How Des Moines Water Works is treating nitrates in the water

Jun 24, 2025

Over the past several weeks, nitrate levels in the Des Moines metro's drinking water have been a hot topic of discussion. A lawn watering ban that was implemented June 12 prompted rumors of a potential ban on drinking tap water.

During a tour of the laboratory in the Des Moines Water Works Fleur Drive Treatment Plant on Tuesday, June 24, Central Iowa Water Works Executive Director Tami Madsen reassured residents that the facility was doing everything in its power to keep drinking water safe. CIWW does not have plans to ban drinking tap water, she said, though its first-ever ban on lawn watering remains in place.

Here's what else we know about the ongoing ban and nitrate testing.

At the Fleur Drive Treatment Plant in Des Moines, samples taken from the Raccoon and Des Moines rivers are filtered before they're run through a machine called an ion chromatograph, said chemist Lisa Morarend. The chromatograph is capable of accurately reading nitrate levels in water up to 0.01 milligrams per liter.

Once water is tested, it flows through removal vessels that contain a sodium chloride-coated resin material. This resin captures the nitrate ions and releases chloride ions into the water in a process called ion exchange. The process works similarly to a home water softener system that removes calcium and magnesium ions in exchange for sodium, according to Melissa Walker, communications adviser for CIWW.

When the nitrate-capturing resin in the removal vessels is cleaned, the waste is sent to the Wastewater Reclamation Authority for further treatment.

Much of the drinking water in Iowa comes from groundwater. However, due to the large area that CIWW serves, that groundwater simply isn’t enough. To supplement that source, CIWW draws in water from the Raccoon and Des Moines rivers.

Anhydrous ammonia, a substance commonly used by farmers to fertilize their fields in the spring, is a water soluble compound. When it rains, the substance is readily converted into nitrate, which is picked up by runoff that quickly moves into rivers and streams.

Due to the nature in which these nitrates are transported from field to river, the rate at which they contaminate local water sources is highly dependent on the amount of rain the area receives in a given year. In times of drought, nitrate levels tend to remain low due to the lack of farm field runoff. However, during periods of continuous rainfall, like what the Des Moines area has seen in recent weeks, nitrate levels trend higher.

“This year we’re having specific problems because we’ve had a number of drought years, so that nitrate didn’t get washed off into the rivers and streams,” Morarend said. “We’re seeing a lot more of it getting washed off into our rivers and it’s hanging around for quite a while.”

This is not an uncommon phenomenon, Morarend said, citing a graph tracking nitrate levels in the Raccoon River starting in 1974 and continuing up through June 23. Nitrate levels in the river spike and fall, but the average nitrate level has hovered around 6 milligrams per liter since 1974.

The facility can adjust how much it treats based on the concentration of nitrates in the water flowing through the plant in order to maximize efficiency.

Earlier this spring, Morarend said, when there was less nitrate runoff, the plant wasn’t operating daily or at full capacity. Now that nitrate levels have increased, the nitrate removal system has been running for more than 65 days.

Water exiting the plant is currently sitting at about 8.01 milligrams of nitrate per liter, compared to the 14.17 milligrams per liter coming into the plant from the Raccoon River, and 13.9 milligrams per liter from the Des Moines River. The safe drinking water standard is 10 milligrams of nitrate per liter.

The average level of nitrates in water in the rivers has been slowly on the rise over the past few decades, according to the graph, but Madsen said Des Moines residents should not be concerned.

“Thankfully the lawn watering ban has been incredibly effective so we have not exceeded the drinking water standard of 10 milligrams per liter,” Madsen said Tuesday. “We do not anticipate needing to put any kind of further restriction or ban whatsoever.”

Morarend said she sees no need for Des Moines residents to purchase in-home water treatment systems, adding doing so has the potential to cause more harm than good. Home testing kits, she added, also are an unnecessary purchase.

“There are some things you want left in the water,” Morarend said. “You have some minerals that are naturally occurring in the water that you want to be there. … Also, if you’re not operating (the home treatment system) properly, you can cause issues with making the water corrosive and harming your plumbing system.”

Though water is safe to drink, with the plant continuing to operate at such a high capacity, CIWW's lawn watering ban remains in place.

Norah Judson is a reporter for the Register. Reach her [email protected].