Deep in rain-swept forests on the Indonesian island of Sumatra, an environmental and human disaster is looming, at a zinc mine ready for digging in mountains considered the most seismically dangerous on Earth. Mining experts are warning that a dam set to hold millions of tons of waste slurry from the mine is almost bound to fail, potentially engulfing Indigenous villagers in their homes just a few hundred feet away, and pouring toxins down rivers and through forests inhabited by critically endangered Sumatran orangutans.

The proposed mine would be dug underground in the Barisan Mountains, the backbone of Sumatra. The area is surrounded by protected forests and villages of the Indigenous Pakpak people, who have long lived in scattered communities across Dairi district in northern Sumatra, and the Toba people, who moved there in the early 20th century.

The $630-million project would be operated by Dairi Prima Mineral (DPM), a joint venture between Indonesian mining giant Bumi Resources, which is owned by the politically well-connected Bakrie family of Indonesia, and the Chinese state-owned China Nonferrous Metal Mining Group.

“I have never see a plan for a tailings disposal facility that showed such callous disregard for human life,” a mining expert says.

It would tap one of the world’s largest and richest unexploited zinc reserves, with seams up to 30 meters thick, removing up to six million tons of zinc and lead ore from a deposit known as Anjing Hitam, Indonesian for Black Dog. The ore would be ground on site, with waste rock removed, and the resulting “concentrate” shipped to refineries in China. Up to three-quarters of the waste, known as tailings, would be mixed with cement and put back into the mine. But the remaining million tons or more of slurry, consisting of about 35-percent water, would be left at the surface behind a tailings dam 80 feet high.

In interviews with Yale Environment 360, two U.S. mining experts said the question is not if the tailings dam will fail – only when. “I have never seen a plan for a tailings disposal facility that showed such callous disregard for human life,” says Steven Emerman, a mining hydrologist formerly at Utah Valley University.

The area downstream of the mine and its tailings dam is home to villages occupied by the Pakpak and the Toba, two tribes of the Batak Indigenous people, who live in the mountains of northern Sumatra. The Toba and Pakpak kept the dense forests closed to outsiders until the mid-19th century. Today, they are mostly farmers, and the villages closest to the mine, such as Parongil and Sopokomil, are reached by all-year roads and have schools, clinics, and mosques.

Bongkaras village, seen from the hills above and at the road entering town, is 2.5 kilometers from the proposed tailings dam site. TONGGO SIMANGUNSONG

Emerman fears a repeat of the disaster at a Brazilian tailings dam two years ago, which killed 270 people. Company maps show that the proposed dam will be just 1,200 feet from Sopokomil. That is closer than many of those who were buried alive in the Brazilian tailings dam disaster.

Local communities have also discovered, to their anger, that the siting of a tailings dam so close to communities would be illegal in China. Following the Brazilian disaster, China banned all new tailings dams within a kilometer of settlements. “Do they think human life in China is more valuable than human life in Indonesia?” asks villager Rinawati Sinaga in a video produced by a Sumatran advocacy group. She says at least 10 villages downstream will be impacted by the mine and its tailings dam.

Emerman and Richard Meehan, a Stanford University engineer specializing in dam design and safety, have joined Sumatran communities and advocacy organizations in calling for the Indonesian government to deny the mine an environmental permit, which locals expect could be issued as soon as next month.

Yale Environment 360 has submitted questions about the project to BPM, to its two owners, to the consultants that compiled the project’s environmental impact assessment, and to the Indonesian government via its London Embassy, and has asked for comment. No responses have been received.

If the project goes ahead, zinc from the mine could soon be incorporated into Chinese-made drive shafts, steering columns, and other auto parts installed in tens of millions of automobiles in the U.S. and worldwide.

DPM obtained a production permit for 30 years’ operation three years ago, but awaits an environmental license from the Indonesian government before it can proceed. Meehan and Emerman contend that the company has left huge technical gaps in the environmental impact assessment (EIA) it submitted to the Indonesian government, in particular about how it will manage tailings and keep the dam and its contents safe. This has made consultations with local communities meaningless, he says.

According to Emerman, DPM has failed to say how much water the mine will need or where it will come from. He estimates it will require up to 5 million cubic meters a year, taking most of the flow of the river on which local villages rely.

The fact that the mine is at the heart of one of the most seismically unstable regions of the earth’s crust is the greatest hazard.

Emerman says the ore, which mainly contains zinc and lead sulphide, will generate large volumes of sulphuric acid when exposed to oxygen and water at the surface. The acid would in turn dissolve the zinc and lead, along with other toxic trace metals in the ore, such as cadmium and arsenic. Unless contained or neutralized, most of the acid drainage would end up flowing north into the Alas River and then south and west through forest to the Indian Ocean.

An independent study by Japanese and Indonesian geologists warned in 2019 that the acid water could acidify soils, rivers, and water supplies, and poison forests and crops over a wide area, and “may endanger lives around the mining sites.” The effects “could extend for centuries post-mine closure,” concluded lead author Tomy Rivai of Kyushu University.

Tongam Panggabean, executive director of Bakumsu, a legal advocacy group in North Sumatra representing local communities, says that the forest “is home to an incredible array of flora and fauna, including critically endangered orangutans.”

The biggest source of contamination is likely to be the contents of the tailings dam, which the experts say has three fundamental failings.

First, it is vulnerable to heavy rain. The Barisan Mountains are among the rainiest places in the world, with annual precipitation between 120 and 200 inches, and storms able to deliver up to 20 inches in 24 hours. The fear is that floodwater rushing into the tailings could turn the thick slurry into liquid that would overflow or breach the dam. The latter is what happened at the Brazilian disaster, which collapsed after weeks of heavy rain liquefied the tailings, causing the dam wall to buckle and collapse.

A Batak woman harvests betel nuts, a major crop in northern Sumatra. TONGGO SIMANGUNSONG

DPM’s EIA, seen by Yale Environment360, says its dam is designed to withstand a hundred-year flood, and its emergency spillways could handle a flood anticipated once every 500 years, albeit by releasing large volumes of toxic sludge into the local environment. But in a detailed analysis of the hydrology of the mine for Inclusive Development International (IDI), a U.S. community-rights advocacy group that is campaigning against the dam, Emerman says such standards are impossible to verify and, in any case, “completely inadequate.” This is both because of the proximity to communities and because the dam will need to remain intact and keep its toxic and potentially liquid contents safe for thousands of years.

The EIA says that after the mine shuts, “the tailings will be covered and made semi-saturated” (meaning partially dewatered). But Emerman says it has not explained how, and he believes it will be impossible, “especially under conditions of high rainfall.” What the company plans is “completely inconsistent with international, Chinese, and Indonesian regulations for tailings dam safety,” he said.

The risk of a dam failure is made more likely, says Meehan, because it will not have stable foundations. DPM states in its EIA that the dam will be built on stable bedrock. But local villager Hodwin Hutasolt said in a video recorded by Bakumsu that “the soil structure is unstable” in the valley where it will be built.

Meehan says the company has produced no borehole data to corroborate its claim, which is contradicted by earlier surveys carried out by Terence Middleton, an Australian prospector, who found that found the entire valley floor is underlain by a thick layer of volcanic ash.

The ash is fallout from a giant volcanic eruption 73,000 years ago at Mount Toba, 30 miles to the east. It covered the entire region in ash tens of feet thick, and much of it remains. Meehan says that the ash would “liquefy” and flow downhill in a strong earthquake, taking the dam and its tailings with it.

Environmental safety is often still a low priority in the mining business, with tailings dams a serious black spot

The fact that that the mine is at the heart of one of the most seismically unstable regions of the earth’s crust is likely the greatest hazard. It is less than 150 miles east of a boundary between geological plates known as the Sunda subduction, which has triggered volcanic mega-eruptions – including Toba and Krakatoa in 1883 – as well as earthquakes, such as the 2004 submarine quake and tsunami that killed 227,000 people, mostly on Sumatra. Also, just a few miles away is the Great Sumatra fault, known for producing earthquakes that last for minutes at a time and have destroyed infrastructure such as dams. In all, the Barisan Mountains have 35 active volcanoes.

Meehan says that “the failure of the tailings dam is a virtual certainty” at some time during the many centuries during which the tailings need to be kept safe. “I cannot avoid the conclusion that within a few decades after the ‘closure’ of the deposit, there will likely be an earthquake-induced sudden failure of a tailings dam, with a disastrous breach sending a wave of liquid mud downstream,” he states in an unpublished report for IDI.

Emerman estimates that such a wave would today engulf Sopokomil within two minutes, and take out the next village downstream, Parongil, within six minutes. Hundreds would die. Yet, he says, the mine’s EIA contains no risk assessment for such a disaster.

Both Meehan and Emerman are retained as consultants by IDI. But other experts contacted by e360 agreed with their broad assessment. “Sumatra is highly seismically active and has high rainfall. That does not mean a tailings dam could not be designed to cope with the conditions, but it would be extremely challenging,” said one British mine-risk engineer, on condition of anonymity. An Indonesian expert, Eko Teguh Paripurno, director of the disaster mitigation study center at the University of National Development “Veteran” Yogyakarta, said that “canceling mining and dam activities is indeed the best option.”

What are the prospects that local communities can stop the project? According to Panggabean, the community lawyer, the Indonesian government has recently granted DPM mining rights in the area until 2048. So it only needs its environmental license to go ahead. Even so, David Pred, director of IDI, hopes to disrupt funding. While most investment is from Chinese banks, one of them, the Postal Savings Bank of China, may be amenable to pressure from the International Finance Corporation (IFC), an arm of the World Bank that has recently invested $300 million in the Chinese bank.

Pred argues that the mine breaches IFC safety and environmental standards and has failed to meet international rules for ensuring that Indigenous communities such as the Pakpak are able to give or withhold their “free, prior, and informed consent” to such projects. He says local communities were given just seven days last July, during the pandemic lockdown, to review the 492-page EIA. They subsequently complained to the IFC’s internal watchdog, the Compliance Advisor Ombudsman, which has been considering the matter since July 2020. “Why does DPM not provide clear and open information to the public?” villager Hutasolt asks. “We have lived in the area for a long time. Our participation should be respected and our voices heard.” If the project goes ahead, he says, it will be “an environmental and social crime.”

A road leading to the mine site, where preliminary clearing work for the tailings dam has already begun. TONGGO SIMANGUNSONG

But the pressure to get digging will be strong. Zinc is in short supply worldwide and demand is growing, both for batteries and as an alloy to prevent steel from rusting. China Nonferrous Metal Mining Group (NFC) has the right to take up to 65 percent of the DPM mine’s production.

After refining the zinc in China, NFC will probably sell much of it to the Wanxiang Group. The world’s largest manufacturer of auto parts is a major customer. It bought 3,000 tons of zinc ingots from NFC in 2018 to produce drive shafts, steering columns, wheel hubs, axles, brakes, and much else for major brands such as Ford, Volkswagen, and General Motors. It claims that “one out of every three cars made today” contains its parts.

Environmental safety is often still a low priority in the mining business, with tailings dams a serious black spot. They make up only a small fraction of all dams worldwide, yet of 14 known dam failures resulting in deaths over the past 11 years, four involved tailings dams. They were responsible for almost two-thirds of resulting deaths.

A survey of more than 1,000 mining facilities published this month by Daniel Franks of the University of Queensland found that a tenth of all tailings facilities worldwide have experienced “stability concerns.” Yet more than a quarter of the companies had not formally considered the consequences of a catastrophic failure.

Meehan believes that the safety record of tailings dams has deteriorated as the mining of lower-grade ores creates more tailings. Franks agrees, predicting that “ever larger tailings storage facilities will continue to be built in locations with ever higher consequences of failure.”