The greatest threat to sustainable groundwater in the Indo-Gangetic Basin is contamination and not depletion, according to a study published online this week in Nature Geoscience. Using groundwater measurements from across the region, the study reveals that over 60% of accessible groundwater is no longer drinkable or usable for irrigation due to high concentrations of arsenic or salt.
The Indo-Gangetic Basin accounts for 25% of global groundwater extraction, supporting the livelihoods and agricultural activities of more than 750 million people in Pakistan, India, Nepal and Bangladesh. Satellite gravity measurements have indicated that groundwater levels are declining across the region as extraction for agriculture increases. However, field measurements are needed to understand groundwater patterns at a local scale.
Alan MacDonald and colleagues report thousands of such groundwater field measurements in combination with existing groundwater datasets to reveal a more diverse picture of groundwater changes across the Indo-Gangetic Basin over the past decade. They find that groundwater levels are falling in 30% of the basin, particularly near major cities, but are stable or even increasing across the other 70% due to recharge from leaky irrigation canals. However, they also find that for groundwater up to a depth of 200 metres - which represents a volume 20 times greater than the combined annual flow of the Indus, Brahmaputra and Ganges rivers - more than 60% is contaminated by arsenic or salt.
In an accompanying News & Views article, Scott Fendorf and Shawn Benner write that the study illustrates “the importance of monitoring both groundwater quantity and quality, in combination with active management, to ensure water and food security for the vast Indo-Gangetic Basin region.”