Rural residents who drink water from private wells are much more likely to have Parkinson’s disease, a finding that bolsters theories that farm pesticides may be partially to blame, according to a new California study.

Nearly one million people in the United States–one of every 300–have the incurable neurological disease. Beginning with a slight tremor, Parkinson’s often progresses to severe muscle control problems that leave patients struggling to walk and talk.

Over the past few years, a growing body of evidence has led experts to suspect that agricultural pesticides can attack developing brains, perhaps in the womb or infancy, leading to Parkinson’s disease later in life. Many insecticides widely used on farms are potent neurotoxins, and lab animals exposed to mixes of them develop Parkinson’s symptoms. In addition, several previous studies of farmers and rural residents have reported a link.

The new study of more than 700 people in California’s Central Valley found that those who likely consumed contaminated private well water had a higher rate of Parkinson’s.

The risk was around 90 percent higher for those who had private wells near fields sprayed with the widely used insecticides propargite or chlorpyrifos.

People with Parkinson’s “were more likely to have consumed private well water, and had consumed it on average 4.3 years longer” than those who did not have the disease, said the scientists, led by UCLA epidemiology professor Beate Ritz, in their study published online last week in the journal Environmental Health Perspectives.

Unlike municipal water supplies, private wells are largely unregulated and are not monitored for contaminants. Many are dug at shallow depths of less than 20 yards, and some of the crop chemicals used to kill pests and weeds can seep into ground water.

The study participants lived in Fresno, Kern and Tulare counties-the heart of California’s farm belt. About 17 percent reported drinking private well water during the study period from 1974 through 1999.