SILVA, Mexico -- In what Mexican and U.S. officials are calling an environmental catastrophe, up to 40,000 migratory birds have died since November after drinking contaminated water at an irrigation reservoir.
Hundreds more are dying every day, raising concerns that nearbyresidents may be exposed to the same toxic hazard.
The 21 species of birds are perishing at a rate scientists say eclipses that of the Kesterson Reservoir disaster in California's San Joaquin Valley. In that crisis, which surfaced in 1983, thousands of birds were poisoned by farm wastewater containing the toxic trace element selenium before the reservoir was drained and filled.
Scientists blame the contamination on wastewater flowing from the vast tannery industry in nearby Leon, a city of more than 867,000 people. Government officials are helping to bury the birds but have not proposed plans to solve the problem.
"These are migratory birds, and they're coming from Canada and the United States. They fly long distances to find their tomb in the Silva Reservoir," said Homero Aridjis, leader of the Group of 100, Mexico's most prominent environmental group.
The dead birds include American coots, American avocets, black-necked stilts, mallards, ruddy ducks, northern shovelers, and both blue- and green-winged teal, biologists say. None is classified as an endangered species, but many are declining, according to the National Audubon Society.
As more dead or ailing birds are found daily in the fetid reservoir, concern is growing that people may be in danger, too. Local doctors are reporting cases of skin rashes, headaches and intestinal problems among children who have played in the water or handled dead or sick birds. Residents say cows, dogs, turtles and snakes have died.
Townspeople walk along the shores with handkerchiefs over their mouths. Children are warned not to touch the water. In December, 40 percent of the schoolchildren were absent with medical problems that school officials believe are related to the contamination.
Industrial wastewater continues to flow into the 7-square-mile reservoir, and despite the overpowering stench, it is still being used to irrigate crops on surrounding farms. So far, authorities say, they have no evidence that the ground water has been contaminated.
About 15 years ago, residents say, the reservoir's once-abundant fish began to die. For the past six years, they say, birds have been found dead along the banks of the reservoir. But it wasn't until last fall, when a severe drought may have concentrated contaminants in the receding water, that hunters began to notice the magnitude of the deaths.
On Jan. 13, Mexico's National Water Commission attributed the mass deaths to large concentrations of the toxic pesticide endosulfan. It called the deaths "a great tragedy," but said the 50-year-old reservoir did not pose a danger to people.
Since then, the agency has not returned phone calls from the news media and has declined to make its studies available to environmental groups and scientists skeptical of the conclusions. Scientists say the evidence they have seen does not correspond to exposure to endosulfan.
Despite its finding, the agency has taken no steps to limit the use of the pesticide. Four other Mexican government agencies charged with protecting the environment said last week that the reservoir disaster was not their responsibility.
Mexican scientists believe the tannery industry in Leon -- where 60 percent of Mexico's leather-curing business is concentrated -- is the probable culprit. Tannery Association officials deny that their industry is responsible, and the state government of Guanajuato has backed the tanneries.
The North American Commission on Environmental Cooperation, set up under the North American Free Trade Agreement (NAFTA) to monitor environmental abuses in Mexico, Canada and the United States, is powerless to investigate the disaster.
"We're watching the situation very closely, and like most people in the area, we're very concerned about it," said commission spokeswoman Rachael Vincent. "But until someone decides to bring a case to us, our hands are tied. . . . This is exactly the sort of case we envisioned pursuing when the commission was formed."
At the reservoir last week, Water Commission employees and environmentalists buried the dead birds and scared away the living by firing blanks and detonating explosives.
Volunteers washed sick birds with soap and clean water, and a veterinary student injected them with antibiotics to ward off infections.
"For me, it's a total disaster; it's an embarrassment for all Mexicans to see the poor birds dying. What did they do to us?" said Sonia Castillo, a veterinary student from Leon who was treating sick birds.
"More than anything, the embarrassment is that no one really says what caused the problem," she said. "Not enough people are coming to help. So far, nothing has changed."