Department of Geosciences
North Dakota State University
GEOLOGY / Chemistry 428/628
Geochemistry
Homework 4

1. Having calculated SI's in Homework 3 "by hand" (i.e. using a spreadsheet), now solve Question 1 using WEB-PHREEQ. Compare your results for SI with the computer modeling results, and suggest explanations for any discrepancies.

2. Practice using WEB-PHREEQ by again solving problem 3 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).

3. The following is a river water that had a pH of 8.01 measured at 9.5 °C (concentrations in mg/L):

Ion	Conc. (mg/L)	Ion	Conc. (mg/L)
Na+	12	Cl-	9.9
K+	1.4	NO3-	0.06
Ca+2	12.2	Fe+2	0.01
Mg+2	7.5	pH	8.01
HCO3-	75.2	T	9.5 °C
SO4--2	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 °C in a container exposed to air.

4. Download PhreeqcI from the USGS web page and learn how to input data and carry out simulations. The output will be identical in format to WEB-PHREEQ (Nothing to hand in for this question).

5. Use WEB-PHREEQ or Phreeqc (Windows, Mac or UNIX version - your choice) to do one or more equilibrium geochemical analyses on a groundwater of your choice, taken from the literature. Provide me with *selected* and relevent portions of your output. Discuss what the implications/importance of your result might be for the groundwater you chose. To earn full credit, choose a significant problem and provide meaningful discussion. Be sure to fully cite the source of your data.

B. Saini-Eidukat
Geochemistry
Geology 428/628