Targeted Wetlands Improve Water Quality
Prior to European settlement, much of north-central Iowa was covered with a prairie ecosystem containing a large number of seasonal and permanent wetlands. To facilitate the cultivation of domesticated crops, thousands of miles of drainage infrastructure including ditches and underground drainage tiles were installed to drain wetlands and convey water off the land. The result today is an outcome of over 100 years of growth and maintenance of a drainage network that supports one of the most productive agricultural landscapes in the world. Good soil fertility, favorable climate, adequate drainage, advanced genetics and skilled crop management produce fantastic yields of corn and soybeans. Unfortunately, this production system can be subject to nutrient losses, particularly nitrogen as nitrate, when water leaves fields through drainage infrastructure.
Recently, state agencies and Iowa State University (ISU) developed an overarching strategy to assess how to reduce nutrient loads leaving the state through waterways that ultimately flow into the Gulf of Mexico. For non point sources, primarily cropland, this document identifies a number of in-field and edge-of-field practices that can reduce losses from fields or remove nitrate within the drainage infrastructure before it enters a larger stream network.
One practice identified that has been used in Iowa for more than a decade is targeted water quality wetlands. Wetlands in general can provide a number of positive water quality and wildlife functions. What makes these wetlands different is that they are sited at the outlet of drainage infrastructure and designed to maximize the removal of nitrate before the water continues downstream. This effort was pioneered by the Iowa Department of Agriculture and Land Stewardship (IDALS) and demonstrated by IDALS and their research collaborators.
Through the Conservation Reserve Enhancement Program (CREP), IDALS worked with United States Department of Agriculture, local soil and water conservation districts, and local drainage districts to construct 72 of these targeted wetlands in north central Iowa counties since 2001. Targeting and designing for maximum water quality and other benefits includes a number of considerations. To respect upstream drainage rights, the permanent wetland pool is at least one foot below any incoming drainage tile. Pool depth is designed to be shallow with no more than 25 percent of the pool area deeper than three feet to encourage rooted wetland vegetation and a mix of deep and shallow water wetland habitat.
New sites are incorporating shallow berms within the wetland pool to discourage short-circuiting of water through the wetland, ensuring the greatest opportunity for nitrate removal. The size of the wetland in relation to the watershed that drains to it is between 0.5 and 2 percent. These wetlands should accommodate a minimum 150 years of sediment storage. Careful planning and design successfully integrates targeted wetlands into this working agricultural landscape.
ISU measures and the impact of these sites through monitoring and modeling. Studies referenced in the Iowa Nutrient Reduction Strategy estimate 52 percent removal of the nitrate in tile water entering a wetland covering only 0.785 percent of the total watershed area.
This program has great landowner interest and more sites continue to be developed as funding is available. The number of potential sites, however, dramatically outweighs current funding resources. Many more sites could be established if additional or alternative funding resources are developed to meet demand.
To date 640 acres of wetland pool and 2160 acres of buffer areas surrounding these targeted wetlands are in place. These 72 sites remove 500 tons (1 million pounds) of nitrogen annually that would otherwise continue downstream, making them some of the hardest working features of the landscape in terms of reducing Iowa’s nutrient load into the Mississippi River basin. Along with these water quality benefits, wetlands provide wildlife habitat and aesthetic values. These benefits are an important part of Iowa’s natural heritage.
This practice is only one approach to the enormous task of reducing export of nitrogen through Iowa waterways. But it demonstrates how an undervalued landscape feature largely removed from the landscape is being thoughtfully repositioned to reduce nutrient export from tile-drained cropland. For more information on targeted wetland monitoring and performance visit CREP’s website.
For more information about the CREP program visit the IDALS water resources bureau.