Prepared by: Robert Evans, Extension Agricultural Engineering Specialist J. W. Gilliam, Professor of Soil Science J. P. Lilly, Extension Soil Science Specialist
Publication Number: AG 473-7
Last Electronic Revision: June 1996 (KNS)
Most wetlands are privately owned, but their protection has become a public concern that is currently focused on the role of wetlands in improving water quality and as a habitat for wildlife. This fact sheet addresses the issue of water quality.
Contrary to popular belief, not all wetlands contribute equally to water quality. While some wetlands impede drainage flow from developed land, filtering out pollutants and greatly improving the quality of the water entering streams, others provide no significant water-quality benefits. This fact sheet explains how wetland type and location influence water quality, and it reviews the cases for development or preservation.
Riparian wetlands are typically narrow, wet areas that are adjacent to streams and are periodically soaked because both surface and subsurface water flows toward them. The soils are usually alluvial (water deposited). Wet areas on interstream divides are generally large and nonalluvial. They result from poor drainage in flat areas where rainfall exceeds evapotranspiration. Even though riparian and interstream divide wetlands may be equally wet, they make different contributions to water quality.
When evaluating how wetlands influence water quality, natural vegetation must be carefully considered. In their natural state, both riparian wetlands and interstream divide wetlands are forested or have other shrub or scrub vegetation. These undeveloped vegetated areas are a prominent feature of North Carolina's landscape, and most rural landscapes are at least 50 percent forested. Many of these areas are wetlands today because better-drained land was cleared and developed first.
Figure 1. Landscape position of riparian wetlanda in the piedmont and coastal plain of North Carolina.
Runoff water from developed areas, like agricultural fields, often contains large amounts of nitrate-nitrogen (NO3-N) and phosphorus, which are nutrients essential to crop growth but harmful to humans and animals in high doses. In addition, excess nutrients in receiving waters can stimulate algae growth and, as a result, deplete the supply of oxygen necessary to fish and disrupt the aquatic food chain. The researchers found that a large percentage of the nitrate-nitrogen was removed from the subsurface flow as it passed through the riparian areas.
In one North Carolina wetland, the nitrate-nitrogen in the shallow groundwater that passed through the riparian vegetation was reduced from approximately 15 milligrams per liter to 2 milligrams per liter or less.(1)(The current EPA drinking water standard is 10 milligrams of NO3-N per liter of water). The reduction occurred as the water passed through the first 30 to 50 feet of the riparian areas.
Researchers estimated that movement through the riparian areas reduced the nitrate-nitrogen content of the upland runoff nearly 85 percent annually, from 27 pounds of NO3-N per acre at the agricultural field edge to approximately 4 pounds per acre at the wetland edge near the streams.(2) Most of the nitrate- nitrogen was removed as the flow passed through the narrow bands of vegetation between the agricultural fields and the small streams. The soils in the large downstream areas of wooded wetland (shown schematically in Figure 2) contributed little to the overall drop in nitrate-nitrogen.
Figure 2. Aerial view showing stream orders and locations of associated riparian wetlands and floodplain.
Most researchers surmise that the nitrate-nitrogen is removed through a biological process known as denitrification. In denitrifcation, soil bacteria convert nitrate-nitrogen to nitrogen gas, which eventually returns to the atmosphere. Although both the narrow areas of vegetation and the large downstream areas can process nitrogen through denitrification, most of this nutrient is removed before the water reaches the downstream areas. Therefore, from the standpoint of water-quality improvement, the location of the wetland (or wet soils) is apparently much more important than either the degree of wetness or the size of the wet area. Soils immediately adjacent to streams (Figure 2), which are wet because they receive surface or subsurface flows from higher elevations, are the most effective at removing nitrate- nitrogen from agricultural and other runoff waters.
Figure 3. Dense vegetation typically found in the transitional area between the agricultural field and riparian forest. Runoff from the field is from left to right, moving toward a stream not visible in the photo.
Figure 4. Coarse sediment deposition in dense vegetation along the field-forest edge.
Figure 5. Sediment deposition thickness and particle-size distribution along floor of riparian wetland.
One study estimated the effect of riparian wetlands on the deposition of sediment and phosphorus leaving agricultural land in surface runoff.(3) The researchers found that most of the sediment was deposited in the riparian area very close to the edge of the fields (Figure 6). Extrapolating the results, they estimated that 85 to 90 percent of the sediment remained trapped in wooded areas (Figure 7) and never reached major streams.
Figure 6. Sediment deposition depth in riparian wetlands (after Cooper, et al., 1987a, 1987b).
Figure 7. Cumulative sediment deposition on floor of riparian wetland (after Cooper, et al., 1987a), and phosphorus deposition and distribution in riparian wetlands (after Cooper, et al., 1987b).
Data gathered in 1991 indicated that a vegetated buffer 13 feet wide trapped approximately 85 percent as much sediment as a buffer 26 feet wide.(4) This suggests that the first few yards of the riparian area are not only the most effective at removing nitrate-nitrogen, they are also the most efficient at trapping sediment.
Another sediment study measured the phosphorus deposited with the sediment in the wooded wetland over a 25-year period.(5) (Nitrate-nitrogen dissolves in water, but phosphorus will attach to soil particles.) The fine sediment deposited around th higher-order streams and in the floodplain swamps contained a higher concentration of phosphorus than the coarse materials found at the edge of the forest. Further, the floodplain deposits contained even more phosphorus than the higher-order stream areas, even though the floodplain accumulated sediment more slowly (Figure 7). The researchers concluded that the large floodplain swamps were more important for retaining phosphorus than for their ability to remove nitrate-nitrogen or trap sediment.
Figure 8. Landscape position of wetlands in lower coastal plain.
Interstream divide soils are wet for two major geographic reasons: (1) The areas are relatively flat, so water moves slowly across the soil surface; and (2) they often are located miles from a naturally occurring drainage outlet, which means that excess rainfall can take several weeks to dissipate. The degree of wetness depends primarily on rainfall and evapotranspiration. During the winter and spring, when rainfall greatly exceeds evapotranspiration, water often pools on the soil surface. During the summer and fall, when evapotranspiration more closely equals rainfall, the soil dries faster and the water table often drops more than 3 feet below the soil surface.
Fresh-but nutrient-poor-runoff from the interstream divide wetlands eventually mixes with the nutrient-rich runoff from the developed lands closer to the streams. This blending dilutes the pollutants in the runoff without diminishing the beneficial amounts of nutrients and sediments reaching the stream. Thus, interstream divide wetlands make a passive contribution to surface water quality.
Undeveloped interstream divide wetlands can be used as a sink for agricultural drainage, but a pump must be used to lift the drainage water over the flat land, and a diffuser canal is needed to distribute it evenly. Without the diffuser canal, runoff channels soon develop, reducing the amount of time the drainage water remains in the wetland and thereby reducing the benefits. Because interstream divide wetlands must be extensively modified to act as a sink, this water-quality benefit is marginal.
Everyone who has studied riparian wetlands agrees that they provide many water-quality benefits. These are the areas that first receive and impede- drainage runoff from developed lands. They are also the areas where shallow groundwater seeps into surface waters. Riparian vegetation traps sediment, removing harmful amounts of nitrate-nitrogen, phosphorus, and pesticides before they can enter streams. For all these reasons, there should be a strong effort to maintain or restore wet, vegetated buffers adjacent to streams.
Although there are valid environmental reasons to preserve or restore some of the large wet areas on interstream divides, water-quality benefits are not among them. Available data suggest that natural interstream divide wetlands have only a passive, limited effect on water quality. To increase their water quality benefits, the wetlands must be extensively modified. On the other hand, when interstream divide soils are drained and used for agricultural production, they retain far more sediments and plant nutrients than many other well-drained North Carolina soils used for the same purpose.(10)
From a water-quality viewpoint, the facts support preserving or restoring riparian areas close to the streams. Then, to maintain a base of highly productive, fertile cropland, some interstream divide wetlands could be developed to compensate for croplands restored as riparian wetlands. The net result would be a general improvement in water quality.