Numerical modelling of the hydrogeochemical behaviour of laboratory-scale waste rocks configurations in improving contaminated neutral mine drainage quality: Implications for designing for closure.

Nickel-contaminated neutral drainage (Ni-CND) has been reported from the Lac Tio waste rock (WR) piles after several years of atmospheric exposure. This issue has prompted the development of innovative geo-engineering strategies to mitigate Ni release and transport. One proposed approach involves configuring the WR piles using a combination of Ni-releasing hemo-ilmenite (HI) and Ni-sorbing anorthosite rocks. Short-term laboratory leaching column tests (504 days) demonstrated that placing an anorthosite layer beneath the HI-WR significantly reduced Ni release, primarily through sorption within the anorthosite. While these results were promising for Ni mitigation, the short duration of testing may not adequately represent long-term geochemical behaviour-an essential consideration for mine closure planning. This underscored the need for predictive tools to assess the long-term drainage quality of the Lac Tio WRs. To address this, the long-term hydrogeochemical behaviour of the column tests was simulated using the reactive transport model MIN3P. The objective was to evaluate the anorthosite layer's effectiveness in sorbing Ni and delaying the onset of Ni-CND, a key concern for sustainable WR management at Lac Tio. Despite simplifications and parameter limitations, the 504-day simulations satisfactorily reproduced laboratory data (pH, Ca, K, Mg, Ni, and SO₄), indicating model reliability and providing valuable insights for the long-term WR management. Extending the simulation to 30 years revealed that Ni sorption sites in the anorthosite became saturated after approximately 16 years, resulting in elevated Ni concentrations in drainage water (up to 0.45 mg/L), approaching regulatory limits and suggesting a potential risk of CND development. Sensitivity analysis with increased anorthosite thickness (and by inference, mass) showed a notable reduction in Ni concentrations (maximum 0.30 mg/L) after 50 years and subsequent delay before any potential onset of CND. However, the mineral volume fractions indicated the long-term potential for CND. The implications of the simulation results in the optimization of waste rock piles design to further mitigate Ni release from waste rocks from the study area was subsequently discussed.