Guwahati: An Assam-based environmentalist Apurba Ballav Goswami on Thursday sought the intervention of the Central Pollution Control Board (CPCB) over alleged hazardous effluent discharge from Numaligarh Refinery Limited (NRL) into the Dhansiri River in Golaghat district.
Concerning the severe ecological damage in Assam, Apurba Ballav Goswami in his letter to the CPCB member secretary stated the findings from a report by AGT Biosciences (OPC) Private Limited, which analyzed a water sample reportedly collected from the NRL’s effluent plant on 11 March this year.
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“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.
Goswami stated that 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 do not represent isolated incidents but signal a potential pattern of pollution that threatens the biodiversity of the Dhansiri River and the communities that depend on it. The elevated BOD indicates excessive organic matter, depleting dissolved oxygen levels critical for aquatic organisms,” Goswami wrote.
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The fish species such as mahseer and catfish, abundant in Dhansiri, require oxygen levels above 4-6 mg/L to survive, Goswami stated. 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.
Goswami added that the high COD level reflected a toxic mix of oxidizable 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.
Moreover, 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.
Meanwhile, in response to the letter of Goswami, NRL denied the allegation of discharging effluents into Dhansiri.
Following the allegation, an NRL official clarified that the NRL treated generated effluents in a modern effluent plant within the refinery and it recycled the processed water for utilization in the refinery itself.
However, the refiner discharged the stormwater occasionally through a stormwater drain. Further, the refinery closely monitored the discharged stormwater quality for compliance with statutory norms. The discharged stormwater flows to several villages and finally into the Kalioni River, 6 km away, not Dhansiri ” the spokesperson said.
The refinery has collected stormwater samples, and the results will come in a few days. NRL maintained compliance with pollution control and environmental protection norms. It will cooperate actively with any agency, including the CPCB, the spokesperson added.