Well-Water Consumption and Parkinson’s Disease in Rural California

INTRODUCTION Investigators have hypothesized that consuming pesticide-contaminated well water plays a role in Parkinson's disease (PD), and several previous epidemiologic studies support this hypothesis. OBJECTIVES We investigated whether consuming water from private wells located in areas with documented historical pesticide use was associated with an increased risk of PD. METHODS We employed a geographic information system (GIS)-based model to estimate potential well-water contamination from agricultural pesticides among 368 cases and 341 population controls enrolled in the Parkinson's Environment and Genes Study (PEG). We separately examined 6 pesticides (diazinon, chlorpyrifos, propargite, paraquat, dimethoate, and methomyl) from among 26 chemicals selected for their potential to pollute groundwater or for their interest in PD, and because at least 10% of our population was exposed to them. RESULTS Cases were more likely to have consumed private well water and to have consumed it on average 4.3 years longer than controls (p = 0.02). High levels of possible well-water contamination with methomyl [odds ratio (OR) = 1.67; 95% confidence interval (CI), 1.00-2.78]), chlorpyrifos (OR = 1.87; 95% CI, 1.05-3.31), and propargite (OR = 1.92; 95% CI, 1.15-3.20) resulted in approximately 70-90% increases in relative risk of PD. Adjusting for ambient pesticide exposures only slightly attenuated these increases. Exposure to a higher number of water-soluble pesticides and organophosphate pesticides also increased the relative risk of PD. CONCLUSION Our study, the first to use agricultural pesticide application records, adds evidence that consuming well water presumably contaminated with pesticides may play a role in the etiology of PD.