As part of a study on carbon dynamics in the local watershed, you collect water from the Willamette River and make the following measurements:   Parameter Measurement Na+ 0

1. As part of a study on carbon dynamics in the local watershed, you collect water from the Willamette River and make the following measurements:

Note: Use Table 4.3 in Stumm and Morgan, chapter 4 to find equilibrium constants for carbonate equilibria and the dissolution of calcite at 10°C.

1. Calculate the alkalinity of the solution. 
2. Estimate the ionic strength of the solution (Don’t forget to include DIC species in your calculation, but you can ignore complexation).
3. Calculate the activity coefficient for singly charged and doubly charged species at this ionic strength.
4. Calculate the concentration of HCO₃^-, H₂CO₃*, CO₃^2-, and DIC.
5. Is the river equilibrated with atmospheric CO₂? (calculate the pCO₂ of the river and compare with modern day atmospheric value, 4e-4 atm). If not, provide a reasonable explanation for why the river is not in equilibrium.

2. ‘Alkali Lake’ in Lake County, Oregon is a ‘soda lake’ with a remarkably basic pH of 11. You measure the total dissolved Si concentration to be 3.3 mM at a temperature of 25°C.

Note: At 25°C, the pKas for the first and second deprotonation of silicic acid (H₄SiO₄) are pKa,₁ = 9.8 and pKa,₂ = 12.6.

1. Estimate the activity of H₄SiO₄ in the solution (you can assume ionic strength is negligible)

2. Is the lake supersaturated or undersaturated with respect to quartz (SiO₂; pKs = 4) at 25°C? Provide a reasonable explanation for why the system is not in equilibrium.

3.  Describe two mineral weathering reactions that add alkalinity to natural waters. Write out the stoichiometric chemical reaction and state how many molar units of alkalinity are added per molar unit of mineral reacted.