In Manson, Iowa, a glass case in the public library contains core samples that look unremarkable until the sample’s history is told: the layers of rock are jumbled, a geological signature of an impact that occurred tens of millions of years ago, long before anyone cared about fluoride.
The town of about 1,600 people is situated on top of the Manson Impact Structure, a crater formed 74.1 ± 0.1 million years ago. Glaciers later smoothed the area, creating flat ground above a deep, disturbed basin of geology that acts simultaneously like a lid and a trap. Groundwater can remain there for very long periods of time, cut off from rapid recharge and left to interact with rock for extended periods. This prolonged underground stay is part of the reason the town’s raw water is renowned among hydrologists as unusually soft and unusually mineral-rich in ways that are important for public health.
“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.” In Manson’s source water, fluoride concentrations have reached 10 milligrams per liter, above the U.S. Environmental Protection Agency’s maximum allowable level of 4 mg/L. To maintain safe levels in treated water, the town built a reverse osmosis system, forcing water through membranes that filter out dissolved substances, including fluoride, on a daily basis. It is a geologic legacy transformed into an engineering necessity.
This is where Manson’s story gets bigger than one crater. A recent, wide-ranging dataset shows that high fluoride is not a one-time thing but a trend related to depth and bedrock. Scientists examining almost 9,000 samples of groundwater determined that untreated concentrations ranged from <0.1 to 11.2 mg/L, with higher concentrations more common in deeper aquifers like Cambrian-Ordovician and Mississippian. In this study, groundwater near the Manson Impact Structure contained significantly higher levels of fluoride, a reverse trend in which an ancient impact structure correlates with a modern-day exposure map. In the same study, a clear line was drawn between regulated and unregulated water. In the former, community water systems typically control fluoride levels within acceptable limits, while an estimated 230,000 Iowans using private water wells face uncertain conditions since private water wells are unregulated and may be tapping deeper, more fluoridated strata.
Across the country, the question of fluoride in water is increasingly being characterized as a debate over adding it to the mix, while the engineering challenge in regions like Manson is actually the removal of excess fluoride when nature overachieves. Even as target levels changed to 0.7 mg/L for community fluoridation, communities with geologic challenges are now grappling with the engineering of treatment systems, monitoring frequencies, concentrate disposal, and the long-term expense of maintaining membranes as designed.
The next change in the town is not only chemical but also hydraulic. A pipeline construction has been approved to transport water from Fort Dodge to Manson, which can meet the average daily demand of 225,000 gallons. The connection came about due to the failure of drilling a new well, which could not provide enough water, making the reliability of water supply the decisive factor even before the chemistry aspect comes into the picture. For the residents of this town, who are so proud of their soft water, the change in water quality will affect the water feel in the pipes, appliances, and laundry.
In the Manson library, visitors will sometimes come looking for the crater and leave talking about water. This encapsulates the quiet lesson of the town: The most important engineering challenges begin as geology, and the solution must continue to function long after the story has been forgotten.
