MIGRATEv10 Example 13: Termination of Primary Leachate Collection System

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MIGRATEv10 Example 13: Termination of Primary Leachate Collection System

Landfill showing increasing leachate mound and contaminant leakage after leachate collection system failure

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Introduction

MIGRATEv10 Example 13 explores a critical long-term scenario in landfill performance:

👉 What happens when the primary leachate collection system (PLCS) stops operating?

This example builds directly on Example 1 by introducing time-dependent changes in hydraulic conditions following landfill closure. It highlights how reduced system performance can significantly impact leakage rates and contaminant migration.

Conceptual Model Overview

The modeled system includes:

  • A RCRA Subtitle D landfill
  • A composite liner system:
    • 60 mil geomembrane
    • 0.9 m compacted clay liner
  • A primary leachate collection system (PLCS)
  • Time-dependent changes in leachate head and Darcy velocity

Key Modeling Objective

This example aims to:

  • Simulate the termination of leachate collection system performance
  • Evaluate how increasing leachate head affects leakage rates
  • Assess long-term impacts on contaminant transport

Landfill and Liner System

ComponentDescription
Geomembrane60 mil (1.5 mm), good contact
Clay Liner0.9 m thick
System TypeComposite liner

This configuration provides strong containment—while the PLCS is functioning.

Source Behavior

Early Operation Phase (0–10 years)

  • Contaminant concentration increases linearly
  • Peak concentration reached at 10 years

Leachate Collection System Performance

During Operation (0–50 years)

  • PLCS is active Downward Darcy velocity:
    va = 5.7 10-5 m/a

👉 This low velocity reflects effective leachate removal

Post-Closure Conditions

At 50 Years: System Termination

  • PLCS stops functioning
  • Cover system is no longer maintained
  • Leachate begins to accumulate

50–70 Years: Transition Phase

  • Leachate mound rises gradually
  • Darcy velocity increases over time

👉 Modeled using linear interpolation divided into 5 steps

At 70 Years: Maximum Leachate Mound

  • Leachate head reaches 25 m
  • New Darcy velocity:

va = 0.0116 m/a

👉 This represents a ~200× increase in leakage potential

Modeling Approach in MIGRATEv10

Step 1: Start with Example 1 Model

  • Same geometry and liner system

Step 2: Define Time-Dependent Source

  • Linear increase in concentration (0–10 years)

Step 3: Apply Initial Flow Conditions

  • Darcy velocity during PLCS operation

Step 4: Introduce System Termination (50 years)

  • Modify boundary conditions
  • Remove leachate collection effect

Step 5: Model Transition Phase (50–70 years)

  • Divide into 5 time steps
  • Gradually increase Darcy velocity

Step 6: Apply Final Conditions (70+ years)

  • Use maximum Darcy velocity
  • Continue simulation

Graphical Output: Depth vs Distance

PDF Report

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Interpretation of Results

1. Early Period (0–50 years)

  • Minimal leakage
  • Effective containment
  • Low contaminant migration

2. Transition Period (50–70 years)

  • Increasing leakage rates
  • Accelerated contaminant movement

3. Long-Term Behavior (70+ years)

  • Significantly higher contaminant flux
  • Greater impact on underlying aquifer

Key Insights

1. Importance of Leachate Collection Systems

  • PLCS plays a critical role in limiting leakage
  • System failure dramatically increases risk

2. Sensitivity to Leachate Head

  • Leakage is highly dependent on:
    • Hydraulic head
    • System maintenance

3. Long-Term Risk

  • Post-closure conditions may pose greater risk than active operation

Practical Implications

This example is highly relevant for:

  • Long-term landfill performance assessment
  • Closure planning and post-closure care
  • Regulatory evaluation of containment systems

Key Takeaways

  • Termination of PLCS leads to significant increases in leakage
  • Time-dependent modeling is essential for realistic predictions
  • Leachate mound height is a critical control parameter
  • MIGRATEv10 can simulate changing system conditions over decades

Final Thoughts

MIGRATEv10 Example 13 highlights a crucial reality in landfill engineering:

Containment systems do not perform the same way over time

By modeling the failure of the leachate collection system, this example demonstrates how long-term conditions can dramatically alter contaminant migration behavior. It underscores the importance of:

  • Proper closure design
  • Long-term monitoring
  • Conservative modeling assumptions

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