Abstract:

Over a mine lifetime, environmental teams face the challenge of assessing the evolution and extent of seepage plumes in aquifers affected by the emplacement of tailing storage facilities (TSF). Seepage generated from a TSF can have a similar water quality range as natural groundwater and vary over time as the recovery process evolves and/or the source of water supply changes. These factors represent a technical challenge in evaluating the effect of seepage on groundwater systems. Seepage plume extent is commonly delimited by evaluating the concentrations or ionic ratios of (semi-) conservative elements used as hydrochemical tracers. However, as the chemistry of tailings and supernatants evolve, the concentration of these elements in seepage may vary through time. Unless properly accounted for, these changes may result in an incorrect understanding of the plume characteristics, leading in an under- or overestimation of its extent. In view of this, isotopic tracers have emerged as a cost-effective tool for assessing plume dynamics. A case study has shown that a change from groundwater source supply to desalinated seawater has altered the long-term utility of the historical hydrochemical tracer of seepage assessment. After evaluation of alternative water quality and isotopic parameters, water isotopes (δ²H and δ¹⁸O) were selected as a new site tracer for assessing the plume extent. The distinctive isotopic signature of desalinated seawater, paired with the effects of isotopic fractionation of evaporating TSF infiltration water, make this isotopic system a robust seepage tracer. For this specific study site, water isotopes not only represent a useful tool to estimate the plume extension but also to support the understanding of the site hydrological system in terms of its natural variability and operational evolution.

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Gregory Lehn