Suspect’s Fingerprints Found in Minnesota Stormwater Ponds; May Be Throughout US

MPCA staff sampling sediment pond in South St. Paul, MN.  Photo by Judy Crane.

The State of Minnesota has found the fingerprints, chemical fingerprints, of a suspected pollutant, coal tar pavement sealants in several Minnesota stormwater ponds.

Most likely these same fingerprints would be found in much of the United States and Canada where coal tar-containing asphalt sealers are commonly used. These results are consistent with a nationwide study from the USGS, as well as academic studies in Missouri, Pennsylvania, and New Hampshire that together provide a weight-of-evidence of the importance of coal tar-based sealants as a major source of PAHs in residential and commercial urban areas where these products are applied to parking lots and driveways. The author of the paper, Dr. Judy Crane of the Minnesota Pollution Control Agency, published the robust work in the Archives of Environmental Contamination and Toxicology, and used an EPA model to compare the pattern, or fingerprint, from a group of 12 PAH compounds in the pond sediments to the fingerprints from eight major source categories of PAHs (polycyclic aromatic hydrocarbons).  Each major source has a relatively-unique fingerprint of PAH compounds.

Dr. Crane used “environmental forensics” to determine the sources of the PAH pollution. After analyzing sediment from 15 stormwater ponds in the Minneapolis-St. Paul (Twin Cities) metropolitan area, about 2/3 of the PAHs were from coal tar-based pavement sealant and a bit less than a third were automobile-related.  The USGS has used the same model on pond sediment across the U.S. and found the contribution from coal-tar based asphalt sealer to average 53%, with a range of 52–72% for two Twin Cities lakes.

Source: MPCA

The State of Minnesota has adopted guidelines, based on human health risk, for the upland disposal of pond sediment.  This level (also known as Soil Reference Value or SRV) was exceeded in 9 out of 15 samples (60% of the time).  As an indicator of aquatic ecological risk, 3 out of 15 (20%) exceeded what is known as the probable effects concentration (PEC) of 22.8 parts per million that would be detrimental to bottom-dwelling organisms (i.e., invertebrates) in sediment.

The high concentrations of PAHs limit the disposal of the material on land and it must be sent to a special landfill at a considerable cost.  If just 10% of these stormwater ponds in the Twin Cities exceed the Industrial SRV value, the cost of disposal could skyrocket to over $1 Billion.  Other states do not have any requirements for the chemical testing or disposal of stormwater pond sediment so PAH-contaminated sediment may be inadvertently disposed of in unsafe ways.  Even though Minnesota’s ban of coal tar pavement sealants when into effect January 1, 2014, it will take decades before the latent effects of PAH contamination are out of the parking lot-to-pond loading cycle.  For more on these costs, please see the previous posts:

• “Nasty Surprise” Awaits Communities with Coal Tar Sealant Pollution
• The High Cost of Cheap Coal Tar

The pro coal tar sealant lobby has attacked the use of this model by others, in what is little more than a diversionary tactic.  It seems like the idea is if we just focus our attention on ratios and scatter plots that we will not ask about the product itself or the exposure it presents right on the parking lot.  After 10 years of discussion, the industry has yet to address this.  When Austin passed its ban, it was clearly stated that it was NOT about which source was highest, but rather which was significant and CONTROLLABLE.  Coal tar sealants fit that category; vehicle emissions do not, space junk, and backyard barbeques do not either.

Please don’t misunderstand.  The research presented here is important to comprehend the magnitude of the problem we are dealing with, but it is NOT where the crux of the debate should belong.

How pavement sealants end up in ponds from the MPCA.