Numerous studies on coal-tar based asphalt sealcoats have linked this product to human health problems and ecological damage (learn more about the issue in this recent blog and this newsletter article, page 7).  HRWC would like municipalities and homeowners to refuse to use coal-tar based sealcoats because of their environmental effects, and a recent study released by the U.S. Geological Survey gives us all another reason to do so.

In this study, researchers applied coal-tar sealcoat to a section of a parking lot, and simulated rain-falls and collected the water runoff both 3 days and 36 days after sealcoat application.  Two common aquatic species (fathead minnows and cladocerans) were exposed to the run-off.  A 1:10 dilution of the run-off (which would be an approximation of conditions in moderately urban streams and ponds) caused a 10% mortality of the fathead minnow and a 60-100% mortality of the cladocerans, with the length of time after sealcoat application not making a difference.

There is much more to this study including looking at the effects of ultraviolet light, alternative sealcoat products, control treatments, and differing treatment lengths.  However, based on the one part of the study given above, it is clear that coal-tar sealcoat is producing toxic rain run-off. Secondly, the sealcoat continues to cause the death of aquatic life 36 days after application (and perhaps further, but this was not included in the study). Application guidelines of coal-tar sealcoat  state that the sealcoat should not be applied if rain is forecast within 24 hours to allow the product time to cure, but after this period the “risk level of runoff drops close to inconsequential.” This study reveals these application guidelines are incorrect.

Readers are welcome to check out the study for themselves; it is technical but not impossible to read. It is also copyright free, so HRWC is able to give the journal article here.