The information contained in this document is a compilation of the best professional judgement of a working group of scientists convened in Raleigh April 8th 10th. For two days, these scientists toured 19 riparian buffer and controlled drainage sites (carefully selected to be representative) within the Neuse River Basin. Discussions were held at each site on the appropriate management practice(s) that should be used under various soil and crop conditions. On the third day, the group synthesized their recommendations for placement and design of riparian buffer and controlled drainage systems.
We are very grateful to all of the scientists who went on the tour of sites and offered their suggestions at each site as well as during the discussion that followed. It was a help to have qualified individuals from the North Carolina Department of Environment, Health and Natural Resources (DEHNR), North Carolina Department of Agriculture (NCDA), United States Department of Agriculture (USDA)Natural Resources Conservation Service (NRCS), and the North Carolina Farm Bureau Federation participate in both the tour and the discussion of recommended best management practices. There was general agreement among the participants listed below that the recommendations in this document are scientifically sound and a reasonable balance between the need to protect water quality and have a productive agriculture. However, there is no implication that this document represents current or future policies of the agencies where the participants are employed.
Special thanks are extended to David Correll and Richard Lowrance, two of the worlds foremost authorities on riparian buffer functions, for spending a week in North Carolina to work on these recommendations; the NCSU Water Quality Group, under the direction of Jean Spooner, who formatted and printed the document and made other contributions; and Janet Young (of the NCSU Water Quality Group) for layout and design. Scientists Contributing to this Document: Visiting Scientist
David Correll Smithsonian Environmental Research Center, Edgewater, MD
Richard Lowrance USDA-ARS, Tifton, GA
North Carolina State University Faculty
Steve Coffey Biological and Agricultural Engineering
Ray Daniels Soil Science
Robert Evans Biological and Agricultural Engineering
Wendell Gilliam Soil Science
David Hardy Cooperative Extension Service
Will Harman Biological and Agricultural Engineering
Steve Hodges Soil Science
Greg Jennings Biological and Agricultural Engineering
Dan Line Biological and Agricultural Engineering
Rob Mikkelsen Soil Science
Deanna Osmond Soil Science
John Parsons Biological and Agricultural Engineering
Wayne Skaggs Biological and Agricultural Engineering
Jean Spooner Biological and Agricultural Engineering
Mitch Woodward Cooperative Extension Service
North Carolina Department of Agriculture Personnel
Ray Campbell Agronomic Division
Jim Cummings Commissioners Office
North Carolina Department of Environment, Health and Natural
Bryan Bledsoe Division of Water Quality
Beth McGee Division of Water Quality
Ted Mew Division of Ground Water
Ling Xu Division of Water Quality
U.S. Department of Agriculture
Roger Hansard Natural Resources Conservation Service
Sheryl Kunickis Natural Resources Conservation Service
Farm Bureau of North Carolina
In response to the deteriorating water quality in the Neuse River, the Environmental Management Commission of North Carolina proposed rules in the summer of 1996 for reducing point source and nonpoint source loads of nitrogen by 30% within five years. In order to achieve this 30% reduction, mandated solutions were proposed for the different pollution sources. For agricultural land uses, the proposed rules required the following combination of best management practices (BMPs): 1) nutrient management and controlled drainage, or 2) nutrient management or controlled drainage and a forested riparian buffer or vegetative filter, or 3) forested riparian buffers and vegetative filter strips when controlled drainage and nutrient management are not utilized (NCDWQ, 1996). These regulations proposed the reestablishment of riparian buffers along both sides of perennial and intermittent surface waters in the Neuse River Basin, exclusive of ditches.
In order to assure that state-of-the-art scientific information was being utilized and incorporated into current recommendations and any future regulations for reduction of nutrients, particularly nitrogen, from agricultural land, a working group consisting of the most knowledgeable experts was formed. The group consisted of out-of-state and North Carolina State University (NCSU) scientists, state and federal agency officials, and interested constituency groups who spent two days touring representative agricultural locations within the Neuse Basin. The focus of the discussions at each site centered on site-specific control of nitrogen. The ideas discussed during the tour were then synthesized, by physiographic region, during an all-day working session following the tour. The results are presented in this paper. It must be remembered, however, that the proposed site-specific recommendations are the result of best professional judgment and as new data are acquired, recommendations may change.
Farmers in the Neuse River Basin should use a combination of BMPs or BMP systems to reduce nitrogen loading to the Neuse. Animal operators have been mandated to implement the state .0200 non-discharge rules by 1998. These rules prescribe the containment of animal waste and the disposal of the waste at agronomic rates. For both livestock operators and row crop farmers, a system of BMPs consists of a combination of practices: nutrient management, controlled drainage, riparian buffers, and stream practices. The mix of BMPs should always consist of nutrient management with one or more additional BMPs. For example, in some locations farmers should use nutrient management and controlled drainage in areas appropriate for controlled drainage. In other locations, producers may combine riparian buffers and nutrient management. While at other locations, controlled drainage, in-stream constructed wetlands, and nutrient management will constitute the appropriate BMP system.
Riparian buffers are vegetative areas next to water resources that protect water resources from nonpoint source pollution and provide bank stabilization and aquatic and wildlife habitat. Natural riparian buffers are composed of grasses, trees, or both types of vegetation. In North Carolina, natural riparian buffers are dominated by trees. If riparian buffers are maintained or reestablished, they can exist under most land uses: natural, agricultural, forested, suburban, and urban. It is agreed among scientists that a corridor of vegetation can be effective at buffering valuable aquatic resources from the potential negative impacts of human use of the adjacent land. The streamside riparian buffer filters nonpoint source pollutants from incoming runoff and provides habitat for a balanced, integrated, and adaptive community of riparian and aquatic organisms (Welsch, 1991). In particular for nitrogen, riparian buffers can reduce nitrate to gaseous nitrogen, thus reducing the amount of nitrogen loaded into surface waters. While there is general agreement about the benefits of buffers, the specific design criteria such as buffer width, types of vegetation, and management are subjects of considerable debate.
Controlled drainage consists of a surface drainage system, a subsurface drainage system, and drainage outlets that are fitted with flashboard risers. The flashboard risers allow water to be drained from the fields in the early spring for field work or to be stored in the fields during the summer or winter. With higher water levels, due to lower flows and denitrification processes, nitrate losses to surface waters are reduced up to 45% (Evans et al., 1991). Under controlled drainage, corn and soybean yield increases of 10% have been measured, although some farmers report even larger increases. Thus, controlled drainage benefits both crop production and water quality.
In-stream wetlands can be created on small streams by impounding or adding a water control structure to the stream. Construction or restoration of in-stream wetlands provides an opportunity to control nonpoint source pollution, regulate water storage, and provide habitat for both aquatic and non-aquatic species. The construction of new or restoration of drained wetlands will create a marsh-like wetland (Gannon et al., 1995). In order for created in-stream wetlands to function most effectively in controlling nonpoint source pollution, they must be located in optimal positions in the watershed. Created wetlands should be placed bordering agricultural fields and high in the watershed, along first and second order streams. Created in-stream wetlands positioned in the upper reaches of the watershed can more effectively reduce runoff and erosion for the entire watershed than can the same acreage of created wetlands put into large wetlands low in the watershed (van der Valk and Jolly, 1993).
In the Lower Coastal Plain and Tidewater regions, the BMP system of choice is controlled drainage and nutrient management. It should be noted, however, that controlled drainage systems do not provide all the aquatic benefits of riparian buffers. In order to insure that anadromous fish can migrate, it is recommended that forested riparian buffers be established along the larger streams in order to protect aquatic habitat. The ideal riparian buffer for Coastal Plain conditions would be 25 feet of forest and then the width of grass necessary to control erosion. If there is no erosion, then a 25-foot tree or shrub buffer would be acceptable. Shrub buffers, which develop naturally when fields are not cultivated, can provide water quality benefits through the denitrification of nitrate, as well as habitat for wildlife species such as quail (Anderson, 1997).
In the Piedmont, the preferred BMP system to control nitrogen consists of riparian buffers and nutrient management because of the rolling topography. In conjunction with other BMPs (terraces, contour farming, conservation tillage, grassed waterways, field borders), buffers can also be used to retard sediment and phosphorus. It should be noted that these BMPs are very good conservation practices, but alone, they are inadequate for reducing nitrogen entry into surface waters. However, because of the abundance of riparian buffers in the Piedmont, most subsurface drainage water from agricultural fields currently passes through riparian buffers. Thus, implementation of additional practices in the Piedmont to improve nitrogen-derived water quality problems should have low priority. Because, however, sediment entry into surface waters in the Piedmont region is of considerable concern, buffers should be continued and maintained. In the Piedmont, 50-foot buffers are recommended between cultivated fields and surface waters to reduce nitrogen, retard sediment and phosphorus, and protect stream integrity. These buffers should consist of 25 feet of grass and 25 feet of forest with the forested area located adjacent to the stream. In some cases, a 25-foot shrub buffer will suffice. However, in low intensity pastures where high rates of fertilizers are not applied and erosion is not a problem, a 15-foot buffer between the grazed area and stream will usually be sufficient.
Because of the rolling topography of the Piedmont, runoff may form channels that cut through riparian buffers, thus reducing their effectiveness in controlling sediment and sediment-associated pollutants, such as phosphorus. A recently developed technique, the level spreader, spreads the incoming water across the length of the buffer, thus reducing the water velocity and improving the performance of the buffer. Level spreaders are outlets for concentrated runoff, constructed to laterally disperse discharge uniformly across a slope. By converting concentrated, erosive flow to diffuse sheet flow, the level spreader stabilizes slopes; better approximates pre-development hydrology of receiving systems; and provides opportunity for greater infiltration, phosphorus removal, and denitrification in the associated filtering BMPs (Gannon et al., 1995; Franklin et al., 1992).
Within the Neuse River Basin, the regions that contribute more nitrogen to surface waters from agricultural activities are the Middle and Upper Coastal Plain. However, BMP systems for controlling nitrogen are not as obvious as the techniques used in the Piedmont or Lower Coastal Plain regions. Many of the fields in these regions are flat, with inadequate natural drainage outlets for productive agriculture. Thus, irregularly spaced ditches have been placed in many fields and many of the natural streams have been channelized to increase flows. The field ditches, which are sufficiently shallow to primarily collect surface runoff, contribute only small amounts of nitrogen to streams and rivers. However, the deeper ditches, which intercept shallow subsurface flows, frequently have nitrate-nitrogen concentrations of 2-7 milligrams per liter. This is the primary mechanism for nitrogen entry into surface waters from agricultural fields in this region. Because of the varied topography and soil types in the Coastal Plain region, nitrogen reductions from agricultural land will need to be achieved by nutrient management and a combination of riparian buffers, controlled drainage, and in-stream wetlands, depending on the site location.
In the Upper and Middle Coastal Plain, the most effective treatment for overall water quality is riparian buffers. Buffer widths needed in this region will generally not be as wide as those required between cultivated areas and streams in the Piedmont. Erosion is usually not as great a problem in the Middle and Upper Coastal Plain as it is in the Piedmont. Therefore, a 25-foot forested riparian buffer will be adequate for significant (greater than 30%) reduction in nitrate reaching the ditches. If, however, erosion is a problem an additional 25 feet of grass should be added to the 25-foot forested riparian buffer.
There is tremendous opposition, with some justification, to creating forested riparian
buffers adjacent to agricultural fields or channelized ditches. In many locations in the
Upper and Middle Coastal Plain, controlled drainage can be used to improve water quality.
However, because the slopes are too large to maintain the water table close to the root
zone in much of the field, little agricultural benefit will be realized. Farmers should be
given the choice of using riparian buffers or controlled drainage, recognizing that they
must meet controlled drainage guidelines as described in this document. As the slope of
the drainage ditch increases, the cost of controlled drainage increases and controlled
drainage likely will become prohibitively expensive when the ditch or stream slopes are
greater than 0.3 to 0.5%.