From 2,200 feet in the air, Boone County looks like a green quilt. Square fields of corn and soybeans are stitched together by county roads. On the horizon, the pattern is interrupted by a wide, blue ribbon. It curves back and forth over the landscape, surrounded by a dark mass of trees on either side.
This is the Des Moines River. Stretching from southwest Minnesota to southeast Iowa, it drains some of the most productive farmland in the U.S. into the Mississippi River.
It’s also one of the main drinking water sources for around 600,000 people in the Des Moines area. Elevated nitrate levels in the Des Moines and Raccoon rivers this spring and summer pushed Central Iowa Water Works (CIWW), the regional drinking water utility, to issue its first lawn watering ban from June 12 to July 21.
The restriction dropped demand by more than 30%, allowing CIWW to pump water through extra treatment steps and stay under the federal safe drinking water limit for nitrate
Nitrates in Iowa’s rivers are not a new challenge. But this year’s consistently high levels and water restrictions for nearly a fifth of the state’s population bring the issue to the forefront.
Water quality experts hope this could be a watershed moment — a chance to rapidly scale up practices that have been proven to help.
Iowa’s unique challenge
Iowa’s 2013 Nutrient Reduction Strategy and the recently released Central Iowa Source Water Research Assessment identify agriculture as the primary source of nutrients in Iowa’s water.
In the Des Moines and Raccoon rivers, around 40% of the nitrogen comes from farm fertilizer. Roughly 20% stems from manure and 20% comes from soybeans and other nitrogen-fixing legumes.
The scale of agriculture is an important factor.
Over 60% of the land in Iowa was planted with corn and soybeans in 2024. Iowa is also the top producer of hogs in the U.S., supplying one-third of the national supply. Nearly 25 million hogs, along with other livestock and poultry, produce a lot of manure.
“The elevated levels [of nitrate] in the rivers are the sort of the telltale,” said David Cwiertny, director of the Center for Health Effects of Environmental Contamination at the University of Iowa. “The fact that we're still seeing the levels that we are tells us that the land has plenty of nitrogen to give, and unfortunately, we're seeing how water systems are having to struggle to manage that.”
Plants need nitrogen to grow, and the main way most plants absorb it is in the form of nitrate. Microbes in the soil and the root nodules of legumes transform atmospheric nitrogen into nitrate. Farmers often supplement this process by adding synthetic fertilizer or manure to their crop fields.
Nitrates that are not absorbed by plants easily move with water. They can accumulate in the soil during periods of drought and flush out with groundwater or through “field tile” into streams and rivers during heavy rains.
Field tile is a system of buried perforated pipes designed to move water away from fields to prevent waterlogged soil. Almost half of Iowa’s crop acres have field tile. Much of it is concentrated in central and north-central Iowa.
While nitrates are essential for most plants and everything that eats them, high concentrations in streams, lakes and rivers contribute to harmful algal blooms and put human health at risk.
“Whenever we start talking about [nitrate], it immediately strays into, ‘Why are you anti-farmer or anti-agriculture?’ And it’s not about that,” Cwiertny said. “It’s about trying to figure out how to fix a system that clearly keeps showing us — now two years in a row — that it’s out of balance and it’s putting a larger section of the population in a really tough position in terms of having potentially their drinking water go away,” he said.
Matt Helmers is a professor of agricultural and biosystems engineering at Iowa State University and director of the Iowa Nutrient Research Center. He said one of the misconceptions about water quality in Iowa is that it’s simply an overapplication problem.
“We can make improvements with the application of manure or commercial fertilizer, but this is really more a function of the annual row crop system that we have,” Helmers said. “We need to look at ways that we can increase that living cover out on the landscape and increase some of the edge of field practices.”
“We can make improvements with the application of manure or commercial fertilizer, but this is really more a function of the annual row crop system that we have.”Matt Helmers, professor of agricultural and biosystems engineering at ISU
Helmers co-authored a recent study that found field tile drainage from corn and soybean fields — without fertilizer applied for six years — had an average annual nitrate concentration of 8 milligram per liter.
Another study found the average nitrate concentration in tile drainage from a plot seeded with prairie was well under 1 mg/L, even when the researchers applied fertilizer.
The results illustrate a fundamental part of the challenge, according to Helmers. He said Iowa is largely covered with “an annual copping system, where once there was a perennial.”
Mimicking nature and leveraging microbes
Cover crops mimic the natural system by keeping more living roots in the soil, especially when crop fields are bare in early spring and late fall, according to Helmers. A winter cereal rye cover crop can reduce nitrates by 30-45% compared to a system without the cover crop.
Incorporating prairie strips is another in-field conservation practice that significantly reduces soil erosion and nutrient pollution, while providing wildlife habitat.
Edge-of-field practices, like bioreactors, saturated buffers and multi-purpose oxbows, can help filter tile drainage before it flows directly into a stream.
North of Ogden in Boone County, farm manager Mick Johnston drove an off-road vehicle across a slope of wildflowers and chest-high grass. On the ridge above, corn tassels swayed from a gentle breeze.
Below, water flowed through Middle Beaver Creek, which connects to Beaver Creek and then the Des Moines River on a path downstream to Des Moines and Central Iowa Water Works' treatment plants.
“This farm has been in the family since it was homesteaded in 1881, and this used to be a swamp,” Johnston said. “They farmed the hilltops, and [in] the marshy areas, they raised cattle.”
Later, people in this area dug canals and added field tile to create better growing conditions for crops, he said. The farm still uses tile drainage, but several years ago, a bioreactor and saturated buffer were added.
Johnston walked with Justin Grieff, the Beaver Creek Watershed coordinator, to an orange flag marking a small gray control box for a denitrifying bioreactor. The sound of rushing water could be heard as Grieff removed the cover.
Tile drainage from a nearby corn field flows through an underground pipe into a large pit of buried wood chips. There, microorganisms consume the nitrate and release nitrogen gas into the atmosphere. The filtered water then moves through a pipe that spills into Middle Beaver Creek.
Bioreactors are typically 100 feet long by 20 feet wide. They can treat 30-80 acres and reduce annual nitrate loss by 43% on average.
In a saturated buffer, tile drainage flows through perforated pipes buried underground alongside a creek. The water trickles out into the soil, where microbes consume nitrate as it seeps toward the creek.
Saturated buffers can remove 42% of the nitrate load on average.
“We try our best to be the best stewards of the ground that we can,” Johnston said. “This is a technology that’s been proven and doesn’t interrupt our current production practices, but it lowers our nitrates into the water.”
The two projects were funded by a U.S. Environmental Protection Agency Farmer-to-Farmer grant. The former watershed coordinator approached Johnston, who regularly attended the Beaver Creek Watershed Management Authority meetings after it formed in 2015.
"We’re in this to grow crops and feed the world, and if there’s something we can do better to help, we’ll do it.”Mick Johnston, farm manager in central Iowa
He said people from Des Moines often came to share their concerns about water quality downstream.
“I think those are very healthy conversations,” Johnston said. “Anytime that we can have a dialogue about a challenge, everybody understands that we’re not in this just to cause problems. We’re in this to grow crops and feed the world, and if there’s something we can do better to help, we’ll do it,” Johnston said.
The state of Iowa is investing in a three-year project in the Beaver Creek Watershed to install more edge-of-field practices and promote cover crops.
Johnston emphasized that farmers would rather be part of a solution than be regulated. He said farmers don’t want to spend money on fertilizer that isn’t used by crops and added that they would like to see more research identifying sources of nitrate in Iowa’s waterbodies.
“We’re using less nitrogen per unit of production now than we were in the '70s and '60s, and we’re managing it better,” Johnston said. “We apply our nitrogen four times a year. We put some on right before we plant and then we apply three more times while the crop is up, so we’re spoon-feeding that crop.”
Accelerating Iowa’s Nutrient Reduction Strategy
Iowa’s Nutrient Reduction Strategy laid out practices that “we can do — that we need to do,” according to Helmers.
“We've seen an increase in the implementation of some of these practices that we want for nitrate reduction, whether that be cover crops or bioreactors, saturated buffers, wetlands,” Helmers said.
He added that the state has set up programs, like a cover crop cost-share, and made investments to support these practices.
“Now I hope that we're at the point to really accelerate that rate of adoption,” Helmers said. “As we think toward the next decade, we need to see even more of these practices on the landscape and raise that sense of urgency that we need to be proactive about this.”
The Nutrient Reduction Strategy lays out several scenarios to reduce nitrogen and phosphorus in Iowa’s rivers by 45%. One of the scenarios includes planting cover crops on 60% of all row crop acres and installing bioreactors and saturated buffers on 60% of all tile-drained acres.
In 2023, around 17% of row crop acres were planted with cover crops. Nearly 300 bioreactors and saturated buffers and at least 125 water quality wetlands were installed through the end of 2022. Thousands more will be needed to meet the state’s nutrient reduction goals.
“We still have a long way to go,” Helmers said.
Nitrate’s health risks
Cwiertny said the challenge with adopting conservation practices and applying fertilizer at the optimal rate is that they are voluntary measures. He said the state needs to ramp up efforts to protect source waters and human health.
“I don't think the state of Iowa wants to ever confront 600,000 people that can't drink their water. I don't think we're ready for that,” Cwiertny said. “What are we going to do to make sure we don't get to that point?”
The EPA set a maximum contaminant level for nitrate in public drinking water systems at 10 mg/L in 1991.
“This was largely done for the way that nitrate interferes with — or can interfere with — how oxygen is transported through the body,” Cwiertny said.
Methemoglobinemia is particularly life-threatening to infants. Often called blue baby syndrome, it’s extremely rare today.
But more research over the last three decades indicates lower-level nitrate exposure increases risks for long-term health effects, including colorectal cancer, thyroid disease and neural tube defects.
“It's really led to questions about whether or not the Safe Drinking Water Act standard of 10 mg/L of nitrogen is health protective enough. And so that's kind of where it's stuck right now,” Cwiertny said. “The science has outpaced the regulation.”
In 2010, the EPA said it was considering a new health assessment for nitrate. But the agency was slow to start the process, Cwiertny said. The Trump administration paused the assessment in 2019, and the Biden administration restarted it in 2023, but it hasn’t moved beyond the first step.
One of the challenges with changing the nitrate regulation is the cost to water treatment systems, Cwiertny said. The EPA is required to conduct a cost-benefit analysis as part of its review process.
“It’s just going to be a difficult lift to ever see the nitrate [maximum contaminant] all get lower, even with the science that's out there,” Cwiertny said.
He added that many Iowans are aware the federal 10 mg/L threshold may not be safe enough. Extended periods of high nitrates can erode trust in public drinking water utilities, leading more people to buy bottled water, which has different regulations.
Water providers should have more conversations with consumers about what it would cost for more nitrate removal, Cwiertny said. He added that the state could also support more testing for private drinking wells and at-home reverse osmosis systems.
