Figure 3: Solutions to the one-component transport equation.

The duration of caprock alteration calculated from equation (4) given the time for the reaction front to develop (τ₀; equation (3)) and the displacement distance of the front, l. Calculations are performed on the basis of Fe in equation (1). Shaded area denotes solutions for 37<q<228 m⁻¹, which gives a haematite dissolution rate, k_R, between 1 × 10⁻¹⁰ and 5 × 10⁻⁹ mol m⁻² s⁻¹ from equation (5) for a diffusion coefficient of 5 × 10⁻¹² m² s⁻¹. Solid line shows median solution with D_e=5 × 10⁻¹² m² s⁻¹ and ΔC₀=2.9 mol m⁻³. Dashed lines show extreme solutions with the minimum estimate of diffusivity of 10⁻¹² m² s⁻¹ and minimum ΔC₀ of 1.9 mol m⁻³ giving maximum reaction times and a high estimate of diffusivity of 1 × 10⁻¹¹ m² s⁻¹, combined with a maximum estimate of ΔC₀ of 3.95 mol m⁻³ giving minimum reaction times. Uncertainties in the duration of caprock alteration propagated from the other variables are small compared with those from the potential range of caprock diffusivities and reaction stoichiometry.