Paving paradise: the peril of impervious surfaces

Environmental Health Perspectives, July, 2005 by Lance Frazer

"In general," says Lalor, "pollutant level tends to vary depending upon the age of the material, and the harshness of the environment in which it is exposed. As material ages and is exposed to high levels of sunlight, temperature extremes, chemicals in the environment such as salt from roads, and so on, leaching out will increase."

If the pollutant source is a coating, then pollution levels decrease with age, but can still have a significant impact, she says. "If you look at the asphalt used in a parking lot, the top coat is quite toxic. So if you have a heavy rain [soon] after the parking lot goes in, it's not unusual to see fish kills dowstream."

Lalor cites research published in volume 35, issue 9 (1997) of Water Science and Technology showing that stormwater from roofs and streets contributed 50-80% of the cadmium, copper, lead, and zinc measured in Swiss combined sewer system flows. Polyester roofing materials shed the highest concentrations of metals, followed by tile roofs, then flat gravel roofs. The Swiss researchers also found PAHs and organic halogens in the roof runoff.

The chemicals released can have a significant impact on environmental and potentially human health. "Some materials, such as metals, are especially toxic to fauna at various stages of their life cycle," says Lalor, "while some organics, particularly petroleum-based organics, can function as pseudoestrogens. So while they may not cause death, they can trigger a significant disruption in the physiology of the organisms exposed to these pollutants."

According to Lalor, although there are mandated tests for urban stormwater discharge, there are currently no tests mandated for building materials to determine their potential for toxics release. "If a community wants to develop around their drinking water source, they should know about release potential from building materials so they can carefully select those with which they build," she says. "We don't yet have the science to support it, but it would be a positive step to be able to go to a builder and say, 'Look, here's a list of twelve building material alternatives that would be most environmentally benign for this site and these conditions.'"

Lalor says New Zealand has been the leader in this sort of study, and that nation is preparing to put regulations in place regarding building materials and environmental impact. But such studies haven't been elevated to a high enough priority in the United States to build the science we need for setting new policies. She adds, "We need to address the entire life cycle of building materials, from what goes into their creation, to the impact of construction on the environment, to the impact of whatever might leach out during their lifetime, to the end-of-life disposal issues."

The Promise of Porous Pavements

Despite the overwhelming body of evidence supporting the negative relationship between impervious surfaces and the environment, no one would seriously suggest that we stop paving streets or building parking lots. What, then, are the options?