Questions: Aqueous Geochemistry

Question 1

Groundwater analysis shows a saturation index (SI) of +0.5 for calcite. What does this indicate about the water's relationship to calcite?

• A. The water is undersaturated and will dissolve calcite
• B. The water is supersaturated with respect to calcite (SI > 0), meaning it has a thermodynamic driving force to precipitate calcite -- though whether precipitation actually occurs depends on kinetics and nucleation
• C. The water is exactly at equilibrium with calcite
• D. The water contains no calcium or carbonate

Answer: (B)

SI = log(IAP/Ksp), where IAP is the ion activity product and Ksp is the solubility product. SI > 0 means IAP > Ksp, so the solution is supersaturated. Thermodynamics predicts precipitation should occur to reduce the dissolved load. However, calcite supersaturation is common in natural waters because precipitation requires nucleation and crystal growth, which may be inhibited by dissolved organic matter, magnesium, or phosphate.

Question 2

The concentration of a dissolved ion (in mg/L or molality) is the appropriate quantity to use in equilibrium calculations for natural waters.

• T. True
• F. False

Answer: False

Equilibrium constants are defined in terms of activities, not concentrations. Activity = concentration x activity coefficient, where the activity coefficient accounts for ion-ion interactions in solution. In dilute waters, activities approximate concentrations (activity coefficients near 1). But in saline waters (seawater, brines, hydrothermal fluids), activity coefficients deviate significantly from 1 due to electrostatic interactions. Using concentrations instead of activities in equilibrium calculations produces incorrect speciation and saturation results for any water with significant ionic strength.

Question 3

Explain why pH is considered a master variable in aqueous geochemistry.

Model answer: pH controls the speciation of virtually every weak acid and base in solution: carbonate species (CO2/HCO3-/CO3 2-), aluminum hydroxide complexes, silica species, phosphate species, and metal-hydroxide complexes all shift their dominant form depending on pH. It also controls mineral solubility -- many minerals (carbonates, hydroxides, sulfides) have pH-dependent solubility that varies by orders of magnitude. Additionally, pH couples to redox through the Nernst equation (H+ appears in most half-reactions). Because pH influences so many equilibria simultaneously, it is the single most diagnostic measurement for characterizing a natural water.

Knowing pH immediately constrains the speciation of carbonate, aluminum, iron, and most other dissolved species -- which is why it is measured first in any water analysis.