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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.)
Writing in the journal Epidemiology, Ward's team reported that its examination of more than 20,000 older women in Iowa showed that those who had consumed water that had nitrate levels of five milligrams per liter or above were three times as likely to develop thyroid cancer as women who consumed water low in nitrates. Five milligrams per liter is half the nitrate concentration that the Environmental Protection Agency deems "safe."
Organic fertilizers such as manure also produce nitrate, but because they break down slowly, plants are able to absorb more of the nitrogen, leaving less to percolate into groundwater. Nitrates can also enter the water table through polluted rain.
"But the largest problem is irrigated agriculture," Jean Moran, professor of earth and environmental science at California State University, told Julia Scott of the San Francisco Chronicle. In some cases, crops absorb only one half of the applied nitrogen, leaving residues to pollute rivers, lakes, and oceans as runoff, or to seep into drinking water. Yet there are no regulations on how much chemical fertilizers farmers can apply to their fields.
Michael Cahn of the University of California has shown that farmers can get by using far less nitrogen fertilizer than they now apply. He frequently tests soil in farmers' fields for excess nitrogen, telling growers that the wasted chemicals represent dollars that are leaching out of their fields.
They also represent proven carcinogens that are being absorbed into our bodies.
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