CEE 680

29 November 1995


Closed book, two pages of notes allowed.

Answer all questions. Please state any additional assumptions you made, and show all work.

1. (40%) Carbonate System. A groundwater that is at pH 8.4 and is in equilibrium with a partial pressure of CO2 of 10-2.5 atmospheres is pumped to the surface. It is immediately mixed in equal proportions with a surface water which has an alkalinity of 100 mg/L as CaCO3. What will the pH of this water be once it has reached equilibrium with the bulk atmosphere?

2. (60%) Precipitation and Dissolution: Magnesium System

A. (20%) Prepare a pC-pH diagram for a water in equilibrium with Mg(OH)2 precipitate.

B. (5%) How high must the pH of a magnesium-containing water be elevated so that the equilibrium soluble Magnesium-Hardness concentration drops to 1 mg/L as CaCO3 (i.e., 0.01 mM)?

C. (10%) What is the total Soluble Magnesium concentration in a pure solution of Mg(OH)2, to which no acid or base has been added?

D. (25%) Prepare a second pC-pH diagram for a water in equilibrium with both Mg(OH)2 and MgCO3 precipitate. Assume a closed system with a total carbonate (CT) concentration of 1 mM. Include a line approximating the total soluble magnesium concentration and show the regions of precipitation for the two solid phases.

Important Equilibrium constants:

Log K

Mg+2 + OH- = MgOH+


Mg(OH)2(s) + 2H+ = Mg+2 + 2H2O


MgCO3(s) = Mg+2 + CO32