CHENNAI: The Tamil Nadu Pollution Control Board (TNPCB) has directed the Chennai Metropolitan Water Supply and Sewerage Board (Metro Water) to plug unauthorised stormwater drains and wastewater canals leading to Chembarambakkam Lake — the major drinking water source for Chennai and its suburbs — after water samples from channels feeding the reservoir showed high levels of organic pollution and faecal contamination.
The TNPCB proceeding was issued recently under Section 33A of the Water (Prevention and Control of Pollution) Act, 1974, following an inspection of 16 sampling points around the lake a few days ago. The Board had collected samples on May 22 and May 27 from canals and lake-border locations and found that several parameters did not conform to prescribed standards.
As per the proceeding, biochemical oxygen demand (BOD) was the most serious concern. Against the prescribed limit of 30 mg/l, BOD levels touched 132 mg/l at one sampling point and 61 mg/l at another. Values of 34 mg/l and 33 mg/l were also recorded at two other locations. The findings indicate significant organic pollution in the channels carrying water towards the lake.
Dissolved oxygen (DO) levels were also found to be poor at several locations. While the standard is at least 4 mg/l, the samples recorded nil DO at two points, while levels of 0.8, 1.1, 1.25 and 1.3 mg/l were recorded at other locations. Such low oxygen levels were noted by the TNPCB as an indication of deterioration in water quality.
Further, the total coliform count was particularly high. Against the standard of 5,000 per 100 ml, counts reached 97,680 and 92,220 at two sampling points. Other samples recorded 32,550, 27,400 and 20,980 per 100 ml, among others.
Nitrate nitrogen was recorded at 31.6 mg/l at one location against the 10 mg/l limit, while dissolved phosphate touched 5.6 mg/l, marginally exceeding the 5 mg/l standard. Total dissolved solids were as high as 4,180 mg/l, against the 2,100 mg/l standard.
The TNPCB said the results indicated that sewage was entering the feeder canals and ultimately the lake. It directed the Metro Water to plug the unauthorised stormwater drains and wastewater canals and submit a report within a week.
Failure to comply, it warned, would invite action under the Water Act. On the other hand, a Metro Water official told TNIE that the water drawn from the lake is being treated before supply and the treated water quality is up to the standards. “Also, steps have been taken along with the WRD to arrest sewage inflow into the lake,” the official added.