Untreated wastewater from drains creates localized pollution hotspots
Ashley Wheaton, SuSanA Secretariat
Few rivers carry as much symbolic, cultural, and economic weight as the Ganga. Flowing across northern India, it sustains agriculture, domestic water needs, and spiritual life for millions. Yet in many urban stretches, the river’s greatest challenge is not upstream hydrology or climate variability but the steady inflow of untreated wastewater through city drains. The pollution of the Ganga is often imagined as a problem of visible discharge that include industrial effluents, floating waste, and untreated sewage entering the river in plain sight. But in cities like Patna, the most significant threats flow quietly through an intricate network of urban drains.
A recent scientific study titled “GIS-driven insights into seasonal water quality shifts: The Ganga River’s journey through Patna’s urban drains (Bihar, India)", published in Cleaner Water (March 2026) by Bhawana Raj, Avinash Dass, Umesh Kumar Singh, and Rajesh Kumar Ranjan, examines this challenge in detail. The study uses geographic information systems (GIS), multivariate statistical techniques, and irrigation water quality indices to trace how Patna’s drains influence the chemistry of the Ganga and how these changes vary across seasons and locations.
The research offers a rare, spatially detailed look at how urban drainage systems shape river health, revealing that the Ganga’s pollution in cities often enters quietly through a network of drains rather than through visible industrial discharges.
Patna’s rivers and drains
Patna, located along the middle stretch of the Ganga basin, is shaped by a complex river system. The Ganga flows along the northern edge of the city, the Sone river lies to the west, and the Punpun river system runs through the central urban area. This hydrological network historically supported flood drainage and agricultural productivity across the fertile alluvial plains of Bihar.
However, rapid urban expansion has transformed these natural drainage systems. With a population exceeding two million, the city produces large volumes of domestic sewage, stormwater runoff, and urban waste. Much of this wastewater enters the Ganga through open drains before any treatment occurs.
Several major drains, including those at Digha, Kurzi, Bhadraghat, and Ghaghaghat, act as direct conduits carrying municipal wastewater, agricultural runoff, and other pollutants into the river. Over time, these drains have become localised pollution hotspots that influence the water quality of the Ganga along the Patna stretch.
Despite this, many monitoring efforts historically focused on the river itself rather than on the drainage channels feeding it. The new study therefore takes a different approach by examining the drain–river interface, where pollution actually enters the system.
Mapping pollution with GIS
To understand the spatial influence of drains on river water quality, the researchers conducted a detailed field sampling programme. Twenty water samples were collected during summer and another twenty during winter from locations along the Ganga, including points upstream and downstream of major drains.
The samples were analysed for a broad range of physicochemical parameters, including:
pH and turbidity
electrical conductivity (EC)
total dissolved solids (TDS)
dissolved oxygen (DO)
major ions such as sodium, potassium, calcium, and magnesium
anions including chloride, nitrate, sulphate, and phosphate
To interpret these results, the study combined laboratory analysis with advanced statistical tools such as principal component analysis and Pearson correlation matrices. GIS mapping was then used to visualise spatial variations in water quality along the river. This integration of field measurements, spatial analysis, and statistical modelling allows researchers to identify pollution hotspots and understand how different drains influence the river at different locations.
Seasonal changes in river chemistry
One of the most significant findings of the study is the strong seasonal variation in water quality. Chemical parameters such as sodium, magnesium, chloride, and total dissolved solids were found to increase during winter compared to summer.
This seasonal shift reflects hydrological dynamics. During summer and monsoon months, higher river discharge dilutes pollutants entering through drains. In winter, however, lower river flows reduce dilution capacity, allowing wastewater inputs to exert a stronger influence on river chemistry.
As a result, the impact of urban drains becomes more pronounced during low-flow periods. Drains that appear relatively minor during high-flow seasons may significantly alter water quality during winter months.
This seasonal dimension underscores the importance of continuous monitoring rather than relying on single-season assessments of river health.
Is the water still fit for irrigation?
Despite the pollution inputs, the study finds that the Ganga’s water along the Patna stretch remains largely suitable for irrigation.
Several indicators support this conclusion. The pH values of the water samples remained within the acceptable range for irrigation recommended by the Food and Agriculture Organisation. Total dissolved solids generally remained below levels considered harmful for agricultural use. Electrical conductivity values also indicated that most river water samples do not pose serious salinity risks for irrigation.
However, the researchers also identified localised areas where water quality deteriorates near drain outlets. Elevated electrical conductivity and sodium concentrations were observed near certain drains, particularly the Kurzi and Bhadraghat drains.
These localised increases could eventually pose risks for agricultural soils if irrigation continues with water containing higher sodium levels. Over time, sodium accumulation in soils can degrade soil structure and reduce permeability.
Thus, while the overall irrigation suitability remains acceptable, the findings suggest that continued wastewater inflows could gradually create soil management challenges in downstream agricultural areas.
What the statistics reveal about pollution source
The study also used multivariate statistical analysis to identify the origins of pollutants entering the river.
Principal component analysis revealed that the river’s water chemistry is shaped by three main influences:
Natural geochemical processes, including mineral weathering and rock–water interaction.
Urban wastewater inputs, including domestic sewage and stormwater runoff.
Nutrient enrichment from agriculture, particularly fertilizer runoff and organic waste.
Strong correlations between sodium, chloride, electrical conductivity, and total dissolved solids indicate that these parameters are influenced by both natural mineral dissolution and urban wastewater discharges. Meanwhile, elevated phosphate and nitrate levels suggest nutrient inputs from agricultural activities and domestic wastewater.
These findings confirm that river pollution in urban areas rarely has a single source. Instead, it reflects a complex interaction between natural geochemical processes and human activities across the watershed.
Why drains matter in river management
One of the most important insights from the study is that river pollution is highly localised. Instead of being evenly distributed along the river, contamination tends to cluster around drain outlets.
This has significant implications for river restoration programmes. Many large-scale initiatives focus on constructing sewage treatment plants or implementing broad river-cleaning schemes. While these interventions are essential, they may not fully address the problem if wastewater continues to enter rivers through multiple unregulated drains.
The Patna study highlights the need to shift from a city-scale approach to a drain-specific strategy for river pollution control. By targeting drains directly, policymakers can reduce pollution loads entering the river more effectively.
Lessons for India’s river cities
The situation observed in Patna reflects a broader challenge facing many Indian cities. Rivers such as the Yamuna in Delhi, the Musi in Hyderabad, and the Sabarmati in Ahmedabad also receive significant pollution through urban drainage systems.
In many cases, drains function as the final link between urban wastewater systems and rivers. Unless these channels are managed properly, even well-designed sewage treatment plants may struggle to prevent pollution.
The Patna study therefore highlights a critical lesson: restoring rivers requires managing urban wastewater networks as much as managing the river itself.
Towards a drain-focused river strategy
The researchers conclude that long-term protection of the Ganga will require stronger monitoring and targeted interventions. They recommend strengthening sewage treatment infrastructure, improving coordination between urban authorities and pollution control agencies, and adopting GIS-based monitoring to identify pollution hotspots.
Equally important is the need for long-term monitoring programmes that integrate irrigation suitability indicators. Since millions of farmers depend on the Ganga for irrigation, understanding how water quality affects soils and crop productivity is essential for sustainable river basin management.
Ultimately, the study demonstrates that the future of the Ganga in cities like Patna will be shaped not only by large river restoration projects but also by the smaller, less visible channels that feed into it. These drains may appear insignificant individually, but together they determine how urban life leaves its imprint on one of the world’s most important rivers.
In the end, protecting the Ganga may depend on paying closer attention to those overlooked intersections—where city drains quietly meet the river.