A fertilizer spill caused a massive fish kill in southwest Iowa last month. According to a recent Iowa DNR press release, agricultural company NEW Cooperative Inc. alerted the Iowa DNR of the inadvertent release of fertilizer on March 11. Around 1,500 tons or 265,000 gallons of liquid nitrogen fertilizer discharged into a drainage ditch before reaching the East Nishnabotna River. The release occurred after a storage tank valve was left open for a weekend. 

Officials immediately responded to the scene to stop the discharge and begin cleanup, but it appears they arrived too late to mitigate the effects of the spill. The liquid nitrogen killed almost all of the aquatic life in a 60-mile stretch of the East Nishnabotna and Nishnabotna Rivers downstream of the spill—all the way to the confluence of the Nishnabotna with the Missouri River in the state of Missouri. 

“I refer to this one as ‘the big one,’” Matt Combes, an ecological health unit science supervisor for the Missouri Department of Conservation, told The New York Times. “A near-total fish kill for 60 miles of a river is astounding and disheartening.”

The Iowa DNR estimates that at least 749,000 fish died in the incident as of March 28. Combes says another 40,000 died in Missouri. In Iowa, the majority of the deceased fish were minnows, shiners, dace, or chub, though thousands of channel catfish and at least 264 flatheads were killed, too, along with carp and sunfish. Combes reports that the fish kill hit catfish and sturgeon in his state. Frogs, snakes, mussels, and earthworms were also impacted. 

“The whole river was full of dead fish,” Iowa angler Todd Meyer, who captured a video of the carnage, told the Iowa Capital Dispatch. “It was just nuts.”

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Officials will continue to assess the ecological impacts of the spill. Iowa state code prohibits the discharge of a pollutants into a river. Iowa DNR field staffers are working with the agency’s legal team to determine potential restitution for the “lost aquatic life.” Meanwhile, officials are cautioning the public against recreating on or near the river.