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
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
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
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
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
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
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
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.