MANAGEMENT MEASURES

Animal Waste Collection
Curb Elimination
Debris Removal
Parking Lot And Street Cleaning Operations
Road Salt Application Control

STRUCTURAL MEASURES

Dry Detention Basins
Infiltration Devices
Oil and Grease Trap Devices
Porous Pavement
Sand Filters
Vegetative Practices

Filter Strips
Grassed Swales

Wetlands, Constructed
Wetlands, Natural and Restored
Wet Retention Ponds

LINKS

REFERENCES

INTRODUCTION

This section discusses best management practices (BMPs) to control or prevent contamination of stormwater runoff from roads. Two distinct types of stormwater management practices are presented. The first subsection addresses the use of management measures (nonstructural practices) to control stormwater pollution. The latter subsections discuss structural measures. The descriptions are general in nature to introduce and explain the practices. More detailed information is available in other sources listed in the References. Sources of material for this section include the Decisionmaker's Stormwater Handbook by N. Phillips (1992), Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban BMPS by T. Schueler (1987), and Stormwater Management Guidance Manual by J.A. Arnold et al. (1993).

BEST MANAGEMENT PRACTICES

MANAGEMENT MEASURES

Preventive measures, sometimes called source controls, are management techniques that reduce the capture of substances by stormwater. Since our discussion is oriented toward users who are experiencing existing water quality problems, it is focused more on mitigative measures, therefore first line planning actions are not discussed here. However, many measures that mitigate existing water quality problems are preventive in nature, and these are discussed in the following section. Such practices use alternative maintenance procedures, education of management and technical personnel, or redesign of structures to reduce the amounts of pollutants entering stormwater and accumulating on impervious areas. Preventive measures are very cost-effective ways to manage stormwater runoff. Usually they require no land area, no construction and can be implemented with moderate effort. The following are examples of such preventive measures.

One of the most effective ways to mitigate stormwater pollution is to prevent potential pollutants from entering stormwater at their sources. It is generally far more effective and less expensive to prevent a pollutant from entering stormwater than it is to try to remove the pollutant after the stormwater has become contaminated. The following practices present methods for using source reduction to control stormwater pollutants.

ANIMAL WASTE COLLECTION
Animal wastes contribute significantly to the numbers of bacteria and organic matter in stormwater runoff. This problem is particularly serious because the wastes are deposited in street gutters where runoff carries the waste directly into streams. Animal wastes can be controlled through ordinances requiring collection and removal of the waste from roadways where it can otherwise be washed directly into receiving waters. The ordinances should include guidance on proper disposal of animal wastes.

CURB ELIMINATION
The elimination of curbs has been shown to reduce pollution entering the aquatic environment. Because curbs function as channels for stormwater, runoff flows at high velocities in curbs, carrying with it much sediment and other pollution. Without curbs, runoff can be spread over large vegetated areas where runoff velocities can be reduced and pollutants can settle out and be taken up by plants or soils. Sections of existing curb can be removed and curb outlets can be installed at regular intervals or in appropriate areas to allow the stormwater to flow onto well-vegetated areas. To avoid erosion, flooding and trash accumulation, the areas to install curb outlets should be carefully chosen and street cleaning programs should be modified to maintain these areas.

DEBRIS REMOVAL
Stormwater control and conveyance structures require frequent debris removal to maintain proper function. Litter and yard wastes can clog inlets, catch basins, and outlets, lead to overflows, erosion and unintended flooding, and make these devices ineffective in stormwater pollutant removal. Grates on inlets and outlets must prevent entry by children but should be easily cleaned by maintenance crews. Municipal facilities maintenance programs and commercial, and industrial stormwater permittees should be required to regularly clean inlets, catch basins, clean-out access points, and outlets. Forebays can be installed where feasible prior to entry into ponds. Forebays are very useful in promoting proper maintenance and cleaning. They are easily cleaned and separate much of the sediment, associated pollutants, and trash and floatables from the main pond. Paving portions of the forebay allows easy access for maintenance equipment. Hard bottoms can also be made permeable through the use of turf blocks or flexible revetment.

PARKING LOT AND STREET CLEANING
Street cleaning is usually performed to improve the appearance of streets and access roads; however, it can reduce pollutants in runoff if it is performed regularly. Another benefit of street cleaning is that pipes and outlets in detention structures and ponds are less likely to become clogged. New street sweeping machines pick up much finer materials than older models, a feature designed to help reduce the transport of sediment-bound pollutants. Disposal of street sweeping wastes may pose a problem because of possible high levels of lead, copper, zinc and other wastes from automobile traffic. Testing of street sweepings may be appropriate to determine appropriate disposal or reuse alternatives. Some municipalities and industries have found that street sweepings can be used as cover in sanitary landfills. Industries could be required to regularly sweep access roads, parking lots, truck aprons and loading dock areas. Homeowners should be educated not to use streets and curbs as disposal areas. Yard wastes and grass clippings can be disposed of in compost piles and the compost used around shrubs and in flower beds.

ROAD SALT APPLICATION CONTROL
In areas where salt is used, reduced application or alternative agents, consistent with the need for safety, will reduce pollution of area water bodies. Sand is an alternative that is less harmful to vegetation and aquatic life. Storage facilities can be constructed or modified to prevent salt exposure to rainfall.

STRUCTURAL MEASURES

There are a number of control measures used to reduce the levels of pollutants in stormwater that has been exposed to pollutant sources. Stormwater control practices can employ inexpensive techniques, such as settling, biological uptake of substances, and infiltration to treat stormwater. These techniques will remove some portion of most common pollutants and the practices presented here will apply to most situations. However, in situations involving very sensitive waters or unusual pollutants, it is possible that more sophisticated techniques will be required to meet NPDES discharge standards or other requirements. For those situations where more sophisticated techniques are needed, you may wish to investigate some of the references listed at the end of the section. This section presents common stormwater control practices in use today. Each BMP includes a general description of the practice, lists a range of pollutant reduction for various pollutants, and presents some general design considerations, advantages and disadvantages.

DRY DETENTION BASINS
Dry detention basins, also called dry detention devices and ponds, temporarily detain a portion of stormwater runoff for a specified length of time, releasing the stormwater slowly to reduce flooding and remove a limited amount of pollutants. They are referred to as "dry detention" because these devices are meant to dry out between rain events. Pollutants are removed by allowing particulates and solids to settle out of the water. Overall pollutant removal in dry detention devices is low to moderate. Important reasons for use of dry detention basins are reducing peak stormwater discharges, controlling floods and preventing downstream channel scouring.

There are several types of dry detention devices, the most common being the dry detention basin and the extended dry detention basin. These are structures which hold a certain amount of water from a storm and which release the water through a controlled outlet over a specified time period based on design criteria. The extended detention basin drains more slowly or may retain a permanent pool of water. The major failure of these basins is that the release of water is often too slow to empty the basin before the next storm. Since the basin is partially full, only a portion of the design runoff volume from the next storm is detained and the remainder is bypassed directly into the stream. With little or no detention, few pollutants are removed from the runoff. Such failures can be prevented through adequate design and maintenance to keep the inlets and outlets open. Many dry basins have partially failed or are not meeting design performance due to clogging of inlets or outlets. Dry detention basin effectiveness is rated low to moderate compared to other stormwater BMPs. Typical dry basin removal efficiencies are listed below for selected pollutants.

Table 4.7 Dry Detention Basin Pollutant Removal
Pollutant	Estimated Removal Efficiency
Plant Nutrients Total phosphorus Total nitrogen	low low
Sediment Total suspended solids	High
Metals Lead Zinc	Moderate to High Moderate
Organic Matter Biochemical and chemical oxygen demand (BOD or COD)	Moderate
Oil and Grease	Low
Bacteria	High

Compiled from Schueler 1987; Schueler, et al. 1992; US EPA 1990; Phillips 1992; Birch, et al. 1992 and others.

Design Considerations. Design of dry detention basins includes locating proper sites for construction of the basin, calculating the appropriate detention time, treatment of the expected range in volumes of stormwater from storms, and maintenance procedures and schedules. The stormwater should be held for at least 24 hours for maximum pollutant removal. Soils should be permeable to allow the water to drain from these basins between storms and the water table should be more than two feet below the bottom of the basin (to avoid a permanent pool of water in the basin during wet weather). A forebay is a section of the basin separated from the main part of the basin by a wall or dike and which receives the incoming stormwater. Forebays help capture debris and sand deposits, which accumulate quickly, and thereby ease routine cleaning.

Advantages. Dry detention basins are capable of removing significant amounts of pollutants and have proven effective at reducing peak storm flows. An appreciable body of knowledge has been accumulated on the design and maintenance of these structures. Detention basins can serve small to rather large developments and are usually readily incorporated into the design of the overall development. Existing dry basins built to control stormwater peak flows can be modified to provide extended detention for stormwater, thus improving pollutant removal.

Disadvantages. Dry basins can be unsightly, especially if floating and other debris accumulate in them. Basins should be located where they are not easily seen or where they can be concealed with landscaping. Dry basins are not very effective in removing pollutants from stormwater; therefore, the receiving water will have limited protection from pollution. Also, many pollutants that settle out are resuspended in the next storm flow and are discharged into the stream. Many dry basins end up with permanent pools of water because runoff from previous storms has not either flowed out or infiltrated before another storm occurs. The standing water can be a nuisance and an eyesore to residents. Because they take up large areas, dry detention basins are generally not best suited for high-density residential developments. Sites must allow easy access for equipment to maintain and clean the basin and remove sediment. The appearance of some dry detention basins has been improved by planting hardy wildflowers in the bottom. Resident acceptance of a "wildflower basin" is much higher than of an unadorned open basin. The maintenance costs associated with dry detention basins are higher than other stormwater treatment devices.

Maintenance. Maintenance of dry detention basins is both essential and costly. General objectives of maintenance are to prevent clogging, prevent standing water and prevent the growth of weeds and wetland plants. This requires frequent unclogging of the outlet and mowing. Normal maintenance costs can range from 3-5% of construction costs on an annual basis (Schueler, 1987). Cleaning out sediment, which is expensive, will be necessary in 10 to 20 years' time. Cleaning involves digging out the accumulated sediment, mud, sand and debris with construction backhoe or other earth-moving equipment.

INFILTRATION (EXFILTRATION) DEVICES
Infiltration refers to the process of water entering into the soil, which indicates the predominant means by which these devices evacuate their treatment volume. There are a number of devices used to treat stormwater that make use of infiltration to remove pollutants and to recharge or replenish the ground water. Infiltration devices include infiltration basins, infiltration trenches and dry wells. The term exfiltration is also frequently used in reference to these BMPs, based on the perspective of the device rather than its setting. Properly designed infiltration devices can closely reproduce the water balance that existed pre-development, providing ground water recharge, control of peak flows from stormwater and protection of streambanks from erosion due to high flows. A significant advantage of infiltration is that in areas with a high percentage of impervious surface, infiltration is one of the few means to provide significant groundwater recharge.

Infiltration devices can remove pollutants very effectively through adsorption onto soil particles, and biological and chemical conversion in the soil. Infiltration basins with long detention times and grass bottoms enhance pollutant removal by allowing more time for settling and because the vegetation increases settling and adsorption of sediment and adsorbed pollutants. Although infiltration is a simple concept, infiltration devices must be carefully designed and maintained if they are to work properly. Poorly installed or improperly located devices fail easily. It is critical that infiltration devices only be used where the soil is porous and can absorb the required quality of stormwater. Maintenance needs for infiltration devices are higher than other devices partly because of the need for frequent inspection. Nuisance problems can occur, especially with insect breeding, odors and soggy ground.

Pollutant removal capability for infiltration basins that exfiltrate the entire amount of captured stormwater is shown below. Other infiltration devices which exfiltrate only part of the captured stormwater (some of the stormwater is discharged to receiving waters on the surface) have lower removal effectiveness.

Table 4.3 Infiltration Devices Pollutant Removal
Pollutant	Estimated Removal Efficiency
Plant Nutrients Total phosphorus Total nitrogen	high high
Sediment Total suspended solids	Very High
Metals Trace metals (sediment-bound)	Very high
Organic Matter Biochemical and chemical oxygen demand (BOD)	Very High
Oil and Grease	High
Bacteria	Very High

Compiled from Schueler 1987; Schueler, et al. 1992; US EPA 1990; Phillips 1992; Birch, et al. 1992 and others

Design Considerations. Some infiltration devices (infiltration trenches, dry wells, and catch basins) can be constructed under parking lots and roads, taking very little land from other uses. Other infiltration devices take up considerable areas, depending on their size and the drainage area served. Locating smaller infiltration devices is fairly easy so that large downstream devices can be replaced with a number of small structures upstream and still achieve the same control of stormwater. Infiltration devices require permeable soils and reasonably deep water tables. Smaller infiltration devices such as dry wells basins can be located near buildings to capture the runoff from roofs and other impervious surfaces.

Advantages. Infiltration devices help replenish the ground water and reduce both stormwater peak flows and volume. Pollutant removal can be very high for many pollutants. Because they take up little land area and are not highly visible, many underground infiltration devices can be located close to residential and commercial areas.

Disadvantages. Infiltration techniques work only where the soils are permeable enough that the water can exit the storage basin and enter the soil. These devices have a high failure rate. Infiltration devices must have sediment removed before the stormwater enters the device to prevent clogging of the soil. The water table must be at least two feet under the bottom of the device.

Maintenance. Maintenance requirements include regular inspections, cleaning of inlets, mowing and possible use of observation wells to maintain proper operation. Infiltration basins and sediment removal devices used to prevent clogging of other infiltration devices must have the sediment removed regularly. If an infiltration device becomes clogged, it may need to be completely rebuilt.

OIL AND GREASE TRAP DEVICES
A number of devices are used to remove oil and grease from stormwater. One type, commonly known as oil-water separators, are mechanical devices manufactured by various industrial equipment manufacturers and usually installed at industrial sites. These devices employ various mechanisms, some of which are proprietary, to separate oil from stormwater, which is then discharged to a treatment plant or to a receiving water. Oil-water separators usually require support from the manufacturer and are best used where these devices can be properly maintained and frequently inspected, such as at industrial sites. Information concerning these devices, their installation, use and requirements can be obtained from the manufacturer or a consultant. Another type of oil and grease removal device is the oil and grease trap catch basin (or oil and grit separator). These catch basins are underground devices used to remove oils, grease, other floating substances and sediment from stormwater before the pollutants enter the storm sewer system. They are usually placed to catch the oil and fuel that leak from automobiles and trucks in parking lots, service stations, and loading areas. A third type of device is a simple skimmer and control structure used at the outlet of a sediment basin (forebay), typically used prior to discharge into a larger detention device. This section discusses the latter two designs.

A popular design for the oil and grease trap catch basin uses three chambers to pool the stormwater, allow the particulates to settle and remove the oil. As the water flows through the three chambers, oils and grease separate either to the surface or sediments and are skimmed off and held in the catch basin. The stormwater then passes on to the storm sewer or into another stormwater pollution control device. Because these devices are relatively small and inexpensive, they can be placed throughout a drainage system to capture coarse sediments, floating wastes, and accidental or illegal spills of hazardous wastes. Oil and grease trap catch basins can reduce maintenance of infiltration systems, detention basins and other stormwater devices. Since these catch basins detain stormwater for only short periods, they do not remove other pollutants as effectively as facilities that retain runoff for longer periods. However, these basins can be effectively used as a first stage of treatment to remove oil and sediment from stormwater before it enters another, larger stormwater pollution control device, such as a wet pond. The second design involves an open sedimentation basin with a skimmer plate extending below the ponding control elevation at the outlet. Stormwater velocities are reduced in this sump, dropping out coarse sediment and separating oils and greases and floatables, which are retained in the basin by the skimmer as the stormwater discharges to a larger detention device or off-site. These sediment sump/skimmers are often designed larger than the underground chambers, have longer detention times, and thus remove more of the sediments and oils and greases.

Pollutant removal varies depending on the basin volume, flow velocity, and the depth of baffles and elbows in the chamber design. Well maintained catch basins should remove the following levels of pollutants, with the open sump/skimmer design showing somewhat higher levels.

Table 4.8 Oil and Grease Trap Pollutant Removal
Pollutant	Estimated Removal Efficiency
Plant Nutrients	None
Sediment Total suspended solids	Low
Metals Trace metals	Low
Oil and Grease	High
Organic Matter	Low
Bacteria	Low

Compiled from Schueler 1987; Schueler, et al. 1992; Phillips 1992; Birch, and Pressley. 1992.

Design Considerations. Oil and grease trap catch basins can be installed in most areas. Drainage areas flowing into the catch basin must be no larger than two acres and the catch basin must be large enough to handle dry weather flows that enter the basin. These catch basins can be installed in almost any soil or terrain, which allows their use near or at the impervious surfaces contributing heavily to the stormwater runoff. Little land area is taken up by catch basins as they require only enough area for proper maintenance. Oil and grease skimmer design is essentially that of a sedimentation basin with a conrol structure discharge that allows for mounting of a skimmer plate. The plate should extend sufficiently below the lowest discharge level to preclude siphoning of the water surface by the discharge.

Advantages. Oil and grease trap catch basins are inexpensive and easily installed in most areas. Since these devices are underground, there should be few complaints concerning appearances. These catch basins can be used very effectively as part of a system of stormwater controls to remove oily pollutants and coarse sediment before they enter another stormwater control device. Also small catch basins can be distributed over a large drainage area, which may prove advantageous over constructing a single large structure downstream. Sediment basins with skimmers are simpler and more easily maintained than the chamber design, tend to be larger and more effective in their role, and allow for photodegradation of hydrocarbons in addition to settling.

Disadvantages. Pollutant removal is low for contaminants other than oil, grease and coarse sediment for both types of systems. Both must have the accumulated sediment removed or cleaned out frequently to prevent sediment-bound pollutants from being stirred up and washed out in subsequent storms. Sediment removal removes the oil and grease because these pollutants eventually bind to the sediment. The chamber type is more difficult to maintain because of its enclosed, underground design, and typically is less efficient than the sediment basin because it tends to be smaller. Odors are sometimes a problem.

Maintenance. Oil and grease trap catch basins require regular inspection and cleaning at least twice a year to remove sediment, accumulated oils and grease, floatables, and other pollutants. Sump/skimmers require periodic but less frequent sediment removal. Wastes removed from these systems should be tested to determine proper disposal methods. The wastes may be hazardous; therefore, maintenance costs should be budgeted to include disposal at a proper site.

POROUS PAVEMENT
Design and Uses: Porous pavement is an alternative to conventional pavement that is intended to reduce imperviousness and consequently minimize surface runoff. Porous pavement follows one of two basic designs. First, it may be comprised of asphalt or concrete that lacks the finer sediment found in conventional cement. This formulation is usually laid over a thick base of granular material (Urbonas and Stahre, 1993). Second, porous pavement may be formed with modular, interlocking open-cell cement blocks laid over a base of coarse gravel (Urbonas and Stahre, 1993). A geo-textile fabric underlying the gravel prevents the migration of soil upward into the gravel bed. Both designs typically include a reservoir of coarse aggregate stone beneath the pavement for stormwater storage prior to exfiltration into surrounding soils (Schueler et al., 1992). Use of porous pavement requires permeable soils with a deep water table. Traffic must be restricted to exclude heavy vehicles. Its use is not advisable in areas expecting high levels of off-site sediment input, including chemicals and sand used in snow removal operations.

Pollutant Removal: The porous pavement itself functions less as a treatment BMP and more as a conveyance BMP to the other necessary component of the design, the underlying aggregate chamber, which functions as an infiltration device. As with other infiltration devices, treatment is provided by adsorption, filtration, and microbial decomposition in the sub-soil surrounding the aggregate chamber, as well as by particulate filtration within the chamber. Operating systems have been shown to have high removal rates for sediment, nutrients, organic matter, and trace metals. These rates are largely due to the reduction of mass loadings of these pollutants through transfer to groundwater (Schueler et al., 1992).

Advantages and Disadvantages: The big disadvantage of porous pavement is that sites have a high failure rate, due to clogging either from improper construction, accumulated sediment and oil, or resurfacing (Schueler et al., 1992). Excessive sediment will cause the pavement to rapidly seal and become ineffective (Urbonas and Stahre, 1993). The modular, interlocking, open-cell concrete block type tends to remain effective for considerably longer than asphalt or concrete porous pavement. Porous pavement must be maintained frequently to continue functioning. Quarterly vacuum sweeping and/or jet hosing is needed to maintain porosity, and this may constitute one to two percent of the initial construction costs (Schueler et al., 1992).

Positive attributes include the diversion of potentially large volumes of surface runoff to groundwater recharge, providing both water quality and quantity benefits. While more expensive than conventional pavement, it can also eliminate the need for more involved stormwater drainage, conveyance, and treatment systems, offering a valuable option for spatially constrained urban sites. Porous pavement may be most beneficial in watersheds with high percentages of impervious surface and high volumes of runoff. Its use is typically recommended for lightly trafficked satellite parking areas and access roads. Increased infiltration at the source (parking lots, etc.) will reduce the both volume of runoff and the delivery of associated pollutants to water bodies.

SAND FILTERS
Sand filters are a type of stormwater control device used to treat stormwater runoff from large buildings, access roads and parking lots. As the name implies, sand filters work by filtering stormwater through beds of sand. Small sand filters are installed underground in trenches or pre-cast concrete boxes. Large sand filters are above-ground, self-contained sand beds that can treat stormwater from drainage areas as much as five acres in size.

To date, the city of Austin, Texas and the state of Florida have made use of the large, above-ground versions of sand filters, while the underground sand filters have been installed in Florida, Maryland, Delaware and the District of Columbia. Both above-ground and underground versions use some form of pre-treatment to remove sediment, floating debris, and oil and grease to protect the filter. After the stormwater passes through the pre-treatment device, it flows onto a sand filter bed. As the stormwater flows through the filter bed, sediment particles and pollutants adsorbed to the sediment particles are captured in the upper few inches of sand.

The underground versions fit in very well in urban areas and on sites with restricted space. Depending on the design, the underground sand filters are practically invisible to casual observers and generally receive few complaints from residents. Maintenance of these sand filters is simple and done manually. Above-ground sand filters are often considered to be eyesores by residents. Thus, these sand filters are best used where they cannot be seen or where hedges or other visual barriers can be installed. Because of the construction techniques used to build above-ground sand filters, large filters are proportionately less expensive than small filters. Construction costs can be kept lower if lightweight equipment is used for maintenance, which reduces the structural reinforcing needed in the filter.

Pollutant removal for sand filters varies depending on the site and climate. Overall removal for sediment and trace metals is better than removal of more soluble pollutants because the filter functions by simply straining small particles out of the stormwater. Table 4.6 lists removal rates.

Table 4.6 Sand Filter Pollutant Removal
Pollutant	Estimated Removal Efficiency
Plant Nutrients Total phosphorus Total nitrogen	Moderate Moderate
Sediment	Very High
Metals Trace metals (sediment-bound)	Very High
Organic Matter Biochemical oxygen demand (BOD)	Moderate
Oil and Grease	High
Bacteria	Moderate

From Schueler, et al. 1992.

Design Considerations. Sand filters remove pollutants by settling out particles in the pretreatment devices and by straining out particles in the filter. Underground sand filters built in two-chambered precast concrete boxes cannot handle large drainage areas. Moderate to large parking lots should be the largest areas drained to underground sand filters. Sand filters constructed underground should have pretreatment or settling chambers that hold 540 cubic feet of water for each acre of drainage area contributing stormwater to the sand filter (Shaver 1992). For two-chambered sand filters, the volume of the filter chamber should equal the volume of the settling chamber and the sand filter bed should be 18 inches deep (Shaver 1992). The surface area of both the settling and filter chambers should have 360 square feet of area for each acre of drainage area (Shaver 1992). Above-ground sand filters, built on the land surface, can handle drainage areas up to five acres in size. The sand filter bed should be 18 inches deep. Above-ground sand filters may use grassed filter strips, grassed swales or large basins to pretreat the incoming stormwater to prevent clogging of the sand filter.

Advantages. Sand filters can be installed underground in urban settings and be kept out of sight, or above ground for large drainage areas. Sand filters can provide effective reduction of the more common urban pollutants in stormwater. Sand filters have demonstrated long lifetimes and consistent pollutant removal when properly maintained. Maintenance for sand filters is simple and inexpensive. Mosquito breeding is usually not a problem, even in underground settling chambers that hold pools of water for long periods. Shaver (1992) reports that oil and grease in the stormwater form a sheen on the water which prevents mosquito growth.

Disadvantages. Sand filters are more expensive to construct than infiltration trenches. If heavy equipment is to be used for maintenance, construction costs are significantly higher. Sand filters on the land surface are considered unattractive. No stormwater detention is provided by sand filters. Sand filters have only limited pollutant removal for a number of pollutants.

Maintenance. Sand filters require frequent but simple maintenance. Maintenance for smaller, underground filters is usually and best done manually. Normal maintenance requirements include raking of the sand surface and disposal of accumulated trash. The upper few inches of dirty sand must be removed and replaced with clean sand when the filter clogs. The pretreatment devices must be cleaned to remove sediment and debris.

VEGETATIVE PRACTICES
Vegetation can be used to reduce the velocity of stormwater, which helps stormwater infiltrate into the soil and settle particulates, as well as prevent erosion. Such use of vegetation occurs in filter strips, grassed swales, riparian areas, and landscaping of wet, dry and infiltration basins. Vegetation is often employed as part of a BMP system, to remove particulates and slow runoff before it enters another treatment device. Two frequently used vegetative measures, filter strips and grassed swales, sometimes called biofilters, are described in this section. Another vegetative measure, the buffer strip or buffer zone, was described above under preventive measures.

Filter Strips: Filter strips are typically bands of close-growing vegetation, usually grass, planted between pollutant source areas and a receiving water. They also can be used as outlet or pretreatment devices for other stormwater control practices. Filter strips can include shrubs or woody plants that help to stabilize the grass strip, or can be composed entirely of trees and other natural vegetation. Such strips or buffers are used primarily in residential areas around streams or ponds. Filter strips do not provide enough runoff storage or infiltration to significantly reduce peak discharges or the volume of storm runoff. For this reason, a filter strip should be viewed as only one component in a stormwater management system. At some sites, filter strips may help reduce the size and cost of downstream control facilities.

Filter strips reduce pollutants such as sediment, organic matter and many trace metals by the filtering action of the vegetation, infiltration of pollutant-carrying water and sediment deposition. Although studies indicate highly varying effectivenesses, trees in strips can be more effective than grass strips alone because of the trees' greater uptake and long-term retention of plant nutrients. Properly constructed forested and grassed filter strips can be expected to remove more than 60 percent of the particulates and perhaps as much as 40 percent of the plant nutrients in urban runoff.

Filter strips fail very easily if they are not maintained regularly. Filter strips function best when they are level in the direction of stormwater flow toward the stream. This orientation makes for the finest sheetflow through the strip, increasing infiltration and filtering of sediment and other solids. To prevent erosion channel formation, a level spreader should be situated along the top edge of the strip. Level spreaders are designed to disperse concentrated flows evenly over a larger area. One type of level spreader is a shallow trench filled with crushed stone. The lower edge of the level spreader must be exactly level if the spreader is to work properly.

Grassed Swales: Grassed swales are earthen channels covered with a dense growth of a hardy grass such as Tall Fescue or Reed Canary grass. Swales are used primarily in single-family residential developments, at the outlets of road culverts, and as highway medians. Because swales have a limited capacity to convey runoff from large or intense storms, they often lead into concrete lined channels or other stable stormwater control structures. Swales may provide some reduction in stormwater pollution through infiltration of runoff water into the soil, filtering of sediment or other solid particles, and slowing the velocity and peak flow rates of runoff. These processes can be enhanced by adding small (4-10 inches high) dams across the swale bottom, thereby increasing detention time.

Pollutants are removed from surface flow by the filtering action of the grass, sediment deposition, and/or infiltration into the soil. The pollutant-removing effectiveness of swales has been assessed as moderate to negligible depending on many factors, including the quantity of flow, the slope of the swale, the density and height of the grass, and the permeability of the underlying soil. Research on grassed swales has found varying levels of pollutant removal ranging from 30 to 90 percent reduction in solids and 0 to 40 percent reductions in total phosphorus loads. Vegetative BMPs can reduce the amounts of the following pollutants in stormwater.

Table 4.2 Vegetative Practices Pollutant Removal
Pollutant	Estimated Removal Efficiency
Plant Nutrients	Low
Sediment	Moderate
Metals Trace metals	Moderate
Organic Matter	Low
Oil and Grease	Moderate
Bacteria	Low

Compiled from Schueler 1987; Schueler, et al. 1992;
US EPA 1990; Phillips 1992; Birch, et al. 1992 and others

Design Considerations. Vegetative practices remove pollutants by encouraging infiltration into the ground, reducing runoff velocity and allowing particles to settle, and by absorbing some pollutants. To be effective, vegetative practices require flat areas that are large in relation to the drainage area, and deep water tables. Swales should have as little slope as possible to maximize infiltration and reduce velocities. Filter strips should not be used where slopes exceed 15 percent; best performance occurs where the slope is 5% or less. The height of grass in filter strips and swales can affect the pollutant removal. Taller grass will slow velocities more but grass cut to a short length may take up more plant nutrients.

Advantages. Vegetative practices are inexpensive and generally easy to maintain with common procedures such as mowing and trimming. Vegetation is usually pleasing to residents. Filter strips and grassed swales are easily located and constructed. Vegetation is highly effective in preventing erosion and thus controlling sediment in stormwater runoff.

Disadvantages. Vegetative practices remove only small amounts of pollutants. These practices do little to control peak storm flows or reduce stormwater volumes.

Maintenance. Maintenance of vegetation includes periodic inspection, mowing, fertilizer application and repair of washed-out areas and bare spots. Filter strip maintenance basically involves normal grass- or shrub-growing activities such as mowing, trimming, removing clippings or replanting when necessary. Strips that are used for sediment removal may require periodic regrading and reseeding of their upslope edge because deposited sediment can kill grass and change the elevation of the edge such that uniform flow through the strip can no longer be obtained. Swale maintenance basically involves normal grass-growing activities such as mowing and resodding when necessary.

WETLANDS, CONSTRUCTED
Interest has steadily increased in the United States over the last two decades in the use of natural physical, biological, and chemical aquatic processes for the treatment of polluted waters. This interest has been driven by growing recognition of the natural treatment functions performed by wetlands and aquatic plants, by the escalating costs of conventional treatment methods, and by a growing appreciation for the potential ancillary benefits provided by such systems. Aquatic treatment systems have been divided into natural wetlands, constructed wetlands, and aquatic plant systems (USEPA, 1988). Of the three types, constructed wetlands have received the greatest attention for treatment of stormwater pollution. Constructed wetlands are a subset of created wetlands designed and developed specifically for water treatment (Fields, 1993). They have been further defined as:

engineered systems designed to simulate natural wetlands to exploit the water purification functional value for human use and benefits. Constructed wetlands consist of former upland environments that have been modified to create poorly drained soils and wetlands flora and fauna for the primary purpose of contaminant or pollutant removal from wastewaters or runoff (Hammer, 1992).

Constructed wetlands as defined here are not typically intended to replace all of the functions of natural wetlands, but to serve as do other water quality BMPs to minimize point source and nonpoint source pollution prior to its entry into streams, natural wetlands, and other receiving waters. Constructed wetlands which are meant to provide habitat, water quantity, aesthetic and other functions as well as water quality functions (termed created, restored, or mitigation wetlands (Hammer, 1994)) typically call for different design considerations than those used solely for water quality improvement, and such systems are not addressed here. In fact, debate continues over the advisability of intentionally combining primary pollution control and habitat functions in the same constructed facilities. Nonetheless, constructed wetlands can provide many of the water quality improvement functions of natural wetlands with the advantage of control over location, design, and management to optimize those functions.

Constructed wetlands vary widely in their pollutant removal capabilities, but can effectively remove a number of contaminants (Bastian and Hammer, 1993; Bingham, 1994; Brix, 1993; Corbitt and Bowen, 1994; USEPA, 1993). Among the most important removal processes are the purely physical processes of sedimentation via reduced velocities and filtration by hydrophytic vegetation. These processes account for the strong removal rates for suspended solids, the particulate fraction of organic matter (particulate BOD), and sediment-attached nutrients and metals. Oils and greases are effectively removed through impoundment, photodegradation, and microbial action. Similarly, pathogens show good removal rates in constructed wetlands via sedimentation and filtration, natural die-off, and UV degradation. Dissolved constituents such as soluble organic matter, ammonia and ortho-phosphorus tend to have lower removal rates. Soluble organic matter is largely degraded aerobically by bacteria in the water column, plant-attached algal and bacterial associations, and microbes at the sediment surface. Ammonia is removed largely through microbial nitrification(aerobic)-denitrification(anaerobic), plant uptake, and volatilization, while nitrate is removed largely through denitrification and plant uptake. In both cases, denitrification is typically the primary removal mechanism. The microbial degradation processes are relatively slow, particularly the anaerobic steps, and require longer residence times, a factor which contributes to the more variable performance of constructed wetlands systems for these dissolved constituents. Phosphorus is removed mainly through soil sorption processes which are slow and vary based on soil composition, and through plant assimilation and subsequent burial in the litter compartment. Consequently, phosphorus removal rates are variable and typically trail behind those of nitrogen. Metals are removed largely through adsorption and complexation with organic matter. Removal rates for metals are variable, but are consistently high for lead, which is often associated with particulate matter.

Constructed wetlands can be expected to achieve or exceed the pollutant removal rates as estimated for wet pond detention basins and dry detention ponds. Generalized ranges of removal for various pollutants are given below.

Table 4.5 Constructed Wetlands Pollutant Removal
Pollutant	Estimated Removal Efficiency
Plant Nutrients Total phosphorus Total nitrogen	High Moderate
Sediment Suspended solids	Very High
Metals Trace metals (sediment-bound)	High
Organic Matter Biochemical oxygen demand (BOD)	Moderate
Oil and Grease	Very High
Bacteria	High

Compiled from Schueler 1987; Schueler, et al. 1992; US EPA 1990; Phillips 1992; Birch, et al. 1992 and others

Design Considerations. The use of constructed wetlands for stormwater treatment is still an emerging technology, hence there are no widely accepted design criteria. However, certain general design considerations do exist. It is important first to drop stormwater inflow velocities and provide opportunity for initial sediment deposition with facilities which can be periodically maintained and which avoid the likelihood of entraining deposited sediment in subsequent inflows. It is important to maximize the nominal hydraulic residence time and to maximize the distribution of inflows over the treatment area, avoiding designs which may allow for hydraulic short-circuiting. Emergent macrophytic vegetation plays a key role, intimately linked with that of the sediment biota, by providing attachment sites for periphyton, by physically filtering flows, as a major storage vector for carbon and nutrients, as an energy source for sediment microbial metabolism, and as a gas exchange vector between sediments and air. Thus, it is important to design for a substantial native emergent vegetative component. Anaerobic sediment conditions should be ensured to allow for long-term burial of organic matter and phosphorus. A controlled rate of discharge is the last major physical design feature. While an adjustable outfall may seem desirable for fine-tuning system performance, regulatory agencies often require a fixed design to preclude subsequent inappropriate modifications to this key feature. The outfall should be fitted with some form of skimmer or other means to retain oil and grease. Plants must be chosen to withstand the pollutant loading and the frequent fluctuation in water depth associated with the design treatment volume. It is advisable to consult a wetlands botanist to choose the proper vegetation.

Location of constructed wetlands in the landscape can be an important factor in their effectiveness. Mitsch (1993) observed in a comparison of experimental systems using phosphorus as an example that retention as a function of nutrient loading will generally be less efficient in downstream wetlands than in smaller upstream wetlands. He also cautioned that the downstream wetlands could retain more mass of nutrients, and that a placement tradeoff might be optimum. Mitsch observed that creation of in-stream wetlands is a reasonable alternative only in lower-order streams, that such wetlands are susceptible to reintroduction of accumulated pollutants in large flow events as well as being unpredictable in terms of stability. Such systems would likely require higher maintenance and management costs.

Constructed wetlands are most effective as part of a BMP system which includes minimization of initial runoff volumes through the positioning of pervious landscaping features, routing of runoff to maximize infiltration, use of pervious pavement, grass swales, swale checks, or other measures, pre-treatment of collected runoff to minimize sediment and associated pollutant loads, and off-line attenuation of larger storm event runoff to optimize wetland performance and minimize downstream erosion-related water quality impacts.

Advantages. Properly constructed and maintained wetlands can provide very high removal of pollutants from stormwater. Constructed wetlands can be used to reduce stormwater runoff peak discharges as well as water quality benefits. Constructed wetlands can serve a dual role in controlling stormwater pollution and providing a pleasing natural area. Wetlands are highly valued by residents; therefore they can be given high visibility, they can serve as attractive centerpieces to developments and recreation areas, and they typically increase property values (Schueler 1987 and Shaver 1992). Constructed wetland systems can provide ground water recharge in the area, thus lessening the impact of impervious surfaces. This recharge can also provide a groundwater subsidy to the surficial aquifer, which can benefit local vegetation and decrease irrigation needs.

Disadvantages. Constructed wetlands may contribute to thermal pollution and cause downstream warming. This may preclude their use in areas where sensitive aquatic species live. They are not a competitive option compared to other treatment methods where space is a major constraint. The ponded water may be a safety hazard to children.

Maintenance. Constructed wetlands have an establishment period during which they require regular inspection to monitor hydrologic conditions and ensure vegetative establishment. Vegetation establishment monitoring and long-term operation and maintenance, including maintenance of structures, monitoring of vegetation, and periodic removal of accumulated sediments, must be provided for to ensure continued function (Wetzel, 1993; Bingham, 1994). Maintenance costs vary depending on the degree to which the wetlands are intended to serve as popular amenities. Frequent initial maintenance to remove opportunistic species is typically required if a particular diverse, hydrophytic regime is desired. Operators of wetlands may need to control nuisance insects, odors, and algae.

WETLANDS, NATURAL AND RESTORED
The many water quality improvement functions and values of wetlands are now widely recognized. At the same time, concern has grown over the possible harmful effects of toxic pollutant accumulation and the potential for long-term degradation of wetlands from altered nutrient and hydraulic loading that can occur with the use of wetlands for water treatment. Because of these concerns, the use of natural wetlands as treatment systems is restricted by federal law (Fields, 1993). Most natural wetlands are considered "waters of the United States" and are entitled under the CWA to protection from degradation by NPS pollution. Natural wetlands do function within the watershed to improve water quality, and protection or restoration of wetlands to maintain or enhance water quality are acceptable practices. However, NPS pollutants should not be intentionally diverted to wetlands for primary treatment. Wetlands must be part of an integrated landscape approach to NPS control, and cannot be expected to compensate for insufficient use of BMPs within the upgradient contributing area. Restored wetlands are subject to the same restrictions as unmodified natural wetlands. Wetlands created from upland habitat for the purpose of mitigating the loss of other wetlands as required by regulatory agencies are generally also subject to the same restrictions as natural wetlands. Constructed wetlands (see WETLANDS, CONSTRUCTED), which have been defined as a subset of created wetlands that are designed and developed specifically for water treatment (Fields, 1993), clearly are not intended for the same protections as natural wetlands, and can serve as valuable treatment BMPs.

WET RETENTION PONDS
Wet retention ponds, also called wet detention basins, or wet basins or ponds, maintain a permanent pool of water in addition to temporarily detaining stormwater. The permanent pool of water enhances the removal of many pollutants. These ponds fill with stormwater and release most of it over a period of a few days, slowly returning to its normal depth of water. Several mechanisms in wet ponds remove pollutants including: settling of suspended particulates; biological uptake, or consumption of pollutants by plants, algae and bacteria in the water; and decomposition of some pollutants. Wet ponds have some capacity to remove dissolved plant nutrients, an important characteristic to protect lakes, rivers and estuaries from eutrophication.

Wet ponds can be used in most locations where there is enough space to locate the pond. Because of the permanent pool of water, wet ponds can remove moderate to high amounts of most pollutants and are more effective in removing plant nutrients than most other devices. Also, the large volume of storage in the pond helps to reduce peak stormwater discharges which, in turn, helps control downstream flooding and reduces scouring and erosion of streambanks.

Construction costs for wet ponds can be somewhat high because the ponds must be large enough to hold the required volume of runoff and to contain the permanent pool of water. Maintenance costs run about 3-5% of the construction cost per year (Schueler 1987). See Appendix L for more information on wet retention ponds.

Pollutant removal is rated as moderate to high compared with other stormwater devices. Typical wet pond removal efficiencies are listed below for each pollutant.

Table 4.4 Wet Pond Pollutant Removal
Pollutant	Estimated Removal Efficiency
Plant Nutrients Total phosphorus Total nitrogen	Moderate to High Moderate
Sediment Total suspended solids	High
Metals Lead Zinc	High Moderate
Organic Matter Biochemical and chemical oxygen demand (BOD or COD)	Moderate
Oil and Grease	High
Bacteria	High

Compiled from Schueler 1987; Schueler, et al. 1992; US EPA 1990; Phillips 1992; Birch, et al. 1992 and others

Design Considerations. Wet ponds should be designed to displace the older stormwater with the newer stormwater, which ensures the proper amount of holding time. If the design is improper, short-circuiting can occur where the newer stormwater flows directly to the outlet, bypassing the main part of the wet pond. Short-circuiting causes the new stormwater to be released too soon, preventing pollutant removal and settling of sediment. Basic considerations for the installation of wet retention ponds are location, the inflow runoff volume, hydraulic residence time, permanent pool size and maintenance. Volumes of stormwater runoff and normal discharge available for the permanent pool must be calculated by trained hydrologists before constructing a wet pond. Long, narrow ponds or wedge-shaped ponds are preferred shapes to minimize short-circuiting of storm flows. These shapes also will lessen the effects of wind, which can stir up sediment and sediment-bound pollutants. Pond shape, depth and surrounding fringe areas must be considered to maximize the effectiveness of the basin. Marsh plants around the pond help remove pollutants, provide habitat and hide debris.

Advantages. Because people find these ponds to be aesthetically pleasing, wet ponds can be sited in both low and high visibility areas. Quite often, residents feel that the permanent pool of water enhances property values as well as the aesthetic value of the area. The outlet must be sized to provide adequate time for pollutant removal, yet discharge the stormwater before the next storm occurs. Wet retention ponds have been used to provide wildlife habitat and they may be a focal point for a recreation area. Wet ponds are one of the most effective and reliable devices for removing pollutants from stormwater.

Disadvantages. One disadvantage of wet retention basins is that they may contribute to thermal pollution and cause downstream warming. This may preclude their use in areas where sensitive aquatic species live. Wet ponds are not well suited to very small developments because of their large size. Wet ponds may flood prime wildlife habitat; and there are sometimes problems with nuisance odors, algae blooms and rotting debris when the ponds are not properly maintained. Wetland plants may need to be harvested or removed periodically to prevent releasing plant nutrients into the water when the plants die. The pool of water presents an attractive play area to children; hence, there may be safety problems.

Maintenance. The maintenance costs of wet ponds are estimated at 3-5% of construction cost per year. Wet ponds require regular inspection, removal of sediment according to a regular schedule of maintenance, regular mowing, and regular cleaning and repair of inlets and outlets. Operators of wet ponds must control nuisance insects, weeds, odors, and algae; inspect and repair pond bottoms; and harvest deciduous vegetation prior to the onset of fall as necessary. Mosquitoes can be controlled in wet ponds with fish of the Gambusia family which eat the mosquito larvae. The Gambusia can survive the winters in North Carolina if the permanent pool is at least three feet deep. Another control method which does not use insecticides is monthly application of briquettes containing bacteria which cause a disease in mosquitoes. The application needs to be done only in the warmer months. The bacteria can be purchased at hardware and garden stores.

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