Silica transport in an aquifer

We can use the calculated reaction rates (Fig. 26.3) to compute how rapidly quartz precipitation seals the fracture. The sealing rate, the negative rate at which fracture aperture changes, can be expressed as 

Next we look at how temperature gradients along an aquifer might affect the silica content of flowing groundwater. We consider a symmetrical aquifer that descends from a recharge area at the surface to a depth of about 2 km and then ascends to a discharge area. Temperature in the calculation varies linearly from 20 °C at the surface to 80 °C at the aquifer s maximum depth. 

To set up the calculation, we take a quartz sand of the same porosity as in the calculations in Section 26.1 and assume that the quartz reacts according to the same rate law (Eqn. 26.1). We let the rate constant vary with temperature according to the Arrhenius equation (Eqn. 26.7), using the values for the preexponential factor and activation energy given in Section 26.2. As in the previous section, we need only be concerned with the time available for water to react as it flows through the aquifer. We need not specify, therefore, either the aquifer length or the flow velocity. 

To model reaction within the descending leg of the flow path, along which water warms with depth, the procedure in REACT is 

to model the ascending limb, we start with the final composition of the descending fluid and let it cool. The corresponding commands are 

5 Calculated silica concentration in a fluid packet flowing through a quartz sand aquifer. The fluid descends from the surface (T = 20°C) to a depth of about 2 km (80°C) and then returns to the surface (20°C). Results are shown for time spans At (representing half of the time the fluid takes to migrate through the aquifer) of 0.1, 1, and ten years. In the latter calculation, the fluid remains near equilibrium with quartz. 

5000 free grams Quartz kinetic Quartz surface = 1000 

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