Aerial view of the Franklin Research and Demonstration Farm. The three constructed wetland complexes are visible, along with other conservation practices used on the farm (woodland savanna, prairie restoration, floodplain wetlands, grassed waterways, and buffers). Photo by Tim Lindenbaum.
Long-term study shows constructed wetlands provide meaningful water quality and wildlife benefits on Central IL farm
By: Adrienne Marino | Water Quality Program Manager, The Nature Conservancy in Illinois
In 2005, The Nature Conservancy (TNC), began a long-term study to understand if, and how much, constructed wetlands can reduce agricultural nutrient losses from tile drainage on a Midwest row crop farm. Recent analyses of data collected over a 12-year period and published in the Journal of Environmental Quality shows that these treatment wetlands are very effective both in removing nitrate and in retaining dissolved phosphorus delivered from adjacent row crop farm fields via tile drains.
Study Objectives and Research Design
TNC’s Dr. Maria Lemke, Director of Science for the Illinois chapter, and Krista Kirkham, Aquatic Ecologist, led the long-term study, designed to:
- Quantify the effectiveness of tile-treatment wetlands for nutrient removal
- Investigate factors affecting nutrient loadings and wetland effectiveness
- Document the longevity of constructed wetlands as a conservation practice to treat tile runoff.
Three wetland complexes were constructed at the Franklin Research and Demonstration Farm in Lexington, IL. Each complex is designed with three wetland cells separated by berms but connected by tiles that run through the berms. Each wetland cell represents 3% of the tile-drained farmland area flowing into the complex (Figure 1). This design allows researchers to measure and compare wetland effectiveness of one cell (3% of drainage area), two cells (6%) and three cells (9%). The study design also called for installation of new tile in the fields draining to the wetlands and the addition of water quality and flow monitoring equipment at the inlet and outlet of each wetland cell.
The Franklin Farm study highlights that edge of field constructed wetlands were highly effective in the removal of nitrate and dissolved phosphorus from tile drainage systems over a 12-year period (Figure 2). The study’s findings are especially notable due to the long-term nature of the research, the focus on wetland-to-tile drainage-area ratio, and the generation of new information about the value of constructed wetlands as a tool for phosphorus removal at sites with suitable soils.
How long will constructed wetlands provide water quality benefits?
Maintaining the infrastructure and personnel to support a research project over many years – in all seasons – can be challenging but it is worth it. This will provide a complete picture of how the wetlands function under a wide range of weather conditions, precipitation patterns, and farming practices. The data from the Franklin Farm show definitively that constructed wetlands begin providing effective nutrient removal soon after implementation, and that they maintain their efficacy throughout the study during both corn and soybean years. They continue to function as expected over a wide range of observed weather and precipitation patterns, including during periods of wet weather, and following periods of drought.
How large does a wetland need to be?
As an edge of field conservation practice, a constructed wetland will often require a producer or landowner to remove land from crop production. A key objective of the Franklin Farm wetland study was to evaluate the effectiveness of tile-treatment wetlands in relation to wetland size and tile-drainage area. In the study, the smallest wetlands, representing just 3% of the tile-drained area, reduced nitrate losses from tiles by 15-38% and captured 53-81% of the dissolved phosphorus. The findings suggest that wetlands do not need to be large to provide a valuable nutrient removal function. Further experience installing more than 20 constructed wetlands on other Central IL farms shows that wetland placement is quite flexible, and that designs can be customized to fit wetlands into less productive areas of fields or into areas that are difficult to farm, helping to reduce some of the economic barriers associated with the practice.
Can constructed wetlands be used to address dissolved phosphorus loss?
The Illinois Nutrient Loss Reduction Strategy, which seeks to achieve 45% reductions of nitrogen and phosphorus loads to the Gulf of Mexico, includes constructed wetlands as a practice expected to reduce nitrogen loads, but does not list the practice as one providing phosphorus-removal benefits. While research on the phosphorus removal efficiencies of constructed wetlands has been limited, there is growing interest and need for understanding which conservation practices can retain dissolved phosphorus carried by tile drains. Dissolved phosphorus is a highly bioavailable form of phosphorus that contributes to rapid plant and algal growth, even in very small amounts.
In the constructed wetlands, which only receive inputs from the inlet tiles, dissolved phosphorus can be taken up by plants and aquatic organisms, providing short-term phosphorus retention, or it can be retained long-term in the soils. Long-term phosphorus retention occurs when soils have high clay content and when they have high levels of aluminum, iron, or calcium.
This research shows that constructed wetlands can be an effective tool for reducing dissolved phosphorus export to surface waters when placed in areas with suitable soils. A soils analysis of potential constructed wetland sites is a critical step for understanding if a wetland is likely to act a phosphorus sink (retains phosphorus) or a source (exports phosphorus).
Current and Future Research
TNC scientists are building on this research to determine if stacking infield conservation practices, such as cover crops and spring/split fertilizer applications, combined with edge of field practices can further reduce nutrient losses. By demonstrating nutrient removal efficiencies of stacked practices, it may be possible to reduce the size of the wetland needed to treat tile drainage, bringing down installation costs and further minimizing loss of farmland acres.
About the Franklin Research and Demonstration Farm
The Franklin Research and Demonstration Farm in Lexington, IL is a collaborative effort between TNC, University of Illinois Champaign-Urbana, Illinois State University , McLean County Soil and Water Conservation District, McLean County Natural Resources Conservation Service and the Franklin family. It serves as a model for sustainable agriculture, innovative research, habitat conservation, public outreach and education, and successful partnerships.