Minebank Run now splashes through Baltimore County along cobblestones instead of concrete channels, and sports lush, green edges where steep, muddy banks once stood.

The restored urban stream is also sending as much as 50 percent less nitrogen into the Gunpowder River and, ultimately, the Chesapeake Bay.

A team of university and federal scientists is excited about improved water quality in Minebank Run and equally excited about its ability to document the cause.

“This is the only stream for which we have such an intensive study, where we can really demonstrate how restoration projects have reduced nitrogen in the stream,” said Paul Mayer, a groundwater scientist who monitors Minebank Run on behalf of the EPA.

Nitrogen—from fertilizers, animal waste, septic tanks and sewer lines—is the top pollutant in the Chesapeake Bay and many of its rivers. The nutrient feeds algae blooms, which inhibit the growth of underwater grasses and rob aquatic creatures of oxygen.

“The achievements at Minebank Run point to stream restoration as an important approach for reducing the amount of nitrogen entering the Chesapeake Bay,” Mayer said. “It can also help us to know which restoration approaches are the most effective and where to apply specific techniques.”

Along with the EPA, partners in the Minebank Run project include the Baltimore County Department of Environmental Protection and Resource Management, University of Maryland Center for Environmental Science, U.S. Geological Survey, and Institute for Ecosystem Studies.

Nitrogen reduction in Minebank Run is tied to a major restoration project that reshaped the entire stream and improved vegetated buffers along its banks.

Minebank Run begins in Towson, at the northern edge of the Baltimore metropolitan area. It flows north past residential areas, corporate offices, the Baltimore beltway, a high school, and a county park before reaching the Gunpowder River.

Minebank Run had suffered the effects of stormwater runoff for decades. Heavy volumes of stormwater runoff, driven by impervious surfaces like roads, rooftops and parking lots, had scoured the channel to unnatural depths.

“A stream is not designed by nature to accommodate that volume of runoff, so it cuts down the channel and erodes beyond its natural banks,” Mayer said.

Exposed, unstable banks rose up to 8 feet above the stream bed in some locations. The channel had migrated onto both public and private properties, stressing nearby sewer lines, bridges and roads.

In 1999, the county began an aggressive restoration effort on Minebank Run. Contractors graded and reshaped the stream to slow the force of runoff and allow it to soak slowly into the flood plain. They removed more than 500 linear feet of concrete stream channel, while widening streamside or “riparian” buffers with more than 3,000 new trees and 6,000 shrubs. The work affected nearly 3.5 stream miles and was completed in 2005 at a cost of approximately $4 million.

“This was a very comprehensive approach to stream restoration, because we incorporated the entire length of stream and we were able to determine the best approach for the entire length,” said Candace Croswell, a restoration manager with the county.

Reducing the force of erosive stormwater runoff was top priority. But Mayer, who studies nitrogen and groundwater issues in the mid-Atlantic region, thought that the project presented an ideal chance to monitor nitrogen removal.

“The timing was right,” Mayer said. “We could get pre-restoration information and follow through with a post-restoration assessment.”

Using wells and nitrogen tracers, Mayer and other scientists continue to study the relationship between groundwater, stream features and nitrogen removal on a weekly, biweekly and seasonal basis.

Their findings demonstrate that the county’s stream restoration project not only prevents up to 50,000 pounds of sediment from entering the stream annually, but reduces the amount of nitrogen in the water, too.

“Levels vary throughout the year, but we’re seeing significant reductions in nitrogen concentration—from 25 to 50 percent,” Mayer said.

Improved buffer areas mean that less nitrogen reaches the stream. But repairing the stream’s hydrology as a whole also boosted the activity of bacteria that remove nitrogen from the water naturally, through a process known as denitrification.

“Most people think of a stream as a tube with water running through it, but it’s not,” Mayer said.

A healthy stream system includes groundwater, flood plains and riparian areas filled with trees, shrubs and grasses. The streambed exchanges water with all of them.

During the exchange, water soaks into riparian areas, where nitrogen-removing bacteria thrive in the decaying, organic matter found among plant roots. These bacteria “breathe” nitrogen instead of oxygen, converting it to a harmless nitrogen gas that is released into the atmosphere and removed from the stream system.

But denitrification was stunted at Minebank Run. The deeply eroded channel severed the streambed and groundwater from the floodplain and riparian area above them. Connections within the stream system were broken.

Sujay Kaushal, a professor at the University of Maryland Center for Environmental Science and research scientist who helps to monitor Minebank Run, said this is a common condition in urban streams.

“Erosion makes the stream sink deeper and deeper, away from the surrounding banks,” Kaushal said. “The groundwater sinks deeper with it and carries pollutants below the riparian area, where the bacteria can’t reach. The banks are so high that stormwater can’t spread over the flood plains, where it would get taken up. Instead, the water just passes through.”

The restoration of Minebank Run repaired these connections.

“Reconnecting the stream to the floodplain was an important aspect of this project, and having the ability to adjust and relocate the channel was important, too,” Croswell said.

The county used a suite of techniques to regrade the stream and its banks to more natural levels, while protecting it from erosion. Concrete channel was stripped away. Meanders were returned to the stream and riparian areas were improved. More than 5,000 tons of stream cobble replaced concrete channels and armored the streambed at key locations. Large stone structures, placed strategically within the stream, help to slow the runoff and direct it once again to the floodplains.

The change has been dramatic.

“The big question is whether to call this stream restoration or stream reconstruction,” Kaushal said.

Croswell said that the restored stream must include bioengineering, using natural materials, to cope with changes in the surrounding environment.

“We can’t restore a stream to what it used to be. But we can recreate what science tells us is the closest thing to a natural system, given the man-made influences around it,” Croswell said.

Documenting the impact of these techniques on nitrogen reduction is the first step in explaining how they can be used most effectively in other settings.

“Minebank Run has been a good start, and work is being done at different sites to see if we can make some generalizations about the benefits of specific restoration features,” Kaushal said. “There are lots of questions we still need to answer. What happens to denitrification as the sandy bank changes to vegetation? How many streams need to be restored to see a nitrogen benefit in a major tributary? Is it more important to restore headwaters or larger streams?”

Such information could be vital in promoting stream restoration projects when they must compete for limited funding. With approximately 100,000 creeks, streams, and rivers draining into the Chesapeake Bay watershed, stream restoration is one of the mostly costly means of reducing nitrogen in the Bay.

“It’s expensive to do stream restoration, there’s no doubt,” Croswell added. “But the cost of not restoring streams is high, too.”

In local settings, effective stream restoration shields property from erosion and protects community investments in roads, bridges and sewer lines. Streamside trees cool and cleanse the water, restoring fish habitat and sending cleaner water toward the Chesapeake Bay.

Croswell said that effective stream restoration must also include improvements for the surrounding watershed and not serve as a quick fix for seemingly isolated problems. In Baltimore County, stream restoration is coupled with projects like rain gardens, neighborhood education programs, wetland creation, stormwater management ponds and improvements to underground storm drains.

For severely damaged urban streams, a holistic approach to stream restoration may be the only way to ensure lasting improvements.

“A major disturbance in the landscape that affects the stream requires a major solution,” Kaushal said.

Research at Minebank Run is helping to document progress and lay the groundwork for future success.

“The goal would be to build the Honda Civic of stream restoration—small enough to fit in the backyard but highly efficient at removing nitrogen,” Kaushal said.