A warning voice on the telephone, a home intrusion, a punch in the face, a pistol barrel prodded against the ear.

The intimidation of Maria do Socorro Silva has come in many forms since she began defending her Amazonian home against the world’s biggest alumina refinery and its local government backers.

As a leader of forest dwellers – indigenous, quilombo and riverine communities – Socorro ought to be terrified. Her home is in Pará, the deadliest state for land activists in Brazil, the most murderous country in the world. Two associates and friends have been killed since December.

But there is fury rather than fear in the eyes of the diminutive, powerful woman as she speaks of the industrial plant that threatens her quilombo, a community established in the forest by African slaves who broke their chains.

“Will we fight this? Yes. Will more die? Yes!” she spits, her hands forming fists. “They kill the water, the air and the animals. They should be put in prison.”

To be in the presence of Socorro is to face a storm. Animated and angry at the refinery company and the government, she also sheds tears over the plight of her community and environment. This - the people and the land - comes first. Only then does she speak of herself, quietly letting slip that she has just been diagnosed with cancer. Test results indicate high levels of lead contamination in her blood.

For 10 years, she has been fighting on multiple fronts: against the Norwegian-owned Hydro Alunorte refinery in Barcarena , against the Albras bauxite mine that supplies it, against powerful land-grabbing politicians, against investors, against first-world consumers who use tin cans, foil, non-stick pans, beer kegs and aeroplane parts without a thought about the environmental and social costs.

For an outsider, it may seem strange at first that a descendent of African rebel slaves should be leading Amazonian forest people in this campaign. But for Socorro, the struggle is deeply personal, as well as historically symbolic. It is about land, race, inequality and justice.

Her ancestors were among the 4.9 million Africans brutally forced to work in homes and plantations in Brazil, the world’s biggest slave-owning nation and the last to abolish the practice. Some rebelled, many fled into remote regions like Barcarena to build their own free refuges, known as quilombos. Today, there are 2,962 of these communities which are home to an estimated 16 million residents (or quilombolas).

Even after emancipation in 1888, the conditions for Afro-Brazilians remained grim. For more than a century, the most the quilombolas could hope for was to be ignored. But in recent years, they have campaigned for legal status, which entitled them to property rights and social benefits. When Socorro’s community was finally acknowledged by the government in 2014, she said it was among the most important moments in her life. “This is a recognition that I am not an invader, I am a quilombola,” she said at the time.

Getting the land was one thing, keeping it another. In Congress, the powerful bancada ruralista (rural lobby) is pushing to overturn the rights of quilombolas and indigenous groups so more land can be taken by agribusiness and mining companies. This is the primary driver of violence in Pará, which has more quilombos than any other state.

Barcarena is no longer isolated. The population has grown with the opening of road and river links, a bauxite mine and industrial zone. It takes only three hours to drive the 40km here from Belém, a journey that once took days.

Among the palm trees, quilombo vegetable patches and indigenous aldeia now have to jostle for space with factories and shops. Tension over space is growing. The expansion of the alumina refinery is at the heart of economic development plans promoted by local politicians. In the way, stands Socorro, her community and their allegations of land-grabbing, pollution and corruption.

Contaminated water in Barcarena, Brazil. Photograph: Thom Pierce / Guardian / Global Witness / UN Environment

Almost a decade ago she began to denounce the refinery, then owned by Brazil’s Vale mining company. One of the tailings pools, she claimed, was built without permission on an environmentally protected area.

“At first, we didn’t realise what was happening because the factory was small. But as it grew, we began to realise. First the fishermen, then the animals. Our plants have been destroyed,” she says. “I’ve been complaining since 2009, but nobody does anything because the mayor is involved. The government are bandits and murderers. They have no respect for us.”

The authorities ignored her warnings, she said, because they were on the side of the company, and, besides, she was a quilombola - arguably the most overlooked group in Brazil. But she kept pressing the case. As president of the Association of Caboclos, Indigenas and Quilombolas da Amazônia (Cainquiama), which represents thousands of local forest-dwellers, she arranged protests and filed two lawsuits against Hydro Norte. She denounced the company to public prosecutors, the Pará state legislative assembly, and the media.

“The Norwegians get rich at our expense. They don’t care about our poor and miserable people,” the matriarch lamented as family and friends looked on at her walled home.

Her life is at risk. Last year, 57 Earth defenders were murdered in Brazil, more than a fifth of the world total, according to Global Witness. The vast majority took place in the Amazon, where rich nature, poor policing and dire corruption breed a culture of strongest-takes-all.

Since December, two Cainquiama leaders have been murdered - Fernando Perreira on 22 December and Paulo Sérgio Almeida Nascimento on 12 March. Police have no leads on the killers. The group’s lawyers believe they are linked to local politicians who want to silence opposition to the industrial plant. So does Socorro.

“Fernando died, Paulo Sergio died, but the truth is that it [the industrial plant] is slowly killing all of us. Every day we drink the water, every day we die a little bit. This is not just now, it has been happening for years. No one else has the courage to denounce it,” Socorro says.

Norsk Hydro - the Norwegian company that owns 92% of Hydro Alunorte and 51% of Albras - is not implicated in the killings and it has condemned the use of intimidation tactics. The company says it operates in accordance with Brazilian criminal law and environmental regulations, and that its plant has not had a negative impact on the health of local residents.

Socorro’s claims about pollution were strengthened on 17 and 18 February when heavy rains flooded the Hydro Alunorte site, leading to reports of contamination of surrounding waters. Researchers from the Evandro Chagas Institute, which is affiliated to the health ministry, discovered high levels of sulphate, chloride and lead in the nearby community of Bom Futuro. Levels of aluminium were more than 30 times Brazil’s legal limit. When the Guardian visited several weeks later, the water was still an unhealthy milky white and residents complained of diarrhoea and stomach pains.

Maria do Socorro Silva: ‘Everyone knows what happened in Barcarena, but they turned a blind eye.’ Photograph: Thom Pierce / Guardian / Global Witness / UN Environment

A subsequent investigation by the authorities found a waste pipe that was not supposed to exist. Courts recognised that one of the waste ponds was built illegally and punished the company by ordering a production cut of 50%. The governor of Pará is demanding $ 250m in damages. Hydro Alunorte has appealed. It says other studies show no contamination from the plant. But it also apologised and promised to provide free medical care and bottled water to more than 1,000 local people.

Socorro feels vindicated, but she is far from satisfied. She believes refinery executives, political leaders and local environmental officials should be punished. “Everyone knows what happened in Barcarena, but they turned a blind eye. They have to be arrested,” she says.

Then, in case anyone might think intimidation might end her campaign, she adds a vow to fight on. “They do not like what we do. That is why we are being threatened,” she says. “But I’m not afraid of the mayor or anyone else. I’m a quilombola. The struggle of slavery runs in my blood.”