Abstract

Approximately 20%, or 967 aggregate extraction operations in southern Ontario operate below the water table. As these sites transition into pit lakes, they are often characterized by low nutrients, steep walls and limited biological activity. These pit lakes represent a unique opportunity for repurposing to create positive social and economic impacts. However, effective post-mining land-use (PMLU) in pit lakes must carefully consider the limnology, geography and technical attributes of each site. 

We propose the implementation of aquaculture as an innovative and sustainable PMLU strategy. By leveraging modern aquaculture techniques such as floating raceways, this addresses many of the historical issues with aquaculture in pit lakes. This technique optimizes production rates while mitigating risks of eutrophication and fish waste accumulation. With proper management, aquaculture creates a positive environmental, economic and social impact by adding nutrients to the oligotrophic lake, generating jobs and producing new local food sources for the community. Moreover, aquaculture may complement other desirable uses of these valuable mine waters, such as recreation, ecological restauration, research and energy production. 

One of the primary challenges in implementing this solution is the identification of eligible sites. Currently, no comprehensive public database of pit lakes exists in Ontario. As such, we developed a GIS database for aggregate pit lakes in Southern Ontario that includes social, technical and economic indicators. To further facilitate the decision-making process, we developed a model that assigns compatibility scores to each site. To do this, we first created detailed polygons for over 100 pit lakes then developed proxy indicators for technical requirements such as lake depth, temperature and contamination risks. For lakes meeting specific criteria, we assessed economic viability by estimating theoretical production areas and benchmarking it against other successful aquaculture operations. Social impacts were estimated using data such as the Canadian Multiple Depravation Index. Our model aggregated all this data, then assigned scores to each pit lake using multi-criteria decision-analysis. According to our model, multiple sites in southern Ontario have been identified as suitable for aquaculture. Promisingly, a reputable Ontario aquaculture operator has verified one of the sites identified by our model as a viable candidate, demonstrating this tool’s effectiveness in supporting stakeholder decisions. 

This practical tool capable of identifying eligible aquaculture sites helps address data limitations for pit lake repurposing and PMLU planning. Future research will further expand our scope beyond southern Ontario and assess other PMLU opportunities associated with valuable mine waters.