Since the earliest known times, the Ganga has represented a sacred force to the people drawing their lifeblood from its waters. In many ways, the river symbolises India itself with its spiritual, cultural and religious importance – Hindus consider it the holiest body of water in the world with sacred pilgrimage sites and holy cities adorning its banks.

But, as one of the worlds’ most populated river basins and a major water resource for the 400 million people inhabiting its catchment, the Ganga today represents one of the planet’s most complex and stressed river systems. This makes understanding and management of the water quality along its 2,500km length an act of humanitarian and geopolitical relevance.

Many factors critically affect water quality along the Ganga including agricultural, industrial and domestic pollution. Large areas of the river catchment suffer from a critical lack of water and sanitation infrastructure, as well as increasing water demands in areas of intense population growth and migration, combined with the severe implications of land use and climate change.

Some aspects of water pollution are more easily observed as the river network evolves, whilst others contribute to an invisible water crisis affecting the life and health of hundreds of millions of people.

Scientists from the University of Birmingham’s Institute of Global Innovation have worked with colleagues from the UK and India to monitor the evolution of water pollution along the entire length of the Ganga and its major tributaries.  Their study – the first attempt at a longitudinal survey of the river system – examined three different sections of the Ganga from its source in the Himalayas to its meeting the Indian Ocean in the Bay of Bengal.

Stefan Krause, Professor of Ecohydrology and Biogeochemistry at the University of Birmingham, outlines the scale of the challenge with respect to its substantial plastic pollution.

“Some 20 rivers are estimated to account for almost 70% of global plastic emissions to the marine environment. Of these, the Ganga River is second only to the Yangtze River, but the scale and complexity of the Ganga network make assessing microplastic contamination along the watercourse, including the attribution of suspected pollution sources, and the fate of plastic within the river network challenging.”

Over six weeks in late 2019, Professor Krause and the team sampled surface water and sediment from more than 80 locations along the river’s entire length - analysing over 1,000 samples for nutrients, metals, organic contaminants, pathogens and faecal indicators, microbial activity and diversity, as well as microplastics.

The scientists analysed post-monsoon spatial patterns of microplastics, taking samples from a variety of hydrological conditions and land-use categories – for example, downstream of large urban areas, rural areas with agriculture and major tributaries, and sites of cultural and spiritual activity. They collected sediment samples from the riverbank, storing the material in glass jars, while they sieved and filtered 100 litre river water samples on-site – later extracting plastic particles between 63 and 5000µm by density separation followed by organic matter digestion. The work helped researchers to identify pollution hotspots and work out the specific size, type and potential origin of microplastics – ultimately better understanding the sources of river pollution and how they become activated.

Researchers did find that concentrations of pollutants did not generally just increase in a downstream direction but revealed pollution hotspots that varied in their locations and downstream footprints for different contaminants – a finding that could hold significant implications for the spatial reach of pollution hotspots and their legacy in time.

Comparing downstream pollution profiles of various contaminants found in surface water and sediment samples allowed the researchers to create an unprecedented multi-dimensional pollution dataset. This body of data will be invaluable in establishing the usefulness of large river network surveys to better understand the impacts of variable pollution sources and develop integrated approaches in water resources and pollution management.

After presenting the first findings of their work at the European Geoscience Union Conference, in Vienna, the team plans to publish the first of several research papers relating to its work on the Ganga in the summer of 2021.

Professor Krause believes the findings of the group – which includes scientists from British Geological Survey, IIT Roorkee, National Institute of Hydrology (India), Mahavir Cancer Institute and Research Centre Patna, Bose Institute, Kolkata and the UK Centre for Ecology & Hydrology, as well as the Universities of Lincoln, Manchester and the West of England - will prove significant for water and pollution experts around the globe.

“Our results reveal the co-existence of distinct pollution hotspots for several contaminants that can be linked to population density and land use near sampling sites and the contributing catchment area. While some hotspots displayed increased concentrations of most contaminant groups, we identified several areas of specific pollutants, including microplastics that could be linked to specific cultural and religious activities.

“Interestingly, longitudinal concentration profiles in sediment and surface water samples do not match, hinting at differences in source as well as transport, accumulation and long-term storage mechanisms. We believe our work will be used as a springboard for future research assessing local microplastic contamination, as well as helping to refine medium and large-scale models estimating the distribution of microplastics in freshwater environments.”