Authors: Landers, M; Usher, B; Faulkner, D
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Mine closure projects are increasingly including consideration of detailed groundwater contaminant models that require inclusion of geochemical conditions, attenuation and plume development. To illustrate these concepts, modelling of a proposed borehole mining (BHM) trial, undertaken as part of a mineral sands development project, is provided as a case study. The BHM trial tests a proposed unconventional mining method, known as borehole mining, and trials the selective in situ removal of ore to reduce the need for overburden removal. The ore is located within an aquifer which resides below the groundwater table and is associated with potentially acid forming (PAF) material. The BHM trial will extract mineralised sands/ore and separate fines (< 53 µm) from the sands. As part of the BHM trial, the fines, neutralised with excess limestone (CaCO₃), and a portion of the ore will be re-injected (with groundwater; 70/30, liquid/solids) back into the cavity space left from extraction. Klohn Crippen Berger was commissioned to determine the likely water qualities resulting from the various scenarios of the borehole re-injection trial. This was assessed using the results of in-field, laboratory geochemical testing and hydrogeochemical modelling. The hydrogeochemical modelling was undertaken using the Geochemist’s Workbench (Bethke & Yeakel 2010) X2t module (2D reactive transport modelling). The models have incorporated the injectate composition (~70% groundwater and ~30% waste materials), aquifer water quality and simplified aquifer mineralogy. Aquifer hydraulic properties were included based on previous hydrogeological characterisation and groundwater modelling investigations. A series of 2D reactive transport models were constructed to simulate two injection options and to undertake sensitivity analysis. This includes aquifer injection for a 5-day period (fines only) and 36-day period (fines and ore), followed by one year of migration under natural groundwater gradients. The purpose of the modelling is to provide confidence in the BHM trial, with the possibility of the results being used to assess contaminant trigger values from the point of injection. Resulting water qualities for the various scenarios were compared to Australian drinking water requirements to provide guidance on the preferred injection methodology, and to assess whether the proposed approach would result in significant water quality impacts in the receiving aquifer. The paper uses this case study to illustrate how reactive transport modelling can assist in mine close assessments.

Keywords: acid mine drainage, aquifer injection, geochemist's workbench, reactive transport modelling

Landers, M, Usher, B & Faulkner, D 2016, 'Two-dimensional reactive transport modelling for waste management — aquifer injection case study', in AB Fourie & M Tibbett (eds), Proceedings of the 11th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 439-451.

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