The Canal From Hell.(water pollution in Mexico City)(Statistical Data Included)

When the rainy season starts this month, dozens of places in Mexico City will overflow with gray flood-water. Drivers will stall on the highways, subway riders will tiptoe through the muck and stores will sell loads of platform shoes. City workers will pump away massive quantities of water, but there is no stopping the inevitable. Since the Aztecs built Mexico City on a lakebed in a bowl of mountains nearly 700 years ago, its rulers have been struggling with the same problem: how to prevent residents from drowning in rainwater and their own effluent- -much of it now swelling out of a canal of sewage in their midst.

Stories of Mexico City's pollution usually begin and end with toxic smog. But the saga of the Grand Canal--the public-works project from hell--offers even more dramatic testimony of man's abuses and nature's revenge. A century ago it was designed as the grand solution: a 29-mile drainage ditch (all but five miles of it aboveground) that carries sewage and rainwater out of the city and under the mountains. It remains a central artery in the drainage system, but exploding population growth long ago overwhelmed its capacity.

Now engineers are battling gravity as well. The canal once flowed downhill, but as the city taps its aquifer for diminishing supplies of drinking water, the clay lakebed is collapsing. The downtown plaza has sunk 29 feet over the past 100 years, and the canal has sunk as well-- forcing the city to pump sewage uphill through a series of locks to get it out of town. The combination of water shortage and sewage excess has city officials quietly discussing a controversial idea: recycling drainwater into drinking water. "The challenge is not trying to convince people of the benefits of this option," says Antonio Dovali, director of the city's drainage system. "It is to convince the people that we don't have options."

A trip down--or, rather, up--the Grand Canal reveals an environmental disaster. In 1940 the metropolitan population was 2 million. Last year it topped 18 million people, generating 13,000 gallons of sewage every second. The big ditch starts in a downtown working-class neighborhood, where it resembles a long and oily lake coated with garbage. During the rains, 28 pumps churn

furiously to push the water forward. Five miles ahead, the Deep Drain branches off: an underground tunnel 30 miles long and 21 feet in diameter that also is beyond capacity.

By the time the Grand Canal reaches the six-mile mark--the line between the city proper and the state of Mexico--the water burps methane and sulfuric acid. After five minutes on the shore your eyes begin to burn, and you know how somebody like Emilio Cortez Mendez feels every day of his life. A decade ago Cortez arrived with his wife and two children from the fields of Veracruz, found an abandoned concrete sewage tunnel and turned it into a rent-free apartment. Since then he and his wife have had four more children, and five more families have erected shanties next to the canal. Cortez, 37, monitors two boxes that send water-level data to a pumping station; in return the city gives him electricity and one spigot of potable water. "I don't live badly," he says. "I have all this, made from cement and concrete."

Multiply Cortez by 200,000--the annual population increase in Mexico City--to understand the growing water shortage. The government is reopening old wells, encouraging conservation, recycling some wastewater for industrial use and repairing leaks: 30 percent of potable water is lost because of pipes that crack under the sinking city. Still, a million people don't have a steady supply of water. In Unidad Valle Ecatepec, the small town that hugs mile 11 of the canal, Abdias Cruz, who runs the water station, turns the water on and off throughout the day to let the cistern refill. The water stays off while he sleeps--though he is never far away. "I don't have money to pay rent," Cruz shouts to be heard over an air compressor, which sits next to two beds, a kitchen table and a ...