🎓 Lesson 8
D5
PHREEQC Essentials: Building Your First Mine Drainage Model
PHREEQC is a free computer program that lets engineers simulate how water and minerals interact underground—like predicting whether mine drainage will become acidic or corrode pipes.
🎯 Learning Objectives
- ✓ Explain the role of PHREEQC in predicting acid mine drainage (AMD) potential from sulfide-rich waste
- ✓ Design a minimal batch model input file (SOLUTION, PHASES, EQUILIBRIUM_PHASES) for a representative mine drainage scenario
- ✓ Analyze output reports to identify dominant dissolved species, saturation indices, and pH-driven mineral stability
- ✓ Apply charge balance error and saturation index thresholds to assess model reliability
📖 Why This Matters
Every year, billions of dollars are spent globally mitigating acid mine drainage (AMD)—a legacy of uncontrolled sulfide oxidation. PHREEQC is the industry-standard tool for *predicting* AMD before it forms, enabling smarter waste placement, cover design, and treatment planning. In this lesson, you’ll build your first model—not as a black box, but as an engineering decision-support tool grounded in thermodynamics and field reality.
📘 Core Principles
Geochemical modeling rests on three pillars: (1) mass conservation (elements must balance), (2) chemical equilibrium (reactions proceed until ΔG = 0), and (3) activity-based thermodynamics (not concentration alone). PHREEQC uses the Debye-Hückel equation to calculate activity coefficients for ionic strength correction, and iteratively solves for speciation using log K values from thermodynamic databases. For mine drainage, key reactions include pyrite oxidation (FeS₂ + 3.5O₂ + H₂O ⇌ Fe²⁺ + 2SO₄²⁻ + 2H⁺), gypsum dissolution, and ferrihydrite precipitation—all governed by their respective equilibrium constants. Understanding which phases are supersaturated (SI > 0) or undersaturated (SI < 0) tells you what minerals may precipitate or dissolve over time.
📐 Saturation Index (SI)
The saturation index quantifies how close a solution is to equilibrium with a mineral phase. It is the primary diagnostic output for predicting mineral precipitation/dissolution in mine drainage models.
Saturation Index
SI = log_{10}(IAP) - log_{10}(K)Measures departure from equilibrium between aqueous solution and a solid phase.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| IAP | Ion Activity Product | dimensionless | Product of activities of dissolved ions in stoichiometric proportions |
| K | Equilibrium Constant | dimensionless | Thermodynamic equilibrium constant for the mineral dissolution reaction at specified temperature |
Typical Ranges:
Stable mineral phase: −0.5 to +0.5
Likely precipitation: > +0.5
Likely dissolution: < −0.5
💡 Worked Example
Problem: Given: Measured [Ca²⁺] = 1.2 × 10⁻³ mol/kgw, [SO₄²⁻] = 2.8 × 10⁻³ mol/kgw, pH = 6.2, temperature = 25°C. Calculate SI for gypsum (CaSO₄·2H₂O). Log K for gypsum = −4.93 (phreeqc.dat, 25°C). Activity coefficients: γ_Ca²⁺ = 0.62, γ_SO₄²⁻ = 0.57.
1.
Step 1: Compute ion activities: a_Ca²⁺ = [Ca²⁺] × γ_Ca²⁺ = 1.2e−3 × 0.62 = 7.44e−4; a_SO₄²⁻ = 2.8e−3 × 0.57 = 1.60e−3
2.
Step 2: Calculate ion activity product (IAP): IAP = a_Ca²⁺ × a_SO₄²⁻ = (7.44e−4)(1.60e−3) = 1.19e−6 → log(IAP) = −5.92
3.
Step 3: Compute SI = log(IAP) − log(K) = −5.92 − (−4.93) = −0.99
Answer:
The result is SI = −0.99, which falls within the undersaturated range (SI < −0.5), indicating gypsum will not precipitate under these conditions.
🏗️ Real-World Application
At the Mt. Milligan copper-gold mine (BC, Canada), PHREEQC batch models predicted neutral drainage from waste rock despite high pyrite content—due to abundant calcite buffering. Field monitoring over 8 years confirmed stable pH ~6.8 and low metal mobility, validating the model’s prediction. The model used measured pore water chemistry (ICP-MS, alkalinity titration) and mineralogy (XRD) to constrain SOLUTION and EQUILIBRIUM_PHASES blocks—demonstrating how lab data directly informs robust predictive modeling.
🔧 Interactive Calculator
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