‘Flushing’ impact: Sept floods cleanse Yamuna

Yamuna's water quality in Delhi reached its best since 2013 due to flooding that diluted pollutants, though experts warn improvements may be temporary.

Water quality in the Yamuna touched its cleanest levels since 2013 in Delhi this September, according to the latest monthly report from the Delhi Pollution Control Committee (DPCC). Experts attributed the rare improvement to the river being in spate during recent flooding, which flushed out pollutants and diluted sewage.

Between August 31 and September 4 this year, Hathnikund barrage discharged more than 100,000 cusecs per hour for five consecutive days. (HT File photo)

Records of Yamuna monitoring have been available since 2013, and the September data stands out as the most dramatic improvement yet. To be sure, this is the first time that sampling was done in the middle of a high-flood spell. In 2023, when Delhi hit its highest-ever Yamuna level of 208.66 metres on July 11, sampling was conducted earlier that month, before the peak.

On September 3, when the river ran high from heavy rains upstream and heavy discharge from the Hathnikund barrage, samples were collected at six of eight monitoring points across the capital. Faecal coliform -- a key marker of sewage contamination – was at 790 most probable number (MPN)/100 ml at Palla, touching a peak of just 3,500 MPN/100 ml at Okhla barrage, close to the permissible limit of 2,500.

For context to how low numbers are, the river recorded 54,000 units last month and a staggering 9.2 million in July. The all-time high of 1.2 billion units was seen in December 2020.

Dissolved oxygen (DO), which typically disappears downstream of Wazirabad, remained between 3.7 mg/l and 5.1 mg/l -- with 5 mg/l the minimum needed to sustain aquatic life. Biological oxygen demand (BOD), another critical indicator, improved significantly. At Okhla barrage, it touched 13.5 mg/l, down from 24 mg/l in August and 70 mg/l in July. At ITO, the BOD dropped to 4 mg/l, a figure not seen in the last decade downstream of Wazirabad.

To be sure, experts attributed this to Delhi seeing its third-highest Yamuna level on record on September 4, at 207.48 metres. The high water diluted pollutants, while flood-control measures meant many drains were shut to prevent backflow, further reducing sewage entering the system.

“This is the first time in 10 years we have DO present throughout the river stretch and BOD dipping so low downstream,” said Pankaj Kumar, a Yamuna activist. He credited both the flood-driven flush and recent improvements in sewage treatment plants (STPs). “STPs are releasing better-quality water than in previous years, which is also contributing,” he added.

DPCC noted that samples could not be collected from Nizamuddin Bridge or Asgarpur due to flooding.

Between August 31 and September 4 this year, Hathnikund barrage discharged more than 100,000 cusecs per hour for five consecutive days. The flow then tapered but left the river temporarily cleansed.

Experts, however, caution the improvement is short-lived. “Samples were taken when a huge flood volume had just reached Delhi, flushing out pollutants. But once the flow reduces, effluents will again dominate,” said Bhim Singh Rawat of the South Asia Network on Dams, Rivers and People. He stressed that while higher e-flows -- at least 23 cusecs as recommended -- are crucial, Delhi must also tackle untreated sewage flowing into drains.