Department of Geosciences
North Dakota State University
GEOLOGY 428/628
Geochemistry
Homework 5
1. Practice using WEB-PHREEQ by again solving problem 1 of homework 3. Input the log P CO2 into the correct box, and click "submit". Calculate the pH of pure H2O in equilibrium with CO2 gas at a pressure of:
a. 1 atmosphere
b. 0.01 atmospheres (approximately the value in soils)
c. 3 x 10 -4 atmospheres (approximately the value in air).
2. Having solved problem 11.9 in Faure "by hand" (i.e. using a spreadsheet), now solve it using WEB-PHREEQ. Compare your results for SI with the computer modeling results, and suggest explanations for any discrepancies.
3. The following is a river water that had a pH of 8.01 measured at 9.5 oC (concentrations in mg/L): Na = 12.0 K = 1.4; Ca = 12.2; Mg = 7.5; Cl = 9.9; HCO3 = 75.2; SO4 = 7.7.
a. What is the relative abundance of each Ca species in this water, in % of total Ca?
b. Calculate and discuss what will happen if you carelessly let the river water warm up to 25 oC in a container exposed to air.
When using the model, think carefully about what should be allowed to "float" during these calculations.
4. Use WEB-PHREEQ to do one or more equilibrium geochemical analyses on a groundwater of your choice, taken from the literature. Discuss what the implications/importance of your result might be for the groundwater you chose. You can earn up to 10 extra credit points by choosing a significant problem and providing meaningful discussion. Be sure to fully cite the source of your data.
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B. Saini-Eidukat
Fall 2008
Geochemistry
Geology 428/628