Nearly a decade ago, Mike Bravard agreed to have a backhoe dig a large hole alongside one of his soybean fields in Greene County near Jefferson. The hole would become the first bioreactor installed in Iowa.

Last week, Bravard’s bioreactor was excavated to remove what was left of the original wood chips that have been working hard and decomposing for nearly a decade.

A denitrification bioreactor is an edge-of-field structure designed to reduce nitrates in subsurface drainage before the water enters a river or stream. It consists of a shallow pit,...