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`Black Water' Makes Valley Bloom

Polluted Mexico City water provides cheap fertilizer but contaminates farm vegetable crops

By Staff writer of The Christian Science Monitor / October 16, 1990


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.

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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.