The gravest threat to groundwater in India isn’t over-exploitation but arsenic and salt contamination.
This is the conclusion of a new study published on Monday in Nature Geoscience, which also challenges several recent reports to say that 70 per cent of the water table in most of the Indo-Gangetic aquifer — among the largest in the world — is ‘stable and mostly improving’.
The authors of the study, “Groundwater quality and depletion in the Indo-Gangetic Basin mapped from in situ observations”, say the unsustainable levels of groundwater extraction are largely limited to urban agglomerations in Punjab, Haryana and Uttar Pradesh.
However, nearly 23 per cent of the 300 BCM (billion cubic metre) is extremely saline and about 40 per cent contaminated by arsenic, says the study, authored by an international team of researchers led by A.M. MacDonald and H.C. Bonsor, which includes K. Gopal of the National Institute of Hydrology, Roorkee and A. Mukherjee, of IIT Kharagpur. In recent years, several reports have warned of alarming groundwater depletion in northwest India and Pakistan based on satellite imagery from the Gravity Recovery and Satellite Experiment (GRACE) that has minutely tracked how gravity varies across the earth since 2002.
For their study, Prof. MacDonald, Principal Geologist with the British Geological Survey, and colleagues analysed groundwater tables from 3,429 wells, high-resolution spatial data and studies within Pakistan, India, Nepal and Bangladesh between 2000 and 2012.
Substantial variations The researchers sought to assess groundwater-level variations, groundwater quality and groundwater storage within the top 200 m of the Indo-Gangetic aquifer.
“Compiled water-table records indicate substantial spatial variability…(This) is unresolvable by GRACE and depends on ground-truth observations,” the authors added.
Canals built in the 19 and 20 centuries significantly influenced groundwater trends, the study says as water accumulated at the origins of the canal tended to “leak out”, leading to high recharge and sometimes floods.
Moreover, geological variations determine how much groundwater is available in a region and only regular groundwater observations would give an accurate picture of water availability, the researchers add.
Independent experts said groundwater trends were highly variable across India and could change year on year.
“Even one year of drought can substantially alter groundwater availability though we’ve known for a while that it’s the northwest where groundwater levels are worrisome,” said G.S. Jha, former chief of the Central Groundwater Board.
According to a review of India’s water resources by the government in December 2015, India has a usable resource of 1,123 BCM/year of which surface water and groundwater is 690 BCM/year and 433 BCM/year respectively. Setting aside 35 BCM for natural discharge, the net annual ground water availability for the entire country is 398 BCM.