Researchers Devise Hidden Dune Filters to Treat Coastal Stormwater Runoff

When it rains, untreated stormwater can sweep pollutants into coastal waters, potentially endangering public health. Now researchers from North Carolina State University have developed low-cost filtration systems that are concealed beneath sand dunes and filter out most of the bacteria that can lead to beach closures.

“It was not economically feasible to use a tract of beachfront property to treat stormwater. Instead, we were able to devise a system that could be installed in an area that was not developable – underneath the dunes,” says Dr. Michael Burchell, an assistant professor of biological and agricultural engineering at NC State and senior author of a paper on the research.

Specifically, the so-called dune infiltration systems reduced the concentration of bacteria in stormwater runoff by 96 percent.

The researchers designed and built two such systems in Kure Beach, N.C. The systems consist of large, open-bottomed chambers that effectively divert the stormwater into dunes, which serve as giant sand filters. The systems are built under dunes that are covered with vegetation. The researchers then launched a three-year study to see how the filters would perform.

“We needed to know if these filters would affect the flow of ground water, affect dune erosion and whether they would effectively limit bacterial pollution,” Burchell says.

The filters “exceeded our expectations” at removing bacterial pollution, Burchell says. And the study found minimal impact on ground water. There were short-term increases in ground water levels during storms, but those increases dissipated in anywhere from a few hours to two weeks.

“And we found that after replanting, the coverage of the dune vegetation actually exceeded what had been there previously – which is important to dune stability,” Burchell says.

Burchell’s team has since installed a third dune infiltration system in a more developed location to see if the system is able to handle higher flow rates, water volume and concentrations of bacteria. That work is ongoing, but early results are promising.

The paper, “Long-term study of dune infiltration systems to treat coastal stormwater runoff for fecal bacteria,” is published in the March 2013 issue of Ecological Engineering. Lead author of the paper is former NC State grad student W. D. Price. Co-authors are Dr. Bill Hunt, an associate professor of biological and agricultural engineering at NC State; and Dr. George Chescheir, an associate research professor of biological and agricultural engineering at NC State. The work was supported by the North Carolina Department of Transportation, and the filtration systems were installed by the Town of Kure Beach.

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Note to Editors: The study abstract follows.

“Long-term study of dune infiltration systems to treat coastal stormwater runoff for fecal bacteria”

Authors: W.D. Price, M.R. Burchell II, W.F. Hunt, G.M. Chescheir, North Carolina State University

Published: March 2013, Ecological Engineering

Abstract: The discharge of untreated stormwater runoff into recreational waters places swimmers at risk of contracting various illnesses and often results in beach closures or swimming advisories. In an effort to safeguard the public, two experimental Dune Infiltration Systems were installed beneath the sand dunes in Kure Beach, NC. The systems diverted stormwater from two existing beach outfalls, which drained 1.9 ha (4.7 ac) and 3.2 ha (8.0 ac) watersheds, into subsurface chambers for temporary storage and infiltration into the existing sand dunes. A 3-year study examined the long-term performance of the two systems during which 14,584 m3 (515,046 ft3) of stormwater was diverted into the dunes, with only 438 m3 (15,457 ft3) bypassing the systems, a nearly 97% capture rating. Enterococci concentrations in stormwater runoff from the watersheds exceeded the state’s single sample maximum for Tier I waters (104 MPN/100 mL) in >70% of samples, with geometric means [greater than or equal to] 278 MPN/100 mL. Groundwater enterococci concentrations tended to remain below the state limit ([less than or equal to] 11% exceedance), with geometric means [less than or equal to] 7 MPN/100 mL. Groundwater monitoring in the control dune, with no direct input of stormwater, had a geometric mean of 5 MPN/100 mL and a 6% exceedance rate. The influences on the local water tables were temporary, as the water table would mound around the systems during infiltration events and dissipate to within pre-storm variations from the control within hours or up to approximately 2 weeks. The Dune Infiltration System appears appropriate for installation in small watersheds (<4 ha (10 ac)), though further research is recommended to identify bacterial removal processes and residence times, and to quantify the lateral extents of the water table mounds.