Forsyth, B, Vega, I, Green, R, Chapman, J, Garvie, A & Noël, M 2026, 'Predictive draindown modelling for a copper heap leach facility', in AB Fourie, G Boggs, J Heyes & M Tibbett (eds), Mine Closure 2026: Proceedings of the 19th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 1-13, https://doi.org/10.36487/ACG_repo/2615_142
(https://papers.acg.uwa.edu.au/p/2615_142_Forsyth/)
Abstract: The moment irrigation of a heap leach facility (HLF) ends it begins its transition to closure. Its draindown in those first years will determine decades of environmental performance. At closure, the leached ore will be acidic, and draindown will require active management until seepage decreases to acceptable levels. 
An innovative pseudo-3D numerical model was developed to estimate draindown from a proposed acid copper HLF after the final irrigation. In the modelling, the HLF was represented as individual cells that are sequentially constructed, irrigated and then allowed to drain. The overall model considered heap shape, stacking sequence, ore properties, liners, cover options and climate. A Python script was developed to integrate the results of multiple 1D simulations into a simplified 3D representation of the HLF.
Key inputs were obtained from material properties measured on leached ore generated in metallurgical column tests and from operational design parameters for the HLF. The soil-water characteristic curve (SWCC) for the heap material was estimated from particle size distribution and bulk density results using the HYDRUS Rosetta model as SWCC data were unavailable. The draindown model was validated by reproducing observed draindown curves from column tests under the applied irrigation regime, achieving close agreement with measured outflows. Heterogeneity was addressed through sensitivity analyses of key hydraulic properties.
The predicted changes in water content over time were used to estimate appropriate oxygen diffusion coefficients for modelling oxygen entry and oxidation, which then informed predictions of long-term water quality.
Results indicated that 4 years after closure, when the final cover is placed, drainage flow will decrease by up to 99%. Approximately 16–57% of the total drainable water would remain after 4 years, and draindown (excluding cover infiltration) could continue for 30–40 years, depending on evaporative losses. Initially, water quality would resemble the pregnant leach solution (pH ~1.2). After rapid draindown, ongoing pyrite oxidation will sustain acidic seepage.
The results highlighted the impact of evaporation on the HLF water balance, and the need to consider consolidation and deagglomeration on hydraulic conductivity. The model outcomes provided the basis for HLF design refinements.