Green Stormwater Infrastructure - Volume 2: Guidebook (2017)

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33 Follow-up information for each BMP type is provided in this section. The nine types of GSI BMPs are as follows: • BMP 1: Bioretention (bioretention cells, rain gardens) • BMP 2: Green Roofs (vegetated roofs, rooftop gardens) • BMP 3: Harvesting and Reuse (rain barrels, cisterns) • BMP 4: Infiltration Galleries (infiltration trenches, infiltration basins) • BMP 5: Porous Pavement (permeable pavement, porous asphalt, pervious concrete, pavers) • BMP 6: Sand Filters (media filters) • BMP 7: Filter Strips (vegetated filter strips) • BMP 8: Bioswales (vegetated swales, grassy swales) • BMP 9: Wetland Treatment Systems (constructed wetlands, conventional stormwater wetlands) Information presented in each BMP-type discussion includes: • What design features may be customized, • Performance potential for pollutant reduction and runoff volume control, • Any potential issues and/or constraints, • Maintenance requirements and any indications that maintenance is needed, • What the replacement needs are, and • General information on costs. Information regarding these BMPs can also be found in the Sustainable Aviation Guidance Alliance database (http://www.airportsustainability.org/). S t e p 4 Understanding GSI Best Management Practices

34 Green Stormwater Infrastructure BMP 1: Bioretention (Bioretention Cells, Rain Gardens) Bioretention BMPs function as soil-and-plant-based filtration devices that remove pollutants through physical, biological, and chemical interactions between the pollutants in the runoff and the plants and organic media in the BMP. These facilities normally consist of a filtration bed, ponding area, organic or mulch layer, and plants (Figure 5). Exfiltration of the stored water in the bio- retention area planting soil into the underlying soils occurs over a period of days when installed as an unlined system. Bioretention practices are implemented primarily on the land- side to treat runoff from terminals, roadways, parking lots, and commercial buildings. Bioretention systems can often serve as a replacement for ornamental landscaping for landside areas. Bio retention systems are typically designed to manage approxi- mately one inch of rainfall within these areas. Bioretention systems can also serve purposes other than stormwater management. Austin– Bergstrom International Airport has installed a series of bioretention systems (as a rain garden) in the median of Spirit of Austin Lane as part of a project to convert the taxicab waiting area to a cell phone lot. Part of the objective of this retrofit was to decrease the road width and provide a measure of traffic calming. Figure 6 shows the Austin–Bergstrom International Airport bio- retention system. Design Features that May Be Customized Proper vegetation can be selected to minimize wildlife attraction based on local climate and growing seasons. Other design features may include an Internal Water Storage Zone if underly- ing soil permeability could result in standing water for more than 48 hours. The composition Suitable Airport Areas Landside: terminals, roadways, parking lots, commercial buildings Climates All Pollutant Removal Sediment; metals; nutrients Runoff Volume Control Infiltration; evapotranspiration Site Hydrology and Other Site Properties High sediment loads issues FAA or Other Regulations Wildlife hazard management Customizable Design Features Vegetation; media; size; underdrains; wildlife controls Primary Maintenance Activities Trash & debris; mulch replacement; pruning; O&M of pipes and drains Figure 5. Bioretention. Source: M. Barrett.

Understanding GSI Best Management practices 35 of the planting media can also be adjusted as needed. In areas with a shallow water table or low permeability soil, bioretention can be implemented with an underdrain. Where rainfall is insuf- ficient to maintain vegetation, the bioretention system may function like a sand filter (e.g., at Fresno Yosemite International Airport). Performance Pollutant Reduction Bioretention systems generally perform well for pollutant removal. Sediment is removed through sedimentation and filtration. Heavy metals are removed by adsorption to clay and organic matter in the soil media. Nutrients are removed via biological activity and vegetation uptake. The issue regarding pollutant removal is that many guidance documents recommend a substantial amount of compost or other organic matter in the filtration media. This can result in the leaching of nutrients and an increase in nutrient concentration as compared to untreated runoff. Runoff Volume Control According to Minneapolis–St. Paul International Airport, volume reduction can be an impor- tant consideration when selecting stormwater BMPs. For example, bioretention installed in an area that contains relatively permeable soils (e.g., greater than 0.5 inch/hour) can be unlined and retain up to 100 percent of runoff volume through infiltration. Load will be substantially reduced because of runoff infiltration into the underlying soils. Even where underlying soils have low permeability, volume can be reduced up to about 30 percent through evapotranspiration. Potential Issues and Constraints Standing water and vegetation associated with this BMP are the major constraints. Wildlife exclusion devices such a wires or netting may be required to prevent or minimize hazardous wildlife attraction. Bioretention systems are better suited for implementation in the landside Figure 6. Bioretention/rain garden at Austin–Bergstrom International Airport. Source: M. Barrett (© 2015). Note: The curb cuts allow water to flow into the planted area and the storm sewer grate to handle the overflow.

36 Green Stormwater Infrastructure areas to minimize wildlife attraction. Martin State Airport considered bioretention during hangar development, but because of hazardous wildlife use concerns, runoff from the hangar area is directed to the drainage system. Cleveland Hopkins International Airport rejected the use of bioretention on the airside of the terminal because of the potential for spills to enter bioretention areas and contaminate groundwater. The filtration media can become clogged over time due to high sediment loads. Consequently, periodic inspections 24 to 48 hours after a major rain event should be conducted. Cleveland Hopkins International Airport indicated that improper construction can also cause early failure. Maintenance Requirements and Indications of the Need for Maintenance Accumulated sediment, trash, and other debris must be removed from the BMP. O&M includes annually replacing the mulch, pruning the vegetation, and replacing dead or diseased plants. Other potential tasks include repairing erosion at inflow points, unclogging the under- drain, and repairing overflow structures. Plants that are appropriate for the site, i.e., suitable to climatic and watering conditions, should be selected for use in the bioretention system. Appro- priately selected plants will aid in reducing the use of fertilizer, pesticides, and water. Routine inspections for areas of standing water and correcting the situation to restore proper infiltration rates are necessary to avoid habitat creation caused by water standing in areas for more than 48 hours, according to Seattle–Tacoma International Airport. Restoring infiltration rates may be as easy as replacing the layer of mulch on top of the media, but replacing the plant- ing media may be required. Replacement Needs Bioretention systems are expected to have a substantial life span when properly maintained; many existing bioretention systems have functioned as designed for as many as 20 years. The activity most likely to compromise a system’s performance is construction in a watershed. Con- struction can result in excessive sediment loads and subsequent clogging. Costs Commercial, industrial, and institutional site costs can range from $10 to $40 per square foot, based on the need for control structures, curbing, storm drains, and underdrains. In any bio- retention system, the cost of plants varies substantially and can account for a significant portion of the airport’s expenditures. While these cost estimates are slightly greater than those of typical landscaping treatment (due to the increased number of plantings, additional soil excavation, backfill material, and use of underdrains), those landscaping expenses that would be required regardless of the bioretention installation should be subtracted when determining the net cost (Low Impact Development Center, Inc. 2007a).

Understanding GSI Best Management practices 37 BMP 2: Green Roofs (Vegetated Roofs, Rooftop Gardens) A green roof is a vegetative layer grown on a rooftop. Green roofs can be installed on a range of buildings, and they can be as simple as a 2-inch covering of hardy groundcover or as complex as a fully accessible park complete with trees. Green roofs provide shade and remove heat from the air through evapotranspiration, reducing temperatures of the roof surface and the surrounding air. On hot summer days, the surface temperature of a green roof can be cooler than the air temperature, whereas the surface of a conventional rooftop can be up to 90°F (50°C) warmer (U.S. EPA 2015). A study performed in Canada showed summer daily temperatures of up to 81°F for a traditional roof compared to only 11°F for a green roof (approximately seven times less) (Liu and Baskaran 2003). Green roofs can also minimize re-roofing costs and energy costs by having greater longevity and improved insulation capabilities; provide increased acoustic buffering from airport traffic noise; provide natural absorption of airborne contaminants; and provide educational value for passengers. The Chicago Department of Aviation (CDA) considers the green roofs installed on 16 facili- ties at O’Hare International Airport to be part of an initiative that ensures its “commitment to sustainability is visible throughout the airport” (CDA 2012, 2015). O’Hare has constructed over 450,000 square feet of vegetated green roofs (one of which is shown in Figure 7) (CDA 2015). Additional examples of green roofs installed at airports are in Frankfurt, Germany; Amsterdam, The Netherlands (Figure 8); and Ibiza, Spain (Figure 9). Green roofs are also used at Chicago Midway International Airport and are being considered for implementation at Minneapolis– St. Paul International Airport and Cleveland Hopkins International Airport. Design Features that May Be Customized Green roofs are generally classified by the depth of the planting media, with intensive green roofs having a relatively thin growing medium (3 to 6 inches) and extensive roofs often having soil deep enough to support small trees. Plant species used should be unattractive to wildlife. Suitable Airport Areas Landside or airside: terminals, facility buildings Climates Not arid or excessively hot Pollutant Removal Minimal; nutrient export Runoff Volume Control Evapotranspiration Site Hydrology and Other Site Properties Flat roof needed FAA or Other Regulations Wildlife hazard management Customizable Design Features Vegetation; media; drainage layer configuration Primary Maintenance Activities Pruning; vegetation replacement Figure 7. Green roof on the FedEx facility at Chicago O’Hare International Airport. Source: greenroofsolutions.com.

38 Green Stormwater Infrastructure Green roofs are typically designed and installed by specialist companies. Each company tends to have proprietary aspects to their designs, with components that can be customized for a par- ticular installation. Performance Pollutant Reduction Green roofs are not generally designed to remove pollutants as they capture rainfall at its point of origin. In some cases, green roofs may decrease water quality, by exporting nutrients contained in the planting medium. Runoff Volume Control The primary driver for green roof implementation is reduction of the amount of stormwater runoff generated. This is often a requirement in areas served by combined sewers (wastewater and stormwater in a single pipe), where overflows and discharge of untreated wastewater occur during rain storms. Green roofs can retain 25 to 90 percent of precipitation, depending on the season. Figure 8. Green roof on the central terminal at Schiphol Airport Amsterdam. Source: ZinCo (2012). Figure 9. Green roof on Ibiza Airport terminal, Spain. Source: International Green Roof Association (2012).

Understanding GSI Best Management practices 39 Potential Issues and Constraints The main concern regarding the use of green roofs is hot and/or dry climates. Several air- ports, including Los Angeles International Airport, San Diego International Airport, Dallas/Fort Worth International Airport, Southwest Florida International Airport, and Naples Municipal Airport, expressed concerns about the use of green roofs due to low rainfall conditions. An important secondary concern is the potential to attract hazardous wildlife and create wildlife habitat. Similarly concerns on FOD are critical. Damage to aircraft from FOD can be very costly. Therefore, O’Hare International Airport green roofs were designed in compliance with wind speed requirements to prevent parts of the plants or bedding from blowing off the roof and onto aircraft movement areas. Maintenance Requirements and Indications of the Need for Maintenance The green roof should be visually inspected and spot-weeded every 2 to 4 weeks during the growing season. Fertilizing should be conducted at least annually in spring for the first 3 to 5 years after installation, and then on a reduced schedule if soil testing and plant performance warrant it. Every 2 to 3 years, the plants should be trimmed to encourage additional leaf development at the plant crown and to spread regenerative cuttings across the green roof during the spring season. Replacement Needs Green roofs are expected to last until the underlying roof needs replacement (up to 40 years). Costs The cost for a green roof can be highly variable depending on design selected. In the United States, costs are estimated to range from $15 to $25 per square foot (Low Impact Development Center, Inc. 2007b). Installation cost data is not available for O’Hare International Airport. The airport estimates that the green roof insulation and decrease in the heat island effect provide an energy savings of $0.20 per square foot per year. The expected increase in the longevity of the underlying roof material is expected to decrease the re-roofing rate from once every 15 to 20 years to 40 to 50 years (CDA 2015).

40 Green Stormwater Infrastructure BMP 3: Harvesting and Reuse (Rain Barrels, Cisterns) Rainwater can be harvested from rooftops, stored using rain barrels and cisterns, and later reused in a variety of ways, e.g., landscape irrigation. Cisterns can store from 200 to over 10,000 gallons, such as the 25,000-gallon cistern at Hartsfield– Jackson Atlanta International Airport. Rainfall harvesting components are generally low cost; how - ever, payback periods can be quite long (e.g., greater than 20 years). Airports that have implemented or plan to implement rain- water harvesting systems include Austin–Bergstrom Inter- national Airport, Hartsfield–Jackson Atlanta International Airport, Cleveland Hopkins International Airport, Minneapolis– St. Paul International Airport, and Gulfport–Biloxi International Airport. In addition to augmenting water supplies, use of the har- vesting and reuse systems also aids in green building recognition. For instance, Austin–Bergstrom International Airport achieved Leadership in Energy and Envi- ronmental Design (LEED) gold certification (which was the primary driver for the project) partially through the use of rainwater harvesting for a new taxi driver waiting building. Figure 10 shows the rain water collection tank located at Austin–Bergstrom International Airport at the taxi waiting area. Design Features that May Be Customized Cisterns come in many shapes, sizes, and materials and can be installed underground to save space. Performance Pollutant Reduction Rainwater harvesting is generally not implemented for pollutant reduction, as any reduction is limited. Suitable Airport Areas Landside or airside: terminals, facility buildings, commercial buildings Climates All Pollutant Removal Minimal Runoff Volume Control Primary objective Site Hydrology and Other Site Properties No limitations FAA or Other Regulations Wildlife hazard management; plumbing code; water rights Customizable Design Features Type of reuse; size; treatment; location Primary Maintenance Activities Control water quality; O&M of gutters and downspouts Figure 10. Rainwater harvesting at Austin–Bergstrom International Airport taxi waiting area. Source: M. Barrett (© 2015).

Understanding GSI Best Management practices 41 Runoff Volume Control Harvesting and reuse allows for up to 100 percent reduction in the volume of runoff from the design storm event. This can be a very important consideration in locations where combined sewer overflows occur. Potential Issues and Constraints Surface materials on the area from which rainwater will be collected affect the quality of captured rainwater, which has implications for the recommended uses of the harvested water. If the roof has asphalt or wooden shingles, harvested rainwater should only be used for non- edible plants, unless the water is treated first. Petroleum or other chemicals from these roofing materials can leach into the rainwater. Roofs with cement, clay, or metal surfaces are ideal for harvesting water for a wide variety of uses. However, the harvested water may require some level of treatment depending upon its intended use. It is recommended that airports check with their local health departments or state health/environmental departments on the acceptable use of harvested rainwater. In some areas, collection of rainwater may be prohibited by water rights laws. For example, Denver International Airport found it was unable to implement rainwater harvesting because a state law requires runoff to be released within 72 hours. Maintenance Requirements and Indications of the Need for Maintenance Gutters and gutter guards should be regularly checked to make sure debris is not entering the rainwater harvesting system. Screens on the rain barrel or cistern should be inspected to make sure debris is not collecting on the surface and that there are no holes allowing mosquitoes or other animals to enter. The inside of the storage tank should be cleaned once a year to prevent buildup of debris. Routine inspections should be performed to ensure that overflow does not pond or otherwise create a water source that could attract hazardous wildlife. Treatment systems for potable water reuse require regular maintenance of filtration and disinfection components to provide reliable and safe service. Replacement Needs Rain storage tanks are expected to last 20 years or more. Costs No airport-specific cost data is available for this BMP. However, according to the Texas Water Development Board, the cost for the cistern alone (the most expensive component) ranges from $0.50 to $4.00 a gallon, depending on the material of manufacture.

42 Green Stormwater Infrastructure BMP 4: Infiltration Galleries (Infiltration Trenches, Infiltration Basins) An infiltration gallery is a stone-filled trench with no outlet that receives stormwater runoff (see Figure 11). Stormwater run- off passes through some combination of pretreatment measures, such as a swale or sediment basin, before entering the trench. Run- off is then stored in the voids between the stones, and slowly infil- trated through the bottom and into the soil matrix. FAA guidance encourages the installation of stormwater infiltration galleries, such as French drains or buried rock fields, because they are less attractive to wildlife than other BMPs (FAA AC 150/5200-33B). The primary pollutant removal mechanism is filtering through the soil matrix. The primary objective of using infiltration galler- ies is to reduce the volume of runoff, which can be an important consideration in areas subject to flooding or combined sewer overflows. Infiltration galleries are common and can be found at a range of airports, each with different rainfall patterns, such as Seattle–Tacoma International Airport, Los Angeles International Airport, and San Diego Inter- national Airport. Figure 12 shows an infiltration area topped by artificial turf at San Diego International Airport. Design Features that May Be Customized The infiltration gallery dimensions are the elements most subject to customization. Gen- erally the trenches are long and narrow. San Diego International Airport has constructed Suitable Airport Areas Landside or airside: terminals, roadways, parking lots, commercial buildings Climates All Pollutant Removal Sediment; metals Runoff Volume Control Infiltration Site Hydrology and Other Site Properties Avoid high water table, low permeable soils, existing groundwater contamination FAA or Other Regulations None Customizable Design Features Depth; surface area; storage mechanism Primary Maintenance Activities Trash and debris removal Source: Schueler (1987). Figure 11. Cross section of infiltration trench.

Understanding GSI Best Management practices 43 shallow infiltration galleries, where nearly one-third of the 30 acres of the new runway has been designed to drain into a 1.75 acre infiltration gallery covered with artificial turf. The infiltration gallery is composed of 3 to 4 inches of crushed rock that captures and infiltrates approximately 18,000 cubic feet of stormwater runoff. This allows the airport to meet the MS4 permit requirement to treat the volume of water from the 85th percentile of a 2-year storm event. Performance Pollutant Reduction Infiltration galleries can reduce pollutant discharge to surface waters as much as 100 percent by infiltrating the entire design storm under certain operating conditions. Pollutant loads in the runoff are reduced by filtering in the soil layer and adsorption of dissolved metals to clay particles in the soil. Los Angeles International Airport is implementing a $30 million infiltration project that will help remove bacteria from the runoff to protect nearby beaches. Runoff Volume Control Infiltration galleries provide up to 100 percent reduction in the volume of runoff from the design storm event, often about 1 inch of rainfall. Potential Issues and Constraints Infiltration galleries cannot be used in areas with a high groundwater table or low perme- ability soils (e.g., Southwest Florida International Airport and Dallas/Fort Worth International Airport). NPDES regulatory requirements may also limit the use of infiltration galleries. For example, at Minneapolis–St. Paul International Airport, GSI BMPs that use infiltration must not receive stormwater runoff from deicing areas per the NPDES permit requirements. At some locations, existing groundwater contamination may preclude the installation of infiltration gal- leries. For example, an area of groundwater contamination (from tetrachloroethylene) near Austin–Bergstrom International Airport precludes any infiltration that could potentially mobi- lize the contaminated groundwater. Infiltration galleries that are deeper than they are wide or long are considered Class V injection wells and permits must be obtained for them. For more information on the Underground Injection Control Program and Class V wells, see the U.S. EPA’s web- site (www.epa.gov/uic). Source: J. Jolley (© 2015). Figure 12. Artificial turf infiltration area at San Diego International Airport west side.

44 Green Stormwater Infrastructure Maintenance Requirements and Indications of the Need for Maintenance The primary maintenance activity for infiltration galleries is the removal of any trash or debris from the surface of the infiltration gallery. This activity is critical in the airside areas to prevent the generation of FOD. Pretreatment is recommended to reduce the sediment load to infiltra- tion galleries. In some cases, excessive sediment can clog the infiltration system requiring it to be completely rebuilt. Replacement Needs An infiltration gallery has a life expectancy of between 5 and 15 years depending on the sedi- ment load (Schueler 1987). Costs The cost for infiltration galleries per treatment area was estimated in 2012 U.S. dollars as follows (Venner et al. 2013): • New construction: $27,300/acre for areas less than 3 acres. $9,700/acre for areas greater than 3 acres. • Retrofit construction: $92,700/acre for areas less than 3 acres. $32,800/acre for areas greater than 3 acres.

Understanding GSI Best Management practices 45 BMP 5: Porous Pavement (Permeable Pavement, Porous Asphalt, Pervious Concrete, Pavers) Porous pavement is a type of pavement that allows rain to pass through the surface. It can be used on both permeable and imper- meable soils. In the latter case, the porous pavement system is designed with an underdrain system. Where soils are sufficiently permeable, the majority of the runoff will infiltrate and minimal discharge of storm water or associated pollutants will occur. The installation of porous pavement that is designed with an under- drain will provide substantial pollutant removal and slow the runoff rate by increasing the runoff time of concentration. There are several types of porous pavement: • Porous asphalt pavement consists of an open-graded coarse aggregate, bonded together by asphalt cement, with sufficient interconnected voids to make it highly permeable to water. • Pervious concrete differs from regular concrete in the proportion of coarse aggregate, the absence of fine material, and the reduced quantity of water in the mix. Pervious concrete has enough void space to allow rapid percolation of rainfall through the pavement. • Pavers themselves are typically impermeable; however, infiltration occurs either in the gaps between the pavers or within openings cast as part of the geometry of the paver. Porous pavement has been used at a number of airports including Los Angeles Interna- tional Airport (largest porous parking lot on the West Coast), San Diego International Airport, Denver International Airport, Hartsfield–Jackson Atlanta International Airport, and Pittsburgh International Airport. The porous pavement surface is typically placed over a highly permeable layer of open-graded gravel and crushed stone, as shown in Figure 13. The void spaces in the aggregate layers act as a storage reservoir for runoff. The liner and underdrain are optional fea- tures that might be required because of structural considerations and/or low soil permeability. Design Features that May Be Customized Design options include selection of pavement type and configuration of the reservoir/structural layer underneath the pavement. Because porous pavements are so permeable, the entire area Suitable Airport Areas Landside or airside: roadways, parking lots Climates All Pollutant Removal Minimal Runoff Volume Control Infiltration Site Hydrology and Other Site Properties Clogs from high sediment loads FAA or Other Regulations Load capacity Customizable Design Features Pavement type; support layer Primary Maintenance Activities Sweeping; weed removal Source: Texas State University, M. Barrett. Figure 13. Example of porous pavement cross section.

46 Green Stormwater Infrastructure does not need to be permeable. Consequently, designs often vary in the amount and location of the permeable sections, as shown in the two examples in Figures 14 and 15. Performance Pollutant Reduction Porous pavement has typically been installed to reduce the volume of runoff, so there is little available data on pollutant concentration reduction. However, in settings that allow some fraction of runoff to be infiltrated, surface discharge of polluted runoff will be similarly reduced. Source: J. Jolley (© 2015). Figure 14. Porous pavement under parking spaces at San Diego International Airport. Source: J. Jolley (© 2015). Figure 15. Porous pavement in center lane of parking lot at San Diego International Airport.

Understanding GSI Best Management practices 47 Runoff Volume Control The volume of runoff is expected to be reduced up to 100 percent when the pavement is located on permeable soils. Potential Issues and Constraints The primary constraints of porous pavement are load-bearing capacity, damage from spills, and vegetation growth. The load-bearing requirements of certain paved areas of the airport, such as taxiways, may preclude using conventional porous pavement at these locations. Porous pave- ment should not be used in areas where there is the potential for spills, such as at loading docks, fueling areas (for equipment and aircraft), and maintenance areas. Vegetation growth through the pavement could attract wildlife and should be prevented and/or removed. At northern loca- tions, snow removal equipment has the potential to damage porous pavement leading to loss of permeability. Porous pavement materials may be at an increased risk of producing FOD due to the openness of the material and potential for dislodging of the aggregate. Though the FOD risk from porous pavement has been noted, several airports—including San Diego International Air- port and Los Angeles International Airport—have used porous pavement extensively in parking lots adjacent to the airfield. Maintenance Requirements and Indications of the Need for Maintenance Maintenance requirements for porous pavements will depend on the environmental con- text and intensity of use and may include periodic weed removal and control, street sweeping, vacuum sweeping, and/or high-pressure washing. The pavement should be inspected after large storms, when puddles will reveal clogging. Replacement Needs Porous pavement is expected to last from 20 to 40 years, depending on the location (Young 2013, Green Building Alliance 2016). Costs The cost for porous pavement is highly variable depending on the type of pavement selected and the required load-bearing capacity. Hartsfield–Jackson Atlanta International Airport esti- mated the cost of porous pavement for its interim parking lot at $250,000. On average, porous pavement costs $2.00 to $6.50 per square foot (Low Impact Development Center, Inc. 2007c).

48 Green Stormwater Infrastructure BMP 6: Sand Filters (Media Filters) Sand filters consist of basins that capture stormwater runoff and then filter the runoff through a bed of sand in the floor of the facility. See Figure 16 for an example sand filter design. This BMP can be configured as either a single basin or as sepa- rate sedimentation and filtration basins. Sand filters should be installed at grade to facilitate drying out of the sand between storm events. The objective of sand filters is to remove sediment and the pol- lutants from the first flush of impervious area runoff. The filtra- tion of nutrients, organics, and coliform bacteria is enhanced by a mat of bacterial slime that develops during normal operations. One of the main advantages of sand filters is their adaptability; they can be used on areas with thin soils, with high evaporation rates, or with low soil infiltration rates, as well as in limited-space areas, and where groundwater requires protection. There have been numerous alterations or variations in the original sand filter design as engineers in various jurisdictions have improved and adapted the technology to meet their specific requirements. Major types include: • Austin sand filter; • District of Columbia underground sand filter; • Alexandria dry vault sand filter; • Delaware sand filter; and • Peat–sand filters, which are adapted to provide a sorption layer and vegetative cover to various sand filter designs (Young et al. 1996). Suitable Airport Areas Landside or airside: terminals, roadways, parking lots, and commercial buildings Climates All Pollutant Removal Sediment Runoff volume control Infiltration Site Hydrology and Other Site Properties High sediment loads issues FAA or Other Regulations Wildlife hazard management Customizable Design Features Media; size; underdrains; pretreatment Primary Maintenance Activities Trash and debris removal; replacement of sand Source: Young et al. (1996). Figure 16. General sand filter design.

Understanding GSI Best Management practices 49 Design Features that May Be Customized Several components of a sand filter can be customized. Depending on the hydraulic head at the site, the ponded water depth may range from less than 1 foot to more than 5 feet, with the largest depths resulting in smaller facility footprint. For example, pretreatment can be provided at the sediment chamber. In addition, sand filter systems can be constructed over an imperme- able liner where infiltration is not desired. Another customization is the addition of vegetation, either on purpose or naturally. For example, as finer-grained material accumulates through time, vegetation may become established in the media, resulting in the filter performing like a bioretention system. San Diego International Airport has constructed a number of high-rate media filters, which are similar to sand filters but are augmented with compost, zeolite, and other media to improve removal of dissolved constituents. Performance Pollutant Reduction Sand filters generally perform well for removal of particles and associated pollutants. Sedi- ment is removed through sedimentation and filtration. The removal of dissolved constituents is generally less effective than sediment removal. Conversion of organic nitrogen to nitrate is commonly observed, so export of nitrate could be expected and should be monitored to prevent contamination of groundwater. Runoff Volume Control Runoff volume control is a function of the permeability of the underlying soils. When the sand filters are located over sandy soils, volume reduction can be significant. Potential Issues and Constraints The main constraint is managing standing water that has the potential to attract hazardous wildlife. In addition, the basins may not be used in the runway safety areas where they would be a hazard for aircraft. Sand filters are easily clogged by high sediment loads, so increased mainte- nance will likely be necessary if there is construction in the catchment of the system. At airports where legacy groundwater contamination exists, infiltration to groundwater via sand filters is not appropriate as there exists the potential to mobilize existing ground- water contaminants. In some cases, this drawback may be avoided. Austin–Bergstrom Inter- national Airport, for example, relies heavily on sand filters for water quality treatment but also experiences groundwater contamination issues. The sand filters at Austin–Bergstrom Inter- national Airport are equipped with underdrains and are not built in areas with groundwater contamination. Maintenance Requirements and Indications of the Need for Maintenance It is recommended that sand filter BMPs be inspected on a quarterly basis and after large storms during the first year of operation. This intensive monitoring is intended to ensure proper operation and to provide maintenance personnel with an understanding for the operational characteristics of the filter. Subsequent inspections can be limited to semi-annually or more often if deemed necessary.

50 Green Stormwater Infrastructure Maintenance of the filter media is necessary when the draw-down time exceeds 48 hours. When this occurs, the upper layer of sand should be removed and replaced with new material meeting the original media specifications. Replacement Needs The expected life span of the sand filter is 50 years. Costs The cost for sand filters per treatment area was estimated in 2012 U.S. dollars as follows (Venner et al. 2013): • New construction: $88,000/acre for areas less than 3 acres. $48,100/acre for areas greater than 3 acres. • Retrofit construction: $113,800/acre for areas less than 3 acres. $62,300/acre for areas greater than 3 acres.

Understanding GSI Best Management practices 51 BMP 7: Filter Strips (Vegetated Filter Strips) Filter strips, also known as vegetated filter strips, are vegetated areas with shallow slopes. Filter strips are designed to treat runoff as overland sheet flow. The dense vegetative cover in the filter strip facilitates conventional pollutant removal through sedimen- tation and infiltration. The primary application for filter strips at airports is management of runoff generated by runways and taxiways. At Seattle–Tacoma International Airport and Austin– Bergstrom International Airport, filter strips have been installed to meet runoff permit requirements. At many airports, runway safety areas often naturally function as filter strips. Vegetated fil- ter strips share some similarities with bioswales (BMP 8); how- ever, they have different geometries. While vegetated filter strips gently slope in one direction, bioswales have the concave shape of a typical swale. Successful performance of filter strips relies heavily on maintaining shallow dispersed flow. To avoid flow channelization and maintain performance, a filter strip should contain dense vegetation with a mix of erosion-resistant, soil-binding species. The most important criteria for selection and use of this BMP are space and slope as shown in Figure 17. Design Features that May Be Customized Filter strip design must be consistent with runway safety area requirements to be used on the airside; consequently, there is little design flexibility. When used to treat runoff from roads on the landside, slope and length can be modified as necessary. Suitable Airport Areas Airside: runways, taxiways, ramp, roadways Climates All except arid Pollutant Removal Sediment; metals; nutrients Runoff Volume Control Infiltration; evapotranspiration Site Hydrology and Other Site Properties No limitations FAA or Other Regulations Wildlife hazard management; runway safety area Customizable Design Features Vegetation; filter slope and length Primary Maintenance Activities Mowing Source: M. Barrett. Figure 17. Vegetated filter strip.

52 Green Stormwater Infrastructure Source: Seattle–Tacoma International Airport. Figure 18. Vegetated filter strips along runway at Seattle–Tacoma International Airport. As mentioned previously, when Seattle–Tacoma International Airport first installed filter strips adjacent to the runway (Figure 18), the airport did not consider them GSI; however, the airport has since retrofitted them to include GSI characteristics. The filter strips now include a minimum of 100-foot filter depth with compost amended soils. Performance Pollutant Reduction Filter strips generally perform well for removal of solids, while removal of dissolved constitu- ents is achieved primarily by runoff infiltrating into the soil. Little reduction in bacteria concen- trations is typically observed, which is likely due to bacteria living in the soil. Runoff Volume Control When filter strips are installed in an area that contains relatively permeable soils, almost all runoff will be retained. In lower permeability soils, infiltration will still reduce runoff volumes by up to 50 percent. Potential Issues and Constraints The primary issue related to the use of filter strips along runways is the potential habitat they provide. This issue can be minimized by regular mowing to keep the grass short. In some cases, compaction along runways may limit the effectiveness of filter strips at these locations. For example, Austin–Bergstrom International Airport noted that compaction can affect the effectiveness of filter strips. Maintenance Requirements and Indications of the Need for Maintenance According to Los Angeles International Airport, filter strips should be mowed as needed for safety and to avoid creation of wildlife habitat. Grassy areas functioning as runway safety areas

Understanding GSI Best Management practices 53 would have to be mowed regardless of whether they were considered filter strips; therefore, there would be no additional O&M costs for this GSI practice. Replacement Needs A filter strip is expected to have an unlimited life span. Costs A filter strip is effectively cost neutral when used to manage runoff from runways and taxi- ways because requirements of a runway safety area (i.e., a stable, low-slope area 250 feet from the runway centerline) naturally create a filter strip.

54 Green Stormwater Infrastructure BMP 8: Bioswales (Vegetated Swales, Grassy Swales) Bioswales are vegetated channels that convey stormwater and remove pollutants by sedimentation and infiltration. They require shallow slopes and soils that drain well. Bioswales are widely used at airports. Examples include Los Angeles International Air- port, Seattle–Tacoma Inter national Airport, San Diego Interna- tional Airport, Hartsfield–Jackson Atlanta International Airport, Denver International Airport, Baltimore Washington Inter- national Airport, and Cleveland Hopkins International Airport. Figure 19 shows the general diagram of the cross section of a bio- swale in a parking area. The vegetation in bioswales is often grass (grassy swales), but it may consist of other plant types selected for site appropriateness or water quality goals (e.g., nutrient removal). Landscaped bio- swales (e.g., with shrubs and other plantings) may sometimes be referred to as forms of bioretention BMP. However, bio retention cells are generally considered to differ from bioswales in geometry; bioswales are linear in shape and have a gentle longitudinal slope. Bioswales share some similarities with vegetated filter strips (BMP 7); however, they have different geometries. While vegetated filter strips gently slope in one direction, bioswales have the concave shape of a typical swale. Bioswales are primarily stormwater conveyance systems. They can provide sufficient con- trol under light to moderate runoff conditions, but their ability to control large storms is lim- ited. Grassy swales in particular can be used as a pretreatment measure for other downstream Suitable Airport Areas Landside or airside: terminals, roadways, parking lots, runways, commercial buildings Climates More than 12" of rain/year Pollutant Removal Sediment; metals; nutrients Runoff Volume Control Infiltration; evapotranspiration Site Hydrology and Other Site Properties Moderate slope needed FAA or Other Regulations Slope meets FAA safety area; wildlife hazard management Customizable Design Features Slope; width; length; water depth; soil; vegetation Primary Maintenance Activities Trash and debris removal; sediment removal; mowing Infiltration Curb cut Source: M. Barrett (© 2015). Figure 19. Bioswale.

Understanding GSI Best Management practices 55 structures, such as bioretention cells. Enhanced bioswales have engineered soils and an under- drain to provide filtration of pollutants. Bioswales can also be incorporated in the design of parking areas. Bioswales can be more aesthetically pleasing than concrete or rock-lined drainage systems and are generally less expensive to construct and maintain. Swales can slightly reduce imper- vious area and reduce the pollutant accumulation and delivery associated with curbs and gutters. Design Features that May Be Customized Pollutant removal capability is related to channel dimensions, longitudinal slope, and amount of vegetation. The plant or grass species selected should be unattractive to hazardous wildlife. Optimal design of these components will increase the contact time of runoff with the soil and plants as it moves through the bioswale, thus improving pollutant removal rates. There is con- siderable latitude in determining bioswale dimensions, including channel width, steepness of the side slopes, and maximum water depth for the design rainfall event. The length of the bioswale can also be an important factor in determining the amount of pollutant removal and the loss of volume via infiltration through the channel bottom. Many designs also include soil amendments to increase the amount of infiltration. Seattle–Tacoma International Airport has chosen a design of overplanting with heavy ground cover to minimize O&M. Performance Pollutant Reduction Bioswales have only moderate pollutant removal generally and, in some cases, are an inexpen- sive way to convey stormwater. They do allow some additional pollutant load reduction by infil- tration into the soil. Little reduction in bacteria concentrations is typically observed. Runoff Volume Control When bioswales are installed in an area that contains relatively permeable soils, load will be reduced substantially because of runoff infiltration into the underlying soils. Potential Issues and Constraints The primary issue related to the use of bioswales is accumulation of sediment. Accumulation of sediment can result in standing, open water within the channel, which must drain within 48 hours to meet FAA wildlife hazard management guidelines. Bioswales may be used within runway safety areas and taxiway safety areas, but the slope must meet FAA safety area requirements. Los Angeles International Airport decided to incorporate a series of bioswales along the taxiway and runway to meet local requirements, but it was not possible in all taxiway locations or along the entire runway because of grade, flow, and soil quality constraints. Maintenance Requirements and Indications of the Need for Maintenance For bioswales dominated by grass (i.e., grassy swales), maintenance is minimal and is largely aimed at keeping the grass cover dense and vigorous. Mowing should be performed routinely, as needed, throughout the growing season to eliminate potential wildlife habitats. Mowing should be done in alternate directions to minimize the occurrence of ruts. Regular mowing should also

56 Green Stormwater Infrastructure include weed control practices, although herbicide use should be kept to a minimum. An inte- grated pest management approach can help reduce chemical use. Sediment accumulating near culverts and in channels needs to be removed when it results in a significant amount of standing water. A healthy dense grass should be maintained in the channel and side slopes. Grass damaged during the sediment removal process should be promptly replaced using the same seed mix used during bioswale establishment. Grass height should be maintained based on the target hazard- ous wildlife species. For bioswales dominated by species other than grass, e.g., shrubs or other plantings, maintenance may be more involved. Replacement Needs With periodic maintenance, a bioswale is expected to last indefinitely. Costs According to Seattle–Tacoma International Airport, capital costs are easy to estimate because a basic swale is a familiar construction practice. Enhanced bioswales, which incorporate an “engineered soil mix,” can be significantly more expensive. San Diego International Airport reported a cost of $650,000 to construct 5.2 acres of enhanced bioswales at the new rental car center (Figure 20). The cost for vegetated swales per treatment area was estimated in 2012 U.S. dollars as follows (Venner et al. 2013): • New construction: $19,500/acre for areas less than 3 acres. $2,300/acre for areas greater than 3 acres. • Retrofit construction: $37,500/acre for areas less than 3 acres. $4,400/acre for areas greater than 3 acres. Source: J. Jolley (© 2015). Figure 20. Bioswale at San Diego International Airport rental car center.

Understanding GSI Best Management practices 57 BMP 9: Wetland Treatment Systems (Constructed Wetlands, Conventional Stormwater Wetlands) Constructed wetlands are facilities that create growing con- ditions suitable for marsh and wetland plants, which may be designed as either surface or subsurface systems (Figure 21). Sub surface wetlands are preferred for airport locations as they substantially reduce hazardous wildlife attraction. Surface wet- lands, with ponded water, can be implemented at airports, but must be designed to reduce habitat either through the use of netting or by ensuring a plant density great enough to eliminate open water. Conventional stormwater wetlands are shallow man-made facilities supporting abundant vegetation and a robust micro- bial population. These facilities are designed to maximize pollut- ant removal through plant uptake, microbial degradation, and settling of solids. Hydrology is one of the most influential factors in pollutant removal due to its effects on sedimentation, aeration, biological transformation, and bottom sediment adsorption. Site con- siderations should include the water table depth, soil/substrate, and space requirements. Because the wetland must have a source of flow, it is desirable that the water table is at or near the surface. Suitable Airport Areas Landside or airside: terminals, roadways, parking lots, commercial buildings Climates Wet Pollutant Removal Sediment; metals; deicing chemicals Runoff Volume Control Little to no reduction Site Hydrology and Other Site Properties High water table; wet climate FAA or Other Regulations Wildlife hazard management Customizable Design Features Vegetation; media; size; drainage layer configuration Primary Maintenance Activities Vegetation management Source: NIWA (2011). Figure 21. Subsurface wetland treatment system.

58 Green Stormwater Infrastructure Design Features that May Be Customized The primary design consideration is to ensure that there is sufficient residence time of the run- off within the wetland to allow biological processes to operate effectively. Consequently, the size of the facility will vary substantially depending on the rainfall characteristics of the area. Wetlands can also be constructed with multiple cells, which improves pollutant removal performance. Side slopes should be steep with a deep open water feature to limit accessibility to wading birds and other wetland-dependent wildlife. Another important design decision is whether the wetland will be surface or subsurface, with the latter preferred to reduce the potential for attracting wildlife. For surface systems, the operator will have to determine the best strategy for limiting wildlife access through either physical exclusion (e.g., netting, bird balls, or wire grids) or through the use of dense vegetation and/or plant species that are unattractive to wildlife. Performance Pollutant Reduction Constructed wetlands provide physical, chemical, and biological water quality treatment of stormwater runoff. Chemical processes include chelation, precipitation, and chemical adsorp- tion. Biological processes include decomposition, plant uptake and removal of nutrients, plus biological transformation and degradation. Constructed wetlands are capable of excellent pol- lutant removal if sized and designed properly. Performance is generally good with respect to settling of the solids fraction and for the dissolved constituents as well, due to active microbial action. In addition to stormwater treatment, subsurface systems have also been used to treat deicing-contaminated runoff at airports such as at Buffalo Niagara International Airport. The Buffalo Niagara International Airport system was designed to reduce BOD5 concentration by greater than 95 percent (Mericas et al. 2009). During the 2010/2011 deicing season, the treat- ment system (Figure 22) removed an average of 98.3 percent of BOD5 at a loading rate of up to 44,000 pounds per day (Wallace and Liner 2011). Runoff Volume Control Generally there is very little volume loss in constructed wetlands because the high water table preferred for these facilities limits infiltration. Subsurface systems may be lined to ensure that water remains available for plants during dry spells, so no infiltration would occur in these systems. Source: Wallace and Liner (2011). Figure 22. Engineered wetland treatment system at Buffalo Niagara International Airport.

Understanding GSI Best Management practices 59 Potential Issues and Constraints The main issue with wetlands is their potential to attract wildlife, primarily birds. Seattle– Tacoma International Airport and Minneapolis–St. Paul International Airport have used netting to cover open water to limit access. Seattle–Tacoma International Airport looked into several strategies to comply without attracting hazardous wildlife. They built a detention pond with the construction of a new runway as well as mitigation wetlands with enhanced features that would normally attract birds. Eleven sterile ponds surround the airport and detain all runoff. Built with liners and nets and located away from vegetation, these ponds do not attract water fowl. While the nets increased costs, they have been effective in preventing wildlife hazards. Seattle–Tacoma International Airport has also addressed the wildlife hazard issue by increasing plant density and planting less diverse, non-fruit-bearing vegetation in its regulated wetland systems. The plant- ings are so dense that they reduce open surface water and make it difficult for birds to move in and out of the wetland, thereby limiting access and resulting in a dramatic reduction in wildlife use. FAA headquarters and local offices have provided assistance. Maintenance Requirements and Indications of the Need for Maintenance Wetlands should be inspected at least twice a year (once during or immediately following wet weather) to evaluate facility operation. When possible, inspections should be conducted during wet weather to determine if the BMP is functioning properly. Replacement Needs The expected life span of a well-designed system is 20 to 30 years. The system may be rejuve- nated at that point by dredging and revegetating the cells (Miller et al. 2003). Costs The cost of a wetland treatment system can be highly variable depending on size, whether a liner is required, and the topography of the site, which affects the amount of grading required. In 2009, Buffalo Niagara International Airport completed an engineered subsurface wetland treatment system primarily to meet the NPDES requirements for glycol. The system consists of (1) a 3,300-square-foot glycol treatment utility building housing pumps and (2) four discrete subsurface wetland cells, each 300 feet long by 167 feet wide by 5.5 feet deep. The total construc- tion cost was approximately $14 million (McFarland Johnson 2013). The cost for wetland treatment systems per treatment area was estimated in 2012 U.S. dollars as follows (Venner et al. 2013): • New construction: $32,800/acre for areas less than 3 acres. $13,700/acre for areas greater than 3 acres. • Retrofit construction: $52,300/acre for areas less than 3 acres. $21,900/acre for areas greater than 3 acres.