Weathering, Soil Chemistry, and Groundwater Notes

Weathering & Chemical Properties of Soil

Learning Outcomes

• Categorize key chemical weathering reactions.
• Discuss how weathering reactions modulate atmospheric temperature over geologic timescales.
• Explain why soils are enriched in different mineral fractions.
• Predict the effect of pH and specific adsorption on the surface charge of soil minerals.
• Describe how features of oxidic, allophanic, and melanic soils relate to soil chemistry.

Reading Material

• van Loon and Duffy
  • Chapter 17: The terrestrial environment (pp. 442-456)
  • Chapter 14: Environmental chemistry of colloids & surfaces - Sections 1-2, 6 (pp. 339-347, 367)
  • Chapter 18.2: Chemical properties of soil (pp. 461-467)

Part 1 - Weathering

• Weathering: The breakdown of rocks, soil, and minerals through contact with Earth's atmosphere, biota, and waters.
• Denudation: Long-term processes causing the wearing away of the Earth's surface, reducing elevation and relief of landforms and landscapes.

Chemical Weathering Processes

• Hydrolysis: Breakdown of rock by acidic water to produce clays and soluble salts.
• Solution: Removal of rock in solution by acidic rainwater; carbonation specifically refers to limestone weathering by rainwater containing dissolved $CO_2$.
• Oxidation: Breakdown of rock by oxygen and water, often giving iron-rich rocks a rusty-colored surface.
• Chelation: Complexation and transport of ions by chelating agents, primarily organic ligands from decaying plant materials.

Hydrolysis Explained

• A general term for processes involving water as an essential reactant.
• Example:
  $2KAlSi<em>3O</em>8(s) + 2H<em>3O^+ + 7H</em>2O \rightarrow Al<em>2Si</em>2O<em>5(OH)</em>4(s) + 4H<em>4SiO</em>4(aq) + 2K^+(aq)$
  (Orthosclase + hydronium + water -> kaolinite + silicic acid + potassium ion)
• Further hydrolysis of Kaolinite:
  $Al<em>2Si</em>2O<em>5(OH)</em>4(s) + 5H<em>2O \rightarrow 2Al(OH)</em>3(s) + 2H<em>4SiO</em>4(aq)$
  (kaolinite + water -> gibbsite + silicic acid)
• Silicic acid $pKa_1 = \sim9.8$ with a solubility of approximately $150 mg L^{-1}$.
• Aluminosilicates can be represented by a general equation:
  $Aluminosilicate(s) + H<em>3O^+ + H</em>2O \rightarrow Clay mineral + H<em>4SiO</em>4(aq) + cation(aq)$
• Agents of weathering ($H<em>3O^+$ and $H</em>2O$) from:
  • $CO_2$ respired by soil microorganisms
  • Low molar mass acids from decomposition of soil organic matter
  • $HNO_3$ (nitric acid) from fertilizers and rainfall
  • $H<em>2SO</em>4$ (sulfuric acid) from rainfall

Temperature – Respiration Relationship in Soils

• Chemical weathering is the dominant process in warm and wet conditions.
• Assuming $OH^-$ and $CO<em>3^{2-}$ are negligible compared to $HCO</em>3^-$, then $[H^+] \approx [HCO_3^-]$.
  $pH = -\log(4.46 \times 10^{-7} \times 3.4 \times 10^{-4})^{1/2} = 4.91$
• Where:
  • $pCO_2 = 1 \times 10^{-2} atm$
  • $KCO_2 = 3.4 \times 10^{-2} mole/L \cdot atm$

Acid/Base Reactions & Buffering

• Dissolution of carbonate minerals:
  $(Ca, Mg)CO<em>3(s) + H</em>2O + CO<em>2 \rightarrow Ca^{2+}(Mg^{2+})(aq) + HCO</em>3^-(aq)$
• Acidity is buffered by the Urey reaction:
  $CaSiO<em>3 + CO</em>2 \rightarrow CaCO<em>3 + SiO</em>2$
  (Igneous rock + carbon dioxide -> Limestone + Quartz)
• Low Temperature (Earth’s surface):
  • Carbon in crystalline forms, buried in ocean floor sediments.
  • Lowers atmospheric $CO_2$ leading to cooling.
• High Temperature (Deep Earth):
  • Melting of crust/sediments, carbon is gaseous.
  • Increases atmospheric $CO_2$ leading to warming.

The Thermostat & Feedback Mechanisms

• The Urey reaction acts as a negative feedback mechanism on geological timescales.
  $CaSiO<em>3 + CO</em>2 \rightarrow CaCO<em>3 + SiO</em>2$
  (Igneous rock + carbon dioxide -> Limestone + Quartz)
• The thermostat broke on Venus, leading to a runaway greenhouse effect with surface temperatures reaching 464°C.
• Venus atmosphere = 96% carbon dioxide (92 atm pressure at the surface)
• There is also photodissociation of water: $H_2O + h\nu \rightarrow H + OH$

Colloids & Soil Chemistry

• Colloids: A heterogeneous mix of inorganic and organic solid particles.
• Interface between dissolved and solid phases in environmental systems.
• Governs ion exchange, nutrient availability, fixation & physical properties
• Size determines reactivity
• Key point: Dispersed colloids account for only a small % of what’s in the water, BUT because of their large surface areas, they are highly reactive and comprise a significant proportion of all the reactions taking place.

Surface Charge

• Colloids adsorb molecules or ions from the surrounding solution through electrostatic attraction.
• Colloids of fixed negative charge (e.g., clay minerals) attract and retain positively charged ions (cations).
• For other solids (e.g., metal oxides, organic matter, sulfides, carbonates), charge depends on the pH of surrounding water.
  • pH < pH_0 = protonated
  • pH > pH_0 = deprotonated
• Clays:
  • Products of the physiochemical weathering of primary minerals (e.g. kaolinite, allophane).
  • Usually of terrestrial origin
  • comprise a suit of aluminosilicate minerals with a layered lattice structure
  • Common structural feature - $SiO_4$ tetrahedra linked in a planar structure by 3 of the O atoms
  • This ‘sheet’ is then joined via the additional oxygen to octahedral units of aluminium surrounded by 6 oxygens or hydroxyl groups
  • Combine to form 1:1 or 2:1 clay mineral structures
• Common Natural Colloids and their $pH_0$:
  • $MnO_2$: $2-4.5$
  • $Fe<em>2O</em>3$ (hydrated, Goethite): $6.5-9$
  • Haematite: $8.5$
  • $Al<em>2O</em>3$ (hydrated): $5-9$
  • Carbonates: $\sim9.5$
  • Humic material: $4-5$

Electrical Double Layer

• Refers to two parallel layers of charge surrounding colloids.
• the surface of the colloid has a net negative charge
• Colloid attracts positive ions in the solution to the region adjacent to the surface.
• The thickness of the layer is defined as: the distance from the surface where the charge is reduced to $1/e = 0.37$ times its original value
• High salt concentrations enables neutralisation of the surface charge
• Compresses of the electrical double layer surrounding a colloid particle
• Reduces electrostatic repulsion
• Fine particles amalgamate,
• Increased size causes particles to drop out of suspension
• In soils, the relative importance of cations occupying exchange sites is: Ca^{2+} > Mg^{2+} > K^+ > Na^+

Cation Exchange Capacity (CEC)

• CEC is the sum of cations that a soil is capable of holding, at a given pH value, available for exchange with the soil solution.
• CEC is used as a measure of potential fertility, nutrient retention capacity, and the capacity to protect groundwater from cation contamination.
• CEC is highly variable → compositional and pH dependent.
• Should be determined at a pH similar to that intrinsic to soil environment
• Expressed as milliequivalent of hydrogen per 100 g of dry soil $(meq^+/100g)$, or the SI unit centi-mol per kg $(cmol^+/kg)$.
• Calculation:
  1. Extract soil sample (e.g., 1.00 g) with $NH_4Cl$ to displace exchange cations.
  2. Determine dissolved $Ca^{2+}$, $Mg^{2+}$, $K^+$, and $Na^+$ by AAS (Atomic Absorption Spectrometry).
  3. Determine $H^+$ by titration.
  4. Calculate total positive charge from cations and convert to $cmol/kg$.

Example Calculation of CEC

• Given:
  • Ca: $30.3 \mu g \cdot mL^{-1}$
  • Mg: $3.2 \mu g \cdot mL^{-1}$
  • K: $2.2 \mu g \cdot mL^{-1}$
  • Na: Not detected
  • $H_3O^+$: $2.60 mmol \cdot L^{-1}$
• Convert concentrations to charge equivalents, multiply by extractant volume, and convert to $cmol/kg$.
• Example calculation: $1.51 cmol^{(+)} kg \cdot soil^{-1}$
• Calculate H3O+ the charge on 1 g if soil is calculated by multiplying the concentration for hydronium ion $(mol \cdot L^{-1})$ by the volume of solution, 0.100 L.
• $2.6 × 10^{-3} mol \cdot L^{-1} × 0.100 L × \frac{1000g}{1.00 g \cdot soil} × \frac{10 cmol}{mol} = 26.0 cmol^{(+)} kg \cdot soil$
• Sum the values for each ion.
• $CEC = 15.1 + 2.6 + 0.6 + 26.0 cmol^{+} kg^{-1} = 44.3 cmol^{+} kg^{-1}$

CEC Ranges and Total Base Saturation (TBS)

• CEC Rating ranges in NZ (cmol/kg):
  • Very high: >40
  • High: 25-40
  • Medium: 12-25
  • Low: 5-12
• Total Base Saturation (TBS):
  $TBS = \frac{Ca + Mg + K + Na}{CEC} \times 100$
  (The more base cations, the more alkaline the soil.)
  Liming involves adding $Ca$ and $Mg$ rich minerals to neutralise soil acidity

New Zealand Soil Types

• A soil is determined by the local geology and its environmental history → develop a variety of physical and chemical properties with depth
• Oxidic Soils:
  • Old soils (>0.5 Ma), strongly weathered, clay-rich (50-90%).
  • Dominated by 1:1 clays (kaolinite/halloysite) and $Fe/Al$ oxides (haematite, gibbsite).
  • Low CEC and very low nutrient reserves $K, Mg, Ca$.
  • May be very acidic (Al toxicity possible).
• Allophanic soils:
  • Moderately aged soils – typically > 20 000 ka with moderate clay contents
  • $\sim10-30$. Rich in allophane -clay mineral formed from the weathering of volcanic glass in tephra.
  • Strongly adsorbs anions e.g phosphorus (P)
  • pH-dependant charge.
  • Positive surface charge increases with lowering pH.
  • Moderate – high CEC
  • Highly productive soils, but requires careful management
• Melanic soils:
  • Derived from rocks high in $Ca$ or $Mg$ (e.g., limestone or mafic/ultra-mafic igneous rocks).
  • High levels of base cations.
  • Clays dominated by smectite or vermiculites.
  • Large surface areas and permanent negative charge.
  • Topsoils black in color (high organic content).
  • Very high CEC.
  • Weakly acidic to alkaline pH (low risk for Al toxicity).

From Rainwater to Groundwater - Learning Outcomes

• Describe and categorise the principal reactions that contribute to alkalinity
• Explain the difference between pH and alkalinity
• Describe how waters evolve along groundwater flow paths
• Explain why soils may be more or less enriched in different mineral fractions
• Discuss the causes and chemical mechanisms underlying the arsenic contamination in groundwater

Groundwater Basics

• Groundwater is the water present beneath Earth's surface in soil pore spaces and fractures of rock formations.

• An aquifer is a rock unit or unconsolidated deposit that yields a usable quantity of water.

• Darcy’s Law:
  $Q = \frac{kA \Delta P}{\mu L}$

  • Q = discharge $(cm^3 s^{-1})$
  • K = Permeability $(cm^2)$
  • A = Cross-sectional area $(cm^2)$
  • $\Delta P$ = Pressure difference $(Pa, Nm^2) = P<em>2 - P</em>1$
  • $\mu$ = Viscosity $(Pa \cdot s)$
  • L = Length $(cm)$

Major Chemical Reactions in Groundwater

1. Dissolution of carbon dioxide
2. Weathering of carbonates (alkalinity)
3. Mineral dissolution and precipitation
4. Sorption
5. Redox processes

Alkalinity and Acid Neutralizing Capacity (ANC)

• Key interaction: mildly acidic rainwater and carbonate minerals
  $CaCO<em>3 + H</em>3O^+ \rightarrow HCO<em>3^- + Ca^{2+} + H</em>2O$
• Alkalinity: A measure of the ability of a water body to neutralize acidity.
  $Alkalinity = [OH^-] + [HCO<em>3^-] + 2[CO</em>3^{2-}] - [H_3O^+]$
• ANC: a broader concept (many other proton-accepting species)

Alkalinity as a Buffer

• pKa1 = 6.35
• pKa2 = 10.33
• For water at pH ~7, the dominant carbonate species is $HCO_3^-$.
• Alkalinity in natural water
  ${[HCO<em>3^-][H</em>3O^+] \over [CO<em>2(aq)]} = K</em>{a1} = 4.5 × 10^{-7}$

Water Status in Terms of Acid-Base Properties

• Soil $CO_2$ is an abundant source of acidity:
  • Chemical weathering (e.g., hydrolysis)
  • Microorganisms
  • Nitrification of fertilizers
  • Acid rain

Mineral Weathering and Dissolution

• Carbonic acid production:
  $CO<em>2(g) + H</em>2O \rightleftharpoons (H<em>2CO</em>3) \rightarrow H^+ + HCO_3^-$
• Carbonate minerals:
  $CaCO<em>3(s) + H</em>2CO<em>3 \rightarrow Ca^{2+} + 2HCO</em>3^-$
• Silicate minerals:
  • Primary Silicate minerals Secondary minerals (by hydrolysis)
• Rock minerals dissolve at widely varying rates depending on their respective resistances to chemical weathering

Factors Influencing Weathering Resistance

• Weathering resistance is determined by the degree of Si-O-Si bonding and ligand exchange kinetics of the mineral-derived cations.
• Minerals with isolated Si tetrahedra have low molar Si-O ratios and weather most rapidly.
• Water-rock interaction time dictates alkalinity and major ion content of groundwater.
• Major Cations: $Na^+, K^+, Ca^{2+}, Mg^{2+}$
• Major Anions: $Cl^-, SO<em>4^{2-}, HCO</em>3^-, NO_3^-$

Groundwater Salinity and Chebotarev Sequence

• Anions (Chebotarev Sequence)
  • $HCO<em>3^- → SO</em>4^{2-} → Cl^-$
• Cations (Matthess Zones)
  • $Ca^{2+} → Mg^{2+} → Na^+$

Saturation Index and Groundwater Chemistry Modeling

• Equilibrium Assumption: Law of Mass Action activities
• Saturation Index (SI):
  $SI = \log \frac{IAP}{K_{sp}}$
• Saturation State:
  • SI < 0: Undersaturated
  • SI ≈ 0: Equilibrium
  • SI > 0: Supersaturated

Factors Affecting Saturation States

• Activities (ionic strength effect)
• Formation of complexes
• Temperature
• Generally, solubility increases with increasing ionic strength.
• $IAP$: Ion Activity Product
• $K_{sp}$:Solubility Product
• Groundwater chemistry modelling with PHREEQC:
  • Saturation Calculations
  • Geochemical Modeling
  • 1-D Reactive Transport Modeling

Redox Processes in Groundwater

• Reduction = gain of electrons
• Oxidation = loss of electrons
• $½O<em>2 + 2H^+ + 2e^- \rightarrow H</em>2O$
• $Fe^{2+} \rightarrow Fe^{3+} + e^-$
• Combining the two half-reactions gives the balanced redox-reaction:
• $2Fe^{2+} + ½O<em>2 + 2H^+ \rightarrow 2Fe^{3+} + H</em>2O$
• Microbially mediated oxidation-reduction reactions play a major role in regulating redox conditions in groundwater systems
• Transfer of electrons from reduced electron donors to oxidised acceptors provides energy
• Organic Carbon (CH2O)
  $CH<em>2O + H</em>2O = CO_2 + 4e^- + 4H^+$
• Has to be combined with an electron acceptor like: $O<em>2, NO</em>3^-, MnO<em>2(s), Fe(OH)</em>3(s), SO_4^{2-}$

Redox Ladder & Arsenic Contamination

• Redox Ladder: Redox sequence in aquifers
  • Evolution of redox conditions along groundwater flow paths.
  • Typically, redox conditions tend to become more reducing with increasing travel distance and residence time of water underground.
• Arsenic Poisoning
  • Main Source: Arsenopyrite (FeAsS)
  • Microbially driven process in anoxic waters
  • Bacterially mediated reductive dissolution of As-bearing Fe (hydr)oxide minerals
  • bacterially mediated reductive desorption of As(V)
  • In oxic waters, As (V) can be mobilised at pH values >8.2 → loss of electrostatic attraction