In California's Central Valley, where verdant fields of fruit and vegetables unfurl under sunny skies, the water that feeds them - and flows into taps across the region - contains a toxic and silent poison.

The very same farmers who have tilled and cultivated the earth for decades in one of America's biggest produce regions have also poisoned it, dumping millions of tons of fertilizer, which has found its way into many of the region's aquifers.

Nitrates, a residue left behind by intensive farming, now lurk in the water in a number of communities, many of them poor and rural.

For years, Cristobal Chavez has been drinking the water directly from the faucet, never imagining that he, his wife and their children were in danger of nitrate poisoning.

The water "tasted normal," said the former truck driver, who lives in the agricultural town of Porterville, is a foster parent and now runs a day care - meaning lots of children have consumed the water in his home over the years.

Several months ago, the Community Water Center nonprofit association discovered that water in the family's well contained twice as many nitrates as the maximum allowed under state standards.

According to a study by the University of California, Davis, some 250,000 people in the region are at risk of excessive exposure to nitrates.

Most of the tainted communities are small and cut off from larger water distribution networks, making them dependent on wells. The majority are poor, and most residents are Latino, with few speaking English well.

The California State Water Resources Control Board, which monitors public distribution systems in large cities, has no jurisdiction over private wells.

One of its branch chiefs, Kurt Souza, said that county authorities are "trying to target the areas they feel are the most critical," but admitted they had probably missed some contaminated spots.

The toxic effects of nitrates are widely recognized by World Health Organization and US health officials, and are particularly dangerous for children and pregnant women.

Nitrates can sometimes cause a deadly blood disorder called "blue baby" syndrome, in which the blood's capacity for carrying oxygenis reduced, in addition to causing increased miscarriages and fetal deformations.

The substance is also thought to play a role in kidney and thyroid problems, and may cause certain cancers.

Nitrates are "a public health emergency," said Jennifer Clary, an advocate at CWC.

According to a CWC report, the rate of blue baby syndrome is 40 percent higher in the Porterville area than the California average.

California's Public Health Department was unable to provide statistics on blue baby syndrome, and says it does not collect data on the broader impact of nitrates in the Central Valley.

"There is a total lack of transparency, this is of a crisis proportion," said Erin Brockovich, the environmental crusader portrayed by Julia Roberts in her Oscar-winning role in the 2000 eponymous Hollywood film.