Chronic arsenic poisoning has long inhibited Bangladesh. The South Asian country has spent decades battling natural occurrences of the toxin that can be found in millions of its shallow wells, the cause of an estimated one in every 20 deaths in the country (around 43,000 deaths a year according to the World Health Organization).

The good news is that incidents of contaminated groundwater in wells are reducing. While a study conducted in 2000 showed around 75 per cent of wells (out of 6,000 surveyed) exceeded World Health Organization guidelines of ten micrograms of arsenic per litre, a recent study found that this number has plummeted to only 30 per cent (of the 50,000 surveyed). 

‘Any sediment buried with plant matter is likely to release arsenic,’ explains Alexander van Geen, research professor at the Lamont-Doherty Earth Observatory, Columbia University. As with other countries downstream of the Himalayas, Bangladesh’s groundwater is in close contact with eroding coal seams packed with sulphide minerals from which arsenic can dissolve into aquifers.

According to van Geen, the poisoning has only occurred in the country since the 1970s, when villagers started installing cheap PVC wells in order to avoid the pathogens found lurking in surface water. They would not have known that they would instead be tapping into subsurface water stores containing hazardous levels of arsenic. 

Depth is key

Deeper wells are far less likely to be contaminated, with 150 metres being a reliable guideline for safe water. Multiple studies have shown that wells exceeding this threshold (there are more than 900) were disproportionately distributed to benefit politically well-connected households. Now, greater awareness of the importance of deeper wells has enabled communities to improve their own health by drilling deeper. 

The need for rice paddies to be flooded means arsenic has also turned up in food, a trend that may get worse. ‘Pumping deep for irrigation would be a concern,’ says van Geen. ‘It would accelerate downward contamination of deep aquifers that are typically low in arsenic, which we are starting to see around Dhaka. Fortunately, irrigation is not subsidised in Bangladesh and hence too expensive for farmers. They should try to use river water instead.’