Ramona Yesenia stood in her town square with two empty jugs, waiting for water to replace the municipal supply contaminated by a chemical spill that turned Mexico's Sonora river orange.

Yesenia is one of 20,000 people left without water since a massive sulfuric acid leak last week at the Buenavista copper mine in northwestern Mexico, one of the largest in the world.

She waited in the sweltering heat with her mother and two daughters for water brought into the town of Arizpe by a tanker truck, but left empty-handed after the truck ran dry, unable to meet the demand from the seven affected towns.

The housekeeper and farm laborer said she was afraid to even eat local food.

"If they kill a cow, we don't know if we can eat it... They say if the (cattle) drink just a little water (from the river), they get infected," she said.

An estimated 40,000 cubic meters (10.6 million gallons) of sulfuric acid, which is used to dissolve copper from ore for processing, leaked out of a holding tank at the mine, owned by leading Latin American mining company Grupo Mexico.

The spill happened on August 6, but the authorities say the company only informed them 24 hours later.

Executives, who blame the spill on "abnormal rains" that caused the acid to overflow its holding tank, insist the government was alerted by email.

Juan Rebolledo, Grupo Mexico's vice president for international relations, downplayed the impact.

"The content of these acids is not toxic in itself," he said on radio network Formula.

"There's no problem, nor any serious consequence for the population, as long as we take adequate precautions and the company pours lime into the river, as it is currently doing."

The mine has dumped 100 tonnes of lime into the Sonora to neutralize the acidity, according to the state government.

But environmentalists say that is not enough to address the health risks posed by the altered metal content of the water.

They have also condemned a government plan to make the company responsible for testing the water for the next five years.

"It's illogical," said local activist Rafael Chavez. "The company would have to be pretty stupid to say 'Yes, I'm contaminating the river.' It will never declare itself responsible for what it did."

The National Human Rights Commission has opened an investigation into the spill, saying it may constitute a human rights violation.

- 'Rotten smell' -

Residents of the affected towns say they received no warning before officials cut off their water, and still do not know exactly how big the accident is or how long it will take to clean up.

Usually the 400-kilometer (250-mile) river is a crystalline ribbon winding its way through the region at this time of year, the rainy season in Sonora.

But the leak turned its waters a reddish orange for a 60-kilometer stretch.

"I took this from the river a couple days after," Octavio Toledano said, displaying a small plastic bottle with a yellowish liquid and reddish sediment at the bottom. "It had a rotten smell."

Jesus Sabori, a resident of nearby Huepac, said the river has been growing "more and more red every day."

"But it was only (Monday) that they told us to keep our animals away," he added.

Pickup trucks with the logos of the National Water Commission and Grupo Mexico can be seen driving around the affected towns, but residents complain they have received little information from the government or the company.

"We're angry because they didn't take the time to tell us either that the spill had happened or that they were cutting off our water," said 70-year-old resident Israel Duran.

"Even if (the mine) creates jobs, it would be better if they closed it if they're going to behave like this every time something happens."

The mine employs 9,000 people and has announced plans to expand, seeking to boost its annual output from 200,000 tonnes of copper to 510,000 by 2016.