GUWAHATI: An Assam-based environmentalist has raised concerns with the Central Pollution Control Board (CPCB) over alleged hazardous effluent discharge from Numaligarh Refinery Limited (NRL) into the Dhansiri river in Golaghat district, warning of severe ecological damage.
In a letter to the CPCB Member Secretary, Apurba Ballav Goswami cited findings from a report by AGT Biosciences (OPC) Private Limited, which analysed a water sample reportedly collected from the NRL’s effluent plant on 11 March this year.
“The results reveal exceedances of CPCB effluent discharge standards for petroleum oil refineries under the Environment (Protection) Rules, 1986, and subsequent guidelines, with severe implications for aquatic life and the long-term health of the Dhansiri river,” the letter stated.
According to Goswami, the analysis showed biochemical oxygen demand (BOD) at 18.0 mg/L, chemical oxygen demand (COD) at 142.0 mg/L, and oil and grease content at 11.5 mg/L, surpassing the CPCB limits of 15 mg/L, 125.0 mg/L, and 10 mg/L, respectively.
“These exceedances are not isolated incidents but signal a potential pattern of pollution that threatens the Dhansiri river’s biodiversity and the communities that depend on it. The elevated BOD indicates excessive organic matter, depleting dissolved oxygen levels critical for aquatic organisms," Goswami wrote.
"Fish species such as mahseer and catfish, abundant in Dhansiri, require oxygen levels above 4-6 mg/L to survive. A BOD of 18.0 mg/L risks creating hypoxic zones, leading to fish kills, suffocation of benthic invertebrates, and disruption of the river’s food web," he added.
He added that the high COD level reflected a toxic mix of oxidisable pollutants from refinery processes, which could impair reproduction, stunt growth, and bioaccumulate in aquatic species, ultimately affecting predators such as birds and mammals.
“The oil and grease content of 11.5 mg/L, though seemingly modest, forms a surface sheen that blocks oxygen exchange, coats fish gills, and smothers aquatic plants, further compounding the ecological stress,” he noted.
Goswami urged the CPCB to conduct a detailed investigation into NRL’s effluent treatment practices and compliance with pollution control standards as of March 2025.
He also called for a cumulative assessment of the discharge’s impact on Dhansiri’s aquatic life and water quality, long-term monitoring of oxygen levels and biodiversity, and stringent corrective measures to ensure BOD, COD, and oil and grease levels remain within permissible limits.
In response, NRL denied discharging effluents into Dhansiri.
“Generated effluents are treated in a modern effluent plant within the refinery and processed water is recycled for utilisation in the refinery itself. However, stormwater is occasionally discharged through a stormwater drain. Discharged stormwater quality is closely monitored for compliance with statutory norms. The discharged stormwater flows to several villages and finally into the Kalioni river, 6 km away, not Dhansiri. Dhansiri is 12 km away,” an NRL spokesperson said.
Following Goswami’s letter, the refinery collected stormwater samples, and the results are expected in a few days.
“NRL is committed to ensuring continued compliance with pollution control and environmental protection norms, and it will actively cooperate with any agency, including CPCB,” the spokesperson added.