Institute of Water Research

Several published sources of data indicate that agricultural tile drains may be responsible for the greatest single contribution of phosphorus delivery to Lake Erie. Phosphorus runoff has been a huge issue in the Lake Erie watershed basin, contributing to increased harmful algal blooms and low oxygen concentrations. A three-year study by the Institute of Water Research with collaborators from Adrian College and Water Resource Monitoring Group, LLC, is focusing on using floating wetland plants and a sand and limestone filter bed in a constructed wetland treatment train to reduce nutrient input from tile drainage before entering a tributary to Lake Erie. The project also aims to increase farmer’s awareness and potential adoption of subsurface nutrient treatment systems.

The site is located in the River Raisin Watershed in southeastern Michigan on a farm field northeast of Adrian, MI, that drains into the south branch of the River Raisin. The treatment area overall is a little over 16 acres in a field that grows alfalfa. Water is collected using a flow paced sampling method with automatic samplers. Drainage control structures enable measurements of water flow and concentrations of total phosphorus (P), soluble reactive phosphorus and nitrate coming out of the tile drain and out of each wetland. Five video cameras allow for continuous visual inspection of the site.

In year one, the phosphorus load coming into the first wetland from the tile drain and then leaving that wetland was reduced by 63% for total phosphorus and 53% for soluble reactive phosphorus. Due to restricted flow in the second wetland’s filter bed, nutrient concentrations were not taken. Four native wetland plants (Scirpus atrovirens, Carex vulpinoidea, Ranunculus hispidus, and Juncus effuses) were selected and planted on floating mats with their roots dangling in the water below the mat.

After one year, all species except for the Ranunculus gained weight. Juncus’ biomass increased 38 times its initial biomass, Scirpus, 12 times, and Carex, five times its initial weight. Total phosphorus concentration was measured separately in both plant roots and shoots. All four plants had more phosphorus in their roots than in their shoots. Compared with the phosphorus concentration in control plants that were dried and weighed at the beginning of the season, the concentration after one year increased by over 78% in both Scirpus and Juncus roots, but only by 31% and 25%, respectively in Carex and Ranunculus roots.

To slow down nutrient saturation in the wetland, at the end of the season, the tops of the plants were harvested and the nutrients will be recycled by either spreading them on the farm field or composting the plant material. As the project moves forward, more species of plants will be monitored to get a better understanding of how effective various plants may be at removing P and N from the wetland. The substrate is being replaced in the second wetland to determine its effectiveness in further reducing phosphorus and nitrogen in the wetland. The long-term result will offer another practice for farmers to consider for reducing nutrient runoff from tile drains that enter receiving streams.

The project has received funding from the Fred A. and Barbara M. Erb Family Foundation, USDA Natural Resources Conservation Service, and US Army Corps of Engineers.