A multi-university research study found that historical gold mining operations between the 1860s and 1940s have left substantial quantities of arsenic- and mercury-rich tailings near abandoned mines in remote and urban areas of Nova Scotia. The study,

A multi-university research study suggests that climate change is bringing harmful chemicals back to the surface of some Nova Scotia lakes, deposited by gold mines that were operational between the 1940s and 1960s. The research showed that large amounts of materials from the tailings have entered the surface waters of downstream aquatic ecosystems at concentrations that present a risk to the benthos community at the bottom of the lakes.

The 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.

Research used paleolimnological approaches to examine long-term trends in sedimentary metal(loid) concentrations, assess potential sediment toxicity, and determine if geochemical recovery has occurred at four lakes located downstream of three productive gold-mining districts. Paleolimnology is a multidisciplinary science that uses the physical, chemical and biological information preserved in aquatic sediments to track past changes in ecosystems.

During the historical mining era, sedimentary total arsenic and mercury concentrations and enrichment factors increased substantially at all downstream lakes that received inputs from tailings. Similarly, chromium, lead, and zinc concentrations increased in the sediments after mining activities began and the urbanization that followed.

The researchers found that sedimentary metal(loid) concentrations had decreased for most elements in recent sediments. However, continued exceedance of Canadian Interim Sediment Quality Guidelines (ISQG) suggest that complete geochemical recovery has not occurred. All sediment samples collected, including intervals from the pre-mining period, had arsenic concentrations that exceeded the CCME ISQG of 5.9 µg/g. Similarly, the total mercury ISQG of 170 ng/g was exceeded in all sediment samples except for six pre-mining samples from one of the four lakes examined.

The CCME ISQGs serve as environmental benchmarks and can be useful tools to assess sediment quality; however, they should be applied with caution and within an appropriate environmental context involving a recognition of “background” or pre-impact levels.

The researchers concluded that concentrations of mining-associated elements have increased from background levels at all study lakes. The researchers further found that their study and previous research highlights the legacy As and Hg pollution of aquatic ecosystems near abandoned gold mines in Nova Scotia and their exceptionally high concentrations relative to other Canadian watersheds where historical gold mining occurred

They concluded that it is likely that surface runoff from tailing fields, urbanization, and climate-mediated changes are impacting geochemical recovery trajectories at the lakes they studied. They note that is has been more than eight more decades since the cessation of major mining activities, yet sedimentary metal(loid) concentrations have not returned to background values. They suggested that complete geochemical recovery may not be a realistic target in severely polluted lakes based on their findings.