This example is a classic scenario used to simulate contaminant transport from a U.S. RCRA Subtitle D landfill with a composite liner system. We’ll break down the setup, key assumptions, model inputs, and interpret the results using graphs and downloadable PDF-style output suitable for reporting.
Overview of the Scenario
The example models a landfill with:
- A composite liner system:
- 60 mil (1.5 mm) geomembrane
- 0.9 m compacted clay liner
- A primary leachate collection system
- Defects in the geomembrane:
- Hole area: 0.1 cm²
- Frequency: 1 per acre
- Constant leachate head: 0.3 m
- Constant contaminant source: 1500 μg/L (VOC)
Leakage is computed internally using the Giroud method, while contaminant transport follows analytical solutions developed by R. Kerry Rowe and colleagues.
Key Input Parameters
| Property | Symbol | Value | Units |
|---|---|---|---|
| Source Concentration | c₀ | 1500 | μg/L |
| Landfill Length | L | 200 | m |
| Leachate Head | – | 0.3 | m |
| Clay Thickness | Hs | 0.9 | m |
| Clay Diffusion | D | 0.02 | m²/a |
| Geomembrane Diffusion | – | 3.0×10⁻⁵ | m²/a |
| Distribution Coefficient | Kd | 0.5 | mL/g |
| Soil Porosity | n | 0.35 | – |
| Dry Density | – | 1.9 | g/cm³ |
| Aquifer Thickness | h | 3 | m |
| Aquifer Porosity | nb | 0.3 | – |
| Base Velocity | vb | 10 | m/a |
Conceptual Model
The model simulates:
- Leakage through liner defects
- Diffusion + sorption in clay liner
- Transport into aquifer
- Down-gradient plume migration
Graphical Output: Depth vs Concentration for 3 Different Times
Interpretation
- Highest concentration near landfill boundary
- Exponential decay with distance
- Controlled by:
- Darcy velocity (vb)
- Dispersion (implicit in model)
- Sorption
PDF Report
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Open in new tab Key Results
- Breakthrough occurs after significant delay due to liner system
- Peak concentrations reduced relative to source
- Plume migration controlled by advection and sorption
- Leakage governed primarily by defect flow (Giroud method)
Conclusions
- Composite liner significantly reduces contaminant flux
- Even small defects dominate leakage behavior
- Sorption in clay plays a critical role in delaying transport
- Model supports long-term performance evaluation of landfill systems
Engineering Insights
- The geomembrane-clay interaction is critical — good contact reduces leakage dramatically
- Defect density assumptions can change results by orders of magnitude
- Kd and porosity strongly influence breakthrough timing
- This example demonstrates why Subtitle D designs rely on redundant containment systems