October 30, 2013
(35 pages including "How-to Guide” and Executive Summary, PDF)
Cities and towns often use constructed wetlands to manage water. Many Iowa municipalities are faced with managing not only stormwater generated within their limits, but also water from agricultural sources originating outside their jurisdiction. Managing urban and agricultural runoff, and the pollutants associated with it, provides unique challenges that are often best met with constructed wetlands. In addition to their benefits as stormwater infrastructure, wetlands also offer value-added opportunities for community and ecosystem improvement. They can be attractive community assets for citizens to enjoy. This report provides an overview of how constructed wetlands function, their ecological benefits, regulations related to wetlands, how to build and manage wetlands, and where to look for funding for a constructed wetland.
Historically, Iowa has seen dramatic land use change. Settlement and agricultural development proceeded rapidly during the second half of the 19th century as native prairies were turned into agricultural fields, and woodlands were harvested for fuel and building materials. Alteration of the landscape continued into the 1900s as extensive draining of wetlands to facilitate agricultural production dried up most of the state’s natural wetlands (Jaynes and James, 2007). Without the deep-rooted prairie vegetation and well-vegetated creeks and streams, erosion and flooding increased. Creeks were straightened to make rivers and creeks more efficient in carrying floodwater, more wetlands were drained and extensive systems of drainage tiles installed to improve drainage and create more arable land. The manner in which water moves through the landscape has drastically changed. Today approximately 98.9 percent of the native prairie and woodland ecosystem has been transformed into an agricultural landscape dotted with a few moderate-size cities and many small towns. The impacts of this transformation are especially evident in surface water, and extend well beyond Iowa’s borders, to the ecosystem effects related to hypoxia in the Gulf of Mexico and to migratory bird populations that rely on wetland habitat in the Central North American flyway.
Surface waters are among society’s most valuable natural resources. They provide recreational and economic opportunities and support many critical ecological functions. When vegetation cover is adequate much of the precipitation infiltrates into the soil where it provides for plants and other organisms and recharges groundwater. If vegetation cover is inadequate, or when the rate of precipitation exceeds the soil’s ability to absorb it, runoff occurs. Many land use activities have decreased the ability of the land surface to absorb precipitation and as a result, runoff amounts and rates are much higher than they were before the advent of towns and modern farming. Flooding in Iowa in 1993, 2008, 2010 and 2011 has demonstrated how dramatically the absorptive native landscape has been altered and replaced by one that efficiently sheds water. Too much runoff leads to flooding, but excess water is not the only problem. Sediment, excess plant nutrients, pesticides, and other pollutants washed and leached from agricultural fields, lawns and urban landscapes degrade surface waters and impair their function. Today more than one-third of the surface waters assessed in Iowa are not able to support their designated uses (U.S. EPA, 2006 http://water.epa.gov/lawsregs/guidance/cwa/305b/96report_index.cfm). A significant portion of this water quality degradation can be tied to agricultural runoff with urban stormwater contributing a minor part. (Heffernan, Galluzzo & Hoyer 2010 http://www.iowapolicyproject.org/2010docs/100927-nutrients.pdf)
In towns and cities stormwater is rainfall and/or snowmelt that runs off roads, parking lots, roofs and other impermeable surfaces. It is usually directed via culverts and storm drains to the nearest ditch, stream, river or lake. In agricultural areas stormwater runoff enters streams and other water bodies via overland flow, rills and gullies and road ditches when the soil’s ability to infiltrate rainfall or snowmelt is exceeded. While agricultural land is the source of most of the contaminated runoff in Iowa, managing stormwater that enters rivers and streams is a mandated responsibility of local government. Flood and erosion control, water quality, community health, and management of greater ecosystem services require a local action.
Water and Watershed
Surface water is closely linked to the surrounding landscape in a variety of ways (Frissell et al. 1986). Flow, or discharge, can respond quickly to precipitation because the watershed serves to collect rainfall or snowmelt from uplands and slopes and route it to surface water through a variety of mechanisms including artificial drainage systems, shallow and deep groundwater pathways, and overland flow. Each of these mechanisms can transport sediment and other pollutants. It generally is overland flow, however, that erodes soil and transports most sediment and compounds, such as phosphorus (P) that are attached to soil particles and transported to surface waters. Whether a given precipitation event will generate overland flow is dependent on soil conditions, the type and extent of vegetation, and the intensity and duration of the precipitation.
During dry periods, water seeping through saturated sediment and rock is the source water for surface water. When the discharge in a stream consists only of inputs from shallow and deep subsurface flow, the stream is said to be at baseflow. During baseflow, erosion and sediment transport are minimal and streams tend to have high water clarity. Compounds that have infiltrated the soil and entered the shallow groundwater, such as nitrates (N), will be transported to the stream with the movement of groundwater. When the level of the shallow groundwater drops below the streambed, streams may go dry. Such streams are referred to as ephemeral, as opposed to perennial streams, which flow continuously. The shallow channels and swales occupied by ephemeral streams often have well-developed vegetation, and such vegetation slows the movement of water and decreases erosion during runoff periods.
The soils and geologic material of a watershed have a strong influence on surface and groundwater. The geology of a watershed influences water chemistry as well as the types and sizes of materials found on bed and banks of streams draining the watershed. Bed and bank material of low-gradient streams often will consist mainly of small particles, such as silt, clay and sand. Larger or higher-gradient streams often contain sandy bed and bank materials as well as gravel bars. The interface between the streambed and the water column is the benthic zone, and it is here that many of the important chemical and biological processes that occur within streams take place.
The riparian zone, the area immediately adjacent to stream channels, also plays a critical role in the health of surface waters. Riparian zones influence the movement of water and sediment via slowing, trapping and redistributing before reaching the stream channel. In the riparian zone subsurface water often passes through organic-rich soils where, given the right conditions, nitrate dissolved in the water is removed. This process, denitrification, also occurs within the bed and benthic zone of healthy streams. Sediment and sediment-attached pollutants can also be removed from runoff in the riparian zone, especially if a good vegetation cover is present.
Surface Water Degradation
In agricultural and urban landscapes degradation of surface waters results from changes in the watershed, riparian vegetation, and chemistry of runoff and groundwater. Activities that cause degradation are called stressors because they place a stress on the health of the system. The type, intensity, and location of the stressors determine the impact on surface waters. Often, multiple stressors occur simultaneously. Likewise, activities to improve the situation can address more than one stressor. For example, a constructed wetland might trap sediment, increase the rate of denitrification, and provide habitat — addressing three common stressors to Iowa’s surface waters.
Why are Nitrogen and Phosphorus a problem in surface waters? Nitrogen and Phosphorus have the same fertilizing effect on algae and aquatic plants that these nutrients have on crops and lawns. The result of nutrient loading to surface waters is eutrophication; excessive growth of algae and aquatic plants. Severe eutrophication, which lowers the amount of oxygen dissolved in the water, kills fish, facilitates harmful algal blooms, causes odor problems, and decreases the recreational and aesthetic value of surface waters. Eutrophication also decreases the diversity of pollution-sensitive animals but may increase the abundance of undesirable species. Surface and shallow groundwater have a natural capacity to process and retain nutrients because of the biological activity of microorganisms in the soil and the benthic zone. However, excessive input of nutrients from the watershed or decreasing the amount of time water spends in contact with the soil through artificial drainage can overwhelm this natural cleansing capacity. When this occurs, surface waters become a conduit for transporting excess nutrients and other pollutants to downstream water bodies, such as lakes, ponds, rivers, and the ocean.
Many of the characteristics of streams, such as temperature, turbidity and sediment size distribution, depend on the flow of water. As a result, changes to the natural hydrology, or patterns in discharge, act as a stressor to stream organisms. The life histories of many aquatic invertebrates and fish are closely tied to particular water temperature, flow and streambed conditions. Modifications to the hydrology of a watershed, such as channelization and tiling, change the flow conditions in the stream. Physically, a stream responds to hydrologic change by channel — adjustments such as by down-cutting, filling, widening, narrowing, or pattern shift. Down-cutting can lead to bank erosion, whereas filling will lead to the loss of channel capacity and flooding. Biologically, changes in hydrology result in loss of aquatic species for which the stream no longer supports suitable flow, temperature, substrate and chemical conditions.
Surface waters that are degraded or impaired typically suffer from multiple stressors, and it can be very difficult to isolate the impact of any one stressor. In the case of some stressors the origin may be in the upland areas of a watershed, whereas other stressors may originate locally. Designing, implementing and assessing programs to improve the health of surface waters should, therefore, consider all the interacting components of water bodies and their watershed as an integrated, temporally dynamic system.
Table 1. Approaches to Restoring the Natural Functions of These Systems
What options do towns and cities have when faced with managing their stormwater and agricultural runoff from outside their jurisdiction? Constructed wetlands are a very effective practice for managing stormwater and improving water quality in urban and rural environments. Constructed wetlands modify peak flow rates and floods in streams and rivers by temporarily storing water and releasing it more slowly than it enters the wetland and/or allowing a portion of the water to infiltrate into the local groundwater.
Constructed wetlands also offer a wide range of cost-free benefits to people and the environment. Properly constructed and maintained wetlands are attractive water features that enhance the local scenery and provide opportunities for a variety of outdoor recreational activities such as hiking, bird watching, nature photography, picnicking, and a variety of other outdoor recreational pursuits. Larger wetlands may provide opportunities for boating, canoeing, kayaking, swimming and fishing, and waterfowl hunting.
Wetlands also contribute to the diversity of habitats. They support wetland plants, amphibians, insects, birds and other wetland species with local habitat and are important components of regional landscapes that support migratory waterfowl (Gallant et al., 2011). Habitat diversity is critical for protecting biodiversity and for ensuring that organisms have suitable habitat and habitat corridors to better adapt to changing climate.
A final cost-free benefit of constructed wetlands is that some of the stormwater they capture infiltrates and contributes to aquifer recharge. This is an important benefit since much of what runs off today’s altered landscape formerly infiltrated and recharged local and regional aquifers. A large percentage of Iowa’s water-supply needs are met by groundwater, and recharge is critical to maintaining the abundance and quality of groundwater. Groundwater also contributes to the state’s streams, springs, lakes, and wetlands year-round, sustaining them and the habitats and industries they support during droughts and dry summer months.
Types of Constructed Wetlands
Stormwater basins are the most widely used method for managing stormwater in Iowa. Basins are designed to collect stormwater and slowly release it at a controlled rate to prevent flooding and erosion in downstream areas. While effective for flood and erosion control, these practices are not very effective for improving the quality of stormwater runoff and thereby preventing impacts to stream biological systems. There are two kinds of stormwater basins: detention basins and retention basins.
The main difference between the two types is whether the basin is designed to have a permanent pool of water — like a traditional “pond.” A low flow orifice controls the water level in these basins. Most of the time the orifice is part of a metal or concrete structure called a riser. A detention, or dry, basin has an orifice level with the bottom of the basin so that all of the water eventually drains out at a controlled rate and it remains dry between storms – hence, a dry basin. Retention basins have a riser with an orifice at a higher point so that it retains a permanent pool of water. Detention basins can provide water quality benefits by reducing the amount of sediment and sediment-attached pollutants entering streams. Some retention basins, known as stormwater wetlands, are designed with significant wetland vegetation that promotes biological activity to reduce the concentration of other pollutants such as Nitrogen and Phosphorous.
Bioswales are swaled drainage courses with gently sloped sides that contain vegetation growing in a permeable material (usually a soil/compost/sand mixture) and/or riprap. The water’s flow path, along with the wide and shallow ditch, is designed to maximize the time that water spends in the swale, which aids the trapping of sediment and attached pollutants and infiltration and biological treatment of soluble pollutants. Swales can be designed with an underdrain (dry swale) or without an underdrain. In the latter case, the bioswale acts much like a long, narrow intermittent wetland.
Bioretention Cells (Bio-cells)
Bio-cells are vegetated depressions that are sized and located to capture and temporarily pond runoff from impervious surfaces such as parking lots and roofs. They are filled with permeable bio-soil to a depth of 42 inches to 48 inches underlain by a perforated drainpipe in a rock bed, covered by sand. They are typically designed to pond from 6 inches to 9 inches following a runoff event and drain down in 12 to 24 hours. Bio-cells are planted to appear garden like, and plantings of deep-rooted native plant species are especially successful. Bioretention cells are very effective at filtering sediment and removing pollutants and excess nutrients from stormwater if properly designed and installed.
Other Biological Treatment Infrastructure
Other measures are available to manage local stormwater, although they will not treat agricultural flows. Green roofs are vegetated roofs that absorb most of the rainfall and thus decrease stormwater runoff at the source. Raingardens are small-scale bio-cells, usually without a subdrain, that collect runoff and allow it to infiltrate. These low-cost measures typically treat runoff from single downspouts or relatively small parking lots or other impermeable surfaces. Curb cuts are bioswales along streets where curbing along the gutter directs runoff into the bioswale. They function to remove sediment and other debris in the runoff and allow some of the runoff to infiltrate into the bioswale’s engineered soil.
For detailed information about these and other stormwater infrastructure, see the Iowa Stormwater Manual.
Information regarding small-scale homeowner or business owner stormwater management is available at Rainscaping Iowa: http://www.rainscapingiowa.org/
Stormwater wetlands vary widely in their ability to remove contaminants. All types of stormwater wetlands are effective at removing sediment. Excess nutrients in stormwater are also effectively removed in wetlands. Nitrogen concentration of stormwater can be reduced by 20 percent to 40 percent in wetlands (Dinnes, 2004). Several processes including denitrification, dilution, temporary nutrient sequestration in soil organic matter, trapping and retention of transported Nitrogen in nutrient-enriched sediments and particulates, and vegetative assimilation operate in wetlands to reduce nutrient concentrations. Heavy metals generally accumulate either in sediments or are associated with organic detritus and may or may not be taken up by plants and animals living in the wetlands. Many pesticides, organic compounds, oils and greases are broken down by microbes in these wetlands. Removal rates for bacteria are fair to good, and many viruses are immobilized or destroyed in wetlands.
Small wetlands have a large edge to surface area ratio and serve to slow water flow through them more than do larger wetlands. Wetland vegetation also slows water flow and attenuates peak runoff flows. In addition, a significant amount of water may be removed from a wetland by evapotranspiration. Because of all these factors, a series of small constructed wetland areas are generally more effective at reducing peak stormwater flows and reducing pollutant levels than a single large wetland.
Before constructing a wetland for stormwater management, the local hydrology must be understood in terms of catchment area and runoff volumes, local recharge or discharge areas, soil types and the seasonal pattern of water flow. In a groundwater recharge area, water in wetlands contributes to the recharge, and care must be taken to ensure that polluted or nutrient-rich waters do not enter and contaminate the ground water. In a groundwater discharge area, polluted stormwater may be diluted with ground water and if the groundwater quality is good, water quality improvement of stormwater entering the wetland will occur.
Many Iowa municipalities are faced with managing not only stormwater generated within their limits, but also runoff from agricultural sources outside their jurisdiction. Constructed wetlands are increasingly being used to manage all types of runoff. In general, constructed wetlands bring stability to the watersheds they serve by reducing storm event energy and decreasing flood events. They improve overall water quality, increase biodiversity, and build economic stability through the reduction of pollution and creation of beautiful community spaces.
The following information provides a general guideline for local governmental and private organizations to construct and properly maintain constructed wetlands. This guide also identifies potential collaborators and funding sources, and outlines the regulations and permitting requirements for some constructed wetlands.
J. Elizabeth Maas owns and operates Transition Ecology, LLC, a small native landscape consulting business in eastern Iowa that provides prairie, wetland and woodland consulting services, design, installation, and establishment practices for native environments, land restoration services, and prescribed fire planning and implementation. She has 11 years of experience as a Restoration Ecologist performing wetland delineations, mitigation and permitting procedures as well as sensitive areas and species assessments. In addition, she is a full-time Biology Instructor for Kirkwood Community College.
E. Arthur Bettis III is an Associate Professor of Soils Geomorphology at the University of Iowa, Geosciences Department. Prior to joining the University of Iowa faculty he worked in several capacities for the Iowa DNR Geological Survey. Dr. Bettis has been actively involved in research on various aspects of Iowa's soils, including wetland soils, and is considered one of the state's authorities on Iowa landscapes and the hydrology and physical processes acting in wetland soils.
We gratefully acknowledge the generous support of the McKnight Foundation and the Fred and Charlotte Hubbell Foundation, which made the preparation of this report possible. While these funders support the research that went into this report, they may not necessarily agree with policy recommendations that are included. Policy recommendations are solely the perspective of the authors and the Iowa Policy Project.