336 amplitude (e.g., 2033, 2048, and 2062) where the maximum pH exceeded 5.0 (three years of the period), while scenario 2 showed a typically low annual range (median 0.023) and an absolute annual maximum of 4.673 without significant alkalizing peaks. In geochemical terms, this pH range is consistent with the spectrum reported for acid mine waters (up to pH ~ 5) and suggests a partially buffered system where moderate acidity and trace element mobilization can coexist. The most diagnostic signal of reactivation under hydroclimatic forcing was sulfate. Scenario 1 exhibited high intermittency and extreme pulses: the annual average sulfate varied widely (minimum of 176.6 mg/L and maximum of 974.0 mg/L), and the annual maximum reached 3946 mg/L (year 2051), with values above 2000 mg/L in 27 of 75 years (~ 36% of the period). In addition, very low annual minimums (down to 9.5 mg/L) were observed, consistent with periods of intense dilution or short-term “flushing” when flow dominates over salt accumulation. In contrast, scenario 2 showed a smooth and monotonous evolution: the annual average increased from 445.7 mg/L (2026) to 576.8 mg/L (2125), with a small annual band that stabilizes below ~2 mg/L after the first decades (Figure 2). This difference implies that transient forcing does not necessarily increase the long-term average sulfate, but it does markedly increase peaks and intra-annual variability, which are the most relevant components for ecological risk and infrastructure requirements (hydraulic sizing and treatment). Table 3 - Comparative summary of annual metrics (pH and sulfate) between scenarios Scenario pH (period average) pH (mi n) pH (ma x) pH range (median ) SO₄²⁻ (period average, mg/L) SO₄²⁻ (min mg/L) SO₄²⁻ (max, mg/L) SO₄²⁻ band (median, mg/L) Variable hydroclimate (2026-2100) 4.572 4.3 93 5.0 75 0.115 451.8 9.5 3946.3 1629.4 Average precipitation (2026-2125) 4.555 4.4 61 4.6 73 0.023 547.1 436.4 576.9 0.6 The comparison between scenarios also shows a change in the relationship between “acidification” and sulfate production. In scenario 2, the Pearson correlation between the annual average pH and the annual average sulfate was strongly negative (r ~ -0.97), consistent with a relatively stationary oxidation/progression trajectory where the advance of sulfate generation accompanies the gradual decline in pH. In scenario 1, this correlation was weak (r ~ -0.15), indicating a decoupling between average pH and average sulfate under wetting-drainage pulses: the system can maintain a relatively stable pH through buffering, while sulfate exhibits highmagnitude pulses (loads) associated with the hydrological regime. This difference is important for monitoring because pH alone can underestimate load changes during transient events, while sulfate more sensitively captures the response to first flush and salt redissolution.
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