Parking Lot Sealants
Parking lot sealcoating is a national obsession. On any fair-weather day, dozens of work crews in cities throughout the nation can be seen spreading gooey black tars over acres of parking lot pavements and driveways–spraying, mopping, or brushing a few years’ shine onto their fading surfaces, dutifully following applicators’ recommendations that the practice be repeated every couple of years. But few people pause to consider what may have become of the previous years’ coatings, abraded away by tire treads, wind, and rain. As the spring paving season heats up, however, the people of Austin, TX, will be regarding shiny new parking lots with a new sense of discernment.
There are two major types of parking lot sealants widely in use throughout the United States; one is based on asphalt, and the other has a coal-tar base. Refined coal-tar sealants are used in some regions of the country, while asphalt sealants are favored in other regions. As of January 1, 2006, coal-tar-based sealants were banned for use in Austin.
Coal tars contain a high concentration of polycyclic aromatic hydrocarbons (PAHs)–environmental pollutants that have been the subject of scrutiny for a number of years. In recent studies researchers have noted increases in sediment PAH concentrations in urban ponds and lakes over time.
Tom Ennis, director of the Spills Response Division of the Watershed Protection and Development Review Department (WPDRD), estimates that “in Austin, 600,000 gallons of parking lot sealcoat is laid down each year.” He says that applicators recommend the sealants be applied every two to three years to maintain the shiny new appearance and integrity of the parking lot surfaces. Ennis adds that the vast majority of sealant “traditionally used in Austin has been coal-tar based, but now using a coal-tar sealant in Austin can net you a $2,000 fine.”
How do you tell the difference between two gooey black substances intended to seal and coat parking lot surfaces to protect them from weathering and wear? For Ennis, this became an important question. “We developed a simple screening test, which we demonstrated on a local newscast to help inform the public of the new policy. The test involves a black light, some vegetable oil, a swab, and the wheel well of the investigator’s vehicle. We know that coal tar leaves a residue that fluoresces yellow under certain wavelengths of black light. We also know from experience that when people who worked in the industry needed to remove the residue from their skin, they would first wipe down with vegetable oil and then look at the stained area under black light. If it glowed yellow, that meant there was still coal-tar residue left behind.”
On the morning of January 4, 2006, the test would undergo its first real-life enforcement application as the WPDRD received its first call for a potential violation of the coal-tar ban. A concerned citizen had observed a contractor applying a substance he suspected might have been one of the banned sealants onto a parking lot surface. The WPDRD Spills Response Division responded to the caller by dispatching a team to investigate. Ennis was confident that by the end of the day he would have the answer. But answers have not always been that easy to come by.
Routine Studies Uncover Problems
It all began innocently enough when in 1993, with the help of an EPA grant, the WPDRD began a comprehensive environmental survey of Austin’s creeks. Nancy McClintock, assistant director for Watershed Protection at the WPDRD, provides a rationale for the project: “In a state like Texas, we don’t have as much water as some places have, and we’ve always been really focused on protecting it. People in Austin always wanted to know that their resources were going to be protected.” The WPDRD planned to test all of the 50 creeks in its jurisdiction for numerous parameters, from physical characteristics to biodiversity to chemical and flow characteristics.
McClintock says the idea “was to get an overall assessment of stream health that could help us in development planning and resource management.” One of the chemicals they tested for was PAH.
A Big Family
PAHs comprise a family of chemical compounds composed of the two elements carbon and hydrogen, arranged in a lattice of interconnecting hexagonal rings, similar in structure to the benzene ring. Like carbon monoxide, they do exist in nature, but they are also created from the incomplete combustion of hydrocarbons. There are about 100 distinct PAHs, and they can be categorized in 17 families. Within each branch of the PAH family of molecules, the benzene-like rings bear distinctive patterns of connections with one another. Some forms of these molecules stretch out as chains, others coalesce into complex polygons, some are a combination of both, and occasionally some PAHs may include traces of elements other than carbon and hydrogen among their constituents. PAH molecules of varying shapes and sizes display distinctive, and sometimes divergent, chemical and physical properties.
PAHs tend to adhere to surfaces. They are hydrophobic and don’t dissolve readily in water, and those derived from coal tar also have a high resistance to petroleum products. Many of the PAH compounds are remarkably chemically stable. All of these are qualities that would make them potentially useful as sealants. Coal-tar-based sealants contain the highest concentration of PAHs–between 20% and 35%–whereas asphalt-based sealants contain about 5% PAHs. But the same characteristics that make sealants containing high levels of PAHs so effective also might make exposure to them hazardous. Once PAHs adhere to an object, they are hard to remove, whether that object is a pebble, a grain of sand, or (if inhaled, ingested, or in contact with) living tissue. Historically, PAHs have been associated with many negative biological impacts, ranging from reproductive effects in wildlife to cancers in humans. In 1775, Dr. Percival Pott traced an epidemic of scrotal cancer in English chimney sweeps to coal-tar residues in soot. Subsequently, the Danish Chimney Sweepers’ Guild and other occupational authorities in many European nations recommended frequent bathing for chimney sweeps, reducing dermatologic exposure levels and illness in those countries (Nature, August 2004, Vol. 4). At least 13 branches of this suite of chemicals have been designated to be of environmental concern or hazardous by the EPA.
Paradoxically, as dangerous as they sound, with sophisticated and sensitive detection methods, PAHs can be found virtually everywhere it is possible to look. Astronomers have even detected them in traces on meteorites and in interstellar space. PAHs can be found among natural sources–spouting from volcanic flows, in the ashes of forest fires–from manmade sources such as vehicle exhaust and coal furnaces, and as a byproduct of the coking processes used in the manufacture of aluminum and steel. They can even be detected on meats grilled over an open flame.
The majority of PAHs in the environment are believed to arise from two primary sources: petrochemical and combustion processes. These are scientifically termed, respectively, petrogenic and pyrogenic. However, they are usually found in low concentrations in the ambient environment and are seldom concentrated enough to be a health concern. But recent studies are in fact showing increasing concentrations of PAHs in the sediments of lakes and reservoirs in the urban environment over time. This became a growing concern for Leila Gosselink, an engineer working with data analysis for the WPDRD. She and her team, as part of their study of Austin’s streambed ecology, had found widely divergent concentrations of PAHs in sediments in certain hot spots along the city’s creeks.
“When we started sampling, there was not a lot of data on PAH levels in small creeks and tributaries of urban watersheds,” Gosselink recalls. She says their sampling had disclosed PAH concentrations varying from “undetectable levels in some creeks to levels of hundreds to thousands of parts per million in others”–a discrepancy, she says, large enough to warrant further scrutiny. “We had these data that showed widely these varying concentrations. At that time there were no guidelines for PAHs in creek sediments. We checked with our colleagues in other cities, and no one had done any studies like this on creek sediments. So we needed someone to help us interpret our data.”
They contacted Peter Van Metre of the US Geological Survey’s (USGS’s) National Water-Quality Assessment Program, who had been working on a study investigating rising PAH concentrations in sediments of 38 lakes nationwide.
“When we informed him of some of the high PAH readings we were getting, he told us our data couldn’t be right, that we must have a problem with our study, or that we must have misplaced our decimals.” But Gosselink was sure of her data.
She and her team agreed to split samples with Van Metre and also did some side-by-side sampling with USGS researchers in the field. When Van Metre sent his samples to USGS labs to have them perform an independent analysis, along with the lab reports, he also received a complaint. “They wanted to know what the heck was in the stuff. They said it gummed up the equipment and the PAH levels were off the charts.”
No one had ever seen PAH readings like this from ambient sources. So Gosselink began the meticulous task of source tracing. Because of the expense of laboratory analysis for PAH, she first did some research to find an inexpensive screening method. Her colleague, Mateo Scoggins, an aquatic biologist with the WPDRD, describes the process. “We used the enzyme-based ELISA method and were able to do the analysis on samples in-house at a cost of between $25 and $50, compared to the usual $150 or more for lab analysis.” They sampled sediments from the creeks and tributaries starting at their mouth at Town Lake on the Colorado River, all the way up the creeks, looking for the source of the PAH contamination. “We tracked sediments up the creek until PAH concentrations started dropping again. So we knew it wasn’t continuing from the main stem. Then we started looking at the small drainages immediately upstream of the area where we were getting the high readings. We got a couple of readings that showed increasingly large concentrations the farther we went up the channel.” Following the trail of the increasing levels, he says, “led us right up to the parking lots. One of our people in the Spills Response Division who had experience in the building trades observed that these were in fact sealed lots.” A picture was beginning to emerge.
But all sealed parking lots were not equal. Some parking lots had PAH runoff levels many times higher than others. Another variable had to be involved. McClintock recalls what she calls the “Aha!” moment. “Tom Bashara from our Spills Division was doing some research and stopped by at a local hardware dealer while puzzling over the question of where these high concentrations might be coming from. He examined containers of two types of parking lot sealcoat and suddenly it struck him–one type of sealant contained coal tars, and coal tars are heavy in PAHs.” All that would be necessary now was to confirm that coal-tar sealants had in fact been applied to the parking lots in question, and the WPDRD would have its answer and could begin looking for a remedy. But things were not going to be that simple.
The Puzzle of Barton Springs
Nestled not far from downtown Austin is a spring-fed pond surrounded by rolling parklands. With waters a temperate 70 degrees year-round, it had become a favorite summertime escape from the sweltering city. The park is well regarded by Austin residents. In fact, Scoggins says, “There are indications the site was visited by humans as far back as prehistoric times, although more recently in the early 1900s, the pond was dammed and the banks reinforced with concrete to convert it to a public swimming hole called Barton Springs Pool.”
McClintock adds that it is special for another reason. “Along with a few other related springs in the area, it’s the only habitat for the endangered Barton Springs salamander.” For her, “The Barton Springs Pool is the heart and soul of the city.”
In January 2003, staff from the WPDRD prepared to announce their conclusions that coal-tar-based sealants were a major contributor to elevated PAH levels in Austin’s creeks, when suddenly a dramatic story hit the media. The Austin American-Statesman, a local daily, citing some of the WPDRD’s data, reported that traces of potentially cancer-causing PAH residues had been detected in the vicinity of Austin’s favorite swimming destination, Barton Springs Pool. The paper’s reports discussed the hazardous nature of PAHs and developed a position contrary to what the WPDRD believed the data would demonstrate. The Statesman made the association between PAH concentrations and toxic waste sites such as retired coal gasification plants, which had heretofore been the only widely discussed source for elevated PAH concentrations. The paper developed a theory that the PAHs in and around Barton Springs Pool were constituents of wastes from one of these plants. It was theorized that wastes containing coal tar had in the past been disposed of on a hillside adjacent to the park and were contributing contaminants to the groundwater, eventually making their way to the pool. Immediately public attention was galvanized. Austin clamored over PAHs and the history and fate of Barton Springs Pool.
McClintock remembers the shock and disappointment that rippled through the community. “It was a difficult time for everyone–people were saddened at the notion that one of their cherished parks was being compared to a toxic site.”
For the community, the question of human health effects from PAHs in the pool was the first priority. From McClintock’s point of view, however, there was never any real human health threat. “Because PAHs tend to cling to sediments and don’t dissolve readily, they are usually there left behind by the water, and therefore they seldom become a drinking-water or a groundwater problem.” Furthermore, “The contaminated sediments in the pool were generally under 15 to 17 feet of water, so the risk of significant contact with them was practically nonexistent. Also, she suspected that “the concentrations of PAHs in those and other contaminated sediments in the area, while above the screening limits for our study, were far below those at which one would expect adverse health impacts.” She was right. In the toxicological report that was made in the aftermath of the incident, one toxicologist who concurred with McClintock’s view is Dr. Richard Beauchamp of Texas Department of Health. Beauchamp was quoted in a press release from the Austin City Council announcing the reopening of the pool, indicating that concentrations were so low that a child playing in the mud, with the most contaminated sediments found covering his hands, arms, and legs for an hour at a stretch, would face no significant health risks. He added, “There is really no risk for a child playing with sediment.”
According to McClintock, “The whole question is exposure.”
Nevertheless, researchers and staff from the WPDRD joined with city, state, and federal officials as they took steps to ensure public safety at the park. The pool was closed to bathers for three months while efforts were made to isolate the source of the contaminants. Core samples were taken along the hillside in question. The Texas Council on Environmental Quality, along with the WPDRD, assessed the results. Samples of sediment from various locations in the nearby creeks and the pool were analyzed. A constellation of factors was to be considered: the historical record, the hydrology of the area, the topography. One factor after another seemed to militate against the possibility of a toxic waste issue at Barton Springs, and together these analyses seemed to indicate a source other than a toxic waste deposit. And once again, sediment sampling in the nearby creeks revealed that the highest concentrations of PAHs were not to be found buried in the core samples taken from the suspected toxic dump on the hillside, but in runoff from the freshly sealcoated parking lot of an apartment complex at the crest of the hill.
Seasonal flooding that sometimes caused the creeks near the springs to top their banks had probably carried residues from the parking lot along with the flow. “The indications were that in this case, what we were dealing with was in fact a surface runoff issue from those parking lots,” says McClintock.
Comparing Risks to Create Policy
While the Barton Springs incident did not endanger public health or safety, it did raise the consciousness of the community to the risks from this ubiquitous contaminant. And it helped confirm the WPDRD’s position that much of the PAHs were coming from parking lots. Nonetheless, the WPDRD needed more information to evaluate the implications of its developing knowledge base. For instance, Van Metre says, “No one had ever identified sealant as a problem; no data existed describing how quickly PAHs washed off of them into sediments. There had been no side-by-side comparisons of sealant products.”
The WPDRD, along with Van Metre, designed a study to answer some of these questions. They would compare PAH yields from parking lots of several different types including sealed, unsealed, and concrete–13 parking lots in all, throughout the city. They had several broad questions they hoped to address: Did freshly applied sealants yield higher concentrations of PAHs than aged or worn coatings? Did parking lot use patterns have any significant impact on PAH yields? What was the prospective PAH yield over time of the various types of sealants?
Another component of their study called for the application of fresh coats of sealants on test plots at the retired airport. These test plots would provide a direct comparison between PAH levels from asphalt sealants and coal-tar sealants. The results of their research were published in the journal Environmental Science & Technology in 2005 (Vol. 39, No. 15).
Van Metre summarizes their methodology. “We contacted several businesses and got permission to sample on their lots. The University of Texas and the city gave us permission to sample on their lots. We went out on Saturdays or Sundays when there weren’t a lot of people driving in and out and marked off an area of about 50 square meters for the study. We took 100-liter jugs of distilled water, clean tubing, and clean pumps and used a light spray through a watering nozzle and just moved gradually down the lot until the water would run down to the spill berms we had set up. This would simulate about one-tenth of an inch of rain. Then we pumped the runoff up into containers as we were doing the washoff. By the time we got to the end of that process, we hadn’t completely cleaned the lot, but we were getting a lot clearer water than when we started, which indicated we were getting most of the loose dirt and particles. Then we did the filtering through Teflon filters, which let us trap the particles in a real fine mesh. This allowed us to separate the water and get the mud right back off so we could measure the chemistry of the sediments directly.” When the sediments were isolated, they had the appearance of a dark brown mud the color of coffee grounds, including particulates of varying sizes from fine dusts to granules the size of a grain of sand and larger.
When tested, the particles in runoff from parking lots with coal-tar-emulsion sealcoat had mean concentrations of PAHs 65 times higher than the mean concentration from unsealed asphalt and cement lots. In fact, PAH yields from the sealed parking lots were “100 times higher than from used motor oil,” which Van Metre says “is the next-most-contaminated stuff out there for PAHs.”
By way of comparison, Van Metre recalled another study of urban PAH levels with which he had been involved. This one focused on PAH runoff from asphalt-shingled rooftops. “One might have expected the asphalt rooftops to be a large source of PAH, but runoff we collected in that study right here in Austin was 200 times lower in PAH yield per area than the sealed lots.”
The test plots where Van Metre had applied fresh coats of the sealants revealed more conclusive data; coal-tar sealants yielded more PAHs by an order of magnitude over the asphalt-sealed plots. According to Van Metre, “An ongoing study in Fort Worth is looking at particles swept off of parking lots and streets. Their findings seem to support what we have seen: much higher concentrations of PAHs coming from the sealed parking lots and much lower concentrations from the roadways, which are generally not sealed, and from unsealed parking lots.” This information gave Van Metre added confidence. “I think it shows it wasn’t just some kind of weird anomaly we were picking up here in Austin.”
The joint study concluded that when all the other sources of urban PAHs that parking lots receive–such as vehicle exhaust, tire particles, leaking motor oil, and atmospheric fallout–are accounted for, the average yield from sealed parking lots is 50 times greater than from unsealed lots. Based on this, statistical projections generated from their study data, and an analysis of four local watersheds in the Austin area, the researchers estimated that the total PAH load from parking lots in those watersheds would be reduced to 5% to 11% of the current loading if all the parking lots were unsealed.
Important Little Bugs
Mateo Scoggins, the aquatic biologist for the WPDRD, undertook a separate line of inquiry that began to look into the biological impacts of PAH exposure on the typical stream ecology. He looked at indicator organisms under several concentrations of PAH exposure. As a benchmark, he used the consensus-based probable effect concentration, the level at which biological impacts are considered likely. He exposed creek-dwelling macroinvertebrate species to low, medium, and high concentrations of PAHs derived from parking lot sealants.
His group conducted three major studies. The three studies involved looking at the effects of PAH exposure under laboratory conditions, in microcosms, and in the field. In the lab study, test organisms were exposed to water dosed with sample scrapings from either asphalt-based sealants or coal-tar-based sealants in low, medium, and high concentrations for a one-month period. In the microcosm study, test samples of creek organisms were seeded in waters spiked at the same three treatment levels. In the field study, communities of organisms were evaluated at various locations in five urban creeks.
Effects were evident at all levels of PAH concentration tested, but at moderate and high concentrations, the negative impacts of PAH toxicity were pronounced. Scoggins says the results of the studies “showed the coal-tar-based sealants to be significantly more toxic to these organisms than the asphalt-based products. When ultraviolet light was added to more closely simulate the aquatic environment in the streambeds, the effects were profound.” Under moderate PAH exposure levels, the samples exposed to coal-tar-based sealants showed zero survivability when dosed with UV light, while those in the asphalt-exposure group survived at a rate of 70% under the same UV dosing. The control groups were not affected by the addition of UV exposure and showed 100% survival. It appeared that coal-tar-based PAHs could be expected to become significantly more toxic when exposed to sunlight, which is unavoidable in the urban aquatic environment.
Furthermore, the study recorded almost complete mortality for some organisms at levels of PAH exposure below some of those actually recorded in the most contaminated streams.
Observations recorded in the actual aquatic environment of the streams supported these findings. One consistent finding was a loss of taxa, a classic measure of ecosystem impairment, in the most severely impacted streams. Scoggins was surprised “that with all the other urban stressors affecting these creeks, the effects of PAH exposure would be so pronounced. Some of these creeks had only a seasonal flow and were so severely impacted by the urban environment that they would not appear in any way to be inviting aquatic environments.” Nevertheless, Scoggins says, “The simple factor of being downstream of one of these sealed lots caused a significant loss of taxa relative to locations on the same creek just upstream of the parking lots.” Among the five streams studied, he says, “The two most highly impacted showed a 21% to 42% loss of taxa. This could have major implications for overall watershed health.”
Scoggins adds, “The organisms that I look at as indicators of stream health, worms and invertebrates who inhabit streams, are definitely being degraded by these PAHs. Though we just look at them as indicators of stream health, if you’re dropping out large percentages of the food base and the food web, obviously you’re going to be influencing other organisms. I feel that the small critters are extremely important from a wide range of perspectives. I’m sure the problem with the bugs in the streams will be passed on further up the food chain–the fishery is probably being influenced by it indirectly.” He notes that “if people could grasp the magnitude of this, it is kind of a big deal.”
The results of these studies, along with a review of policy and federal regulations, led the WPDRD first to suggest a voluntary ban on coal-tar-based sealants and finally to propose legislation to the city council for a mandatory ban on coal-tar-based sealants–the first in the nation. The ban, enacted by the City of Austin in November 2005, went into effect on January 1, 2006. McClintock says response has been mostly positive so far from the community. In fact, during the period of the voluntary ban, she says, one of the major local sealant applicators, Wheeler Coatings, indicated its intention to switch over to using an asphalt-based product. “They committed that they would not apply the coal-tar-based product any longer in the Barton Creek watershed.” Also, her staff members tell her that they’re no longer seeing the product on the shelves in local hardware stores. McClintock also says the department will be “developing a communication plan for working with the other cities that are adjacent to Austin whose runoff may be affecting our watershed to help them understand our new policy.”
Closing the Loop
On the afternoon of Friday, January 7, 2006, Ennis met with Sharon Cooper, manager of the Spills Response investigative team; they discussed the progress of the enforcement effort after one week of implementation. With one investigation concluded, Ennis was able to say, “So far, so good.” The investigative team from the Spills Division was able to report the satisfactory result that the sealant sample taken from the complaint site sample did not fluoresce under black light, confirming that it was not the banned coal-tar sealant, and the contractors were allowed to proceed with their work. Ennis was pleased to see the department’s outreach efforts had been effective: “Even though it turned out not to be a violation, the citizen knew enough about the ban from our media coverage to pick up the phone and ask us to find out what was going on out there. We tested, talked with the contractor, and verified that they were using an asphalt sealant and not one of the banned products. I understand that the citizen was happy to see us respond so quickly and to learn that the appropriate product was being placed.”
Ennis doesn’t think the coal-tar sealants will be missed and sees big opportunities for using less-toxic alternatives on the hundreds of acres of parking lots previously sealed with coal tar that may come in need of repair the future. “As those pavements need resealing, repair, and rehabilitation, we’ll be looking at putting down less-toxic alternatives. Those with the need to seal will still have the option of using asphalt sealants. And then,” Ennis adds, “there’s the option of not sealing at all. The preferred option would be to just have a concrete pavement, which would have the added benefit of reducing the urban heat island effect. There are pavers. There’s even a technique called “˜white-topping’ in which a 1- or 2-inch layer of high-strength concrete is laid over asphalt.” With all these options to consider, this year’s spring paving season in Austin could be one of the most interesting and environmentally innovative ever.