SRK Consulting assisted with the development of closure plans for portions of a mining Development Area located in Central Laos. To support closure planning, it is important to identify which pits and dumps represent significant contaminant sources, and to understand how effectively contaminant release can be mitigated as part of the closure design.
A catchment delineation process was undertaken in which flow paths and catchment areas were defined to reflect the Life of Mine (LOM) topography, landforms and features within individual precincts. A closure water balance model (CWBM) was constructed to allow assessment of impacts for closure options at the precinct, sub catchment and catchment scale.
Stochastic rainfall was developed based on the available climatic data for the years 1994 2015 and showed good correlation with long term averages. Runoff from natural (i.e. non mining) areas for the CWBM was developed using the Australian Water Balance Model (AWBM). Unique calculations of hydrologic fluxes were conducted within GoldSim for mine pits, backfilled pits, water management structures, waste rock landforms and non mining areas. Within the CWBM, flows from natural areas and precincts were aggregated at nominated water quality assessment points to provide flow estimates for water quality predictions.
In general, pit voids and water management structures follow a distinct seasonal pattern, with ephemeral outflows during the wet season, and no outflow during the dry season. The assessment predicts that this seasonal pattern will be maintained after closure, such that the pit voids and water management structures maintain perennial lakes through the dry season, and overflow during the wet season. In addition, most pit lakes will be gaining water bodies, i.e. receiving significantly more seepage from groundwater than they are losing to groundwater and will spill.
