed to excess floodwaters over the average number of times a wetland
is flooded in a given period). Changes in the hydroperiod can affect such
processes as nutrient transformation and availability (Hammer, 1992);
responses of biota, including both enrichment of species and degradation
of species diversity with succession to a different vegetative community,
(Zimmerman, 1987); and amphibian egg and larval development (Richter
et al., 1991). Changes in the hydroperiod Can be measured by the average
change in water level occurring in the wetland (Azous, 1991).
Seasonably is also a characteristic of hydroperiod. Some wetlands have
water year-round, while others may become dry during the summer
period. Reduced groundwater base flows are frequently cited as a conse-
quence of urbanization and may result in extending the length of the dry
period in wetlands, with seasonally affected groundwater sources poten-
tially impacting the life cycles of species dependent on the water column
(Azous, 1991).
A major hydrologic feature of coastal salt marshes and freshwater tidal
marshes is the periodic tidal inundation. The tides act as a stress by
causing submergence, saline soils, and soil anaerobiosis. The tides act as
a subsidy by removing excess salts, reestablishing aerobic conditions, and
providing nutrients (Mitsch and Gosselink, 1986). The periodic tidal
inundations influence the species mat occur in the wetland because of the
water depth and duration of flooding. Salinity is also a major factor in
influencing what vegetation is found in the wetland, with a salinity
gradient generally high in the low marsh and decreasing as the elevation
increases. If the salinity in the adjacent waterbody is less than 5 parts
per thousand (ppt), salt marsh vegetation is replaced by freshwater
plants (Mitsch and Gosselink, 1986).
An important function of wetlands is their role in changes mat occur in
water quality. Many complex chemical and biological processes that
affect water quality occur in wetlands. The occurrence and timing of these
processes are determined by the wetland type and the hydrologic regime
of the wetland. Wetland water quality processes include:
• ' . *>
• Sedimentation,
• Filtration,
« Adsorption,
• Ion exchange,
WATER
QUALITY/BENTHIC
PROCESSES
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February 1993
• Precipitation, and
• Biodegradation. "
Sedimentation is the principal mechanism by which suspended solids are
removed from the water column. Sedimentation is directly related to me
size of the paniculate, the rate and type of flow through the wetland, and
toe residence time of the paniculate. Wetland systems that have long
hydraulic residence times allow most settleable solids to be removed by
sedimentation. As particle size decreases, solids in the water column
become more difficult to remove by sedimentation. Wetlands with dense
stands of vegetation also enhance sedimentation by decreasing the veloc-
ity of water flowing through them.
Filtration occurs as suspended pollutants are physically trapped by vege-
tation, biota, and sediments in the wetland Reduced velocities and dense
vegetation promote greater pollutant removal. Removal of pollutants by
filtratiou-ferojugh soils is effective in removing organic matter, phospho-
rus, bacteria^aad suspended material.
Adsorption is a physical process by which dissolved pollutants adhere to
suspended particulates or Onto bottom sediments and the surfaces of
vegetation. It is also a factor in removing nutrients and heavy metals
through sedimentation. Suspended organic and inorganic materials have
a strong tendency to adsorb other pollutants, such as refractory organics,
hydrocarbons, bacteria, and viruses. Since these substances are adsorbed
onto suspended solids, they, too, are effectively deposited with me trapped'
sediment (Chan et al., 1981; Silverman, 1983 in PSWQA, 11986). Bom
particulates and their associated contaminants can be considered pollut-
ants. Thus the removal of sediments from the water column by wetlands
reduces the potential impact on receiving waters.
The excess water in wetland soils, along with biological and chemical
activities, can change me soils from an aerobic to an anaerobic system,
with manyresultantchemical (reduction-oxidation) transformations in the
wetland. The chemical transformations are governed by pH and redox
potentials (Eh) and determine the state of the nutrient, mineral, or heavy
metal entering the water column in the.wetland or infiltrating the ground-
water The relationship between Eh andpH manifests itself in chemical
speciation; e.g., the predicted pH level necessary to precipitate iron or
manganese is much higher at low Eh levels man at higher Eh levels
(Faulkner and Richardson, 1989).
Nitrogen and phosphorus speciation are two of die most important chemi-
cal transformations occurring in wetlands. Of the many elements neces-
sary to sustain biotic production in wetlands, nitrogen presents special
research challenges because of its chemical versatility. This versatility is
expressed in the various valence states nitrogen can occupy (-3 to +5), in
the intricate array of biotic and abiotic transformations in which nitrogen
participates, and by the fact mat, like few other elements, nitrogen occurs
naturally in soluble and gaseous phases (Bowdao, 1987). In a wetland,
only a fraction of available nitrogen is removed by plants, with the most
effective removal by Ditrification/denitrification (Knight et al., 1986). A
limiting factor for nitrogen removal is anoxia. In aerobic substrates
ammonia is oxidized to nitrate by nitrifying bacteria; Nitrates (NO3) are
10
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February 1993
then converted ,to free nitrogen in the anoxic zones by denitrifying
bacteria. . . ,
; • ..'•-.- -:.&.-. ... .51 .,&. ,
•.*"'. "' • ' ':
Phosphorus removal in wetlands systems occurs from adsorption, absorp-
tion, compiexation, precipitation, and burial. Removal rates are highest
in systems where a significant clay content is present (Watson et al., 1988).
Another factor affecting phosphorus removal is the presence of iron,
aluminum, or calcium. For example,^Richardson (1985) found that the
phosphorus adsorption capacity of a wetland soil can be predicted by
measuring the extractable aluminum content of the soil. Removal effec-
tiveness is limited by the contact surface area of the substrate and the root
zone. ' ', , . • . • . • - , •
Wedands provide a valuable source of food and habitat, and wetlands often
become a focal point for varied wildlife populations within a particular
region. Wetland vegetation also provides nesting material and sites for
numerous birds and mammals Some fish rely on vegetation clumps as
sites for depositing their eggs and as nursery areas for fry (Atchesonetal.,
1979).
Most wetlands receive extensive use by animals characteristic of terres-
trial or purely aquatic environments, while many unique organisms are
restricted to wetland environments (Mitsch and Gosselink, 1986). Wet-
lands are also important habitats for a disproportionately high number of
endangered and threatened plant, mammal, bird, reptile, amphibian, and
fish species. Some aquatic organisms may use wetlands seasonally as a
spawning ground and nursery for theirypung, spending mostof their adult
lives in deeper waters. Amphibians, reptiles, and invertebrates usually
undergo an aquatic phase mat requires water for breeding, egg develop-
ment, and larval growth. Some reptiles and amphibians are able to adapt
to fluctuating water levels (Mitsch and Gosselink, 1986), whereas others
may experience changes in breeding patterns and species composition due
to water level fluctuations (Azous, 1991). Wetlands are also used daily
by birds and terrestrial animals for diurnal and nocturnal food foraging. '
Many birds that inhabit both terrestrial and wetland habitats are frequently
found in the highest numbers in the diverse, productive habitats of
wetlands (NWTC, 1979).
The wetland vegetative community is determined by climate and wetland
hydrology. Wetland plant species are established based on their water
regime requirements and on the natural hydroperiod of the wetland (van
der Valk, 1981). Plant species and diversity, in turn, have a direct effect
on which wildlife will use the site. Species diversity and abundance may
'vary greatly among different wetland locations and within a single wet-
land Some wetlands—acidic bogs, monotypic cattail (Typha) marshes,
and many saltwater wetlands—can have high abundance but low plant
species diversity. Others, such as riverine swamps and fresh/brackish
marshes, have high diversity.
Many emergent plant species are sensitive to changes in water levels in
excess of the wetland's natural hydroperiod (Mitsch and Gosselink, 1986;
Stockdale, 1991). Excess depths, frequencies;, and duration of inundation
BIOLOGIC/HABITAT
FUNCTIONS
11
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February 1993
UNRESOLVED ISSUES
in wet seasons, or water deficiencies in dry seasons, have the potential to
alter the vegetative community and, thus, the wildlife that use the wetland
and the benthic and aquatic organisms that depend on the wetland. These
hydrologic changes can also directly affect some animals, such as am-
phibians, that have distinct preferences for placing their eggs in the water
column (RichteretaJ., 1991). '
Although much recent research has been directed at understanding the
processes that control wetland functions, many questions remain to'be
resolved, particularly with respect to wetland functions as habitat. Among
these unresolved issues are the following:
• In-depth knowledge of the totality of wetland functional support,
taking into consideration such factors as nutrient flows, hydrology,
trophic dynamics, community structure, and population distribu-
tion and abundance, is not available for most wetland types.
• A greater understanding of habitat processes and functions and
how changes in these functions affect the support of living organ-
isms is needed.
• New and improved methods are needed to measure and assess the
habitat functions of wetlands.
-------
February 1993
3, STORMWATER
CHARACTERISTICS
As human activities alter the watershed landscape, adverse impacts to
receiving waters may result from changes in the quality and quantity of
stormwater runoff. Umnanaged storm surges increase discharges during
runoff-producing storm events. These discharges result in a predictable
change of waters flowing to those receiving waters. If left unmanaged,
the hydraulic impacts associated with the: increased water volumes may
be several orders of magnitude higher than the impact of the undisturbed
watershed. In addition to causing runoff volume impacts, stormwater
can also be a major source of nonpoint source pollution in many water-
sheds.
Six main source activities contribute to surface water runoff pollution:
• Agriculture,
1 = -"f ' '
• Silviculture,
• Mining,
, • Construction,
• Urban activities, and
• Atmospheric deposition.
The first five ai-e the traditional sources; the sixth, atmospheric deposi-
tion, has only recently been recognized as a major contributor of some
types of nonpoint source pollution in certain regions of the country. The
type and quality of pollutants carried by storm runoff, commonly result-
ing in nonpoint source pollution of receiving waters, are highly variable
(USEPA, 1984). The pollutant characteristics of stormwater runoff are
largely based on land use characteristics (as illustrated in Table 2) and
vary with the duration and the intensity of rainfall events (Metropolitan
Washington Council of Governments, 1980). Table 2 illustrates the
.variability of pollutant loads associated with stormwater runoff. For
example, Table 2 shows mat loads of suspended sediment vary consid-
erably within aland use and between land uses. Pollutant characteristics
from stormwater runoff also vary regionally.
The remainder of this chapter focuses specifically on the chemical and
hydrologic characteristics of urban stormwater. Knowledge of these
characteristics is necessary to understand and predict the potential impacts
such discharges may have on natural wetlands. The potential impacts of.
urban stormwater discharges on natural wetlands are discussed in Chapter 5
of mis document
13
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February 1993
Table 2. Examples of Pollutant Characteristics Found in Stormwater Runoff From Various
.; Land Uses in the Great Lakes Region
Land Use
General Agriculture
Cropland
Improved Pasture
Forested/Wooded
Idle/Perennial
General Urban
Residential
Commercial
Industrial
Developing Urban
Suspended
Sediment
(kg/ha-yr)
5-8000
30-7500
50-90
2-900
9-900
300-2500
900-4000
75-1000
750-2000
>10,000*
Total
Nitrogen
(kg/ha-yr)
0.8-75
6-60
5-15
1-8
0.6-7
8-10
6-9
3-12
3-13
90*
Total
Phosphorus
(kg/ha-yr)
0.1-9
03-7
0.1-0.6
0.03-0.7
0.03-0.7
0.5-4
0.6-1
0.09-0.9
0.9-6
>10»
Lead
(kg/ha-yr)
0.003-0.09
0.006-0.007
0.005--0.02
0.01-0.05
0.01-0.05
02-0.6
0.08a
- 03-1.0
,b . •
3.0-7.0
Ajmy one value reported.
^
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February 1993
One significant effect of urbanization is to increase pollutant runoff loads
over predevelopment levels. During a storm event, land surfaces, includ-
ing impervious surfaces, are washed clean by the rainfall and the resulting
runoff .creates an increased loading of pollutants to receiving streams
(Livingston, 1989). Pollutant concentrations in urban runoff vary consid-
erably, both during the course of a storm event and from event to event at
a given site, from site to site within a given urban area, and from one urban
area to another across the country. This variability is the result of vari-
ations in rainfall characteristics, differing watershed features that affect
runoff quantity and quality, and variability in urban activities (Wpodward-
Clyde Consultants, 1 990). Table 3 presents ranges of urban runoff pollut-
ant concentratons based on results of the Nationwide Urban Runoff
Program (NURP) as reported in Woodward-Clyde Consultants (1990).
Values reported in Table 3 represent the mean of event mean concentration
pollutant values for the median, 10th perceatile, and 90th percentile sites
in the NURP data. Potential sources of urban runoff pollutants are
presented hi Table 4. The principal types of pollutants found in urban
runoff from these various sources include:
• Sediment . . ? • ' •
• Oxygen-demanding substances (organic matter)
• Nutrients
- phosphorus
- nitrogen
• Heavy metals
- copper
-zinc
- omens
Pesticides
Hydrocarbons
- PAHs
-others
Temperature
Trash/debris
The most important factor in determining the quantity of runoff that .will
result from a given storm event is the percent imperviousness of the land
cover. Other factors include soil infiltration properties, topography, vege-
tative cover, and previous conditions (Woodward-Clyde Consultants,
1990).
, The factors mat influence the hydrologic characteristics of stormwater
are dependent on the phase of urbanization of an area. During the
construction phase, the hydrology of a stream changes in response to
initial site clearing and grading. Trees that had interrupted rainfall are
felled. Natural depressions that temporarily ponded water are graded to
a uniform slope. The thick humus layer of the forest floor that had
CHEMICAL
CHARACTERISTICS
HYDROLOGIC
CHARACTERISTICS
15
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February 1993
Table 3. Ranges in Pollutant Concentrations Found in Urban Runoff
Mean Concentration in Runoff
Constituent
Total Suspended Solids (mg/L)
BOD (mg/L)
COD (mg/L)
Total Phosphorus (mg/L)
Soluble Phosphorus (mg/L)
Total Kjedahl Nitrogen (mg/L)
Nitrate-Nitrogen (mg/L)
Total Copper fyig/L)
Total Lead Qlg/L)
Total Zinc (jig/L)
lOthPercentile
Urban Site
35
6.5
40
0.18
0.10
0.95
0.40
15
60
80
Median
Urban Site
125
12
80
0.41
0.15
2.00
0.90
40
165
210
90th Percentile
Urban Site
390
20
175
0.93
0.25
4.45
2.20
120
465
540
SOURCE: Woodward-Clyde Consultants, 1990.
Table 4. Sources of Urban Runoff Pollutants
Source
Pollutant of Concern
Erosion
Atmospheric Deposition
Construction Materials
Manufactured Products
Plants and Animals
Nonstormwater Connections
Accidental Spills
Sediment and attached soil nutrients, organic
matter, and other adsorbed pollutants.
Hydrocarbons emitted from automobiles, dust,
aromatic hydrocarbons, metals, and other
chemicals released from industrial and
commercial activities.
Metals from flashing and shingles, gutters and
downspouts, galvanized pipes and metal plating,
paint, and wood preservatives.
Heavy metals; halogenated aliphatics; phthalate
esthers; PAHs; other volatiles; and pesticides and
•phenols from automobile use, pesticide use,
industrial use, and other uses.
Plant debris and animal excrement
Inadvertent or deliberate discharges of sanitary
sewage and industrial wastewater to storm
drainage systems.
Pollutants of concern depend on the nature of
the spill.
SOURCE: Based in pan on Woodward-Clyde Consultants, 1990.
'16
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February 1993
absorbed rainfall is scraped off or eroded away. Having lost much of its
natural storage capacity, the cleared and graded site can no longer prevent
rainfall from being rapidly converted to runoff (Schueler, 1987).
\ . •'
'' • ^ .... - . ,
After construction is completed, rooftops, roads, parking lots, sidewalks,
and driveways make much of the site impervious to rainfall. Unable to
percolate into the soil, rainfall is converted into runoff. The excess runoff
becomes too great for the existing drainage system to handle. As a result,
the drainage network must be improved to direct and convey the runoff
away from the site (Schueler, 1987).
The following changes in stream hydrology in a typical, moderately
developed watershed were summarized by Schueler (1987):
• Increased peak discharges compared to predevelopment levels
(Leopold, 1968; Anderson, 1970);,
• Increased volume of storm runoff produced by,each storm in
comparison to predevelopment conditions;
• Decreased time needed for runoff to reach the stream (Leopold,
1968), particularly if extensive drainage improvements are made;
• Increased frequency and seventy of flooding;
• Reduced sircamflow during prolonged periods of dry weather due
to.the reduced level of infiltration in the watershed; and
• Greater runoff velocity during storms, due to the combined effect.
of higher peak discharges, rapid time of concentration, and the
smoother hydraulic surfaces that occur as a result of development
17
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February 1993
18
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February 1993
STORMWATER CHARACTERKIICS
ANDINWETLAND
To comprehensively evaluate die impacts and potential for use of natural
wetlands for the storage and treatment of urban stormwater runoff, it is
essential to understand the regional variations, both in stormwater runoff
and in natural wetland types, that exist throughout the Nation. The
following sections briefly summarize these differences.
The characteristics of precipitation events control die timing, Volume, and
intensity of urban stormwater runoff. The U.S. Department of Agriculture,
Soil Conservation Service (SCS) developed dimensionless rainfall distri-
butions using U.S. Weather Bureau data (McCuen, 1989). The distribu-
tions are based on generalized rainfall volume-duration-frequency
relationships and indicate that there are four geographically distinct rain-
fall regions hi the United States, illustrated in Figure 1. Figure 2 is a
dimensionless hydrograph that shows the hourly fraction of total rainfall
that falls in a 24-hour period for each rainfall distribution type (Ferguson
and Debo, 1990).
Climatic variations result in different storm intensities for each rainfall
distribution type. Figure 3 illustrates the major climatic regions of North
America (Ahrens, 1982) and shows mat the four SCS rainfall distribution
types have very different climatic regimes. Figure 4 shows die frequency
of thunderstorms experienced nationwide, given in days per year when
thunderstorms are observed (Ahrens, 1982). It is obvious why Type IA,
with 20 percent of the rainfall volume falling during me 8th hour of a
24-hour storm, and Type m, with 55 percent of the rainfall volume falling
during the 12th hour of a 24-hour storm, have very different stormwater
characteristics. Type IA regions have Maritime and Mediterranean cli-
mates with onshore winds producing mild, wi£t winters with frequent, light
precipitation; dry summers; and very few thunderstorms annually. Typi-
cal Type IA storms are long, steady periods of relatively light rainfall.
Type m regions are coastal areas with a humid subtropical climate, with
adequate precipitation throughout the year, cold to mild winters, and hot
and humid summers with frequent thundershowers. Typical Type III
storms are short, high-intensity rainfall events.
• , ' *'
Seasonally of precipitation, the time of year in which the precipitation
falls, is another important factor that must be considered when evaluating
REGIONAL DIFFERENCES
IN STORMWATER
CHARACTERISTICS
19
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February 1993
Figure 1. Approximate geographic areas for SCS rainfall distributions (Adapted from McCuen, 1989)
0.6
0.0
0 2 4 6 , 8 10 12 . 14 16 18 20 22 24
Time during 24-hour storm, lu.
Figure 2. Hourly fraction of total rainfall within a 24-hour period for each rainfall distribution Hype
(Ferguson and Debo, 1990)
20
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February 1993
A Moist
tropicci
cknur.e
3 .Dry
ciimc:e
C Most ciunote
mild winter
D Moist chmotes
with sever* ,
winters
E Polar
H Highland
7r=c:csi wet Af
Tropics: wet and dry Aw
desert cr arid BW .
Steppe or senu-arid BS
V.anr.eClc
Coastal Med«erran<»an Csb
Interior Moditerranoan C«a
Humid subtropical Cfa
»
Hunud continental hot sumrr.er Die
Humid cononental warm summer Bib
Subpolar Die
Polar tundra ET t •,
^ciox icecap EF
H
Figuie3. Major climatic regioas of Nordi America (after Koppen)(Ahrens, 1982)
21
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February 1993
10
70
Figure 4. Average number of days each year on which thunderstorms are observed throughout the
United States (Ahrens, 1982)
stonnwater runoff. Water available during the growing season has a
differcmimpacttomthatofwaterthatbecomfisavailablewhentheplanJs
are donnanL Precipitationthat fells as snow during the winter months
and melts during the spring thaw has a profound effect on local stream
levels. Figure 5 gives the monthly distribution of precipitation in the
United States.
When precipitation intensity and frequency are combined with water-
shed cover characteristics, the runoff characteristics of a region canbe
estimated. Runoff estimations are useful for estimating impact to a
natural wetland from stonnwater. Figure 6 gives average annual
runoff in inches for the United States (Chow, 1964).
REGIONAL DIFFERENCES
IN WETLAND TYPES
The United States has a wide range of wetland types mat result from the
interaction of many separate environmental variables. The characteristics of
wetlands derive fiomand are conttolledby twoinfisirreJatedfactnrs: (1) origin
and (?) regional plimatie fjK*oiy,
The origin of a wetland, and the resulting topography, affects and deter-
mines critical wetland aspects such as elevation, drainage, and soils.
Wetlands are created by one or more bask processes: geological forces
(tectonic, volcanic, and glacial activities); erosion and sedimentation;
animal activity; and human activities (OTA, 1984; Hammer, 1992). The
second major controlling factor that leads to the formation of regional
wetland types is climate. Because hydrology is critical in establishing
22
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February 1993
Figure 5. Month-to-month variation of precipitation in the United States (UJS. Weather Bureau)
(Chow, 1964)
Figure 6. Average annual runoff in the United States (Chow, 1964)
23
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February 1993
'maintaining a wetland (Mitsch and Gosselihk, 1986), the variable climates
in the United States have contributed to the formation of distinctly
different wetland types (OTA, 1984). Rgurc? shows the major physiog-
raphic regions of the United States, and Table 5 gives the geographic
locations of wetland types in the United States (OTA, 1984). The relation-
ships of the -various wetland types to their origin, hydrology, soils, and
vegetation are summarized in Table 6. -
RELATIONSHIP BETWEEN
REGIONAL
CHARACTERISTICS OF
STORMWATERAND
WETLANDS
The relationship between stormwater characteristics and wetland type
will influence the degree and character of the impacts to natural wetlands
that may result from urban stormwater discharges. The regional differ-
ences in stormwater hydrology and wetland types summarized above can
play a significant role in determining such impacts. Table 7 presents the
wetland types that occur in each of the SCS rainfall distribution areas to
illustrate a method of describing the relationship between stormwater
characteristics and wetland types. Byidentifymgffooseregionsofthecountry
t.AtfcnUe CoasJal Zone
2. Guff Coastal Zone
3. Atlantic Coastal Rats
4. Gulf Coastal Flats
5. Guff-Atlantic Holing Plain
6. Lower Mississippi Alluvial Plain
7. Eastern Highlands
8. Dakota-Minnesota Drift and Lake-bed Flab
9. Upper Midwest
10. Centra! Kite and Plains
11. Rocky Mountains
12. Intarmontane
13. Pacific Mountains
Figure 7. Physiographic regions of the United States (Mitsch and Gosselink, 1986)
24
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February 1993
Wetland Type
TableS. Locations of Wetland Types in the United States
Primary Regions3 States
inland freshwater marshes
Bogs
Timdra
Wooded swamps
Bottom land hardwood
Coastal salt marshes
Mangrove swamps
Tidal freshwater marshes
Prairie pothole region: Eastern
Highlands (7); Upper Midwest (9);
Dakota-Minnesota drift and lake bed
flats (8); Central Hills and Plains (10)
West Coast: Pacific Mountains (13)
Upper Midwest (9); Gulf-Atlantic Rolling
Plain (5); Gulf Coastal Flats (4); and
Atlantic Coastal Flats (3)
Central Highland and Basin; Arctic
Lowland; and Pacific Mountains
Upper Midwest (9); Gulf Coastal Flats (4);
Atlantic Coastal Flats (3); and Lover
Mississippi Alluvial Plain (6)
Lower Mississippi Alluvial Plain (6);
Atlantic Coastal Flats (3); Gulf-Atlantic
Rolling Plain (5); and Gulf Coastal
Flats (4)
Atlantic Coastal Zone (1); Guff Coastal
Zone (2); Eastern Highlands (7); Pacific
Mountains (13)
Guff Coastal Zone (2)
Atlantic Coastal Zone (1) and Flats (3);
Guff Coastal Zons (2) and Flats (4)
New York and New Jersey to North
Dakota and eastern Montana;
Washington, Oregon, northern California
'Wisconsin, Minnesota, Michigan, Maine,
Florida, North Carolina
Alaska
Minnesota. Wisconsin, Michigan, Florida,
Georgia, South Carolina. North Carolina,
Louisiana -
Louisiana. Mississippi, Arkansas,
Missouri, Tennessee, Alabama, Florida,
Georgia, South Carolina, North Carolina,
Texas
All coastal States, but particularly me
Mid-and South Atlantic and Gulf Coast
States
Florida and Louisiana
Texas, Louisiana, Mississippi, Alabama,
Florida, all of the Atlantic coastal states
'Numbers in parentheses refer to the geographic regions in the United States identified in Figure 7.
SOURCE: Adapted from OTA, 1984 and Mitsch and Gosselink, 1986.
in which, rainfall is characterized by the fraction of rain that falls per hour
duringa24-hourperiod(e.g^ rainfall is more or less evenly distributed during
a 24-hour period or ig rhararfrTJyrd hy a gradual hirilH-iip nf rainfall |
by a brief period of relatively intense rainfall and gradual dissipation) and the
wetland types that occur in those regions, storm hydrology can be linked to
wetland type. Also, the actual hydroperiod characteristics of natural wetlands
depend on specific watershed land use and wetland morphology, soils, and
biological nature in addition to regional climate. The effects that regional
differences in wetland type and stormwater characteristics may have on
impacts on natural wetlands that receive stormwater are briefly discussed in
Chapters.
25
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February 1993
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SCSType
Table 7. Wetlands Present in SCS
Wetland Type
Type Rainfall Distribution Areas
State
Type I
TvpelA
Typell
TypeJJI
Tundra
Coastal salt marsh
Tidal freshwater marsh
Coastal salt marsh
Tidal freshwater marsh
Inland freshwater marsh
Bogs
Wooded swamps
Bottomland hardwood
Coastal salt marsh
Mangrove swamps
Tidal freshwater maJSfi7 ' •
^>
Coastal salt'marsh
'•Mangrove swamp
Tidal freshwater marsh
Inland freshwater marsh
AK,ffl
WA.OR.CA
CA,AK
WA,OR,CA,AK
WA.OR.CA
NY, PA, OH, NO. IN, WI, IL, MN,
ND,SD,MT,WA,OR
WI.MN.MIiME.FL.NC
MN, WI, MI, FL, GA, SC, NC, LA
LA, MS, AR, MO, TN, AL, FL, GA, SC,
NC.TX
DE.MD.VA
FL
FL,VA,MD,DE
ME, NH, MA, RI, CT, NY, NJ, NC, SC,
GA, northern FL, AL, MS, LA, TX
FL,AL,MS,LA,TX
TX, LA, MS, AL, FL, GA, NC, NJ, NY,
CT,RI,MA,NH,ME
ME, NH, MA, RI, CT, NY, NJ, VA, NC,
SC, GA, FL, AL, MS, LA, TX, AK, OK
28
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February 1993
5. POTENTIAL IMPACTS OF
URBAN STORMWATER RUNOFF
ON NATURAL WETLANDS
Stormwaler runoff has the potential of influencing natural wetlands in four
major areas: wetland hydrology, wetland soils, wetland flora and fauna,
and wetland water quality. There is little doubt that urban stormwater
discharges can affect wetlands; however, the long-term impacts on natural
wetlands from urban stormwater discharges axe not known at this time.
Perturbations to wetland hydrology can cause fluctuations in the character
of the ecosystem mat are seen as changes In the species composition and
richness, primary productivity, organic deposition and flux, and nutrient
cycling (Livingston, 1989). Naturally occurring quantities of runoff with
seasonal fluctuations are essential for the maintenance of a wetland, and
moderate amounts of nutrients and sediment in the runoff can increase a
wetland's productivity (Stockdale, 1991). However, excessive stormwa-
ter discharge on a continuous basis has the potential to alter wetland
hydrology, topography, and the vegetative community (Johnson and
Dean, 1987 in Stockdale, 1991). A few investigations that look at the
potential impacts to natural wetlands from stormwater discharges have
been initiated Some of these impacts have been identified and others
require further investigation. This chapter examines the nature of changes
to wetland hydrology, soils, and water quality attributed to stormwater
runoff and the perceived effects on die biologic community.
As a result of urbanization, the quantity and quality of stormwater runoff
diange due to physical changes occurring in the watershed. The quantity
of water entering a wetland as stormwater inmoff is dependent on factors
such as drainage basin area, imperviousness of the drainage basin, routing
of stormwater within the drainage basin, and climate (Lakatos and McNe-
mar, 1987). Increased impervious area in the watershed (from building
construction, roadways, and parking lots), removal of trees and vegeta-
tion, and soil compaction can increase the quantity of urban stormwater
ranoff (Schueler, 1987). Water velocity also increases, in general, as the
degree of urbanization increases (Viessman et al., 1977). These same
activities will potentially cause decreased infiltration of stormwater to
groundwater, resulting in decreased base flow.
One basis for determining the impacts to a wetland from stormwater
runoff is the wetland's natural hydroperiod. Impacts will also vary
depending on the wetland type and size and whether the runoff is inter-
cepted before entering the wetland. Brinson (1988) characterized wet-
lands, geomorphologically, in three major categories:
HYDROLOGIC CHANGES
29
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February 1993
• Basin — Wetlands typically in headwater regions that capture
drainage from small areas and may receive precipitation as the
primary source of water. They are characterized by vertical fluc-
tuations of the water table, a long hydroperiod, low hydrologic
energy, and low nutrient levels. Plant communities are usually
concentric zones of similar vegetation.
• Riverine—Wetlands that occur throughout the landscape and are
primarily affected by water flowing downstream. Riverine wet-
lands typically have short hydroperiods, high hydrologic energy,
and high nutrient levels. Plant communities are usually parallel to
the direction of water flow.
• Fringe — Wetlands that are usually located at the base of a
drainage basin and next to a large body of water. They generally
have a long hydroperiod, high hydrologic energy, and variable
nutrient loads. Fringe wetlands are also usually influenced by
frequent flushing by bidirectional waterflow. Many fringe wet-
lands are located in estuarine areas. Zonation of vegetation is
usually perpendicular to the direction of water flow.
Although these classifications are very general, Branson (1988) acknow-
ledges that classification of many wetlands is not clear-cut and the
definitions tend to overlap.
Known impacts to wetlands associated with increased storm runoff in-
clude change in wetland response time, change in water levels in the
wetland, and change in detention time of the wetland. The response time
is the time it takes for a wedand's water depth to begin to rise in response
to a storm event occurring in the watershed. The wetland's water depth
will begin to rise sooner as the infiltration capability of die watershed
decreases. The greater the amount of runoff entering the wetland soon
after the storm event, the greater the water level fluctuation (WLF)
(Azous, 1991). On the other hand, the mcreased runoff at ths expense of
infiltration may cause local water tables to be reduced along with reducing
base flows of local streams (USEPA, 1985). Reduction in groundwater
base flows has the potential effect of extending the length of dry periods
in wetlands with seasonally affected groundwater sources, potentially
impacting the life cycles of die species dependent on the water column
(Azous, 1991).
Increased impervious surface areas have die effect of increasing flood
peaks during storms and decreasing low flows between storms (Stockdale,
1991). Larger peak flows can result in scoured streambeds as the beds
enlarge to accommodate larger flows. Associated impacts include in-
creased sediment loading to bordering vegetated wetlands and reduction
offish spawning habitat (Canning, 1988). In addition to increased flows,
urbanization can increase die velocity of die stormwater entering die
wetland, which can result in biotic disturbances (Stockdale, 1991). Dis-
rupted flow patterns and channeling can result in decreased pollutant
removal efficiencies (Morris et al., 1981), and due changes in velocity will
determine deposition as well as eroded areas (USEPA, 1985).
Changes in average water levels or duration or frequency of flooding will
also alter species composition of plant and animal communities and
30
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February 1993
distribute pollutants more extensively throughout the wetland (Stockdale,
1991). Cooke (1991) states that species richness is affected by increases
in water level fluctuation, with decreased species richness associated with
higher water level fluctuations than are found in natural systems. The
flood tolerance and sensitivity of different plant species vary greatly and
will dictate the response to flooding stress. Responses of vegetation to
WLF are discussed in the Biologic/Habitat Impacts section.
• - • . . , . . . ... . •
As stormwater runoff passes through a wetland, its quality often changes
and the changes tend to be variable and difficult to predict The ability of
a wetland to remove pollutants from water has typically been the predomi-
nant reason cited to promote the use of wetlands for stormwater runoff
treatment Studies have been conducted to determine the pollutant re-
moval capacity of natural wetlands (Schiffer, 1989; ABAC, 1979; Hickok
et al.', 1977), and recent studies have tried to address the impacts of
stormwater runoff on wetlands (Homer, 1988; Cooke, 1991; Reinelt and
Homer, 1991). Table 8 gives an example of seasonal water quality
characteristics for stormwater from various land uses. Note the differ-
ences from season to season (e.g., total phosphorus for undeveloped land
was 22 mg/L, 0.37 mg/L, and 030 mg/L from spring to fall) and between
land uses. Changes in water quality as stormwater runoff passes through
a natural wetland are examined in mis chapter by discussing physical,
chemical, and biological changes separately.
The predominant physical water quality parameters of concern are tem-
perature, conductivity, and suspended solids (Reinelt and Homer, 1991).
As urbanization increases, these parameters typically increase in storm-
water runoff and likewise in wetlands (Reinelt and Homer, 1991). In-
creases in water temperature are attributed to wanning of runoff as it
passes over wanned impervious surfaces. Conductivity increases are
related to increases in the total dissolved solids mat typically are found in
stormwater runoff. *
Of these three physical parameters, suspended solids are typically the
pollutant of concern, primarily because solids tend to settle within the
wetland., A good example of sedimentation rates achievable in wetlands,
in which a wetland was found to trap 16,009 kg of sediment per year, is
illustrated by Hickok et al. (1977). These results represented a reduction
of 94 percent of the suspended solids entering mat wetland on an annual
basis. Hickok et al. (1977) pointed out that total suspended solids are
often the parameter mat exceeds effluent requirements of stormwater
runoff. Other authors (ABAC. 1979; Schiffer, 1989) have also reported
nigh percentage removals of suspended solids from stormwater runoff
passing through a wetland.
WATER QUALITY
CHANGES
31
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February 1993
Table 8. Comparison of Stonnwater Runoff Quality
SPRING 1975 SUMMER 1975
FALL 1975
Drainage
Group
Undeveloped
Low-Density
Residential
Business/Commercial
Urban Roadway
TP
(mg/L)
2.2
2.4
2.0
1.9
NHa-N
(mg/L)
3.33
3.87
2.85
2.55
TSS
(mg/L)
780
559
580
614
TP
(mg/L)
037
0.73
0.22
0.09
NHa-N
(mg/L)
4.13
5.44
5.13
2.86
TSS
(mg/L)
1200
3800
374
200
TP
(mg/)L
0.30
0.42
0.22
0.25
NHa-N
(mg/L)
4.33
4.97
4.00
3.81
TSS
(mg/L)
70
60
68
100
Average
2.1
3.15 633
035 4.39 1394
0.30 4.28
75
Note: Above concentrations are based on weighted values calculated from specific runoff events that occurred
during the study period. ' ' ,
TP = Tottl phosphorus .
NHj-N = Ammonia nitrogen
TSS = Total suspended solids
SOORCE:HickoketaI,1977. -
Chemically, water quality parameters of concern can be broken down into
nutrients, metals, and other toxics. Nutrients include phosphorus and
nitrogen and are generally linked to eutrophication problems in receiving
waters. Metals present in stormwater runoff may include copper, chro-
mium, cadmium, nickel, lead, iron, manganese, and zinc. Other metals
may be present depending on the specific activities within the drainage
basin feeding the wetland. Miscellaneous toxics that may be present in
Stonnwater runoff include pesticides, hydrocarbons, and organic com-
pounds. Table 9 compares three wetlands used to treat Stonnwater runoff
and gives an indication of the variability of pollutant removal between
wetlands. This table shows some of the variability found between differ-
ent wetlands. For example, phosphorus decreases about 79 percept in the
Wayzata wetland, increases about 6 percent in the Palo Alto wetland, and
decreases about 87 percent in the Island Lake wetland.
The fate of chemicals entering a wetland is highly variable and depends
on many chemical and physical factors (Richardson, 1989). At times,
wetlands serve as a sink for pollutants, which are stored in the wetland.
Wetlands can also transform pollutants from one form to another. The
transformation may be from a desirable to an undesirable state, or the
converse can occur. The complex chemical reactions mat occur in wet-
lands change with time; Forexample,apollutantbeingstoredfflaweuand
can become a transformed pollutant that is subsequently exported from
the wetland (Richardson, 1989).
Biological changes in water quality for wetlands receiving Stonnwater
runoff typically are reported as changes in the bacteriological quality
of the water. Homer (1988) reported that bacterial indicator
(fecal conforms and enterococci) within wetlands increased in numbers
in more highly urbanized watersheds. The levels reported by Homer
(1988) did not exceed water quality standards and were in areas not
typically used directly by humans. However, wetlands with elevated
bacterial levels that discharge to shellfish areas may be of concern.
32
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February 1993
Table 9. Water Quality Changes From Several Wetlands Receiving Stormwater Runoff
Wayzata3
Pollutant In (mg/L)
Phosphorus 1.12
Total
Nitrogen
Total
Ammonia 4.07
Ammonia & Organic
Wayzata
Out (mg/L)
0.24
5.87
PaloAltob
In (mg/L)
0.36
3.67
Palo Alto
Out (mg/L)
038
2.30
Island
Lakec
In (mg/L)
0.23
0,23
1.4
. Island
.Lake
, Out (mg/L)
0.-03
0.01
0:82
Solids
Total Suspended
Solids
Metals
313
119
290
37.5
ZJDC
Lead
Copper
Cadmium
Nickel
Chromium
"ABAC, 1979.
•SchSffer, 1989.
0.012
0.041
0.017
0/0009
0.0022
0.0025
0.0033
0.0003
0.16
0.05
<0.01
6.07
0.05
0.02
<0.06
0.075
0.018
0.008
<0.001
0.0075
0.605
..
0.025
0.0003
0.001
<0.001
0.0045
0.490
Reinelt and Homer (1991) found that the water columns in wetlands with
a flow-through (more' channelized) character in .urbanized areas had
higher bacterial levels than more quiescent open-water systems. The
difference was attributed to settling of sediment, and the adsorbed bacte-
ria, out of the water column in the open-water systems. Reinelt and
Homer (1991) compared levels of chlorophyll a, an indicator of algal
growth, in several wetlands and found that open-water wetland systems
had higher levels than those of other systems.
Physical, chemical, and biological qualities of the soil substrate change
in wetlands as they are subjected to stonnwater runoff. Soils are storage
facilities for many potentially toxic compounds including heavy metals.
Urban stonnwater input has the potential to change the pH and redox
potential of soils, rendering many toxins available from the storage pool
so that they can have an immediate effect on wetland soils, both in situ
and potentially downstream (Cooke, 1991). The rate of metal accretion
and the degree of burial in the sediments are critical factors in determining
'the loadings that can be endured by wetlands without damage (USEPA,
1985). Physical property changes of wetland soils due to stonnwater
runoff such as texture, particle size and distribution, and degree of
saturation are not well documented in the literature. Some of the physical
WETLAND SOIL CHANGES
33
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February 1993
properties will be affected by other processes that occur due to stormwater
runoff. For example, as sediment is deposited in the wetland, the soil will
take on the characteristics of the sediment addition. As the hydrology
changes in a wetland, the soil moisture patterns may also change to reflect
new conditions.
Similar to the physical properties, the chemistry of wetland soils change
as processes change in the wetland. Chemical property changes typically
reflect sedimentation patterns as documented by Schiffer (1989) and
ABAC (1979) and are illustrated in Table 10. The findings of Homer
(1988), relative to the greater accumulation of some metals in some zones,
were high heavy metal accumulation occurring in the inlet zone of
wetlands affected by urban runoff. Wetland soils typically act as a sink
for nutrients and metals, as evidenced in Tables 11 and 12. Note the large
differences in constituent concentrations for phosphorus, nitrogen, and
some metals. Another chemical process that occurs in wetlands is the
adsorption of some chemicals to the existing soil particles in the wetland
(Richardson, 1989). Chemical processes in wetlands are also transient
As water chemistry changes, pollutants that are stored in wetland soils can
be transformed from solid to dissolved phases and become exported from
a wetland. For example, as the soil-water interface becomes anaerobic,
the redox potential changes, and pollutants like phosphoiius are trans-
formed from solid to dissolved phases.
Table 10. Summary of Mean Soil and Sediment Chemistry Data as a Function of Sampling Location,
December 1978 (in mg/kg unless noted) •
Source of EC Organic
Validation pH (mmhos/cm) Carbon TKN
Available
NH3-N Total-P P Cu Pb
Lateral Position
LowerMarsh 4.9 21 2.0 458 72
Middle Marsh 5.9 10 2.1 372 37
UpperMarsh 5.7 18 23 1320 52
Vertical Position
0-8inches 5.9 10 2.0 1,002 39
8-16incbes 5.5 17 2.4 495 .70
16-24 inches 5.2 22 2.1 670 51
Vegetative Cover
Pickleweed 4.7 25 2.0 948 40
SaltBnsh 5.1 13 2.0 388 ' 30
RyeGrass 4.7 9 2.0 995 30
No Vegetation 7.7 17 2.6 545 120
(Stream Channel)
605
667
700
710
707
550
2.7
73
7.4
10.0
4.5
2,9
14
14
23
19
17
15
28
38
70
48
56
33
•23
26
25
27
25
22
719
747
645
506
8.7
72
5.9
1.2
15
17
13
23
. 32
41
27
88
23
25
24
27
SOURCE: ABAG, 1979.
34
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February 1993
Table 11. Distribution of Selected Constituents in Water, Sediments, and Ground water at the
/ Silver Star Road Study Area
Water Column fmg/L')
Constituent
Specific conductance
pH-labb
Ammonia nitrogen
Nitrogen ammonia
plus nitrite
Phosphorus
Total organic
carbon
Cadmium
Chromium
Copper
Iron
Lead
Zinc
pH units. - • t
'Detection level
Pond Inlet
145
, 13.
0.8
0.10
0.06
15
<0.001C
<0.003
0.01
- ' — • .
0.034
0.06
' a
Wetland Inlet
144
7.1
0.2
0.10
0.10
15
<0.001C
<0.001C
<0.01C
.• — ;
0.026
0.05
Wetland Outlet
153
6.9
0.4
0.10
0.08
15
<0.002
<0.002
<0.01C
—
0.010
6.03
Pond
^__ -
• .". —
92
9
1,100
• — - '
<6
20
49
4,400
620
250
Wetlands
r —
14
6
260'
• -r- ' . . '
,/
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February 1993
table 12. Median Values of Selected Constituents in the Water Column and Values for One Sample of Bed
Sediments at the Island Lake Wetland
Constituent
Water Column (mg/Ll
Inlet Outlet
Sediments (mg/kg)
Specific conductance*
pH - laboratory1*
Nitrogen, ammonia
Nitrogen, ammonia plus organic
Phosphorus
Total organic carbon
Chromium
Copper
Iron
Lead
Zinc
140
7.1
023
1.4
0.23
7.5
0.001°
0.0075
0.008
0.605
0.018
0.075
100
6.9
0.01
0.82
0.03
203
0.001C
0.0045
0.001
0.490
0.003
0.025
803
9,600
2,250,
2
40
26.5
4350
390
175
at 25 °C.
pH rate-
cDeiecti on level.
SOURCE: Scliffer. 1989.
frequency and duration of inundation experienced in the wetland from
excess stormwater discharge (Stockdale, 1991; Azous, 1991; Cooke,
1991, USEPA, 1985). USEPA, (1985) icports that marked changes in
water depth and frequency of inundation can result in changes in plant
species composition and can affect plant production, as well as influence
dissolved oxygen in the water column and in the soils. The tolerance to
water depth changes varies with each plant species and will dictate the
response to flooding stress (Stockdale, 1991). Local ecotypes within a
species may also vary in their tolerance to flooding and soil saturation
(Tiner, 1991).
Increasing flood frequency or water level fluctuations could cause the
mortality of certain plant species while favoring theproductivity of others.
'Stockdale (1991) in his literature'review states that the character of
wetland vegetation and riparian areas is primarily governed by the flood-
ing regime (Thibodeau and Nickerson, 1985), with periodic inundation
promoting richer and more abundant species composition man either
constant dry or constant flooded conditions (Conner el al., 1981; Gomez
and Day, 1982). Determining plant responses to these stresses is difficult
because direct responses (physical damage) and indirect responses
(physiological responses to direct impacts) are numerous and often simul-
taneous (Koslowski, 1984 in Azous, 1991).
36
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February 1993
Plant species are generally specific in their requirements for germination,
and many are sensitive to flooding effects once established (Nierihg, 1989
in Azous, 1991). Mature trees may survive inundation, whereas the same
water level fluctuations may retard or limit the establishment of seedlings
and saplings (Stockdale, 1991). Newton (j.989) and Stockdale (1991) list
the relative flood tolerance of woody plants. Little information is avail-
able on the effects of hydroperiod on emergent plants, though Kadlec
(1962) found that several species of emergent plants were tolerant of
lengthy dry periods (Azous, 1991). Because the tolerance to flooding,
intermittent and prolonged, varies so widely among and within plant
species, it is hard to extrapolate from the literature what the impact on a
certain plant species within a community will be. Some information,
however, is known about hydroperiod impacts on individual species
(Stockdale, 1991):
• Typhaspp.- survive well under fluctuating conditions.
« Phalaris arundinaceae — has a wide tolerance to WLF, but does
not survive long periods of inundation during the growing season
• Spiraea douglasii — highly tolerant of fluctuating groundwater
tables.
• Carexspp.—highly specific in hydrologic preferences.
Homer (1988) found that emergent zones of palustrine wetlands receiving
urban runoff in the Pacific Northwest were dominated by an opportunistic
exotic grass (Phalaris arundinaceae) while unimpacted wetland plant
communities were composed of a more diverse group of species. Ehren-
feid and Schneider (1990) found a relationship between stonnwater
discharge and changes in plant community composition in the New Jersey
Pinelands; there was a reduction in indigenous wetland species and
colonization of exotic species due to changes in hydrology, water quality,
or both. Wetland plant species may have a limited ability to migrate in
tfae face of persistently raised water levels; many species can spread only
through clonal processes under such conditions because of seed bank
dynamics (van derValk, 1991). The result maybelowercdplantdiversity
over the wedand-to-upland gradient
Azous (1991) reports that many Pacific Northwest amphibians undergo
an aquatic phase that requires water for breeding, egg development, and
larval growth. Changes in wetland water level may alter the quantity and
quality of amphibian habitat, triggering changes in breeding patterns and
species composition (Minton, 1968 in Azous, 1991). Egg development
may be impacted by a decline in WLF by potential exposure and desicca-
tion when stranded on emergent vegetation (Lloyd-Evans, 1989 in Azous,
L991). WLF may also cause changes in water temperatures, which may
impact egg development (Richter et al.,'1991).
Freshwater hydrologic disturbances were also correlated to responses of
fish and macrobentbic assemblages (Nordby and Zedler, 1991). In the
study, two coastal marshes with different hydrology, one of which was
impounded from tidal action, were compared. Results show that the fauna
was most depleted where the hydrologic disturbances were the greatest,
37
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February 1993
REGIONAL DIFFERENCES
with the trends over the course of the study being reduced species richness
and abundance.
Among potential impacts brought up by workshop participants was the
mortality of eggs or young of waterfowl due to flooding during the nesting
period. Also, continuity of habitat around wetlands receiving stormwater
may be important in allowing wildlife free movement and refuge during
storm events.
Wetland mammal populations may potentially be affected by change in
hydroperiod because of the loss of vegetative habitat and the increased
potential for disease organisms and parasites due to shallower, wanner
base flow conditions (Lloyd-Evans, 1989 in Azous, 1991).
Changes in water quality (chemistry and sediment loading) have the
potential to affect the vegetative community structure and to reduce the
availability of plant species preferred by fish, mammals, birds, and
amphibians for food and shelter (Lloyd-Evan:;, 1989 in Azous, 1991;
MitschandGosselink, 1986; Weller, 1987 in Azous, 1991). For example,
Azous (1991) found that plant community richness, evenness, and domi-
nance were negatively correlated with the presence of total organic carbon
in the water column. Further studies are needed to determine the levels
of heavy metal concentrations in the water column that will affect the plant
species diversity in the wetland.
Despite the fact that little work has been documented on the effects of
water quality changes on aquatic organisms, such changes have the
potential to impact life cycles. The ability of aquatic organisms, espe-
cially amphibians, to readily absorb chemicals suggests that they are
responsive monitors ofwetland conditions ORichterandWisseman, 1990).
Richter et aL (1991) state that significant negative correlations were found
between amphibian species richness and water column conductivity.
Negative changes in water quality and potential accumulation in soils and
macrobentbic organisms suggest mat bioaccumiilation may occur in the
hhri and mammal enmmiinififtg Farther studies am required to determine
whether bioacciimnlation is occurring and to what degree.
The habitat requirements, life histories, and species assemblages of wet-
land communities are relatively unknown at this time, requiring further
investigation before impacts from stormwater discharges into wetlands
can be determined.
The degree and character of impacts to natural wetlands due to urban
stormwater runoff described above will vary from region to region and
even from site to site. These impacts will vary due to regional differences
in storm events, wetland types, watershed characteristics, and pollutant
loads. For example, geographical areas with Type Et and HI rain distribu-
tions (see Chapter 4) are those in which relatively intense rainfall occurs
over a relatively brief period of time. Certain wetland types that occur in
•these regions (e.g., coastal wetlands and seasonally wet areas) may be
particularly vulnerable to stormwater discharges characterized by this
type of rainfall. In addition to regional climate, other factors including
watershed land use and wetland morphology, soils, and biological nature
38
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February 1993
influence actual wetland, hydroperiod characteristics and changes that
may occur in these characteristics as a result of stormwater discharges.
For example, Reinelt and Homer (1990) found that water level fluctuation
patterns of wetlands depend on such factors as watershed use (e.g., level
of urbanization), wetland bathymetry, vegetation, inlet and outlet condi-
tions, and others. Hie authors found that the level of urbanization and
wetland outlet conditions appear to be the most significant factors influ-
encing water level fluctuation.
Regional differences not only will result in differences in observed
changes to wetland hydrology, water quality, and soils, but also will-
influence changes that occur in wetland vegetation, benthic organisms,
and the wildlife functions of the wetland. No attempt will be made in this
paper to characterize the regional differences in impacts to wetlands that
occur or may occur as a result of stormwaier discharges. In most cases,
the exact nature of such impacts is not known. Regional differences in
impacts will occur, however, and will need to be considered in any
stonnwater management program.
Wetlands have historically been impounded for a variety of management
purposes, but the primary reason has been wildlife and habitat manage-
ment Treatment of stonnwater runoff in natural wetlands by impounding
all or part of a wetland is currently being used as a method for stonnwater
management (Livingston, 1988; ABAC, 1991). For example, some im-
pounded tidal wetlands in California are used to detain stonnwater during
rainfall events for later gradual release to the San Francisco Bay during
low tides (ABAC, 1991). When a wetland is partly or wholly impounded
for stonnwater management, its water quality improvement, flood attenu-
ation, sediment retention, or groundwater recharge capabilities are being
exploited, possibly at the expense of other wetland functions such as
habitat for fish and wildlife.
An impoundment is defined as a body of water confined by a dam, dike,
floodgate, or other barrier (USEPA, 1989). Often the impoundment of a
wetland (e.g., for stonnwater treatment) results in changes in the wetland.
These changes may result in such extreme modifications that the func-
tional characteristics of a wetland, such as hydrology, soils, or water
quality, are affected. Such modifications may include the placement of
water control structures within a wetland, or at its outlet, which changes
thenaturalhydrologyestabh'shedinthatwetland. Asnotedintbcprevious
sections of this document, many functions of a wetland depend on its
characteristic hydrologic regime. Thus, changes in the hydrology of a
wetland can result in changes in other functional attributes. For example,
impoundment of coastal wetlands may decrease water circulation; in some
cases, circulation may become almost nonexistent (Devoe and Baugh-
man, 1986). As water circulation changes, water quality may change as
well, which can result in changes in temperature, dissolved oxygen,
salinity, pH, and nutrient levels of effluents from wetlands. Another
impact noted in impounded wetlands includes increased sedimentation^
which produces changes in the characteristics of the substrate and smoth-
ers vegetation (Copeland, 1974; Dean, 1975).
IMPOUNDMENTS
39
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February 1993
HYDROLOGIC CHANGES
WATER QUALITY CHANGES
Although there are few research data available describing the impacts of
impounding wetlands for stormwater treatment, it is likely that research
from general studies on impounded wetlands may be used to predict
potential changes in natural wetlands resulting from the combination of
impoundments and increased stormwater inflows. Stormwater, depend-
ing on the land uses within the watershed over which it flows, can vary in
water quality and quantity. This section begins with a description of how
impoundments change the hydrology, water quality, soils, and wild-
life/habitat of natural wetlands. Then, by considering the typical constitu-
ents found in stormwater coupled with changes found when wetlands are
impounded, the resulting impacts on wetland!; that are impounded for
treatment of stormwater runoff are discussed.
Modifying natural wetlands with impoundments may result in radically
different hydrologic regimes that are not ecologically sound (Frederick-
son, 1982). ^Manipulating a wetland to enhance certain habitats or to
attract certajp species has been shown to degrade the wetland over a long
period of time. Managed wetlands often lack seasonal and long-term
water level fluctuations (Frederickson, 1982). The ability to vary water
levels allows the depth of water in the impounded wetland, the length of
the drawdown period, and the amount of exchange during flooding and
drawdown to be controlled (Thompkins, 1986). Studies have shown,
however, that water circulation, patterns within impounded wetlands
appear to be responsible for many of the differences between these
systems and natural wetlands. The degree to which wetlands export
carbon and nutrients is dependent in large part on the hydrologic charac-
teristics of a system. Changes in these characteristics as the result of
impoundment or hydrologic manipulation can change mis export Re-
duced circulation in impoundments can result in higher water tempera-
tures and increased evaporation rates during the summer, as well as
fluctuations in dissolved oxygen and salinity and other changes associated
with water quality. The manipulation of wetland hydrology can also
directly influence the availability of aquatic habitat and indirectly affect
invertebrates through the physiological responses of hydrophytes (Reid,
1982). Therefore, an understanding of the amount and timing of water
exchange is important to the success of these systems.
Although few studies directly relating stormwater inflow to water quality
changes in impounded wetlands were identified in the literature, some
comparisons between impounded and open wetlands can be considered.
Suspended particles are typically the pollutant of concern. Sedimentation
of suspended particles is one of the principal mechanisms of pollutant
removal in a wetland. The rate and degree of sedimentation are directly
related to the flow characteristics of a particular wetland (Brown, 1985).
The ability to manage many of the flow characteristics within an im-
pounded wetland can result in more efficient removal of suspended
sediments in the system. However, the loss of water storage capacity in
impounded wetlands as the result of increased sedimentation (Le., filling
in of the impounded basin due to settling solids) must be considered to
properly manage impounded wetlands used for stormwater control. High
influent concentrations of sediment from stormwater runoff can also
-------
the taponnded
Total Hiosphoms 0^88 0307
Dissolved Phosphorus 0312 0.152
Qrftophosphonis 0^29 p.137
T«alKjeIdahl Nitrogen 2.742 2390
N5nate and Nitrite 0356 0.176
Mtrogen .
Ammonia Nitrogen 0^30 1.061
Scspended Solids 45.4 303
0.763 0^42
0.173 0.121
0.162 0.071
2^10 2.163
0.455 0^27
0.217
43.8
4.52
0^62 0^45
0.135 0.098
0.102 0.103
2.127 2J540
Q-235 0.117
0.185 1369
Z87
0.587 0341
0.092 0.124
0.076 0.186
3.120 1.650
0.360 0.502
203.6 395
0^24
0.254 0.127
0^90 0.112
1.470 2.170
0^03 0.063
0.082 0.445
113.0 27.0
SOURCE: Baiten, 1986.
™^
luent ooncentralions of water quality
41
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February 1993
carbon. The nature of the carbon export was not determined. Differences
in the quality of organic matter being exported are important in determin-
ing the overall impacts of impounded versus open wetlands on adjacent
receiving waters (McKeller, 1986).
Salinity in impounded systems has been shown to fluctuate as the result
of several factors, including reduced circulation, increased evaporation,
and a lack of exchange. Diked wetlands in the San Francisco Bay exhibit
alternating periods of hypersaline and freshwater conditions in response
to winter rains and summer evaporation. Winter rains dilute and leach
salts in the upper soil .profile, and summer evaporation brings the salts to
the surface (ABAG, 1991). Conditions of varying salinity,. pH, and
oxygen in impounded wetlands also occur as a result of water control and
management techniques (Wenner, 1986)1 Decreased flushing rates in late
summer can contribute to deteriorating oxygen levels within a system. In
addition, lack of vertical mixing due to water depth can result in the
development of anoxic conditions in an impoundment.
Shallow impoundments have dissolved oxygen dynamics different from those
of unimpounded streams or lakes. In north central Minnesota shallow
impoundments have been shown to lose much of their oxygen during
ice-over (Veny, 1982). Resulting low redox potentials cause massive
migrations of nutrients out of the bottom sediments into the overlying
water. The enriched layer of water can encompass the entire depth of the
impoundment (Verry, 1982).
Dissolved oxygen levels are also affected by the surges in organic matter
that can occur in wetland impoundments. Strong development of thermal
stratification does not occur in shallow impoundments, and as a result the
decay of organic matter can occur throughout the water column When
organic matter is introduced into the system due to natural conditions or
management practices, large fluctuations in dissolved oxygen can occur
(Verry, 1982). Figure 8 shows oxygen fluctuations in a shallow water
impoundment in Minnesota over the period of a year. During the summer,
concentrations move between the upper and lower shaded areas rapidly
as organic matter is processed and wind mixing or photosynthesis occurs.
Dashed lines depict areas of limited data.
Water temperatures in shallow impoundments have also been shown to vary
compared to those of adjacent natural marshes and free-flowing streams in
north central Minnesota. WhOe minimnm temperatures in impoundments
were shown to be the same as those in natural systems, the maximum
temperatures were as much as 5 degrees Celsm, higher in me impounded
wetlands (Verry, 1982). The higher maximum temperatures were shown to
be associated with surface-water-fed impoundmerais that were stagnant, with
diminished depths and little or no water flow. Decreased water depths and
flow were associated with dry weather conditions or intentional management
o'rawdowns (Verry, 1982). Tneefiectofhigherwatertemperatureinshallow
impoundments on downstream water temperatunss was also examined by
Veny (1982). Temperatures were shown to drop from 24.5 "C to 203 °C
within 20 meters of a shallow impoundment outlet Oferry, 1982). Tempera-
tures remained the same farther downstream. The rapid decrease in water
temperature leaving the impoundment was attributed by Veny to streamside
shading and groundwater influx. The difference ill temperature downstream
42
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February 1993
MONTHS
Figure 8. Oxygen fluctuation!; in a shallow water impoundment in Minnesota (Veny, 1982)
from the outlet was close to the temperature difference between the
impoundment and the natural stream.
The increased capability to control flow characteristics and exchange in
impounded wetlands can result in a more efficient removal of suspended
sediment from the water column. Increased sedimentation and manage-
ment of water levels in impounded wetlands can affect the soils wifliin
the system. Lack of daily flushing by tides in impounded wetlands in
South Carolina resulted in a greater accumulation of organic material in
the soils as compared to adjacent natural marshes (May and Ztelinski,
1986). Surrace sediments within the tidal wetland impoundments ranged
from silry clays to clayey silts. Sediments that accumulate in impounded
wetlands may contain a higher percentage of fine-grained particles be-
cause of decreased energy levels and increased water retention time. The
higher percentage of finer-grained sediments affects the textural charac-
teristics of .the soils developing in the system.
Suspended organic and inorganic materials have a tendency to adsorb
pollutants, such as heavy metals, nutrients, hydrocarbons, bacteria, vi-
ruses, and refractory organics (Stockdale, 1991). These materials may
then be deposited with the sediments, affecting the overall characteristics
of the soils in the impoundment. Toxicants are generally found to be
associated with finer-grained particles that are less than 246 microns
(Oberts, 1977). Impoundments that cause sedimentation of finer-grained
particulates would result hi the incorporation of these toxicants into
SOIL CHANGES
43
-------
February 1993
developing soils. Bioaccumulation of contaminants by fish and wildlife
s .-. could occur as a result of the buildup of materials in the soil. Improper
management techniques in impoundments used for stonnwater runoff
could result in the reintroduction of these toxicants to the water column
during turbulent conditions associated with storms or high-flow events.
High percentages of silt-sized particles in combination with low sedimen-
tary flushing of smaller particle sizes can result in decreased oxygen levels
in impounded wetlands. Oxygen depletion can result and in brackish or
saltwater environments may be accompanied by the accumulation of
sulfides (Wenner, 1986). The accumulation of organic material can also
result from oxygen depletion. Management of water levels in impounded
wetlands can also cause leaching and oxidation of marsh soils. If soils
are not kept moist, sulfides can become oxidized to form sulfuric acid and
cat clays (Wenner, 1986). The development of acid sulfate soils or cat
clays can result in a soil pH of 3 5 or less. Soil samples taken in a brackish
marsh impoundment that had been dewatered oh South Island in South
Carolina had pH values ranging from 32 to 8.3, depending on whether
the soils were kept wet or allowed to dry (Wilkinson, 1970)..
BIOLOGIC/HABITAT IMPACTS
Changes in the types and diversity of vegetation in wetlands have been
shown to occur as the result of the impoundment of these systems.
Additional changes in vegetation could be expected as the result of
stonnwater discharges to impounded wetland systems. As mentioned
above, plant communities and individual species appear to be affected by
water depth, frequency and duration of flooding, and water quality.
Studies in impounded marshes along Florida's east coast showed that
excessive or prolonged flooding in wetland impoundments resulted in the
stressing or killing of existing high marsh vegetation in the systems
(Carlson and Carroll, 1985).
Another basic change associated with the impoundment of intertidal
marshes is the conversion from a wetland dominated by emergent vege-
tation to a system dominated largely by submerged macrophytes, benthic
algae, and phytoplankton (Kelly et al., 1986). Although total community
production in managed wetland impoundments in South Carolina was
shown to be similar to total production in adjacent open marshes, the
contributions to productivity of the various plant communities (marsh
grasses, benthic macrophytes and macroalgae, and phytoplankton) were
shown to differ considerably between the impounded and open marshes
(Marshal and McKeller, 1986).
Wilkinson (1970) conducted studies on a newly flooded brackish im-
poundment on South Island in South Carolina to determine vegetative
succession in the system. Water depths in the wetland impoundment
ranged from 12 to 24 inches. During a 3-year study period, the relative
abundance of some species changed drastically with the distribution of
plants into zones associated with water depth. Ruppia maritima, a sub-
merged aquatic grass, became the most successful plant after flooding.
Scirpus robustus was the most successful emergent plant Distichlis
spicata, a salt grass associated with higher portions of salt marshes,
decreased in abundance after flooding and eventually disappeared from
the impoundment; Spartina cynosuroides, a shallow water emergent, was
44
-------
February 1993
reduced in area of coverage to the very shallow margins of the impound-
ment (Wilkinson, 1970). , ,; .'.'•;. ;
In freshwater impoundments in South Carolina where water levels are
maintained, floating and submergent species have been shown to become
the dominant vegetation in succession. The dominant species vary ac-
cording to water depth, but Utriadaria, a submerged aquatic plant,
Lfmna, a floating aquatic plant, Nymphaea, a floating leaved aquatic
plant, and Ceratophyllum, a submerged aquatic plant, are usually the most
common species (Miglarese and Sandifer, 1982).
Changes in salinity associated with the impounding of salt marshes also
result in changes in vegetation patterns. Restriction of tidal inundation
resulting from the impoundment of a 20-bectare tidal marsh in Stonington,
Connecticut, resulted in a succession from a S/?artzna-dominated marsh
tooo&donaaatedbyPhragmitesaustralisaadTyphdeaigustifolia. Lower
soil water salinity associated with tidal restrictions in the study area
resulted in a change in vegetation from salt marsh Wan emergent fresh-
water wetland to a-brackish water wetland (Sinicropr et al., 1990).
Brackish conditions resulted from an attempt to control Phragmites by
raising salinities in the impoundment Similar or more rapid changes
could occur in the vegetation of salt marsh impoundments with the
-'• introduction of stormwater discharges to these systems.
Changes in the hydrologic character of impounded versus open wetland
systems can result in a depletion of fauna. The introduction of stormwater
runoff or water control objectives, resulting in hydrologic disturbances in
impounded wetlands, could result in the development of stressful habitat
conditions. Since a limited number of species can adapt to conditions of
changing salinity, pH, temperature, and dissolved oxygen, low species
richness could result The lack of interchange between impounded wet-
lands and adjacent waters could also result in a reduction of species
richness due to the inability of many fauna to access the impounded
wetland. Studies have shown that wetland impoundments can signifi-
cantly affect the diversity and richness of fish species in comparison to
adjacent natural wetlands (Devoe and Baughman, 1986).
Studiestodeterniinefishspeciesdiversitywereconductedonasaltmarsh
impoundment in Indian River County, Florida, in 1979. Prior to the study
the impoundment was managed for mosquito control. During the study
period the impoundment did not receive pumped estuarine water and the
water levels in the impoundment were allowed to fluctuate with weather
conditions. Initial surveys conducted on the impoundment indicated mat
at least 11 species offish were present in the impounded marsh (Gilmore
et al., 1981). Arid conditions during the study period resulted in hyper-
saline conditions and the dewatering of large portions of the impound-
ment. Salinities ranged from 2 to 125 ppt, water temperatures were from
14 to 34 6C, and oxygen levels varied from 1.2 to 14.4 ppm (Gilmore et
al., 1981). The number of species collected in the impoundment after
dewatering was reduced to four. Figure 9 shows the monthly distribution
of fish in relation to salinity, rainfall, and water levels in the impoundment
during the study period (Gilmore et al., 1981). Fluctuations in rainfall,
salinity, and water depth during the month of September were the result
of Hurricane David.
45
-------
February1993
SPECIES
A. mftchll!
M. ertionticus
C. voriegotus
F. confluentus
F. groncSJ
G. offinis
P. krtipinna
M. peninsulae
C. undedmalfc
G. boaci
G. tobustum
M. gutosus
No. o* species
55
5
40
30
20
10
0
' 60
•£ 50
|t> 40
- 3" (-,n
E
u
o
C
o
ec
20
Q.* 10
0
•1.
"2-5,
1-6-15,
•16-50
J-51-100.
N
•100+, rainfall
o
v>
Figure 9. Monthly distribution of fishes in Impoundment No. 12 and pond water levels during 1979 (data from Flor-
ida Medical Entomology Laboratory and Indian River County Mosquito Control District). Some water re-
mained in two ponds during April but was not measured because the permanent calibrated stake was completely
exposed. (GilmoreetaL, 1981)
-------
Februan-1993
Earlier studies conducted on the marsh by Harrington and Harrington in
1966 prior to and 30 months afteranitial impoundment showed a reduction
in the number of fish species from 16 to 5. Studies also indicated a change
in feeding habits to a reliance on plant materials by three of the remaining
species in the impoundment (Harrington and Harrington, 1982).
Shallow impoundments in north central Minnesota with reduced or stagnant
water Sows were determined not to be well suited for fish populations during
die summer and over winter because of rapid and wide fluctuations in
dissolved oxygen levels Cverry, 1982). Maximum water temperatures in the
impoundments were also shown to be above the upper level for normal trout
growth. Maximum temperatures in wetland impoundments with Sowing
water didnot exceed maximum growth temperatures for several other species
of fish in the region including walleye, yellow perch, large-mouth bass,
northern pike, carp, shad, crappie, white perch, spotted bass, white bass, and
catfish (Vary, 1982).
Potentially rapid or large changes in water temperature associated with
stonnwater inflow could cause thermal stress to fish in shallow impound-
ments. Impacts to aquatic insects resulting from temperature fluctuations
in a system are possible because of their general inability to compensate
for or acclimate to the temperature changes. Fluctuations in water tem-
perature regimes of from 2 to 3°C could potentially eliminate some
sensitive species (Galli and Dubose, 1990).
Changes in the pH of water in wetland impoundments associated with
management practices or the introduction of stonnwater can also affect
the biota in impounded systems. Most organisms are adapted to function
within particular pH ranges, and abrupt or substantial variations in pH can
have adverse effects on aquatic life usually in the form, of reduced
productivity and increased mortality (Newton, 1989). Most urban stonn-
water is slightly acidic; The variable nature of stonnwater inflow could
result in abrupt changes in thepH of an impoundment Lowered soil water
pH associated with drawdown in impounded brackish or.saltwater
marshes can affect densities of invertebrates such as molluscs and crusta-
ceans. Species that depend on alkaline conditions for shell development
may be affected if low pH occurs at the sediment or soil-water interface
(Wenner, 1986).
The use of impounded wetlands by water birds has been shown to be high
in several systems. Studies conducted on South Carolina impoundments
indicated that high water bird use was directly related to season, manage-
ment practices, impoundment size, and availability of resources (Epstein
and Joyner, 1986).
Newly impounded brackish wetlands on South Island, South Carolina,
were studied by Wilkinson (1970) over a 3-year period to determine plant
succession and waterfowl use. Five impoundments with different hydro-
logic controls—fully flooded, slowly rising, tidal, saturated soil, and
dry—were observed Use of the impoundments by waterfowl was rated
based on the estimated number of observed waterfowl. Observed num-
bers of 1 to 10 were rated as poor, 10 to 30 as fair, 30 to 60 as good, and
above 60 as excellent (Wilkinson, 1970); Waterfowl observations were
made twice a week during the fall and winter. The fully flooded impound-
ment was the most used by waterfowl. Use of the wetland impoundment
47
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February 1993
REGIONAL DIFFERENCES
SUMMARY
with rising water levels was rated good, and use of the tidal impoundment
was good to poor. Use of the impoundment with saturated soil was rated
as fair to poor, and use of the dry impoundment was rated as poor
(Wilkinson, 1970).
In Minnesota, Wisconsin, and Michigan, surveys of impoundments indi-
cated that after initial flooding the diversity and density of birds increased
due to increased edge, productivity, nest cavities, and perch sites (Rakstad
and Probst, 1982). The increase in amount of edge and degree of inter-
spersion of habitat types also resulted in use by greater numbers and kinds
of wildlife including muskrats, racoons, red fox, river otter, mink, and
water shrew (Rakstad and Probst, 1982). After several years, however,
the density and diversity of wildlife has been shown to have decreased in
many impoundments. This decrease has been shown to be due in part to
vegetative succession in the impoundments.
Some management techniques applied to wetland impoundments have
been shown to be successful in maintaining or enhancing use by wildlife
in several cases. The water quality salinity and hydrology requirements
of different fish and wildlife species vary, and therefore management
techniques applied to wetland impoundments to increase or enhance
habitat for one species may have adverse impacts on others (Hynson et at,
1985).
Regional differences that affect impounded wetland systems are similar
to those mat affect natural wetlands. The methods, timing, and period of
drawdowns depend largely on the geology, hydrology, soils, and climate
of an impoundment site. For example, soils in arid regions with low
rainfall tend to accumulate salts in their upper profiles. As a result,
drawdowns or evaporation in arid-region impoundments can result in the
development of hypersaline conditions. Such conditions would be less
likely to occur in humid regions. In addition, northern regions are more
likely to be affected by the ice-over of impoundments in winter than are
southern regions. These regional and site-specific characteristics, in
addition to others, all exert controls on the inflow, outflow, and quality of
water in an impoundment.
Shallow-water impoundments have been shown to be both potentially
beneficial and potentially detrimental to the functions of the impounded
wetland systems. The increased ability to manipulate the hydrology in
impoundments (Le., water levels and flow) allows management tech-
niques to be designed to enhance or control specific aspects of the systems.
For example, water levels can be controlled to enhance the growth of
certain vegetative species and in turn attract certain waterfowl or wildlife.
Flow within the impoundments can be controlled to promote increased
sedimentation of pollutants from inflowing stormwater. However, alter-
ing the hydrology in a natural system by impoundment or through the
management of impounded systems can change me functional processes
of the system. As mentioned, techniques applied to impoundments to
enhance or control one aspect within the system can result in adverse
impacts to others. Changes in the characteristics of the hydrology, water
quality, soil, vegetation, and fauna in the impoundment can result
-------
February 1993
As the result of urbanization, in many areas low- to .moderate-intensity
storms can produce large volumes of runoff. Because of the variable
nature of stormwater runoff flow, the ability of impounded wetlands to
remove nutrients, suspended solids, and heavy metals may vary by season,
from storm to storm, or within the same storm (ABAC, 1991). Impound-
ments may act as a sink for the constituents of stormwater under certain
conditions or as a source under others. Variations in the characteristics of
stormwater inflow will also have varying impacts on the components of
impoundments. Changes in the characteristics of the soil, water quality,
and hydrology in impoundments will occur and, in turn, will affect the
biota in the impounded wetland. The potential bioaccumulation of pol-
lutants for fish and wildlife as the result of stonnwater inflows remains
unclear (Meiorin, 1986). The effects of impounding wetlands and ma-
nipulating impoundment conditions, along with the potential impacts of
stonnwater discharges on the characteristics of the soil, vegetation, water
quality, and fauna in the systems, need to be further studied.
~ ' ••*•"- ' . ' "
Because the use of natural wetlands for stormwater management purposes
is relatively new, considerable uncertainty exists concerning the impact
of the quantity and quality of stormwater runoff on natural wetlands.
Several issues related to mis topic are presented below.
• Better understanding of the long-term impacts of water level
fluctuations on wetlands and wetland functions, particularly habi-
tat Junctions, is needed.
• Thresholdlevelsformevolumeandqualityofstormwaterentering
and being stored in a wetland before functions are impacted need
to be identified.
• Better understanding of the long-term impacts of water and sedi-
ment quality changes on wetland biola is needed.
• The potential benefits to natural wetlands (Le., enhancement) due
to stormwaterdischarges need to be better understood and consid-
ered in stormwater ma
Increased recognition and understanding of regional differences
and concerns associated with natural wetlands, including hydro-
logic differences and wetland types, are needed.
Increased understanding of the public health risks associated with
the storage of urban stonnwater in natural wetlands is needed.
There is a lack of understanding and methods to measure and
assess how changes in wetland processes due to urban stonnwater
discharges affect the support of biological communities.
.
Several unresolved issues were raised at the January 1992 workshop in
Clearwater, Florida. These include:
UNRESOLVED ISSUES
49
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February 1993
UNRESOLVED
IMPOUNDMENT ISSUES
More research is needed on the effects of impounding various
wetland types for stormwater treatment
There is a need to study the functioning of wetlands at the water-
shed and landscape levels and to plan and manage them with the
broader environment in mind. The trade-offs between the benefits
and impacts to various habitats when wetlands receive sKonriwater
need to be addressed.
As mentioned, the potential use of wetland impoundments for the en-
hancement of treatment of stormwater runoff has been considered as an
option for stormwater management Several information needs related to
this practice (in addition to those discussed in the previous section) are
listed below:
• Better understanding of the amount and timing of water exchange
in impoundments in order to improve water circulation patterns in
the system is needed.
• Increased understanding of techniques to improve the exchange
and circulation between impounded wetlands and open systems is
needed. .
• Better understanding of the technical aspects of the long-term
management of impounded wetlands for optimal stormwater con-
trol needs to be developed.
• Better imd^rstand^g of me impacts of the constituents of storm-
water on the water quality, soils, vegetation, and fauna of impound-
ments is needed.
• Better understanding of die functional differences between im-
poundedandopen wetlandsisneededsotbatcomparisonsbetween
the systems can be made.
• Better understanding of how different management techniques
affect the long-term stability of impoundments and the faunausing
them is needed
• More research on impounded wetlands in general and their effects
on adjacent open wetlands is needed. -
50
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February 1993
6. STORMWATER
MANAGEMENT PRACTICES AND
NATURAL WETLANDS
Urban stonnwater runoff has been recognized as a major contributor to
nonpoint' source pollution in surface waters and groundwaters. The
control of urban runoff has been the subject of Federal and State programs,
including the Nationwide Urban Runoff Program (KURP) of the mid-
1980s. Section 319 of the 1987 amendments to the Clean Water Act
(CWA) requires States to identify waters that, without further action to
control nonpoint sources, cannot be expected to attain the water quality
standards or goals of the Act States weie also to submit programs for
management of nonpoint source pollution. This is not a regulatory
program, however, and it does not ensure that sources of nonpoint pollu-
tion are controlled (Ehom, 1990). EPA has issued guidance on various
measures for controlling nonpoint source pollution, including stonnwater
runoff. This guidance, and other Federal and State legislation, has led to
me development of stonnwater management programs and suggestions
for managing urban stonnwater runoff, on both a watershed level and a
site-specific level. This chapter discusses urban stonnwater management
programs and implementation tools for controlling adverse impacts from
stormwater runoff, including the relationship of such programs and tools
to natural wetlands. '.
While the Federal government provides guidance for the control of
nonpoint source pollution, the only Federal regulations for stormwater
runoff are promulgated through the NPDHS permitting process (section
402 of the Clean Water Act). Section 402 authorizes EPA to issue permits
to discharge pollutants into waters of the United States if States do not
have an approved NPDES permit program in place. The majority of the
States are NPDES delegated; therefore, most stonnwater controls are
implemented at the State and local government levels. In addition, as
part of the Coastal Zone Act Reauthorization Amendments of 1990,
Congress created anew section 6217, which requires States with approved
coastal zone management programs to develop and implement coastal
nonpoint pollution control programs. The National Oceanic and Atmos-
pheric Administration (NOAA) and EPA recently developed guidance to
implement these requirements. They are .also responsible for reviewing
and approving State programs and providing technical assistance to the
FEDERAL AND STATE
STORMWATER
MANAGEMENT
PROGRAMS
51
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February 1993
CONTROLOF ADVERSE
IMPACTS
States. Under section 303, EPA has issued guidance for States to develop
water quality standards. This guidance will have an impact on regulated
stormwater discharges to wetlands. The State standards must address
wetlands as waters of the State, set appropriate narrative and numeric
criteria, and establish an antidegradation policy. States can establish
narrative or numeric hydrologic and biologic criteria that address storm-
water impacts.
Construction of certain stormwater management systems, including the
impoundment of natural wetlands, may involve the discharge of dredged
or fill material into waters of the United States, which Include wetlands.
These discharges are regulated under section 404 of the Clean Water Act,
which is administered by the Army Corps of Engineers and EPA. Ques-
tions regarding the applicability of section 404 to stormwater activities
need to be addressed prior to initiation of construction.
The extent and requirements of State and local stormwater management
programs vary. Many, but not all, States have stormwater management
programs, and many local .governments are required by the State to
develop stormwater management programs consistent with or stronger
than the State guidelines. In some States, local controls for stormwater
runoff are the only controls in place, and they have been developed
voluntarily.
For those states with stormwater management programs, some allow the
use of natural wetlands as pan of a permitted stormwater treatment
system. Many states do not have enabling legislation to allow this, but
they realize that runoff impacts wetlands by default and have therefore
developed general guidelines for wetlands. Some state guidelines aim to
prevent direct discharge of stormwater to natural wetlands without appro-
priate pretreatment. The broad range of requirements and general criteria
that some states have developed for using natural wetlands in stormwater
management is presented in Table 14.
Few states have formal administrative rules or regulations that address
hydrological and chemical changes that may occur to wetlands as a result
of stormwater discharges. At this time, most states that have stormwater
management programs address the use of wetlands for treating urban
runoff on a case-by-case basis.
As stated earlier, urban runoff is considered a major source of nonpoint
pollution. Because of this, numerous structural and nonstnictural meas-
ures for controlling nonpoint source pollution have been developed and
implemented in both urban and urbanizing areas. Most structural controls
are designed to mitigate adverse impacts from stormwater runoff. Exam-
ples include exfiltration trenches, infiltration structures, retention and
detention systems, wet ponds, sediment traps, porous pavement, and oil
and grease separators. While these controls address potential water
quality problems, there are also measures for controlling impacts from
increased water quantity. Baffles, dissipaters, and control gates are often
used to control the quantity of water entering a stormwater management
system. More fundamentally, retention/detention vaults, tanks, and ponds
52
-------
February 1993
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February 1993
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55
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February 1993
ural Welltnds
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56
-------
February 1993
are provided to attempt to limit peak discharges at levels no higher than
"redevelopment levels. ,.. ,
I , ' • " • ' '
In addition to these measures, there are notistmctural alternatives to prevent
nonpoint source pollution. Some of these may be implemented by them-
selves; others incorporate structural alternatives. Nonstructural control
measures are those which do not require construction or maintenance, and
they include such practices as developing regional comprehensive stonnwa-
ter management prograrns, planning future development or redevelop-
ment in such a way as to minimize stormwater runoff, limiting the amount
of impervious surface in new and retrofitted development, requiring
setbacks from surface water and wetlands, to protect their environmental
integrity, siting infrastructure so as not to encourage development in
environmentally sensitive areas that are critical to maintaining water
quality, requiring the use of best management practices, inspecting storm-
water management systems and erosion control structures to ensure that
they are functioning properly, and providing education programs to at-
tempt to reduce individual contributions to stormwater problems.
In most instances, stormwater management systems involve more than
one practice. To effectively manage urban runoff, a series of measures
may be used. Many States that do allow the use of wetlands require that
other control measures be used as weD, usually prior to discharge to the
wetland. -
As discussed in Chapter 5, alteration of the wetland environment may
occur if wetlands are used for the treatment of urban stormwater runoff.
However, adverse impacts can be minimized, both from a site-specific
and watershed-wide perspective.
The use of wetlands for treating urban stormwater runoff is not an isolated
activity; it is usually part of a larger stonnwater management system
addressing both water quality and quantity. Some States have stormwater
management prograins; however, the use of natural wetlands as part of
stormwater management systems may not fce specifically regulated by the
State. If the use of natural wetlands as pant of stonnwater management
systems involves the discharge of dredged or fill material to waters of the
United States, it would be regulated under section 404 of the Clean Water
Act Water quality, water quantity, and physical modification can impact
a wetland system. These parameters do not necessarily follow political
"boundaries; therefore, overall watershed planning can help to anticipate
and prevent adverse impacts from urban stonnwater runoff.
Unresolved issues related to stormwater management practices include
the following:
• Many States do not nave statewide stonnwater management pro-
grams; consequently, stonnwater runoff to natural wetlands occurs
without consideration of the impacts to the system. This occur-
. rence may be contradictory to wetland preservation efforts in some
States. '
CONCLUSIONS
UNRESOLVED ISSUES
57
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February 1993
• Most Federal and much State guidance incorporating stonnwater
. controls addresses water quality and water quantity separately.
* Watershed management practices need to be implemented to mini-
mize the impacts of stonnwater discharges on natural wetlands.
• The circumstances under which stonnwater discharges to natural
wetlands should be allowed need to be identified.
• Many local jurisdictions require regional stonnwater ponds for
new developments. Wetlands are often the only remaining unde-
veloped land and are the lowest points in the landscape to receive
stonnwater runoff. Are there alternatives that minimize impacts
and meet stonnwater management objectives?
The need for integration of various local, State, and Federal authorities
with jurisdiction over wetlands and stonnwater discharges was raised at
the January 1992 workshop.
58
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February 1993
Although wetlands have long been recognized for their flood control and
water quality improvement functions, there is increasing concern that
unrestricted use of natural wetlands as receptacles for point and nonpoint
sources of pollution, such as urban stormwater, will have adverse effects
on wetlands and wetland biota. These impacts will vary from site to site
and region to region. However, if natural wetlands are the ultimate
receiver of stormwater runoff, either inadvertently or by design, the
potential impacts of such discharges need to be better understood and
management practices need to be designed to minimize these impacts. •
This issue paper has identified several unresolved issues related to the use
of natural wetlands for urban stormwater control. These and other issues
were discussed at the Wetlands and Stormwater Workshop held in January
1992 in Clearwater, Florida. The purpose of these discussions was to
share ideas and opinions and make recommendations on how to best
manage the discharge of urban stormwater to natural wetlands. Informa-
tion from the workshop has been incorporated into this issue paper. The
unresolved issues identified in this paper are summarized below.
• In-depth knowledge of the totality of wetland functional support,
taking into consideration such factors as nutrient flows, hydrology,
trophic dynamics, community structure, and population distribu-
tion and abundance, is not available for most wetland types.
• A greater understanding of habitat processes and functions and
how changes in these functions affect the support of living organ-
isms is needed. . ,
• New and improved methods are needed to measure and assess the
habitat functions of wetlands. '
Better understanding of the long-term impacts of water level
fluctuations on wetlands and wetland functions, particularly habi-
tat functions, is needed.
7. SUMMARY
WETLAND FUNCTIONS
IMPACTS OF
STORMWATER
DISCHARGES
59
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February 1993
MANAGEMENT OF
STORMWATER
DISCHARGES
• Threshold levels for the volume and quality of stormwater entering
and being stored in a wetland before functions are impacted need
to be identified.
• Better understanding of the long-term [impacts of water and sedi-
ment quality changes on wetland biota is needed.
• The potential benefits to natural wetlands (i.e., enhancement) due
to stormwater discharges need to be better understood and consid-
ered in stormwater management
• Increased recognition and understanding of regional differences
and concerns associated with natural wetlands, including hydro-
logic differences and wetland types, are needed:
• Increased understanding of the public health risks associated wish
the storage of urban stormwater in natural wetlands is needed.
• There is a lack of understanding and methods to measure aid
assess how changes in wetland processes due to urban stormwater
discharges affect the support of biological communities.
Many States do not have statewide stormwater management pro-
grams; consequently, the .discharge of urban runoff to natural
wetlands occurs without consideration of the impacts to the sys-
tem. This occiinence may be contradictory to wetland preserva-
tion efforts in some States.
Most Federal and much State guidance incorporating stormwater
controls addresses water quality, not water quantity. The potential
changes in hydrology and their impact must be addressed as well.
Watershed management practices need to be implemented to mini-
mize the impacts of stormwater discharges on natural wetlands.
The circumstances under which stormwater discharges to natural
wetlands should be allowed need to be identified.
60
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February 1993
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68
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GLOSSARY1
Absorption: A process in which one material takes up and retains another: to take a substance, as water or nutrients.
into the body through the skin or mucous membranes or, in plants, through root'hairs.
Adsorption: The ability to attract and concentrate upon surfaces molecules of gases, liquids, and dissolved solids; the
adhesion of molecules to the surfaces or liquids with which they are in contact. Many pollutants adsorb to sediment
• particles and are transported by these particles.
Aggressive plant species: Opportunistic species of inferior biological value that tend to Outeompete more desirable
forms and become dominant Term applied to native species; invasive is term applied to non-native species with
similar characteristics. »
Alkalinity: A measure primarily of the carbonate or carbon dioxide-related compounds in water. The lower the
alkalinity, the less capacity the water has to absorb acids without becoming more acidic. Therefore, alkalinity is a
measurement of the buffering capacity of water.
Ammonia (NHs): Amtrogen-contairing compound that may indicate recently decomposed plant or animal material.
Antecedent soU condition: The sofl moisture condition at the startof a storm event The soil moisture condition prior
to a storm event influences the amount of runoff. ,
Beneficial uses: Uses of a waterbody that provide benefits to human users, such as swimming, fishing, boating fish
spawning and rearing, water supply, and wildlife habitat -
Best Management Practice (BMP): A method, activity, maintenance procedure, or other management practice for
reducing me amount of pollution entering a waterbody. BMPs generally tall into two categories: source control
BMPsandstormwatertreatmentBMPs. The tenn originated fix>m the ndes and regulations developed pursuant to
section 208 of the Federal Clean Water Act(40 CFR 130).
Bioaccumulation: The process by which a contaminant accumulates in the tissues of ah individual organism. For
example, certain chemicals jn food eaten by a fish tend to accumulate hi its liver and other tissues. (See
biomagnification.) • • .
Biochemical oxygen demand (BOD): An index of the quantity of oxygen-demanding substances (organic matter
subject to bacterial decay) in a sample as measured by a specific test Although not a specific compound. BOD i<
defined as a conventional pollutant under the Federal dean Water Act During bacterial decay and digestion
processes, oxygen is used, reducing dissolved oxygen levels in the water column. Sources of BOD include sewace
treatmentandseptic tank effluents, oil andgrease,pesticides, organics of natural origin, andany otherdecomposabie
material. Sewage effluents fromsecondary treatment have a BOD level of 30 mg/L. Urban runoffcan have a BOD
level equal to or greater than that of sewage effluents. (See chemical oxygen demand (COD).)
Bioengraeering: Restoration or reinforcement of slopes and stream banks with living plant material.
Adapted from Stockdale, 1991.
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Febniarv 1993
Biofiltration: The processes by which stormwater or wastewater receives treatment through interaction with
vegetation and the soil surface. These processes include (1) sheet flow over a broad, vegetated surface area
(filter strip): (2) flow at some depth through a vegetated channel, or'swale; and (3) use of small, created
wetlands, developed specifically for local stormwater management purposes.
Biomagnification: The process by which concentrations of contaminants increase (magnify) as they pass up the food
chain so that each animal in the chain has higher tissue concentrations than did its food. For example, concentrations
of certain contaminants can increase as they are passed from herring to salmon to seals.
Bioturbation: The activities of burrowing macroinvertebrates (such as oligocheate worms) that result in the re-expo-
sure of buried sediments (and associated contaminants) to the water-sediment interface. Bioturbation can be an
important factor in the release of phosphorus from lake sediments.
Bog: A shrub peatland dominated by ericaceous shrubs (e.g.. labrador tea. bog laurel, cranberries); sedges (Carex spp.):
and mosses (e.g.. Sphagnum spp.) and usually having a saturated water regime or a forested peatiand dominated
by evergreen trees (usually spruces, hemlocks, or firs) and/or larch (Larix laricina).
Channel flow: Observable movement of surface water (due to gradient currents) in a confined, concentrated zone.
Includes intermittent channels.
Chemical oxygen demand (COD): A measure of the amount of oxygen required to oxidize (with a strong chemical
oxidant) organic and oxidizable inorganic compounds in water. Both BOO and COD are two different tests that
provide relative measures of demand on oxygen resources.
Chlorosis: Discoloration of vegetation due to environmental stress, such as from nutrients, water quality, or lack of
water.
Chronic effects: Adverse effects likely to occur over a prolonged period of exposure to a pollutant, such as altered
growth, reduced reproduction, decreased survival success, or mortality.
Constructed wetland: A wetland intentionally created from a non-wetland site for the sole purpose of wastewater or
stormwater treatment These wetlands are not normally considered waters of the United States or waters of the
State. (See created wetland.) .
Contaminant: A substance that is not naturally present in the environment or is present in amounts that can, in sufficient
concentrations, adversely affect the environment. A contaminant in such concentrations becomes a pollutant. (See
pollutant)
Conventional contaminant: As specified underthe Clean Water Act, conventional contaminants are suspended solids.
coliform bacteria, biochemical oxygen demand, pH. and oil and grease. Today a large number of toxic contaminants
are of concern in addition to the conventional contaminants.
Created wetland: A wetland intentionally created from a non-wetland site to produce or replace natural habitat (e.g..
a compensatory mitigation project). These wetlands are normally considered waters of the United States or waters
of the State. (See also restoration, enhancement, constructed wetland.)
Cumulative impacts: The combined environmental impacts that accrue from a series of similar or related individual
actions, contaminants, or projects. Although each action may seem to have an acceptable impact, the combined
effect can be severe.
Degraded (disturbed) wetland (community): A wetland (community) in which the vegetation, soils, and/or hydroloux
have been adversely altered, resulting in lost or reduced functions and values. Generally, implies topogruphu
isolation; hydrologic alterations such as hydroperiod alteration (increased or decreased quantity of water), dikm-j
channelization, and/or outlet modification; soil alterations such as the presence of fill or soil removal and«r
compaction; accumulation of toxicants in the biotic or abiotic component of the wetland; and/or plant specie*
richness with dominance by invasive weedy species. .
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Denitrification: The. biological transformation (reduction) of nitrate-nitrogen into nitrogen gas, which then enters the
atmosphere; a mechanism whereby nitrogen is removed from wetlands. (See nitrification.)
Detention: The temporary holding of stormwater from a site, with release at a slower rate than it is collected by a
drainage facility system.
Dissolved oxygen (DO): A measure of the amount of oxygen available for biochemical activity in a given amount of
water. Adequate levels of DO are needed to support aquatic life. "~ "
Emergent vegetation: Plants dominated by erect, rooted, herbaceous angiosperms that may be temporarily or
permanently flooded at the base but do not tolerate inundations of die entire plant or, if tolerant, donot flower when
submerged (e.g., bulrushes, cord grasses). ,
Enhancement: Actions performed to improve the condition of an existing degraded wetland so that the functions it
provides are of a higher quality. (See created wetland, restoration.)
Erosion: The wearing away of land surface by wind or water. Erosion occurs naturally from weather or runoff but can
be intensified by land-clearing practices. - •* ' •'.''..-
Estuarine: Pertaining to deepwater tidal habitats and adjacent tidal wetlands that are usually semi-enclosed by land
but have open, partially obstructed, or sporadic access to the ocean and in which ocean water is at least occasionally
diluted by freshwater runoff from UK: land. r
Fen: Apeat-fonningwedandthatreceives nutrients from sources other than precipitation, usually through ground water
movements. Its peat and water are acid neutral.
IHtration: The process of filtering.
• • f , - '
First flush: Phenomenon observed after a prolonged dry spell in which the concentration of pollutants in runoff is
higher in the earlier stages of a storm event
Functions and vahies: Wetlands are important because they provide many intrinsic ecological functions (water quality
maintenance, fish and wildlife habitat, etc.) and socioeconomic values (flood and erosion control, groundwater
recharge and water supply, recreation, education, research, food production, etc.). Functions generally refer to the
ecological (physical, chemical, and biological) processes or attributes of a wetland without regard for their
importance tosociety. Valuesprefer to wetland processes or attributes mat are valuable or beneficial to society.
-' , ' .
Groundwater: That portion of the water below the ground surface that is under greater pressure than atmospheric
pressure; that part ofJoe subsurface water that isin the zone of saturation. .
Heavy metals: Metallic elements, such as mercury, lead, nickel, zinc, and cadmium, that are of environmental concern
because they do not degrade over time. Althoughmany are necessary nutrients, they are sometimes magnified in
me food chain and in high concentrations can be toxic to life.
Hectare (ha): 2.47 acres, or 10,000 square meters.
Herbaceous: Plant material characterized by the absence of wood.
Hydric soils: A soil that is saturated, flooded, of ponded long enough during the growing season to develop anaerobic
conditions hi the upper part
Hydrology: The properties, distribution, and circulation of water. Wetland hydrology is the total of all wetness
characteristics in areas that are inundaited for a sufficient duration to support hydrophytic vegetation.
Hydroperiod: The seasonal occurrence of flooding and/or soil saturation; the depth, frequency, duration, and seasonal
pattern of inundation/flooding in a riparian zone or wetland.
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February 1993
Hydrophytic vegetation: Plant life growing in water or on a substrate that is at least periodically deficient in oxygen
as a result of excessive water content
Impervious surface: A hard surface area that either prevents or retards the entry of water into the soil mantle as under
natural conditions prior to development, and/or a hard surface area that causes water to run off the surface in greater
quantities or at an increased rate of flow from the flow present under natural conditions prior to development.
Indirect or secondary impacts: Impacts removed from the direct area and time at which the disturbance or
development occurs. ,
Infiltration: The downward movement (seepage) of water from the ground surface into the subsoil.
Invasive weedy plant species: Opportunistic species of inferior biological value that tend to outcompete more desirable
forms and become dominant Term applied to non-native species (compare to aggressive plant species).
Lacustrine wetlands: Wetlands and deepwater habitats with all of the following characteristics: (1) situated in a
topographic depression ordammedriver channel: (2) lacking trees, shrubs, persistent emergents, emergent mosses,
or lichens with greater than 30 percent areal coverage; and (3) total area exceeds 8 ha (20 acres). Similar wetland
and deepwater habitats totaling less than 8 ha are also .included in the lacustrine system if an active wave-formed
or bedrock shoreline feature makes up all or pan of the boundary, or if the depth in the deepest part of the basin
. exceeds 2 meters (6.6 feet) at low water.
Loading, loading rate: The total amount of material (such as pollutants ) entering a system from all sources. Measured
as a rate in weight per unit time. ' .
Marsh: A common term applied to describe treeless wetlands characterized by shallow water and abundant emergent,
floating, and submergent wetland flora. Typically found in shallow basins, on lake margins, along flow gradient
rivers, and in low-energy tidal areas. Waters may be fresh, brackish, or saline.
Metals: Elements found in rocks and minerals that are naturally released to the environment by erosion, as well as
generated by human activities. Certain metals, such as mercury, nickel, zinc, and cadmium, are of environmental
concern because they are released to the environment in excessive amounts by human activity. They are generally
toxic to life at certain concentrations. Since metals are elements, they do not break down in the environment over
time and can be incorporated into plant and animal tissue. .
Mitigation: Term that encompasses a broad array of activities when applied to wetlands management Mitigation
describes the efforts to lessen, or compensate for, the impacts of a development project The process of mitigation
follows a preferred sequence of options, as defined by the National Environmental Policy Act (NEPA) of 1969:
a.
Avoiding the impact altogether by not taking a certain action or parts of an action;
b. Minimizing impacts by limiting the degree or magnitude of the action and it implementation;
c. Rectifying the impact by repairing, rehabilitating, or restoring the affected environment;
d. Reducing or eliminating the impact over time by preservation and maintenance operations during the life
of the activity; and
e. Compensating for the impact by replacing or providing substitute resources or environments.
The principle of mitigation is implemented in such a way as to prevent any net losses of wetland functions and
values. . ,
Monitor: To systematically and repeatedly measure something in order to track changes.
Monotypic: Composed of only one species.
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February 1993
!- °^d nitr°gen SpeCieS' ^ method Used to measure *e concentration of nitrate
and mtnte yields a good estimate of the concentration of nitrate, the form of nitrogen preferred by aquatic plants
because nitnte is usually present only in small quantities in water.
Nitrification: The process of oxidizing nitrogen compounds to nitrites and nitrates, usually by bacterial action. -
Ni^:«ln±>ntPl^T^eUptoVTOUSf0nnS- ^^g^^ie in wetlands can include several nitrogen
sinks with nitrogen being lost as a gas, adsorbed to soil particles, and incorporated into organic material.
Nonpoint source (NFS) poDution: Typically defined as pollution that is not discharged through pines but rather
°f "^ °VCr a ta«B — ' N°nP°int ««« can bSwSffi-SL^SS
r ^ ThK 1S dlstift8uished fo>mpoint source pollution. Common sources of nonpo in
flw^
Nonsfructural controls: Techniques used to manage stormwater ninoffttot do not tequiie physical alteration of the
PAHs: See polynudear (polycyclic) aromatic hydrocarbons.
li^nc H ii " ^^f^*^^0™"*^^8^^
'
,
Vehide faels' ^ricating oils and greases, tars,
erae "^ ^^ partial'y burned faels S^oVveU
Sr
B, which is conducted by measuring the concentration ofhydr«>i:«.i
r^r^T*- PnKlmeasureao°ascalefiom 1 to 14, with 1 indicating the most acidic. 7 indicatingneutral
and 14 the most basic or alkaune. The pH of water influences many of thelypes of cheim<^ reactiZ tS^ Hi
. occur in it
Phenol: A caustic poison composed of acidic compounds that are generally derived from aromatic hydrocarbons
Phosphorus: A nonmetailic element that occurs widely and is essential to the growth of aquatic organisms as ucl
all forms of life. In aquatic environments, phosphorus is often the nutrient that limits the growth that a txxh
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February 1993
water can support. Additions of phosphorus to wetlands can cause increased vegetative growth and modifications
to community composition. Phosphorus can be reduced in wetland systems by plant uptake and by adsorption to
soil and organic material. ''
Point source: A source of pollutants from a single point of conveyance such as a pipe. For example, the discharge
pipe from a sewage treatment plant or a factory is a point source. See nonpoint source for comparison.
Pollutant: A contaminant in a concentration or amount that adversely alters the physical, chemical, or biological
properties of the environment. The term includes pathogens, toxic metals, carcinogens, oxygen-demanding
materials, and all other harmful substances. With reference to nonpoint sources, the term is sometimes used to
apply to contaminants released in low concentrations from many activities that collectively degrade water quality.
As defined in the Federal Clean Water Act, pollutant means dredged spoil; solid waste; incinerator residue; sewage;
garbage; sewage sludge; munitions; chemical wastes; biological materials; radioactive materials; heat; wrecked or
discarded equipment; rock; sand; cellar dirt; and industrial, municipal, and agricultural waste discharged into water.
Porynuckar (polycyclk) aromatic hydrocarbons (PAHs or PNAs): A class of complex organic compounds, having
more than one benzene ring, some of which are persistent and cancer-causing. These compounds are formed from
the combustion of organic material and are ubiquitous in the environment. PAHs are commonly formed by the
combustion of gasoline and by forest fires. They often reach the environment through atmospheric fallout and
highway runoff.
Prerreatment: The treatment of wastewater to remove contaminants prior to discharge into a municipal sewage system.
or the treatment of stonnwater (such as in a grassy swale or sediment trap) prior to discharge downstream.
Primary Treatment: A basic wastewater treatment method that uses settling, skimming, and (usually) chlorination to
remove solids, floating materials, and pathogens from wastewater. Primary treatment typically removes about 35
percent of BOD and less than half of the metals and toxic organic substances.
Priority pollutants: Substances listed by EPA under the Federal Clean Water Act as toxic and having priority for
regulatory controls. The list includes metals (13), inorganic compounds (2), and a broad range of both natural and
artificial organic compounds (111).
Receiving bodies of water: Creeks, streams, rivers, lakes, and other bodies of water into which surface waters (and
treated or untreated wastes) are directed, either naturally or in man-made ditches or open systems.
Recharge: The flow to groundwater from the infiltration of surface and stonnwater runoff.
Redox potential: A rneasure of trie mtensity of oxidation or reduction of a chemical or biological system. Theredox
potential of hydric soils indicates me state of oxidation (and hence the availability) of several nutrients. For
example, phosphorus is more soluble under anaerobic conditions.
Refractory organks: A term recently developed to identify a broad lumping of man-made organic chemicals that are
refractory; that is, they resist chemical or bacterial decomposition. Included in this class are many pesticides.
herbicides, household and industrial cleaners and solvents, photofinisning chemicals, and dry-cleaning fluids.
Regional detention facility: A stonnwater quantity control structure designed to correct the existing excess surface
water runoff problems of a basin.
Restoration: Actions performed to reestablish wetland functional characteristics and processes that have been lost bv
alterations, activities, or catastrophic events in an area mat no longer meets the definition of a wetland. (See
enhancement, created wetland.)
Retention: The collection and holding of surface and stonnwater runoff with no surface outflow.
Retention/Detention (R/D) facility: A type of drainage facility designed (1) to hold runoff for a considerable length
of time and then release it by evaporation, plant transpiration, and/or infiltration into the ground or (2) ito hold runoff
74 '.
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February 1993
for a short period of time and then release it to the surface and stormwater system. Most facilities do both to some
degree. . > (
Retention time: The ratio of wetland volume/average outflow rate (approximately) unless the soil infiltration rate is
relatively nigh. '
% Riparian areas: Areas bordering streams, lakes, rivers, and other watercourses. These areas have high water tables
and support plants requiring saturated soils during all or part of the year. Riparian areas include both wetland and
•
zones.
Runoff: that portion of the precipitation on a drainage area ten is discharged overland from the areato stream channels
or drainage systems. • . V""""MS
Scour: The cl^ga^ digging actiom of flowing water, especiaUy the downwa^
away mud and sdt on the outside of a curve or during a flood. ' ««««» sweeping
Sediment Fragmented material that originates from weathering and erosion of rocks or unconsolidated deposits and
« ; transported by, suspended in, or deposited by water. Certain contaminants tend to collect on and adhere to
.... seoiment particles. . - -. . ^ . ' . - ,
^
^
~,
Silt can clog gravel beds and prevent successful-salmon spawning. ^ awmnraiB.
l: Control of runoff waters before m^^
jac^tlandorwateicom^orisroutedmtodrain/sewersj^^
imaaa
. stormwater that involve altering the flow, velocity, duration, and other
characteristics of runoff by physical ^^ means, e.g., construction of a detention dam and weir.
Surface water: Water present above the substrate or soil surface.
Suspended solids: Organic or inorganic particles that are suspended in and carried by the water. The term includes
sand,mud,andctoyParticles,aswellasSolidsinwastewater. High levels of suspended solids can clog the breathing
giUsofsomefishaddsuffocatethem. When suspended solids settle to stream^ndlake bottoms theySclof
salmon spawning gravels, suffocating salmon egp anoVor preventing future spawning. Clay and silt sedimem
parddes generally carry other pollutants adsorbed to their surface, mclucimg i*troleum hydniarbons, refractorv
oramcs, esticides and heav at— j uv-uouub, renaciory
orgamcs, pesticides, and heavy metals..
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February 1993
Swale: Asfaallow drainage conveyance with relatively gentle side slopes, and generally with flow depths less than one
toot. Water quality swales are open vegetated drainage channels intended to optimize water quality treatment of
surface and stormwater runoff according to specific design criteria.
Swamp: A forested wetland with a shallow water table; palustrine forested wetlands, dominated by woody vegetation
greater than 6 meters (20 feet) in height. ' s=^"un
Total suspended soIids(TSS): A measure of the amount of suspended solids found in the water column. "
Toxic: Poisonous, carcinogenic, or otherwise directly harmful to life.
Toxicsobstancesand toxicants: Chemical substances, such as pesticides, plastics, heavy metals, detergents, organics
chlorine, oil, and industrial wastes, that are poisonous, carcinogenic, or otherwise directly harmful to life.
Ibeataent: Chemical, biological, or mechanical procedures applied to an industrial or municipal discharge or to other
sources of contamination to remove, reduce, or neutralize contaminants.
Tbrfaidi^Aii1easureoftheamountof material suspended in the water. Increasing the turbidity of the wafer decreases
the depth to which light can penetrate. High levels of turbidity over extended periods are harmful to aquatic life.
Volatile: Readily vaporizable at a relatively low temperature.
Wastewater: Effluent from a sewage treatment plant. , . < '
1' cbcmical-andPhysica» conditions of a waterbody; measure of a waterbody's ability to
0n Withi° Which water drains into a Particular river, stream, or body 6Y water A
' ""l *" b°dy °f Water "* Whicfa
are
W«ler table: The upper surface of groundwater in the zone of saturation.
transitional between torestrial and aquatic systen^ ttat have a water table usually at or near the
surfece or a shallow covering of water, hydric soils, and a prevalence of hydrophytic vegetation. Note that there
76
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