Thomas Boving, assistant professor of geosciences at the University of Rhode Island, may have just solved the problem by using a cheap and readily available material: shredded aspen wood.
Boving and graduate student Wei Zhang are evaluating the effectiveness of the storm water detention ponds at the Gano Street ramp to I-195 in Providence, built in 1999 as part of the highway's relocation project. During storms, water is carried from the roadway and surrounding urban areas to the three ponds, which are designed to filter out contaminants before the water reaches Narragansett Bay.
"The goal of the project is to find out if the ponds are doing what they're supposed to do," said Boving. "And during a shower in May, they appeared to capture most of the pollutants well."
But he noted that heavy rains cause more pollutants to run off the road, and the faster flow of water into and out of the ponds during these storms is expected to reduce the ponds' effectiveness.
"Most of the contaminants in roadway run-off are attracted to suspended organic material and sediments, which then settle to the bottom of the ponds," he said. "But if the flow rate is too fast, like during a heavy storm, there may not be enough time for the solids to settle before flowing out and into the bay."
So Boving began searching for a material that could be used in conjunction with the ponds to filter out the contaminants that don't settle to the bottom. Knowing that the contaminants cling to organic material, he tested shredded wood, which is available commercially in the southwestern U.S. for use in evaporative cooling systems.
Boving used pyrene as a test contaminant. A human carcinogen, pyrene is a polyaromatic hydrocarbon (PAH) and a significant component of contaminated roadway run-off. PAHs are the byproduct of combustion, and come from smokestacks, automobile tailpipes, chimneys and outdoor grills, among other places. In a laboratory experiment, Boving pumped water contaminated with pyrene through the shredded wood, and found the wood effective at removing 97 percent of the pyrene from the water. Although the wood absorbed less pyrene over time, he concluded that shredded wood is effective if replaced every 30 to 60 days. Boving calculated that up to 100 pounds of shredded wood would be needed each month at the Gano Street ponds. "I was very encouraged by what I found with this first test," Boving said. "It fulfills all of the requirements for a successful technology - it's non-toxic, cheap, available, and public acceptance of these filters is likely very good since no one is concerned about putting wood in water."
His next step is to test pine - the cheapest wood available -- and other types of wood to see if one is more effective than another. He will also evaluate how wood filters other organic contaminants, though he expects it will be equally effective on most contaminants. Once the laboratory experiments are completed, he plans to field-test his system at the Gano Street site next year.