Water: Nonpoint Source Success Stories
North Carolina: Section 319 Success Stories, Vol. III
|Story 1 | Story 2 | State Water Quality Site|
Wetland Systems Reduce Nitrogen Concentrations
412 West Queen Street
Edenton, NC 27932
Kristopher Bass and
|Primary Sources of Pollution:|
|Primary NPS Pollutants:|
|60 percent reduction in nitrate nitrogen|
|33 percent reduction in ammonia nitrogen|
|9.5 percent reduction in TKN, 20 percent reduction in total nitrogen|
|55 percent increase in total phosphorus|
In northeastern North Carolina, excess rainfall is typically removed from developed areas by an existing network of field ditches and canals, often bypassing natural riparian areas before entering creeks and streams. As a result, the nutrients and sediment in storm water are often carried directly to the nutrient-sensitive river and estuarine waters.
Installing constructed wetlands
In an effort to control water flow and improve water quality, constructed wetlands were installed to intercept two ditches draining approximately 600 acres of a surrounding agricultural and urban watershed in the town of Edenton, North Carolina. The drainage area included a hospital, a shopping center, residential areas, and several hundred acres of agricultural land. In addition to the two inlet ditches, one small side ditch, several tile drains, and possible groundwater movement also contributed to the wetland.
The wetland systems are considered "constructed" wetlands because the natural relief or lack of relief is not conducive to implementing a traditional riparian system. Wetlands were created in existing drainage canals by installing water control structures and planting several native wetland species.
Educational opportunities were also provided for school groups, scout troops, and civic groups. Two field days, four educational meetings, and one training workshop for agency personnel and consultants were held.
The project demonstrated that wetlands with small wetland/watershed area ratios can provide significant water quality benefits for nitrogen, although phosphorus increased. Monitoring and data collection at this site were conducted from 1996 to 1999. The integration of grab and automatic sampling schemes resulted in more than 1,000 water quality samples. Concentrations of all forms of nitrogen were reduced significantly between the inlets and the wetland outlet over the evaluation period. The highest drop in concentrations was achieved for nitrate nitrogen (NO3-N, 60 percent), with lower declines for ammonia nitrogen (NH4-N, 33 percent) and total Kjeldahl nitrogen (TKN, 9.5 percent) levels. Total nitrogen concentrations were 20 percent lower at the wetland outlet.
Phosphorus levels increased 55 percent between the inlets and the outlet. The liberation of phosphorus bound in the wetland substrate and organic matter apparently negated any sorption or uptake occurring within the wetland. At some point in the future, phosphorus equilibrium might be reached, leading to no net increase at the outlet. Thus far, however, no decline has been observed.
Nitrate and ammonium nitrogen concentrations dropped as much through the wetland during the dormant months as during the growing season. TKN concentrations were lowered only during the winter months. The observed increase in phosphorus concentrations between wetland inlets and the outlet was significantly larger during the summer months than in the dormant periods.
Public acceptance of the project was excellent, attributed to the pleasing aesthetics of the sites. A variety of wildlife continues to flourish in the wetland.
For more information on the project, go to www.bae.ncsu.edu/ research/evans_web/ etd/klbass.pdf.
Durham Soil and Water Conservation District
721 Foster Street
Durham, NC 27701
|Primary Sources of Pollution:|
|Primary NPS Pollutants:|
|ecosystem protection practices (stream rehabilitation)|
|revegetation of 600-foot reach|
|decrease in sulfate levels|
|flow-reducing structures installed along 1,584 feet of streambank|
Goose Creek is the major stream draining east-central Durham, North Carolina. The creek is a tributary of Ellerbe Creek, identified in the 1993 and 1998 Neuse River Basin Management Plans as not supportive of its intended uses. The watershed is in an old, well-established low-income neighborhood with little opportunity for landscape modification to alter runoff quantity or quality.
The channel was extremely degraded with hardened channel control structures, including concrete fiber fill lining and vertical rock wall channel banks. The hardened urban flow channels were extremely conducive to carrying large quantities of sediment at a very high velocity.
Three-phase creek restoration
Restoration of Goose Creek involved installing ecosystem protection practices, or EPPs (stream rehabilitation), to reduce sediment, reduce thermal fluctuation, and increase dissolved oxygen. Recommended EPPs were derived from typical stream restoration techniques and modified for the Goose Creek system. The project was designed to rehabilitate more than 2,100 feet of the stream, in three continuous treatment phases.
Phase I included the installation of 25 log structures in an 884-foot-long concrete-lined fiber fill channel. The concrete-lined channel provided no water quality protection or vegetation to reduce flow. The log structures provided channels to break up storm flow energy; variety of flow allows for deposit behind the logs and storage of sediment. This phase of the project included the addition of soil and planting of wetland vegetation to provide shade and some nutrient uptake in the cement-lined area. A group of volunteers planted willows and wetland plants along a 600-foot reach of the project.
Phase II of the project occurred in a public park but was constrained by vertical rock walls on both sides. Four rock cross veins were installed along a 700-foot reach to break up energy and increase dissolved oxygen in this low-gradient channel. The cross veins reduce stress on the rock walls by transferring flow toward the center of the channel.
Phase III of the project was to involve a section of the stream that runs through an industrial and commercial area. This phase was not completed within the scope of the 319 grant primarily because of the need to perform underground soil remediation at an industrial site. However, Phase III has received funding from the North Carolina Clean Water Management Trust Fund and is projected to be completed after the soil remediation is performed, possibly by the end of 2001.
Meeting the challenge
Phases I and II of the project were completed in February of 1999. The education portion of the project, which is coordinated through the Durham Soil and Water Conservation District Office, is ongoing.
The project is unique in that it has employed stream restoration techniques in an extremely constrained situation to create a sustainable creek ecosystem. The term "ecosystem protection practice" is appropriate, considering the initial channel condition. Installation of these practices through an elementary school and public park will increase education opportunities in this low-income neighborhood. The enhancements will improve public perception about the stream and potentially reduce litter and other pollutants to the system.