Storm water run-off: a significant pollution source

Patricia Drummey samples water

The nature of storm water makes it too expensive to treat for contaminants. “It doesn’t come along very often, and when it does, there’s a lot of it. As a result, building a treatment plant for these large, intermittent flows is often impractical,” says Dr. Greg Characklis, associate professor of environmental sciences and engineering at Carolina’s School of Public Health.

But storm water is a significant source of pollution.

“The clean water act of 1972 mostly addressed ‘point sources’ of pollution, such as municipal wastewater and industrial discharges,” Characklis says. “We did a reasonably good job of cleaning those up, but we didn’t get as much improvement in water quality as we thought we would.” The next logical sources of contamination are ‘non-point sources,’ like storm water runoff, that are more difficult to identify and often contribute to water quality problems. After washing over lawns, driveways and parking lots, storm water runoff can carry with it chemical fertilizers, motor oil residues, even bacteria from pet and wildlife waste, all of which then run into drains that often lead directly to waterways. From there, these contaminants can end up in drinking water reservoirs or favorite swimming spots.

Concerns about this have resulted in federal and state regulations mandating that storm water be treated. In most cases, this involves relatively inexpensive approaches, such as diverting storm water into settling basins. There, in the best case, the icky stuff — dirt, sand and contaminants that stick to them — settles out, leaving cleaner water to flow to the rivers and lakes.

But what about bacteria, which contribute to poor water quality at elevated concentrations and which, in some cases, can make people sick? These organisms are one of the most common non-point source contaminants but are not dense enough to settle out quickly on their own.

“If some of these organisms stick to the dirt and other denser particles, they might be effectively removed by these basins,” Characklis says.

Understanding the extent to which these organisms associate with denser particles can be useful when building models for predicting where and when a stream or river will experience high levels of microbial contamination. However, relatively little is known about interactions between microbes and particles in storm water or how these interactions might change once storm water enters other water bodies, Characklis says.

Characklis is working with Dr. Mark Sobsey, Kenan Distinguished University Professor of environmental sciences and engineering, and research assistant professor Dr. Chip Simmons to learn how microbes behave in storm water. They want to know, for example, what proportion of microbes actually stick to dirt and other particles.

Answering such questions will help them predict how the public’s health can be affected by large storms. “These organisms end up in very different places depending on whether or not they’re stuck to particles,” Characklis says. “Therefore, information on microbe-particle attachment will help us predict where we might have the biggest water quality problems after a storm.”

Environmental sciences and engineering graduate students in Characklis’ lab, including Leigh-Anne Krometis, a doctoral student, and Patricia Drummey and Adrienne Cizek, both master’s students, do the dirty job of sampling from retention basins. “We couldn’t do this without the energy and ingenuity of graduate students,” Characklis says. The success of current and past efforts here in North Carolina has more recently led to a project with the New York Department of Environmental Protection, which is interested in tracking microbial movement in water supply reservoirs that serve New York City.

Sobsey is lending his expertise to identify organisms in water. Characklis developed a calibrated centrifugation technique to separate water samples to give a more accurate picture of the fraction of microbes that stick to particles (and may therefore settle out).

Past studies often have used physical filtration, based solely on particle size, as a means of separating particles and attached microbes. But Characklis’ separation method takes into account density as well as size. “If you really want to find out about settling behavior, knowing something about both size and density is important,” he says.

This work is funded by the N.C. Department of Environment, Health and Natural Resources and the New York City Department of Environmental Protection.

— by Angela Spivey

Carolina Public Health is a publication of the University of North Carolina at Chapel Hill School of Public Health. To subscribe to Carolina Public Health or to view the entire Fall 2007 issue in PDF, visit www.sph.unc.edu/cph.