329 PREDICTIVE MODELING OF ACID DRAINAGE UNDER CLIMATE-INDUCED HYDROLOGICAL SHIFTS *J.Costa1, M. Canicoba1, L. Rivera1, L. Contreras1, D. Lucano1, C. Iparragirre1, M. Rocco1 1 Department of Water Resources, Anddes Asociados, Lima- Peru, (*Presenting author: jesus.costa@anddes.com) ABSTRACT Acid drainage remains one of the most persistent environmental challenges in postmining landscapes, and its long-term behavior under climate change remains insufficiently addressed in conventional closure planning. This study presents an integrated predictive framework that couples reactive transport modeling in PHREEQC with transient hydroclimatic forcing derived from climate projections (RCP 4.5), to evaluate the response of a reactive mine waste deposit under non-stationary hydrological conditions. The model represents monthly infiltration pulses, gas-water exchange, mineral kinetic reactions, and progressive depletion of reactive phases in a one-dimensional discretized column. Two scenarios were evaluated: (i) variable hydroclimatic forcing (2026-2100) and (ii) stationary average precipitation (20262125). Results indicate that long-term average pH values remain comparable between scenarios (4.57 vs. 4.56), but the hydroclimate-driven scenario exhibits substantially higher intra-annual variability and episodic excursions, including transient pH maxima above 5.0. The most significant difference is observed in sulfate dynamics: while the stationary scenario shows a gradual monotonic increase from ~446 to ~577 mg/L with minimal annual variability, the transient scenario produces pronounced pulses, with annual maxima reaching 3946 mg/L and variability bands exceeding 1600 mg/L in median terms. Approximately 36% of modeled years under transient forcing exceeded 2000 mg/L sulfate. Correlation analysis further reveals a strong negative coupling between pH and sulfate under stationary conditions (r ~ -0.97), whereas this relationship weakens substantially under hydroclimatic variability (r ~ -0.15), indicating partial decoupling between acidity and solute loading during episodic flushing events. Iron and arsenic display smoother long-term trends compared to sulfate, highlighting that climate-induced risk amplification is primarily associated with transient solute pulses rather than gradual compositional shifts. The findings demonstrate that incorporating climate variability into reactive transport modeling fundamentally alters risk diagnostics, shifting emphasis from mean conditions to extreme events and peak loads, thereby supporting climate-resilient mine closure design and monitoring strategies. KEYWORDS Acid Mine Drainage; Reactive Transport Modeling; Climate Change; RCP 4.5; Hydroclimatic Variability; Mine Closure; PHREEQC; Sulfate Pulses; First Flush; Climate Resilience
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