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Chris Hay with bioreactor

Chris Hay, environmental scientist with the ISBA, gave a presentation on bioreactors to those attending the Rock Creek Watershed Project tour.

OSAGE | The Iowa Soybean Association (ISBA) is assisting farmers in the Rock Creek Watershed in developing bioreactors and saturated buffers.

The two conservation practices are geared to reduce nitrates that can leave fields and infiltrate streams and rivers. The goal is to place a combined total of 25 bioreactors and saturated buffers in the watershed.

Chris Hay, environmental scientist with the ISBA, recently gave a presentation on bioreactors to the Rock Creek Watershed Project tour.

Each bioreactor, which can serve from 40 to 80 acres of drainage area, is placed at the end of a field. An inlet and an outlet control structure are then placed at the ends of a 20-by-100-foot pit which is 4-1/2 feet deep. The pit is filled with quarter-inch to one-inch woodchips and a geo fabric is then placed over the chips. Finally, 12 inches of topsoil is then placed over the fabric and seeded with grass.

A field’s tile drainage system is connected to the inlet control structure, and the outlet control at the opposite end of the pit is hooked to an outlet tile which transfers processed water to a natural water drainage source, such as a stream.

The bioreactor does not process surface runoff, only water that has been collected in tile lines. The water flows from the field’s tile through the control structure and into the woodchips. In ideal conditions the water, which carries nitrates from the farmland, will stay in the woodchips from four to eight hours before it exits.

Contrary to some beliefs, nitrates are not filtered off by the woodchips, rather microorganisms feeding on the woodchips in the tightly enclosed pit draw oxygen from the nitrates in the water. As the organisms extract the oxygen from the nitrates, only nitrogen gas remains and it’s released naturally into the atmosphere. Research has proven that a bioreactor extracts an average of 43 percent of the nitrates from the intake water that comes from the field’s tile lines.

When the ground becomes heavily saturated with water and tile lines run full, a bypass system allows excess water to bypass the normal process of the bioreactor, so there are no drainage problems in fields. The cost of a bioreactor is from $8,000 to $10,000 but once constructed there is minimal maintenance.

“The woodchips are only replaced every 10 to 20 years,” said Hay. There are conservation funds available to help defray the initial construction costs.

Kurt Hoeft, district conservationist for the Natural Resources Conservation Service, said, “Bioreactor tests conducted in the Lake Hendricks Watershed near Riceville have shown a 50 to 60 percent reduction in nitrates when inlet and outlet water was tested."

A relatively new practice for eliminating nitrates from streams is the use of saturated buffers. Water from a field’s exit tile line is routed through a water level control structure, which reroutes the water back into a perforated tile that runs parallel to a stream. The perforated tile then allows the water to flow back into the soil which will filter more of the nitrates before the water naturally works its way back into the parallel stream.

The tile is located in a grassland strip next to the stream, and grasses use much of the nitrates and water before they leach into the stream. Limited data on this practice shows a 50 percent reduction in nitrates.

The Iowa Soybean Association has set up the Rock Creek Watershed as a pioneer in these two types of conservation.

“When the project is completed, the Rock Creek Watershed will have a combined total of 25 bioreactors and saturated buffers, which will be more than any other watershed in the US,” said Hoeft.

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