Iowa farms use low-tech filters to remove nitrates contaminated with fertilizer from water

Iowa farms are using low-tech systems to filter nitrates from water. Polk County is paying for farms to install the systems in order to ensure a cleaner water supply in the state.

Nick Helland’s farm in central Iowa is no different from any other farm nearby on a chilly day in March. Corn stubble stretches up a gravel-road and over a small hill, to the northern horizon.

Look closely and you’ll see mud patches where crews, a few years ago, buried bioreactors (low-tech systems) and buffers along streams that remove nitrates in the water flowing from Helland’s field to Big Creek and then on to the Des Moines River.

The underground devices are effective. The underground devices work.

Polk County makes it easy for farmers by handling all logistics and arrangements and paying $1,000 per site. In the last two years the number of installations has exploded, from a few to 104.

Helland explained, “They paid for all of the installation and I was paid.” It’s a no brainer for me to install this on my farm. I will have better water quality downstream.

It is now up to counties to fund and launch similar initiatives to reduce runoff on Iowa’s 10,000,000 acres of tiled-drained farmland, and to combat the multi-billion-dollar problem of nitrogen pollution in the state.

Manure and nitrogen-based fertilizers can cause excessive levels of nitrates to be found in the groundwater, which can be toxic for livestock and humans. Since decades, chemical fertilizers and animal waste sprayed onto fields have caused high levels of nitrates in waterways throughout Iowa and the Midwest. Farmers can now use modern tractors to assess the soil and only apply as much fertilizer is needed. However, it is still common for farmers to overspray.


Why? It’s not hard to understand. Fertilizer doubles the yield of corn, which is a king crop here and is planted on 90 million acres in this area. Farmers want to ensure their crops are getting enough nutrients. The quick drainage systems, also known as tiles but in reality plastic pipes, that are under so many fields and shunt excess water into streams add to the problem.

Many studies have shown that low-tech systems can remove up to half of the nitrate from runoff, before it reaches the waterways. In bioreactors the water is passed through a mound of wood chip that breaks down a large amount of nitrate. In buffers, the water moves along a grassy strip parallel to a stream.

A surplus of nitrates and phosphorus in rivers and streams encourages algae growth and other plants that reduce oxygen levels in the water. This also blocks sunlight. Industrial farming practices have altered waterways, straightening streams, and removing wetlands. This is bad news for the fish who need clear water with slower currents.

On March 28, 2023 near Roland, Iowa, a worker shovels wood chip into a bioreactor trench on a farm field. Low-tech systems, such as bioreactors and buffers along streamsides, help filter out nitrates before they reach rivers and streams. (AP Photo/Charlie Neibergall)

Humans are also affected. Drinking water contaminated with nitrates can lead to blue baby syndrome, where the infant’s blood lacks oxygen. According to the state, more than half of Iowa’s rivers, lakes and streams are too polluted for aquatic life, fishing or swimming.

Iowa contributes the most to nitrates flowing into the Gulf of Mexico, causing the dead zone in the Gulf by reducing oxygen for marine life over several thousand square kilometers.

In 2008, the Iowa agriculture and natural resource departments joined Iowa State University to develop a strategy for the reduction of dead zones. Iowa’s legislator has rejected any proposals that would require farmers to reduce their runoff.


According to a 2019 estimation, Iowa’s nitrogen runoff has not been significantly reduced 15 years after the program began. In some ways, the problem has gotten worse as farmers have planted more corn and soybeans due to strong commodity prices. Iowa’s hog population has increased to 24 million pigs, which is roughly three times the number of pigs in any other state. This means that more manure will be spread on farmland.

In Polk County the frustration with nitrate contamination reached a boiling point in 2015. The agency responsible for providing drinking water to over 600,000 residents in the Des Moines region went to court to protest the millions it had to spend on filtering unsafe levels of nitrates from drinking water sourced from the Des Moines River and Raccoon River. The lawsuit was dismissed by a judge against three counties in northwest Iowa, stating that the Legislature should address the issue.

Polk County officials, without the hope of a state mandate, sought to cooperate with agricultural groups. In order to understand why so few farmers installed bioreactors or buffers along streamsides, part of the study was done. They discovered an inefficient installation system that was costly and time-consuming for farmers who had to hire contractors and seek reimbursement.

Polk County’s Solution: Take care of all the details to make things easy for farmers and group projects for economies-of-scale. The new process is 15% cheaper than the old one, even with the $1,000 incentive to encourage farmers to sign up. It costs less than $10,000 to create a saturated buffer and as much as $15,000 to build a bioreactor.

John Swanson Polk County’s Water Resources Supervisor said, “We realized we were doing things wrong for six years before we achieved our success.”

Iowa Agriculture Secretary Mike Naig has supported Polk County’s efforts and encouraged them elsewhere. He has been a strong opponent of requiring farmers filter runoff. He promoted bioreactors, buffers, and other conservation measures at an event held in Story County north of Des Moines. Conservation officials there have adopted this new program.

Naig explained, “We make it easy for the landowner to accept and then bring in the resources.” These are almost 100% paid for. It doesn’t matter which way you look at it, work must be done. And if willing landowners, producers and others are willing to help, then that is much more effective.

Clean water advocates point out that Iowa requires thousands of systems to be added every year, and not hundreds. They also question whether voluntary efforts will reach a small portion of farms in the state, let alone those of other states.

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“There are a lot people doing great work,” said Alicia Vasto. She is the director of the Iowa Environmental Council’s water program. The truth is, it’s not happening at the speed and scale necessary to solve the problem.

The projected costs of scaling up are staggering. A 2017 analysis revealed that the upfront costs for reducing nitrogen and phosphate runsoff could reach $4 billion. This would include over 100,000 bioreactors for runoff on 2/3 of tile-drained farms, as well other solutions like cover crops.

Swanson, Polk County’s official, now works with state officials on building more wetlands. These wetlands cost more, require more space, but filter more runoff than bioreactors or buffers. Helland is interested in a wetlands on his land and he wants farmers to contribute more. However, he believes that efforts should be voluntary. He said that each farm is unique and that requiring farmers to take action could lead to more problems.

Jerry Hill, a farmer for 52 years who attended the Story County meeting, is considering installing a bioreactor near a creek bordering his property. He liked the idea that filtering water would be inexpensive to him.

Hill stated that “we’re going have to do better work keeping things clean.” What I heard is that what they are doing now is the best.

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