When the rain begins to fall in Southern California, the freeways get slippery because the water lifts the oil and grease that has collected on the pavement. The initial cloudburst also breaks loose tons of pollutants, sending them down the storm drains and into the ocean, according to Michael K. Stenstrom, professor of civil and environmental engineering at UCLA's Henry Samueli School of Engineering and Applied Science.
Stenstrom and his team are currently trying to ascertain how much higher the concentration of pollutants is in the first part of the storm-water runoff. If the data support his theory that the first burst of storm water contains the most potent concentration of toxins, Stenstrom said, "we have the opportunity for some natural treatment systems."
His goal is to find a way of reducing the amount of pollutants reaching the ocean. He hopes to use the data he is collecting "to save money because there is such a high volume of storm water that you can't possibly treat it all."
This is the fourth year of a $1.2 million research project, funded by Caltrans.
Mixed with the crankcase drippings and partially burned fuels that become polynuclear aeromatic hydrocarbons (PAHs), some of which are carcinogenic, the toxic stew that makes up storm-water runoff also contains heavy metals such as zinc, copper, nickel and chrome, Stenstrom said. Some of these are the products of automobile corrosion; some comes from brake pads.
"When a car hasn't been washed in a while, you can see a layer of black powder around the wheels. That powder is from the brake pads and can be loaded with heavy metals," Stenstrom said.
Stenstrom noted that some PAHs can be found in soot-like, micron-size particles. He cited diesel engines as one of the main sources of these particles.
Because conventional storm drains were designed to prevent flooding, they provide what Stenstrom calls "the biggest, slickest pipe to the ocean."
"If you want to prevent floods, that's the way to do it. Get the water down to the ocean," he said. "But when you push it down to the ocean that quickly, you transport all the contaminants along with it."
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Instead, Stenstrom said, "Let's try to take some of the water -- as much as we can -- and hold it up on the land, giving the pollutants a chance to be removed and maybe, just maybe, we'll be able to reclaim some of it, too."
To accomplish this, Stenstrom advocates constructing bioinfiltration basins along the freeway shoulders. These would consist of a trench about two feet wide and three or four feet deep. It would be filled with gravel and topsoil and covered with some type of grate that allows water to flow freely into the trench. Because the gravel is about 50 percent porous, each cubic yard of gravel can hold a cubic yard of water.
"When it starts to rain, the first really dirty water flows into the bioinfiltration basin, goes down into that gravel and topsoil and it stays there," Stenstrom said. "The rest flows off because the gravel is full."
This natural and cost-effective method would collect the first hour's worth of storm-water runoff. In addition to trapping the pollutants, Stenstrom said, this method could detoxify them, as well.
If allowed to filter into the soil, the top layer of soil will remove both heavy metals and PAHs, Stenstrom said.
"You can trap them there for many, many years and the PAHs will biodegrade over time," Stenstrom said. "There are organisms that will detoxify PAHs if you just keep them in the soil."
Such a method could treat the dirtiest water at the lowest possible cost, Stenstrom said. "We're trying to be clever and save our money and treat the most."
As the water seeps out, the gravel lets in air that promotes the growth of aerobic bacteria, creating an effective, natural treatment system.
Heavy metals are absorbed by the soil, where they remain trapped. The soil need not be replaced for at least 20 -- and possibly as long as 50 -- years, Stenstrom said.
Similar bioinfiltration basins are being used in other parts of the world including the East Coast of the United States and many Asian cities.
Stenstrom said he has become an advocate for the use of bioinfiltration basins to treat storm-water runoff. "I've been pushing this idea," he said. "A lot of what I'm trying to do is convince people in public office."