LUANDA, Angola — First, the river turned red. Then dead fish by the ton floated up to the surface. Then thousands of people started getting sick.
Now, 12 people have died in the Democratic Republic of Congo, in what researchers have called “an unprecedented environmental and human disaster” along the Kasai River, a southern tributary of the mighty Congo River.
Researchers and officials with the Congolese government say that the cause was a toxic leak upstream, from the biggest diamond mine in Angola, run by Catoca, a joint venture owned by Endiama, the Angolan state mining company, and the Russian mining giant Alrosa.
The company admitted in a statement last month that there was a leak from its facility, but said that it was only water and sand — not anything toxic.
In addition to the 12 fatalities, about 4,500 people got sick from diarrhea as a result of the pollution and nearly one million were affected overall, said Eve Bazaiba, Congo’s minister of environment and sustainable development in a news conference on Thursday.
“It’s a total destruction of ecosystems, especially aquatic biodiversity,” said Ms. Bazaiba, who traveled to the region.
She said people living near the water noticed around July 26 that something strange was happening on the Tshikapa River, which flows northward from Angola, where it is spelled Chicapa, then into Congo, emptying into the Kasai.
At first, they thought small-scale diamond miners were causing the problem, she said. But then, on July 31, the situation grew worse.
“They noticed that there were dead fish. Lots of dead fish — tons and tons of them, floating on the river,” Ms. Bazaiba said.
A team dispatched to the area reported back that two hippopotamuses had also died. “Everyone was panicking,” she said.
The government warned people not to eat the fish, and took samples of the water to be tested in laboratories in Kinshasa, Congo’s capital. A week later, the results came back. The water sample contained heavy metals — nickel and iron — and its pH levels were off, according to the minister.
“It’s practically acid,” she said. “It sucks the oxygen out of the water. There’s no life there anymore.”
Researchers at the Congo Basin Water Resources Research Centre at the University of Kinshasa called the pollution of the Kasai river basin “an unprecedented environmental and human disaster.” In a report released in mid-August, they said they had been tracking the spill from its source in the provinces of Lunda Norte and Lunda Sul in Angola since July 15, and that it took 15 days to reach the city of Tshikapa, at the confluence of the Tshikapa and Kasai rivers. It said that two million people were at risk.
The immediate consequences of the disaster so far, the report said, included water pollution, poisoning and loss of aquatic fauna and flora, waterborne diseases for riparian communities, the disruption of fishing and navigation activities, and lack of access to domestic water services.
It warned that the pollution could spread downstream to the section of river that flows through the vast metropolis of Kinshasa, one of Africa’s most populous.
Ms. Bazaiba said she was hoping the voluminous waters of the Congo River — second only to those of the Amazon — would dilute the pollution by the time it reached the capital, adding that the waters were beginning to clear.
Meanwhile, the government is trying to ascertain the source of the pollution, she said, but has to follow procedure, as it originated in a foreign country.
“We don’t know exactly whether it was an accident,” she said, “or if was known about.”
Ms. Bazaiba said that the Angolan government and the company acknowledged that the pollution had come from Catoca mine. She added that Congo would seek compensation under the “polluter pays principle.”
But the Angolan government has not said anything on the matter publicly. An official in the ministry of environment, tourism and culture, who was not authorized to speak publicly and spoke on condition of anonymity, said the ministry had not received any official information from Congo’s government. The official said that the only information the ministry had was what it gleaned from the media, and investigations are still underway.
An employee of the company, who was not authorized to address the matter and spoke on condition of anonymity, denied that Catoca had confirmed the Congolese government’s allegation that there had been a toxic spill.
The Catoca mine produces three quarters of Angola’s diamonds. One of its owners, the Russian firm Alrosa, has in recent years been trying to increase sales in the United States.
In a statement last month, the company admitted that there had been a “rupture in the pipeline that works as a spillway.” But it said that all that had leaked into the river was a mixture of sand and water. It said that a survey had been done and that “the situation recorded did not represent a risk to the population’s lives.”
The company employee said that Catoca did not use the heavy metals described by the Congolese minister.
“There could not be toxic materials originating from the Catoca mine because the mine does not use such materials,” the employee said. “It was a concentration of sand and water, or to be clear, it was mud.”