EESC20 Geochemistry Assignment 1

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University of Toronto, Scarborough *

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Biology

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Feb 20, 2024

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1 EESC20 Geochemistry Assignment #1: Ion speciation and geochemical computer modelling Due date: Monday, October 2, 2023 before 1pm Please submit your assignment on Quercus before 1pm. Reminder: all assignments will be screened for plagiarism when submitted. Part A (25 marks total): Ion Speciation Calculations Do not use the computer model to complete this part of the assignment – show all calculations and list any assumptions where appropriate. The Athabasca River (Alberta, Canada) has the following chemical composition: pH = 5.2 [Ca 2+ ] = 13.7 mg/L [Cl - ] = 0.55 mg/L [Mg 2+ ] = 1.6 mg/L [SO 4 2- ] = 6.1 mg/L [Na + ] = 0.94 mg/L [HCO 3 - ] = 0.9 mg/L [K + ] = 0.3 mg/L [PO 4 3- ] = 4.25 mg/L Question A1 (10 marks): Convert the concentrations of the ions listed above in units of mol/L. Question A2 (5 marks): Calculate the ionic strength and electrical conductivity of this solution. Question A3 (6 marks): Calculate the activity for Ca 2+ and SO 4 2- using the Davies equation. Question A4 (4 marks): Gypsum (CaSO 4(s) ) has a K sp = 10 -4.60 . Is this water undersaturated or saturated with respect to Gypsum?
2 Part B (40 marks): Geochemical Computer Modelling Please use Visual Minteq to complete part B of the assignment. Students can access Visual Minteq 3.1 using 2 different methods: 1) Download: https://vminteq.lwr.kth.se/download/ onto your computer (Windows only) OR 2) Access Visual Minteq on the UTSC Student Computer Labs in BV - Please ensure that you include your Visual MINTEQ model output in an Appendix in addition to completing the questions listed below (marks will be deducted for missing output) . You should export your model results to Excel and then append the output to your assignment. - Show all your calculations where applicable and include units for all values where applicable. A soil solution has the following composition: pH = 6.7 [Ca 2+ ] = 114 mg/L [Cl - ] = 103 mg/L [Mg 2+ ] = 74 mg/L [SO 4 2- ] = 141 mg/L [Na + ] = 8 mg/L [CO 3 2- ] = 289 mg/L [K + ] = 12 mg/L [NO 3 - ] = 112 mg/L [NH 4 + ] = 8 mg/L [PO 4 3- ] = 88 mg/L Question B1 (3 marks): Calculate the ion balance for this soil solution. Comment (based on the calculations of your results) whether or not you think that the ion balance is acceptable. Explain why or why not. Question B2 (2 marks): What is the ionic strength and electrical conductivity of this soil solution? Question B3 (2 marks): What is the percentage of free (not speciated) calcium (ie: Ca 2+ ) in comparison to the total calcium? Question B4 (2 marks): What is the percentage of free (not speciated) chloride (ie: Cl - ) in comparison to the total chloride? Question B5 (4 marks): Compare the amount of not speciated (free) calcium (question B3) to the amount of not speciated (free) chloride (question B4). Is the extent of speciation different? Why or why not? What would you predict (take an educated guess, do not actually run the simulation) would happen to these ions if the ionic strength increased doubled or tripled? Question B6 (3 marks): List the 2 mineral (solid) phases that are closest to equilibrium. Explain how you selected these.
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