337 The additional parameters included in Figure 2 (iron and arsenic) showed smoother and comparable trends between scenarios. Iron gradually increased in both cases from values of around 0.9 mg/L to 1.7-1.8 mg/L towards the end of each period, with relatively low variability in scenario 1 compared to the sulfate signal. Arsenic remained in the range of 10⁻³-10⁻² mg/L, with moderate oscillations in scenario 1 (small bands relative to the axis) and a slightly increasing trend in scenario 2. These levels are consistent with the literature, which warns that even waters close to circumneutral pH can maintain relevant concentrations of trace elements (including As), so risk assessment cannot be based solely on pH. The numerical and graphical evidence (Table 3 and Figure 2) confirms that incorporating transient hydroclimatic variability qualitatively changes the “type” of response: from a smooth and cumulative evolution (scenario 2) to a behavior dominated by pulses (scenario 1). This pattern coincides with field observations reported for acid drainage/acid rock, where during prolonged dry periods acidity and metals gradually increase, and during the initial phase of rainfall events, especially after droughts, sudden increases (“first flush”) are observed due to the dissolution and entrainment of accumulated soluble salts; Subsequently, concentrations may decrease due to dilution and temporary depletion of the salt reservoir, before restarting the concentration cycle during the next dry period. Likewise, reviews on climate change and acid drainage indicate that changes in seasonality and hydrological extremes can modify the composition, transport, and magnitude of pollutant loads, reinforcing the need to evaluate scenarios with non-stationary hydrology.
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