DACHANG, CHINA (WASHINGTON POST) - Day and night, overfilled trucks rumble over Nanjiu Road in the saw-toothed hills that stretch to the Vietnam border. It's a procession at the heart of one of China's most hazardous industries.

The trucks load up on metal ore in the valley below, where 13 miners died in October in underground shafts laden with tin, copper and zinc. Then the trucks motor up the mountain towards belching smelters - the culprit, researchers say, behind arsenic levels in Dachang's dust reaching more than 100 times the government limit.

Across southern China - far from the affluent coasts and Beijing's gaze - a vast metals industry has fed the country's manufacturing boom and sated global demand for components used in products from smartphone batteries to electric motors to jet airframes.

China's production of a basket of metals such as aluminium, copper, lead and zinc, known as base or nonferrous metals, has soared as the country became the world's factory floor.

Combined output was 57 million tonnes last year, up from six million tonnes in 1998, according to the China Nonferrous Metals Industry Association.

But some of the country's most isolated, impoverished communities are paying the price.

In Guangxi, a balmy southern region that has some of China's most concentrated mineral deposits, large tracts of farmland lay wasted by run-off carrying cadmium and lead. Metal miners toil in shafts deadlier than China's notorious coal pits.

Villagers roll up their sleeves to show deformities caused by ingesting food contaminated by heavy metals. Residents wait daily for shipments of fresh water.

In the past decade, China's top leaders have steadily tightened regulations on the metals industry, including introducing the country's first soil pollution law last year.

After an eight-year study that began as a state secret, the Chinese government said in 2014 that 20 per cent of the country's farmland was contaminated and a third of its surface water unfit for human contact.

Top officials said last month that they had set aside US$4 billion (S$5.4 billion) to clean up contaminated soil - similar to the US Superfund - yet it's a fraction of the US$1 trillion that some Chinese experts predict is needed.

A review of soil and water data, interviews with environmental researchers, and a 804km journey through Guangxi illustrated how the sheer financial cost is a small part of the challenge facing China.

"Central leaders may have a great vision," said Mr Song Guojun, a former environmental-protection official who studies policy at Renmin University. "But at the local level, there is no transparency, no upward accountability, no money."

As a result, metal producers appear to operate with a degree of impunity - and leave a toxic trail - as they transform crude mountain ore into the essential nuggets of modern life. There are zinc slabs for coating steel, copper cathodes for wires and transformers, and grains of nickel matte, a step in making purified nickel used in batteries and other products.

From his soot-smeared home on Nanjiu Road, Mr Wei Shujian has watched the trucks multiply since the 1970s.

"They are unstoppable," the farmer growled, wheezing from an incurable lung disease caused by dirty air.

Mr Wei nodded towards the hillside, where a huge elevator reached deep into the source of fortune and grief: the mines.

In 1986, Chinese officials informed leader Deng Xiaoping of Guangxi's wealth of aluminium deposits. "It must be done!" Deng famously snapped back.

Two decades on, Xinfa Group, an aluminium conglomerate from northern China, has brought US$2.4 billion in investments to sleepy Jingxi, the next county over from Daxin.

"You can see this from space," said Mr Huang Qi as he strode across a dam holding back a small valley filled with goopy red mud.

It was a reservoir of spent bauxite - aluminium ore - left by Xinfa, which has been locked in repeated disputes with locals for 10 years. Three times in the past 18 months, waste has seeped out of such reservoirs, jamming underground rivers, flooding village streets and rendering the local reservoir water undrinkable.

In June, dozens of locals blockaded a Xinfa facility for three days to demand water before they were dispersed by police.

Mr Chen Wenxi, a Beijing-based environmentalist, helped sue Xinfa on local villagers' behalf in August 2018, seeking US$2.8 million in damages.

A preliminary hearing in local court in June lasted 15 minutes, he said. Mr Chen has sought Xinfa's environmental records, but the government denied him on the grounds that they were state secrets.

"There are biases, certain political factors, when one side has so much money and the other side is so poor," Mr Chen said.

Xinfa, whose chairman sits in China's National People's Congress, has been named a "core enterprise" in Jingxi's five-year development plan.

Mr Huang Lituo, a Jingxi deputy propaganda chief, acknowledged several "unavoidable" industrial accidents involving Xinfa. The local government would hold it accountable for the cleanup, he told The Post.

In recent months, the local government has fined Xinfa US $15,000 for illegal prospecting. Officials have also ordered the company to truck in potable water to communities stranded without it.

But the impact of Xinfa's presence is undeniable. Last year, Xinfa contributed more than US$100 million in tax revenue, more than any other source.

"If not for Xinfa, we couldn't carry out poverty alleviation, build schools, build kindergartens, build medical clinics," said Mr Huang Lituo.

Mr Huang Hua, a villager in his 30s, saw bitter irony in depending on the firm for survival.

"We were fighting a water war with Xinfa," he said. "Now if Xinfa moved away, we might actually die of thirst."

Mr Huang Hua looked out a car window at the Caterpillar excavators chipping away in the distance, turning verdant mountain faces into rust-coloured terraces.

He wondered what Beijing was like and what he could possibly do to get help from the Chinese president himself.

"I wish Xi Jinping would see this," he said, referring to China's leader.

Then he quoted a proverb suggesting central authorities can have little sway over local affairs: "But the mountains are high and the emperor is far."