Using ecological risk assessment to establish closure criteria

Using ecological risk assessment to establish closure criteria

Does an open pit lake represent a toxic threat to wildlife after closure? If a former process pond is converted into an evapo-transpiration (ET) basin to manage heap leach and tailings drainage, does it create soil conditions that are hazardous to vegetation and soil organisms? These questions are increasingly being asked during the feasibility and permitting stages of mining projects, and the answers are guiding water management and closure strategies for mines around the world, to the benefit of the environment.

For nearly 20 years, the U.S. Environmental Protection Agency (EPA) focused on assessing potential human health risks at industrial waste sites including abandoned mine sites. However, in the last decade, equal consideration is being given to livestock, wildlife, and vegetation at risk of exposure to residual chemical compounds. Understanding the risks from exposure to metals and developing appropriate mitigation measures are key to extending corporate stewardship of the environment beyond mere regrading of heaps and backfilling of ponds, and fosters the guiding principles of sustainable development.

Decision-makers at all levels rely on predictive modelling of future site conditions in determining the profitability and potential liability of mining projects. While these models have limitations, assessing environmental risk is complicated by variability in the natural occurrence of metals in soils and water, and the fundamental requirement for metals in every biological organism. Background concentrations and sometimes the speciation or valence state of metals must also be considered before properly assessing a hazard, and the evaluation must include the uncertainties associated with natural biological variability.

Both the EPA’s “Guidance for Ecological Risk Assessment” (1998) and the International Council on Mining & Metals, “Metals Environmental Risk Assessment Guidance” (2007), provide environmental professionals an extensive set of risk assessment tools to quantify the potential threat to the ecosphere from mining. During project development, these tools are used to factor potential future liabilities, including mitigation measures, in costing projects. New, authoritative studies on the toxicity of inorganic compounds add evidence to wildlife Toxicity Reference Values, the cornerstone of ecological risk assessment. Moreover, exposure factors measuring wildlife food consumption are constantly being refined, increasing their validity.

SRK Consulting (U.S.) continues to apply these techniques in assessing ecological risks at mining sites throughout Nevada, including at a gold and copper mine where the potential for elevated ecological risks was a factor in designing the long-term, post-closure, fluid management system, which needed to exclude wildlife from exposure to proposed evaporation cells. More recently, evaluation requests have come from projects in Pakistan, Australia, and Tanzania, demonstrating the expanding use of ecological risk as an essential tool for effective closure management.

Mark Willow:

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