Rainwater infiltration in an aquifer

In a first reactive transport model (Bethke, 1997), we consider the reaction of silica as rainwater infiltrates an aquifer containing quartz (SiC 2) as the only mineral. Initially, groundwater is in equilibrium with the aquifer, giving a SiC 2(aq) concentration of 6 mg kg-1. The rainwater contains only 1 mg kg-1 Si02(aq), so as it enters the aquifer, quartz there begins to dissolve, 

In order to observe details of how the model responds to the onset of flow, we have set in the final commands an output interval on a logarithmic scale, and a small initial time step. 

We can see this point by evaluating the characteristic time required for chemical reaction to change the concentration of dissolved silica, and that required to affect the mass of quartz grains. These intervals are the relaxation times (s) for the groundwater and aquifer minerals (Lasaga and Rye, 1993 Lichtner, 1996). In 

In our example, the relaxation time for the groundwater is 2.7 x 106 s, according to Equation 27.4, or about 0.1 yr. The relaxation time for the aquifer (Eqn. 27.5), in contrast, is 4.0 x 1012 s, or 130 000 yr. The infiltrating fluid adjusts to reaction with the aquifer on a time scale six orders of magnitude shorter than the scale on which the aquifer adjusts to the fluid. Whereas we could observe the chemical evolution 

We can repeat the simulation for a range of flow velocities, by adjusting the time span of the simulation. For a simulation spanning one year, for example, we type the xlT commands 

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