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Box it up

Researchers say ‘control boxes’ could limit nitrate run-off from tile drains

Tile drainage is a boon in a wet spring but it can also increase nitrate run-off into nearby waterways and eventually lakes and even the ocean.

The answer, when the problem was first discovered in the 1980s, was to develop “edge of field” practices, keeping saturated strips of natural landscape near streams to remove nitrates, says Tom Isenhart, a professor at Iowa State University.

“The initial study on saturated riparian buffers was promising,” says Isenhart. “And it led the USDA to develop a conservation standard for the practice.”

The problem is those practices weren’t working in many areas, because the drains had been installed decades ago to run under the buffers.

Isenhart and his colleagues hit on a solution: to use a control structure to divert a portion of this water into the soil beneath the buffer strips.

“We excavated the tile outlet within the buffer strip,” says Isenhart. “Then we installed a control box with outlets attached to new perforated pipe that serves as a distribution pipe.”

Since the first installation they’ve expanded their study to include more sites with different soil conditions.

Isenhart and his team calculated the costs of nitrate removed for various nitrate removal systems. The cost per pound of nitrate removed for saturated riparian buffers was $1.33 (or per kilogram, $2.94), with a 40-year lifetime. Denitrifying wood chip bioreactors have a lifespan of around 10 years (before the wood chips need replacing) and have a cost of $0.95 per pound (or per kilogram, $2.10) of nitrogen removed.

“Saturated riparian buffer strips are comparatively simpler, cheaper, and quicker to install than many systems,” says Isenhart. “But they are not suitable for all fields.”

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