DWARFED by lush, 8-foot stalks of corn, farmer Armando Guerrero proudly credits the canals of pungent ``black waters,'' used to irrigate the crops here. The Mezquital Valley, some 50 miles north of Mexico City, was once so bony the only major ``crop'' was cactus. Farmers - too poor or too stubborn to leave - called this the ``valley of tears.'' Today, it's a verdant 50,000-hectare (123,500 acre) valley sown with corn, beans, barley, alfalfa, chiles, tomatoes, and cucumbers. Most of the harvest winds up on the dinner plates of Mexico City's 19 million residents.
But the same water Mr. Guerrero praises for bringing prosperity to the valley is polluting it. The Mezquital Valley (agricultural District 03) is the last stop for Mexico City's waste water. Three long tunnels, the latest completed in 1975, carry to the valley almost every drop flushed or washed down a drain in the world's largest metropolis (more than 17 million people) - up to 415 cubic meters (109,871 gallons) of water per second.
The water is dark, with putrid torrents foaming from nonbiodegradable household detergents, largely untreated sewage, and a mixture of industrial waste waters laden with grease, acids, heavy metals, and unknown chemicals. A recent report by the federal Secretariat of Agriculture and Water Resources warns that there should be ``major restrictions'' on use of this water because of the high level of carbonates and sodium. But the report ambiguously states that conditions have shown a ``remunerative crop'' can be grown with this water.
When asked about the contaminants, one agricultural official shrugs. ``We Mexicans tend to develop an immunity to these things,'' he says
But the industrial wastes from the 35,000 factories in Mexico City are posing a health hazard by contaminating the crops with heavy metals, according to several studies. The most recent, done in 1987, showed vegetables tested in the area had lead levels two times as high as the maximum allowed by the United States Environmental Protection Agency (EPA). Levels exceeded those allowed by Mexico's federal health code, according to a report by a team of Mexican university researchers.
The EPA doesn't list maximum levels for cadmium or chromium in vegetables, but it does have limits for water. The levels found by the Mexican researchers ``are at least several orders of magnitude over the maximum,'' says Stephen Lovejoy, an agricultural water-quality expert at Purdue University in West Lafayette, Ind.
Contaminated vegetables are not unique to the Mezquital Valley. Aida Zapapta Cruz, a biologist at Universidad Nacional Autonoma de Mexico at Zaragoza, is doing a study on vegetables growing in a Mexico City park. Preliminary tests show lead levels 45 to 60 percent above world maximum norms, she says.
About 500 tons or 25 percent of all vegetables sold in Mexico City are contaminated with heavy metals, estimates Alfonso Garzon Santibanez, head of the Independent Farmers Central, a large labor union. He blames the federal government for allowing the continued contamination of the Lerma and Tula Rivers, which run through five states.
While the food contamination problem is getting worse here, it's not an environmental problem unique to Mexico, says David Barkin, economics professor at Universidad Autonoma Metropolitana in Mexico City.
``In developing and developed countries, farmers want to get hold of sewage as a cheap fertilizer,'' says Mr. Barkin, who is doing research on strawberry crops irrigated with sewage water in the state of Michoacan. ``Also, increasing water scarcity [therefore higher water costs] means farmers are looking at more creative sources of irrigation. But at the same time, industrial pollution of these waters is getting worse,'' he says.
The source of the contaminants in the Mezquital Valley is clear: 60 percent of Mexico City's water pollution is due to industrial discharges. Some 5,000 companies are creating 80 percent of the problem, according to the Secretariat of Urban Development and Ecology (SEDUE), Mexico's version of the EPA.
Armed with a two-year-old, tougher environmental protection law, SEDUE's inspectors are cracking down on polluters. Agreements have been reached in the last couple of years with more than 700 factories to reduce pollution and recycle water. SEDUE temporarily closed 370 operations - including companies such as N'estles and Pepsi-Cola - until agreements were reached on antipollution devices. Typically, SEDUE allows companies five months to install equipment.
``We're giving them a fright. We're putting pressure on the contaminators,'' says Rene Altamirano, SEDUE's director of prevention and control of environmental polution.
But he says many companies wait until the SEDUE inspector arrives to test their discharge before taking steps to comply with the law.
The agency is woefully understaffed and underfunded. Mr. Altamirano's office has an annual budget of $3.15 million to combat air, water, and solid-waste pollution across the entire country. ``It's absurd,'' he says.
Altamirano enviously notes the World Bank and Japan are loaning the Mexican government money to tackle Mexico City's more widely publicized air pollution problem, but ``not one credit yet has been received for water pollution.''
ECOLOGISTS say the lack of funding leaves low-paid SEDUE inspectors open to bribes. And, the heavy workload means follow-up inspections don't always get done.
``SEDUE has only closed a small number of factories compared to the total number of polluters,'' says Homero Aridjis, founder of the Group of 100, an influential ecological organization.
Altamirano counters that SEDUE isn't supposed to close companies. It's purpose is to ensure that factories operate within the guidelines given. He notes that pollution technology is expensive and many companies still perceive it as an unnecessary cost rather than an investment.
Mexican business organizations are now lobbying for a tax break on pollution-control equipment. Altamirano sees that as a step in the right direction. ``Undoubtedly, attitudes are changing,'' he says.
Indeed, as part of a Lerma River restoration program, the National Water Commission announced last month plans to spend $45.1 million to build 46 water-treatment plants along the river. The goal, says director Fernando Gonz'alez Villarreal, is to cut contamination levels by 50 percent in three years.