A stinky water problem that prompted complaints from dozens of Halifax residents has returned, and finding a permanent fix could take years and cost millions of dollars.

The problem is geosmin, a by-product of algae that isn't harmful, but can can give off an earthy smell that some people can detect in their water.

Geosmin was first detected in Halifax water in October 2012.

"It was the first time we detected it in our water supply in the 35 years of operating this facility," said Dr. Alisha Knowles, water quality manager for Halifax Water.

The problem has been traced to Pockwock Lake, the main water supply for people on the Halifax side of the harbour.

Last year, the geosmin smell faded after about four months. But it resurfaced again in October of this year. Last month, Halifax Water brought in an environmental specialist.  

"They specifically looked in Pockwock Lake itself," Knowles said.  

"They looked at other water bodies in our watershed and other tributaries that are actually flowing water into Pockwock Lake. And we're really trying to figure out what you just asked: why this is suddenly happening after 35 years of operating a water treatment plant."

Fix will cost $3M to $15M

Knowles says the Pockwock water treatment facility is not currently equipped to deal with geosmin. "To remove geosmin in this facility, we would require a new process. Geosmin is not currently removed by conventional water treatment processes."

Knowles says there are two types of technology that could be used. She says depending on the option chosen, the cost could be anywhere from $3 million to $15 million.

"One thing about geosmin is that although it is a significant taste and odour concern, it is predominantly, or solely an aesthetic concern. It is not a public health concern for our customers," Knowles said. "We want to be sure that if we're putting a treatment process in, we're not going to have any unintended consequences happen for this plant."

Preliminary results from the water testing are expected next week. Then planning must begin to determine the best way to deal with the geosmin.

"Then we can develop some kind of early detection warning systems so that we have early indicators of when geosmin's going to occur," she said.