People whose drinking water came from newer groundwater had a higher risk of developing Parkinson's disease than those whose drinking water came from older groundwater, according to a preliminary study released March 2, 2026, that will be presented at the American Academy of Neurology's 78th Annual Meeting taking place April 18-22, 2026, in Chicago and online. The study does not prove that newer groundwater causes Parkinson's disease; it only shows an association.
The study looked at the age of groundwater. It also looked at aquifers, the sources from which groundwater was drawn. An aquifer is an underground layer of porous rock, silt or sand that holds and transports groundwater.
"One way to examine our exposure to modern pollution is through our drinking water," said study author Brittany Krzyzanowski, PhD, of Atria Research Institute in New York City who conducted this research while at Barrow Neurological Institute in Phoenix, Arizona, and is a member of the American Academy of Neurology. "Newer groundwater, created by precipitation that has fallen within the past 70 to 75 years, has been exposed to more pollutants. Older groundwater typically contains fewer contaminants because it is generally deeper and better shielded from surface contaminants. Our study found that groundwater age and location is a potential environmental risk factor of Parkinson's disease."
The study included 12,370 people with Parkinson's disease and more than 1.2 million people without the disease who were matched for factors like age, sex and race and ethnicity. All participants lived within three miles of 1,279 groundwater sampling sites across 21 major U.S. aquifers.
Researchers looked at groundwater age, aquifer type and drinking water source, such as municipal groundwater systems or private wells, as potential indicators of exposure to neurotoxic contaminants.
Carbonate aquifers are the most prevalent in the United States, consisting mostly of limestone with water stored in fractures and channels. They often contain groundwater that is more vulnerable to surface contamination due to rapid flow through fractures.
Glacial aquifers, formed when glaciers advanced and retreated more than 12,000 years ago, are composed of sand and gravel with water stored in the gaps. These aquifers tend to promote more diffuse flow and natural filtration.
In the U.S., carbonate aquifers are common in parts of the Midwest, South and Florida, while glacial aquifers are found mostly in the Upper Midwest and Northeast.
Among people with Parkinson's, 3,463 got their drinking water from carbonate aquifers, 515 from glacial aquifers and 8,392 from other aquifers. Among those without Parkinson's, 300,264 got their drinking water from carbonate aquifers, 62,917 from glacial aquifers and 860,993 from other aquifers.
After adjusting for factors such as age, sex, income and air pollution, people whose drinking water came from municipal groundwater systems or private wells that draw from carbonate aquifers had a 24% higher risk of developing Parkinson's disease than those whose water came from all other aquifers. They also had a 62% higher risk when compared to people whose water came from glacial aquifers.
The protective effect of older groundwater was found only when water was sourced from carbonate aquifers. For each one-standard-deviation increase in groundwater age, the risk of Parkinson's disease declined by approximately 6.5%. Researchers also found that newer groundwater, from the past 75 years, in carbonate systems was associated with an 11% higher risk of Parkinson's disease compared to groundwater older than 12,000 years from the ice age.
"We speculate that the apparent protective effect of older groundwater is seen mainly in carbonate aquifers because these systems can show a clearer contrast between newer and older water," said Krzyzanowski. "In these aquifers, newly recharged groundwater is more vulnerable to surface contamination, while older groundwater can remain cleaner if it is separated from recent inputs by a confining layer."
"In contrast, glacial aquifers tend to slow groundwater movement and naturally filter contaminants as water travels underground," said Krzyzanowski. "As a result, differences in contamination between newer and older groundwater in these aquifers may be smaller and therefore harder to detect."
Krzyzanowski noted that people can usually find out where their drinking water comes from through their local water utility or, for private wells, through state or county groundwater resources.
"This study highlights that where our water comes from, including the age of groundwater and the type of water source, could shape long‑term neurological health," said Krzyzanowski. "While additional research is needed, bringing together knowledge about groundwater and brain health may help communities better assess and reduce environmental risks."
A limitation of the study was that it assumed all individuals living within a three-mile radius of a sampling site shared the same aquifer characteristics and groundwater age as the sampled location.
The study was supported by AAN Clinical Research Training Scholarship, American Brain Foundation and The Parkinson's Foundation.
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