The nitrogen in chemical fertilizer does two things incredibly well. It supercharges crop growth, and it produces nitrates, chemicals that are ultra-soluble in water and easily pass through soil to accumulate in groundwater. Once there, nitrates can persist for decades and increase in concentration as more fertilizer is added.

Ingestion of nitrates by infants has been shown to lower levels of oxygen in the blood, leading to the potentially fatal blue-baby syndrome. And several studies have shown that consumption of nitrate-contaminated water can cause cancers in animals. But a recent report by a team led by Mary H. Ward of the National Cancer Institute for the first time links nitrates directly to thyroid cancer in humans.

This is bad news for anyone who drinks water from wells-more than half the nation’s population, both urban and rural, according to a study done by the United States Department of Agriculture (USDA). And since chemical fertilizers came into general use only in the last 60 years, the problem could become much more severe. (Click here to read the USDA report and see how your area rates on a map showing areas of high and low nitrate contamination.)