1223 science, and geochemistry) provides a proactive strategy for managing DAM in a changing climate, contributing to safer and more sustainable mine closures in the long term. 2. METHODOLOGY The geochemical model developed to simulate the behavior of reactive fill material is based on the integration of advective-dispersive transport processes, thermodynamic equilibrium reactions, and heterogeneous mineral kinetics under unsaturated flow conditions. Given the inherent complexity of modeling partially saturated porous media with multiple reactive lithologies, a sequential approximation approach was adopted that combines three complementary strategies to represent unsaturation without resorting to the complete resolution of the Richards equation: (i) a simplified Lagrangian mixture between “new water + resident water” in each infiltration pulse, implemented with MIX based on a meteoric solution and representative interstitial water solutions; (ii) a scheme equivalent to series reactors using TRANSPORT (discretized 1-D column) and monthly time steps that capture variable pulses and contact times; and (iii) a gas-water transfer control to impose a vertical gradient of O2(g)/CO2(g) availability, consistent with a more reactive upper layer and a predominantly anoxic underlying volume, a common condition in granular packages with limited gas diffusion (low O2(g) infiltration). The thermodynamic basis used in the development of this work was XXX, as it is considered the most self-consistent in acid drainage models with high metal content. In addition, the temperature ranges used vary/are (to be defined) within the range of standard conditions at 25°C (or, failing that, within the climatic temperature range of the study area, stating averages and max-min). This last condition was operationalized by differentiating the gaseous contact of the surface cell from the rest of the domain, using EQUILIBRIUM_PHASES with contrasting partial pressures of O₂(g) and CO₂(g), maintaining the thermodynamic consistency of the system. Table 1 - Gas conditions imposed by layer (EQUILIBRIUM_PHASES) Layer Cells log1sΩ#(æ) log1Ω#(æ) Interpretation Reactive upper zone 1 -3.5 -0.67 Contact with atmosphere, active oxidation Anoxic zone 2-50 -2.0 -10.0 Limited O₂ (diffusion/consumption), relative reducing conditions Note: Controlling O₂ as the “master variable” is consistent with the kinetics of pyrite oxidation under wet conditions and with the DAM literature, where oxygen access and the presence of water control the effective oxidation rate. The reactive domain was discretized as a 1-D column of 50 cells (Δz = 1 m), with an upper cell acting as an inlet boundary and a lower zone with restricted oxygen availability. The chemistry of meteoric water was defined as SOLUTION 0, and resident waters (pore water) were represented by standard solutions (Groups A-E) with contrasting compositions, each with an initial water mass (-water) associated with its relative contribution to the package. Each monthly infiltration pulse mixes a fraction of meteoric water (SOLUTION 0) with the resident pore water of the package (constructed from representative solutions by lithology) using a MIX block. Unsaturation is controlled by a parameter f_m (mixing fraction) or by a water content proxy θ_m, such that: SOLUTIONmix(ï)=à- SOLUTIONrain +(1−à-) SOLUTIONpore…(1) Reactive mineralogy was obtained by X-ray diffraction analysis (quantitative XRD) and converted to initial kinetic reagent inventories in PHREEQC. For each phase T, the initial moles were calculated from the XRD mass percentage and a dry mass basis representative of the modeled material, according to: Å ) = m% !<< Mdry Ms% (1)…(2)
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