The authorities are investigating a black discharge seen in a canal in Potong Pasir.
Videos uploaded on the Nature Society (Singapore) Facebook page last Saturday morning, and shared with The Straits Times, show an inky black substance flowing into the canal, which then spreads and clouds the water rapidly.
In response to ST's queries, national water agency PUB said it received a report of discharge in an outlet drain at Potong Pasir Avenue 1 at 10.20am on Saturday.
"PUB officers and contractors went down to investigate immediately," the national water agency said yesterday, adding that investigation was in progress.
There was no discharge found at the site when the officers arrived as it might have been diluted with water in the canal, PUB said.
"Online water quality sensors and in-situ water quality readings showed that water quality is within the normal range. We will continue to closely monitor water quality over the next few days," a PUB spokesman said.
The National Environment Agency (NEA) said it was aware of the matter and is investigating.
In the videos, a construction site and a Lian Beng Group crane can be seen adjacent to the canal. ST understands that a condominium is being built at the site, and that the canal leads to the Kallang River.
Mr Ted Ng, 34, who frequents the area to photograph wildlife, said he spotted the black liquid flowing into the canal twice, first at around 9.30am, and the second time between 10.30am and 11am.
Several residents nearby had also whipped out their phones to take pictures and videos, added Mr Ng, an administrator who works in the finance industry.
He said he was concerned that birds, fish and otters in the area would be affected. His friend later alerted the authorities, he said.
Veteran wildlife expert Subaraj Rajathurai said the presence of a black discharge could mean oil or inner soil from the site.
ST has also contacted Lian Beng Group for more information.