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Iowa town’s drinking water shaped by 74-million-year-old crater
As national debate over fluoride intensifies, Manson, Iowa faces the opposite problem: naturally excessive levels
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In a small northwest Iowa town built atop a 74-million-year-old meteorite crater, even the drinking water carries remnants of ancient history.
The crater, known as the Manson Impact Structure, traps groundwater that has been sealed underground for thousands of years. Over long stretches of time, that water dissolves minerals from surrounding rock, leaving the city of Manson, population 1,600, with naturally high fluoride levels that continue to puzzle scientists and shape local life.
“It’s some of the oldest, softest water we’ve ever studied,” said Keith Schilling, State Geologist at the Iowa Geological Survey and the IIHR-Hydroscience & Engineering. “That long contact time underground increases the minerals in the water, including fluoride.”
Fluoride has long been added to public water supplies because the mineral strengthens tooth enamel to protect people who drink the water from cavities. But in Manson’s source water, fluoride levels have reached 10 milligrams per liter (mg/L), far above the Environmental Protection Agency’s (EPA) maximum legal limit of 4 mg/L.
To make the water safe to drink, the state of Iowa mandated Manson install a reverse osmosis treatment system, which filters out contaminants, including fluoride, by pushing water through a semi-permeable membrane. After the system was added, fluoride levels dropped within federal safety limits.
Work will soon be completed on a pipeline to move water from the nearby city of Fort Dodge to Manson to make sure the smaller town has enough supply.
New study of fluoride in Iowa water
A statewide study published earlier this year, led by Darrin Thompson of the University of Iowa’s Center for Health Effects of Environmental Contamination, revealed deeper aquifers across Iowa often contain elevated fluoride levels. The study analyzed thousands of samples from both municipal and private wells and found a consistent trend: the deeper the groundwater, the higher the fluoride.
The deepest aquifer studied, the Cambrian-Ordovician, which supplies Manson’s groundwater, has an average well depth of 475 meters and showed the highest mean fluoride concentration at 1.18 mg/L. Statewide, the maximum raw groundwater concentration detected reached 11.2 mg/L. These findings suggest that many communities across Iowa, and possibly beyond, may face similar challenges with elevated fluoride levels in their natural water sources.
“Some of these wells are tapping very old groundwater,” Thompson explained. “In certain areas, fluoride levels can exceed what’s considered safe.”
The findings come at a time when fluoride in public water systems is drawing renewed attention. Unlike most communities that add fluoride to promote dental health, towns like Manson face the opposite problem: naturally excessive levels that require treatment. National recommendations have also shifted.
National debate over fluoride
In 2015, the U.S. Public Health Service lowered the ideal fluoridation target from 1.2 mg/L to 0.7, reflecting a more cautious approach to balancing benefits and risks. Public debate, meanwhile, has only intensified. U.S. Secretary of Health and Human Services Robert F. Kennedy Jr. is among the most prominent critics, warning of possible health concerns and calling for an end to water fluoridation.
His position has energized anti-fluoride movements in several states. Utah and Florida recently passed laws banning the addition of fluoride to public water systems, citing concerns about neurodevelopmental risks for children who drink water with 1.2 mg/L or higher of fluoride. The research review by the National Toxicology Program found there was not enough evidence to show harm to children’s IQs at 0.7 mg/L in drinking water.
Iowa’s public health experts and dentists continue to support fluoridation when levels are controlled.
Dr. Steven Levy, a leading fluoride researcher at the UI College of Dentistry, said fluoridated water remains one of the most effective and equitable public health tools available.
“It’s especially important for kids and for communities without reliable dental access,” Levy said. “We’ve seen reductions in cavities for decades.”
In places like Manson, where fluoride occurs naturally at higher levels, routine testing and treatment systems are necessary to ensure the water remains within safety limits, he said. “It is important to know the fluoride levels in your local water, whether it is coming from natural sources or public systems.”
Schilling believes the growing national conversation has created both confusion and an opportunity for better education.
“There’s misinformation out there, but there are also real questions about naturally high fluoride that deserve attention,” Schilling said. “The key is monitoring.”
Manson preparing to import water
While researchers analyze the science and shifts in public health guidance, the people living above the crater experience these issues in far more personal ways.
Alex Hanson, director of the Manson Public Library, grew up in town and said residents have always known that something unusual lies beneath their feet. What she did not realize until working at the library was how little of that story is taught in schools.
“We kind of all know about it, but I learned a lot more when I started working at the library,” Hanson said. “There is a lot they don’t teach us about it at school, which I always thought was weird.”
Inside the library, visitors can view core samples from the crater that show the disrupted rock layers beneath the town. Hanson said the samples often spark curiosity among tourists, though most locals rarely think much about the impact structure.
“Most people don’t really care that much,” she said. “They think it’s cool, like ‘I live in a crater,’ but we don’t actually live in a crater anymore. A glacier filled it in, so it is just really flat here.”
The naturally high fluoride levels and unusually soft water are widely known in town, and many residents are protective of their local supply. Still, change appears inevitable. Manson is preparing to receive water piped in from Fort Dodge, to ensure a stable water supply — a move Hanson said many residents are unhappy about. The new water is expected to reach homes next year.
“It is going to be a huge adjustment for all of us,” she said. “We have naturally soft water, and Fort Dodge doesn’t.”
Hanson said she and other staff members occasionally meet visitors who come to the library specifically because of the crater’s geology and water chemistry. But tourism has never been a major focus in town.
“It is usually people from out of town who look us up and learn we are in an impact site. Locals do not really come in to learn about it,” Hanson said.
Manson’s blend of ancient geology, unique water chemistry, and evolving public health guidance has placed the town at the center of a larger statewide question. As Iowa continues researching its groundwater, communities are being asked to consider how to balance the dental benefits of fluoride with naturally occurring risks and a rapidly shifting national conversation.
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