The Indo-Gangetic basin (IGB), an alluvial aquifer system that is one of the world’s most important freshwater resources and home to the largest surface-water irrigation system, is getting extensively contaminated, new research shows. Contamination is in fact beginning to outstrip the rapid depletion of groundwater in the area.

The IGB system, formed by eroded sediments from the Himalayas and redistributed by the Indus, Ganges and Brahmaputra river systems, forms a flat fertile plain across Pakistan, northern India, southern Nepal and Bangladesh. There are many aquifers within the IGB, both geographically and at different depths. Of late, the focus has been on the alarming depletion of groundwater due to its over-extraction from the IGB, which now accounts for a quarter of global groundwater withdrawals, to sustain agricultural productivity in Pakistan, India, Nepal and Bangladesh.

However, new research published in the journal Nature Geoscience, based on high-resolution on-the-spot records of groundwater levels, abstraction and groundwater quality, shows that “sustainable groundwater supplies are constrained more by extensive contamination than depletion.” The scientists examined the composite data of all groundwater to a depth of 200 meters across the IGB.

Groundwater levels are falling in 30% of the IGB aquifer system, amounting to a net annual depletion of five to 11 trillion litres (or cubic kilometres) each year, the study found. But within 60% of the aquifers, access to potable groundwater is restricted by excessive salinity or arsenic.

“Depletion of groundwater, while being lesser in magnitude, occurs in urban centres where large populations reside,” lead author Scott Fendorf, Huffington Professor of earth science and chair of earth system science at Stanford University, told The Wire.

Previous satellite-based observations were “poorly constrained” by ground-based studies that can provide insights into the groundwater dynamics, and increasing salinisation and nitrate contaminations in groundwater, says the research team. Their analysis of groundwater in the IGB, in contrast, is based on on-the-ground measurements. The scientists analysed several years of groundwater level records from 3,429 water wells and additionally compiled existing high-resolution datasets and studies in Pakistan, India, Nepal and Bangladesh. According to them, this is the first time that the status of groundwater across the IGB alluvial aquifer has been analysed, entirely from local on-site (in situ) measurements.

Arsenic worries

“Arsenic is the most significant contaminant of the basin: 37% of the groundwater has hazardous levels of arsenic,” Fendorf says. “High salt content, which is problematic for agriculture and for the taste of drinking water, affects 23% of the groundwater.”

The Nature paper also says that “tackling the arsenic issue is more complex”. Arsenic in very old aquifers – dating to the Pleistocene age (between 11,700 and 2.5 million years ago) is generally found between 100 and 200 metres below the surface. These aquifers are a safe drinking water source. However, large-scale localised pumping in many areas for irrigation leads to arsenic-laden water being drawn from upper aquifers. High arsenic levels were also reported in the irrigation groundwater in the Indo-Gangetic plain in a 2015 study by the National Botanical Research Institute (NBRI), Lucknow. It had been published in the journal Environmental Monitoring and Assessment.

The scientists monitored arsenic content in irrigation groundwater and paddy soil, and the accumulation of arsenic in the roots and grains of different paddy varieties grown in the arsenic-contaminated middle Indo-Gangetic plains of northern India. Their results showed the highest arsenic contamination in the irrigation groundwater and in paddy soil, occurring at levels that significantly exceeded the recommended threshold values of 100 micrograms per litre recommended by the European Union and 20 milligrams per kg recommended by the UN Food and Agriculture Organisation.

Falling groundwater levels

The team also assessed groundwater-level variations, groundwater quality and groundwater storage within the top 200 metres of the IGB aquifer system. It found that the water-table within the IGB alluvial aquifer “is typically shallow (less than five metres below ground level) and relatively stable since at least 2000 throughout much of the basin, with some important exceptions.” In areas of high groundwater extraction – in northwest India and the Punjab region of Pakistan – the water table can be more than 20 metres below ground level and is falling at rates of more than one millimetre every year.

According to the report, groundwater levels are deep and falling beneath many urban areas, and particularly in large groundwater-dependent cities such as Lahore, Dhaka and Delhi. “Within the many urban areas that house much of the region’s population, substantial groundwater drawdown is observed and current extraction is not sustainable,” the study says. The scientists suggest the use of rainwater harvesting, among others, to replenish groundwater levels and help to curb the continued decline observed over since the 1980s. Limiting water loss due to evaporation from agricultural fields where groundwater depletion is a concern will also serve to help preserve sustainable levels, it adds.

Moreover, shallow and rising water tables are found in the lower Indus, parts of the lower Bengal basin, and in places throughout the IGB aquifer as a result of leakage from canals, rivers and irrigation. The largest depletion occurred in areas of high abstraction and consumption in northern India and Pakistan: Punjab, Haryana, Uttar Pradesh states in India, and the Punjab region in Pakistan.

Downstream, in the lower Indus and within the Sindh, groundwater is accumulating. As a result, the land has become increasingly waterlogged and less water has been flowing to the sea.

The latest research findings add to concerns expressed in a 2015 United Nations World Water Report that India, China, Nepal, Bangladesh and Pakistan (four of these are in the IGB) alone account for almost half of the world’s total groundwater use. Referring to the Asia Pacific region, the UN report says that coastal cities such as Kolkata, Dhaka, Jakarta and Shanghai are experiencing saltwater-intrusion in groundwater supplies due to “uncontrolled groundwater abstraction as a result of the inadequacy of the public water supply system.”

The report adds that natural contaminants found in the region’s groundwater systems include arsenic, fluoride and iron, while human-made contaminants come from fertilisers and pesticides used in agriculture, for mining and in tanneries.

Further, research from the Massachusetts Institute of technology (MIT), Boston, published in the journal PLOS ONE in 2015, cautions that “there is a high risk of severe water stress in densely populated watersheds” in Asia by 2050, compared to recent history. It goes on to add later, “There is strong evidence to suggest that in the absence of autonomous adaptation or societal response, a much larger portion of the region’s population will live in water-stressed regions in the near future.”