POLLUTEv10 Example 18: Modeling Phase Change in a Secondary Leachate Collection System

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Introduction

POLLUTEv10 Example 18 demonstrates the application of the Phase Change special feature to simulate contaminant transport within a landfill system that includes a secondary leachate collection system (SLCS). This example is particularly valuable for environmental engineers and hydrogeologists interested in modeling multi-phase contaminant behavior across unsaturated and saturated zones.

The scenario focuses on the migration of dichloromethane (DCM) under conditions where phase partitioning occurs, influenced by Henry’s Law in the unsaturated zone. The model incorporates biological decay, layered hydrogeology, and controlled flow conditions to illustrate how phase change affects contaminant transport.

⚠️ Important Note: This example is hypothetical and intended for instructional purposes only. It should not be used as a direct template for real-world landfill modeling without proper hydrogeologic expertise.

Conceptual Model Overview

The modeled system consists of:

A landfill containing a constant concentration source of DCM
A primary liner
A secondary leachate collection system (SLCS) with distinct unsaturated and saturated zones
A secondary liner
An underlying 1-meter thick aquifer

Key Feature: Phase Change

A phase change occurs at the interface between:

Unsaturated zone (0.2 m thick)
Saturated zone (0.1 m thick)

This interface governs how DCM transitions between vapor and dissolved phases, significantly impacting transport behavior.

Hydrogeologic and Transport Assumptions

1. Layered Structure of the SLCS

The secondary leachate collection system is divided into two conceptual layers:

Layer	Zone Type	Thickness
Layer 1	Unsaturated Zone	0.2 m
Layer 2	Saturated Zone	0.1 m

This layered approach allows POLLUTE to simulate phase-dependent transport properties.

2. Contaminant Properties (DCM)

Constant concentration source in the landfill
Subject to biological decay
Exhibits phase partitioning in the unsaturated zone

Biological Decay Rates

Zone	Half-Life
Landfill	10 years
Primary liner	40 years
Secondary liner	40 years
Aquifer	40 years
Secondary leachate system	No decay

3. Diffusion Coefficients

Diffusion varies significantly between zones:

Zone Type	Diffusion Coefficient
Unsaturated Zone	300 m²/year
Saturated Zone	100 m²/year

The higher diffusion in the unsaturated zone reflects enhanced vapor-phase mobility, while the saturated zone value represents increased mixing conditions.

4. Phase Change Parameter

The phase behavior of DCM in the unsaturated zone is governed by:

Henry’s Constant = 0.1

This parameter controls the equilibrium between dissolved and vapor phases, making it critical for accurately simulating contaminant partitioning.

5. Flow Conditions

Darcy velocity through primary liner: 0.003 m/year
Darcy velocity through secondary liner: 0 m/year

This creates a controlled system where flow is limited to the primary liner and lateral movement occurs within the saturated portion of the SLCS.

Modeling Approach in POLLUTEv10

To implement this example in POLLUTEv10:

Step 1: Define Layered Geometry

Input two layers for the SLCS:
- Unsaturated (0.2 m)
- Saturated (0.1 m)

Step 2: Assign Transport Properties

Set diffusion coefficients for each layer
Define Darcy velocities for liners

Step 3: Enable Phase Change Feature

Activate the Phase Change option
Input Henry’s Constant (0.1) for the unsaturated zone

Step 4: Apply Decay Parameters

Assign appropriate half-lives to each domain
Ensure no decay is assigned to the SLCS

Step 5: Define Source Conditions

Use a constant concentration boundary at the landfill base

Step 6: Run Simulation

Analyze concentration profiles across layers
Evaluate the impact of phase change on transport

Graphical Output: Depth vs Concentration

PDF Report

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Interpretation of Results

This example highlights several important behaviors:

1. Enhanced Transport in Unsaturated Zone

Due to higher diffusion and vapor-phase mobility, DCM can migrate more rapidly in the unsaturated portion.

2. Mixing-Dominated Saturated Zone

The lower diffusion coefficient still reflects significant mixing, leading to lateral spreading within the SLCS.

3. Impact of Phase Partitioning

Henry’s Constant controls how much DCM transitions into vapor form, influencing:

Vertical flux
Concentration gradients
Breakthrough timing

4. Role of Biological Decay

Decay reduces concentrations over time, particularly in the landfill and underlying aquifer, but not within the SLCS.

Key Takeaways

The Phase Change feature in POLLUTEv10 is essential for modeling contaminants that partition between phases.
Accurate representation of unsaturated vs. saturated conditions is critical.
Henry’s Law plays a central role in volatile contaminant transport.
Layered modeling allows for realistic simulation of landfill systems.
This example reinforces the importance of site-specific parameterization.

Final Thoughts

Example 18 provides a powerful demonstration of how POLLUTEv10 can simulate complex environmental processes involving multi-phase transport and layered hydrogeology. While simplified, it introduces key concepts necessary for advanced contaminant migration modeling.

However, real-world applications require:

Detailed site characterization
Calibration against field data
Expert interpretation of results

The Phase Change option should only be used when the user fully understands the physical processes and assumptions involved.

Learn more about our Contaminant Transport Modeling Solutions

POLLUTE and MIGRATE Contaminant Modeling and Landfill Design