Introduction to Hydrogeology (GEO 346C) Lecture 6a: Hydrogeochemistry Instructor: Bayani Cardenas TA: Travis Swanson and John Nowinski 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

GEO346C, Cardenas

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)  g/L (  g 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, Cardenas

Sources of chemicals in ground water 1)Natural sources Rocks and minerals SiO 2 + 2H 2 O -> H 4 SiO 4 0 CaCO 3 + H + -> Ca 2+ + HCO 3 - Atmosphere CO 2 (g), O 2 (g), N 2 (g) CO 2 (g) + H 2 O HCO H + Organic carbon CH 2 O + O 2 -> CO 2 (aq) + H 2 O 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, Cardenas

How do natural waters get chemical constituents? Chemical reactions in natural waters 1)Precipitation/ dissolution 2)Acid/ base reactions 3)Complexation 4)Reduction/ oxidation 5)Surface reactions (sorption/ desorption) 6)Microbial processes GEO346C, Cardenas

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 K eq or K sp ) 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) K eq values are taken from tables. Precipitation/ Dissolution GEO346C, Cardenas

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

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

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

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)  G r 0 is the Gibbs standard free energy for the reaction (kJ/mol)  G r is the Gibbs free energy for the reaction under actual conditions standard -> P=1 atm, and T=25C GEO346C, Cardenas

 G r < 0, the reaction proceeds to the right (spontaneous)  G r > 0, the reaction proceeds to the left (non-spontaneous)  G r = 0, the reaction is at equilibrium Gibbs free energy cC + dD = yY + zZ O GEO346C, Cardenas

Revisiting Thermodynamics  G r 0 is the Gibbs standard free energy for the reaction  G f 0 is the Gibbs free energy of formation for the reactants and products standard -> P=1 atm, and T=25C GEO346C, Cardenas

Revisiting Thermodynamics  enthalpy T temperature of the system S entropy

Enthalpy  H r 0 is 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)  H r 0 < 0, exothermic, releases energy (heat)  H r 0 > 0, endothermic, takes in heat How does enthalpy change with temperature? T 1 and T 2 are two different temperatures V’ant Hoff equation GEO346C, Cardenas

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

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

Chemical reactions in natural waters 1)Precipitation/ dissolution 2)Acid/ base reactions 3)Complexation 4)Reduction/ oxidation 5)Surface reactions (sorption/ desorption) 6)Microbial processes GEO346C, Cardenas

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, and neutral when pH=7. Many processes (eg precipitation/ dissolution, reduction/ oxidation) are pH dependent. GEO346C, Cardenas

Acid/ Base Reactions Is water an acid or a base? H 2 O ↔ H + + OH - It’s both a base and an acid, it’s an ampholyte. What is the pH of pure water? K eq =K w = = [H + ][OH - ] H 2 O Charge balance or electrical neutrality Charge from cations (+)= charge from anions (-) z i is absolute value of charge, m i is molal concentration GEO346C, Cardenas

What is the pH of pure water? K eq = = [H + ][OH - ] H 2 O [H + ]=[OH - ] = [H + ] 2 =[H + ][H + ] = [H + ] pH=-log[H + ] pH=7 GEO346C, Cardenas

Acid/ Base Reactions and Carbonate chemistry CaCO 3 ↔ Ca 2+ + CO 3 2- K sp = [Ca 2+ ][CO 3 2- ] [CaCO 3 ] K P CO2 is partial pressure of CO 2, it is convenient to express this in atm GEO346C, Cardenas

What is the pH of water in equilibrium with the atmosphere? Conditions: Temperature= 25˚C P CO2 = atm (at sea level), P CO2 is partial pressure of CO 2 GEO346C, Cardenas

The Keeling Curve

CO 2 and natural waters GEO346C, Cardenas

CO 2 and natural waters GEO346C, Cardenas

What is the pH of river water running through a channel incised in limestone? Conditions: Temperature= 25˚C P CO2 = atm (at sea level), P CO2 is partial pressure of CO 2 GEO346C, Cardenas

Carbonate chemistry and pH GEO346C, Cardenas H 2 CO 3 HCO 3 -1 CO 3 -2

Carbonate chemistry and pH GEO346C, Cardenas

Solubility of carbonates GEO346C, Cardenas

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

Soil and river water chemistry in area with volcanic rocks GEO346C, Cardenas

Spring water chemistry in area with carbonate rocks GEO346C, Cardenas

Chemical reactions in natural waters 1)Precipitation/ dissolution 2)Acid/ base reactions 3)Complexation 4)Reduction/ oxidation 5)Surface reactions (sorption/ desorption) 6)Microbial processes GEO346C, Cardenas

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 -, SO 4 2-, PO 4 2- and CO Organic compounds may also act as ligands. The cations are typically metals. The difference between a complex and salt is that a complex is in solution while salts precipitate as solids. Simple complex: Mn 2+ + Cl - = MnCl + GEO346C, Cardenas

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, Cardenas

Complexation Reactions Formation of inorganic complexes are fast and we don’t need to worry about kinetics. Therefore, we can apply equilibrium thermodynamics concepts. Mn 2+ + Cl - = MnCl + K MnCl+ = [MnCl + ] [Mn 2+ ][Cl - ] GEO346C, Cardenas

Complexation Reactions Complexation reactions occur in series with the minor species typically neglected. Cr 3+ + OH - = Cr(OH) 2+ Cr(OH) 2+ + OH - = Cr(OH) 2 + Cr(OH) OH - = Cr(OH) 3 0 Cr(OH) OH - = Cr(OH) 4 - and so on… GEO346C, Cardenas

Chromium Complexation Reactions Cr 3+ + OH - = Cr(OH) 2+  1 = [Cr(OH) 2+ ]= [Cr 3+ ][OH - ] Cr OH - = Cr(OH) 2 +  2 = [Cr(OH) 2 + ]= [Cr 3+ ][OH - ] 2 Cr OH - = Cr(OH) 3 0  3 = [Cr(OH) 3 0 ]= [Cr 3+ ][OH - ] 3

Complexation Reactions GEO346C, Cardenas

Complexation Reactions In reality, multiple metals (cations) form multiple complexes with different ligands. (Pb) T =(Pb 2+ ) + (PbCl 2 0 ) + (PbCl 3 - ) + (PbOH + ) +(PbCO 3 0 ) Solubility enhancement