Oil-eating bacteria have been lurking in Saint John's Marsh Creek for about a year, but no need to worry. The bacteria were put there to slowly eat a kilometre-long "blob" of creosote, a harmful, chemical mass of carbon.

"As someone who's been in the creek firsthand for years and years, even when there was raw sewage, the creosote was always looming there as this, quite literally, dark ominous blob, which existed there that you did not want to disturb," said Graeme Stewart-Robertson, the executive director of Atlantic Coastal Action Program ​​​​​Saint John.

Marsh Creek is a substantial part of the east Saint John watershed, running almost the length of Rothesay Avenue to Courtenay Bay.

The non-profit organization has been testing the water quality of the infamously dirty creek since 1995. Since then, it has worked to eliminate raw sewage that used to be drained there and remove debris such as old shopping carts.

Now it's tackling the so-called blob.

The mass lies a metre deep below the creek bed behind the Canada Post mail processing plant on Rothesay Avenue. Between 1940 and 1970, the Joseph A. Likely lumber yard, the former occupant of the post office land, used creosote as a wood preservative for telephone and power poles.

The coal tar liquid that forms the blob contains more than 300 different chemicals, some of which are carcinogenic or caustic. Contact with the substance can lead to skin lesions, rashes and even kidney and liver failure.

Graeme Stewart-Robertson, the executive director of ACAP Saint John, has been testing the oil-eating bacteria on the 'blob' for the past year. (CBC)

ACAP Saint John discovered the creosote in the late 1990s, but there wasn't an effective way to treat the mass other than by dredging the creek or pumping it out somehow. Dredging the sediment would have cost an estimated $10 million.

But last year, ACAP started working with a petroleum company called Novozymes in Pennsylvania, which creates groups of bacteria that can degrade hydrocarbons.

"We've been doing small pilots using hundreds of these spikes in small areas to see what the impact is," Stewart-Robertson said, adding that the group injects the "spikes" into the creek about every two weeks.

So far, the bacteria are changing the hydrocarbon levels in the creek bed, but Stewart-Robertson said he's unsure how many bacteria will be needed to completely eliminate the hydrocarbon.

"I believe it is the most cost-effective means of dealing with this historic degradation of an ecosystem which we have taken for granted for so long," he said. "But we don't know the full answer yet."

The team is planning on expanding its efforts to another site in Saint John's north end but will also continue its work at Marsh Creek.

"Marsh Creek has represented such a negative on our city's image for a long time that a lot of people don't even know that it was once contaminated with sewage and is now this much cleaner watercourse," Stewart-Robertson said.

He said the contamination of the chemicals has a harmful impact on fish and waterfowl that land in the creek and can be a risk in a large municipality — a problem that still exists despite the work the group has done.

"No matter how many positive things I say about Marsh Creek now, [the contamination] still exists, but there are solutions that we can work on together," Stewart-Robertson said.