An international team of scientists created a "hazard map" of locations at risk using water samples collected from almost 1,200 sites across the country, the majority of them from hand and motor pumps.
Using statistical modeling, the team also assessed environmental factors that may affect the movement of arsenic and calculated the size of populations at risk of exposure. They concluded that there was widespread arsenic contamination in the Indus Plain, from where 50 million to 60 million people get their groundwater.
"This alarmingly high number of people likely affected demonstrates an urgent need to test all drinking water wells in the Indus Plain," the team said in a statement.
The researchers have urged authorities to treat affected wells accordingly. Arsenic does not have a smell or a taste, and there are no short-term symptoms after consuming it. However, drinking contaminated water regularly could lead to serious illnesses including lung cancer and cardiovascular diseases, the team said.
The study "doesn't mean every well in the area is affected, but the easiest thing to do and the first obvious step is to test each and every well," said Joel Podgorski of the Swiss Federal Institute of Aquatic Science and Technology, the study's lead author. "Once it's known that a well has arsenic in it, people can shift to a safe water source. Or a filter could be used."
'Humans have exacerbated the issue'
But use of either measurement would not change the number of people who the team believes to be at risk, Podgorski said, with most supplies in the area breaching 50 micrograms per liter.
Podgorski added that the arsenic naturally occurred in the groundwater but that "it's possible in some cases that humans have exacerbated the issue."
"We've alluded in the study to a correlation between arsenic levels and high organic pollution in groundwater," he said. "Sometimes, human and animal waste getting into a shallow well can make the issue worse locally, as it causes a different type of of arsenic release."
The team also found a correlation between irrigated areas and high arsenic levels in the water, though Podgorski stressed that this could be a coincidence.
Beyond further testing, the team suggested emergency measures such as health intervention programs for those affected.
The need to know more
"We need to know now how many people are affected as well as at risk," said Mohammad Shamsudduha, a research fellow and hydrogeology expert at University College London who was not involved in the research. "I'm quite convinced by their data, as they've collected so many samples."
He added that the fact that the group has linked contamination to the number of people exposed is...
Read More: Arsenic in water puts up to 60 million in Pakistan at risk, study says