1224 where w_i is the mass percentage by XRD, M_“dry” is the dry mass of the calculation basis, and MW_i is the molar mass of the compound. In PHREEQC, these moles were assigned with -m within KINETICS. The reactive surface area and/or geometric factors were incorporated using -parms as multipliers of the rate law (RATES), based on the physical parameters adopted for the particulate material (average size 0.1 mm, porosity 0.3, saturation 1) and an effective specific surface area (140 m²/L) that controls the scaling of rates as a function of the volume of reactive water. The reference kinetic constants were compared with widely used compilations in water-mineral interaction geochemistry to maintain traceability of orders of magnitude. Hydrological variables were incorporated as monthly transient forcing in the TRANSPORT block, evaluating two scenarios: (a) historical average precipitation (baseline) and (b) RCP 4.5 (intermediate mitigation). The monthly series came from climate projections conditioned to scenarios, treated as projections (not forecasts) and subsequently converted to effective infiltration using a simplified water balance, consistent with the IPCC distinction between scenario-dependent projection and initial statedependent prediction. Monthly effective infiltration was calculated as: Z eff,- =max[0, L- −YS0ï−:-](2) where P_mes is monthly precipitation, reference evapotranspiration, and Lm groups losses due to evapotranspiration, runoff, and storage (depending on data availability). This infiltration was translated into (i) the -water parameter of the meteoric solution, proportional to the infiltrated volume of the month with respect to the total reactive water in the system, and (ii) a monthly contact time implemented as -time_step (s) in TRANSPORT, calculated from the infiltrated volume and the equivalent average flow for the month: ΔM - = _eff,- P- (3) Table 2 shows a representative example of the monthly calculation (baseline scenario) and the time_steps used as input (seconds), consistent with the monthly pulse scheme. In the RCP scenarios, the same procedure was applied to the projected series, preserving the traceability of hydrological conversion and geochemical forcing: Table 2 - Example of monthly hydrological parameterization (baseline scenario): precipitation, aggregate losses, effective infiltration, and time_step Month Precipitation L- (mm) Losses :- =L-− Z eff,-(mm) Effective infiltration Z eff,-(mm) time_step (s) time_step in days (d) January 45.3 24.9 20.4 386640 4.48 February 38.7 21.3 17.4 348480 4.03 March 42.1 23.2 18.9 362880 4.20 April 15.2 8.4 6.8 259200 3.00 May 8.9 4.9 4 172800 2.00 June 5.1 2.8 2.3 86400 1.00 Reactive transport was performed with flux-flux boundary conditions, 50 cells, one shift per month, and conservative mixing/dispersion parameters (cell length 1 m, low longitudinal dispersivity, and explicit molecular diffusion) to avoid excessive numerical dispersion and favor the geochemical signal of hydrological forcing. The outputs were extracted with SELECTED_OUTPUT (pH, pe, alkalinity, equilibrium phases, and kinetic reactants) and USER_PUNCH, which converts total concentrations to mg/L by multiplying by molecular weights, maintaining PHREEQC's internal mass control.
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