Leachate Breakthrough Time Calculation

This tool estimates the breakthrough time, which is the time required for leachate to travel through a compacted clay liner under the force of gravity. This calculation is a critical aspect of landfill design, as it helps determine the long-term effectiveness of the containment system in protecting the underlying groundwater from contamination.

The equation models advective transport, which is the process where contaminants are carried along by the bulk movement of the seeping water. The time is directly proportional to the liner's thickness and its effective porosity, and inversely proportional to the hydraulic conductivity and the driving head.

Design Stage Applicability

• Preliminary & Concept Design: This calculation is fundamental for the initial sizing of a landfill's liner system. It allows engineers to determine the required thickness and material quality (hydraulic conductivity) of the clay liner to meet regulatory requirements or a specific design life (e.g., 100 years).
• Detailed Design & Risk Assessment: In the detailed design phase, this calculation provides a baseline for the liner's performance. While more complex models that include diffusion and dispersion might be used for a full risk assessment, this advective travel time is often a primary compliance metric required by environmental agencies.

Limitations

• Ignores Diffusion & Dispersion: This model only accounts for advection and ignores other contaminant transport mechanisms like molecular diffusion (movement from high to low concentration) and hydrodynamic dispersion (spreading due to varying flow paths). These other mechanisms can cause contaminants to appear earlier than predicted, making this equation potentially non-conservative.
• Assumes Uniformity: The calculation assumes the liner is a single, uniform (homogeneous) layer that is fully saturated. In reality, imperfections, cracks, or variations in compaction can create preferential flow paths, leading to a much faster breakthrough time.
• Parameter Sensitivity: The result is highly sensitive to the values of hydraulic conductivity (K) and effective porosity (α), which can be difficult to measure accurately and may vary significantly.