The pavement sealcoating industry has faced growing regulatory pressure since a 2005 study in Austin, Texas, suggested that refined tar-based sealers (RTS) were a primary source of polycyclic aromatic hydrocarbons (PAHs) in urban waterways. This finding led to bans and restrictions in several jurisdictions, raising questions about the future of sealcoating practices across the United States. In response, the Pavement Coatings Technology Council (PCTC) sponsored a series of independent scientific studies to examine whether refined tar-based sealers are truly a major contributor to PAH pollution. The results, published in peer-reviewed journals, challenge the assumptions underlying the regulatory push and offer pavement contractors and specifiers a more complete understanding of PAH sources in the urban environment. For professionals working in Asphalt Pavement Engineering Mix Design Construction Methods Rehabilitation, understanding the science behind these regulatory debates is essential for making informed decisions about materials and practices.
Understanding PAHs and the Origins of the Sealcoating Controversy
What Are Polycyclic Aromatic Hydrocarbons?
Polycyclic aromatic hydrocarbons are a class of chemical compounds that occur naturally in the environment. They are formed whenever organic matter is burned incompletely. PAHs are found in a wide range of everyday sources, including vehicle exhaust, power plant emissions, wood smoke, forest fires, grilled foods, and used motor oil. They are also present in products derived from coal and petroleum, including refined tar-based pavement sealers. It is well established that refined tar-based sealers contain PAHs, but the critical question for the industry has always been whether these sealers are a significant source of PAHs in streams, rivers, and lakes compared to other sources.
The 2005 Austin Study and Its Regulatory Consequences
The controversy began with a study conducted in Austin, Texas, which showed that soil and sediment samples collected near parking lots treated with refined tar-based sealer sometimes contained elevated levels of PAHs. This study was the first to propose a direct link between PAHs in refined tar-based sealers and PAHs found in rivers, streams, and lakes. However, a critical piece of data was overlooked: sediment samples from Austin streams that had been collected and analyzed just a few years before the study was conducted. These pre-existing data were not incorporated into the analysis.
Despite this omission, the city of Austin banned the use of refined tar-based sealers within city limits, effective January 1, 2006. Other municipalities followed suit, and the sealcoating industry found itself defending a core product against regulatory action based on what the PCTC and its scientific partners would later argue was incomplete and flawed analysis. The stakes were high for contractors, material suppliers, and property owners who relied on refined tar-based sealers for durable, cost-effective pavement protection.
PCTC-Sponsored Research: Key Studies and Findings
In response to the regulatory challenges, the Pavement Coatings Technology Council commissioned independent scientific studies to investigate whether refined tar-based sealers actually contribute meaningfully to PAH levels in the environment. Two major lines of research emerged, both published in peer-reviewed scientific journals.
Before-and-After Analysis of Austin Stream Sediments
The first study took advantage of a natural experiment created by Austin’s ban. A local environmental company had collected sediment samples from Austin streams in October 2005, just before the ban took effect, and again in April 2008, more than two years after the ban was in place. If refined tar-based sealers were the principal source of PAHs in urban sediments, then PAH concentrations in Austin streams should have declined measurably after the ban removed that source. The results, published in the journal Environmental Forensics under the title “Polycyclic Aromatic Hydrocarbons in Austin Sediments after a Ban on Pavement Sealers,” showed that total PAH concentrations did not change significantly between the two sampling periods. This finding directly contradicts the claim that refined tar-based sealers are the dominant source of PAHs in urban waterways.
Environmental Forensics and PAH Fingerprinting
The second line of research employed environmental forensics, a scientific discipline that uses chemical fingerprinting techniques to identify the sources of contaminants in the environment. The PCTC engaged Dr. Paul Boehm, a leading expert in PAH forensics whose methods had previously been used in litigation following the Exxon Valdez oil spill to distinguish PAHs from spilled oil versus naturally occurring sources. Dr. Boehm and his team applied forensic fingerprinting methods to determine whether the PAH signature of refined tar-based sealers matched the PAH signature found in urban sediments nationwide.
How PAH Fingerprinting Works
PAH fingerprinting relies on the fact that different sources produce different mixtures of individual PAH compounds. The relative proportions of these compounds create a chemical profile, or fingerprint, that can be used to trace the origin of PAHs in an environmental sample. For example, PAHs from vehicle exhaust have a different fingerprint than PAHs from coal tar or wood smoke. By comparing the fingerprint of refined tar-based sealer to the fingerprints of sediment samples from urban waterways, scientists can determine whether the sealers are a likely source of the PAHs found in those sediments.
In a September 2010 article published in Stormwater magazine, Boehm and his team presented their initial evaluation. The refined tar-based sealer showed a distinct PAH fingerprint that did not match the fingerprint of most environmental samples. The only exception was sediments collected in the immediate vicinity of sealcoated lots, where localized contributions were detectable. The article concluded that atmospheric deposition of combustion products, not pavement sealcoating, is the most significant pathway introducing PAHs to urban sediments. Preliminary results presented at the annual meeting of the Society for Environmental Toxicology and Chemistry in November 2010 reinforced this conclusion: PAHs in sediment samples collected nationwide were more similar to PAHs found in atmospheric particles than to PAHs associated with refined tar-based sealers.
Comparing PAH Sources: What the Evidence Reveals
The PCTC-sponsored studies have helped clarify the relative contributions of different PAH sources in urban environments. When all sources are considered together, a clear picture emerges: refined tar-based sealers are not the primary driver of PAH levels in urban sediments.
| PAH Source | Relative Contribution to Urban Sediments | Primary Transport Pathway |
|---|---|---|
| Atmospheric deposition (vehicle exhaust, power plants) | Majority of PAH input | Air to land to water |
| Used motor oil and automotive fluids | Moderate | Runoff from roads and parking lots |
| Tire wear particles | Moderate | Runoff and airborne particulates |
| Wood smoke and biomass burning | Moderate, seasonal | Atmospheric deposition |
| Refined tar-based pavement sealers | Minor, localized | Runoff from sealcoated surfaces |
| Naturally occurring coal and petroleum deposits | Variable by region | Erosion and groundwater |
Why Early Studies Reached Different Conclusions
The PCTC contends that the flawed conclusions of the original 2005 Austin study arose from incomplete data analysis. By not incorporating pre-existing sediment data and failing to account for the many other sources of PAHs in urban environments, the study created a misleading impression that refined tar-based sealers were the primary culprit. Subsequent publications by the same research group attributed volumes of PAHs to refined tar-based sealers that were supported by no factual data. These imaginary volumes were then fed into mass balance models, originally developed for atmospheric science, to produce percentages of RTS-source PAHs in sediments that had no empirical basis. As the old saying goes, garbage in, garbage out.
The Scientific Consensus on PAH Sources
The peer-reviewed research sponsored by the PCTC aligns with the broader scientific understanding that PAHs in urban environments come from many sources, with combustion-related inputs being the most significant. Studies of Residential Glass Technology Low E Coatings Gas Fills demonstrate how modern building materials are evaluated through rigorous performance testing, a standard that should apply equally to environmental claims about pavement products. Similarly, industry advocacy efforts such as those described in Nibs Building Industry Advocacy How the Consultative Council show how construction trade organizations engage with policymakers to ensure that regulations are based on sound science. The slower pace of Technology Adoption Home Building Construction Industry Lags Digital transformation in construction serves as a reminder that evidence-based decision-making takes time to influence established practices, but it remains essential for sound policy.
Implications for the Pavement Maintenance Industry
Regulatory Landscape and Contractor Considerations
The PCTC research has practical implications for contractors and specifiers in the pavement maintenance industry. While some jurisdictions have moved to restrict or ban refined tar-based sealers, the scientific basis for such actions remains contested. Contractors who understand the full picture can make informed decisions about the products they use and can engage more effectively with regulators and clients.
Key Takeaways for Industry Professionals
- Refined tar-based sealers contain PAHs, but they are not the primary source of PAHs in urban sediments. Combustion sources such as vehicle exhaust and power plant emissions contribute the majority of PAHs found in waterways.
- The Austin ban on refined tar-based sealers did not result in a measurable decrease in PAH concentrations in local stream sediments, indicating that other sources dominate.
- PAH fingerprinting techniques demonstrate that the chemical signature of refined tar-based sealers does not match the PAH signature found in most urban sediment samples nationwide.
- Regulatory decisions affecting pavement maintenance products should be based on comprehensive data that accounts for all sources of PAHs, not single-source studies.
- Industry organizations such as the PCTC continue to fund independent research to ensure that environmental policy is grounded in sound science.
Recommendations for Pavement Maintenance Contractors
- Stay informed about the evolving science on PAH sources and refined tar-based sealers. The regulatory landscape continues to change as new research becomes available.
- Document the products and application methods used on each project. Detailed records help demonstrate compliance with local regulations and support the industry in defending science-based practices.
- Engage with industry organizations such as the PCTC and National Pavement Expo to access the latest research, training, and regulatory updates.
- When discussing sealcoating options with clients, explain the full context of PAH sources rather than focusing narrowly on sealers alone.
- Monitor local and state regulations in your service areas. Some jurisdictions have adopted bans or restrictions, while others have decided not to act based on the weight of scientific evidence.
The debate over refined tar-based sealers and PAH pollution illustrates the importance of rigorous, independent science in environmental policy. The PCTC-sponsored studies provide a more complete picture of PAH sources in urban environments and demonstrate that refined tar-based sealers are not the primary contributor to PAH contamination that early studies claimed. For pavement maintenance professionals, understanding this science is essential for making informed material choices, navigating regulatory challenges, and continuing to provide durable, effective pavement protection. As research continues and forensic methods become more refined, the industry is well positioned to advocate for evidence-based regulation that accurately reflects the true sources of environmental contaminants.