Geochemistry?
Part I Scenario: Aquo Care Environmental (ACE®) has contracted with Agassiz Lake Association to complete an analysis of water quality conditions that will inform a restoration program for the lake. Muskellunge (Esox masquinongy) stocking efforts have been yielding poor survival rates since the early 1990's. The Agassiz Lake Association board is prepared to consider aggressive measures to improve water quality in ways that enhances the fishery. They have contracted with ACE® to advise them about possible treatment approaches of their lake to meet this goal.

Key chemical results from recent sampling through the winter ice cover:
Total dissolved sulfide = 1 x 10-4 M
Total dissolved inorganic carbon (DIC or CT) = 1 x 10-3 M
Total dissolved organic acid (AT) = 5 x 10-4 M

Important Assumptions:
>Greatest fish stress is associated with the winter ice-cover season. Treatment options will likely focus on this period.
>Assume that the entire water column is isothermal at 4oC (i.e. the ice covered epilimnion is relatively thin). >Atmospheric exchange of gases is highly constrained by the ice cover - carbonate and sulfide are nonvolatile at this time of year. Therefore, use constant CT and constant ST for carbonate and sulfide systems throughout this exam.
> Microbial decomposition of organic matter in the soils and sediments generates the organic acid buffer system. Model the dissolved organic acid buffer system as if it were acetic acid with a pKa = 4.76 at 4oC.
>All of the DIC (CT) comes from respiration CO2 weathering of carbonates in the soils and sediments (see Homework 4).
>All the sulfide derives from hydrogen sulfide production by microbial decomposition.

Tasks:
1. Demonstrate how thermodynamic constants are determined for the carbonate system appropriate for the Agassiz Lake study. Ignore activity corrections (I → 0). Show your work for each calculation. Summarize the results in a simple table with ΔGo, ΔHo, Ka, Ka', pKa and pKa' for each reaction.

2. Prepare a set of mole balance equations that match those that MINEQL+ would use in its solution to this problem. Using electroneutrality checks, demonstrate that the terms of the mole balance equations are correct and that the total molar amounts of inputs are correct.

3. Prepare a log C - pH diagram that summarizes acid/base chemistry for the wintertime water column. Remember that the ACE® team will use this in the final report to the Agassiz Lake Association.

4. Determine the equilibrium pH for the wintertime system. Show the details of your work. Label this system point on your log C - pH diagram.

5. Calculate the CB and CA for the wintertime lake and reconcile these results with inputs given above. Show the details of your work.