The water table has either been stable or rising between 2000 and 2012 across 70 per cent of the aquifer in the Indo-Gangetic Basin that is spread over more than 250 million hectres across India, Bangladesh and Pakistan. In the remaining 30 per cent of the aquifer, ground water levels are falling, amounting to a net annual depletion of around 9 km3 in the past decade. And the areas of decline are near the highly populated areas.
The volume of groundwater up to 200 metres depth in the Basin is 20-30 times the combined annual ﬂow of the three major rivers — Indus, Brahmaputra and Ganges, says a study published today (August 29) in the journal Nature Geoscience.
According to A.M. MacDonald, the first author of the paper from the British Geological Survey, Edinburgh, U.K., the new evidence comes from in situ records of groundwater levels, removal and quality within the top 200 metres of the aquifer. These were assessed by mapping specific yield from lithological and hydrogeological data, and compiling.
But that is where the good news ends.
More than the declining water table in 30 per cent of the aquifer, the most widespread threat to groundwater is rather the poor quality of water. About 60 per cent of groundwater in the Indo-Gangetic Basin aquifer is contaminated either with excessive salinity or arsenic.
Of the 30,000 km3 (with an error margin of 14,000 km3) of groundwater present in the aquifer, about 23 per cent has salinity greater than 1,000 mg per litre in the Indo-Gangetic Basin’s upper aquifers. So high is the salinity in this region that water is unfit for agriculture use and drinking purposes. Add to this another 37 per cent of the aquifer that is contaminated with arsenic at toxic concentration.
Origin of salinity
Saline intrusion, marine transgressions in the past, dissolution of evaporite layers and excessive evaporation of surface or shallow groundwater are some of the natural processes responsible for the excessive salinity seen in the aquifer. And irrigation and shallow water tables have only exacerbated the natural salinity. “The widespread salinity in the Indus Basin and Upper Ganges is terrestrial in origin and formed by a combination of natural and anthropogenic activities,” they write.
Arsenic contamination begins in the Himalayas and is transported to the basins and deltas. Once buried, arsenic is slowly released from the sediments and leads to contamination of the groundwater.
Arsenic contamination is restricted to the uppermost 100 metres across the floodplains in the southern Bengal Basin, they say. In this region, the arsenic concentration is more than 100 microgram per litre. Arsenic concentration is less extreme at over 10 microgram per litre in Assam, southern Nepal, the Sylhet trough in eastern Bangladesh and Holocene sediments along the course of the river Ganges and Indus.
Even at the lowest concentration, arsenic is harmful to humans and makes the water unsuitable for domestic use. The arsenic contamination gets compounded when it gets accumulated in the soil irrigated with contaminated groundwater and the subsequent accumulation of arsenic in food grains grown in such contaminated soil.
According to the paper, previous work has shown that intensive pumping of shallow groundwater can indeed remove arsenic from the aquifer. In fact, irrigation from shallow depths tends to protect the deeper part of the aquifer from getting contaminated. Groundwater contamination by arsenic at toxic levels will persist for decades to centuries, notes an accompanying News piece in the same issue of the journal.
The silver lining is that aquifer at depths greater than 100 metres generally has lower arsenic concentration unlike in the shallow depths. This raises the possibility of pumping out groundwater from greater depths to avoid toxic arsenic concentration. Withdrawal from a few deep wells holds promise for a sustained supply of drinking water, the authors note. Though water from greater depths tends to have high salinity, it may be less harmful than toxic arsenic seen at shallow depths.
“Protecting those deeper aquifers that remain potable should be a regional priority,” writes Scott Fendorf from Stanford University and Shawn Benner from Boise State University, Idaho, U.S. in the News piece.