In mid-2017, SRK was appointed to prepare a closure plan to pre-feasibility study (PFS) level for a mine in Southeast Asia. The mine area had more than 40 pits, some backfilled, and 13 waste rock dumps. To support closure planning, it was important to identify the pits and dumps that represented significant contamination sources, and to develop effective mitigation measures to manage the ongoing release of contaminants as part of the closure design.
Available information for the site included drill hole data, geological block models, current topographic surfaces, future pit and dump designs, outcomes from geochemical characterisation studies, and water quality monitoring of data surface water and groundwater. Conceptual models were developed for distributing reactive materials and contaminant sources within key pits and dumps. The most significant sources identified for producing post-closure solute were expected to be related to oxidising sulfide minerals present in unsaturated materials in above-ground waste dumps, pits backfill located above the long-term water table, and pit walls that remain exposed after closure, i.e. those not covered by backfill, or remaining above the long-term pit lake levels.
Field-scale solute production rates (inferred from laboratory column test results) were combined with water flows to calculate water chemistry at the source and downstream after mixing with other water sources. Additional project work included a site water balance, including representation of surface and groundwater flows at the site. The water quality results were used to examine the range of possible mine-impacted water chemistries for (i) base case flows and solute loadings – to reflect the condition of the site before implementing active closure measures, and (ii) post-closure design conditions – to examine the effect of closure measures, such as placing low infiltration covers on dumps. Using the site-wide water and solute load balance, it was possible to identify the optimal closure measures for mitigating water quality impacts on to downstream receptors (drinking water resources).
The geochemical assessment formed a key part of a multi-disciplinary approach to support the development of overall closure designs for this large, complex site in tropical, high precipitation conditions.
