A HANDBOOK OF CONSTRUCTED WETLANDS

A HANDBOOK OF CONSTRUCTED WETLANDS

a guide to creating wetlands for: AGRICULTURAL WASTEWATER DOMESTIC WASTEWATER COAL MINE DRAINAGE STORMWATER

in the Mid-Atlantic Region

Volume

1

ACKNOWLEDGMENTS

Many people contributed to this Handbook. An interagency Core Group provided the initial impetus for the Handbook, and later provided guidance 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 Timothy Murphy, USDA - NRCS, Harrisburg, PA Glenn Rider, Pennsylvania Department of Environmental Resources, Harrisburg. PA

Melanie Sayers, Pennsylvania Department of Agriculture, Harrisburg, PA Fred Suffian, USDA - NRCS Philadelphia, PA Charles Takita, Susquehanna River Basin Commission, Harrisburg, PA Harold Webster, Penn State University, DuBois, PA.

Many experts on constructed wetlands contributed by providing information and by reviewing and commenting on the Handbook. These Individuals included: Robert Bastian. EPA .WashinSton, DC Robert Knight, CH2M HILL, Gainesville, FL Fran Koch, Pennsylvania Department of William Boyd, USDA - NRCS. Lincoln, NE Environmental Resources, Harrisburg, PA Robert Brooks, Penn State University, University Park, PA Eric McCleary, Damariscotta, Clarion, PA Donald Brown, EPA, Cincinnati, OH Gerald Moshiri, Center for Wetlands and Eco-Technology Application, Gulf Breeze, Dana Chapman, USDA - NRCS, Auburn, NY FL Tracy Davenport, USDA -NRCS, Annapolis, John Murtha, Pennsylvania Department of MD Environmental Resources, Harrisburg. PA Paul DuBowy, Texas A & M University, Robert Myers, USDA - NRCS, Syracuse, NY College Station, TX Kurt Neumiller, EPA, Annapolis, MD Michelle Girts, CH2M HILL, Portland, OR Richard Reaves, Purdue University, West Robert Hedin, Hedin Environmental, Lafayette, IN Sewickley, PA William Sanville, EPA, Cincinnati, OH William Hellier. Pennsylvania Department of Environmental Resources, Hawk Run, PA Dennis Sievers, University of Missouri, Columbia, MO Robert Kadlec, Wetland Management Services, Chelsea, MI Earl Shaver, Delaware Department of Natural Resources and Environmental Douglas Kepler, Damariscotta. Clarion, PA Control, Dover, DE Robert Kleinmann, US Bureau of Mines, Pittsburgh, PA

Daniel Seibert, USDA - NRCS, Somerset, PA Jeffrey Skousen, West Virginia University, Morgantown. WV Peter Slack, Pennsylvania Department of Environmental Resources, Harrisburg, PA Dennis Verdi, USDA - NRCS, Amherst, MA Thomas Walski, Wilkes University, WilkesBarre, PA Robert Wengryznek, USDA - NRCS, Orono, ME Alfred Whitehouse, Office of Surface Mining. Pittsburgh, PA Christopher Zabawa, EPA, Washington, DC.

This document was prepared by Luise Davis for the USDA-Natural Resources Conservation Service and the US Environmental Protection Agency-Region III, in cooperation with the Pennsylvania Department of Environmental Resources. Partial funding has been provided with nonpoint 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 constructed wetlands for the treatment and control of nonpoint sources of pollutants. Each state agency should be consulted to determine specific programs and restrictions in this regard.

VOLUME 1 TABLE OF CONTENTS

CHAPTER 1. INTRODUCTION . . . . . . . . . . . . . . . . . . ................................................._................................................., .5 CHAPTER 2. CONSTRUCTED WETLANDS AS ECOSYSTEMS ......................................................................... 7 What Are Wetlands? ......... . . ................................................................................................................ 7 Wetland Functions and Values.. ...................................................................... . .................................. 7 Components of Constructed Wetlands ...............................................................................................

8

Water. ................................................................................................................ ............................ Substrates, Sediments, and Litter ................................................................................................ Vegetation ..................................................................................................................................... Microorganisms ............................................................................................................................. Animals.. ............................................................ ..........................................................................

8 8 8 9 9

Aesthetics and Landscape Enhancement .................................................................................. 10

CHAPTER 3. CONSTRUCTED WETLANDS AS TREATMENT SYSTEMS ........................................................ 11 How Wetlands Improve Water Quality ........................................................................................ 11 Advantages of Constructed Wetlands.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Limitations of Constructed Wetlands .................................................................................. 11 Types of Constructed Wetlands ........................................................................................................ 12 13 Surface Flow Wetlands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Subsurface Flow Wetland .........................................................................................................

Hybrid Systems .......................................................................................................................... Winter and Summer Operation ........................................................................................................ Creation of Hazard ............................................................................................................................ Change and Resilience ............. . ........................................................................................................

13 13 13 14 14

CHAPTER 4. GENERAL DESIGN OF CONSTRUCTED WETLANDS ................................................................ 17 Design Considerations ....................................................................................................................... 17 Planning ............................................................................................................................................ 17 Site Selection .... ................................................................................................................................ 18 Land Use and Access.. ............................................. .................................................................. 18 Land Availability ........................................................................................................................ 18 Topography ................................................................................................................................. 19 Environmental Resources.. ....................................................... ................................................. 19 Permits and Regulations ................................................................................................................... 19 Structures ............................................................. .......................................................................... 20 20 Cells ............................................................................................................................................ 20 Liners .......................................................................................................................................... Flow Control Structures ............................................................................................................. 20 Inlets ............................................................................................................................. 21 Outlets .............................................. ..................................................................................... 22 System Lifetimes .................................................................................................................. 23

Chapter 5. HYDROLOGY ..................................................................................................................................... Climate and Weather .........................................................................................................................

25 25

Hydroperiod ........................................................................... ......... ................................................... 25 Hydraulic Residence Time ............................................................................................................... Hydraulic Loading Rate .................................................................................................................... Groundwater Exchange ..................................................................................................................... Evapotranspiration ............................................................................................................................

26 26 26 26

Water Balance ...................................................................................................................................

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Chapter 6. SUBSTRATES.. ................. . ................................................................................................................ Soil ..................................................................................................................................................... Sand and Gravel ................................................................................................................................ Organic Material ............. .................................................................................................................

29 29 39

Chapter 7, VEGETATION ................................. . .................................................................................................. Selecting plants .......................................................... ...................................................................... Surface Flow Wetlands ............................................................................................................... Subsurface Flow W e t l a n d s ................................................................. . .......................................

31 31 31 34 34 34 34 34 35 35 36

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Sources of Plants ......................................................................................................... . ..................... Seeds ..................................................... . ..................................................................................... Wetland Soil .... ........................................................................................................................... Rhizomes,Tubers. and Entire Plants .......................................................................................... When To Plant.. ................................................................................................................................. Site Preparation ................................................................................................................................ How To Plant ..................................................................................................................................... Surface Flow Wetlands.. ........................................... . ................................................................. 36 Subsurface Flow Wetlands .................................................................... . . .................................. 36 Establishing and Maintaining Vegetation ......................................................................................... 36 Chapter 8. CONSTRUCTION .............................................................................................................................. Construction Plans ............................................................................................................................ Pre-Construction Activities .............................................................................................................. Construction Activities ......................................................................... . ......................................... Inspection, Startup, and Testing ........................................................................................................

39 39 39 39

Chapter 9: OPERATION, MAINTENANCE, AND MONITORING ..................................................................... Operation and Maintenance .............................................................................................................. Operation and Maintenance Plan ............................................................... .:. ............................ Hydrology ................................................................................................................................... Structures .................................................................................................................................... Vegetation ................................................................................................................................... Muskrats ..................................................................................................................................... Mosquitoes .................................................................................................................................. Monitoring ........................................................................................................................................ Monitoring Plan.. ........................................................................................................................ Monitoring for Discharge Compliance.. ..................................................................................... Monitoring for System Performance .......................................................................................... Monitoring for Wetland Health ..................................................................................................

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REFERENCES .I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .........................

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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 .................................................................. .12 Figure 2. Inlet and outlet designs ....................................................................................................................... 21 Figure 3. Influent splitter box .............................................................................................................................. 22

Natural processes have always cleansed water as it flowed through rivers, lakes, streams, and wetlands. In the last several decades, systems have been constructed to use some of these processes for water quality improvement. Constructed wetlands are now used to improve the quality of point and nonpoint sources of water pollution, including stormwater runoff, domestic wastewater, agricultural wastewater, and coal mine drainage. Constructed wetlands are also being used to treat petroleum refinery wastes, compost and landfill leachates, fish pond discharges, and pretreated industrial wastewaters, such as those from pulp and paper mills, textile mills, and seafood processing. For some wastewaters, constructed wetlands are the sole treatment; for others, they are one component in a sequence of treatment processes. One of the most common applications of constructed wetlands has been the treatment of primary or secondary domestic sewage effluent. Constructed wetland systems modelled after those for domestic wastewater are being used to treat the high organic loads associated with agriculture. A large number of wetlands have been constructed to treat drainage from active and abandoned coal mines and more than 500 such systems are operating in Appalachia alone. The use of constructed wetlands to control stormwater flows and quality is a recent application of the technology and the number of such systems is increasing rapidly. The treatment of wastewater or stormwater by constructed wetlands can be a low-cost, lowenergy process requiring minimal operational attention. As a result of both extensive research and practical application, insight is being gained into the design, performance, operation, and maintenance of constructed wetlands for water quality improvement. Constructed wetlands can be sturdy, effective systems. However, to be effective, they must be carefully designed, constructed, operated, and maintained.

This Handbook has been prepared as a general guide to the design, construction, operation, and maintenance of constructed wetlands for the treatment of domestic wastewater, agricultural wastewater, coal mine drainage, and stormwater runoff in the mid-Atlantic region, The Handbook is not a design manual. The use of constructed wetlands to improve water quality is a developing technology. Much is not yet understood, and questions remain regarding the optimal design of wetland systems and their longevity. As our experience with these systems increases, the information offered here will be replaced by more refined information. The Handbook should be used with this clearly in mind. The Handbook is divided into five volumes. This, the first, provides information common to all types of constructed wetlands for wastewater and runoff. It is to be used in conjunction with an accompanying volume that provides information specific to a particular type of wastewater or runoff. The other volumes in the series are Volume 2: Domestic Wastewater, Volume 3: Agricultural Wastewater, Volume 4: Coal Mine Drainage, and Volume 5: Stormwater Runoff. While constructed wetlands are being used to treat other kinds of wastewater, such as industrial wastewaters, a discussion of these applications is beyond the scope of this Handbook. However, the information presented here may be useful in developing other applications. A number of conferences on constructed wetlands have been held recently. The proceedings of these conferences include experimental and operational data from wetland systems built to treat a number of different kinds of wastewaters and runoff, and present detailed discussions of process kinetics and system design. Proceedings from three well-known conferences are: Moshiri, G. A. (ed.) 1993. Constructed Wetlands for Water Quality Improvement. CRC Press, Boca Raton, FL. 632 pp.

V O L U M E 1: G E N E R A L 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 on the Use of Constructed Wetlands in Water Pollution Control. Cambridge, UK, 24-28 September. WRc, Swindon, Wiltshire, UK. 605 pp. Hammer, D. A. (ed.) 1989. Constructed Wetlands for Wastewater Treatment: Municipal, Industrial and Agricultural. Lewis Publishers. Chelsea, MI. 831 pp. Conferences and published’ information continue to become available as more constructed wetland systems are built and monitored.

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CHAPTER 2 CONSTRUCTED WETLANDS AS ECOSYSTEMS

Constructed wetlands for water treatment are complex, integrated systems of water, plants, animals, microorganisms, and the environment. While wetlands are generally reliable, self-adjusting systems, an understanding of how natural wetlands are structured and how they function greatly increases the likelihood of successfully constructing a treatment wetland. This chapter provides an overview of the major components of wetland ecosystems and of the most important processes that affect water treatment.

WHAT ARE WETLANDS? Wetlands are transitional areas between land and water. The boundaries between wetlands and uplands or deep water are therefore not always distinct. The term “wetlands” encompasses 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, freshwater or salt - have one characteristic in common: the presence of surface or near-surface water, at least periodically. In most wetlands, hydrologic conditions are such that the substrate is saturated long enough during the growing season to create oxygen-poor conditions in the substrate. The lack of oxygen creates reducing. (oxygen-poor) conditions within the substrate and limits the vegetation to those species that are adapted to low-oxygen environments. The hydrology of wetlands is generally one of slow flows and either shallow waters or saturated substrates. The slow flows and shallow water depths allow sediments to settle as the water passes through the wetland. The slow flows also provide prolonged contact times between the water and the surfaces within the wetland. The complex mass of organic and

VOLUME 1

inorganic materials and the diverse opportunities for gas/water interchanges foster a diverse community of microorganisms that break down or transform a wide variety of substances. Most wetlands support a dense growth of vascular plants adapted to saturated conditions. This vegetation slows the water, creates microenvironments within the water column, and provides attachment sites for the microbial community. The litter that accumulates as plants die back in the fall creates additional material and exchange sites, and provides a source of carbon, nitrogen, and phosphorous to fuel microbial processes.

WETLAND FUNCTIONS AND VALUES Wetlands provide a number of functions and values. (Wetland functions are the inherent processes occurring in wetlands; wetland values are the attributes of wetlands that society perceives as beneficial.) While not all wetlands provide all functions and values, most wetlands provide several. Under appropriate circumstances. constructed wetlands can provide: water quality improvement flood storage and the desynchronization of storm rainfall and surface runoff cycling of nutrients and other materials habitat for fish- and wildlife passive recreation, such as bird watching and photography active recreation, such as hunting education and research aesthetics and landscape enhancemerit.’

G ENERAL C O N S I D E R A T I O N S

7

COMPONENTS OF CONSTRUCTED WETLANDS A

constructed wetland consists of a properlydesigned basin that contains water, a substrate, and, most commonly, vascular plants. These components can be manipulated in constructing a wetland. Other important components of wetlands, such as the communities of microbes and aquatic invertebrates, develop naturally.

W

ATER

Wetlands are likely to form where landforms direct surface water to shallow basins and where a relatively impermeable subsurface layer prevents the surface water from seeping into the ground. These conditions can be created to construct a wetland. A wetland can be built almost anywhere in the landscape by shaping the land surface to collect surface water and by sealing the basin to retain the water. Hydrology is the most important design factor in constructed wetlands because it links all of the functions in a wetland and because it is often the primary factor in the success or failure of a constructed wetland. While the hydrology of constructed wetlands is not greatly different than that of other surface and near-surface waters, it does differ in several important respects: l

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small changes in hydrology can have fairly significant effects on a wetland and its treatment effectiveness because of the large surface area of the water and its shallow depth, a wetland system interacts strongly with the atmosphere through rainfall and evapotranspiration (the combined loss of water by evaporation from the water surface and loss through transpiration by plants) the density of vegetation of a wetland strongly affects its hydrology, first, by obstructing flow paths as the water finds its sinuous way through the network of stems, leaves, roots, and rhizomes and, second, by blocking exposure to wind and sun.

S U B S T R A T E S, SE D I M E N T S,

AND

LITTER

Substrates used to construct wetlands include soil, sand, gravel, rock, and organic materials such as compost. Sediments and litter then accumulate in the wetland because of the low water velocities and high productivity typical of wetlands. The substrates, sediments, and litter are important for several reasons: l

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they support many of the living organisms in wetlands substrate permeability affects the movement of water through the wetland many chemical and biological (especially microbial) transformations take place within the substrates

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substrates provide storage for many contaminants the accumulation of litter increases the amount of organic matter in the wetland. Organic matter provides sites for material exchange and microbial attachment, and is a source of carbon, the energy source that drives some of the important biological reactions in wetlands.

The physical and chemical characteristics of soils and other substrates are altered when they are flooded. In a saturated substrate, water replaces the atmospheric gases in the pore spaces and microbial metabolism consumes the available oxygen. Since oxygen is consumed more rapidly than it. can be replaced by diffusion from the atmosphere, substrates become anoxic (without oxygen). This reducing environment is important in the removal of pollutants such as nitrogen and metals.

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Both vascular plants (the higher plants) and non-vascular plants (algae) are important in constructed wetlands. Photosynthesis by algae increases the dissolved oxygen content of the water which in turn affects nutrient and metal

Constructed wetlands attract waterfowl and wading birds, including mallards, green-winged teal, wood ducks, moorhens, green and great blue herons, and bitterns. Snipe, red-winged blackbirds, marsh wrens, bank swallows, redtailed hawks, and Northern harriers feed and/or nest in wetlands.

A ESTHETICS

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L ANDSCAPE

ENHANCEMENT While wetlands are primarily treatment systems, they provide intangible benefits by increasing the aesthetics of the site and enhancing the landscape. Visually, wetlands are unusually rich environments. By introducing the element of water to the landscape, constructed wetlands, as much as natural wetlands. add diversity to the landscape. The complexity of shape, color, size, and interspersion of plants, and the variety in the sweep and curve of the edges of landforms all add to the aesthetic quality of the wetlands. Constructed wetlands can be built with curving shapes that follow the natural contours of the site, and some wetlands for water treatment are’ indistinguishable, at first glance, from natural wetlands.

V O L U M E 1: G E N E R A L C O N S I D E R A T I O N S

reactions. Vascular plants contribute to the treatment of wastewater and runoff in a number of ways: they stabilize substrates and limit channelized flow

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they slow water velocities, allowing suspended materials to settle they take up carbon, nutrients, and trace elements and incorporate them into plant tissues they transfer gases between the atmosphere and the sediments leakage of oxygen from subsurface plant structures creates oxygenated microsites within the substrate their stem and root systems provide sites for microbial attachment theycreate litter when they die and decay. Constructed wetlands are usually planted with emergent vegetation (non-woody plants that grow with their roots in the substrate and their stems and leaves emerging from the water surface). Common emergents used in constructed wetlands include bulrushes, cattails, reeds, and a number of broad-leaved species.

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ICROORGANISMS

A fundamental characteristic of wetlands is that their functions are largely regulated by microorganisms and their metabolism (Wetzel 1993). Microorganisms include bacteria, yeasts, fungi, protozoa, rind algae. The microbial biomass is a major sink for organic carbon and many nutrients. Microbial activity: l

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transforms a great number of organic and inorganic substances into innocuous or insoluble substances

alters the reduction/oxidation (redox) conditions of the substrate and thus affects the processing capacity of the wetland is involved in the recycling of nutrients.

Some microbial transformations are aerobic (that is, they require free oxygen) while others are anaerobic (they take place in the absence of free oxygen). Many bacterial species are facultative anaerobes, that is, they are capable of functioning under both aerobic and anaerobic conditions in response to changing environmental conditions. Microbial populations adjust to changes in the water delivered to them. Populations of microbes can expand quickly when presented with suitable energy-containing materials. When environmental conditions are no longer suitable, many microorganisms become dormant and can remain dormant for years (Hilton 1993). The microbial community of a constructed wetland can be affected by toxic substances, such as pesticides and heavy metals, and care must be taken to prevent such chemicals from being introduced at damaging concentrations.

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Constructed wetlands provide habitat for a rich diversity of invertebrates and vertebrates. Invertebrate animals, such as insects and worms. contribute to the treatment process by fragmenting detritus and consuming organic matter. The larvae of many insects are aquatic and consume significant amounts of material during their larval stages, which may last for several years. Invertebrates also fill a number of ecological roles; for instance, dragonfly nymphs are important predators of mosquito larvae. Although invertebrates are the most important animals as far as water quality improvement is concerned, constructed wetlands also attract a variety of amphibians, turtles, birds, and mammals.

VOLUME 1: GENERAL C ONSIDERATIONS

9

CHAPTER 3 CONSTRUCTED WETLANDS AS TREATMENT SYSTEMS

A constructed wetland is a shallow basin filled with some sort of substrate, usually soil or gravel, and planted with vegetation tolerant of saturated conditions. Water is introduced at one end and flows over the surface or through the substrate, and is discharged at the other end through a weir or other structure which controls the depth of the water in the wetland.

HOW WETLANDS IMPROVE WATER 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 array of microorganisms (most importantly bacteria). The mechanisms that are available to improve water quality are therefore numerous and often interrelated. These mechanisms include:

ADVANTAGES OF CONSTRUCTED WETLANDS Constructed wetlands are a cost-effective and technically feasible approach to treating wastewater and runoff for several reasons: l

. operation and maintenance expenses (energy and supplies) are low l

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chemical transformation adsorption and ion exchange on the surfaces of plants, substrate, sediment, and litter

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breakdown and transformation of pollutants by microorganisms and plants

The most effective treatment wetlands are those that foster these mechanisms. The specifics for the various types of wastewater and runoff are discussed in the wastewater-specific volumes.

they provide habitat for many wetland organisms they can be built to fit harmoniously into the landscape they provide numerous benefits in addition to water quality improvement, such as wildlife habitat and the aesthetic enhancement of open spaces they are an environmentally-sensitive approach that is viewed with favor by the general public.

LIMITATIONS OF CONSTRUCTED WETLANDS

uptake and transformation of nutrients by microorganisms and plants predation and natural die-off of pathogens.

wetlands are able to tolerate fluctuations in flow

In addition:

settling of suspended particulate matter filtration and chemical precipitation through contact of the water with the substrate and litter

operation and maintenance require only periodic, rather than continuous, on-site labor

. they facilitate water reuse and recycling.

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wetlands can be less expensive to build than other treatment options

There are limitations associated with the use of constructed wetlands: l

VOLUME 1: GENERAL

they generally require larger land areas than do conventional wastewater treatment systerns. Wetland treatment may be economical relative to other options only where land is available and affordable.

CONSIDERATIONS

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performance may be less consistent than in conventional treatment. Wetland treatment efficiencies may vary ‘seasonally in response to changing environmental conditions, including rainfall and drought. While the average performance over the year may be acceptable, wetland treatment cannot be relied upon if effluent quality must meet stringent discharge standards at all times.

Also, the use of constructed wetlands for wastewater treatment and stormwater control is a fairly recent development. There is yet no consensus on the optimal design of wetland systems nor is there much information on their long-term performance.

the biological components are sensitive to toxic chemicals, such as ammonia and pesticides

There are several types of constructed wetlands: surface flow wetlands, subsurface flow wetlands, and hybrid systems that incorporate surface and subsurface flow wetlands. Constructed wetland systems can also be combined with conventional treatment technologies. The types of constructed wetlands appropriate for domestic wastewater, agricultural wastewater, coal mine drainage, and stormwater runoff are discussed in the wastewaterspecific volumes.

flushes of pollutants or surges in water flow may temporarily reduce treatment effectiveness they require a minimum amount of water if they

are to survive. While wetlands can tolerate temporary drawdowns, they cannot withstand complete drying.

TYPES OF CONSTRUCTED WETLANDS

Water level is above the ground surface; vegetation is rooted and emerges above the water surface: waterflow is primarily above ground

WETLAND PLANTS AND WATER SOIL LINER

Surface Flow Wetland

NATIVE SOIL

Water level is below ground; water flow is through a sand or gravel bed; roots penetrate to the bottom of the bed

WETLAND PLANTS SOIL. SAND. AND GRAVEL LINER

Subsurface Flow Wetland

NATIVE SOIL

Figure 1. Surface flow and subsurface flow constructed wetlands (from Water Pollution Control Federation 1990).

V OLUME 1 : GENERAL C ONSIDERATIONS

SURFACE

FL O W W ETLANDS

A surface flow (SF) wetland consists of a shallow basin, soil or other medium to support the roots of vegetation, and a water control structure that maintains a shallow depth of water (figure 1). The water surface is above the substrate. SF wetlands look much like natural marshes and can provide wildlife habitat and aesthetic benefits as well as water treatment. In SF wetlands, the nearsurface layer is aerobic while the deeper waters and substrate are usually anaerobic. Stormwater wetlands and wetlands built to treat mine drainage and agricultural runoff are usually SF wetlands. SF wetlands are sometimes called free water surface wetlands or, if they are for mine drainage, aerobic wetlands. The advantages of SF wetlands are that their capital and operating costs are low, and that their construction, operation, and maintenance are straightforward. The main disadvantage of SF systems is that they generally require a larger land area than other systems.

S UBSURFACE FL O W W

ETLANDS

A subsurface flow (SSF) wetland consists of a sealed basin with a porous substrate of rock or gravel. The water level is designed to remain below the top of the substrate. In most of the systems in the United States, the flow path is horizontal, although some European systems use vertical flow paths. SSF systems are called by several names. including vegetated submerged bed, root zone method, microbial rock reed filter, and plant-rock filter systems. Because of the hydraulic constraints imposed by the substrate, SSF wetlands are best suited to wastewaters with relatively low solids concentrations and under relatively uniform flow conditions. SSF wetlands have most frequently been used to reduce 5-day biochemical oxygen demand (BOD5) from domestic wastewaters. The advantages cited for SSF wetlands are greater cold tolerance, minimization of pest and odor problems, and, possibly, greater assimilation potential per unit of land area than in SF systems.

It has been claimed that the porous medium provides greater surface area for treatment contact than is found in SF wetlands, so that the treatment responses should be faster for SSF wetlands which can, therefore, be smaller than a SF system designed for the same volume of wastewater. Since the water surface is not exposed, public access problems are minimal. Several SSF systems are operating in parks. with public access encouraged. The disadvantages of SSF wetlands are that they are more expensive to construct, on a unit basis. than SF wetlands. Because of cost, SSF wetlands are often used for small flows. SSF wetlands may be more difficult to regulate than SF wetlands, and maintenance and repair costs are generally higher than for SF wetlands. A number of systems have had problems with clogging and unintended surface flows.

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Single stage systems require that all of the removal processes occur in the same space. In hybrid or multistage systems, different cells are designed for different types of reactions. Effective wetland treatment of mine drainage may require a sequence of different wetland cells to promote aerobic and anaerobic reactions. as may the remoral of ammonia from agricultural wastewater.

WINTER AND SUMMER OPERATION Wetlands continue to function during cold weather. Physical processes, such as sedimentation. continue regardless of temperature, providing that the water does not freeze. Many of the reactions take place within the wetland substrate, where decomposition and microbial activity generate enough heat to keep the subsurface layers from freezing. Water treatment will continue under 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 C O N S I D E R A T I O N S

Rates of microbial decomposition slow as temperatures drop and the wetland may need to be made larger to accommodate the slower reaction rates. For agricultural wetlands, which rely on microbial activity to break down organic wastes, it may be prudent to store the wastewater in the pretreatment unit during the cold months for treatment during the warm months. The high flows that are common in winter and spring because of snowmelt, spring rains, and high groundwater tables can move water so quickly through a wetland tEat there is not enough retention time for adequate treatment. Because removal rates are much higher during warm weather, the agricultural wetland can often be smaller than if the water were treated year-round. Wetlands lose large amounts of water in the summer through evapotranspiration. The adequacy of flow in the summer must be considered since it will affect water levels in the wetland and the amount of wetland effluent available for recycling (if this is part of the design). A supplemental source of water may be required to maintain adequate moisture in the wetland.

CREATION OF HAZARD The question of hazard arises from the fact that, in ecological terms, everything must go somewhere. Wetlands are able to degrade, transform, or assimilate many contaminants, such nitrogen, and are sinks for some materials. For persistent materials, such as phosphorous and metals, wetland sinks may become sources if not properly constructed and managed. The extent to which wetlands retain contaminants such as phosphorous and metals is an important unknown factor, as are the conditions under which wetlands may release stored contaminants. Bioaccumulation and biotoxicity in treatment wetlands is not clearly documented nor understood.

Persistent compounds can be a concern, depending on the constituents in the wastewater. For instance, mine drainage contains metals and stormwater carries hydrocarbons deposited on paved surfaces. Heavy metals are often sequestered in wetland sediments that may be washed out of wetlands during storms, thereby providing only a lag time in pollutant dispersal. Transport of toxic materials in this way is a concern, as is the transport of phosphorus, an extremely important factor in the over-enrichment of surface waters. The question of hazard underscores the importance of designing and operating constructed wetlands properly and monitoring them periodically.

CHANGE AND RESILIENCE All ecosystems change over time. Wetlands for wastewater treatment can be expected to change more quickly than most natural wetlands because of the rapid accumulation of sediment, litter, and pollutants. Some natural variability is also inherent in all living systems and is to be expected. The change in species composition as ecosystems mature is known as succession. In general, species diversity increases as ecosystems mature. Diversity (the number of species within a habitat, such as a wetland) is often considered a measure of ecosystem resilience (the ability of the system to accept disturbance): as the number of species increases, so does the complexity of the interactions of the different species with each other and with their environment; the greater the number of interactions, the more resilient the system is as a whole and the broader its capacity to adapt to change. In wastewater treatment wetlands, the stresses of high wastewater loadings can lead to dominance by a few aggressive, highly tolerant species, such as cattail and common reed, which may eventually eliminate other species. If wildlife habitat values are important to the

V OLUME 1 : GENERAL C O N S I D E R A T I O N S

project, intervention to maintain diversity may be necessary. If habitat values are not important, changes can be allowed to proceed without interference as long as the wetland continues to treat the water to acceptable levels. Any ecosystem, natural or constructed, has limits to its ability to accept disturbance. The performance of constructed wetland systems may change over time as a consequence of changes in the substrate and the accumulation of pollutants in the wetland. Constructed wetlands must be monitored periodically for evidence of stress so that remedial action, if necessary, can be taken.


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CHAPTER 4 GENERAL DESIGN OF CONSTRUCTED WETLANDS

DESIGN CONSIDERATIONS Despite a large amount of research and published information, the optimal design of constructed wetlands for various applications has not yet been determined. Many constructed wetland systems have not been adequately monitored or have not been operating long enough to provide sufficient data for analysis. Among the systems that have been monitored, performance has varied and the influences of the diverse factors that affect performance, such as location, type of wastewater or runoff, wetland design, climate, weather, disturbance, and daily or seasonal variability, ‘have been difficult to quantify. In general, wetland designs attempt to mimic natural wetlands in overall structure while fostering those wetland processes that are thought to contribute the most to the improvement of water quality. Mitsch (1992) suggests the following guidelines for creating successful constructed wetlands: keep the design simple. Complex technological approaches often invite failure. design for minimal maintenance. design the system to use natural energies, such as gravity flow. design for the extremes of weather and climate, not the average. Storms, floods, and droughts are to be expected and planned for, not feared. design the wetland with the landscape, not against it. Integrate the design with the natural topography of the site. avoid over-engineering the design with rectangular basins, rigid structures and channels, and regular morphology. Mimic natural systems. give the system time. Wetlands do not necessarily become functional overnight and several years may elapse before performance reaches optimal levels. Strategies that try to short-circuit

the process of system development or to overmanage often fail. l

design the system for function, not form. For instance, if initial plantings fail, but the overall function of the wetland, based on initial objectives, is intact, then the system has not failed.

PLANNING A conceptual planning phase is essential. Wetlands can be designed in a variety of system types and configurations to meet specific wastewater needs, alternative sites are often available, and a variety of local, native plant species can be chosen. Every site is unique and the design of a constructed wetland system will be site-specific. The planning phase consists of characterizing the quantity and quality of the wastewater to be treated, determining the discharge standards to be met, selecting the site, selecting system type and configuration, and specifying the design criteria to be met by the detailed engineering plans. Economic factors include the land area required. the type of water containment, the control and transport of water through the system, and vegetation. Setting and prioritizing the objectives of the wetland system is key to the creation of a successful system. The characteristics of a local natural wetland should be used as a model for the constructed wetland, modified to fit the needs of the project and the specifics of the constructed wetland site. A constructed wetland should be designed to take advantage of the natural features of the site and to minimize its disturbance. Wetland shape is dictated by the existing topography, geology, and land availability. The number of cells depends on topography, hydrology, and water quality. On level sites, cells can be created with dikes. On sloping sites, cells can be terraced. A site-sensitive design that incorporates existing features of the site reduces the amount


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of earthmoving required and increases the visual attractiveness of the site. Earth grading and shaping can blend newly created landforms into the existing landscape. Basins and channels can be curved to follow the natural contours of the site. Various types of vegetation can be planted in and around a constructed wetland to reduce erosion, screen views, define space, control microclimate, and control traffic patterns. Planning should be oriented toward the creation of a biologically and hydrologically functional system. Plans should include clear goal statements and standards for success. The possible future expansion of the operation should be considered. Plans should include detailed instructions for implementing a contingency plan in case the system does not achieve its expected performance within a specified time. Plans should be reviewed and approved by the appropriate regulatory agencies.

SITE SELECTION Selecting an appropriate location can save significant costs. Site selection should consider land use and access, the availability of the land, site topography, soils, the environmental resources of the site and adjoining 1and, and possible effects on any neighbors. The site should be located as close to the source of the wastewater as possible, and downgradient if at all possible so that water can move through the system by gravity. While a wetland can be fitted to almost any site, construction costs can be prohibitively high if extensive earthmoving or expensive liners are required. A site that is well suited for a constructed wetland is one that: l

is conveniently located to the source of the wastewater Provides adeqyate space

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is gently sloping, so that water can flow through the system by gravity

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contains soils that can be sufficiently compacted to minimize seepage to groundwater

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is above the water table

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is not in a floodplain

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does not contain threatened or endangered species

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does not contain archaeological or historic resources.

LAND U

SE AND

ACCESS

Access is an important consideration, The wetland should be placed so that the water can flow by gravity. If the odors or insects could be a problem, as with some agricultural wastewaters, the wetland should be placed as far from dwellings as possible. The site should be accessible to personnel, delivery vehicles, and equipment for construction and maintenance. For agricultural and some domestic wastewaters, the wetland may be installed on private land. The landowner must be carefully chosen. It is essential that the landowner is cooperative and fully understands the limitations and uncertainties associated with a developing technology such as constructed wetland treatment. The current and future use and values of adjoining land also will affect the suitability of a site for a constructed wetland. The opinions of area residents and those of environmental and public interest groups should be considered. A large buffer zone should be placed between the wetland and neighboring property. The wetland should not be placed next to the edge of the property.

L A N D A V A I L AB I L IT Y The effectiveness of a constructed wetland in treating wastewater or stormwater is related to the retention time of the water in the wetland. The usefulness of a constructed wetland may therefore be limited by the size of the wetland needed for adequate retention time. The site selected should be large enough to accommodate present requirements and any future expansion.

V O L U M E 1: GENERAL C O N S I D E R A T I O N S

T O P O G RA P H Y Landform considerations include shape, size, and orientation to the prevailing winds. While a constructed wetland can be built almost anywhere, selecting a site with gradual slopes that can be easily altered to collect and hold water simplifies design and construction, and minimizes costs. Previously drained wetland areas, including prior converted (PC) agricultural sites, may be wellsuited for a constructed wetland since the topography is usually conducive to gravity flow. The appropriate regulatory agencies must be contacted before disturbing any PC site. Since the best location for a constructed wetland is a low, flat area where water flows by gravity, it is important to ensure that the area is not already a wetland: not all wetlands have standing water throughout the year. The Natural Resources Conservation Service (NRCS), the US Fish and Wildlife Service, or state regulatory personnel should be contacted to determine whether or not a site contains jurisdictional wetlands.

EN V I R O N M E N T A L R E S O U R C E S

such as the Agricultural Stabilization and Conservation Service (ASCS) crop compliance photography and county soil survey information, can be useful in identifying hydric soils and drained wetlands that may be difficult to detect otherwise. Surface and groundwater considerations include possible flooding and drainage problems, location and depth of aquifers, and the location, extent, and classification of receiving waters such as streams and groundwater. A constructed wetland should not be sited on a floodplain unless special measures can be taken to limit its impact on the floodway. Floodplain elevations can often be determined from sources such as Federal Flood Insurance maps or from the Federal Land Management Agency. Landuser input may be the best source of information for assessing previous hydrologic conditions. US Fish and Wildlife Service and state natural resource agencies should be contacted regarding the potential for significant habitat, or habitat for rare or endangered species. The possible presence of archaeological resources should be verified.

PERMITS AND REGULATIONS

To avoid damaging important resources on the site, the presence or absence of significant environmental resources must be determined. Sources of information that can be helpful in selecting a site include the US Geological Survey Topographic Quadrangle maps, and National Aerial Photography Program (NAPP) and and National High Altitude Photography Program (NHAPP) photographs. Geographical information system (GIS) maps are also available. The National Wetlands Inventory (NWI) maps and the County Soil Survey with the list of county hydric soils should be checked for possible locations of existing wetlands. However, the NW1 maps are based on aerial photography and may not show small wetlands or the less obvious wetlands (wet meadows, vernal pools, and some forested wetlands) and the NW1 information should be field-checked by a wetlands scientist. Historical aerial photography,

V OLUME

The appropriate agency(ies) must be contacted to determine the regulatory requirements for a proposed constructed wetland and its discharge. Work in a waterway or natural wetland requires a permit. Discharges to natural waters also require a permit. In some zoned communities, zoning approval may be required. Any stormwater plan must meet local and state stormwater regulations. Some local ordinances have incorporated stormwater provisions which must be complied with. Stormwater regulations vary from place to place and should be consulted before developing a stormwater management plan. The regulatory status of a proposed stormwater wetland, and its relationship to streams and any nearby natural wetlands, must be discussed with the state and/or federal wetland permitting agency before site plans are decided upon.

1: GENERAL C ONSIDERATIONS

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STRUCTURES CELLS Wetlands can be constructed by excavating basins, by building up earth embankments (dikes), or by a combination of the two. Dikes must be constructed of soils with adequate fine-grained material that will compact into arelatively stable and impervious embankment. The dikes should be high enough to contain the expected volume plus ample freeboard to accommodate occasional high flows as well as the buildup of litter and sediment over time. To ensure long-term stability. dikes should be sloped no steeper than 2H:lV and riprapped or protected by erosion control fabric on the slopes. An emergency spillway should be provided. If multiple cells are used, divider dikes can be used to separate cells and to produce the desired length-to-width ratios. On steep sites, they can be used to terrace cells. Dikes can also be used to control flow paths and minimize short-circuiting. Finger dikes are often used to create serpentine flow paths and can be added to operational systems to mitigate short-circuiting. Finger dikes can be constructed of soils, sandbags, straw bales, or treated lumber. Bottom slopes are generally not critical. An exception may be mine drainage wetlands that use subsurface flow through deep beds of compost to induce sulfate reduction; these cells should slope about 1 - 3% upstream. Bottoms should be relatively level from side to side. Muskrats can damage dikes by burrowing into them. Although muskrats generally prefer to start their burrows in water than is more than 3 ft deep, they can be a problem in shallower waters. Muskrats can be excluded by installing electric fence low to the ground or by burying muskrat-proof wire mats in the dikes during construction.

LINERS Constructed wetlands must be sealed to avoid possible contamination of groundwater and also to

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prevent groundwater from infiltrating into the wetland. Where on-site soils or clay provide an adequate seal, compaction of these materials may be sufficient to line the wetland. Sites underlain by karst, fractured bedrock, or gravelly or sandy soils will have to be sealed by some other method. It may be necessary to have a laboratory analyze the construction material before choosing a sealing method. On-site soils can be used if they can be compacted to permeability of