EESC20 Geochemistry Assignment 1

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University of Toronto, Scarborough *

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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.

3 Question B7 (6 marks): Which mineral (solid) phases have formed after speciation? List all of these and explain how you have derived this from the model output. Question B8 (3 marks): If this soil is impacted by acid rain and the pH decreases to 4.5, does this change the minerals you identified in question B7? Explain why or why not. Hint: you will need to rerun the speciation model using the different pH. Question B9 (10 marks): This soil site is being considered for waste amendment (use the solution conditions listed at the beginning of Part B to answer this question). It is predicted that the waste will add the following to the soil solution: [Cd 2+ ] = 30 mg/L and an additional amount of chloride: [Cl - ] = 30 mg/L (remember to add this to what is already present in the soil solution). Given that cadmium is a heavy metal and could pose environmental risk, assess all forms of cadmium when it is mixed in the soil via waste application. In your assessment and using the Visual Minteq output, identify: What happens to the added cadmium? What are the main forms of cadmium that are in solution and in the solid phase? Question B10 (5 marks): Based on your answer in question B9, do you think that the added cadmium will be mobile in the soil? Which species would you monitor and track using geochemical modelling (Visual Minteq)? Is there a concern for contamination of nearby surface water or groundwater? Explain your answer.

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