THERE are no more green-billed teals or northern pintail ducks floating on the dull, steel-gray Silva Reservoir. The water's edge begins to tell the story.
Mixed in among the detritus of everyday human consumption strewn along the reservoir's fetid shore are bills, bones, and feathers of more than 25,000 North American migratory birds that have perished since mid-December at their winter home.
Environmentalists in Leon, a shoe-manufacturing and leather-processing center in central Mexico's Guanajuato State, are still arguing over why the egrets, ibises, and grebes died this year.
What is not open to debate is that the deaths have drawn attention to the complete impunity with which Leon, surrounding towns, and local industries have fouled the waters on which the area depends.
In this era of the North American Free Trade Agreement (NAFTA) and heightened transnational environmental concerns, the tragedy has also taken on international overtones since the birds - from 21 different species, many on United States endangered lists and a few protected by a ``tripartite'' (US, Canada, and Mexico) agreement - will not be returning north.
``This should be an important lesson for our northern neighbors that it is not good enough to protect these birds in the United States or Canada,'' says Homero Aridjis, president of the prominent Mexico City environmental organization, Grupo de los Cien (Group of 100). ``If they spend the winter in other countries, they may never come back.''
Environmentalists and residents around the 100-year-old Silva Reservoir were quick to blame the birds' demise on the leather and chemical plants that dump their untreated waste into streams that feed Silva and nearby reservoirs.
But the National Water Commission (CNA) says its studies found high concentrations of the chemical endosulfan, a chlorine compound used as a pesticide, in the ducks it studied. The commission emphasizes that neither local tanneries nor farmers are known to use the chemical.
That explanation met with heavy skepticism from environmentalists and local experts. ``Before the CNA spoke of 17 suspicious factories that might be responsible, but now they come up with a pesticide that exonerates all the known polluters,'' Mr. Aridjis says. ``I think they're trying to cover up.''
Martha Ramirez, a Leon biologist with the Ecological Foundation of Guanajuato, says the CNA earlier reported finding chrome in the ducks. Chrome oxide is a toxic chemical used heavily in the tanning industry. ``We don't accept the pesticide explanation,'' she says, ``but we fear the investigation into other possible causes has been slowed by economic and political interests.''
In Leon, some four hours north of Mexico City, more than 500 leather and related factories operate. Les leather industry uses many chemicals to prepare some 15,000 hides processed here every day, but the city and plants still do not treat any of their waste water. The city is scheduled to open its first water-treatment plants - one for household waste water, one for industry - in February 1996. In the meantime, waste pours into local streams and rivers.
ONE polluter is the Medina Torres shoe-sole factory, located upstream from Silva Reservoir. Owned by members of the family of Guanajuato Gov. Carlos Medina Placencia, the factory was cited two years ago for its direct discharge of chemical wastes into an adjacent stream.
According to CNA officials, a fine was assessed but ``put off'' following an agreement by the company to develop water-treatment facilities. That wasn't done, however, and two weeks ago a new fine was assessed.
``That one won't be paid, either,'' says a local journalist. ``These companies are untouchable.''
In part because the leather industry directly or indirectly is the source of their economic welfare, local residents were slow to react as Silva and other reservoirs grew increasingly polluted over recent years. Even when Silva's winter birds began dying this year, local environmentalists had trouble drawing volunteers to help.
But Ms. Ramirez says that's changing. ``Now we have people asking what they can do, volunteering to set off firecrackers to chase healthy ducks away from the reservoir, or to help care for the sick ducks we have here,'' she says, standing under a plastic awning of the foundation's makeshift duck-treatment camp.
``People are starting to realize that if the ducks are dying, they are affected, too,'' she says, noting that more residents are coming forward to report skin rashes and boils.
``The waters are undoubtedly contaminated, we don't deny that,'' says CNA spokesman Jose Carmen Guerrero, acknowledging that more than just ducks are affected. He says agencies like his are doing what they can to remedy the problem, but adds, ``We have to remember Mexico is not a first-world country.''
But some local residents say Mexico is failing to enforce its own water-pollution laws. And it is this allegation, if proven, that could potentially bring NAFTA's environmental arm down on Les polluters.
``Our commission was created to deal with cases like this,'' says Mexican Victor Lichtinger, executive director of the North American Commission for Environmental Cooperation, the Mon- treal-based agency formed a year ago to address environmental issues within NAFTA countries.
``What makes us able to become involved is a government not adequately enforcing its own laws,'' Mr. Lichtinger says. He notes that local citizens must file a complaint before the commission can investigate, but the new and relatively unknown agency has yet to hear from anyone about Silva Reservoir.
Ramirez says some US environmental organizations and the US Fish and Wildlife Service have already taken an interest in the Silva tragedy. Some of the dead birds carried FWS bands identifying them as members of an endangered species.
Yet while she appreciates those groups' concern, she says the real answer lies in raising local awareness about the environment. ``If these ducks dying can start that to happen,'' she says, ``maybe it served some purpose.''