Water: Coastal Zone Act Reauthorization Amendments
C. Management Measure for Vegetated Treatment Systems
Promote the use of engineered vegetated treatment systems such as constructed wetlands or vegetated filter strips where these systems will serve a significant NPS pollution abatement function.
This management measure is intended to be applied by States in cases where engineered systems of wetlands or vegetated treatment systems can treat NPS pollution. Constructed wetlands and vegetated treatment systems often serve a significant NPS pollution abatement function. Under the Coastal Zone Act Reauthorization Amendments of 1990, States are subject to a number of requirements as they develop coastal NPS programs in conformity with this management measure and will have flexibility in doing so. The application of management measures by States is described more fully in Coastal Nonpoint Pollution Control Program: Program Development and Approval Guidance, published jointly by the U.S. Environmental Protection Agency (EPA) and the National Oceanic and Atmospheric Administration (NOAA) of the U.S. Department of Commerce.
As discussed in Section I.E of this chapter, vegetated treatment systems (VTS), by definition in this guidance, include vegetated filter strips and constructed wetlands. Although these systems are distinctly different, both are designed to reduce NPS pollution. They need to be properly designed, correctly installed, and diligently maintained in order to function properly.
The term NPS pollution abatement function refers to the ability of VTS to remove NPS pollutants. Filtering sediment and sediment-borne nutrients and converting nitrate to nitrogen gas are examples of the important NPS pollution abatement functions performed by vegetated treatment systems.
a. Vegetated Filter Strips
The purpose of vegetated filter strips (VFS) is to remove sediment and other pollutants from runoff and wastewater by filtration, deposition, infiltration, absorption, adsorption, decomposition, and volatilization, thereby reducing the amount of pollution entering surface waters (USDA, 1988). Vegetated filter strips are appropriate for use in areas adjacent to surface water systems that may receive runoff containing sediment, suspended solids, and/or nutrient runoff. Vegetated filter strips can improve water quality by removing nutrients, sediment, suspended solids, and pesticides. However, VFS are most effective in the removal of sediment and other suspended solids.
Vegetated filter strips are designed to be used under conditions in which runoff passes over the vegetation in a uniform sheet flow. Such a flow is critical to the success of the filter strip. If runoff is allowed to concentrate or channelize, the vegetated filter strip is easily inundated and will not perform as it was designed to function.
Vegetated filter strips need the following elements to work properly: (1) a device such as a level spreader that ensures that runoff reaches the vegetated filter strip as a sheet flow (berms can be used for this purpose if they are placed at a perpendicular angle to the vegetated filter strip area to prevent concentrated flows); (2) a dense vegetative cover of erosion-resistant plant species; (3) a gentle slope of no more than 5 percent; and (4) a length at least as long as the adjacent contributing area (Schueler, 1987). If these requirements are met, VFS have been shown to remove a high degree of particulate pollutants. The effectiveness of VFS at removing soluble pollutants is not well documented (Schueler, 1987).
b. Constructed Wetlands
Constructed wetlands are typically engineered complexes of saturated substrates, emergent and submergent vegetation, animal life, and water that simulate wetlands for human use and benefits (Hammer et al., 1989). According to Hammer and others (1989), constructed wetlands typically have four principal components that may assist in pollutant removal:
- Substrates with various rates of hydraulic conductivity;
- Plants adapted to water-saturated anaerobic substrates;
- A water column (water flowing through or above the substrate); and
- Aerobic and anaerobic microbial populations.
3. Management Measure Selection
This management measure was selected because vegetated treatment systems have been shown to be effective at NPS pollutant removal. The effectiveness of the two types of VTS is discussed in more detail in separate sections below.
a. Effectiveness of Vegetated Filter Strips
Several studies of VFS (Table 7-9) show that they improve water quality and can be an effective management practice for the control of nonpoint pollution from silvicultural, urban, construction, and agricultural sources of sediment, phosphorus, and pathogenic bacteria. The research results reported in Table 7-9 show that VFS are most effective at sediment removal, with rates generally greater than 70 percent. The published results on the effectiveness of VFS in nutrient removal are more variable, but nitrogen and phosphorus removal rates are typically greater than 50 percent. The following are nonpoint sources for which VFS may provide some nutrient-removal capability:
- Cropland. The primary function of grass filter strips is to filter sediment from soil erosion and sediment-borne nutrients. However, filter strips should not be relied on as the sole or primary means of preventing nutrient movement from cropland (Lanier, 1990).
- Urban Development. Vegetated filter strips filter and remove sediment, organic material, and trace metals. According to the Metropolitan Washington Council of Governments, VFS have a low to moderate ability to remove pollutants in urban runoff and have higher efficiency for removal of particulate pollutants than for removal of soluble pollutants (Schueler, 1987).
With proper planning and maintenance, VFS can be a beneficial part of a network of NPS pollution control measures for a particular site. They can help to reduce the polluting effects of agricultural runoff when coupled with either (1) farming practices that reduce nutrient inputs or minimize soil erosion or (2) detention ponds to collect runoff as it leaves a vegetated filter strip. Properly planned VFS can add to urban settings by framing small streams, ponds, or lakes, or by delineating impervious areas. In addition to serving as a pollution control measure, VFS can add positive improvements to the urban environment by increasing wildlife and adding beauty to an area.
b. Effectiveness of Constructed Wetlands
Constructed wetlands have been considered for use in urban and agricultural settings where some sort of engineered system is suitable for NPS pollution reduction.
A few studies have also been conducted to evaluate the effectiveness of artificial wetlands that were designed and constructed specifically to remove pollutants from surface water runoff (Table 7-10). Typical removal rates for suspended solids were greater than 90 percent (Table 7-10). Removal rates for total phosphorus ranged from 50 percent to 90 percent. Nitrogen removal was highly variable and ranged from 10 percent to 76 percent for total nitrogen.
Like vegetated filter strips, constructed wetlands offer an alternative to other systems that are more structural in design for NPS pollution control. In some cases, constructed wetland systems can provide limited ecological benefits in addition to their NPS control functions. In other cases, constructed wetlands offer few, if any, additional ecological benefits, either because of the type of vegetation installed in the constructed wetland or because of the quantity and type of pollutants received in runoff. In fact, constructed wetlands that receive water containing large amounts of metals or pesticides should be fenced or otherwise barricaded to discourage wildlife use.
As discussed more fully at the beginning of this chapter and in Chapter 1, the following practices are described for illustrative purposes only. State programs need not require implementation of these practices. However, as a practical matter, EPA anticipates that the management measure set forth above generally will be implemented by applying one or more management practices appropriate to the source, location, and climate. The practices set forth below have been found by EPA to be representative of the types of practices that can be applied successfully to achieve the management measure described above.
- a. Construct VFS in areas adjacent to waterbodies that may be subject to suspended solids and/or nutrient runoff.
A survey of the literature on the design, performance, and effectiveness of VFS shows that the following factors need to be considered on a site-specific basis before designing and constructing a vegetated filter strip:
- The effectiveness of VFS varies with topography, vegetative cover, implementation, and use with other management practices. In addition, different VFS characteristics such as size and type of vegetation can result in different pollutant loading characteristics, as well as loading reductions. Table 7-9 gives some removal rates for specific NPS pollutants based on VFS size and vegetation.
- Several regional differences are important to note when considering the use of VFS. Climate plays an important role in the effectiveness of VFS. The amount and duration of rainfall, the seasonal differences in precipitation patterns, and the type of vegetation suitable for local climatic conditions are examples of regional variables that can affect the performance of VFS. Soil type and land use practices are also regional differences that will affect characteristics of surface water runoff and thus of VFS performance. The sites where published research has been conducted on VFS effectiveness for pollutant removal are overwhelmingly located in the eastern United States. There is a demonstrated need for more studies located in different geographic areas in order to better categorize the effects of regional differences on the effectiveness of VFS.
- Vegetated filter strips have been successfully used in a variety of situations where some sort of BMP was needed to treat surface water runoff. Typical locations of VFS have included:
- Below cropland or other fields;
- Above conservation practices such as terraces or diversions;
- Between fields;
- Alternating between wider bands of row crops;
- Adjacent to wetlands, streams, ponds, or lakes;
- Along roadways, parking lots, or other impervious areas;
- In areas requiring filter strips as part of a waste management system; and
- On forested land.
VFS function properly only in situations where they can accept overland sheet flow of runoff and should be designed accordingly. If existing site conditions include concentrated flows, then BMPs other than VFS should be used. Contact time between runoff and the vegetation is a critical variable influencing VFS effectiveness. Pollutant-removal effectiveness increases as the ratio of VFS area to runoff-contributing area increases.
- Key elements to be considered in the design of VFS areas follow:
- Type and Quantity of Pollutant. Sediment, nitrogen, phosphorus, and toxics are efficiently removed by VFS (see Table 7-9). However, removal rates are much lower for soluble nutrients and toxics.
- Slope. VFS function best on slopes of less than 5 percent; slopes greater than 15 percent render them ineffective because surface runoff flow will not be sheet-like and uniform. The effectiveness of VFS is strongly site-dependent. They are ineffective on hilly plots or in terrain that allows concentrated flows.
- Native/Noninvasive Plants. The best species for VFS are those which will produce dense growths of grasses and legumes resistant to overland flow. Use native or at least noninvasive plants to avoid negatively impacting adjacent natural areas.
- Length. The length of VFS is an important variable influencing VFS effectiveness because contact time between runoff and vegetation in the VFS increases with increasing VFS length. Some sources recommend a minimum length of about 50 feet (Dillaha et al., 1989a; Nieswand et al., 1989; Schueler, 1987). USDA (1988) has prepared design criteria for VFS that take into consideration the nature of the source area for the runoff and the slope of the terrain. Another suggested design criterion that can be found in the literature is for the VFS length to be at least as long as the runoff-contributing area. Unfortunately, there are no clear guidelines available in the literature for calculating VFS lengths for specific site conditions. Accordingly, this guidance does not prescribe either a numeric value for the minimum length for an effective filter strip or a standard method to be used in the design criteria for computing the length of a VFS.
- Detention Time. In the design process for a vegetated filter strip, some consideration should be given to increasing the detention time of runoff as it passes over the VFS. One possibility is to design the vegetated filter strip to include small rills that run parallel to the leading edge of the vegetated filter strip. These rills would serve to trap water as runoff passes through the vegetated filter strip. Another possibility is to plant crops upslope of the vegetated filter strip in rows running parallel to the leading edge of the vegetated filter strip. Data from a study by Young and others (1980), in which corn was planted in rows parallel to the leading edge of the filter strip, show an increase in sediment trapping and nutrient removal.
- Monitoring of Performance. The design, placement, and maintenance of VFS are all very critical to their effectiveness, and concentrated flows should be prevented. Although intentional planting and naturalization of the vegetation will enhance the effectiveness of a larger filter strip, the strip should be inspected periodically to determine whether concentrated flows are bypassing or overwhelming the BMP, particularly around the perimeter. The vegetated filter strip should also be regularly inspected to determine whether sediment is accumulating within the vegetated filter strip in quantities that would reduce its effectiveness (Magette et al., 1989).
- Maintenance. For VFS that are relatively short in length, natural vegetative succession is not intended and the vegetation should be managed like a lawn. It should be mowed two or three times a year, fertilized, and weeded in an attempt to achieve dense, hearty vegetation. The goal is to increase vegetation density for maximum filtration. Accumulated sediment and particulate matter in a VFS should be removed at regular intervals to prevent inundation during runoff events. The frequency at which this type of maintenance will be required will depend on the frequency and volume of runoff flows. Also, if the soil is moderately erodible in the drainage area, additional precautions should be taken to avoid excessive buildup of sediment in the grassed area (NVPDC, 1987). Development of channels and erosion rills within the VFS must be avoided. To ensure effectiveness, sheet flow must be maintained at all times. The maintenance of VFS located adjacent to streams is especially important since sediment bypassing a VFS and entering a coastal waterbody will cause problems for the spawning and early juvenile stages of fish.
- Type and Quantity of Pollutant. Sediment, nitrogen, phosphorus, and toxics are efficiently removed by VFS (see Table 7-9). However, removal rates are much lower for soluble nutrients and toxics.
Dillaha and others (1989b) showed that many of the VFS installed in Virginia performed poorly because of poor design and maintenance. Consider including one or more of the following items in a VFS maintenance program to make the performance of any VFS more efficient:
- Adding a stone trench to spread water effectively across the surface of the filter;
- Keeping the VFS carefully shaped to ensure sheet flow;
- Inspecting for damage following major storm events; and
- Removing any accumulation of sediment.
- b. Construct properly engineered systems of wetlands for NPS pollution control. Manage these systems to avoid negative impacts on surrounding ecosystems or ground water.
Several factors must be considered in the design and construction of an artificial wetland to ensure the maximum performance of the facility for pollutant removal:
Hydrology. The most important variable in constructed wetland design is hydrology. If the proper hydrologic conditions are developed, the chemical and biological conditions will, to a degree, respond accordingly (Mitsch and Gosselink, 1986).
Soils. The underlying soils in a wetland vary in their ability to support vegetation, to prevent percolation of surface water into the ground water, and to provide active exchange sites for adsorption of constituents like phosphorus and metals.
Vegetation. The types of vegetation used in constructed wetlands depend on the region and climate of the constructed wetland (Mitsch, 1977). When possible, use native plant species or noninvasive species to avoid negative impacts to nearby natural wetland areas. There are several guides for the selection of wetland plants such as the Midwestern Guide to Flora (USDA) or the Florida Department of Environmental Regulation's list of suggested wetland species.
Influent Water Quality. Characterization of influent water quality, such as the types and magnitude of the pollutants, will determine the design characteristics of the constructed wetland.
Geometry. The size and shape of the constructed wetland will influence the detention time of the wetland, the flow rate of surface water runoff moving through the system, and the pollutant removal effectiveness under "typical" conditions.
Pretreatment. Constructed wetlands should contain forebays to trap sediment before runoff enters the vegetated area of the constructed wetland system. Baffles and diversions should be strategically placed to prevent trapped sediment from becoming resuspended during subsequent storm events prior to cleanout.
Maintenance. Constructed wetlands need to be maintained for optimal performance. Since pollutant removal is the primary objective of the constructed wetland, vegetation and sediment removal are two of the more important maintenance considerations. Properly designed constructed wetlands should not need any maintenance of vegetation. Constructed wetlands must be managed to avoid any negative impacts to wildlife and surrounding areas. For example, non-native or undesirable plant species must be kept out of adjacent wetlands or riparian areas. Contamination of sediments due to toxics entering the constructed wetland must also be controlled. The Kesterson National Wildlife Refuge in California is an excellent example of a case in which selenium contamination in wetland sediments was found to cause deaths and deformities in visiting waterfowl (Ohlendorf et al., 1986). Forebays and deep water areas should be inspected periodically, and excess sediment should be removed from the system and disposed of in an appropriate manner. Other routine maintenance requirements include wildlife management, mosquito control, and debris and litter removal (Mitsch, 1990; Schueler, 1987). As debris and litter collect in the detention basins and vegetated areas, they need to be routinely removed to prevent channelization and outflow blockage from occurring. The area around the constructed wetland should be mowed periodically to keep a healthy stand of grass or other desirable vegetation growing. Structural repairs and erosion control should also be done when needed.
Effectiveness of Constructed Wetlands
Table 7-10 summarizes the pollutant-removal effectiveness of constructed wetland systems built for treatment of surface water runoff. In general, constructed wetland systems designed for treatment of NPS pollution in surface water runoff were effective at removing suspended solids and pollutants that attach to solids and soil particles (refer to Table 7-10). The constructed wetland systems were not as effective at removing dissolved pollutants and those pollutants that dissolve under conditions found in the wetland. When the overall effectiveness data are compared among systems, no discernible trends are apparent. Although attempts to correlate removal effectiveness with an area or volume ratio have not shown any significant trends, the constructed wetlands listed in Table 7-10 still served a valuable role in pollutant removal. Total solids removal ranged from 63 percent to 94 percent among the five systems. Nitrogen removal was not as effective, with effectiveness ranging from 10 percent to 76 percent. Phosphorus removal ranged from 37 percent to 90 percent among the constructed wetland systems compared in this document.
Whether constructed wetlands and VFS are used individually or in series will depend on several factors, including the quantity and quality of the inflowing runoff, the characteristics of the existing hydrology, and the physical limitations of the area surrounding the wetland or riparian area to be protected.
A schematic drawing of a system of filter strips and constructed wetland placed in the path of the existing surface water supply to a stream is shown in Figure 7-2.
5. Costs for All Practices
The use of appropriate practices for pretreatment of runoff and prevention of adverse impacts to wetlands and other waterbodies involves the design and installation of vegetated treatment systems such as vegetated filter strips or constructed wetlands, or the use of structures such as detention or retention basins. These types of systems are discussed individually elsewhere in this guidance document. Refer to Chapter 4 for a discussion of the costs and effectiveness of detention and retention basins. The purpose of each of these BMPs is to remove, to the extent practicable, excessive levels of NPS pollutants and to minimize impacts of hydrologic changes. Each of these BMPs can function to reduce levels of pollutants in runoff or attenuate runoff volume before the runoff enters a natural wetland or riparian area or another waterbody.
Several source documents contain information on costs for vegetated treatment systems. Nieswand and others (1989) published costs for vegetated filter strips employed as part of watershed management strategies for New Jersey. Costs varied over a wide range depending on whether the method of installation involved seeding, sodding, or hydroseeding. Another source of cost information on filter strips is EPA's NWQEP 1988 Annual Report: Status of Agricultural Nonpoint Source Projects (1988).
The most comprehensive source of cost data for filter strips was obtained from the USDA ASCS, which provides cost share reimbursement each year to individual farmers for a variety of practices contained in the National Handbook of Conservation Practices (1988). Information was obtained from USDA on the costs in each State for work performed in accordance with Specification No. 393 (Filter Strips) in the National Handbook for the base year of 1990. Based on these data, a total of 914 filter strip projects were installed with cost share assistance in 28 States. The total cost of these projects was $833,871.00. The total combined length of all projects was 6,443,800 linear feet. If an average width of 66 feet is assumed for the filter strip, then an average cost per acre is calculated at $85.41 per acre, in 1990 dollars.
For constructed wetlands, examples of cost data are as follows:
- Lake Jackson, Florida: A cost of $80,769 was reported in 1990 for design and construction of a 9.88-acre constructed wetland for treatment of urban nonpoint runoff (Mitsch, 1990).
Cost in 1990 dollars - $ 8,175.00/acre
- Greenwood Urban Wetland, Minnesota: A cost of $20,370 was reported in 1990 for design and construction of a 27.2-acre wetland for treatment of urban nonpoint runoff (Mitsch, 1990).
Cost in 1990 dollars - $ 748.89/acre
- Broward County, Florida: A cost range of $10,000 to $100,000 per acre (1992) was given for constructing surface water runoff wetlands on sites of new developments. The average cost for constructing a wetland was given as $20,000. The costs represent mucking (depositing organic material substrate) and planting emergent wetlands plants. Site monitoring adds $10,000 to $12,000 per year for sites up to 10 acres. (Goldasich, Broward County Office of Natural Resources Protection, personal communication, July 1992).
Cost in 1990 dollars - $19,200/acre
It is important to note that the type of constructed wetland facility described in this guidance is for treatment of urban or agricultural runoff. To avoid confusion, costs of wetlands constructed for other purposes, particularly for municipal wastewater treatment, were not considered.
As illustrated by the three examples cited above, the cost per acre of constructed wetlands facilities will vary from site to site. One reason is that certain items of work have economies of scale that are rather limited. For example, costs for site surveys, design, gaining access to the site, mobilization of equipment, and installation of sediment and surface water runoff controls do not necessarily increase in proportion to the size of the project. Other factors that affect costs are regional variations in suitable plant species, treatment of existing surface water flow patterns, and detention/retention capacity.
Based on the cost data contained in the source documents, costs are reported below for three realistic hypothetical scenarios of systems of constructed wetlands and vegetated filter strips.
- One filter strip at a cost of - $ 129.00
- Includes design and installation of a grass filter strip 1,000 feet long and 66 feet wide.
- Most effective at trapping sediments and removing phosphorus from surface water runoff.
- One constructed wetland at a cost of - $ 5,000.00
- Includes design and installation of a constructed wetland whose surface area is 0.25 acre in size. The constructed wetland is planted with commercially available emergent vegetation.
- Most effective at removing nutrients and at decreasing the rate of inflow of surface water runoff.
- One combined filter strip/constructed wetland $ 5,129.00