Using constructed wetlands to protect Louisiana water quality

Linda Benedict, Dans, Darinda, Millhollon, Eddie P.  |  3/1/2010 10:44:35 PM

Eddie P. Millhollon and Darinda R. Dans

Nearly 8 million tons of nitrogen in commercial fertilizers are applied annually in the Mississippi-Atchafalaya River Basin, which makes up about 40 percent of the contiguous United States. The Mississippi and Atchafalaya rivers drain the basin, carrying runoff containing nearly 3.5 billion pounds of nitrogen annually into the Gulf of Mexico.

The influx of nitrogen and other nutrients into the Gulf stimulates algal growth. As algae and other organic matter settle to the bottom of the Gulf, the decomposition process depletes the water of oxygen, resulting in a hypoxic zone a zone having less than 2 parts per million of dissolved oxygen. These low oxygen levels adversely affect aquatic life, sometimes resulting in death, which is why the hypoxic zone is often referred to as a “dead zone.”

This zone reached a record area of more than 8,400 square miles in 2002. Increases in the use of fertilizer, particularly nitrogen fertilizer, in the Mississippi-Atchafalaya River Basin have been implicated as the cause of the increasing size of the hypoxic zone. The creation and restoration of wetlands between farmland and streams have been suggested as one method to reduce nitrogen movement into the Gulf of Mexico.

Louisiana is known for its extensive system of wetlands, bayous, rivers and lakes enjoyed by generations of people for outdoor recreation, giving rise to the state’s slogan of “Sportsman’s Paradise.” Experts estimate Louisiana had more than 25,000 square miles of wetlands during the colonial period of the 1780s. Over the next 200 years, as the population of Louisiana grew, wetlands, regarded as having little value, were drained or filled at a rate of 58 square miles per year so they could be used for agriculture or urban development.

By the 1980s, the area occupied by wetlands had declined by nearly half to 13,726 square miles. Between 1986 and 1997, however, the rate of annual wetland loss in the United States declined dramatically, and wetlands had a small increase from 1998 to 2004, largely because of increasing awareness of their environmental importance. The reversal in the decline resulted from restoration of existing wetlands and creation of new wetlands. Because 80 percent of the loss of U.S. coastal wetlands has occurred in Louisiana, much of the current restoration efforts are focused here.

The Red, Mississippi and Atchafalaya rivers are the three largest rivers in Louisiana. The Red originates in the Texas Panhandle and travels through northwest Louisiana, flowing south-southeast until it joins the Atchafalaya and Mississippi rivers. The Red River drains nearly 7,800 square miles of land area within the Red River Basin in Louisiana and has the highest sediment load in the United States.

The Louisiana Department of Environmental Quality (LDEQ) has divided Louisiana into water quality subsegments, 71 of which are within the Red River Basin. In 2004, the LDEQ indicated 75 percent of the water bodies within the basin were impaired, meaning they did not support their designated use for fish and wildlife propagation.

The Flat River, a tributary to the Red River, extends from approximately 34 miles north of Shreveport to Loggy Bayou, south of Bossier City, draining approximately 117 square miles of land, 60 percent of which is agricultural. The Flat River is one of the water bodies LDEQ identified as impaired. In 2008, the U.S. Environmental Protection Agency (EPA) concluded that for the water quality of the Flat River to meet designated use standards, the amount of nitrogen entering the river must be reduced by 30 percent and phosphorus by 62 percent. In addition, the amount of sediment entering the river must allow the concentration of dissolved oxygen to be maintained at 5 milligrams per liter during the summer.

Constructed wetlands provide an effective and economical way of improving effluent water quality through biological and physical means and are being used worldwide to cleanse effluent from municipalities, rural communities and single dwellings, as well as agricultural and urban storm runoff. The layout of the LSU AgCenter Red River Research Station in northwest Louisiana is an ideal location to determine if a constructed wetland can improve the quality of runoff water before it enters an impaired Louisiana water body.

The station is divided into 4- to 8-acre sections for growing crops such as cotton, soybean, corn, wheat and grain sorghum. Runoff from nearly 400 acres on the station flows to the southeast corner and then enters the Flat River (Figure 1). In 2003, LDEQ provided funding through the Federal Clean Water Act to design and construct a wetland to accommodate that runoff. The primary objectives of this project were to determine if it is practical to construct a wetland of sufficient size to successfully detain runoff from 400 acres of agricultural land and to determine the effectiveness of this system in reducing nutrient and sediment loads before the runoff enters a water body.

The design of the constructed wetland system is shown in Figure 2. Runoff enters the system by way of three grassed drainage ditches. At the point where the ditches enter the wetland, basins were created to allow some of the sediment to settle out before the runoff enters a shallow wetland of approximately 4.5 acres with an average depth of 18 inches.

The shallow wetland was planted with six native wetland plant species and three cultivated grass species. Plants and microorganisms in the shallow wetland provide the biological processes necessary for improving water quality. Depth in the shallow wetland is regulated by two v-notch weirs attached to two 48- inch culverts that connect it to a deep wetland, the final stage of the wetland system.

The deep wetland is approximately 10 acres and has a maximum depth of 9 feet. The deep wetland provides anaerobic conditions for important biological transformations of unwanted chemicals and serves as a “polishing” area where remaining sediment can settle out and further improvements in water quality can occur. As in the shallow wetland, the depth of the deep wetland is regulated by two v-notch weirs attached to two 48- inch culverts. From the culverts, runoff flows to the Flat River.

The effectiveness of the constructed wetland system in improving water quality is determined by sampling at four locations along the path of runoff through the system using automatic water samplers (Figure 2). In addition, samples are collected from the shallow and deep wetlands during times when runoff is not flowing through the system to monitor changes in water quality caused by runoff retention in these two areas.

Figure 3 illustrates the system’s effectiveness in improving the quality of water runoff during the spring and summer in 2008 when field activity, including fertilizer application and soil cultivation, was at its peak. The EPA had stated that sediment flow into the Flat River needed to be reduced for dissolved oxygen content to be maintained at 5 milligrams per liter during the summer, and sediment content in the runoff, as measured by changes in total suspended solids in the shallow and deep wetlands, was reduced an average of 90 percent.

The constructed wetland system also was effective in reducing nutrient levels in runoff before release into the Flat River.

Fertilizer, especially nitrogen, in runoff into tributaries that flow into the Gulf of Mexico has been implicated as the primary cause of the hypoxic zone that occurs every summer off the Louisiana coast. This study monitored two forms of nitrogen. Total kjeldahl nitrogen, which is a combination of organic nitrogen, ammonia and ammonium, was reduced 65 percent as runoff moved from the shallow to the deep wetland and then finally to the Flat River. Nitrate level reductions were greater in 2008 (67 percent) compared to 2009 (1.1 percent), but levels in both years were less than 0.5 milligrams per liter, which is significantly lower than the 10 milligrams per liter maximum allowed in drinking water by the EPA.

Two forms of phosphorus also were measured. Comparing total phosphorus – or all forms of detectable phosphorus present – indicated a 59 percent reduction between the shallow and deep wetlands. Orthophosphate, which is the form of phosphorus readily available to plants, was reduced 45 percent.

Louisiana’s vast network of streams, bayous, rivers, lakes and wetlands supports an incalculable wealth of wildlife and recreational resources. These natural resources must be protected for future generations to enjoy. It’s been said that wetlands are nature’s water-filtering system, and the results of this research demonstrate the wisdom in that statement. The 14.5-acre constructed wetland system in this research project provided significant improvements in the quality of water runoff from more than 400 acres of agricultural land. This is equivalent to 3.6 acres of wetland for every 100 acres of runoff. This system will continue to be monitored to determine the longevity of its effectiveness in improving water quality of runoff from the research station into the Flat River.

 Eddie P. Millhollon, Associate Professor, and Darinda R. Dans, Research Associate, Red River Research Station, Bossier City, La.

(This article was published in the winter 2010 issue of Louisiana Agriculture.)

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