that recycled materials be used for airport access roads and non-FAA regulated pavements (Airport Cooperative Research Program, 2011). However, other contaminants potentially present in these waste materials, such as polycyclic aromatic hydrocarbons (PAHs) and metals, should be considered when evaluating potential alternatives to RMA (NJDEP, 2019).

Some alternatives to RMA offer similar durability benefits, such as asphalt cement that is mixed with non-rubber polymers to improve the strength and elasticity of pavement (IDOT, 2005). Use of these materials may support sustainability goals where recycled waste plastics are used as modifiers (Jexembayeva, Konkanov, Aruova, Kirgizbaev, & Zhaksylykova, 2024). The FAA has evaluated the performance of different asphalt-modifying polymers and found that different modifiers can provide an increased level of protection and rutting performance from heavy aircraft (Newman, 2004; Rushing, 2018). However, some of these alternative polymers may also contain anti-degradant chemicals such as 6PPD.

6.3 Conventional Stormwater Best Management Practices

Physiochemical properties reported for 6PPD indicate it has a density like water, a relatively low solubility in water, and a tendency to adhere to soil or organic matter particles (Navickis-Brasch et al. 2022). The solubility of 6PPD-q appears to be higher than 6PPD in water, suggesting the breakdown product, 6PPD-q, may be more likely to travel in the dissolved phase than 6PPD. 6PPD and 6PPD-q are documented to have low vapor pressure and high melting points, suggesting they are less likely to melt or evaporate at ambient temperatures. However, due to the tendency for 6PPD to adsorb to soil, sediments, and suspended particulates, 6PPD can travel on particles in the air (OSPAR Commission, 2006). 6PPD and 6PPD-q have similar physiochemical properties that determine their fate and transport through the environment. Characterizations of both compounds indicate a tendency for fat solubility and an affinity to bind to soil and organic matter, which would suggest that they would not leach to groundwater once they are bound in the soil.

Additional studies of 6PPD and 6PPD-q in environmental soil conditions are needed to determine how they move between soil and other mediums. While 6PPD and 6PPD-q appear to have an affinity for soil or organic matter, both contaminants are moderately non-polar compounds which indicates that transport of the contaminants in the dissolved phase is possible but not anticipated to be probable (Navickis-Brasch et al. 2022).

The breakdown of 6PPD and 6PPD-q in the environment is likely determined by the medium (soil, water, etc.) and the physical conditions (temperature, pH, chemical composition, etc.) The estimated half-lives of 6PPD and 6PPD-q are 75 days in soil and 337 days in sediment (soils below water), based on Fugacity Model results (EPA, 2021b). In water, the half-life of 6PPD tends to range from about 3 hours to less than a day, with shorter half-lives observed in waters with higher temperatures and higher concentrations of heavy metals (Ecology, 2022). Conversely, longer half-lives of up to four weeks were observed for 6PPD in waters with lower temperatures and lower pH (Ecology, 2022). The half-life of 6PPD-q in water is less well documented, but studies indicate that it is longer than that of 6PPD.

Flow control and treatment best management practices (BMPs) are physical, structural, or mechanical devices designed to reduce pollutants in stormwater discharge by improving water quality and/or controlling water volume and flow. These BMPs may be permanent structures or temporary controls, such as those used for erosion and sediment control. Permanent flow control and treatment BMPs include bioretention cells, infiltration ponds, and proprietary treatment devices (Ecology, 2022). Temporary construction BMPs include silt fence, temporary sedimentation basins, and straw wattles (Ecology, 2022).

Construction BMPs are not intended to achieve the treatment performance levels of permanent BMPs, but they are designed to reduce solids in stormwater during land-disturbing activities.

Source control BMPs are procedures and activities that prevent pollutants from entering stormwater through physical separation and management of potential pollutants. Source control BMPs include education and outreach programs, operation and maintenance activities such as street sweeping and line cleaning, and construction source control BMPs such as soil stabilization and proper material storage and handling (Ecology, 2022).

According to assessments by the Washington State Department of Ecology for mitigation of 6PPD in road runoff, the BMPs with the highest potential to reduce 6PPD and 6PPD-q in stormwater are those that utilize infiltration, dispersion, and biofiltration (when combined with adsorptive media such as bioretention soil or compost). BMPs with a medium potential to reduce 6PPD and 6PPD-q in stormwater are those that utilize sedimentation and filtration processes (Ecology, 2022). These types of BMPs remove contaminants by capturing particulates to which chemicals, such as 6PPD and 6PPD-q, are bound. Sedimentation removes solids by reducing stormwater velocity and allowing particulates to settle out of suspension. Filtration removes solids by physically capturing them as stormwater moves through a device or structure. The effectiveness of solids removal BMPs is impacted by particle size and density, with larger and heavier particles being easier to remove. TRWP larger than fine silt (25 to 62.5 µm) are expected to be removed through sedimentation and filtration (up to 500 µm), while smaller TRWP or fine silts and clays are not expected to be captured through filtration and sedimentation (Ecology, 2022). Successive treatment using sedimentation or filtration followed by polishing treatment using adsorption or flocculation may be required to fully remove 6PPD and 6PPD-q from stormwater.

According to the 6PPD in Road Runoff Assessment and Mitigation strategies,

Any BMP processes (infiltration, sorption, or settling/sedimentation) which did not fall under the criteria defined for high or medium potential to reduce 6PPD or 6PPD-q were defined as BMPs or processes with a low potential to reduce these contaminants. BMPs that fall under this category may also include BMPs which cannot be located near a roadway or other source of 6PPD or 6PPD-q. No information was found during the review of sources on physiochemical properties indicating other stormwater treatment processes could reduce the contaminants.

At airports, stormwater BMPs that utilize infiltration, dispersion, and filtration are most likely to be installed due to wildlife and aircraft safety regulations.

6.4 Innovative stormwater Best Management Practices

6PPD-q has been shown to degrade slower than 6PPD in water, however studies indicate that 6PPD-q degrades more rapidly when exposed to sunlight (Caltrans, 2024). In natural river water, the half-life of 6PPD-q is between 13 to 16 days; however, when exposed to sunlight irradiation, the half-life of 6PPD-q in water dropped significantly to 2.6 hours (Caltrans, 2024). This indicates that photodegradation has an impact on how rapidly concentrations 6PPD-q naturally diminish in the environment. Additionally, 6PPD-q has been shown to break down through microbial processes, although bacterial degradation of 6PPD can also create new 6PPD-q. Biodegradation has been found to be the primary loss pathway for 6PPD-q in saturated wetland soils, while low-oxygen (and therefore low-microbial) environments resulted in a significant increase in 6PPD-q concentrations (Caltrans, 2024) (Q. Xu et al., 2023).

Natural stormwater treatment systems that promote microbial activity and are exposed to sunlight may be effective at removing 6PPD-q; however, conditions such as soil saturation, ponding, and oxygen depletion must be considered. Studies of stormwater treatment using bioretention have shown effective 6PPD-q removal up to 98%, however a large surface area is required for optimal performance (Caltrans, 2024). In urban areas where TWRP and 6PPD/6PPD-q are more likely to accumulate, large bioretention systems are often infeasible. Smaller natural systems that both infiltrate and convey stormwater, such as bioswales and biostrips (vegetated strips), may be more feasible in urban areas and near roadways.

Studies of high flow rate biofilters with proprietary media have exhibited 6PPD-q removal rates up to 87 percent, with some effluent concentrations below the coho salmon LC50 (Herrera, 2024). These same studies also tested horizontal bed media filters, membrane filters, and cartridge-based media filters, all with proprietary media; these were not as effective at removing 6PPD-q, with a maximum removal rate of 54 percent (Herrera, 2024). Stormwater treatment BMPs that utilize bioretention and adsorption seem to have the highest potential to remove 6PPD-q, especially with media that is high in organic materials.

Additionally, studies of pervious (or porous) pavement and asphalt have shown effective 6PPD-q removal in stormwater, with greater than 75% removal in one study in Oregon (Caltrans, 2024). However, pervious pavement is not feasible for areas with high volumes of traffic or areas that convey large vehicles and equipment.

Further research is needed to determine the specific treatment mechanisms that most effectively remove both 6PPD and 6PPD-q in stormwater, and the best ways to implement these treatment mechanisms in areas where TWRP are likely to accumulate. Further investigations are also needed to understand how 6PPD and 6PPD-q interact with other contaminants in stormwater, and how that might impact treatment performance.

6.5 Enhanced Foreign Object Debris and Runway Rubber Removal

As discussed in Section 4.4.2, many FOD and runway rubber deposit removal techniques displace materials by sweeping, blowing, or washing them from airport pavements and onto surrounding grassy areas rather than capturing them for disposal. These removal methods could transport particles containing 6PPD and 6PPD-q to soil, via air, or stormwater conveyance systems where they can enter water bodies and other environmental media either directly or by leaching contaminants from the debris. Where practical, equipment can be integrated with collection systems such as vacuum recovery systems to recover debris as it is being removed.

6.6 Waste Management and Disposal

FOD and removed rubber disposal practices are not well documented, but it is expected that they are handled as part of an airport’s overall waste management program that could differ among airports. Segregating materials that contain 6PPD, 6PPD-q, and other contaminants from FOD streams as well as separating recyclable materials may offer opportunities for enhanced removal of these products from the airport environment as well as diversion from landfills. Efforts to improve aircraft tire designs that increase tire durability and longevity provide the opportunity to reduce the amount of tire wear particle waste generated with each landing and limit the release of 6PPD and 6PPD-q to the environment.

Aircraft tires are typically not owned by airlines. Rather, they are leased from the tire manufacturer. After approximately 250 landings or depending on wear, airlines ship the tires back to the manufacturer for retreading, where they are then sent back to the airlines for reuse. An aircraft tire can be retreaded up to four to seven times, which is more than truck or other road vehicle tires can be. Once they can no longer

be retreaded, aircraft tires are recycled by the manufacturer (Michelin, 2017; Rogers, 2016). Since aircraft tires are generally not retreaded or discarded at airports, their contribution to an airport’s waste stream is not expected to be significant. However, tires from ground support equipment are typically managed at airports, and routine maintenance of these vehicles produces waste tires.

Consistent with airport sustainability focus and waste reduction goals, airport waste managers may be directed to segregate and recycle certain wastes. For example, the New York and New Jersey Port Authority specifically requires the airports it manages to segregate tires for recycling (Port Authority NY NJ, 2024). In 2011, the Denver International Airport reported recycling 1,093 tires and the Minneapolis–St. Paul International Airport reported recycling 15.2 tons of tires (FAA, 2013). It is important to note that recycled tires may still pose downstream environmental impacts as recycled tire rubber products such as athletic turf pellets, playground mulches, and certain consumer products are known to contain 6PPD and 6PPD-q (ITRC, 2024).