Problem Set #6 – Using PHREEQC
Due Friday April 12, 2013
Answer the following questions using PHREEQC. For all questions, be sure to use the
phreeqc.dat data file. Please do not simply give me the printouts from the calculations answer the questions on your own paper and make them neat and clear (you may cut and
paste the correct information if you like). In question 1, please include the excel print
out.
(1) Answer the following questions using the following chemical analyses. Assume
HCO3- = alkalinity.
Ion
Ca
Mg
Na
K
mg/L
109
24
117
7
Ion
HCO3
SO4
Cl
T
mg/L
183
238
171
25
A. How far off is the charge balance? Is this acceptable? Which is greater,
anions or cations?
B. What is the value for CO2?
C. Create and print out an excel spread sheet that shows:
(i) concentrations of the three most concentrated ion pairs in the water.
(ii) saturation state of minerals of the water.
(2) A. Equilibrate pure water with dolomite.
a) How much dolomite dissolved?
b) What are the saturation states of aragonite and calcite in this water?
B. Equilibrate pure water with dolomite and CO2 at a partial pressure of 10-2 atm in an
open system.
a) Does the extra CO2 change how much dolomite dissolved?
b) If so, how much and in which direction?
C. Equilibrate pure water with dolomite and CO2 at a partial pressure of 10-2 atm in a
closed system.
a) How much dolomite dissolved?
b) If different from part B, why?
(3) Equilibrate pure water with dolomite and halite
a) How much halite dissolved?
b) How much dolomite dissolved?
c) Why do these values differ for the solution in equilibrium with dolomite
(Question 2A)?
(4) Create the following simulations steps.
a) Calculate the concentrations of species in a “carbonate water” that equilibrates
with PCO2 = 10-2 in an open system and is in equilibrium with calcite.
b) Calculate a “surface water” that is in equilibrium with the atmosphere (PCO2 =
10-3.5) and quartz. What is the saturation state of this water with respect to amorphous
quartz and chalcedony?
c) Mix the two waters in the following proportions:
i) 70% “carbonate water”, 30% “surface water”. What are the Ca and Si
concentrations and activities in this mixture?
ii) 30% “carbonate water”, 70% “surface water”. What are the Ca and Si
concentrations and activities in this mixture?
d) How does the saturation state of calcite, aragonite, quartz, and chalcedony vary
in the two mixtures? Discuss why there may be these differences.