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A HANDBOOK OFCONSTRUCTED WETLANDS
a guide to creating wetlands for:AGRICULTURAL WASTEWATER
DOMESTIC WASTEWATER
COAL MINE DRAINAGE
STORMWATER
in the Mid-Atlantic Region
1Volume
ACKNOWLEDGMENTS
Many people contributed to this Handbook. An interagency Core Group provided the initial impetus for the Handbook, and later providedguidance and technical input during its preparation. The Core Group comprised:
Carl DuPoldt, USDA - NRCS. Chester, PA
Robert Edwards, Susquehanna River Basin Commission,Harrisburg, PA
Lamonte Garber, Chesapeake Bay Foundation, Harrisburg. PA
Barry Isaacs, USDA - NRCS, Harrisburg, PA
Jeffrey Lapp. EPA, Philadelphia, PA
Melanie Sayers, Pennsylvania Department of Agriculture, Harrisburg, PAFred Suffian, USDA - NRCS Philadelphia, PA
Charles Takita, Susquehanna River Basin Commission, Harrisburg, PAHarold Webster, Penn State University, DuBois, PA.
Timothy Murphy, USDA - NRCS, Harrisburg, PA
Glenn Rider, Pennsylvania Department of EnvironmentalResources, Harrisburg. PA
Many experts on constructed wetlands contributed by providing information and by reviewing and commenting on the Handbook. TheseIndividuals included:
Robert Bastian. EPA .WashinSton, DCWilliam Boyd, USDA - NRCS. Lincoln, NERobert Brooks, Penn State University,
University Park, PADonald Brown, EPA, Cincinnati, OHDana Chapman, USDA - NRCS, Auburn, NYTracy Davenport, USDA -NRCS, Annapolis,MDPaul DuBowy, Texas A & M University,
College Station, TXMichelle Girts, CH2M HILL, Portland, ORRobert Hedin, Hedin Environmental,
Sewickley, PA William Hellier. Pennsylvania Department of
Environmental Resources, Hawk Run, PARobert Kadlec, Wetland Management
Services, Chelsea, MI
Douglas Kepler, Damariscotta. Clarion, PARobert Kleinmann, US Bureau of Mines,
Pittsburgh, PA
Robert Knight, CH2M HILL, Gainesville, FLFran Koch, Pennsylvania Department of
Environmental Resources, Harrisburg, PAEric McCleary, Damariscotta, Clarion, PAGerald Moshiri, Center for Wetlands and
Eco-Technology Application, Gulf Breeze,F L
John Murtha, Pennsylvania Department ofEnvironmental Resources, Harrisburg. PA
Robert Myers, USDA - NRCS, Syracuse, NYKurt Neumiller, EPA, Annapolis, MDRichard Reaves, Purdue University, West
Lafayette, INWilliam Sanville, EPA, Cincinnati, OHDennis Sievers, University of Missouri,
Columbia, MOEarl Shaver, Delaware Department of
Natural Resources and EnvironmentalControl, Dover, DE
Daniel Seibert, USDA - NRCS, Somerset, PAJeffrey Skousen, West Virginia University,
Morgantown. WVPeter Slack, Pennsylvania Department of
Environmental Resources, Harrisburg, PADennis Verdi, USDA - NRCS, Amherst, MAThomas Walski, Wilkes University, Wilkes-
Barre, PARobert Wengryznek, USDA - NRCS, Orono,
MEAlfred Whitehouse, Office of Surface
Mining. Pittsburgh, PAChristopher Zabawa, EPA, Washington, DC.
This document was prepared by Luise Davis for the USDA-Natural Resources Conservation Service and the US Environmental ProtectionAgency-Region III, in cooperation with the Pennsylvania Department of Environmental Resources. Partial funding has been provided withnonpoint source management program funds under Section 319 of the Federal Clean Waler Act.
The findings. conclusions, and recommendations contained in the Handbook do not necessarily represent the policy of the USDA - NRCS,EPA - Region III, the Commonwealth of Pennsylvania, or any other state in the northeastern United States concerning the use of constructedwetlands for the treatment and control of nonpoint sources of pollutants. Each state agency should be consulted to determine specificprograms and restrictions in this regard.
VOLUME 1TABLE OF CONTENTS
CHAPTER 1. INTRODUCTION . . . . . . . . . . . . . . . . . . ................................................._................................................., 5.
CHAPTER 2. CONSTRUCTED WETLANDS AS ECOSYSTEMS ......................................................................... 7What Are Wetlands? 7......... . . ................................................................................................................Wetland Functions and Values.. ...................................................................... . .................................. 7Components of Constructed Wetlands ............................................................................................... 8
Water. ................................................................................................................ ............................ 8Substrates, Sediments, and Litter ................................................................................................ 8Vegetation ..................................................................................................................................... 8Microorganisms ............................................................................................................................. 9Animals.. ............................................................ .......................................................................... 9Aesthetics and Landscape Enhancement .................................................................................. 10
CHAPTER 3. CONSTRUCTED WETLANDS AS TREATMENT SYSTEMS ........................................................ 11How Wetlands Improve Water Quality ........................................................................................ 11Advantages of Constructed Wetlands.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Limitations of Constructed Wetlands .................................................................................. 11Types of Constructed Wetlands ........................................................................................................ 12
Surface Flow Wetlands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Subsurface Flow Wetland ......................................................................................................... 13Hybrid Systems .......................................................................................................................... 13
Winter and Summer Operation ........................................................................................................ 13
Creation of Hazard ............................................................................................................................ 14
Change and Resilience ............. . ........................................................................................................ 14
CHAPTER 4. GENERAL DESIGN OF CONSTRUCTED WETLANDS ................................................................ 17
Design Considerations ....................................................................................................................... 17
Planning 17............................................................................................................................................Site Selection .... ................................................................................................................................ 18
Land Use and Access.. .................................................................. 18.............................................Land Availability ........................................................................................................................ 18Topography ................................................................................................................................. 19Environmental Resources.. ....................................................... ................................................. 19
Permits and Regulations ................................................................................................................... 19Structures ............................................................. .......................................................................... 20
Cells ............................................................................................................................................ 20Liners .......................................................................................................................................... 20
Flow Control Structures ............................................................................................................. 20
Inlets ............................................................................................................................. 21Outlets .............................................. ..................................................................................... 22
System Lifetimes .................................................................................................................. 23
Chapter 5. HYDROLOGY ..................................................................................................................................... 25Climate and Weather ......................................................................................................................... 25Hydroperiod ........................................................................... ......... ................................................... 25Hydraulic Residence Time ............................................................................................................... 26Hydraulic Loading Rate .................................................................................................................... 26Groundwater Exchange ..................................................................................................................... 26Evapotranspiration ............................................................................................................................ 26Water Balance ................................................................................................................................... 26
Chapter 6. SUBSTRATES.. ................. . ................................................................................................................ 29Soil ..................................................................................................................................................... 29Sand and Gravel ................................................................................................................................ 39Organic Material ............. ................................................................................................................. 30
Chapter 7, VEGETATION ................................. . .................................................................................................. 31Selecting plants .......................................................... ...................................................................... 31
Surface Flow Wetlands ............................................................................................................... 31Subsurface Flow W e t l a n d s ................................................................. . ....................................... 34
Sources of Plants ......................................................................................................... . ..................... 34Seeds ..................................................... . ..................................................................................... 34Wetland Soil .... ........................................................................................................................... 34Rhizomes,Tubers. and Entire Plants .......................................................................................... 34
When To Plant.. ................................................................................................................................. 35Site Preparation ................................................................................................................................ 35How To Plant ..................................................................................................................................... 36
Surface Flow Wetlands.. ........................................... . ................................................................. 36Subsurface Flow Wetlands .................................................................... . . .................................. 36
Establishing and Maintaining Vegetation ......................................................................................... 36
Chapter 8. CONSTRUCTION .............................................................................................................................. 39Construction Plans ............................................................................................................................ 39Pre-Construction Activities .............................................................................................................. 39Construction Activities ......................................................................... . ......................................... 39Inspection, Startup, and Testing 40........................................................................................................
Chapter 9: OPERATION, MAINTENANCE, AND MONITORING ..................................................................... 41
Operation and Maintenance .............................................................................................................. 41
Operation and Maintenance Plan ............................................................... .:. ............................ 41Hydrology ................................................................................................................................... 41
Structures .................................................................................................................................... 41
Vegetation ................................................................................................................................... 42
Muskrats ..................................................................................................................................... 42Mosquitoes .................................................................................................................................. 42
Monitoring ........................................................................................................................................ 43
Monitoring Plan.. ........................................................................................................................ 43Monitoring for Discharge Compliance.. ..................................................................................... 43Monitoring for System Performance .......................................................................................... 43Monitoring for Wetland Health .................................................................................................. 44
REFERENCES .I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......................... 45
PHOTOGRAPHS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......................... 47
LIST OF TABLES
Table 1. Emergent plants for constructed wetlands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
LIST OF FIGURES
Figure 1. Surface flow and subsurface flow constructed wetlands .................................................................. .12Figure 2. Inlet and outlet designs ....................................................................................................................... 21Figure 3. Influent splitter box .............................................................................................................................. 22
Natural processes have always cleansedwater as it flowed through rivers, lakes, streams,and wetlands. In the last several decades,systems have been constructed to use some ofthese processes for water quality improvement.Constructed wetlands are now used to improvethe quality of point and nonpoint sources ofwater pollution, including stormwater runoff,domestic wastewater, agricultural wastewater,and coal mine drainage. Constructed wetlandsare also being used to treat petroleum refinerywastes, compost and landfill leachates, fishpond discharges, and pretreated industrialwastewaters, such as those from pulp and papermills, textile mills, and seafood processing. Forsome wastewaters, constructed wetlands are thesole treatment; for others, they are one compo-nent in a sequence of treatment processes.
One of the most common applications ofconstructed wetlands has been the treatment ofprimary or secondary domestic sewage effluent.Constructed wetland systems modelled afterthose for domestic wastewater are being used totreat the high organic loads associated withagriculture. A large number of wetlands havebeen constructed to treat drainage from activeand abandoned coal mines and more than 500such systems are operating in Appalachia alone.The use of constructed wetlands to controlstormwater flows and quality is a recent applica-tion of the technology and the number of suchsystems is increasing rapidly.
The treatment of wastewater or stormwaterby constructed wetlands can be a low-cost, low-energy process requiring minimal operationalattention. As a result of both extensive researchand practical application, insight is being gainedinto the design, performance, operation, andmaintenance of constructed wetlands for waterquality improvement. Constructed wetlands canbe sturdy, effective systems. However, to beeffective, they must be carefully designed,constructed, operated, and maintained.
This Handbook has been prepared as ageneral guide to the design, construction, opera-tion, and maintenance of constructed wetlandsfor the treatment of domestic wastewater, agri-cultural wastewater, coal mine drainage, andstormwater runoff in the mid-Atlantic region,The Handbook is not a design manual. The useof constructed wetlands to improve water qualityis a developing technology. Much is not yetunderstood, and questions remain regarding theoptimal design of wetland systems and theirlongevity. As our experience with these systemsincreases, the information offered here will bereplaced by more refined information. TheHandbook should be used with this clearly inmind.
The Handbook is divided into five volumes.This, the first, provides information common toall types of constructed wetlands for wastewaterand runoff. It is to be used in conjunction withan accompanying volume that provides informa-tion specific to a particular type of wastewater orrunoff. The other volumes in the series areVolume 2: Domestic Wastewater, Volume 3:Agricultural Wastewater, Volume 4: Coal MineDrainage, and Volume 5: Stormwater Runoff.While constructed wetlands are being used totreat other kinds of wastewater, such as indus-trial wastewaters, a discussion of these applica-tions is beyond the scope of this Handbook.However, the information presented here may beuseful in developing other applications.
A number of conferences on constructedwetlands have been held recently. The proceed-ings of these conferences include experimentaland operational data from wetland systems builtto treat a number of different kinds of wastewa-ters and runoff, and present detailed discussionsof process kinetics and system design. Proceed-ings from three well-known conferences are:
Moshiri, G. A. (ed.) 1993. Constructed Wetlandsfor Water Quality Improvement. CRC Press, BocaRaton, FL. 632 pp.
V O L U M E 1: GENERAL C O N S I D E R A T I O N S 5
Cooper, P. F., and B. C. Findlater (eds.) 1990.Constructed Wetlands in Water Pollution Control.Proceedings of the International Conference onthe Use of Constructed Wetlands in Water Pollu-tion Control. Cambridge, UK, 24-28 September.WRc, Swindon, Wiltshire, UK. 605 pp.
Hammer, D. A. (ed.) 1989. Constructed Wetlandsfor Wastewater Treatment: Municipal, Industrialand Agricultural. Lewis Publishers. Chelsea, MI.831 pp.
Conferences and published’ information continueto become available as more constructed wetland
systems are built and monitored.
V O L U M E 1 : GE N E R A L C O N S I D E R A T I O N
CHAPTER 2CONSTRUCTED WETLANDS AS ECOSYSTEMS
Constructed wetlands for water treatmentare complex, integrated systems of water,plants, animals, microorganisms, and theenvironment. While wetlands are generallyreliable, self-adjusting systems, an understand-ing of how natural wetlands are structured andhow they function greatly increases the likeli-hood of successfully constructing a treatmentwetland. This chapter provides an overview ofthe major components of wetland ecosystemsand of the most important processes that affectwater treatment.
WHAT ARE WETLANDS?
Wetlands are transitional areas between landand water. The boundaries between wetlands
and uplands or deep water are therefore notalways distinct. The term “wetlands” encom-
passes a broad range of wet environments,including marshes, bogs, swamps, wet meadows,tidal wetlands, floodplains, and ribbon (riparian)wetlands along stream channels.
All wetlands - natural or constructed, fresh-water or salt - have one characteristic in com-mon: the presence of surface or near-surfacewater, at least periodically. In most wetlands,hydrologic conditions are such that the substrateis saturated long enough during the growingseason to create oxygen-poor conditions in thesubstrate. The lack of oxygen creates reducing.(oxygen-poor) conditions within the substrateand limits the vegetation to those species that areadapted to low-oxygen environments.
The hydrology of wetlands is generally oneof slow flows and either shallow waters orsaturated substrates. The slow flows and shal-low water depths allow sediments to settle as thewater passes through the wetland. The slowflows also provide prolonged contact timesbetween the water and the surfaces within thewetland. The complex mass of organic and
VOLUME 1 G ENERAL C ONSIDERATIONS
inorganic materials and the diverse opportunitiesfor gas/water interchanges foster a diverse commu-nity of microorganisms that break down or trans-form a wide variety of substances.
Most wetlands support a dense growth ofvascular plants adapted to saturated conditions.This vegetation slows the water, creates microenvi-ronments within the water column, and providesattachment sites for the microbial community.The litter that accumulates as plants die back inthe fall creates additional material and exchangesites, and provides a source of carbon, nitrogen,and phosphorous to fuel microbial processes.
WETLAND FUNCTIONSAND VALUES
Wetlands provide a number of functions andvalues. (Wetland functions are the inherentprocesses occurring in wetlands; wetland valuesare the attributes of wetlands that society per-ceives as beneficial.) While not all wetlandsprovide all functions and values, most wetlandsprovide several. Under appropriate circumstances.constructed wetlands can provide:
water quality improvement
flood storage and the desynchronization of stormrainfall and surface runoff
cycling of nutrients and other materials
habitat for fish- and wildlife
passive recreation, such as bird watchingand photography
active recreation, such as hunting
education and research
aesthetics and landscape enhancemerit.’
7
COMPONENTS OFCONSTRUCTED WETLANDS
A constructed wetland consists of a properly-designed basin that contains water, a substrate, and,most commonly, vascular plants. These componentscan be manipulated in constructing a wetland. Otherimportant components of wetlands, such as thecommunities of microbes and aquatic invertebrates,develop naturally.
W A T E R
Wetlands are likely to form where landformsdirect surface water to shallow basins and where arelatively impermeable subsurface layer prevents thesurface water from seeping into the ground. Theseconditions can be created to construct a wetland. Awetland can be built almost anywhere in the land-scape by shaping the land surface to collect surfacewater and by sealing the basin to retain the water.
Hydrology is the most important design factor inconstructed wetlands because it links all of thefunctions in a wetland and because it is often theprimary factor in the success or failure of a con- structed wetland. While the hydrology of con-structed wetlands is not greatly different than that ofother surface and near-surface waters, it does differ inseveral important respects:
l small changes in hydrology can have fairly signifi-cant effects on a wetland and its treatment effec-tiveness
l because of the large surface area of the water andits shallow depth, a wetland system interactsstrongly with the atmosphere through rainfall andevapotranspiration (the combined loss of water byevaporation from the water surface and lossthrough transpiration by plants)
l the density of vegetation of a wetland strongly affectsits hydrology, first, by obstructing flow paths as thewater finds its sinuous way through the network ofstems, leaves, roots, and rhizomes and, second, byblocking exposure to wind and sun.
S U B S T R A T E S, SE D I M E N T S, A N D L I T T E R
Substrates used to construct wetlands in-clude soil, sand, gravel, rock, and organicmaterials such as compost. Sediments and litterthen accumulate in the wetland because of thelow water velocities and high productivitytypical of wetlands. The substrates, sediments,and litter are important for several reasons:
l they support many of the living organisms inwetlands
l substrate permeability affects the movement ofwater through the wetland
l many chemical and biological (especiallymicrobial) transformations take place withinthe substrates
l substrates provide storage for manycontaminants
l the accumulation of litter increases the amountof organic matter in the wetland. Organic matterprovides sites for material exchange and micro-bial attachment, and is a source of carbon, theenergy source that drives some of the importantbiological reactions in wetlands.
The physical and chemical characteristics ofsoils and other substrates are altered when theyare flooded. In a saturated substrate, waterreplaces the atmospheric gases in the porespaces and microbial metabolism consumes theavailable oxygen. Since oxygen is consumedmore rapidly than it. can be replaced by diffusionfrom the atmosphere, substrates become anoxic(without oxygen). This reducing environment isimportant in the removal of pollutants such asnitrogen and metals.
V E G ET A T I O N
Both vascular plants (the higher plants) andnon-vascular plants (algae) are important inconstructed wetlands. Photosynthesis by algaeincreases the dissolved oxygen content of thewater which in turn affects nutrient and metal
Constructed wetlands attract waterfowl andwading birds, including mallards, green-wingedteal, wood ducks, moorhens, green and greatblue herons, and bitterns. Snipe, red-wingedblackbirds, marsh wrens, bank swallows, red-tailed hawks, and Northern harriers feed and/ornest in wetlands.
AESTHETICS AND LANDSCAPE
E N H A N C E M E N T
While wetlands are primarily treatmentsystems, they provide intangible benefits byincreasing the aesthetics of the site and enhanc-ing the landscape. Visually, wetlands are unusu-ally rich environments. By introducing theelement of water to the landscape, constructedwetlands, as much as natural wetlands. adddiversity to the landscape. The complexity ofshape, color, size, and interspersion of plants,and the variety in the sweep and curve of theedges of landforms all add to the aestheticquality of the wetlands. Constructed wetlandscan be built with curving shapes that follow thenatural contours of the site, and some wetlandsfor water treatment are’ indistinguishable, at firstglance, from natural wetlands.
V O L U M E 1: GE N E R A L C O N S I D E R A T I O N S
reactions. Vascular plants contribute to thetreatment of wastewater and runoff in anumber of ways:
they stabilize substrates and limitchannelized flow
they slow water velocities, allowing sus-pended materials to settle
they take up carbon, nutrients, and traceelements and incorporate them into planttissues
they transfer gases between the atmosphereand the sediments
leakage of oxygen from subsurface plantstructures creates oxygenated micrositeswithin the substrate
their stem and root systems provide sitesfor microbial attachment
theycreate litter when they die and decay.
Constructed wetlands are usually plantedwith emergent vegetation (non-woody plantsthat grow with their roots in the substrateand their stems and leaves emerging fromthe water surface). Common emergentsused in constructed wetlands includebulrushes, cattails, reeds, and a number ofbroad-leaved species.
M I C R O O R G A N I S M S
A fundamental characteristic of wetlands isthat their functions are largely regulated bymicroorganisms and their metabolism (Wetzel1993). Microorganisms include bacteria, yeasts,fungi, protozoa, rind algae. The microbialbiomass is a major sink for organic carbon andmany nutrients. Microbial activity:
l transforms a great number of organic andinorganic substances into innocuous orinsoluble substances
l alters the reduction/oxidation (redox) condi-tions of the substrate and thus affects theprocessing capacity of the wetland
l is involved in the recycling of nutrients.
Some microbial transformations are aerobic(that is, they require free oxygen) while othersare anaerobic (they take place in the absence offree oxygen). Many bacterial species are faculta-tive anaerobes, that is, they are capable of func-tioning under both aerobic and anaerobic condi-tions in response to changing environmentalconditions.
Microbial populations adjust to changes inthe water delivered to them. Populations ofmicrobes can expand quickly when presentedwith suitable energy-containing materials. Whenenvironmental conditions are no longer suitable,many microorganisms become dormant and canremain dormant for years (Hilton 1993).
The microbial community of a constructedwetland can be affected by toxic substances,such as pesticides and heavy metals, and caremust be taken to prevent such chemicals frombeing introduced at damaging concentrations.
A N I M A L S
Constructed wetlands provide habitat for arich diversity of invertebrates and vertebrates.Invertebrate animals, such as insects and worms.contribute to the treatment process by fragment-ing detritus and consuming organic matter. Thelarvae of many insects are aquatic and consumesignificant amounts of material during theirlarval stages, which may last for several years.Invertebrates also fill a number of ecologicalroles; for instance, dragonfly nymphs are impor-tant predators of mosquito larvae.
Although invertebrates are the mostimportant animals as far as water quality im-provement is concerned, constructed wetlandsalso attract a variety of amphibians, turtles,birds, and mammals.
VOLUME 1: GENERAL CONSIDERATIONS 9
CHAPTER 3CONSTRUCTED WETLANDS AS TREATMENT SYSTEMS
A constructed wetland is a shallow basinfilled with some sort of substrate, usually soil orgravel, and planted with vegetation tolerant ofsaturated conditions. Water is introduced at oneend and flows over the surface or through thesubstrate, and is discharged at the other endthrough a weir or other structure which controlsthe depth of the water in the wetland.
HOW WETLANDS IMPROVEWATER QUALITY
A wetland is a complex assemblage of water,substrate, plants (vascular and algae), litter(primarily fallen plant material), invertebrates(mostly insect larvae and worms). and an arrayof microorganisms (most importantly bacteria).The mechanisms that are available to improvewater quality are therefore numerous and ofteninterrelated. These mechanisms include:
l settling of suspended particulate matter
l filtration and chemical precipitation throughcontact of the water with the substrate andlitter
l chemical transformation
l adsorption and ion exchange on the surfaces ofplants, substrate, sediment, and litter
l breakdown and transformation of pollutants bymicroorganisms and plants
l uptake and transformation of nutrients bymicroorganisms and plants
l predation and natural die-off of pathogens.
The most effective treatment wetlands arethose that foster these mechanisms. The specif-ics for the various types of wastewater andrunoff are discussed in the wastewater-specificvolumes.
ADVANTAGES OFCONSTRUCTED WETLANDS
Constructed wetlands are a cost-effective andtechnically feasible approach to treating waste-water and runoff for several reasons:
l wetlands can be less expensive to build thanother treatment options
. operation and maintenance expenses (energyand supplies) are low
l operation and maintenance require onlyperiodic, rather than continuous, on-site labor
l wetlands are able to tolerate fluctuations inf l o w
. they facilitate water reuse and recycling.
In addition:
l they provide habitat for many wetland organisms
l they can be built to fit harmoniously into thelandscape
l they provide numerous benefits in addition towater quality improvement, such as wildlifehabitat and the aesthetic enhancement of openspaces
l they are an environmentally-sensitiveapproach that is viewed with favor by thegeneral public.
LIMITATIONS OFC O N S T R U C T E D W E T L A N D S
There are limitations associated with the useof constructed wetlands:
l they generally require larger land areas thando conventional wastewater treatment sys-terns. Wetland treatment may be economicalrelative to other options only where land isavailable and affordable.
VOLUME 1: GENERAL CONSIDERATIONS 17
they require a minimum amount of water if they
l performance may be less consistent than inconventional treatment. Wetland treatmentefficiencies may vary ‘seasonally in response tochanging environmental conditions, includingrainfall and drought. While the average perfor-mance over the year may be acceptable, wetlandtreatment cannot be relied upon if effluentquality must meet stringent discharge standardsat all times.
l the biological components are sensitive to toxicchemicals, such as ammonia and pesticides
l flushes of pollutants or surges in water flow may temporarily reduce treatment effectiveness
are to survive. While wetlands can toleratetemporary drawdowns, they cannot withstand
Also, the use of constructed wetlands for waste-water treatment and stormwater control is a fairlyrecent development. There is yet no consensus onthe optimal design of wetland systems nor is theremuch information on their long-term performance.
TYPES OF CONSTRUCTEDW E T L A N D S
There are several types of constructed wetlands:surface flow wetlands, subsurface flow wetlands,and hybrid systems that incorporate surface andsubsurface flow wetlands. Constructed wetlandsystems can also be combined with conventionaltreatment technologies. The types of constructedwetlands appropriate for domestic wastewater,agricultural wastewater, coal mine drainage, andstormwater runoff are discussed in the wastewater-
complete drying. specific volumes.
Water level is above the ground surface; vegetation is rooted and emergesabove the water surface: waterflow is primarily above ground
WETLAND PLANTS AND WATER
SOIL Surface Flow WetlandLINER
NATIVE SOIL
Water level is below ground; water flow is through a sand or gravelpenetrate to the bottom of the bed
WETLAND PLANTS
SOIL. SAND. AND GRAVELLINER
NATIVE SOIL
Subsurface Flow Wetland
Figure 1. Surface flow and subsurface flow constructed wetlands(from Water Pollution Control Federation 1990).
bed; roots
VOLUME 1 : GENERAL CONSIDERATIONS
S U R F A C E F L O W W E T L A N D S
A surface flow (SF) wetland consists of ashallow basin, soil or other medium to support theroots of vegetation, and a water control structurethat maintains a shallow depth of water (figure 1).The water surface is above the substrate. SFwetlands look much like natural marshes and canprovide wildlife habitat and aesthetic benefits aswell as water treatment. In SF wetlands, the near-surface layer is aerobic while the deeper watersand substrate are usually anaerobic. Stormwaterwetlands and wetlands built to treat mine drainageand agricultural runoff are usually SF wetlands.
SF wetlands are sometimes called free watersurface wetlands or, if they are for mine drainage,aerobic wetlands. The advantages of SF wetlandsare that their capital and operating costs are low,and that their construction, operation, and mainte-nance are straightforward. The main disadvantageof SF systems is that they generally require a largerland area than other systems.
S U B S U R F A C E F L O W W E T L A N D S
A subsurface flow (SSF) wetland consists of asealed basin with a porous substrate of rock orgravel. The water level is designed to remain belowthe top of the substrate. In most of the systems in theUnited States, the flow path is horizontal, althoughsome European systems use vertical flow paths. SSFsystems are called by several names. includingvegetated submerged bed, root zone method, micro-bial rock reed filter, and plant-rock filter systems.
Because of the hydraulic constraints imposedby the substrate, SSF wetlands are best suited towastewaters with relatively low solids concentra-tions and under relatively uniform flow conditions.SSF wetlands have most frequently been used toreduce 5-day biochemical oxygen demand (BOD5)from domestic wastewaters.
The advantages cited for SSF wetlands aregreater cold tolerance, minimization of pest andodor problems, and, possibly, greater assimilationpotential per unit of land area than in SF systems.
It has been claimed that the porous mediumprovides greater surface area for treatment contactthan is found in SF wetlands, so that the treatmentresponses should be faster for SSF wetlands whichcan, therefore, be smaller than a SF system de-signed for the same volume of wastewater. Sincethe water surface is not exposed, public accessproblems are minimal. Several SSF systems areoperating in parks. with public access encouraged.
The disadvantages of SSF wetlands are thatthey are more expensive to construct, on a unitbasis. than SF wetlands. Because of cost, SSFwetlands are often used for small flows. SSFwetlands may be more difficult to regulate than SFwetlands, and maintenance and repair costs aregenerally higher than for SF wetlands. A number ofsystems have had problems with clogging andunintended surface flows.
H Y B R I D S Y S T E M S
Single stage systems require that all of theremoval processes occur in the same space. Inhybrid or multistage systems, different cells aredesigned for different types of reactions. Effectivewetland treatment of mine drainage may require asequence of different wetland cells to promoteaerobic and anaerobic reactions. as may the re-moral of ammonia from agricultural wastewater.
WINTER AND SUMMEROPERATION
Wetlands continue to function during coldweather. Physical processes, such as sedimenta-tion. continue regardless of temperature, providingthat the water does not freeze. Many of the reac-tions take place within the wetland substrate,where decomposition and microbial activitygenerate enough heat to keep the subsurface layersfrom freezing. Water treatment will continueunder ice. To create space for under-ice flow,water levels can be raised in anticipation of freeze,then dropped once a cover of ice has formed.
V OLUME 1: GENERAL CONSIDERATIONS
Rates of microbial decomposition slow astemperatures drop and the wetland may need tobe made larger to accommodate the slowerreaction rates. For agricultural wetlands, whichrely on microbial activity to break down organicwastes, it may be prudent to store the wastewa-ter in the pretreatment unit during the coldmonths for treatment during the warm months.The high flows that are common in winter andspring because of snowmelt, spring rains, andhigh groundwater tables can move water soquickly through a wetland tEat there is notenough retention time for adequate treatment.Because removal rates are much higher duringwarm weather, the agricultural wetland canoften be smaller than if the water were treatedyear-round.
Wetlands lose large amounts of water in thesummer through evapotranspiration. The ade-quacy of flow in the summer must be considered
since it will affect water levels in the wetland andthe amount of wetland effluent available forrecycling (if this is part of the design). A supple-mental source of water may be required to main-tain adequate moisture in the wetland.
CREATION OF HAZARD
The question of hazard arises from the factthat, in ecological terms, everything must gosomewhere. Wetlands are able to degrade,transform, or assimilate many contaminants,such nitrogen, and are sinks for some materials.For persistent materials, such as phosphorousand metals, wetland sinks may become sourcesif not properly constructed and managed. Theextent to which wetlands retain contaminantssuch as phosphorous and metals is an importantunknown factor, as are the conditions underwhich wetlands may release stored contami-nants. Bioaccumulation and biotoxicity intreatment wetlands is not clearly documentednor understood.
Persistent compounds can be a concern,depending on the constituents in the wastewa-ter. For instance, mine drainage contains metalsand stormwater carries hydrocarbons depositedon paved surfaces. Heavy metals are oftensequestered in wetland sediments that may bewashed out of wetlands during storms, therebyproviding only a lag time in pollutant dispersal.Transport of toxic materials in this way is aconcern, as is the transport of phosphorus, anextremely important factor in the over-enrich-ment of surface waters. The question of hazardunderscores the importance of designing andoperating constructed wetlands properly andmonitoring them periodically.
CHANGE AND RESILIENCE
All ecosystems change over time. Wetlands forwastewater treatment can be expected to changemore quickly than most natural wetlands becauseof the rapid accumulation of sediment, litter, andpollutants. Some natural variability is also inher-ent in all living systems and is to be expected.
The change in species composition as eco-systems mature is known as succession. Ingeneral, species diversity increases as ecosys-tems mature. Diversity (the number of specieswithin a habitat, such as a wetland) is oftenconsidered a measure of ecosystem resilience(the ability of the system to accept disturbance):as the number of species increases, so does thecomplexity of the interactions of the differentspecies with each other and with their environ-ment; the greater the number of interactions, themore resilient the system is as a whole and thebroader its capacity to adapt to change.
In wastewater treatment wetlands, thestresses of high wastewater loadings can lead todominance by a few aggressive, highly tolerantspecies, such as cattail and common reed, whichmay eventually eliminate other species. Ifwildlife habitat values are important to the
V OLUME 1 : GENERAL CONSIDERATIONS
project, intervention to maintain diversity maybe necessary. If habitat values are not important,changes can be allowed to proceed withoutinterference as long as the wetland continues totreat the water to acceptable levels.
Any ecosystem, natural or constructed, haslimits to its ability to accept disturbance. Theperformance of constructed wetland systemsmay change over time as a consequence ofchanges in the substrate and the accumulation ofpollutants in the wetland. Constructed wetlandsmust be monitored periodically for evidence ofstress so that remedial action, if necessary, canbe taken.
V OLUME 1: GENERAL CONSIDERATIONS 15
CHAPTER 4GENERAL DESIGN OF CONSTRUCTED WETLANDS
DESIGN CONSIDERATIONS
Despite a large amount of research and pub-lished information, the optimal design of con-structed wetlands for various applications has notyet been determined. Many constructed wetlandsystems have not been adequately monitored orhave not been operating long enough to providesufficient data for analysis. Among the systemsthat have been monitored, performance has variedand the influences of the diverse factors that affectperformance, such as location, type of wastewateror runoff, wetland design, climate, weather,disturbance, and daily or seasonal variability, ‘havebeen difficult to quantify.
In general, wetland designs attempt to mimicnatural wetlands in overall structure while foster-ing those wetland processes that are thought tocontribute the most to the improvement of waterquality. Mitsch (1992) suggests the followingguidelines for creating successful constructedwetlands:
keep the design simple. Complex technologicalapproaches often invite failure.
design for minimal maintenance.
design the system to use natural energies, such asgravity flow.
design for the extremes of weather and climate,not the average. Storms, floods, and droughts areto be expected and planned for, not feared.
design the wetland with the landscape, notagainst it. Integrate the design with the naturaltopography of the site.
avoid over-engineering the design with rectangu-lar basins, rigid structures and channels, andregular morphology. Mimic natural systems.
give the system time. Wetlands do not necessar-ily become functional overnight and severalyears may elapse before performance reachesoptimal levels. Strategies that try to short-circuit
the process of system development or to over-manage often fail.
l design the system for function, not form. Forinstance, if initial plantings fail, but the overallfunction of the wetland, based on initial objec-tives, is intact, then the system has not failed.
PLANNING
A conceptual planning phase is essential.Wetlands can be designed in a variety of systemtypes and configurations to meet specific wastewa-ter needs, alternative sites are often available, anda variety of local, native plant species can bechosen. Every site is unique and the design of aconstructed wetland system will be site-specific.
The planning phase consists of characterizingthe quantity and quality of the wastewater to betreated, determining the discharge standards to bemet, selecting the site, selecting system type andconfiguration, and specifying the design criteria tobe met by the detailed engineering plans. Eco-nomic factors include the land area required. thetype of water containment, the control and trans-port of water through the system, and vegetation.Setting and prioritizing the objectives of the
wetland system is key to the creation of a success-ful system. The characteristics of a local naturalwetland should be used as a model for the con-structed wetland, modified to fit the needs of theproject and the specifics of the constructed wet-land site.
A constructed wetland should be designed totake advantage of the natural features of the siteand to minimize its disturbance. Wetland shape isdictated by the existing topography, geology, andland availability. The number of cells depends ontopography, hydrology, and water quality. Onlevel sites, cells can be created with dikes. Onsloping sites, cells can be terraced.
A site-sensitive design that incorporatesexisting features of the site reduces the amount
37V OLUME 1: GENERAL CONSIDERATIONS
Provides adeqyate space
of earthmoving required and increases the visualattractiveness of the site. Earth grading andshaping can blend newly created landforms intothe existing landscape. Basins and channels canbe curved to follow the natural contours of thesite. Various types of vegetation can be planted inand around a constructed wetland to reduceerosion, screen views, define space, controlmicroclimate, and control traffic patterns.
Planning should be oriented toward thecreation of a biologically and hydrologicallyfunctional system. Plans should include cleargoal statements and standards for success. Thepossible future expansion of the operationshould be considered.
Plans should include detailed instructionsfor implementing a contingency plan in case thesystem does not achieve its expected perfor-mance within a specified time. Plans should bereviewed and approved by the appropriateregulatory agencies.
SITE SELECTION
Selecting an appropriate location can savesignificant costs. Site selection should considerland use and access, the availability of the land,site topography, soils, the environmental resourcesof the site and adjoining 1and, and possible effectson any neighbors. The site should be located asclose to the source of the wastewater as possible,and downgradient if at all possible so that watercan move through the system by gravity. While awetland can be fitted to almost any site, construc-tion costs can be prohibitively high if extensiveearthmoving or expensive liners are required.
A site that is well suited for a constructedwetland is one that:
l is conveniently located to the source of thewastewater
l is gently sloping, so that water can flow throughthe system by gravity
The effectiveness of a constructed wetland intreating wastewater or stormwater is related to theretention time of the water in the wetland. Theusefulness of a constructed wetland may therefore belimited by the size of the wetland needed for adequateretention time. The site selected should be largeenough to accommodate present requirements and anyfuture expansion.
V O L U M E 1: GENERAL C ONSIDERATIONS
l contains soils that can be sufficiently compacted tominimize seepage to groundwater
l is above the water table
l is not in a floodplain
l does not contain threatened or endangered species
l does not contain archaeological or historic resources.
L A N D U S E A N D A C C E S S
Access is an important consideration, The wet-land should be placed so that the water can flow bygravity. If the odors or insects could be a problem, aswith some agricultural wastewaters, the wetlandshould be placed as far from dwellings as possible.The site should be accessible to personnel, deliveryvehicles, and equipment for construction and mainte-nance.
For agricultural and some domestic wastewaters,the wetland may be installed on private land. Thelandowner must be carefully chosen. It is essentialthat the landowner is cooperative and fully under-stands the limitations and uncertainties associatedwith a developing technology such as constructedwetland treatment.
The current and future use and values of adjoiningland also will affect the suitability of a site for aconstructed wetland. The opinions of area residentsand those of environmental and public interest groupsshould be considered. A large buffer zone should beplaced between the wetland and neighboring property.The wetland should not be placed next to the edge ofthe property.
L A N D A V A I L AB I L IT Y
T O P O G RA P H Y
Landform considerations include shape, size,and orientation to the prevailing winds. While aconstructed wetland can be built almost anywhere,selecting a site with gradual slopes that can be easilyaltered to collect and hold water simplifies designand construction, and minimizes costs.
Previously drained wetland areas, includingprior converted (PC) agricultural sites, may be well-suited for a constructed wetland since the topogra-phy is usually conducive to gravity flow. Theappropriate regulatory agencies must be contactedbefore disturbing any PC site.
Since the best location for a constructed wetland isa low, flat area where water flows by gravity, it isimportant to ensure that the area is not already awetland: not all wetlands have standing water through-out the year. The Natural Resources ConservationService (NRCS), the US Fish and Wildlife Service, orstate regulatory personnel should be contacted todetermine whether or not a site contains jurisdictionalwetlands.
EN V I R O N M E N T A L R E S O U R C E S
To avoid damaging important resourceson the site, the presence or absence of significantenvironmental resources must be determined.Sources of information that can be helpful in select-ing a site include the US Geological Survey Topo-graphic Quadrangle maps, and National AerialPhotography Program (NAPP) and and National HighAltitude Photography Program (NHAPP) photo-graphs. Geographical information system (GIS) mapsare also available.
The National Wetlands Inventory (NWI) mapsand the County Soil Survey with the list of countyhydric soils should be checked for possible locationsof existing wetlands. However, the NW1 maps arebased on aerial photography and may not show smallwetlands or the less obvious wetlands (wet mead-ows, vernal pools, and some forested wetlands) andthe NW1 information should be field-checked by awetlands scientist. Historical aerial photography,
such as the Agricultural Stabilization and Conser-vation Service (ASCS) crop compliance photogra-phy and county soil survey information, can beuseful in identifying hydric soils and drainedwetlands that may be difficult to detect otherwise.
Surface and groundwater considerationsinclude possible flooding and drainage problems,location and depth of aquifers, and the location,extent, and classification of receiving waters suchas streams and groundwater. A constructed wet-land should not be sited on a floodplain unlessspecial measures can be taken to limit its impacton the floodway. Floodplain elevations can oftenbe determined from sources such as Federal FloodInsurance maps or from the Federal Land Manage-ment Agency. Landuser input may be the bestsource of information for assessing previoushydrologic conditions.
US Fish and Wildlife Service and state naturalresource agencies should be contacted regardingthe potential for significant habitat, or habitat forrare or endangered species. The possible presenceof archaeological resources should be verified.
P E R M I T S A N D R E G U L A T I O N S
The appropriate agency(ies) must be contactedto determine the regulatory requirements for aproposed constructed wetland and its discharge.Work in a waterway or natural wetland requires apermit. Discharges to natural waters also require apermit. In some zoned communities, zoningapproval may be required.
Any stormwater plan must meet local and statestormwater regulations. Some local ordinanceshave incorporated stormwater provisions whichmust be complied with. Stormwater regulationsvary from place to place and should be consultedbefore developing a stormwater management plan.The regulatory status of a proposed stormwaterwetland, and its relationship to streams and anynearby natural wetlands, must be discussed withthe state and/or federal wetland permitting agencybefore site plans are decided upon.
VOLUME 1: GENERAL CONSIDERATIONS 1 9
STRUCTURES
CELLS
Wetlands can be constructed by excavatingbasins, by building up earth embankments (dikes),or by a combination of the two.
Dikes must be constructed of soils with ad-equate fine-grained material that will compact intoarelatively stable and impervious embankment.The dikes should be high enough to contain theexpected volume plus ample freeboard to accom-modate occasional high flows as well as thebuildup of litter and sediment over time. Toensure long-term stability. dikes should be slopedno steeper than 2H:lV and riprapped or protectedby erosion control fabric on the slopes. An emer-gency spillway should be provided.
If multiple cells are used, divider dikes can beused to separate cells and to produce the desiredlength-to-width ratios. On steep sites, they can beused to terrace cells. Dikes can also be used tocontrol flow paths and minimize short-circuiting.Finger dikes are often used to create serpentine
flow paths and can be added to operational sys-tems to mitigate short-circuiting. Finger dikes can
be constructed of soils, sandbags, straw bales, ortreated lumber.
Bottom slopes are generally not critical. Anexception may be mine drainage wetlands that usesubsurface flow through deep beds of compost toinduce sulfate reduction; these cells should slopeabout 1 - 3% upstream. Bottoms should be rela-tively level from side to side.
Muskrats can damage dikes by burrowing intothem. Although muskrats generally prefer to starttheir burrows in water than is more than 3 ft deep,they can be a problem in shallower waters. Musk-rats can be excluded by installing electric fencelow to the ground or by burying muskrat-proofwire mats in the dikes during construction.
L I N E R S
Constructed wetlands must be sealed to avoidpossible contamination of groundwater and also to
prevent groundwater from infiltrating into thewetland. Where on-site soils or clay provide anadequate seal, compaction of these materials maybe sufficient to line the wetland. Sites underlainby karst, fractured bedrock, or gravelly or sandysoils will have to be sealed by some other method.It may be necessary to have a laboratory analyzethe construction material before choosing asealing method. On-site soils can be used ifthey can be compacted to permeability of<10 8 ft/sec (<10-6 cm/sec). Soils that containmore than 15% clay are generally suitable.Bentonite, as well as other clays, provide adsorp-tion/reaction sites and contribute alkalinity. TheSCS (now the NRCS) South National TechnicalCenter (SNTC) Technical Note 716, “Design andConstruction Guidelines for Considering Seepagefrom Agricultural Waste Storage Ponds and Treat-ment Lagoons” (1993) and its companion SNTCTechnical Note 717, “Measurement and Estimationof Permeability of Soils for Animal Waste StorageFacilities:’ (1991) provide guidance in determiningwhen in-situ soils will adequately meet seepagecontrol needs.
Synthetic liners include asphalt, synthetic butylrubber, and plastic membranes (for example, 0.5 to10.0 mil high density polyethylene). The linermust be strong, thick, and smooth to prevent rootattachment or penetration. If the site soils containangular stones, sand bedding or geotextile cushionsshould be placed under the liner to prevent punc-tures. The liner should be covered with 3 - 4 inchesof soil to prevent the roots of the vegetation from penetrating the liner. If the wetland is to be usedfor mine drainage, the reaction of the clay orsynthetic liner should be tested before it is usedsince some clays and synthetics are affected bysome acid mine drainages.
F L O W C O N T R O L S TR UC TUR ES
Water levels are controlled by flow controlstructures. Flow control structures should besimple and easy to adjust. They should allowflexibility so that processes can be optimized
20 V O L U M E 1: GENERAL C O N S I D E R T I O N S
initially and adjusted later in response to systemchanges. ‘Multiple inlets must be fully andindependently adjustable to ensure an evendistribution of flow. Structures should be sizedto handle maximum design flows and should belocated for easy access and to minimize short-circuiting. Boardwalks and piers make accesseasy and reduce disturbance of the wetland.PVC pipe is recommended.
If the wetland will be accessible to the public,or located in an isolated area where it will bevulnerable to vandalism, inlet and outlet struc-tures should be enclosed in lockable concrete
structures or manholes to avoid damage or tamper-ing with water level settings. Structures must beprotected against damage by animals. Measuresinclude installing covers or wire mesh over open-ings, and enclosing controls, gauges and monitor-ing devices in pipes or boxes.
In le ts
Inlets at SF wetlands are usually simple: anopen-end pipe, channel, or gated pipe whichreleases water into the wetland (figure 2). Thesmaller the length-to-width ratio, the more impor-
Inlet with Swiveling Tees Inlet with GabionInlet with BuriedDistributor Pipe
. ‘ iS l o t t e d o r P e r f o r a t e d
Soil. Sand or Gravel * Soil. Sand or Grave
Soil Cover Over LinerPLAN VIEW
P R O F I L E -
rWat
Plan View of OutletStructure withSwiveling Standpipe
Structure withCollapsible Tubing
Structure with SwivelingStandpipe
Figure 2. Inlet and outlet designs (modified from Watson and Hobson 1989).
V O L U M E 1 : GE N E R A L C O N S I D E R A T I O N S 2 1
tant equal flow distribution becomes. Accessibleand easily adjustable inlets are mandatory forsystems with small length-to-width ratios.
Inlet structures at SSF systems include surfaceand subsurface manifolds, open trenches perpen-dicular to the direction of flow, and simple single-point weir boxes. A subsurface manifold avoidsthe buildup of algal slimes and the consequentclogging that can occur next to surface manifolds,but is difficult to adjust and maintain. A surfacemanifold, with adjustable outlets provides themaximum flexibility for future adjustments andmaintenance, and is recommended. A surfacemanifold also avoids back-pressure problems.The distance above the water surface of the wet-land is typically 12 - 24 inches. The use of coarserock (3 - 6 inches, 8 - 16 cm) in the entry zoneensures rapid infiltration and prevents pondingand algal growth. To discourage the growth of
algae, open water areas near the outlet should beavoided. Shading with either vegetation or astructure in the summer and some thermal protec-tion in the winter will probably be necessary.
A flow splitter will be needed for parallelcells. A typical design consists of a pipe, flume,or weir with parallel orifices of equal size at thesame elevation (figure 31. Valves are impracticalbecause they require daily adjustment. Weirs arerelatively inexpensive and can be easily replacedor modified. Flumes minimize clogging inapplications with high solids but are more expen-sive than weirs.
OutletsAt SF wetlands, the water level is controlled
by the outlet structure, which can be a weir,spillway, or adjustable riser pipe. A variable-
Influent to Splitter Box (10”)
Valved Drain to Marsh ( Sampling Cinduit (6”)
Influent toMarsh (l0”)
Influent to Splitter
Sampling Conduit (6”)
Box (l0”)
otch Weir
Influent
Valved Drain (4”)
SECTION A
toMarsh (l0”)
SECTION B
Figure 3. lnfluent splitter box(modified from Watson and Hobson 1989).
V O L U M E 1: GENERAL C ONSIDERATIONS
height weir, such as a box with removablestoplogs., allows the water levels to be adjustedeasily. Spillways are simple to construct but arenot adjustable; incorrect water levels can lead towetland failure and correcting spillway height canbe difficult.
Weirs and spillways must be designed to passthe maximum probable flow. Spillways shouldconsist of wide cuts in the dike with side slopesno steeper than 2H:1V and lined with non-biode-gradable erosion control fabric. If high flows areexpected, coarse riprap should be used. Vegetatedspillways overlying erosion control fabric providethe most natural-looking and stable spillways.Weirs or spillways should be used for mine drainage wetlands since pipes tend to clog withdeposits of iron precipitates.
Adjustable riser pipes or flexible hoses offersimple water level control (figure 2). A PVCelbow attached to a swivel offers easy control ofthe water level. If pipes are used, small diameter(<l2 inch) pipes should be avoided because they
clog with litter.At SSF wetlands, outlets include subsurface
manifold, and weir boxes or similar gated struc-tures. The manifold should be located just abovethe bottom of the bed to provide for completewater level control, including draining. The useof an adjustable outlet, which is recommended tomaintain an adequate hydraulic gradient in thebed, can also have significant benefits in operatingand maintaining the wetland. The surface of thebed can be flooded to encourage the developmentof newly planted vegetation and to suppressundesirable weeds, and the water level can belowered in anticipation of major storms and toprovide additional thermal protection againstfreezing in the winter. The design of SSF bedsshould allow controlled flooding to 6 inches(15 cm) to foster desirable plant growth and tocontrol weeds. A perforated subsurface manifoldconnected to an adjustable outlet offers the maxi-mum flexibility and reliability as the outlet devicefor SSF systems. Since the manifold is buried andinaccessible after construction, careful gradingand subbase compaction are required during
VOLUME 1
construction, and clean-out risers in the line mustb e p r o v i d e d .
The final discharge point from the wetlandsystem should be placed high enough above thereceiving water that a rise in the water level in thereceiving water, for instance after a storm, will notinterfere with the flow of water through the wet-land.
SYSTEM LIFETIMES
A constructed wetland used for wastewatertreatment may have a finite lifetime which will bedetermined by wastewater loadings, the capacity ofthe wetland to remove and store contaminants, andthe buildup of litter. A number of systems havebeen operating for more than 20 years with little, ifany, loss of effectiveness. Long-term data on theperformance of constructed wetlands are beingacquired as more systems are being monitored forlonger periods of time. Data from the few con-structed wetland systems that have provided long-term data show that treatment performance forpollutants that are broken down in wetlands, suchas BOD5, total suspended solids (TSS), and nitro-gen, does not decrease as long as loadings arereasonable, and the wetland system is designed’.,built, and maintained with care.
For pollutants that are retained within awetland, such as phosphorous and metals, thecapacity of the wetland to remove and store thepollutants may decrease over time. The buildup ofthese substances must be monitored periodically toassess the wetland’s performance. Wetlands can besized to accommodate the accumulation of depos-its. It is generally assumed that deposition ofcontaminants in sediments and litter constitutes arelatively long-term sink for contaminants. Ifnecessary, wetland sediments and litter can beremoved periodically and the wetland rebuilt withfresh substrate.
GENERAL CONSIDERATIONS 23
CHAPTER 5HYDROLOGY
The hydrology of a constructed wetland isperhaps the most important factor in its effec-tiveness. However, the design of constructedwetland treatment systems is still in a state offlux and there remain a number of uncertaintiesthat will not be answered until the results oflonger and more numerous operational studiesbecome available. Many wetland designs havebeen based on the design used for conventiona)ponds and land treatment systems. While thedesign of conventional systems is usually basedon hydraulic residence time (and therefore watervolume), some wetland treatment systems showa more consistent correlation with area andhydraulic loading rate than with hydraulicresidence time (R. Kadlec, pers. comm.). Thisseems reasonable since a wetland is a shallowwater system with large surface area in relationto its volume, and receives energy inputs (sun,rain, propagules, gases) on an area1 basis that isnot related to volume. Also, because of the depth limits of wetland plants, the biomass ofmicrobes attached to plants and sediments doesnot increase proportionally to depth except in anarrow range. The design guidelines presentedin this Handbook are thus tentative.
Hydrologic factors in wetland design pertainto the volume of water, its reliability and ex-tremes, and its movement through the site.Hydrologic considerations include climate andweather, hydroperiod, hydraulic residence time,hydraulic loading rate, groundwater exchanges(infiltration and exfiltration), losses to theatmosphere (evapotranspiration), and overallwater balance.
CLIMATE AND WEATHER
Because wetlands are shallow water bodiesopen to the atmosphere, they are strongly influ-enced by climate and weather. Rainfall, snowmelt,spring runoff, drought, freeze, and temperature canall affect wetland treatment.
The high flows caused by heavy rains andrapid snowmelt shorten residence times. Theefficiency of a wetland may therefore decreaseduring rainfall and snowmelt because of increasedflow velocities and shortened contact times. Highflows may dilute some dissolved pollutants whileincreasing the amount of suspended material assediments in the wetland are resuspended andadditional sediments are carried into the wetlandby runoff. The first flush of runoff from a storm,often carries much higher pollutant concentrationsthan flows later in the storm. Taylor et al. (1993)found that intense storms during summer, whenconditions were generally dry, often had greaterimpacts on treatment than storms during othertimes of the year, when conditions were generallywetter. Snowmelt and spring runoff can resus-pend and export stored pollutants. Jacobson(1994) found that runoff during spring may carrymore than half the annual nitrate and phosphorusexported during the year and suggests that wetlandmanagement should focus on this time of the year.Runoff in excess of maximum design flows shouldbe diverted around the wetland to avoid excessiveflows through the wetland.
Minimum temperatures limit the ability ofwetlands to treat some, but not all, pollutants..Wetlands continue to treat water during coldweather. However, freezing temperatures inwinter and early spring can reduce treatment if thewetland either freezes solid or a cover of iceprevents the water from entering the wetland. Ifunder-ice water becomes confined, water veloci-ties may increase, thereby reducing contact times.
HYDROPERIOD
Hydroperiod is the seasonal pattern of waterlevel fluctuations and is described by the timing,duration, frequency, and depth of inundation. Thehydroperiod of a wetland results from the balanceof inflow, outflow, and storage. Hydroperioddetermines the availability of water throughout the
V OLUME 1: G ENERAL CONSIDERATIONS 25
year, the extreme wet and dry conditions that can beexpected, the extent of storage and drainage that maybe required, and the criteria to be used in designingthe water control facilities. While hydroperiod canbe engineered to control surface flow and to reduceits variability, the hydroperiod of a wetland will bestrongly affected by seasonal differences in precipita-tion and evapotranspiration.
HYDRAULIC RESIDENCE TIME
The hydraulic residence time (HRT) of a treat-ment wetland is the average time that water re-mains in the wetland, expressed as mean volumedivided by mean outflow rate. If short-circuitingdevelops, effective residence time may differsignificantly from the calculated residence time.
HYDRAULIC LOADING RATEHydraulic loading rate (HLR) refers to the
loading on a water volume per unit area basis.[loading = (parameter concentration)(water vol-ume/area)].
GROUNDWATER EXCHANGEThe movement of water between a ‘wetland add
groundwater will affect the hydrology of thewetland. Constructed wetlands for domesticwastewater, agricultural wastewater, and minedrainage are usually lined to avoid-possible con-tamination of groundwater. If the wetland isproperly sealed, infiltration can be considerednegligible.
Many stormwater wetlands are sealed so thatthat water needed to support the wetland will beretained between storms. Other stormwater wet-lands are designed to intercept groundwater toensure sufficient baseflow. In this case, the wet-land will receive groundwater when the watertable is high and may discharge to groundwaterwhen the water table is low.
EVAPOTRANSPIRATION
Evapotranspiration (ET) is the combined waterloss through plant transpiration and evaporationfrom the water surface. In wetlands, the amount ofsurface area is large relative to the volume of waterand ET is an important factor. Also, many wetlandplants do not conserve water during hot, dryweather as most terrestrial plants do, and cantransfer considerable amounts of water from awetland to the atmosphere in summer. If ET lossesexceed water inflows, supplemental water will berequired to keep the wetland wet and to avoidconcentrating pollutants to toxic levels.
Estimates of ET values vary widely. The WaterPollution Control Federation (1990) suggests that,for wetlands that are continuously flooded, ET cangenerally be estimated as being equal to lakeevaporation, or approximately 70% to 80% of panevaporation values. (Rainfall and pan evaporationdata can be obtained from National Oceanic andAtmospheric Administration in Asheville, NC, orfrom local weather stations.) Kadlec (1993) foundthat dense stands of emergent vegetation reducedthe total water loss from prairie potholes andconcluded that the vegetation removed less waterthrough transpiration than would have evaporatedfrom open surface water. Other data indicate thatmost wetlands show ET to be equal to or slightlyless than pan evaporation and that experimentsthat show higher ET rates have been conducted ontoo small a scale to compensate for edge effects.
WA T E R B A L A N C E
The overall water balance for a constructedwetland is an account of the inflow, storage, andoutflow of water. Water inflow to the wetlandincludes surface water (the wastewater orstormwater), groundwater infiltration (in unlinedwetlands), and precipitation: Storage is thesurface water plus that in the pore spaces of thesubstrate. Outflow comprises evaporation fromthe water surface, transpiration by plants, effluentdischarge, and exfiltration to groundwater. During
2 6 V O L U M E 1: GENERAL C ONSIDERATIONS
design and operation, the wetland water balance isimportant for determining conformance withdesired limits for HLR, hydroperiod range, HRT,and mass balances. A simple water balanceequation for a constructed wetland is expressed as:
S = Q + R + I - O - E T (5.1)
Where: S = net change in storageQ = surface flow, including wastewater
or stormwater inflow,R = contribution from rainfallI = net infiltration (infiltration less
exfiltration)O = surface outflowET= loss due to evapotranspiration.
Equation 5.1 can be used to calculate waterbudgets for daily, monthly, or yearly intervals.Detailed water balances can be prepared withsite-specific monitoring data collected duringpilot- or full-scale operation of the wetland. Iflarge seasonal variation is expected, monthlydata are essential.
A number of factors can be used to manipu-late the water budget:
l the volume of water released from the wetlandcan be varied
l evapotranspiration rates can be altered byshading, windbreaks, and the selection andmanagement of vegetation around the wetland
l storage capacity can be adjusted with watercontrol structures
l in SF wetlands, storage capacity can be in-creased by excavating deep pools or decreasedby adding fill.
V OLUME I : GENERAL CONSIDERATIONS 2 7
CHAPTER 6SUBSTRATES
Wetland substrates support the wetlandvegetation, provide sites for biochemical andchemical transformations, and provide sites forstorage of removed pollutants. Substrates includesoil, sand, gravel, and organic materials.
SOILMany soils are suitable for constructed wet-
lands. Soil properties that should be considered inselecting soils include cation exchange capacity(CEC), pH, electrical conductivity (EC), texture, andsoil organic matter.
The pH of the soil affects the availability andretention of heavy metals and nutrients. Soil pHshould be between 6.5 and 8.5. The EC of a soilaffects the ability of plants and microbes to processthe waste material flowing into a constructedwetland. Soils with an EC of less than 4 mmho/cmare best as a growth medium.
The surface area of the soil particles and‘the electrical charge on the surfaces of the soilparticles account for much of a soil’s activity., In thenortheastern United States, most soils carry a netnegative charge, thus providing electrostatic bond-ing sites for positively charged ions (cations), suchas Ca2+, Mg2+, Fe2+, A13+ and Mn2. These cations onthe soil surface can exchange with other cations inthe soil solution, hence the term cation exchange.CEC measures a soil’s capacity to hold positivelycharged ions and varies widely among differentsoils. The CEC of a soil that will be used asa planting medium should be greater than15 meq/l00 g of soil.
The redox potential of the soil is an importantfactor in the removal of nitrogen and phosphorus.A reducing substrate must be provided to promotethe removal of nitrate and ammonia. The removalof iron and manganese from mine water alsorequires a reducing environment.
A soil’s capacity to remove and retain contami-nants is a function of soil-water contact. Sandy orgravely soils have high K (porosity) values and
water moves quickly through the soil. In contrast,the finer textures of silty or loamy soils promotelonger soil-water contact. Flow through well-decomposed organic soils and most clays is slow.
The soil must provide enough organic matter.to fuel plant growth and mocrobial activity,particularly during startup. Wetlands are oftenbuilt with infertile site soils, and organic amend-ments, such as compost, leaf litter, or sewagesludge, must be incorporated into the substrate.
Soil texture affects root growth and the reten-tion of pollutants. Sandy, coarse-textured soilshave a low potential for pollutant retention butlittle or no restriction on root growth. Thesesoilshold plants well but are low in nutrients. Addi-tions of organic matter to coarse textured soilshave been shown to improve plant survival andgrowth during the first several years while theorganic litter is beginning to build up within thewetland. Medium textured or loamy soils are agood choice, as these soils have high retention ofpollutants and little restriction on plant growth.Loamy soils are especially good because they aresoft and friable, allowing for easy rhizome and rootpenetration. Dense soils, such as clays and shales,should be avoided because they may inhibit rootpenetration, lack nutrients, and have low hydrau-lic conductivities.
Soils with a high clay content aid in phospho-rous retention but their low nutrient content maylimit growth and development, although such soilsmay be suitable for wetlands used for nutrient-richwastewaters, such as agricultural and domesticwastewaters. Organic admendments will berequired. Soils with greater extractable aluminumhave greater potential for phosphorous assimila-tion than do organic soils, making them wellsuited for domestic wastewater treatment. Highlyorganic soils enhance sulfate reduction and ionicadsorption and are well suited for mine drainagewetlands.
Although peats are common in natural wet-lands, they are not the preferred soil for establish-ing constructed wetlands. Peats can release
V O L U M E 1 : GE N E R A L C O N S I D E R A T I O N S 29
organic acids, which contribute to low pH.Also, when flooded, peats have a soft, loosetexture that may not provide adequate supportfor plants.
The county soils maps, which are availablethrough libraries or through the county NRCSoffices, show the major soil types present andtheir relationship to site topography. The soilsmaps include a general description of the soilcharacteristics. However, the NRCS soils mapscannot be relied upon for detailed, site-specificinformation for several reasons:
ORGANIC MATERIAL
Stabilized organic material, such as spentmushroom compost, sawdust, hay or straw bales,and chicken litter, have been used as organicsubstrates. Organic material provides a source ofcarbon to support microbial activity. Organicmaterial also consumes oxygen and creates theanoxic environments that are required for sometreatment processes, such as nitrate reduction andthe neutralization of acidic mine drainage.
l the NRCS data are averages and estimatestallied over many acres of ground
l most soils units include inclusions that maydiffer in significant ways from nearby soils
l soils vary with depth, that is, they are stratified.If the wetland is to be excavated, it is importantto know the characteristics of the soil at theexcavated depth.
Soils should ‘be analyzed before they are usedin the wetland. Site-specific information on thehydraulic conductivity and permeability of the sitesoils must be made through field data collection.Laboratory soil analyses should include claycontent and type of clay, percent organic matter,and mineral content.
SAND AND GRAVELConstructed wetlands receiving with water
high in nutrients, such as domestic and agricul-tural wastewaters. can be built with sand or gravel.Sand is an inexpensive alternative to soil andprovides an ideal texture for hand planting.Gravel can also be used. Many domestic sewageSSF wetlands in the United States have usedmedia ranging from medium gravel to coarse rock.Sands and gravels dry out quickly and may needto be irrigated to maintain water levels while thevegetation is becoming established.
3 0 V OLUME 1 : GENERAL C ONSIDERATIONS
CHAPTER 7VEGETATION
The function of plants in constructed wetlandsis largely to grow and die: plant growth provides avegetative mass that deflects flows and providesattachment sites for microbial development; deathcreates litter and releases organic carbon to fuelmicrobial metabolism. In addition, plants stabilizesubstrates while enhancing its permeability, andplants add greatly to the aesthetic value of thewetland. A dense stand of vegetation appears tomoderate the effects of storms.
SELECTING PLANTS
The plants that are most often used in con-structed wetlands are persistent emergent plants,such as bulrushes (Scirpus), spikerush(Efeocharis), other sedges (Cyperus). rushes(Juncus), common reed (Phragrnites), and cattails(Typha). Not all wetland species are suitable forwastewater treatment since plants for treatmentwetlands must be able to tolerate the combinationof continuous flooding and exposure to wastewa-ter or stormwater containing relatively high andoften variable concentrations of pollutants. Anumber of species that have been used success-fully in the northeastern United States are listedtable 1.
in
For wastewater treatment wetlands, the par-ticular species selected are less important thanestablishing a dense stand of vegetation. Anyspecies that will grow well can be chosen. Forstormwater wetlands, species should be chosen tomimic the communities of emergent plants ofnearby natural wetlands. For both wastewater andstormwater wetlands, native, local species shouldbe used because they are adapted to the localclimate, soils, and surrounding plant and animalcommunities, and are likely to do well.
NRCS conservation agents and state personnelcan recommend species for constructed wetlands.
S U R F A C E F L O W W E T L A N D S
In wetlands constructed to treat domesticsewage, agricultural wastewaters, and otherwastewaters relatively high in organic matter,bulrushes (either softstem or common three-square) are often used because they are tolerant ofhigh nutrient levels and because they establishreadily but are not invasive. Arrowhead andpickerelweed have also been used successfully inagricultural wetlands. Blueflag iris can be plantedalong wetland edges to provide color. Cattails andcommon reed have been used frequently becauseof their high tolerances for many types of wastewa-ter, but both have disadvantages. Cattails areinvasive. Since cattail tubers are a favorite food ofmuskrats, cattails are susceptible to damage bymuskrats. Also, Surrency (1963) found thatcattails were subject to attack by insects similar toarmy worms and suggests that cattails may not bethe best choice for agricultural wetlands. Commonreed is a highly aggressive species that can elimi-nate other species once it is introduced. It pro-duces abundant windborne seed and spreadsreadily to natural wetlands. It is becoming aproblem in the Northeast and should not be usedwithout approval from the regulatory agency.
For agricultural wastewater wetlands, theammonia tolerances of the species must be consid-ered. Wetland species vary in their ability totolerate ammonia. Plants may be able to toleratehigher concentrations of ammonia if the plants areslowly acclimated to it.
For stormwater wetlands, the goal should be adiverse assemblage of plants. A diverse vegetationis aesthetically pleasing and may be more likely toresist invasive species, to recover from distur-bance, and to resist pests than a less diverse stand.The numbers of wildlife attracted to a wetlandgenerally increases as vegetation diversity in-creases.’ The State of Maryland guidelines forstormwater wetlands suggest planting two primaryspecies (some combination of arrowhead, commonthree-square, or softstem bulrush) and three other
V O L U M E 1 : GENERAL C ONSIDERATIONS 31
Table 1. Emergent plants for constructed wetlands(adapted from Schueler 1992 and Thunhorst 1993).
Recommended Species Maximum NotesWater Depth*
Arrow arumPeltandra virginica
12 inches Full sun to partial shade. High wildlife value.Foliage and rootstocks are not eaten by geeseor muskrats. Slow grower. pH: 5.0-6.5.
Arrowhead/duck potatoSaggitaria latifolia
12 inches Aggressive colonizer. Mallards and muskratscan rapidly consume tubers. Loses much waterthrough transpiration.
Common three-squarebulrushScirpus pungens
Softstem bulrushScirpus validus
6 inches Fast colonizer. Can tolerate periods of dryness.High metal removal. High waterfowl andsongbird value.
12 inches Aggressive colonizer. Full sun. High pollutantremoval. Provides food and cover for manyspecies. of birds. pH: 6.5-8.5.
Blue flag irisIris versicolor
3 - 6 inches Attractive flowers. Can tolerate partial shadebut requires full sun to flower. Prefers acidicsoil. Tolerant of high nutrient levels.
Broad-leaved cattail**Typha latifolia
Narrow-leaved cattail**Typha angustifolio
12-18 inches
12 inches
Aggressive. Tubers eaten by muskrat andbeaver. High pollutant treatment, pH: 3.0-8.5.
Aggressive. Tubers eaten by muskrat andbeaver. Tolerates brackish water. pH : 3.7-8.5.
Reed canary grassPhalaris arundinocea
Lizard’s tailSaururus cernuus
6 inches
6 inches
Grows on exposed areas and in shallowwater. Good ground cover for berms.
Rapid grower. Shade tolerant. Low wildlifevalue except for wood ducks.
PickerelweedPontedaria cordata
Common reed** Phragmites australis
12 inches
3 inches
Full sun to partial shade. Moderate wildlifevalue. Nectar for butterflies. pH: 6.0-8.0.
Highly invasive; considered a pest species inmany states. Poor wildlife value. pH: 3.7-8.0.
V O L U M E 1: GENERAL C ONSIDERATIONS
ommended Species MaximumWater Depth*
Notes
Soft rushJuncus effusus
3 inches Tolerates wet or dry conditions. Food for birds.Often grows in tussocks or hummocks.
SpikerushEleocharis palustris
3 inches Tolerates partial shade.
SedgesCarex spp.
3 inches Many wetland and several upland species.High wildlife value for waterfowland songbirds.
Spatterdock 5 ft,Nuphar luteum 2 ft minimum
Tolerant of fluctuating water levels. Moderatefood value for wildlife, high cover value.Tolerates acidic water (to pH 5.0).
Sweet flagAcorus calamus
3 inches Produces distinctive flowers. Not a rapidcolonizer. Tolerates acidic conditions.Tolerant of dry periods and partial shade.Low wildlife value.
Wild riceZizania aquatica
12 inches Requires full sun. High wildlife value (seeds,plant parts, and rootstocks are food for birds).Eaten by muskrats. Annual, nonpersistent.Does not reproduce vegetatively.
l These depths can be tolerated, but plant growth and survival may decline under permanent inundation at thesedepths.
l **Not recommended for stormwater wetlands because they are highly invasive, but can be used in treatmentwetlands if approved by regulatory agencies
V OLUME 1: GENERAL CONSIDERATIONS 3 3
secondary species (see table 1) to enhance short- andlong-term development and to reduce the invasion byundesirable plants such as common reed (Livingston1989).
S U B SU R F A C E F L O W W E T L A N D S
Many of the SSF constructed wetlands in theUnited States have used bulrush, common reed,cattail, or some combination of the three. About40% of the operational SSF systems use onlybulrush. Common reed has been widely used inBritish and European systems; however, it is a highly invasive species that can be very difficult toeradicate once started and a number of states nowprohibit its use. Some SSF systems have beenplanted with a diverse vegetation similar to that of anatural marsh.
SOURCES OF PLANTS
Seeds, seedlings, entire plants, or parts of plants(rootstocks, rhizomes, tubers, or cuttings) can be usedto establish wetland vegetation. While many wetlandplants produce wind-borne seeds, vegetative spreadby stolons and runners is common since seeds gener-ally will not sprout under water. Many emergentshave rhizomes, rootstocks, or tubers which, althoughthey are primarily food storage organs, can generatenew plants.
S E E D S
Seeds are the least expensive but also the leastreliable approach to planting. Seeds are generallybroadcast on the saturated surface of the wetland.Seeds can also be scattered by shaking ripe spikes ofplants over the wetland surface. Germination isunpredictable. Propagation by seeds requires anexposed, wet surface on which the seeds can germi-nate. Water levels can be raised as the plants grow,but the leaves must remain above water since theplants must be able to photosynthesize and transpireif they are to grow.
Seed stands are typically difficult to establishbecause scarification (abrasion of seed coat) andstratification (exposure to cold) requirements arelargely unknown and because seeds are easilymoved about by rain. However, many wetlandspecies produce abundant wind-borne seed andwill appear quickly on newly exposed surfaces ifthere is another wetland in the area to act as asource of seed.
W ETLAND SO I L
The seeds of many wetland species remainviable for many years buried in sediments. Soilfrom a nearby wetland can be used as a source ofplants since this soil will contain seeds of a numberof native species that are well-adapted to localconditions. Approval must be obtained from theappropriate agency before removing wetland soil.Soil must not be taken from natural wetlandswithout a permit.
Cores (3 - 4 inch, or 8 - 10 cm, in diameter) ofwetland soil from the donor marsh can be trans-planted to the constructed wetland. Cores areexcellent sources of seeds, shoots, and roots ofvarious wetland plants and will promote the devel-opment of diverse wetlands. The disadvantages ofsoil cores are the time and cost associated withcollecting, transporting, and planting the soil mass.Also, the soil is likely to contain propagules ofundesirable, as well as desirable, species. If thecores are taken from a wetland dominated by aspecies that spreads by rhizomes (such as cattails),the resulting wetland will probably be dominated bythat species since earthmoving cuts the rhizomesinto pieces, each of which can produce a new plant.
Once in place, the soil should be kept moist, butnot flooded, until the seeds germinate.
RHIZOMES, TUBERS ,AND E NTIRE PLANTS
Plant materials include entire plants and plantparts, such as rhizomes and tubers. These materi-
3 4 V O L U M E 1: GENERAL C ONSIDERATIONS
als are generally obtained from commercialnurseries or donor wetland sites (on-site nurser-ies or nearby constructed wetlands).
Plants should be obtained from localsources. The US Army Corps of Engineers(1993) recommends that plants should be trans-ferred from areas within 100 miles latitude, 200miles longitude, and 1,000 feet in elevation butnotes that ecologists are expressing concernabout the unknown consequences of relocatinggenetic stock to new areas. For example, plantsbecome adapted to local pathogens, as well asbeneficial mutualistic species, and their survivaland growth are diminished when they aretransplanted to different areas. The State ofFlorida now recommends a 50-mile radius forobtaining plants.
Locally-grown nursery stock is generally themost reliable and ecologically appropriate wayto obtain plants. The NRCS Plant Materials
Centers test and develop plants for various‘applications and can provide plants. On-sitenurseries established to provide plants for theproject offer convenience, reliability, and lowcost. Outdoor beds can be constructed at the-site of the construction project. The beds usu-ally consist of pits 12 - 20 inches (30 - 50 cm)deep dug into the soil, lined with plastic film toprevent water loss, and partially filled with 8 - 12inches (20 - 30 cm) of soil. Water inlets andoutlets for the beds should be adjustable so thatwater levels can be varied to accommodate thedifferent stages of germination and growth.
Rhizomes and tubers are usually collectedin the late fall after growth has stopped or inearly spring before new growth begins. Theentire root system should be taken along withsome soil. including the soil helps to inoculatethe constructed wetland with microbes fromthe donor wetland.
Rhizomes generally are cut into lengths oftwo to three nodes, placed in moist peat or sand,and stored at cool temperatures (40 oF, 4 - 5 o C )until planting. Storage at cool temperaturesstratifies the tubers and rhizomes (prepares them
V O L U M E 1
for growth) and enhances growth after springplanting. A common pitfall is to store plantsunder conditions that are too dry or too warm.Collecting plants in the spring and plantingthem immediately at the new site may reducethe mortality of rhizomes and rootstocks, butcollection is more difficult because of the highwater levels in most wetlands in the spring.
Manual collection is more common thanmachine collection, which is not practical unlesslarge quantities of plants are needed or the projectis a long-term one. Manual harvesting equipmentincludes modified garden tools, bags, buckets, andboats or canoes. Vegetative propagules are usuallyobtained by hand-digging whole plants and cuttingapart roots, rhizomes, and tubers. At sites wherewater levels can be controlled, machinery can beused to dig underground parts.
Private wetland nurseries are becoming morewidespread and can custom-propagate stock forwetlands if given enough advance notice. Somecommercial nurseries will recommend species andwill do the planting.
WHEN TO PLANT
In the Northeast, the planting period typicallybegins after dormancy has begun in the fall andends after the first third of the summer growingperiod has passed. Fall dormant planting isrecommended for tubers and rootstock and is verysuccessful for bulrushes, rushes, and arrowhead.Sedges and cattails are grown more successfully inthe spring after dormancy has been broken. Plant-ing early in the spring growing season is generallysuccessful. The NRCS Field Office TechnicalGuide (FOTG) or local nurseries can be consultedabout planting times.
SITE PREPARATION
For SF wetlands, the site should be disked orharrowed to break up compacted soil once thewetland has been shaped and graded. The bedshould then be shallowly flooded to settle the soil
GENERAL CONSIDERATIONS 35
and level the bed. An extended settling period(a year or to the next growing season) should beprovided, if possible. Constructed wetlands areoften built in the fall and left flooded over thewinter. The bed is then dewatered (but not com-pletely dried) shortly before planting to produce asoft, moist soil.
HOW TO PLANT
Planting is usually done by hand. Few sophis-ticated planting techniques have been applied towetland planting.
S U R F A C E F L O W W E T L A N D S
Dormant propagules, such as tubers andrhizomes, are planted by simply placing themdeep enough in the substrate to prevent them fromfloating out of the medium. Approximately1 - 2 inches (2 - 5 cm) of stem should be left on thetubers so the plants can obtain oxygen through thestem when the wetland is flooded. Tubers ofarrowhead and softstem bulrush are often plantedin this way.
For bareroot plants and tubers, a tree plantingbar (dibble) or tile spade is a good tool. A slit ismade in the substrate, the propagule inserted, andthe slit sealed. The propagule must be planteddeep enough to prevent it from floating out of theplanting hole. For tall plants such as cattails, thestems should be broken over or cut back to 1 ft toprevent windthrow. Wetland cores and pottedplants are placed in small holes dug with a shovel.Temporary anchoring may be needed if the sub-strate is’ soft, the plants are buoyant, or erosioncould disturb the existing system.
Vegetative propagules are usually spaced at1 - 3 ft (0.3 - 1 m) intervals, depending on howrapidly the project must be completed. Clusteredrather than ,uniform arrangements provide speciesand spatial diversity and better simulate a naturalwetland. If the plants are planted in rows, runningthe rows perpendicular to the direction of flow
will improve coverage and reduce channelingwhile the vegetation is filling in. When the plantshave been placed, enough clean water should beadded to maintain good saturation of the substrate,but not flood it. After new growth has reached4 - 5 inches (10 - 12 cm), the water level can beraised. The water must not overtop the plants forextended periods or the plants will die.
S U B S U RF A C E F L O W W E T L A N D S
Most SSF wetlands are planted by hand. Theuse of individual root/rhizome material withgrowing shoots at least 8 inches (0.2 m) long isrecommended. If mature, locally availableplants are used, they can be separated intoindividual root/rhizome/shoot units containingroot, rhizome, and shoot, with the mature stemcut back to <1 ft (<0.3 m) before planting.
The root/rhizome should be placed in themedium at a depth equal to the expected opera-tional water level. The growing shoot shouldproject above the surface of the media. Toencourage deeper root penetration, some Euro-pean systems lower the water level in the bed inthe fall. In Europe, three years is necessary forthe roots of common reed to reach their 2 ftpotential depth. An alternative to lowering thewater level in the entire wetland is to divide thesystem into two or more parallel cells and toalternate drawdown between the cells to encour-age root penetration in the dormant cell.
ESTABLISHING ANDMAINTAINING VEGETATIONIn SF wetlands, water level is the most critical
aspect of plant survival during the first year afterplanting. A common mistake is to assume thatbecause the plant is a wetland plant, it can toleratedeep water. Frequently, too much water createsmore problems for wetlands plants during the firstgrowing season than too little because the plantsdo not receive adequate oxygen at their roots. For
V OLUME 1: GENERAL C ONSIDERATIONS
best survival and growth of small stalks(1 - 2 inches) during the first growing season,the substrate should only be saturated, notflooded. As the plants become well-established(2 - 3 months), water levels can be raised.
Mechanical protection may be needed toprevent animals from damaging newly establishedplants. Canada geese cause significant depredationby grazing on young shoots and seedlings and byuprooting rhizomes and tubers. Deer and black-birds can also damage newly established seedlings.Muskrats feed on the fleshy tubers of plants such ascattails and can quickly demolish a cattail wetland.Preventive methods include planting throughchicken wire fence fastened over the surface of thesubstrate to prevent animals from excavating tubersand rhizomes.
Plantings should be allowed to become wellestablished before the wastewater is introduced intothe system since the plants need an opportunity toovercome the stress of planting before other stressesare introduced. The water must supply enoughnutrients to support plant growth. If not, a solutionof commercial nutrient supplement should beadded. Satisfactory establishment may take fromseveral months to one or two full growing seasons.The plants may not begin to reach maturity andequilibrium until late in the second growing season.A gradual rather than sudden increase in theconcentration of the wastewater applied reducesshock to the vegetation. Alternatively, if plants arereadily available and inexpensive, some die-off andreplanting can be planned for in order to apply thewastewater sooner.
Water level management is key to maintainingwetland vegetation. Despite relatively broad depthtolerances, freshwater plants often sort by smallvariations in water depth, producing the apparentzonation of vegetation along the shores of marshes.Most wetland species are adapted to daily orseasonal fluctuations in water level but mostwetland plants can tolerate neither extendedperiods of flooding nor drying of their roots.
Water quality also affects the health andsurvival of wetland plants. High nutrient loads,high or low pHs, high dissolved solids concentra-tions, and buildup of heavy metals and othertoxics can affect the vegetation in wetlands.Constant pollutant loads work against speciesdiversity and favor pollution-tolerant species suchas cattails. In wetlands constructed to treatdomestic wastewater and mine drainage, Kadlec(1989) and Webster et al. (1994) found that plantdiversity declined and dominance by cattailsincreased as the wetlands aged.
Harvesting or winter burning of above-ground.biomass is sometimes used as a means of removingnitrogen and carbon and maintaining the wetlandvegetation in a log (growth) phase of high physi-ological activity to enhance removal, but maydisrupt the wetland and the maturation of theplant community. Decisions as to whether or notto harvest will depend on the objectives of theproject and will be site-specific.
V O L U M E 1: GENERAL C ONSIDERATIONS 3 7
CHAPTER 8CONSTRUCTION
Wetlands should be designed and constructedto provide reliability and ‘safety. Standard engi-neering techniques should be used. It is impor-tant to use a skilled contractor since elevationsmust be accurate to assure proper hydraulicregimes, and compaction requirements must bemet to control infiltration and exfiltration and toensure berm stability. It is also important to havesomeone on site who is familiar with the plans,tolerances, and overall wetland objectives toanswer the questions that always arise duringconstruction.
CONSTRUCTION PLANS
Construction plans and specificationsdeveloped from treatment area requirementsand siting investigations should be carefullyreviewed. The level of detail depends onthe size and complexity of the wetland, thephysical characteristics of the site, and therequirements of the regulatory agencies. Ata minimum, construction plans must have suffi-cient detail for accurate bid preparationand for construction.
A pre-bid conference with potential contrac-tors is recommended to explain the concept,goals, and requirements of the project. Thismeeting can be effective in soliciting accuratebids from qualified contractors.
PRE-CONSTRUCTION ACTIVITIES
A preconstruction conference should always beheld to interpret and explain the intent of the plansto the operator and the contractor. Many contrac-tors who are experienced with other kinds ofconstruction may have had little experience inbuilding wetlands. Construction plans, specifica-tions, and field layout must portray to the operatorand the contractor the desired work. Because ofwide variations in conditions and experience, plans
may vary from very simple plans and a fewstakes in the ground to complicated plans withdetailed specifications and extensive staking.Pre-construction activities should be consistentwith the size and complexity of the site andadequate to assure orderly and effective con-struction.
CONSTRUCTION ACTIVITIES
Construction includes building access roads;clearing; constructing basins and dikes; piping andvalving; planting; and seeding. liming, fertilizing,and mulching dikes and disturbed areas. Avaluable reference document for constructedwetlands is the Engineering Field Handbook (SCS1992), especially Chapter 13: “Wetland Restora-tion, Enhancement. or Creation”. EPA’s 1993publication Wetland Creation and Restoration:Status of the Science, Chapters 2, 3, 4, 13, and 17,is also recommended. Both are available throughthe National Technical Information Service (NTIS),US Department of Commerce, 5285 Royal Road, Springfield, VA, 22161; telephone (703) 487 4650.
Use of the correct type and size of heavyequipment is crucial for proper and cost-effectiveconstruction. Under- and over-sized equipmentcan result in time and cost overruns. It is a goodpractice to show the equipment operator(s) the siteduring the planning stages to obtain his or heropinion on equipment, time requirements, andpotential problems.
Construction must precisely follow the engi-neering plan ‘if the system is to perform properly.If shallow sheet flow is desired, lateral bed slopeshould not vary by more than 0.1 ft from high spotto low spot since large slope or surface variationscan cause channeling, especially in systems withhigh length-to-width ratios. Permeability specifi-cations must be followed carefully to preventleakage into or out of wastewater wetlands, if thisis part of project specifications. If synthetic liners
V O L U M E 1: GENERAL C O N S I D E R A T I O N S 39
are required, installation should follow preciselythe manufacturer’s instructions for beddingmaterial. sealing (liner-to-liner and liner-to-pipingand control structures), and material placement ontop of the liner.
SSF systems depend on high hydraulicconductivities in the substrate, and specialprovisions must be taken to avoid compactingand rutting of the substrate during construction.The importance of using low-ground-pressureequipment in the wetland and of controllingsmall machine or foot traffic to reduce compac-tion must be made clear to the contractor. Walkboards should be placed on the substrate duringhand planting:
The hydraulic performance of both SF andSSF systems can be significantly influenced byimproper construction, and short-circuiting offlow can often be traced to improper construction.In particular, SSF beds must be carefully con-structed because of the heavy traffic involved inplacing the rock or sand medium. In severalcases, carefully graded beds were seriously dis- rupted when the trucks delivering the rock wereallowed access to the beds during wet weather.One solution is to prohibit trucks from driving onthe medium. Trucks should back in to unload atthe edge of the area already covered. Subgradescan also be stabilized with geotextiles.
Muskrats and beavers can burrow into dikes orobstruct discharge pipes. Muskrat damage can beminimized or prevented by installing hardwarecloth (metal screen) vertically in the dikes or byriprapping both upstream and downstream slopes.The initial cost is small compared to replacing thedike. If beavers are likely to be a problem, statewildlife personnel should be consulted to developa plan to prevent their unwanted damming.
INSPECTION, STARTUP,AND TESTING
Before accepting the final product, the wet-land should be flooded to design depth and allcomponents, such as pumps and water control
structures, should be thoroughly tested to ensurethat they are operating properly and to check thatwater levels and flow distributions meet expecta-tions.
During the initial operation, any erosion andchanneling that develops should be eliminated byraking the substrate and filling by hand. Rills onthe dike slopes and spillways should be filled withsuitable material and thoroughly compacted.These areas should be reseeded or resodded andfertilized as needed. If there is seepage under orthrough a dike, an engineer should be consulted todetermine the proper corrective measures. Startupof a new wetland system is a critical time. Systemstartup comprises the filling and planting of thewetland and a period in which the soil/media,plants, and microbes adjust to the hydrologicconditions in the wetland. Like all living systems.wetlands are better able to tolerate change if theyhave been allowed time to stabilize initially.
After the initial stabilization period, a gradualincrease in wastewater flow to allow the system toadjust to the new water chemistry is often wiserthan immediately operating at the ultimate flow.In Europe, a full growing season is often allowedbefore wastewater is added. Much shorter stabili-zation periods (several weeks to several months)are typical in the United States. Wastewatershould not be added until the plants have shownnew growth, indicating that the roots have recov-ered from transplanting. Highly concentratedwastewaters, such as some agricultural wastes.will require a more gradual introduction than willless concentrated waters, such as stormwater orpre-treated sewage effluent.
40 V O L U M E 1: GE N E R A L C O N S I D E R A T I O N S
CHAPTER 9OPERATION, MAINTENANCE, AND MONITORING
OPERATION ANDMAINTENANCE
Wetlands must be managed if they are toperform well. Wetland management should focuson the most important factors in treatment perfor-mance:
l providing ample opportunity for contact of thewater with the microbial community and withthe litter and sediment
l assuring that flows reach all parts of the wetland
l maintaining a healthy environment for microbes
maintaining a vigorous growth of vegetation.
OPERATION AND MAINTENANCE PLAN
Operation and maintenance (O&M) should bedescribed in an O&M plan written during thedesign of the constructed wetland system. Theplan can be updated to reflect specific systemcharacteristics learned during actual operation.The plan should provide a schedule for routinecleaning of distribution systems and weirs, dikemowing and inspection, and system monitoring.The plan should specify those individualsresponsible for performing and paying formaintenance. The plan should address:
l setting of water depth control structures
l schedule for cleaning and maintaining inletand outlet structures, valving, and monitoringdevices
l schedule for inspecting embankments andstructures for damage
l depth of sediment accumulation before re-moval is required
l operating range of water levels, includingacceptable ranges of fluctuation
the supplemental water source to be used toensure adequate water levels during establish-ment and operation
wastewater application schedule, if this is partof the system design. An application scheduleshould be selected that is both convenient andrelatively continuous. Short, high-flow dis-charges to a wetland are more likely to erode ordamage established vegetation than lowervelocity, more continuous flows.
scheduling discharges to or from the wetland,recycling/redirecting flows, or rotating betweencells, if such are part of the design.
HYDROLOGY
In SF wetlands, water should reach all parts ofthe wetland surface. The wetland should be peri-odically checked to ensure that water is movingthrough all parts of the wetland, that buildup ofdebris. has not blocked flow paths, and that stagnantareas have not developed. The importance ofassuring adequate water depth and movementcannot be over-emphasized. Stagnant water de-creases removal and increases the likelihood ofmosquitoes and unsightly conditions. Flows andwater levels should be checked regularly.
SSF wetlands should be checked to see thatsurface flow is not developing.
S T R U C T U R E S
Dikes, spillways, and water control structuresshould be inspected on a regular basis and immedi-ately after any unusual flow event. Wetlandsshould be checked after high flows or after rapid icebreak-up; both can scour substrates, particularly atoutlets. Any damage, erosion, or blockage shouldbe corrected as soon as possible to prevent cata-strophic failure and expensive repairs.
VOLUME 1: GENERAL CONSIDERIATIONS 41
V EGETATION
Water level management is the key to deter-mining the success of vegetation. While wetlandplants can tolerate temporary changes in waterdepth, care should be taken not to exceed thetolerance limits of desired species for extendedperiods of time. Water depth can be increasedduring the cold months to increase retention timeand to protect against freezing. Alternating flowsand drawdown may help to oxidize organic matterand to encourage the recruitment of new plantsinto the wetland. Vegetative cover on dikes shouldbe maintained by mowing, and fertilizing orliming, as needed. Frequent mowing encouragesgrasses to develop a good ground cover withextensive root systems that resist erosion, andprevents shrubs and trees from becoming estab-lished. The roots of shrubs and trees can createchannels and subsequent leakage through theberm.
Vegetation should de inspected regularly and.invasive species should be removed. Herbicidesshould not be used except in extreme circum-stances, and then only with extreme care, sincethey can severely damage emergent vegetation.
MUSKRATS .
Muskrats and other burrowing animals candamage dikes and liners. If wire screening was notinstalled in the dikes, a thick layer of gravel, rock,or bentonite over trouble spots may inhibit bur-rowing. If damage continues, the animals mayhave to be trapped and removed for temporaryrelief until wire screen can be installed. Burrowscan also be sealed by placing bentonite clay in theentryway and adding water to the bentonite to sealthe opening.
M OSQUITOES
Mosquitoes are common in natural wetlandsand can be expected in constructed wetlands.However, in the mid- atlantic states, mosquitoes
are usually not a major problem in constructedwetlands.
The best approach to avoiding mosquitoproblems in constructed wetlands is to createconditions in the wetland that are not attractive tomosquitoes or are not conducive to larval develop-ment. Open, stagnant water creates excellentmosquito breeding habitat, and stagnant, highnutrient water is ideal for larval development.Flowing water and a covered water surface mini-mize mosquito development.
Control methods include unblocking flows toeliminate stagnant backwaters, shading the watersurface (females avoid shaded water for egg-laying). and dispersing floating mats of duckweedor other floating plants.
Purple martins, swallows, and bats can eatthousands of adult mosquitoes every day, soproviding purple martin houses, swallowperches, and bat boxes will reduce the numberof mosquitoes.
Mosquitofish (Gambusia) can be introduced toprey on mosquito larvae. The green sunfish(Lepornis cyanellus), a native. hardy, and aggres-sive mosquito-eating fish, can be used in areas thatare too cold for mosquitofish. Some control isprovided by the larvae of insects. such as dragon-.flies, which prey on mosquito larvae.
The control of mosquitoes with insecticides,oils, and bacterial agents such as Bti (Bacillusthuringiensis isroelensis) is often difficult inconstructed wetlands. The use of insecticides inconstructed wetlands with large amounts oforganic matter is ineffective because the insecti-cides adsorb onto the organic matter and becausethey are rapidly diluted or degraded by the watertraveling through the wetland. Chemical treat-ment should be used with caution because it ispoorly understood and runs the risk of contami-nating both the wetland and the receiving stream.Before beginning any involved control procedures,every aspect of the wetland system and the sur-rounding area should be carefully inspected,perhaps with the aid of a good vector controlspecialist. The inspection should include such
42 V O L U M E S: GENERAL C ONSIDERATIONS
minor components as old cans, discarded tires,undrainable depressions in wooded areas, hollowstumps, water control structures, open piping, andanywhere else that standing water can accumulate.Mosquito problems often originate from some smalland frequently overlooked pocket of standing waterrather than from the wetland as a whole.
the specific objectives of monitoring
organizational and technical responsibilities
tasks and methods
data analysis and quality assurance procedures
schedules
MONITORINGMonitoring is an important operational tool that:
l provides data for improving treatment performance
reporting requirements
resource requirements
budget.
l identifies problems M O N I T O R I N G F O R
D I S C H A R G E C O M P L I A N C E
l documents the accumulation of potentially toxicsubstances before they bioaccumulate
• determines compliance with regulatory require-ments.
Monitoring is needed to measure whether thewetland is meeting the objectives of the wetlandsystem and to indicate its biological integrity. Moni-toring the wetland can identify problems early on,when intervention is most effective. Photographs canbe invaluable in documenting conditions. Photo-graphs should be taken each time at the same loca-tions and viewing angles.
Monitoring for compliance with the limita-tions of the discharge permit represents the mini-mum of sampling and analysis a requirements. Afixed weir at the outlet provides a simple meansof measuring flow and collecting water samples.The parameters to be monitored and the frequencyof data collection will be set by the terms of thepermit.
M O N I T O R I N G F O R
S Y ST E M P E R F O R M A N C E
The level of detail of the monitoring will dependon the size and complexity of the wetland system andmay change as the system matures and its perfor-
mance becomes more well known. As a minimum.lightly-loaded systems that have been operatingsatisfactorily may only need to be checked every month and after every major storm. Those that areheavily loaded will require more frequent and de-tailed monitoring.
Wetland system performance is usually as-sessed by determining:
l hydraulic loading rates
l inflow and outflow volumes
l water quality changes between inflow andoutflow
M O N I T O R I N G P L A N
V OLUME 1: GENERAL CONSIDERATIONS
A written monitoring plan is essential if continu-ity is to be maintained throughout the life of theproject, which may span many decades. The moni-toring plan should include:
l excursions from normal operating conditions.
The effectiveness of contaminant removal canbe determined from the difference between influ-ent loads (inflow volume x contaminant concen-tration) and effluent loads (discharge volume xcontaminant concentration). The parameters ofconcern may include:
l clearly and precisely stated goals of the projectl domestic wastewater: BOD,, nitrogen, phospho-
rus, total suspended solids, heavy metals,bacteria (total or fecal coliform)
43
l agricultural wastewater: BOD,, nitrogen, phos-phorus, total suspended solids, pesticides,bacteria (total or fecal coliform)
l mine drainage: pH, iron, manganese, aluminum,total suspended solids, sulfate
l stormwater: total suspended solids, nitrogen,phosphorus. heavy metals, vehicle emissionr e s i d u e s
Surface water sampling stations should belocated at accessible points at the inlet and outlet,and, depending on the size and complexity of thesystem, at points along the flow path within thewetland. Surface water quality stations should bepermanently marked. Boardwalks can be installedto avoid disturbing sediment and vegetation whilesampling. If the wastewater could contain toxicpollutants, such as pesticides or heavy metals,sediments should be sampled once or twice a yearto monitor the potential buildup of contaminantsin the wetland sediments. The effluent should besampled during high storms and high springrunoff flows to assure that sediments are beingretained in the wetland. Groundwater should also
be monitored once or twice a year to ensure thatthe wetland is not contaminating groundwater.
M O N I T O R I N G F O R
W E T L A N D H E A L T H
The wetland should be checked periodically toobserve general site conditions and to detect majoradverse changes, such as erosion or growth ofundesirable vegetation. Vegetation should bemonitored periodically to assess its health andabundance. For wetlands that are not heavilyloaded, vegetation monitoring need not be quanti-tative and qualitative observations of the site willusually suffice. Large systems and those that areheavily loaded will require more frequent, quanti-tative monitoring. In general, more frequentmonitoring also is required during the first fiveyears after the wetland is installed.
Species composition and plant density areeasily determined, by inspecting quadrats (square
plots, usually 3 ft x 3 ft) within the wetland atselected locations. A lightweight, open frame ofwood or PVC pipe is laid on the wetland and thenumber of stems of each species present withinthe frame is counted. Changes of concern includean increase in the numbers of aggressive nuisancespecies, a decrease in the density of the vegetativecover, or signs of disease.
The vegetation in constructed wetlands issubject to gradual year-to-year change, just as innatural wetlands. There may be tendency forsome species to die out and be replaced by others.Temporary changes, such as the appearance ofduckweed or algae, can occur in response torandom or seasonal climatic changes. Becausevegetative changes are often slow, they may not beobvious in the short-term, and good record-keeping becomes essential.
The buildup of accumulated sediment andlitter decreases the available water storage capac-ity, affecting the depth of the water in the wetlandand possibly altering flow paths. Sediment, litter,and water depths should be checked occasionally.
44 V O L U M E 1: GE N E R A L C O N S I D E R A T I O N S
REFERENCES
Cooper, P. F., and B.C. Findlater (eds.). 1990.Constructed Wetlands in Water Pollution Control.Proceedings of the International Conference onthe Use of Constructed Wetlands in Water Pollu-tion Control, Cambridge, UK, 24-28 September.WRc, Swindon, Wiltshire, UK. 605 pp.
EPA (Environmental Protection Agency). 1993.Wetland Creation and Restoration: Status of theScience. Washington, DC.
Hammer, D. A. (ed.). 1989. Constructed Wetlandsfor Wastewater Treatment: Municipal, Industrialand Agricultural. Lewis Publishers, Chelsea, MI.831 pp.
Hilton, B. L. 1993. Performance evaluation of aclosed ecological life support system (CELSS)employing constructed wetlands. pp 117-125 inConstructed Wetlands for Water Quality Improve-ment, G. A. Moshiri (ed.). CRC Press, Boca Raton,FL.
Jacobson, M. A. 1994. Prairie Wetland Restorationand Water Quality Concerns. Master’s thesis,University of Minnesota, MN. 101 pp.
Kadlec, J. A. 1993. Effect of depth of flooding on_ summer water budgets for small diked marshes.
Wetlands 13:1-9.
Kadlec, R. H. 1989. Wetlands for treatment ofmunicipal wastewater. pp 300-314 in WetlandsEcology and Conservation:, Emphasis in Pennsyl-vania, S. K. Majumdar, R. P. Brooks, F. J. Brenner,and R. W. Tiner, Jr. (eds.). The PennsylvaniaAcademy of Science, Philadelphia, PA.
Livingston, E. 1989. Use of wetlands for urbanstormwater management. pp 253-262 in Con-structed Wetlands for Wastewater Treatment, D.A. Hammer (ed.). Lewis Publishers, Chelsea, MI.
Mitsch, W. J. 1992. Landscape design and the roleof created, restored and natural riparian wetlandsin controlling nonpoint source pollution. Eco-logical Engineering l(1992): 27-47.
Moshiri, G. A. (ed.). 1993. Constructed Wetlandsfor Water Quality Improvement. CRC Press, BocaRaton, FL. 632 pp.
SCS (Soil Conservation Service). 1991. Measure-ment and Estimation of Permeability of Soils forAnimal Waste Storage Facilities. Technical Note717, South National Technical Center (SNTC),Ft. Worth, TX. 35 pp.
SCS (Soil Conservation Service). 1992. Engineer-ing Field Handbook, Chapter 13: Wetland restora-tion, enhancement, or creation. 210-EFH, l/92.
SCS (Soil Conservation Service). 1993. Design andConstruction Guidelines for Considering Seepagefrom Agricultural Waste Storage Ponds andTreatment Lagoons. Technical Note 716, SouthNational Technical Center (SNTC), Ft. Worth, TX.unpaged.
Schueler, T. R. 1992. Design of Stormwater Wet-land Systems: Guidelines for Creating Diverseand Effective Stormwater Wetland Systems in theMid-Atlantic Region. Metropolitan Council ofGovernments, Washington, D.C. 127 pp.
Surrency D. S. 1993. Evaluation of aquatic plantsfor constructed wetlands. pp 349-357 in Con-structed Wetlands for Water Quality Improve-ment, G. A. Moshiri (ed.). CRC Press, Boca Raton,FL.
Taylor, H. N., K. D. Choate, and G. A. Brodie. 1993.Storm event effects on constructed wetlanddischarges. pp 139-145 in Constructed Wetlandsfor Water Quality Improvement, G. A. Moshiri(ed.). CRC Press, Boca Raton, FL.
Thunhorst, G. A. 1993. Wetland Planting Guidefor the Northeastern United States. Environmen-tal Concern, Inc. St. Michaels, MD.
US Army Corps of Engineers. 1993. Selectionand acquisition of wetland plant species forwetland management projects. WRP Tech. NoteVN-EM-2.1. US Army Engineer WaterwaysExperiment Station, Vicksburg, MS. 5 pp.
V O L U M E I: GE N E R L A L C O N S I D E R A T I O N S 45
4 6
Water Pollution Control Federation. 1990.Natural Systems for Wastewater Treatment.Manual of Practice FD-16, Chapter 9. Alexan-dria, VA.
Watson, J. T., and J. A Hobson. 1989. Hydraulicdesign considerations and control structuresfor constructed wetlands for wastewatertreatment. pp 379-391 in Constructed Wet-lands for Wastewater Treatment, D. A. Hammer(ed.). Lewis Publishers, Chelsea, MI.
Webster, H. J., M. J. Lacki, and J. W. Hummer.1994. Biotic development comparisons of awetland constructed to treat mine water with anatural wetland system. pp 102-110 in Vol-ume 3, Proceedings International Land Recla-mation and Mine Drainage Conference andThird International Conference on the Abate-ment of Acidic Drainage, Pittsburgh, PA. April24-29, 1994. Bureau of Mines Spec. Publ.S P 0 6 C - 9 4 .
Wetzel, R. G., 1993. Constructed wetlands:scientific foundations are critical. pp 3-7 i nConstructed Wetlands for Water QualityImprovement, G.-A. Moshiri (ed.). CRC Press,Boca Raton, FL.
1. Constructed wetlands, such as this stormwaterwetland, provide aesthetics and wildlife habitat aswell as water quality improvement and stormwaterc o n t r o l .
2. Constructed wetlands are a cost-effectivemeans of removing metals from minedrainage. The black deposits aremanganese precipitates.
3. Site surveys are necessary for properdesign and construction.
4. Including a deeper, open water areaincreases the wildlife value of thewetland and may increase ammoniaremoval.
5. An organic substrate is often used for wetlandsthat will treat coal mine drainage.
6. This anoxic limestone drain, which is being builtwith the help of the National Guard, will addalkalinity to acidic drainage from an abandonedcoal mine before wetland treatment.
7. Perforated inlet pipes are a simple way todistribute wastewater across the width of thecell.
8. Construction usually requires heavyequipment.
9. Straw bales can be used to create long flow pathsas part of the initial design, as here, or they canbe added later to correct short-circuiting.
10. A V-notch weir is simple to construct but-does not allow water levels to be adjustedeasily.
12. After excavation, grading, and compaction.a newly-constructed wetland is ready forplanting.
11. Structures may need to be protected fromvandals and animals.
13. After flooding the wetland to settle thesubstrate, the wetland should be drainedfor planting.
15. A cattail rhizome.
14. Water levels can be raised gradually as theplants grow.
16. Dense vegetation promotes sedimentationand pollutant removal.
NOTES
V O L U M E 1: GENERAL C ONSIDERATIONS 5 1
NOTES
5 2 V O L U M E 1 : GE N E R A L C O N S I D E R A T I O N S
G L O S S A R Y
abiotic ......................... not involving biological processes
aerobic ........................ requiring free oxygen
algae .......................... primitive green plants that live in wet environments
ALD.... . ...................... anoxic limestone drain
........................... acidic mine drainage
AML.. .......................... abandoned mine lands
anaerobic .................... a situation in which molecular oxygen is absent; lacking oxygen
anoxic ......................... without free oxygen
aquifer.. .............. ......... a permeable material through which groundwater moves
aspect .......................... the ratio of length to width
AWMS ........................ animal waste management system
baseflow.. .................... the portion of surface flow arising from groundwater; the between-storm Bow
biomass ....................... the mass comprising the biological components of a system
biotic ............................ the living parts of a system; biological
BMP ............................ Best Management Practice
BOD............................ biochemical oxygen demand, often measured as 5-day biochemical oxygen demand (BOD5); the consump-tion of oxygen by biological and chemical reactions
CEC ............................. cation exchange capacity
community (plant) ..... the assemblage of plants that occurs in an area at the same time
denitrification ............ the conversion of nitrate to nitrogen gas, through the removal of oxygen
detritus ....................... loose, dead material; in wetlands, largely the leaves and stems of plants
emergent wetland.. ..... a wetland dominated by emergent plants, also called a marsh
EC................................ electrical conductivity
effluent ....................... the surface water flowing out of a system
emergent plant ........... a non-woody plant rooted in shallow water with most of the plant above the water surface
ET ................................ evapotranspiration
evapotranspiration ..... loss of water to the atmosphere by evaporation from the water surface and by transpiration by plants
exfiltration .................. the movement of water from a surface water body to the ground
exotic species ............. not native; introduced
HLR.............................. hydraulic loading rate: loading on a unit area basis
HRT ............................. hydraulic residence time; average time that moving water remains in a system
hydric soil .................. a soil that is saturated, flooded, or ponded long enough during the growing season to develop anaerobicconditions in the upper part of the soil
hydrolysis ................... chemical decomposition by which a compound is resolved into other compounds by taking up the ele-ments of water
bydroperiod ................ the seasonal pattern of changes in water level
infiltration .................. the movement of water from the ground into a surface water body
influent ....................... the surface water flowing into a system
karst ............................ irregular, pitted topography characterized by caves, sinkholes, and disappearing streams and springs, andcaused by dissolution of underlying limestone, dolomite, and marble
marsh .......................... an emergent wetland
microbe ....................... microscopic organism; includes protozoa, bacteria, yeasts, molds, and viruses
microorganism . . . . . . . . . . . term often used interchangeably with microbe
native species . . . . . . . . . . . . . one found naturally in an area; an indigenous species
nitrification . . . . . . . . . . . . . . . . the conversion of ammonia to nitrate through the addition of oxygen
non-persistent plant . . . a plant that breaks down readily after the growing season
non-vascular plant . . . . . a plant without differentiated tissue for the transport of fluids; for instance, algae
NPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . nonpoint source
organic matter . . . . . . . . . . . . matter containing carbon
oxidation . . . . . . . . . . . . . . . . . . . . the process of changing an element from a lower to a higher oxidation state by the removal of anelectron(s) or the addition of oxygen
pathogen . . . . . . . . . . . . . . . . . . . . . a disease-producing microorganism
peat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . partially decomposed plant material, chiefly mosses
perennial plant ........... a plant that lives for many years
permeability . . . . . . . . . . . . . . . the capacity of a porous medium to conduct fluid
persistent plant . . . . . . . . . . a plant whose stems remain standing from one growing season to the beginning of the next
redox . . . . . . . . . . . . . . . . . . . . . . . . . . . reduction/oxidation
reduction . . . . . . . . . . . . . . . . . . . . the process of changing an element from a higher to a lower oxidation state, by the addition of anelection(s)
rhizome . . . . . . . . . . . . . . . . . . . . . . . a root-like stem that produces roots from the lower surface and leaves, and stems from the upper surface
riparian . . . . . . . . . . . . . . . . . . . . . . . pertaining to the bank of a stream, river, or wetland
SAPS . . . . . . . . . . . . . . . . . . . . . . . . . . . successive alkalinity-producing system
SF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . surface flow
SSF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . subsurface flow
stolon . . . . . . . . . . . . . . . . . . . . . . . . . . a runner that roots at the nodes
scarification . . . . . . . . . . . . . . . . abrasion of the seed coat
stratification . . . . . . . . . . . . . . . treatment of seed by exposure to cold temperatures
succession . . . . . . . . . . . . . . . . . . the orderly and predictable progression of plant communities as they mature
transpiration . . . . . . . . . .. . . . . the process by in which plants lose water
tussock . . . . . . . . . . . . . . . . . . . . . . . . a hummock bound together by plant roots, especially those of grasses and sedges
t u b e r . . . . . . . . . . . . . . . . . . . . . . . a short thickened underground stem having numerous buds or “eyes”
TSS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . total suspended solids
vascular plant . . . . . . . . . . . . . a plant that possesses a well-developed system of conducting tissue to transport water, mineral salts, andfoods within the plant
wrack . . . . . . . . . . . . . . . . . . . . . . . . . . plant debris carried by water
A B B R E V I A T I O N S A N D C O N V E R S I O N F A C T O R S
MULTIPLY
ac, acre
cfs, cubic foot per second
cfs, cubic foot per second
cm, centimeter
cm/sec, centimeter per second
OF, degree Fahrenheit
ft, foot
ft2, square foot
ft3, cubic foot
ft/mi, foot per mile
fps, foot per second
g/m2/day, gram per square meter per day
gal, gallon
gal, gallon
gpm, gallon per minute
ha, hectare
inch
kg, kilogram
kg/ha/day, kilogram per hectare per day
kg/m2, kilogram per square meter
L, liter
L, liter
lb, pound
lb/ac, pound per acre
m, meter
m2, square meter
m3, cubic meter
m3, cubic meter
m3/ha/day, cubic meter per hectare per day
mm, millimeter
mi, mile
BY
0.4047
440.831
2.8317 x l0-2
0.3937
3.28 x 10-2
5/9 (0F - 32)
0.305
9.29 x 10-2
2.83 x l0-2
0.1895
18.29
8.92
3.785
3.765 x 10-3
6.308 x l0-2
2.47
2.54
2.205
0.892
0.2
3.531 x 10-2
0.2642
0.4536
1.121
3.28
10.76
1.31
264.2
106.9
3.94 x l0-2
1.609
T O O B T A I N
ha, hectare
gpm, gallon per minute
m3/s, cubic meter per second
inch
fps, foot per second
oC, degree Celsius
m, meter
m2, square meter
m3, cubic meter
m/km, meter per kilometer
m/min, meter per minute
lb/ac/day, pound per acre per day
L, liter
m3, cubic meter
L/s. liter per second
ac, acre
cm, centimeter
lb, pound
lb/ac/day, pound per acre per day
lb/ft2, pound per square foot
ft3, cubic foot
gal, gallon
kg, kilogram
kg/ha, kilogram per hectare
ft, foot
ft2, square foot
y d3, cubic yard
gallon, gal
gallon per day per acre, gpd/ac
inch
kilometer, km
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