Boring into groundwater impact predictions of hardrock mines in Canada reveals inconsistent modelling timelines, monitoring plans, and consideration of climate change

Hardrock mining activities can impact groundwater through contaminant releases and altered water tables that last up hundreds of years. Environmental impact assessment (IA) is a planning tool used to predict potential effects and inform decision-making of proposed mining projects. The number and size of hardrock mines in Canada is increasing, with many major projects in the IA process. Despite the potential for long-lasting impacts of mines on groundwater quality and quantity, post-decision audits of groundwater effects assessment during IAs in Canada are very scarce. We consulted groundwater effects assessments for 16 hardrock mines assessed under the Canadian Environmental Assessment Act, 2012, to investigate (1) temporal boundaries and approaches to impact prediction; (2) specificity of long-term monitoring plans; and (3) how climate change was accounted for. A limited number (31%, n = 5) of mining projects presented numerical modelling of groundwater impacts with a clearly-stated time horizon. No projects incorporated quantitative assessments of climate change within predictions. Contaminant exceedances were predicted at 5 projects (31%; n = 5), though forecasted exceedance durations were not provided. Sufficiently detailed post-closure groundwater monitoring plans were proposed for 38% of projects (n = 6), with a median duration of only 5 years. Our findings highlight considerable shortcomings with recent IA processes for predicting and monitoring impacts on groundwater as a valued component for hardrock mines. We provide specific recommendations for how groundwater impact predictions can improve technical rigor. If implemented, these recommendations would support informed decision-making about proposed hardrock mines and build trust with stakeholders and rightsholders.