Restoring Streams May Reduce Nitrogen Pollution in Chesapeake Bay

Baltimore County’s Dead Run looks more like a ditch than a meandering stream these days. Like many of the tributaries that flow through the Chesapeake’s increasingly developed landscape, Dead Run suffers from what some scientists call “urban stream syndrome.” Runoff from roads, parking lots, and shopping malls sends large quantities of stormwater racing into these tributaries. No longer slowed by fields and forests, this deluge erodes streambeds and diminishes the streams’ natural ability to absorb water and the nutrients it carries.

When too many nutrients ultimately enter the Chesapeake, they contribute to the overgrowth of algae and oxygen-deprived dead zones. Now a new study funded by Maryland Sea Grant will explore whether restoring the natural hydrology of suburban and urban streams like Dead Run can help to stem the flow of nutrient pollution into the Bay.

Sujay Kaushal, an ecologist at the University of Maryland Center for Environmental Science (UMCES), has recently begun to examine the best way to restore a stream’s natural ability to sequester or remove nutrients, particularly nitrogen. He and a team of colleagues will compare how quickly restored and unrestored streams in Baltimore County remove nitrogen from the water and how much they release to the atmosphere through a process called denitrification.

Denitrification converts inorganic nitrogen such as the nitrate or nitrite found in fertilizers into nitrogen gas — the most abundant gas in our atmosphere. Living things, including harmful algal and bacterial cells, can use nitrate and nitrite to grow and reproduce, but few can use nitrogen gas.

Employing a method called isotope tracing, Kaushal and his colleagues will measure denitrification rates in five streams in the Patapsco and Gunpowder River watersheds including restored streams, degraded streams that have not been restored, and a forested stream that will act as a control or reference site.

Will certain stream restoration features foster nitrogen removal more so than others? Kaushal suspects so. For example, areas where stream water is directed out of the stream and into connected floodplain areas and oxbow ponds — temporary reservoirs designed to hold and slow stormwater — show promise in their ability to promote denitrification.

Kaushal’s preliminary work suggests that restoring streams may in fact increase denitrification rates. “If we can engineer denitrification ‘hotspots’ and slow the flow of water in the stream and its banks,” Kaushal says, “we can get back some ability of the watershed to remove nitrogen.”

Results from this and similar projects will be integrated into a large database to help estimate how much nitrogen can be removed by stream restoration projects and to determine the most economical strategies for nitrogen removal. The team will also estimate how many and what types of suburban and urban streams and rivers would need to be restored to substantially reduce nitrogen loads to the Chesapeake and its tributaries.

The research will cover important territory in restoration science. According to a separate effort led by Margaret Palmer, UMCES Chesapeake Biological Laboratory director and a collaborator on this project, despite more than $400 million invested in stream restoration in the Chesapeake Bay watershed since 1990, few efforts actually document the effect of stream restoration on nutrient removal.

Gary Shenk of the Environmental Protection Agency’s Chesapeake Bay Office notes that research like this is key to making good management decisions. He says Kaushal has worked hard to include resource managers in the planning stages of the project. “Sujay is working on an area that we really need to focus on,” he said. “It’s a gap that needs to be filled.”

-- Jessica Smits

Learn more: See Maryland Sea Grant's page about watershed restoration.