Underground drainage tiling apparently failed and formed a sinkhole in a corn field in northwest Iowa that allowed manure to flow into a creek, where it killed small fish for miles, according to the Iowa Department of Natural Resources.

The manure came from Roorda Dairy west of Paullina, which has about 4,200 dairy cattle, DNR records show. The dairy was irrigating a field with manure and didn’t notice the damaged tile line because of standing corn, said Tom Roos, a senior environmental specialist for the department.

“With all these rain events, a tile line must have burst,” he said.

Paullina is in O’Brien County in far northwest Iowa, parts of which have suffered from severe flooding in recent weeks. The pressure from influxes of water into tile drains can cause weak areas to rupture, after which soil can flow into the tiling and create a sinkhole.

On Wednesday, someone noticed dead fish in nearby Mud Creek and reported it to state law enforcement. A DNR investigation found that the manure had been flowing into the creek through the drainage tiling and killed fish along about 9 miles of the stream, until it met another creek.

It’s unclear how many gallons of manure might have spilled into the creek and how many fish it killed, Roos said. An investigation is ongoing.

The dairy promptly plugged a culvert to prevent more manure from reaching the creek, Roos said. He did not know how long the spill had occurred.

Roorda Dairy paid a $5,000 fine to the DNR in 2022 for starting construction on a manure digester without state approval. Digesters use bacteria to consume the manure and emit flammable gases such as methane that can be used to heat or power farm operations.