Some Nova Scotia lakes still dealing with contamination from old gold mines

Nova Scotia's historic gold mining era ended decades ago, but the legacy of contamination continues.

In one lake in Dartmouth, N.S., it is being aggravated by recent environmental changes like warming temperatures, according to a new study.

"After a certain number of years we should expect some level of ecosystem recovery," said environmental scientist Branaavan Sivarajah. "But our study suggests that ecosystem recovery takes a lot longer."

Sivarajah and other researchers dated and measured contaminants in sediment in five Nova Scotia lakes inside three gold mining districts.

Their study, published in the journal FACETS, found sedimentary metal concentrations have not returned to background values after eight decades of gold mining and eight decades since it ended.

"This suggests that complete geochemical recovery may not be a realistic target in severely polluted lakes," the study reports.

What they looked at

The scientists took lake-bed core samples and, using radiometric dating, were able to trace levels of arsenic, mercury and other metalloids before, during and after gold mining.

Four lakes — Lake Charles, Lake Thomas and Muddy Pond in Dartmouth and Gegogan Lake in Guysborough — were downstream from mining operations and received mine tailings.

Loon Lake in Dartmouth was upstream and did not receive tailings. It was used for comparison purposes.

Tailings are a sand-like substance left over after rock containing arsenic and gold is crushed and spread over liquid mercury to remove the gold. Both elements are poisonous.

Unexpected elevations

As expected, concentrations subsided since mining stopped in most cases. But there were two exceptions.

Sediment in Lake Gegogan, downstream from Goldenville in Guysborough County, contains mercury concentrations as high as the peak mining period. Researchers believe tailings washing into the lake still contain high levels of the chemical.

The situation is worse at Lake Charles. Now in an urban area, the lake received tailings from gold mining at Montague in Dartmouth.

Arsenic levels in Lake Charles have actually risen since 2010.

Sivarajah said low oxygen levels caused by warming have likely triggered a chemical reaction that separates arsenic from binding compounds.

Algae and other matter falling to the bottom and decomposing is reducing oxygen levels.

"We are seeing a shift from lake sediments acting as sinks for contaminants to becoming sources because of various environmental changes that are happening presently," he said.

"In the case of Lake Charles, we are seeing that recovery can also be influenced by contemporary environmental changes related to urbanization, related to climate change."

The thresholds

Mercury at Lake Gegogan and arsenic at Lake Charles exceed the guidelines set by the Canadian Council of Ministers of the Environment, a federal benchmark for environmental contamination.

Levels also exceeded another measure of potential biological impact known as Probable Effects Concentration Quotient, which incorporates six other elements that are potentially toxic to aquatic organisms.

"In [Lake] Charles, it exceeds by a large amount," said Sivarajah.

Both at Montague and Goldenville, tailing fields are used by off-road vehicles "which has accelerated the erosion of tailings that are moved into aquatic ecosystems downstream," the study said.

Even in the absence of mining pollution, urbanization — like residential development and road construction — could also increase contaminants.

Loon Lake, which was spared the effects of mine tailings, has seen seven metalloid concentrations increase. Six of them have crossed the Canadian Council of Ministers of the Environment guidelines, the report said, and the Probable Effects Concentration Quotient "is also moving towards possible biological effects threshold."

The report calls for more monitoring to track the movement of tailings "given expectations of climatic changes."