1221 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 post-mining 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 1. INTRODUCTION Acid drainage is a chronic environmental problem that arises when sulfide-rich materials oxidize in the presence of water and oxygen. This process generates acidic water loaded with sulfate and metals, whose impact can persist for decades or centuries (Nordstrom, 2011; Blowes et al., 2014). This phenomenon can be natural in origin (acid rock drainage, or ARD) or associated with mine components or materials (exposed tailings, waste dumps, deposits of unsuitable material, etc.) which, due to their granulometry and the presence of sulfides, react with environmental conditions (acid mine drainage, or AMD) after the mine is closed. In high Andean regions of Peru, which are highly vulnerable to climate extremes, the risk of AMD can be amplified by changes in precipitation and temperature patterns (Nordstrom, 2011; Blowes et al., 2014). Extreme climate variability (from prolonged dry seasons to torrential rains) can drastically alter the geochemical dynamics of these deposits. For example, it has been observed that after long dry periods
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