Road Ditches Reduce Nutrients

May 3, 2019

Keith Schilling stands in a road ditch examining soil samples on a table.

IIHR’s Keith Schilling led a recent study examining the role of road ditches in nitrate removal.

New research suggests that water quality challenges in Iowa and nationally would be even greater if it weren’t for a little appreciated feature of the landscape — road ditches.

“Road ditches are human-made, hydrologic pathways we take for granted,” says IIHR Research Engineer Keith Schilling, state geologist and director of the Iowa Geological Survey at the University of Iowa, who led the study funded by the Iowa Nutrient Research Center at Iowa State.

“They are everywhere, criss-crossing our state,” Schilling says. “Our research shows they are already working to help filter our water and raises the question: How can we better realize their potential?”

The scientists studied six ditches along paved and gravel roads in eastern Iowa’s Lime Creek Watershed in Buchanan County, where nearly 80 percent of the land is in row crops. Using detailed geographic information system data, Schilling’s team estimated that about 30 percent of the watershed drains to road ditches. They analyzed soils, vegetation, and surface and groundwater levels in the ditches. They monitored water quality upstream, midstream, and downstream, looking at levels of nitrate-nitrogen, phosphorus, dissolved oxygen, salts, and heavy metals.

Based on similar studies in other parts of the country, Schilling says the team was not surprised to find soil and groundwater conditions within the ditches favorable for denitrification. They were surprised to see the extent of ditches’ processing capacity. Nitrate concentrations decreased an average of 60 percent in subsurface water from upstream to downstream locations in four of the six ditches. In the other two, nitrogen levels in water coming into the ditches were low enough that the ditches’ additional nutrient processing capacity did not have a significant impact.

At nearly all sites, the ditches had rich, loamy, organic soils and shallow water tables that provided adequate anaerobic, or oxygen-poor, subsurface conditions for denitrification. “We found this occurring at levels comparable to wetlands constructed to filter nitrate from agricultural drainage,” Schilling says.

While the ditches were effective at removing nitrate, the research found they did little to alter phosphorus levels in water flowing into them. Nor did they reduce concentrations of heavy metals, which were generally the same as in surrounding soils.

Schilling says the ditches in the research were typical of Iowa, but he cautioned that this was a limited study. Further research could explore increasing ditches’ nitrate reduction capacity through modifications like increasing retention time and infiltration of flow. This could be done, he says, by adding features such as check dams or swales or by using two-stage ditch designs that have shelves planted with vegetation to increase interaction of water with biologically active plants and microbes. Also, where ditches have the potential to handle more water, additional drainage could be directed into them.

“Many of these changes could be relatively inexpensive and easy to incorporate into road ditch design and maintenance projects,” says Schilling.

Another way to increase a ditches’ nutrient processing ability may be to populate the ditches with different plant communities. Though the research did not find vegetation in the ditches — primarily introduced species like brome and reed canary grass — to significantly influence water quality at the study sites, they suggest this deserves further study.

Two research papers from the study were recently published in the Science of the Total Environment. Schilling’s co-authors were Matthew Streeter, soil scientist with IIHR—Hydroscience & Engineering at the University of Iowa; Martin St. Clair, professor of chemistry at Coe College; and Justin Meissen, research and restoration program manager at the University of Northern Iowa’s Tallgrass Prairie Center.

Schilling says the team is communicating its findings to transportation agencies and other groups, including county road engineers who have led the way on roadside vegetation management. He hopes to develop pilot projects to test design modifications that enhance nitrate processing in ditches.

“This research shows road ditches are already inadvertently functioning as an edge-of-field conservation practice in many of our watersheds,” Schilling says. “I am hopeful that, with just a little more research and planning, we can manipulate them to do even more for us — and for a very reasonable cost.”