Drinking water from newer sources, particularly from carbonate aquifer systems, was associated with an increased risk for Parkinson’s disease (PD), preliminary results of new research suggest.

PD risk was 24% higher among individuals whose drinking water came from municipal groundwater systems or private wells on carbonate aquifers — the most common aquifer type in the US — compared with those whose drinking water came from all other aquifer types.

That risk rose to 62% when compared with drinking water sourced from older glacial aquifers.

As newer groundwater is from shallower sources, it typically contains more environmental contaminants such as pesticides and industrial chemicals than older groundwater, researchers said.

The results add to a growing body of evidence that suggests environmental factors — in this case drinking water — may play an important role in PD risk, lead investigator Brittany Krzyzanowski, PhD, Atria Research Institute, New York City, told Medscape Medical News.

“The findings support growing evidence that environmental factors — especially contaminants in water — may play a role in the risk for this disease,” said Krzyzanowski, who conducted the research while at Barrow Neurological Institute in Phoenix.

The research will be presented at the American Academy of Neurology (AAN) 2026 Annual Meeting in April.

Closer Look at Groundwater

Although environmental neurotoxins have been linked to PD, the contribution of groundwater exposure has been poorly understood.

For their analysis, investigators used Medicare data on 12,370 patients with PD and 1,224,174 control individuals without the disease, matched for age, sex, and race or ethnicity. Participants lived within 3 miles of 1279 groundwater sample sites across 21 principal aquifers in the US.

Researchers evaluated groundwater age, drinking water source (municipal vs private well), and the type of aquifer — an underground layer of rock, sand, or gravel that stores and moves groundwater.

Glacial aquifers, formed when glaciers advanced and retreated thousands of years ago, tend to promote relatively diffuse water flow and natural filtration. Carbonate aquifers allow water and contaminants to move quickly through cracks, researchers said.

Carbonate aquifers are the most common aquifer type in the US and are found in parts of the Midwest, the South, and Florida. Glacial aquifers are mostly located in the Upper Midwest.

Among patients with PD in the study, 3463 got their drinking water from carbonate aquifers, 515 from glacial aquifers, and 8392 from other aquifers. Among the controls, 300,264 got their drinking water from carbonate aquifers, 62,917 from glacial aquifers, and 860,993 from other aquifers.

Age of Water Matters

After adjusting for age, sex, race, smoking status, use of medical resources, income, urban or rural residence, and air pollution, investigators found that individuals living in areas sourcing drinking water from municipal groundwater systems or private wells on carbonate aquifers had the highest overall risk for PD compared with those drinking water sourced from all other aquifer types (adjusted odds ratio [aOR], 1.24; 95% CI, 1.18-1.30).

The risk rose to 62% when these individuals were compared with those whose water was sourced specifically from glacial aquifer types (aOR, 1.62; 95% CI, 1.45-1.81).

Groundwater from carbonate aquifers formed within the past 25-75 years (Anthropocene-aged groundwater) had a greater risk for PD (aOR 1.11; 95% CI, 1.02-1.20) than glacial-era groundwater (Pleistocene-aged groundwater) formed more than 11,800 years ago.

“Anthropocene-aged groundwater entered the ground in the modern era, when human activity has strongly influenced the environment, roughly in the last several decades, while Pleistocene-aged water is much older — tens of thousands of years old,” said Krzyzanowski.

The authors noted the protective effect of older groundwater was seen only within carbonate aquifer systems. With these aquifers, for each 1-SD increase in groundwater age, the risk for PD declined by about 6.5%.

“Understanding environmental exposures of patients could help researchers and clinicians better understand why the disease develops,” Krzyzanowski said.

Likely Mechanism?

The most likely biological mechanism linking groundwater to PD involves exposure to contaminants such as pesticides or industrial chemicals found in drinking water.

“This can damage dopamine-producing neurons in the brain, leading to Parkinson’s disease,” said Krzyzanowski.

“Some environmental exposures linked to Parkinson’s disease have also been studied in other neurodegenerative diseases, but more research is needed to know whether the same groundwater patterns affect conditions like Alzheimer’s disease,” she added.

As groundwater age and aquifer type affect how contaminants move and persist anywhere, the results would likely apply outside the US.

“However, the exact risk patterns could differ depending on local geology, land use, and type of environmental exposures in other countries,” Krzyzanowski said.

The new findings are most relevant for scientists, public health officials, and water system managers who monitor environmental risks, said Krzyzanowski. Improving the water system involves monitoring groundwater contaminants, strengthening water treatment, and protecting aquifers from pollution, she said.

“For individuals, the study highlights the importance of protecting drinking water sources and reducing contamination,” she said.

Krzyzanowski and her colleagues plan to identify specific contaminants in groundwater that may be driving the association with PD and to examine these patterns in other populations and regions.

Really Fascinating

The study was well conducted and offers “really fascinating” findings, said Michael S. Okun, MD, professor of neurology and executive director of the Norman Fixel Institute for Neurological Diseases, University of Florida, Gainesville, Florida.

“It does not prove cause and effect, but it raises important environmental questions,” said Okun, who was not part of the new study.

Okun noted that for carbonate aquifers, the risk for PD was found to be significantly elevated, especially when the water was relatively new.

“This suggests the possibility that where we live and what’s in our water may play a role in brain health,” said Okun, who also is a medical advisor at the Parkinson’s Foundation and author of the book The Parkinson’s Plan.

The study is one of the largest of its kind, said Okun, who noted as one of its strengths the adjustment for age, smoking, income, air pollution, and other confounders.

“The findings support the idea that environmental exposures, including drinking water, deserve closer attention in Parkinson’s prevention,” he said.

The study was funded by the Parkinson’s Foundation, the American Brain Foundation, and the American Academy of Neurology. Krzyzanowski and Okun reported having no relevant conflicts of interest.