Wetlands and Water Quality

Brian K. Miller, Department of Forestry and Natural Resources

Wetlands once made up 25 percent of Indiana. Many of these 5.6 million acres were located in the fertile farmground of northern Indiana. Early in the 19th century, landowners began using open ditches and tiles to drain large areas of wetlands. They then converted the drained soil to agricultural production. Since then, nearly 86 percent of Indiana's wetlands have been drained or filled.

Wetlands are areas characterized by saturated or nearly saturated soils most of the year. Wetlands serve a number of important environmental functions. Location, soil type, surface and ground water movement determine which of the following functions a particular wetland may serve.

Flood Water Retention

Usually located in depressions, wetlands receive surface runoff during storms. Water collects in these areas and contributes to stream flow when full or through ground water movement. Wetlands act as a holding area for large quantities of surface water which can be slowly released into a watershed. A one acre wetland, one foot deep, can hold approximately 330,000 gallons of water. When wetlands are removed, storm water runs directly into the watershed, increasing flooding.

Nutrient and Sediment Filtering

Often found in areas of intense agricultural production, wetlands play an important role in maintaining local water quality. Wetlands preserve water quality by removing nitrogen, phosphorus and pesticides from agricultural runoff.

Table 1. Common Wetland Aquatic Plants

Cattail PondweedDuckweed
Spikerush Naid Watermeal
SmartweedWatermilfoilWater Hyacinth
KnotweedBladderwort Water Lily

Chemicals and nutrients can enter a wetland through surface water and sediment, or through ground water. The major inorganic nutrients entering wetlands are nitrogen and phosphorus. In the wetland, nitrogen and phosphorus are removed from the surface water and transferred to the sediment, wetland plants or atmosphere. Some agricultural pesticides used in the Midwest can also be carried to the wetland through surface runoff.

Nitrates are lost from upland sites primarily through subsurface drainage. In the wetland, nitrates are absorbed by plants or converted (through an anaerobic process called denitrification) to nitrogen gas and lost to the atmosphere. Nitrate-N is efficiently removed from wetland surface waters by aquatic plants.

Ammonium-N enters wetlands primarily through surface runoff. In the wetland, ammonia is absorbed by plants or converted to nitrogen gas through volatilization. Nitrification can also occur, changing ammonia into nitrites and nitrates. The nitrate form of nitrogen is more readily removed from surface water by wetland plants than the ammonium form.

Phosphorus, organic nitrogen and some metals (iron or aluminum) usually attach to sediment and are carried by runoff to the wetland. By holding water, a wetland allows sediment and large particles to settle on the wetland bottom. The root systems of wetland plants then absorb nutrients from the sediment. Much like phosphorus, some pesticides also bind to sediment materials. Surface runoff carries the sediment materials to the wetlands and deposits them on the wetland bottom.

A particular wetland's function may change seasonally. During the growing season, in the summer and early fall, emergent and submerged aquatic plants (Table 1) take up large quantities of nutrients from water and sediment. Algae and floating plants absorb nutrients from surface water. These plants essentially convert the wetland into a "nutrient sink," by taking nutrients from the water and sediment and retaining them as plant material. By taking up and holding nutrients during the summer, wetlands decrease the possibility of contamination downstream (Figure 1).

Figure 1. Sink

When these plants die, a large portion of the nutrients return to the water and sediment from decaying plant material. During this period (in late fall and early spring), wetlands serve as a nutrient source when water flows from the wetlands to ecosystems downstream (Figure 2).

Figure 2. Source

In most cases nutrients are recycled within the wetland. Emergent and submerged plants bring nutrients from the sediment into the water column, acting as "nutrients pumps." Algae and floating plants serve as "nutrient dumps" by taking nutrients from the water and depositing them back in the sediment when they die and settle on the bottom.

The cycle breaks when nutrients are removed from the wetland system, occurring when nutrient-rich water flows out of the wetland. The release of nitrogen gas to the atmosphere by denitrification, ammonia volatilization or possibly nitrification of ammonia also causes nutrients to be lost.

A wetland's natural filtering ability can become overloaded, disrupting the nutrient cycle. Steps can be taken to prevent overload by reducing nutrients and chemicals lost from agricultural fields.

Management Practices to Reduce Runoff and Leaching

The movement of nutrients and chemicals by sediment and surface runoff to wetlands can be reduced by conservation tillage and other common soil erosion control practices. These practices include: grass waterways, vegetative filterstrips, contouring and terracing. Incorporating fertilizers and chemicals reduces runoff by removing these substances from the runoff mixing zone.

Adjusting the timing and rate of fertilizer application to coincide with crop needs decreases nitrate leaching. Nitrate losses from animal waste can be reduced by timing of manure application, diverting feedlot runoff to grass filterstrips and limiting livestock's access to surface water.

Ground Water Exchange

Ground water and surface water are linked through wetlands. The following explains how wetlands impact surface water quality and also affect ground water quality.

Wetlands with recharge capacity collect runoff water during storms and slowly release the water into ground water supplies (Figure 3). Wetlands therefore make positive contributions to soil moisture in agricultural settings. Without wetlands acting as a catch basin, damage from flooding and water erosion will likely increase.

Figure 3. Water in wetlands, located above the water table, enters into ground water supplies if the underlying soils allow movement.

In locations where the water table slopes away from the wetland, surface water in the wetland is relatively temporary. Because much of the volume may be contributed to recharge of ground water supplies. Draining these wetlands eliminates their recharge capacity and may adversely affect the surrounding soil moisture during dry periods.

Where the water table slopes toward the wetland, ground water discharges into the wetland (Figure 4). The water in this wetland is relatively permanent. Draining wetlands with ground water discharge capacity actually increases ground water discharge initially. However, over an extended period local water tables may be lowered.

Figure 4. Wetlands located lower than the water table can receive ground water discharge.

Seasonal rainfall patterns may influence the direction of ground water flow within a wetland. During the spring, when water inputs are high, the wetland water level may be higher than the water table. At this time, the wetland acts as a point of recharge as water seeps from the wetland into the ground water. As the summer progresses, wetland water levels might drop to a level below the water table. Ground water then flows back into the wetland, which now serves as a point of ground water discharge (Figure 5).

Figure 5. In many instances the same wetland may serve both functions. The water table slopes into a portion of the wetland and slopes away from the rest of the wetland. Where this "through flow" condition exists, wetlands are often referred to as semipermanent.

Wildlife Protection

The appearance, character and function of wetlands vary depending on the depth of the water, length of flooding and characteristics of the surrounding land. The different types of wetlands provide a unique array of habitats for many species of wildlife (Table 2).

Wetlands which do not contain standing water all year still provide valuable wildlife habitat. The vegetation growing around the wetland edge serves as food and cover for many wildlife species, particularly during migration.

As an example, many small aquatic invertebrates are produced during the wet spring period. They survive the dry months by going into a dormant stage. These invertebrates hatch the following spring when the wetland contains water. The hatching usually coincides with migratory waterfowl's northward journey.

Shallow water wetlands, which hold water throughout the year, contain emergent, submerged and floating vegetation throughout most of the marsh. The vegetation supports a variety of wildlife species (Table 2).

Table 2. Benefits To Some Common Wildlife Species Provided By Wetland Vegetation

TypePlants around wetland edges Emergent, submerged and floating vegetation
in shallow water areas

Requirement:Food and CoverFood and Cover
 rabbits waterfowl & broods
 quailmuskrats mink otters fish insects
 pheasants song birds song birds: red-winged blackbird,
 Hydrilla common yellow throat, marsh wren

Submerged and emergent plants around the edges and shallow areas of deep water wetlands, provide food and cover for wildlife. In addition, the deep water area may furnish a suitable habitat for fish and often offers a source of recreation such as fishing, canoeing and swimming.

Preserving Wetlands

Wetlands play an important role in the freshwater system. They positively contribute to the quality of both surface and ground water supplies. In addition, wetlands provide habitat to many different species of wildlife.

In 1988, the U.S. Fish and Wildlife Service established a program in Indiana to assist landowners in restoring wetlands. For more information on the Wetland Restoration program contact: U.S. Fish and Wildlife Service, 718 N. Walnut Street, Bloomington, IN 47401, 812/334-4261.


Van Der Valk, A., Northern Prairie Wetlands,
Iowa State University Press, ed. 1989, Ames, Iowa, 400 pp.

Mitsch, W.J. and J.G. Gosselink, Wetlands,
Van Nostrand Reinhold, New York, New York, 1986, 537 pp.

This material is based upon work supported by the U.S. Department of Agriculture, Extension Service, under special project number 90-EWQI-1-9242.

Cooperative Extension work in Agriculture and Home Economics, state of Indiana, Purdue University, and U.S. Department of Agriculture cooperating; H. A. Wadsworth, Director, West Lafayette, IN. Issued in furtherance of the acts of May 8 and June 30, 1914. The Cooperative Extension Service of Purdue University is an affirmative action/equal opportunity institution.