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.