PrintExample 9.1
A bedrock column is 10.0 meter deep with a porosity of 0.20. Mineral composition is simple with 80% v/v of quartz and 20% v/v of K-Feldspar. As the rainwater infiltrates through the rock, these primary minerals dissolve whereas a secondary mineral kaolinite precipitates, as listed in Table 1.
Conditions
The annual net runoff (precipitation – evapotranspiration) is 0.5 meter/year. Although we know that not all runoff infiltrates through the bedrock, for simplification here we assume the runoff flows through the bedrock and exits from the bottom of the bedrock. Assume the diffusion coefficients of all species are 1.0E-10 m2/s, and dispersivity is 0.1 m. The cementation factors are the default values of 1.0.
Let’s assume the initial pore water composition (all concentrations in molal): pH 7.0, Cl- (charge balance), K+ (4.0E-5), Na+ (4.0E-5), Al3+ (4.0E-6), HCO3- (1.0E-5), and SiO2(aq) (8.0E-5). One of the major drivers of chemical weathering is acidity. The acidity in soils comes primarily from the soil CO2 gas from root respiration and organic acids from root exudates. Here we set the total HCO3- concentration (equivalent to total inorganic carbon (TIC)) in the rainwater to 0.3 mM, which is equilibrated to soil gas CO2 of 9000 ppmv.
Question:
Assume chemical weathering occurs for 0.1 million years. Please set up the simulation in Crunchflow. Plot the profiles of pH, Saturation index $(\mathrm{SI}=[\log10(\mathrm{IAP}/\mathrm{K_{eq}})])$, $\phi$ ( volume of mineral/ volume of total porous media) of minerals and $\tau$ values of K and Si (relative to Zr in this soil column, we will explain later) at t = 0, 5,000, 10,000, 20,000, 40,000, and 100,000 years. How does the chemical composition and physical properties (e.g., porosity, permeability) evolve over time?
Note:
Setting up this lesson will involve what you learned from the lesson on mineral dissolution and precipitation, and the lesson on 1D transport. If needed please revisit these two lessons.
| Primary mineral dissolution | log10Keq |
log10k (mol/m2/s) |
Specific surface Area (m2/g) |
|---|---|---|---|
|
$\mathrm{SiO}_{2(\mathrm{s})}\text{(quartz)}\ \leftrightarrow\mathrm{\ SiO}_2\text{ (aq) }$ |
-4.00 | -13.39 | 0.01 |
| $\begin{array}{l} \mathrm{K} \mathrm{AlSi}_{3} \mathrm{O}_{8} \text { (K-Feldspar) }+4 \mathrm{H}^{+} \leftrightarrow \\ \mathrm{Al}^{3+}+\mathrm{K}^{+}+2 \mathrm{H}_{2} \mathrm{O}+3 \mathrm{SiO}_{2}(\mathrm{aq}) \end{array}$ |
-0.28 |
-13.00 -9.80aH+0.5 -10.15aOH-0.54 |
0.01 |
| Secondary mineral precipitation | |||
| $\begin{array}{l} \mathrm{Al}_{2} \mathrm{Si}_{2} \mathrm{O}_{5}(\mathrm{OH})_{4} \text { (Kaolinite) }+6 \mathrm{H}^{+} \leftrightarrow \\ 2 \mathrm{Al}^{3+}+2 \mathrm{SiO}_{2} \text { (aq) }+5 \mathrm{H}_{2} \mathrm{O} \end{array}$ |
6.81 | -13.00 | 0.01 |
*: Thermodynamic and kinetic parameters from EQ3/6 database [Wolery, 1992]. The three different rate constant values indicate the rate constants for neutral, acidic and basic kinetic pathways, respectively.