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Iowa needs to cut the amount of nitrate in five rivers -- including the Cedar River -- because of the threat it poses to drinking water, according to the U.S. Environmental Protection Agency.
The Iowa Department of Natural Resources has evaluated that nitrate threat based on how frequently it emerges -- but a recent EPA decision says no frequency is tolerable. Stream nitrate can be highly variable and often spikes during wetter months, when rainfall flushes the crop nutrient from farm fields into rivers.
Nitrate is an important fertilizer for corn, but in can damage aquatic life and is unsafe for humans to consume in certain concentrations.
The Iowa DNR long has used a so-called "10 percent rule" to determine whether it needs to take broader action: when river nitrate exceeds the EPA's safe drinking limit of 10 parts per million more than 10 percent of the time.
But the EPA said in a letter to the state last week that any exceedance of the limit should trigger a plan to cut nitrate because it is toxic. The Iowa DNR has pushed back, saying the change "is not supported by new scientific or regulatory rationales," according to EPA documents.
It's unclear effect the change might have. The Iowa DNR has the power to regulate the amount of nitrate that leaves certain facilities, such as wastewater treatment plants, but those can have a much smaller impact on stream quality than does agriculture. The department cannot force farmers to use less fertilizer or to take measures to prevent nitrate from leaving their fields.
And with President-elect Donald Trump set to return next year to the White House, the EPA could reverse course altogether. He has aimed to relax environmental regulations he considers onerous.
The potential change comes amid a federal review of lists of impaired waters, which states submit to the EPA every two years.
In Iowa, that includes hundreds of stream segments, lakes and wetlands that do not meet water quality standards for recreation, public water supplies and the protection of aquatic life.
As part of the review, the EPA identified six stream segments that it says the Iowa DNR assessed with "the non-defensible use of the 10 percent rule" that the department had commonly used in regard to nitrate. They include:
All of those rivers are primary sources of drinking water for the cities.
Des Moines Water Works draws river water more directly -- and it has a large nitrate-removal facility -- but the others use shallow wells that are recharged by the rivers. The shallow wells buffer the cities from some of the nitrate pollution. The water filters through sediment, where microbes consume the nitrate, before it is pumped from the ground.
The cities can mitigate the incoming nitrate concentrations by switching to less-polluted wells or by using deeper, backup wells that don't have the pollution.
Nitrate concentrations peaked in May in the Cedar River just north of Cedar Rapids at more than 17 parts per million, according to Iowa Water Quality Information System data.
But the highest concentration the city measured in its well water this year was 8.9 parts per million, said Roy Hesemann, the city's utilities director. The city's treated water typically has a similar concentration when it is sent into the distribution system to homes, schools and businesses.
That was a higher concentration than usual, in part because nitrate had accumulated in farm fields in recent drought years and was released by a wet start to the year, Hesemann said. But it was lower than the maximum concentration allowed by the EPA for drinking water, a limit meant to prevent high human exposures that can cause acute problems such as a lack of oxygen in the blood. Chronic nitrate exposure also has been linked to cancers.
"There's always some level of nitrates, even in the dead of winter," Hesemann said of the Cedar River.
There is an existing water restoration plan to reduce nitrate in the Cedar River that was developed by the Iowa DNR about 18 years ago.
Its goal is to reduce the total maximum daily load of nitrate, and the department identified agriculture as the source for most of it. At the time, the Iowa DNR estimated that about 45 percent of the nitrate was from fertilizer, 27 percent was from legumes, such as soybeans, and 14 percent was from manure.
Soybeans have a symbiotic relationship with certain bacteria, which feed on the plant's carbohydrates and produce nitrogen compounds the plant can use. But they can also leach from the ground.
All told, about 86 percent of nitrate in the Cedar was attributable to agriculture. About 9 percent was estimated to be from so-called "point sources" that are regulated by the Iowa DNR.
"The TMDL program is really handcuffed," said Dave Cwiertny, director of the Center for Health Effects of Environmental Contamination at the University of Iowa. "All this is really doing is telling us that, 'Yeah, the water is not in good shape.'"
The plan for the Cedar River sought to reduce ag-related nitrate by about 37 percent through voluntary measures by farmers. An update on that progress wasn't immediately available, but the department moved two years ago to withdraw the plan.
In 2022 the department cited data that -- using the 10 percent rule -- showed the river wasn't impaired to an extent that required a TMDL plan.
There was significant opposition to abandoning the plan, and ultimately the Iowa DNR did not follow through with it.
"We took a step back just so we could reevaluate everything," said Lori McDaniel, the department's water quality bureau chief. "Now we're reviewing the EPA's decision and how they're looking at the data, so we haven't made any new decisions going forward."
The EPA is soliciting public comments about its recent decision regarding the rivers until Dec. 13. The agency said written comments should be emailed to [email protected].