Porous asphalt could be salt of the earth for budget, environment

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October 22, 2006

By Terry Date

Porous asphalt looks and smells like regular asphalt, but its tiny holes could save towns and businesses money - and be better for the environment.

The University of New Hampshire Stormwater Center is studying the material to see if it can be used to reduce run-off from parking lots. This winter, the UNH center will see if its experimental porous asphalt parking lot needs less salt for de-icing than a typical lot does.

Already, porous asphalt parking lots have been installed in South Portland, Maine, and Wilmington, Mass. And many more are on order, including one for the Hugh Gregg Coastal Conservation Center in Stratham, said Robert Roseen, director of the UNH Stormwater Center.

The stormwater center, a regional testing, education and outreach center, has provided tours to more than 1,000 people, including representatives of more than 170 governmental agencies and to businesses, UNH senior scientist Thomas Ballestero said.

The center shows towns and businesses how to forego traditional means of collecting stormwater into drains that direct it to retention ponds - or swales - that channel it to brooks and rivers.

With porous asphalt - in which fine sand has been removed from the asphalt mix - rain and snow-melt seep through the surface, and the water is cleaned as it filters through sand or gravel on the way to recharging the groundwater.

Roseen said porous asphalt treats stormwater as a resource, "as opposed to piping it out as a nuisance."

Ballestero said porous asphalt has been around for a number of decades, but there is limited data on how it performs.

The UNH study, now in its third year, is collecting hard data on how water filtered beneath the porous material tests for contaminants such as metals, microorganisms, sediments and nutrients. EPA run-off standards commonly measure for these four contaminants, he said.

This winter, UNH will see if less salt can be used in de-icing the porous parking lot. At least anecdotally, it appears that much less salt is needed to treat the porous material, since salt crystals tend to remain on the porous asphalt.

On regular asphalt, salt washes away with the run-off.

If the porous material requires less salt, it could cost less money to de-ice lots and be better for the environment.

In high concentrations, salt is toxic to freshwater organisms, Roseen said. The salt level in run-off is commonly seen as high as one-third that of sea water, he said.

"The reason why this is so important is because stormwater treatment doesn't remove salt," Roseen said. "(The use of porous asphalt) could be a very valid strategy."

Ballestero and Roseen both caution that the material isn't a panacea. The material isn't usable at gas stations or other locations with the risk of hazardous material spills.

Porous asphalt, however, could help towns meet new EPA standards and make for a cleaner environment.

Ballestero said development can't be stopped, but people can do a better job of environmental stewardship and retrofitting old mistakes.