Legacy impacts of contaminants from historical gold mining in Nova Scotian lakes

Visiting a remote lake in Nova Scotia, the scene may look inviting. However, most do not consider the pollution a lake may have received decades ago or its potential impacts to lake health today. In Nova Scotia, Canada, lakes often experienced a complex history of environmental changes following European colonization.

Hundreds of gold mines began operations around the mid-1800s in Nova Scotia.  About a century later, almost all were no longer in operation. Yet evidence of their presence on the landscape can be seen by historical tailings fields. Today, there are roughly 360 inactive gold mines located throughout the province, often concentrated in mining districts. Those few which continue to operate have moved away from the harmful mining techniques once used to process and extract gold from crushed rock. However, the ecological impacts of historical mining activities have an environmental legacy which may continue to harm Nova Scotia’s freshwater ecosystems and their surrounding environments to this day.

Contaminant warning sign in the historical Montague Gold District

For example, mercury amalgamation was once used and involved exposing crushed gold-bearing ore to mercury for separation of gold from the ore. Historical gold mining thus produced vast amounts of toxic sludge called tailings that contained mercury and arsenic. Tailings were then disposed of in the surrounding environments, often in surface water and thus pollution was transported to nearby and downstream aquatic ecosystems. Toxic contaminants associated with historical gold mining presumably have negative impacts on the ecosystem and organisms living within it. Yet, little is known about ecological harm and whether aquatic ecosystems have recovered from pollution occurring over a century ago.

Tailings transport by flowing water in the historical Montague Gold District. Image credit to Mike Parsons

To study pollution associated with historical gold mining, we will use lake sediments as a natural archive of environmental changes experienced by lakes in Nova Scotia. A sediment core is extracted from the bottom of a lake using basic lake coring equipment. Sections of the core can be aged and thus represent different time periods. Materials and contaminants released into the lake settle in the bottom over time thus making lake sediments a good archive of historical mining pollution. We will also use aquatic invertebrates preserved within the lake sediments to understand organisms' responses to lake conditions in pre-mining, mining and, post-mining time periods. 

Sediment core retrieved from a mining-impacted lake in Nova Scotia. You can observe the grey-colored tailings that are obvious between typical lake sediments

Remains and fragments of various biological organisms are preserved in lake sediments and can be used to infer past changes. Zooplankton are microscopic primary consumers found in the water column of most lakes. They are efficient algal grazers and are the key link between primary producers and higher trophic levels. Cladocera are the main group of zooplankton found in lake sediments. The fragmented remains of different species of Cladocera can be observed throughout the sediment core. Remains can be identified to the species level for most zooplankton and used to estimate diversity and abundance of key species during different time periods. Several pelagic zooplankton are sensitive to pollution and so species composition and abundances reflect ecological response to historical mining activities.

Chironomids are aquatic invertebrates that spend most of their life living at the water-sediment interface. Their chitinous head capsules are resistant to decay and are found in lake sediments, often at high abundances despite pollution occurring. We can extract their head capsules from sediment and identify the different taxa. Chironomids are a diverse group of benthic invertebrates adapted to different ecological conditions, including those associated with mining pollution and low levels of dissolved oxygen in the bottom waters. Changes in chironomid diversity and abundances thus reflect environmental conditions at the time of deposition in the sediment core.

Coverslip with dozens of chironomid head capsules and mandibles of chaoborids

Sedimentary remain of a Cladocera. Remains of invertebrates found in lake sediments can be identified using microscopes and taxonomic guides

Although mines in Nova Scotia have been shut down since ~1950, high contaminant levels are reported in soils, organisms, and aquatic sediments near inactive mines. This suggests that legacy pollution may prevent geochemical and ecological recovery in the absence of further mitigation or clean-up of historical pollution. Tracking the levels of contaminants throughout the aged sediment core and comparing it to the diversity and abundances of invertebrate bioindicators is an important step to assess environmental change and ecosystem recovery in Nova Scotian lakes. With these data, we will understand how contaminated tailings effected the aquatic ecosystem and determine if our study lakes are showing signs of recovery.