Saint John's Little River Reservoir is once again open to the public after an oily-looking mystery substance was spotted on the surface of the water.

On Monday, the city ordered the popular swimming area off Loch Lomond Road closed after lifeguards spotted a film on the water.

The city took samples and found that results from a hydrocarbon test indicated there were no abnormal levels in the water.

Scientist Tim Vickers, the executive director of the Atlantic Coastal Action Program, said he believes the slick that closed the beach could have been sunscreen, or oil and gas.

Scientist Tim Vickers

Scientist Tim Vickers said he believes the slick that closed the beach could have been sunscreen, or oil and gas. (CBC)

But Suzanne Labelle thinks differently. She walks regularly on the trails around the reservoir and said she spotted a similar sheen to the one that closed the beach to swimmers.

"We've seen, as I said earlier, oil slicks that are five to six feet long and three to four feet wide,” said Labelle.

"I've seen empty containers of oil and antifreeze containers scattered in this area where we are right now,” she said.

Oily suspect

Vickars said he believes the substance Labelle noticed is naturally occurring, and likely not what closed the beach.

Oily slick

Suzanne Labelle walks regularly on the trails and noticed this slick near the reservoir. (CBC)

“We certainly broke it apart and it tended to stay in very sharp, distinct lines, as opposeed to man-made hydrocarbons, which will pool and come back together very quickly," he said.

Vickers said he believes it may be iron or manganese interacting with bacteria, a substance which looks very similar to oil and gas.

"They're identical, they catch the sun and it's a tie-dye hydrocarbon sheen," he said.