In just one example of the widespread pollution, a dark blue sludge fills a ditch behind the high-tech Grupo Navarra factory, where jeans are bleached and dyed for brands made by Levi Strauss & Co and Gap Inc.

E.J. Bernacki, a Levi Strauss spokesman based in San Francisco, said Grupo Navarra had failed an independent audit of its laundry facilities last year.

"They were not in full compliance and we did give them a corrective action plan," he said, adding that the Levi Strauss policy was to help factories that do not meet its standards to correct the problem.

"If they do not make any changes or show interest in compliance with our regulations we would begin a process of disengagement," Bernacki said.

No one at Grupo Navarra, which is controlled by a Mexican businessman, was available to comment.

Mexico is popular with garment firms because it is close to the United States, meaning a quick turnaround on fast-changing fashion lines.

Since the sector's peak in the 1990s, many firms have left for cheaper China, but hundreds of thousands of Mexicans still work in assembly plants.

In Tehuacan, 118 miles (190 km) southeast of Mexico City, about 35,000 people work in garment factories.

Water from the denim laundries runs through Tehuacan, where it mixes with municipal sewage and is discharged untreated in a foaming green torrent to a river that feeds irrigation systems in the downstream village of San Diego Chalma.

ITCHY SKIN AND SORE THROATS

Farmer Mariano Barragan, 67, uses the water on his few acres of corn planted in fields a few minutes' drive from the center of Tehuacan.

"Sometimes it comes out blue, sometimes yellow, sometimes black," said Barragan, crumbling between his fingers the bluish gray crust the dirty water leaves on the soil.

"I know when the chemicals are strong because the leaves shrivel and my skin starts itching."

Barragan said health authorities have told him not to plant tomatoes and root vegetables because of a risk of contamination. But corn is permitted and is sold locally and to buyers from Mexico City.

Locals say they do not know if the waste water presents a long-term risk to their health, but some complain of chemical odors that irritate their throats.

"They let the strong chemicals out at night. It wakes you up because it catches in your throat," said Gerardo Diaz, who lives next to an open sluice bringing effluent from a small jeans laundry.

Most major jeans firms now require their suppliers to use water treatment plants and monitor waste water for dangerous substances.

Grupo Navarra uses a modern treatment system and last week the water coming from the factory was clear. However, activists say the company does not always switch the plant on.

"This is clear evidence that Grupo Navarra lies," said local rights activist Martin Barrios, digging a stick into the slimy indigo-colored mud.

Industry leaders in Tehuacan blame most of the pollution on the dozens of small unregulated laundries that wash, bleach and dye jeans for Mexican brands.

"We all know Mexican firms demand less than the international brands," said Javier Lopez, spokesman for the city's industry chamber.

"Sometimes the attitude is that the water is contaminated anyway by unregistered factories and animal waste."

Tehuacan is also a center for pig and poultry farming.