PHYSIOGRAPHIC REGIONS AND RECOMMENDED BMPS
North Carolina is divided into three physiographic regions: the Coastal Plain, the Piedmont, and the Mountains.
The headwaters of the Neuse River are located in the Piedmont, north of Durham. Most of the Neuse River drainage area, however, lies within the Coastal Plain. The majority of the nitrogen load contributed to the Neuse River from agricultural sources is derived from the Coastal Plain area because (1) agriculture is much more intense and a much higher percentage of the land area is planted to fertilized crops; (2) much more of the drainage water leaves agricultural areas as subsurface flow; (3) a much higher percentage of subsurface drainage from agricultural fields is intercepted by ditches and thus does not flow below riparian buffers, and; (4) the nitrogen from Coastal Plain drainage areas enters the Neuse closer to where the excess nitrogen in the river (lower Neuse River Basin) causes environmental problems.
Some of the nitrogen load from nonpoint sources into the Neuse is due to overland runoff. Most nitrogen, however, is leached through the soil into ground water that then flows into the ditches or small, first-order streams that feed the Neuse. The majority of the nitrogen pollution that enters any major river, and in this case the Neuse, is derived from smaller order streams (Leopold et al., 1964). If nitrogen loads from agricultural activities are going to be reduced, systems of BMPs, whether they be forested riparian buffers, controlled drainage, or stream modifications used separately or in conjunction with nutrient management, must be utilized.
The land area of the Coastal Plain comprises 45% of the State of North Carolina. This region is rather flat in most areas, but the topography, or land surface, has more rolling hills in the western portion of the Coastal Plain. Elevations in the Coastal Plain range from sea level to 660 feet (200 meters). The soils in the Coastal Plain are relatively uniform compared to soils found in the Piedmont.
The Coastal Plain geology consists mostly of marine sedimentary rocks. This rock is overlain by fluvial (waterborne) deposits. Sand and clay are the primary sediment types, although some limestone occurs in the southern portion of the Coastal Plain. Coastal Plain soils developed from sandy to clayey unconsolidated marine and fluvial deposits. These deposits are primarily sand and clay from the ocean and rivers that have been laid down over many thousands of years. They are called unconsolidated because they have not hardened into large beds of rock.
An aquitard, or confining layer, exists approximately 6 to 30 feet below most North Carolina Coastal Plain soils throughout the area. This aquitard restricts the movement of ground water downward and lateral flow of shallow, unconfined ground water contributes approximately 70% of the stream flow in this region. As a consequence, nitrate that enters the ground water in the Coastal Plain can become part of the surface water pollution problem.
The Coastal Plain can be divided into distinct regions: the Lower Coastal Plain; the Tidewater and Barrier Island regions, which are subdivisions of the Lower Coastal Plain; the Middle Coastal Plain; and the Upper Coastal Plain. The Upper Coastal Plain grades into the Piedmont just east of Raleigh.
Lower Coastal Plain and
Lower Coastal Plain. The Lower Coastal Plain is a wide, flat plain that extends from the Atlantic Ocean
west to the Goldsboro area. The many slow-moving streams in this area indicate that the water table is high and the soils are generally poorly drained. Large areas of poorly to very poorly drained soils exist in this region. These poorly drained soils are on flood plains directly adjacent to streams and also occur far away from the stream in the middle of the interfluve areas. Figure 9 illustrates the relationship between landscape position and water table depth. Soils are often more than 5 feet thick, but many have poorly defined loamy and clayey B horizons. Soils that are more shallow frequently have sandy C horizons. Figure 9. Water table depth in relationship to stream location.
Tidewater Region. The Tidewater region, in the northeast portion of North Carolina, is a part of the Lower Coastal Plain. The Barrier Island region occurs along the entire coast of North Carolina. The Tidewater region has distinctively broad, rolling plains separated by widely spaced streams and estuaries.
Recommended Practices in the Lower Coastal Plain to Control Nonpoint Source Pollution to the Neuse
Producers have installed over 3,000 controlled drainage structures that drain approximately 300,000 acres in the Lower Coastal Plain of North Carolina. These structures both increase yield and reduce nitrogen losses to surface water by 40-50%. Because controlled drainage is effective in reducing nitrogen, it is recommended that producers in the Lower Coastal Plain utilize controlled drainage, in conjunction with nutrient management, to reduce nitrogen losses from agricultural fields. Even though controlled drainage has been generally accepted by producers, is cost shared by the State, and recommended by both the Natural Resource Conservation Service and the North Carolina Cooperative Extension Service, there are still large areas that could benefit from controlled drainage.
In conjunction with controlled drainage, all field ditches should have 3-to 6-foot grass riparian buffers, composed of grass or natural herbaceous vegetation, on either side of the ditch. The general guide for width determination is that the width of the vegetated buffer on each side of the ditch should equal or exceed the ditch depth. Field borders on ditches are important to maintain the ditch structure and provide some reduction in sediment and nutrients derived from surface flow. Routine mowing or use of herbicides to selectively manage woody vegetation, along with sediment removal from the ditch and buffer, are acceptable recommended maintenance practices.
Controlled drainage is only effective for water quality if the water
depth in the controlled drainage ditches is managed at the correct height, particularly
during the winter (Figure 10). Evans et al. (1991) have prepared controlled drainage
guidelines which are designed to both improve water quality and increase crop yields. We
believe that these guidelines are very practical for most crop and soil situations when
fields are fallow. These guidelines recommend setting the control structure at 12-18
inches below the soil surface during the winter. Holding the water table at 12-18 inches
may cause problems in some situations (such as excessive ditch bank sloughing in fields
with unstable sandy subsoil or excessively wet conditions during the winter growing season
when wheat is planted). Thus, we are hesitant to recommend that it be required for water
to be held at this height. Instead, we recommend that the control boards be placed and
managed such that the average water elevation in the ditch throughout the length of the
channel be no lower than 36 inches below the land to obtain credit for using controlled
drainage for reducing nitrogen entry into surface water. Maintaining controlled water
levels during the growing season does have some water quality benefits, but most benefits
are obtained from December through March. While 36 inches below the land surface is the
general recommendation, short-term adjustments (both higher and lower) are acceptable when
necessary to accommodate routine production activities, such as tillage, planting,
harvesting, ditch maintenance, as well as abnormal weather conditions. Figure 10.
Because controlled drainage does not provide all of the aquatic benefits that riparian buffers do, we recommend that larger streams in the region still have riparian buffers. If this is done, anadromous fish can still migrate from the ocean into the streams. If controlled drainage is used in the fields, woody vegetation buffers adjacent to the large collector ditches are not expected to have any significant effect on nitrogen transported by the stream, but the overall heath of the stream would be better. This is because most nitrogen leaving agricultural fields originates from the headwater areas of streams and drainage ditches. If nitrogen enters surface water in the field ditches, riparian buffers along the larger stream will not help prevent its movement downstream. Natural streams which have been channelized should have woody vegetation adjacent to them whenever practical for the overall health of the stream.
Middle and Upper Coastal Plain
Middle Coastal Plain. The most critical area for nitrogen reduction from agricultural practices is the Middle
and Upper Coastal Plain because this area contributes the most nitrogen per unit area of cultivated land and a high percentage of the area is in cultivated crops. On the average, an acre of farm land in this region contributes the most nitrogen to surface water because the soils are naturally better drained than those in the Lower Coastal Plain, resulting in transport through natural subsurface flow of residual nitrogen not utilized by crops. Most of the larger streams in the region are protected by riparian buffers, but much of the subsurface water from agricultural fields enters surface water through field ditches that were once natural drainageways. These natural drainageways have been channelized and the riparian vegetation removed to improve the hydraulic efficiency and reduce flooding. There is some reduction of nitrogen from the ground water in the region of the ditch but not as much removal as occurs when the water passes below a riparian buffer.
The Middle Coastal Plain has smooth, gently rolling, plateau-like uplands that slope toward the ocean, and gentle-to-steep valley slopes. Figure 11 shows a typical landscape for the Middle Coastal Plain. Figure 11. Landscape Middle Coastal Plain (from Daniels et al., 1984).
Figure 12 shows the relationship between landscape position, water table depth, and soil series in the Middle Coastal Plain.Figure 12. Relationship between landscape position, water table depth, and soil series in the Middle Coastal Plan (from Daniels et al., 1984).
Figure 13. Relationship between soil and saprolite (from Daniels et al., 1984).
Recommended Practices in the
Middle and Upper Coastal Plain to Control
Nonpoint Source Pollution to the Neuse
Even though this region loses more nitrogen to surface waters than other agricultural areas of the State, methods for reducing nitrogen inputs from agriculture in the Middle and Upper Coastal Plain are not as obvious as the techniques used in the Piedmont or Lower Coastal Plain. Many fields in the Middle and Upper Coastal Plain 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. Field ditches, which are sufficiently shallow and collect primarily 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 mg L-1. This is the primary mechanism for nitrogen entry into surface waters from agricultural fields in the Middle and Upper Coastal Plain. Because of the varied topography and soil types in the Middle and Upper Coastal Plain, nitrogen reductions from agricultural land will need to be achieved by nutrient management and a combination of riparian buffers, controlled drainage, and created in-stream wetlands, depending on the site locations.
As stated earlier, the most effective treatment for overall water quality is riparian buffers. However, there is tremendous opposition, with some justification, to putting riparian buffers adjacent to agricultural fields or channelized ditches. Alternatively, controlled drainage can be used to improve water quality but, because of the slopes in these regions, little agricultural benefit will be realized from the controls. The greater the slopes in an area, the greater the cost per unit of treatment area when controlled drainage is used. Regardless of financial benefits, landowners must manage these controlled drainage structures to protect water quality. Without proper management, the structures will be ineffective. If given a choice, many farmers in this region may chose to pursue controlled drainage rather than have riparian buffers adjacent to their drainage ditches.
The ideal riparian buffer for Coastal Plain conditions would be the recommended 25 feet of forest plus the width of grass necessary to control erosion. However, the width of the buffers needed in the Middle and Upper Coastal Plain will generally not be as wide as those required between cultivated areas and streams in the Piedmont. Erosion is usually not as significant a problem in the Middle and Upper Coastal Plain as it is in the Piedmont: grass riparian buffers for sediment reduction are not required in most fields. A 25-foot forested riparian buffer will be adequate for significant (greater than 30%) reduction in nitrate reaching the ditches (Figure 5).
Another riparian buffer type that can be used successfully in this region of North Carolina is a shrub buffer. Shrub buffers, which develop naturally in North Carolina when cleared areas are left undisturbed, can provide water quality benefits through the denitrification of nitrate (Figure 14). In addition, shrubby vegetation provides habitat for wildlife, such as quail (Anderson, 1997). Using a mechanical system that cuts small wounds in trees and wipes the wound with herbicide, shrubs can be maintained and growth of invasive tree species retarded. Figure 14. Shrub buffer.
Shrub riparian buffers can be maintained more cheaply than grass riparian buffers. Cost analyses demonstrate that maintaining shrub vegetation costs $13.00 per linear mile of ditch while custom mowing costs approximately $25.00 per linear mile (Anderson et al., 1996). The biggest drawback to the use of shrub riparian buffers is their appearance, which some producers find offensive.
The question continually asked is, can the same reduction in nitrate be achieved with a grass riparian buffer as with a forested riparian buffer? There are insufficient data, at this time, to compare the function of both types of buffers. It is a scientific best professional judgement that woody plants work better in the Coastal Plain because of acidic subsoils. Grasses tend to be more sensitive to acidic conditions and it is believed that they are more shallow rooted than woody vegetation under acidic soil conditions. Roots of deeply rooted woody vegetation are increasingly likely to interact with the ground water that passes through the riparian buffer area. However, deep rooted grasses, if they are acid tolerant, would probably work as well. Since there are no data at this time to support grass vegetation on the Coastal Plain, we recommend that riparian buffers designed to reduce nitrate be composed of woody herbaceous plants.
The primary factors involved in determining the appropriateness of
controlled drainage or riparian buffers are slope and depth to ground water table. In
order to obtain water quality benefits from controlled drainage, the water table along the
length of the drainage channel should be kept at least 36 inches or closer to the surface
of the soil so that a saturated zone is established near the surface to promote
denitrification (Figure 15). If the slope in the channel is greater than 0.2% or 0.3%,
controlled drainage could be used, but the cost becomes prohibitive. On most of the soils
within the Upper or Middle Coastal Plain, controlled drainage has little or no effect on
crop yields, thus the benefits are strictly for the protection of water quality. Figure 15. Depth of ground water for effective nitrate control.
Deciding which BMPs should be combined to control nitrogen will generally need to be made on a site-by-site basis. In order to help determine what BMPs should be utilized, the decision tree presented below should be used. This decision tree represents best professional judgements. Because there is a lack of empirical evidence on the placement of created in-stream wetlands, site-specific recommendations are not being made for their installation.
Decision Tree for
Determining BMP Usage
The word Piedmont means foot of the mountain. Thus, the Piedmont region lies at the foot of the mountains, between the Mountain and Coastal Plain regions. The Piedmont covers 39% of States area and has a rolling-to-hilly topography. Elevations in the Piedmont range from 295 feet to 1509 feet. The geology of the Piedmont is very complex and, as a results, the soils are very complex. Eight geologic belts, composed of areas with similar rock types and geologic histories, exist in the Piedmont (Figure 16). Figure 16. Mineralogy from Chapel Hill to Raleigh (from Daniels et al., 1984).
Recommended Practices in the Piedmont to
Control Nonpoint Source Pollution to the Neuse
Because of the rolling topography of the Piedmont, nutrient management and riparian buffers are the most suitable BMPs to reduce nitrogen loading to the Neuse. The use of buffers, in conjunction with other BMPs (such as terraces, contour farming, conservation tillage, grassed waterways, field borders) should also be used to retard sediment and phosphorus. It should be noted that while these latter BMPs are very good conservation practices, alone, they are inadequate for reducing nitrogen entry into surface waters. However, agricultural systems in the Piedmont are contributing the least amount of the nitrogen to the Neuse River Basin because agricultural activities are much less intense than agricultural production in the Coastal Plain in the Piedmont. In addition, subsurface drainage water from most agricultural fields in the Piedmont is currently passing through riparian buffers. Thus implementation of additional measures in the Piedmont to improve nutrient water quality problems should have low priority. However, sediment entry into surface waters in the Piedmont region is of considerable concern, buffers should be continued and maintained.
To reduce nitrogen, retard sediment and phosphorus, and protect stream integrity on
cultivated fields, 50-foot buffers are recommended. These buffers should consist of 25
feet of grass and 25 feet of forest, with the forested area being adjacent to the stream
(Figure 17). The 50-foot buffer will generally be required in the Piedmont whereas a
25-foot buffer will sometimes be adequate in the Coastal Plain. Because Piedmont soils are
generally steeper and have a higher clay content in the surface horizon, there is more
surface runoff and potentially higher erosion rates associated with these soils. Thus, a
combination of grass and forested riparian buffer is generally recommended. However,
practices such as conservation tillage can greatly increase infiltration and reduce
surface runoff. In situations where a 50-foot buffer would cause a significant loss of
tillable land (the authors have seen few examples in the Piedmont where this would be the
situation), a 25-foot forested riparian buffer, used in conjunction with conservation
tillage, would likely provide adequate protection from sediment to surface waters. Figure 17. Recommended riparian buffer.
Because of the rolling topography of the Piedmont, runoff may form channels which cut through riparian buffers, thus reducing their effectiveness in controlling sediment and sediment-associated pollutants, such as phosphorus. Particularly in the Piedmont, but in any location where sediment loss is a problem, grass riparian buffers must be maintained. This will generally mean that they must be reworked every three to eight years and that the sediment that has been deposited in or just in front of the buffer must be redistributed. Unmaintained grass riparian buffers will almost always become ineffective.
Level Spreader. 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 and are constructed
to laterally disperse discharge uniformly across a slope. A level spreader consists of a
long, narrow trench, and is often used to disperse runoff from a diversion (Figure 18).
The outlet lip must be of uniform elevation and should be constructed in stable,
undisturbed soil. The outlet area should have a uniform slope and be well vegetated to
avoid rechanneling flows (Franklin et al., 1992). Figure 18. Detailed
cross-section of level spreader trench for dispersing runoff along the contour.
The water quality value of level spreaders is the reduction of runoff discharge peaks and enhanced sediment and nutrient removal efficiency of riparian buffers (Figure 19). 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 down slope (Gannon et al., 1995).
Level spreaders are almost always used in combination with other BMPs. Consequently, no evaluation of the effectiveness of spreaders themselves was found, and little attention has been given to their ability to improve the function of associated BMPs. However, Franklin et al. (1992) routed runoff from two no-till wheat fields into level spreaders above forested riparian buffers in the Piedmont of North Carolina. Over a 3- and 10-month period, they measured 6 to 8% reductions in the total surface discharge-to-rainfall ratios and 36% drops in peak discharge rates through the level spreader system. They also found proportional reductions of 75% ammonia, 29% Kjeldahl nitrogen, 17% nitrate, 38% ortho-phosphorus, 32% total phosphorus, and 47% total suspended solids through the level spreader riparian buffer system. At the same experiment sites, Verchot et al. (1995) found enhanced denitrification potential in the riparian buffer soils downgradient from the level spreaders as compared to the potential in soils just outside the level spreader zone. Figure 19. Design of level spreader used for dispersing agricultural runoff through a forested riparian buffer.
Level spreaders should be inspected and repaired, if necessary, after every rainfall until vegetation is established. Runoff containing high sediment loads should be treated in a sediment-trapping device before release into a level spreader.
In the Piedmont, on well maintained, low-maintenance pastures that generate little
sediment and phosphorus losses, smaller riparian buffer widths may be considered. One line
of trees or a 15-foot forested riparian buffer will adequately reduce nitrate-nitrogen
levels (Figure 20). Unless grazing animals are fenced out of the riparian buffer, cattle
trodding will continue to degrade or destroy fish habitat. In addition, reduced shading,
because of the narrower forest width, will increase stream temperatures and diminish food
sources (detritus), thus ultimately reducing aquatic habitat and aquatic organisms. Figure 20. Pasture stream with fenced riparian buffer. Top of Page / Table
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