Research by the School of Environmental Studies (SES), Jadavpur University, Calcutta, has revealed that the tubewell of a single rural water supply scheme in Malda in West Bengal had pumped out as much as 143 kg of arsenic in 1991. Though it is proved to be coming out of underground rocks, scientist have not fully understood what causes it. They have suggested that large-scale ground water extraction for agriculture may be one of the reasons. Another view is that due to the high use of phosphate-based fertilisers, the arsenate of the soil might have leached into the aquifers.

Some scientists believe that heavy ground water extraction permits air to enter the underground aquifer and the oxygen is responsible for the degradation of the arsenic-rich source.

DIPANKAR CHAKRABORTI, the director of SES, suggests investigating the role of nitrate fertilisers as well as certain microbes in breaking down pyrite (a compound of iron and sulphur). Pyrite is often rich in arsenic and the acid released during the breaking down process allows it to leach into the water.