Melissa Rigney Baxter
Special to The Daily Reporter
Sometimes Mother Nature knows best.
Technological advances in concrete and filtration are offering new methods for managing runoff from highways. But rain gardens and ponds remain among the most effective ways to keep pollution out of waterways.
Pollutants are created by vehicles, road deterioration, and sand and salt.
Designing highways to accommodate polluted runoff is a crucial part of keeping lakes and rivers clean, reducing flood risk and making sure the water supply is potable.
Low tech pollution solution
Highway swales are a low-tech yet effective way to manage runoff. They’re especially useful on long stretches of highway outside urban areas, said Roger Bannerman, environmental specialist with the Wisconsin Department of Natural Resources.
Michelle Gerrits, erosion and sediment control specialist with the Wisconsin Department of Transportation, said swales with engineered soils and specific plantings let unwanted sediment settle in the swale before the runoff becomes part of the water table.
“Swales are how a huge percentage of highways in Wisconsin deal with (runoff),” Bannerman said.
Detention ponds serve a similar purpose in residential developments, collecting water and letting particles settle.
Another low-tech solution is a rain garden such as the one recently installed in Lodi. City volunteers maintain the rain garden, which captures and filters storm water while adding an aesthetic feature.
The downside of such natural solutions is that swales, detention ponds and gardens require adequate space. In the case of detention ponds, Gerrits said, they can be a highway hazard if too close to the road.
Creating concrete to filter water
New concrete technology has been partially successful in mitigating runoff.
Porous pavement is an option for parking lots or streets with low traffic volumes, Gerrits said.
Bannerman said some municipalities experimented with porous paver blocks for residential streets. The method was successful in getting water off the streets and filtering it.
“No one’s tried it in heavy traffic though,” Bannerman said. “I’m pretty sure they couldn’t take (the traffic volume).”
Porous concrete also requires ongoing maintenance, which increases expense.
“It seems to work, but there are the usual concerns about maintenance,” Bannerman said. “Maintenance is always the thing that helps make that final decision.”
Hydrodynamic separators show partial success
In 2008, WisDOT tested two high-tech systems – the Vortechs system and the StormFilter system. Both devices were placed beneath a well-traveled, elevated section of Interstate 794 in Milwaukee.
The Vortechs system is a hydrodynamic settling device that removes pollutants by flotation and sedimentation. StormFilter removes pollutants by filtration and sedimentation.
Both systems funnel water to a large concrete box and then filter it to remove sediment and pollutants.
Gerrits said the systems easily removed bigger particles, but removal of smaller clays was not up to DNR standards.
“We tested quite a few (systems), and they are good in certain situations but not as well as we hoped,” Gerrits said.
Bannerman said the devices that include sand filtering reduce pollutants best.
Mimicking Mother Nature is goal
The space constraints of urban areas often make swales and detention ponds difficult or impractical, but Bannerman said they remain the most reliable option for removing sediment.
Added to that, Bannerman said, is the struggle to meet the goals set by the DNR at the lowest cost possible. “One of my roles as a scientist here is making sure when someone installs something, they have the right expectations,” he said. “Some people can oversell their product. If you have space, the job becomes much easier.”
Wisconsin agencies such as WisDOT and the DNR network with communities around the country to find out what systems work best, said Judy Horwatich with U.S. Geological Survey, a government organization that studies natural resources. She said companies are developing filters that might better meet DNR requirements.
“We are going to be testing bioremediation devices, where water goes through a couple of feet of soil, filtering through and then coming out,” Horwatich said.
However, so far the superior methods require space and time for pollutants to settle instead of being swept through to local waterways.
“It keeps coming back to Mother Nature,” Gerrits said.