Wetlands are going underground and trapping water.

Researchers at the University of Minnesota (U of M) are testing subsurface nutrient-retention basins to purify agricultural drainage water. Like surface-flow wetlands, these basins are designed to trap excess nutrients and sediment, keeping them out of streams and lakes, says Jeff Strock, a U of M soil scientist who is leading the Minnesota research.

Nutrient-retention basins are modeled after subsurface-flow constructed wetlands. As tile water leaves the field, it passes through a bed of soil or gravel, where nutrients are captured and broken down. The basins can be tucked along field edges or ditches, where they don't interfere with cropping. And they may be more efficient than similarly sized surface wetlands, Strock says. The research is part of efforts across the Corn Belt to develop new drainage practices that protect water quality and make economic sense for farmers.

“There is much interest in off-site methods for treating agricultural runoff,” Strock says.

Nutrients coming from Midwest farm fields are one source of poor water quality in the nation's rivers and lakes, and hypoxia — oxygen deprivation — in the Gulf of Mexico.

Nitrate concentrations in agricultural streams frequently exceed drinking water standards, says William Crumpton, a wetlands expert at Iowa State University. “Concentrations in tile drainage water are commonly more than double the drinking water standard.”

In some states, “tile discharge water is being looked at as a point source of pollution,” which could be regulated in the future, says Troy Huntley, program director for the Missouri Corn Growers Environmental Resources Coalition (ERC). That adds impetus to farmers' efforts to improve drainage water quality, he says.

Natural wetlands do a good job of purging pollutants. However, water-filled potholes are often in inconvenient spots — like the middle of fields — “making it hard to farm around them,” Strock says. And restored wetlands that take cropland out of production are expensive for farmers. Edge-of-field practices could be more appealing, Strock says. “The key is to work with approaches that farmers would be willing to adopt.”

STROCK IS TESTING three pairs of basin designs at the U of M Southwest Research and Outreach Center near Lamberton in southwestern Minnesota. Each basin is ½ acre and receives both surface and tile drainage water from 185 acres of corn and soybean ground.

In the vertical-flow system, drainage water enters the basin at ground level and flows down through 2 ft. of permeable soil into narrowly spaced tile. In the horizontal-flow system, drainage water enters the basin below ground level and moves sideways and down through a mixture of soil, sand and gravel. Gates regulate water levels and outflow rates, controlling how long water is retained in the basins.

Water-loving plants growing in the basin beds consume some of the excess nutrients in the drainage water, Strock says. But the main clean-up is carried out by anaerobic microorganisms in the saturated soil, which convert nitrate-N to harmless N gas — just like in a waterlogged cornfield.

The performance of these subsurface treatment systems will be compared to a pair of surface-flow basins, which have permanent pools of water. “We're expecting little sediment loss and good N removal and phosphorus reduction,” Strock says.

The effectiveness of wetlands varies a lot with size, water volume, landscape position, climate, soils, artificial drainage and nutrient loads, Crumpton says. Subsurface-flow basins, with their greater underground area for denitrification, could prove more efficient, acre-for-acre, than similar-sized surface wetlands, says engineer Chuck Brandel of I & S Group, Mankato, MN, who designed the basins. “That's what we hope to learn with the Lamberton research.”

Subsurface-flow wetlands are already being used to clean up wastewater and feedlot runoff, Brandel notes. But the Minnesota experiments are among the first to try this technology on a field scale to treat tile drainage water, he says. Brandel sees them best suited to tight spots “along open ditches and tile mains, where you've got a lower outlet.”

CARMEN START, A RETIRED farmer from Ostrander, MN, has such a spot. It's less than an acre of low-lying ground along a county ditch in southeast Minnesota. The 1916 ditch had fallen into disrepair decades ago, Start says, so the site often flooded and has never been cropped. In 2007, the ditch was rebuilt, and now “farmers are getting their fields back,” says Rick Morrison, Mower County drainage inspector and engineering technician.

Start's wet spot would be a perfect location for a vertical-flow basin to intercept drainage water before it enters the restored ditch, Morrison says. Plans are in the works for a subsurface wetland, which will incorporate an underground trench filled with woodchips to increase the basin's denitrification capacity.

“Rick contacted me to ask if I'd be interested in this,” Start says. Subsurface-flow basins could also be placed underneath grassy ditch buffers, which are now required in Minnesota and other states, says Mark Dittrich, conservation drainage senior planner for the Minnesota Department of Agriculture. One idea is running tile along the ditch for 700 or 1,000 ft., Dittrich says. An outlet control structure would allow the water level to be raised “2-3 ft., right underneath the vegetation that can use the water.”

Because subsurface-flow wetlands aren't permanently wet, they could also be used for grazing, hay or biomass crops, Brandel adds. However, they wouldn't be as good as surface wetlands for wildlife habitat, he notes.

MEASURES TO IMPROVE drainage water quality will have to make financial sense for farmers, “because the land is a farmer's livelihood,” says the ERC's Troy Huntley.

The ERC's Hypoxia Education and Stewardship project is sponsoring research on edge-of-field wetlands at the University of Missouri's Hundley-Whaley research farm near Albany. Wetland cells ranging in size from 0.5% to 1.5% of the drained area are being compared. Researchers want to determine the most cost-effective size for northwest Missouri, Huntley says.

Strock adds, “Farmers want to do the right thing, and we're trying to give them options.”