Some 50 million people are at risk of arsenic poisoning from contaminated groundwater in Pakistan's Indus Valley, far more than previously thought, according to a new study.

Pakistan is aware of the growing problem, with arsenic levels rising in some areas as people increasingly and indiscriminately draw from the country's underground aquifers, said Lubna Bukhari, who heads the government's Council for Research in Water Resources.

“It's a real concern,” she said. “Because of lack of rules and regulations, people have exploited the groundwater brutally, and it is driving up arsenic levels.”

The authors of the study developed a map highlighting areas of likely contamination based on water quality data from nearly 1,200 groundwater pumps tested from 2013 to 2015, and accounting for geological factors including surface slope and soil contents.

They determined some 88 million people were living in high-risk areas.

Given that about 60-70 per cent of the population relies on groundwater, they calculated that roughly 50 million maybe even 60 million were potentially affected. That's equal to at least a third of the 150 million already estimated by the World Health Organisation to be drinking, cooking and farming with arsenic-laced water worldwide.

“This is an alarmingly high number, which demonstrates the urgent need to test all drinking water wells in the Indus Plain,” with hotspots around the densely populated cities of Lahore and Hyderabad, said the study's lead author, Joel Podgorski, of the Swiss Federal Institute of Aquatic Science and Technology, known as Eawag.

The findings were published Wednesday in the journal Science Advances.

The high-risk area mapped out in the study broadly covers the middle and lower reaches of the Indus River and its tributaries, before they empty into the Arabian Sea.

Scientists had expected this area might be affected. Similar geographical areas along the Ganges River in neighbouring India and Brahmaputra in Bangladesh also contain pockets of arsenic contamination.

Normally, that arsenic would stay in the ground. But in the last few decades, South Asian countries concerned with pathogen-infused surface water have been pumping enormous volumes of groundwater, causing the water tables to drop drastically and tapping into new water pockets tainted by the colourless, odourless toxin.

The WHO considers arsenic concentrations above 10 micrograms per litre to be dangerous. Pakistan's guideline is five times that, and many of its wells test much higher.

Arsenic is naturally occurring and kills human cells causing skin lesions, organ damage, heart disease and cancer. There is no cure for arsenic poisoning.

“This study is important because it draws attention to an overlooked yet solvable problem of vast magnitude affecting the health of millions of villagers,” said geochemist Alexander van Geen of Columbia University's Lamont-Doherty Earth Observatory, who was not involved in the study.

He said the patterns it identifies are broadly consistent with data he and other researchers have collected from some 10,000 well tests in the region.

One of those researchers, Abida Farooqui, assistant professor of environmental sciences at Islamabad's Quaid-i-Azam University, said the new study's sample size may be too small to draw clear conclusions.

“The study revealed very important and an emerging problem of arsenic in the country,” Farooqui said. But “only 1,193 samples have been used to predict the situation in the whole Indus Valley, which is unrealistic.”

In any case, no map can tell villagers whether a specific well is contaminated. Arsenic concentration varies widely from pump to pump, and the only way to know for certain is to test each one.

Shallow wells are less likely to be tainted. Deeper ones, such as those run by the government's Drinking Water Filtration sites, may be more at risk.

This makes the problem especially acute for thousands of city-dwellers who have no access to clean water and rely on what the government supplies. At one Islamabad neighbourhood filtration site on Wednesday, resident Ali Hasan said the struggle was real.

“It's the government's job to provide us with clean drinking water, but everywhere we have to travel to find clean water,” Hasan said while filling a large plastic jug to take home to his neighbourhood.

A survey submitted to Pakistan's parliament last year suggested nearly 80 per cent of water sources in 2,807 villages across 24 districts were contaminated with bacteria or other pollutants, to levels that were unsafe to drink.

Now, “the presence of arsenic in drinking water is becoming a widespread health problem,” said Luis Rodrguez-Lado, a chemist with the University of Santiago de Compostela in Spain who was not involved in the study. Yet “there is a general lack of information” about which areas in Asia are most at risk.

For Pakistan, Bukhari said, the problem is now urgent. Her department is already working with the United Nations Children's Fund to provide cheap anti-arsenic water filters to poor villagers in the worst-affected areas.

“We should immediately discourage the indiscriminate ground water exploitation,” she said, noting that even city-dwellers with municipal water access were digging tube wells “to have a lavish supply of water.” But the country also needs to test countless tube wells and identify which have tapped into arsenic, possibly determining which depths might be safer, she said.

If researchers can find a depth at which “there is no arsenic, we can dig wells that stop before the water is contaminated,” Bukhari said.