New Delhi, Jul 31 (PTI) Thousands of years before Delhi became a sprawling metropolis, the Yamuna quietly carved its way across the plains, repeatedly changing course and leaving behind layers of sand, silt, clay and gravel that formed the floodplains stretching along the river in the city today.
While often seen as vacant land vulnerable to encroachment, these floodplains continue to perform a critical function beneath the surface, naturally filtering polluted river water before it reaches Delhi’s drinking water supply, according to a first of its kind study.
“The Yamuna has been flowing and meandering across its floodplains for thousands of years. Rivers naturally change course over time, and they need space to continue doing so in the future. Floodplains should therefore be left to the river, as they are essential not only for the river’s health but also for recharging and protecting groundwater,” Manish Kumar, one of the researchers told PTI.
As river water slowly passes through the sediment deposited by the river over centuries, many harmful pollutants are reduced before the water reaches Ranney wells, which supply drinking water to parts of the national capital.
The researchers detected 57 organic micropollutants in the Yamuna, including pharmaceutical residues, pesticides, chemicals used in personal care products and cosmetics, endocrine-disrupting compounds, plastic-related chemicals known as phthalates, fatty acids, food additives, hormones and traces of hospital waste.
However, once the river water entered the floodplain, both the number of pollutants and their concentrations declined steadily. Wells farther from the Yamuna consistently contained fewer pollutants than those closer to the river, showing that the natural filtration process becomes more effective with increasing distance.
Among the pharmaceutical compounds detected, one of the highest concentrations was recorded for 3-acetamido-5-bromobenzoic acid, which reached 45.2 micrograms per litre in the river during the non-monsoon season but was detected in only four Ranney wells and always below one microgram per litre.
Another commonly used painkiller, diclofenac, reached 19.4 micrograms per litre in river water during the monsoon but dropped to around 0.2 micrograms per litre in the Ranney wells where it was detected.
Ketoprofen, another anti-inflammatory medicine, was detected in river water but not in any Ranney well, suggesting it was almost completely removed during underground filtration. Other medicines, including clofibric acid, ibuprofen and gemfibrozil, also showed substantial reductions before reaching the wells.
The study reported a similar trend for pesticides. While several pesticides were present in relatively high concentrations in the Yamuna, almost all were reduced to below one microgram per litre in the Ranney wells. Only simazine and aldrin occasionally exceeded that level, while alpha-endosulfan, terbuthylazine, atrazine, heptachlor and metribuzin also declined significantly.
The floodplains also reduced concentrations of triclosan, chemicals used in cosmetics and personal care products, phthalates, endocrine-disrupting compounds and residues linked to hospital waste.
The process, known as riverbank filtration, occurs when river water slowly seeps through underground layers of sand, silt, clay and gravel before entering wells. During this journey, particles are trapped, many chemicals stick to the soil, microorganisms break down contaminants and the water mixes with groundwater, improving its quality.
Kumar further said that among the most striking observations was the significant breakdown of contaminants such as antibiotics, pesticides, hormones including estrogen, and common painkillers like ibuprofen through microbial activity.
The researchers described riverbank filtration as an inexpensive and sustainable natural pre-treatment system for drinking water but stressed that it cannot replace proper sewage treatment.
The researchers collected water samples from the Yamuna and eight Ranney wells over 13 months during both monsoon and non-monsoon seasons. The wells, located between around 1.2 km and 4 km from the river, allowed the researchers to compare how water quality changes as it travels underground through the floodplain.
Ranney wells are underground collector wells built close to rivers that collect water after it naturally seeps through surrounding sand and soil instead of drawing it directly from the river. Delhi Jal Board operates several such wells along the Yamuna floodplains.
The study focused on organic micropollutants, these chemicals enter rivers through untreated sewage, industrial wastewater, agricultural runoff and household activities, and many are not completely removed by conventional sewage treatment plants.
The study explains that central Delhi is underlain by a young alluvial aquifer formed through repeated flooding and shifts in the Yamuna’s course over thousands of years. These deposits of sand, silt, clay, pebbles and kankar form a freshwater aquifer between 30 and 85 metres thick from which the Ranney wells draw water.
The researchers said these geological conditions make riverbank filtration possible. Without the floodplains created by the Yamuna over centuries, Delhi would not have the same natural underground filtration system that supplements its drinking water supply.
He further said that, encroachments on floodplains can disrupt these natural processes, contaminate groundwater, and undermine the critical ecological functions that floodplains perform.
“Protecting these areas is therefore essential for the long-term health of both the river and the city’s water resources,” he said.
They concluded that while riverbank filtration provides an effective, low-cost and sustainable first step in improving water quality, protecting the floodplains and expanding wastewater treatment remain essential for Delhi’s long-term water security.
The study was conducted by researchers from IIT Roorkee, Dr. Vishwanath Karad MIT World Peace University, Auburn University in the US, Tecnológico de Monterrey in Mexico, and Tianfu Yongxing Laboratory in China. PTI SGV MDO MDO
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