Nutrient Pollution Reductions From Urban Stream Restoration Quantified

"The key to expanding urban stream restoration efforts nationwide is being able to quantify the environmental benefits gained from those efforts," said UMCES Chesapeake Biological Laboratory researcher Dr. Sujay Kaushal. "This research is opening the door to a new technology that has the potential to help improve water quality in our urban environment."

Using state-of-the-art techniques in a long-term study, Kaushal's team injected stable isotope tracers into restored and unrestored sections of an urban stream, and measured how microbes in the streambanks naturally absorb nitrate and convert it into inert nitrogen gas. By analyzing those samples, the team was able to determine in-the-field nitrogen reductions by stream microbes through a process known as denitrification.

The research showed that stream restoration techniques that "reconnected" the banks to the stream doubled nitrogen removal rates by microbes, and reduced nitrogen levels in ground water by 40%, contributing to significantly lower nitrogen levels in the stream compared to unrestored conditions. Getting water out of the stream channel into denitrification "hot spots" in floodplain wetlands helped improve water quality.

Nationwide, there is a growing need to reduce the amount of nutrients flowing into our coastal waters and restoration efforts are booming in areas adjacent to large urban population centers with acute nitrogen pollution problems, such as near Chesapeake Bay, Long Island Sound, Puget Sound and the Gulf of Mexico.

"Miles of streams will likely need to be restored in upcoming years as our nation's failing infrastructure needs repair," Kaushal said. "Much like our study sites, most of this aging infrastructure was built before current stormwater practices were adopted. When repairing the aging bridge supports and sewer lines that share urban streambeds, public works managers can easily make restoration design changes to improve stormwater management and also increase the stream's ability to reduce nitrogen pollution flowing downstream."

"The science of restoration ecology is still in its infancy, and a great deal of knowledge is needed to achieve objectives," Kausal added. "Large-scale nitrogen reductions are needed along with improved stream restoration techniques that treat water flowing from polluted streams to coastal waters. The trick will be for scientists to figure out what works and what doesn't as we rebuild our cities for future generations."

The article, "Effects of Stream Restoration on Denitrification in an Urbanizing Watershed," appears in the April edition of Ecological Applications, a journal of the Ecological Society of America. This work was supported by the U.S. Environmental Protection Agency Office of Research and Development and the National Science Foundation's Long-Term Ecological Research program.