Introduction to Hydrogeology (GEO 346C) Lecture 6a: Hydrogeochemistry

1. Introduction to Hydrogeology (GEO 346C) Lecture 6a: Hydrogeochemistry Instructor: Bayani Cardenas TA: Travis Swanson and John Nowinski www.geo.utexas.edu/courses/geo346c/ For this part of the course, we will use the following text: Fundamentals of Ground Water, 2003 by Schwartz and Zhang The relevant chapters are Ch. 17-19.

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3. Measures of chemical constituents: review Mass solute per mass solvent parts per million (ppm) parts per billion (ppb) Mass solute per volume solvent mg/L (mg solute/ L solvent) mg/L (mg solute/ L solvent) Mole-based concentration molarity M (mole solute/ L solvent) molality m (mole solute/ kg solvent) Equivalents-based concentration eq=mol  z, z=absolute value of charge eq/L N (normality, equivalent per L of solvent) meq/ L GEO346C, UT@Austin, Cardenas

4. Sources of chemicals in ground water • Natural sources • Rocks and minerals • SiO2 + 2H2O -> H4SiO40 • CaCO3 + H+ -> Ca2+ + HCO3- • Atmosphere • CO2(g), O2 (g), N2 (g) • CO2 (g) + H2O <-> HCO3- + H+ • Organic carbon • CH2O + O2 -> CO2(aq) + H2O • 2) Anthropogenic sources • Waste leaching • Landfills • Hazardous waste disposal/ storage • Industrial waste • Mine waste • Radioactive waste • Spills • Gasoline spills • Acid and base reagent spills • Organic chemical spills • Atmospheric fallout • Acid rain • Radioactive elements (bomb testing) GEO346C, UT@Austin, Cardenas

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6. How do natural waters get chemical constituents? Chemical reactions in natural waters • Precipitation/ dissolution • Acid/ base reactions • Complexation • Reduction/ oxidation • Surface reactions (sorption/ desorption) • Microbial processes GEO346C, UT@Austin, Cardenas

7. Precipitation/ Dissolution Law of mass action and chemical equilibrium cC + dD = yY + zZ C & D are reactants; Y & Z are products c, d, y, and z are number of moles for each For dilute solutions, the equilibrium distribution is described by: where K (aka Keq or Ksp) is the equilibrium constant or solubility product and (Y), (Z), (C), and (D) are the molal or molar concentrations for reactants and products. Technically, the values in parentheses are “activities” but we will assume that these are equal to concentrations (ie the solutions are dilute) Keq values are taken from tables. GEO346C, UT@Austin, Cardenas

8. Equilibrium versus kinetics Kinetics-based approaches are used when the reactions haven’t reached equilibrium. We will only consider reactions that are at equilibrium. equilibrium GEO346C, UT@Austin, Cardenas

9. Deviations from Equilibrium Ion activity product (IAP) where (Y), (Z), (C), and (D) are the reported sample molal or molar concentrations cC + dD = yY + zZ If IAP < Keq, the reaction is progressing from left to right. If IAP > Keq, the reaction is progressing from right to left. If IAP = Keq, the reaction is at equilibrium (reactions in both directions occur at equal rates) GEO346C, UT@Austin, Cardenas

10. Deviations from Equilibrium Ion activity product (IAP) cC + dD = yY + zZ If IAP/Keq < 1, the water is undersaturated with respect to the mineral. If IAP/Keq > 1, the water is supersaturated with respect to the mineral. If IAP/Keq = 1, the water is saturated with respect to the mineral. GEO346C, UT@Austin, Cardenas

11. Revisiting Thermodynamics Gibbs free energy Gibbs free energy is the energy needed by the reaction in order for it to take place. cC + dD = yY + zZ R is the gas constant (8.314x10-3 kJ/mol-K) T is absolute temperature (Kelvin, K) DGr0is the Gibbs standard free energy for the reaction (kJ/mol) DGris the Gibbs free energy for the reaction under actual conditions standard -> P=1 atm, and T=25C GEO346C, UT@Austin, Cardenas

12. Gibbs free energy cC + dD = yY + zZ O DGr < 0, the reaction proceeds to the right (spontaneous) DGr > 0, the reaction proceeds to the left (non-spontaneous) DGr = 0, the reaction is at equilibrium GEO346C, UT@Austin, Cardenas

13. Revisiting Thermodynamics DGr0is the Gibbs standard free energy for the reaction DGf0is the Gibbs free energy of formation for the reactants and products standard -> P=1 atm, and T=25C GEO346C, UT@Austin, Cardenas

14. Revisiting Thermodynamics H enthalpy T temperature of the system S entropy

15. Enthalpy DHr0is the standard enthalpy for the reaction (kJ/mol) (enthalpy is part of the internal energy of a system; heat gained or lost by a system during a reaction at constant pressure) DHr0< 0, exothermic, releases energy (heat) DHr0 > 0, endothermic, takes in heat How does enthalpy change with temperature? V’ant Hoff equation T1 and T2 are two different temperatures GEO346C, UT@Austin, Cardenas

17. Solubility- equilibrium concentration of a dissolved species What is the solubility of AgCl in pure water? AgCl ↔ Ag+ + Cl- Ksp=10-9.8= [Ag+][Cl-] [AgCl] Ksp=10-9.8= [Ag+][Cl-] One equation, two unknowns! Mass/ charge balance [Ag+]=[Cl-] 10-9.8=[Ag+][Ag+] [Ag+]=[Cl-]=(10-9.8)1/2=10 -4.9 or 1.26 x10-5 mol/L GEO346C, UT@Austin, Cardenas

18. Common-ion effect What is the solubility of AgCl in 0.1 M NaCl? For X moles of Ag+, there are X+0.1 moles of Cl-. Ksp=10-9.8= [Ag+][Cl-] 10-9.8= [X][X+0.1] 10-9.8= [X]2+0.1[X] [X]>>[X]2 [X] =10-8.8 [Ag+]=10-8.8 or 1.58 x10-9 mol/L in 0.1 M NaCl In pure water, it is 1.26 x10-5 mol/L Common-ion effect – the solubility of a salt reduced when one of the ions (+ or -) is already present in solution GEO346C, UT@Austin, Cardenas

20. Chemical reactions in natural waters • Precipitation/ dissolution • Acid/ base reactions • Complexation • Reduction/ oxidation • Surface reactions (sorption/ desorption) • Microbial processes GEO346C, UT@Austin, Cardenas

21. Acid/ Base Reactions Acid/ Base Reactions- involves the transfer of hydrogen ion (H+) and/ or (OH-) among the ions present in the aqueous phase The concentration of (H+) determines the pH of the solution. pH=-log(H+) A solution is acidic when pH<7, basic when pH>7, and neutral when pH=7. Many processes (eg precipitation/ dissolution, reduction/ oxidation) are pH dependent. GEO346C, UT@Austin, Cardenas

22. Acid/ Base Reactions Is water an acid or a base? H2O ↔ H+ + OH- It’s both a base and an acid, it’s an ampholyte. What is the pH of pure water? Keq=Kw= 10-14 = [H+][OH-] H2O Charge balance or electrical neutrality Charge from cations (+)= charge from anions (-) zi is absolute value of charge, mi is molal concentration GEO346C, UT@Austin, Cardenas

23. What is the pH of pure water? Keq= 10-14 = [H+][OH-] H2O [H+]=[OH-] 10-14 = [H+]2 =[H+][H+] 10-7 = [H+] pH=-log[H+] pH=7 GEO346C, UT@Austin, Cardenas

24. Acid/ Base Reactions and Carbonate chemistry K 10-14 10-1.46 10-6.35 10-10.33 CaCO3 ↔ Ca2+ + CO32-Ksp= [Ca2+ ][CO32-] 8.48 10-8.48 [CaCO3] PCO2 is partial pressure of CO2, it is convenient to express this in atm GEO346C, UT@Austin, Cardenas

25. What is the pH of water in equilibrium with the atmosphere? Conditions: Temperature= 25˚C PCO2=10-3.5 atm (at sea level), PCO2 is partial pressure of CO2 GEO346C, UT@Austin, Cardenas

26. The Keeling Curve

27. CO2 and natural waters GEO346C, UT@Austin, Cardenas

28. CO2 and natural waters GEO346C, UT@Austin, Cardenas

29. What is the pH of river water running through a channel incised in limestone? Conditions: Temperature= 25˚C PCO2=10-3.5 atm (at sea level), PCO2 is partial pressure of CO2 GEO346C, UT@Austin, Cardenas

30. Carbonate chemistry and pH HCO3-1 CO3-2 H2CO3 GEO346C, UT@Austin, Cardenas

31. Carbonate chemistry and pH GEO346C, UT@Austin, Cardenas

32. Solubility of carbonates GEO346C, UT@Austin, Cardenas

33. Solubility of metal oxides and hydroxides (e.g., Al(OH)3 and Fe(OH)3, PbO) GEO346C, UT@Austin, Cardenas

34. Soil and river water chemistry in area with volcanic rocks GEO346C, UT@Austin, Cardenas

35. Spring water chemistry in area with carbonate rocks GEO346C, UT@Austin, Cardenas

37. Chemical reactions in natural waters • Precipitation/ dissolution • Acid/ base reactions • Complexation • Reduction/ oxidation • Surface reactions (sorption/ desorption) • Microbial processes GEO346C, UT@Austin, Cardenas

38. Complexation Reactions A complex is an ion that forms by combining simpler cations, anions, and sometimes, molecules. In complexes, the anions are referred to as ligands including many of the common inorganic species found in natural waters such as Cl-, F-. Br-, SO42-, PO42- and CO32-. Organic compounds may also act as ligands. The cations are typically metals. Simple complex: Mn2+ + Cl- = MnCl+ The difference between a complex and salt is that a complex is in solution while salts precipitate as solids. GEO346C, UT@Austin, Cardenas

39. Complexation Reactions Complexes are important because they facilitate the dissolution of metals and transport of metals. Some metals may be immobile as simple cations, but they may be more mobile when part of a complex. This results in good and bad things. Some metal deposits, e.g., Pb, Zn and U, accumulate as mineral deposits from complexes. However, some metals which would normally be bound in minerals and sediments and not be in aqueous phase, may be mobile and spread in pristine water resources when as a complex. GEO346C, UT@Austin, Cardenas

40. Complexation Reactions Formation of inorganic complexes are fast and we don’t need to worry about kinetics. Therefore, we can apply equilibrium thermodynamics concepts. Mn2+ + Cl- = MnCl+ KMnCl+= [MnCl+] [Mn2+][Cl-] GEO346C, UT@Austin, Cardenas

41. Complexation Reactions Complexation reactions occur in series with the minor species typically neglected. Cr3+ + OH- = Cr(OH)2+ Cr(OH)2+ + OH- = Cr(OH)2+ Cr(OH)2+ + OH- = Cr(OH)30 Cr(OH)30 + OH- = Cr(OH)4- and so on… GEO346C, UT@Austin, Cardenas

42. Chromium Complexation Reactions Cr3+ + OH- = Cr(OH)2+ b1= [Cr(OH)2+]=1010.0 [Cr3+][OH-] Cr3+ + 2OH- = Cr(OH)2+ b2= [Cr(OH)2+]=1018.3 [Cr3+][OH-]2 Cr3+ + 3OH- = Cr(OH)30 b3= [Cr(OH)30]=1024.0 [Cr3+][OH-]3

43. Complexation Reactions GEO346C, UT@Austin, Cardenas

44. Complexation Reactions In reality, multiple metals (cations) form multiple complexes with different ligands. (Pb)T=(Pb2+) + (PbCl20) + (PbCl3-) + (PbOH+) +(PbCO30) Solubility enhancement