“It may be the size of the Asian rivers, large because they drain the Himalayas, that makes the pollution so prominent.”
Lakshmi Supriya is a freelance science writer based in Bengaluru.
In vintage crime novels, there is often somebody murdered by slow poisoning, and arsenic has been a common weapon of choice. It works the same way in your body – slowly killing you – if it is present in the water you drink beyond a certain threshold. This is why it’s disturbing that, according to a new study, the groundwater along the densely populated Indus river basin in Pakistan is severely contaminated with arsenic, putting the health of over 50 million people at risk.
Arsenic occurs naturally in Earth’s crust. It is used by humans in some alloys in car batteries and semiconductors, as well as to make some pesticides and herbicides. Certain inorganic compounds that contain arsenic are highly toxic. Exposure in small doses causes headaches, dizziness, diarrhoea and changes in skin colouration. When the poisoning becomes acute, convulsions, vomiting and muscle cramps can be caused. Prolonged exposure to arsenic affects various organs – including the lungs, skin and the kidneys – leading to various types of cancers and ultimately death. Arsenic in the soil accumulates in plants, especially in leafy vegetables and apples, and may inhibit plant growth. However, it is at its deadliest to humans when it pollutes groundwater used for drinking or irrigation. It has been estimated that about 200 million people worldwide use such arsenic-contaminated water.
Investigations into the quality of groundwater from the previous decade have revealed that the large river basins in South Asia contain harmful levels of arsenic. The Ganga-Brahmaputra delta in India and Bangladesh and the Red River basin in Vietnam are greatly affected.
The effects of drinking arsenic-contaminated water in India emerged most prominently in the early 1980s in West Bengal and, over time, in other states in the Gangetic plains, such as Bihar and Uttar Pradesh, and in the Brahmaputra basin, including in Assam and Manipur.
The areas usually affected are low-lying, where the movement of water is slow and its flow carries a large amount of sediments. This reduces the amount of oxygen available in the water, which forms the stable and more water-soluble compound arsenite. Once the oxygen levels begin to drop, arsenic is released from the sediments and into the water, and increasing its concentration in groundwater. “It may be the size of the Asian rivers, large because they drain the Himalayas, that makes the pollution so prominent,” John McArthur, a geochemist at University College London who has been studying arsenic contamination around the world, told The Wire.
While several small-scale studies have found that the groundwater in several areas in Pakistan is loaded with arsenic, the full extent of the problem has remained out of focus. “The original motivation for the country-wide sampling campaign came about from wanting to identify the full scope of the arsenic contamination problem in the country, which is what our study has accomplished,” Joel Podgorski, of the Swiss Federal Institute of Aquatic Science and Technology, Switzerland, and the lead author of the new study, told The Wire.
Between 2013 and 2015, the researchers collected more than 1,100 water samples throughout the country from both household pumps and municipal and agricultural tube wells, and analysed these samples for arsenic and other elements. Using this and previously published data, along with hydrological and topographical data for the country, they were able to visualise the full extent of the problem.
According to the World Health Organisation, more than 10 micrograms of arsenic per litre of water is hazardous to health. Pakistan’s official guidelines recommend an upper limit of 50 micrograms per litre of drinking water.
Podgorski and colleagues found that the water was heavily contaminated along multiple points of the Indus. As it made its way across the length of Punjab (especially along the banks of the tributaries Ravi and Sutlej), entered northern Sindh and emptied into the Arabian Sea south of Karachi, arsenic levels frequently crossed the 50 micrograms mark. In northern Sindh, a cluster of samples showed more than 200 micrograms of the metal per litre of water. Beyond the plains watered by the river and its tributaries, the arsenic levels were within safe limits.
According to Podgorski, there are several reasons the Indus basin is so plagued by arsenic – apart from the river’s sluggish flow in the plains, which causes arsenic to accumulate in aquifers. The sediments being washed out from the Himalayas are still relatively young, less than 10,000 years old. Compared to older sediments that would have already leached out their arsenic from the sediments, these have been exposed to the environment only recently, and are still in the process of releasing their arsenic into the water.
Specifically, they found a strong correlation between high arsenic concentrations and the pH of the soil. A higher pH causes arsenic to be released easily from the sediments and into the water. It may be possible that in the arid central plains of Pakistan, the highly alkaline soil (corresponding to higher pH) enhances the release of arsenic from the sediments, especially to water near the surface, which could then migrate to deeper sources.
“The fact that irrigation correlates highly with arsenic contamination in the Indus valley leads us to speculate that it may be contributing to – but not exclusively causing – the problem by raising the pH of the soil through evaporation and transporting the released arsenic to the aquifer depth,” Podgorski explained.
The study estimates that the high level of contamination puts about 50-60 million people living along the Indus basin in the Sindh and Punjab provinces, including the densely populated cities of Lahore and Hyderabad, at risk of drinking toxic water.
“For those living within the Indus Plain, their water supply should be tested for arsenic, since not all wells in this area are contaminated,” Podgorski said. “In fact, contamination can be so heterogeneous that wells within the same village can have both safe and unsafe concentrations of arsenic. Only once all of the wells have been tested is it possible to know from which wells it is safe to take drinking water.” Once toxic wells have been identified, either that well should be closed or arsenic filters should be installed.
But despite the scale of the problem, both Podgorski and McArthur agree that the contamination will not spread to any new areas, as rivers only flow downhill. However, McArthur said that arsenic-rich water can spread to more areas within the same basin at a rate of a few sq. metres per year in the big delta and alluvial plains.
The extent of the problem today is significant and affects a large number of people, and McArthur says that the first step in helping the affected people is for the government to recognise that there is a problem. Once it does, then it could help ensure that people get piped water from local arsenic-free wells or from contaminated sources that have been treated.