Directing tile water through a grass buffer can significantly improve drainage water quality. This new conservation drainage practice, called a “saturated buffer,” removes nitrates from subsurface drainage water at low cost – without affecting farm field drainage.
A 1,000-ft. saturated buffer along Bear Creek in Story County, IA, removed 100% of the nitrate N that was diverted into it, says Dan Jaynes, a soil scientist at the USDA National Laboratory for Agriculture and the Environment in Ames. That amounted to 550 lbs. nitrate that never reached Bear Creek.
“We’re more than pleased,” Jaynes says. “The practice was more effective than we expected.”
These are theresults from the first year in a three-year saturated buffer study by the Leopold Center for Sustainable Agriculture.
Riparian buffers – a combination of grasses, shrubs and sometimes trees planted along a waterway – are very good at filtering sediment, phosphorus and pesticides from surface runoff.
Buffers are also good at cleaning up nitrates, Jaynes says, but in the heavily tiled Midwest, that function is lost. Water moves from farm fields into subsurface tile lines and is piped under the buffer directly to the outlet. “The buffer is bypassed, so there is no chance for interaction.”
With this new practice, a shallow lateral tile line intercepts the main tile line and redirects a portion of the flow through the length of the buffer, as shallow groundwater. At Bear Creek,the saturated buffer consists of 1,000 ft. of 4-in. perforated drain tile, placed 2.5 ft. deep along the outer edge of the 80-ft.-wide buffer. At this site, the creek bed is several feet lower than the buffer.
A control box at the junction of the main and lateral lines raises the water table under the buffer. The diverted water percolates through the soil, where it is denitrified or taken up by plants, the same as in a natural wetland.
Excess tile water flows into the waterway like normal, “so we don’t impact drainage in the field,” Jaynes says. Last year, the 1,000-ft. buffer handled about 60% of tile flow from a 50-acre cornfield.
The cost of the Bear Creek saturated buffer, expected to last 20 years, was $4,960. In the first year of operation, it cost about 45¢/lb./N removed/year, Jaynes says. That’s very competitive with other N removal practices. Constructed wetlands, for example, cost about $1.32/lb. of N removed, he says.
Saturated buffers are “a great opportunity to build on existing infrastructure,” says Harold Reetz, a consultant from Monticello, IL, who often speaks to farm groups about conservation drainage planning. An estimated 2 million miles of buffers have been planted along waterways in farm country, he says. “There are many areas where this could work.”
Iowa alone has more than 39,000 miles of buffered streams. If just 10% of buffers were saturated, Jaynes says, the practice could keep at least 11 million lbs. N out of streams and rivers each year.
The Bear Creek saturated buffer research will be expanded this year to nine sites in Iowa, Illinois and Indiana through a $400,000 grant from the NRCS and the Agricultural Drainage Management Coalition (ADMC), an industry consortium. The practice will be tested on different soils, topography, cropping systems, drainage water volumes and buffer types, says ADMC’s Leonard Binstock, project manager.
He’s hopeful that saturated buffers could eventually qualify for conservation payments, just as riparian buffers do. “This looks like a real boon to drainage water quality coming from Midwest farm fields,” Binstock says. “It’s a low-cost fix we can put on existing buffer systems to greatly improve water quality.”
For an informational video on the saturated buffer project, see http://bit.ly/LeopoldCenter
•1,000 ft. of 4-in. tile installed $2,300
•Water control box 1,100
•Design 100
•Capital/opportunity cost (4%, 20 yrs) 1,460
Total Cost $4,960
•Amortized over 20 years $248/year
•First year cost/lb. N removed 45¢/lb. N