This myth was covered some time ago on this site in post entitled Industry "Study" Says No Effect in Austin From Sealant Ban and is still worth a read if you're interested in the topic. Here are the reasons why the watershed effects are not more immediate:
- Two separate studies have shown that the wear-off rate of coal tar sealants is less than 10% per year. If no additional sealer is used, then it will take around 10 years for the sealant to stop yielding its pollutant load to the adjacent environment.
- Some of the PAHs are extremely stable chemicals that don't break down easily in the environment. EPA calls them "persistent." These are the same chemicals that make up the majority of toxic sites in the US even though many industrial operations have been closed for decades.
- Streams were sampled in the industry study and they tend to be "flashy," meaning that with high velocity rain events there is a lot of mixing of old sediment and new sediment deposits creating a high degree of uncertainty.
- When looking at other banned chemicals like DDT, the USGS says that a half-life of about 15 years is expected. This means that the level of a banned substance will take about 15 years to reduce to one half of its pre-ban level.
- Below are two videos, one with the USGS and one with the City of Austin where both explain the long term view required when looking for watershed-level effects of a coal tar sealant ban.
Parking Lot Effects
The more we've learned about coal tar sealant, the more I would compare a sealant ban to stopping smoking. Most likely you heard that physical healing and restoration happens after a smoker quits. More specifically, improvement can be immediate, some will take a few years, and others can take as much as 20 years. For example it is reported that within 20 minutes from quitting smoking, there is a reduction in the smoker's blood pressure.
One immediate benefit from ceasing the use of coal tar sealers is the reduction in air emissions during application. Look at USGS's analysis of volatilization which showed massive airborne discharges of PAHs from freshly applied coal tar sealants in the first 24 hours. In other words, the product drying conditions cause a rush of toxic PAHs above it. Is toxic too strong a word for this release? Actually no, since 1 acre of curing coal tar sealant meets a toxic release standard of 1 pound of the most toxic PAH, benzo(a)pyrene in 24 hours.
However, a coal tar sealant ban is like quitting but living with a smoker and being exposed to second hand smoke (see Thank You for Not Sealing). While the intensity may not be the same, exposure to sealant dust and debris will continue for years as the old sealer wears away. The best solution is to remove the ongoing source of exposure through shotblasting (see What to Do if CTS is on My Driveway?!). That link also discusses the second best approach to minimizing exposure is to put a non-coal tar sealer on top of it.
The problem is that while most of us would never think of exposing our children to second hand smoke at school, commercial area or home, they can be very much exposed to high-levels of PAH pollution from coal tar pavement sealants at those same locations that creates a similar risk. Let's work together to eliminate the unnecessary risk to our communities!