POLLUTEv10 Example 14: Modeling a Landfill with Primary and Secondary Leachate Collection Using Passive Sink

Introduction

POLLUTEv10 Example 14 demonstrates an advanced application of contaminant transport modeling using the Passive Sink special feature. This scenario simulates a landfill equipped with both primary and secondary leachate collection systems, incorporating layered hydrogeologic conditions and a finite contaminant source.

The example is particularly useful for environmental engineers and hydrogeologists seeking to understand how engineered systems influence contaminant migration in subsurface environments.

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Conceptual Model Overview

The model represents a hypothetical landfill with the following characteristics:

• Finite mass of a conservative contaminant
• Primary and secondary leachate collection systems
• Underlying aquifer with fixed outflow
• Use of a passive sink to simulate leachate removal

Key Assumptions

• Peak leachate concentration is reached early
• No increase in concentration over time
• Flow conditions are steady-state
• Transport includes advection and diffusion

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Source Term Characterization

The landfill contains waste with the following properties:

• Waste thickness: 6.25 m
• Waste density: 600 kg/m³
• Contaminant fraction: 0.2%

Total Contaminant Mass per Unit Area

$$m_{tc} = 0.002 \times 600 \times 6.25 = 7.5 \, \text{kg/m}^2$$

Reference Height of Leachate

$$H_r = \frac{m_{tc}}{c_0} = \frac{7.5}{1} = 7.5 \, \text{m}$$

Where:

• $c_0 = 1000 \, \text{mg/L}$

Because peak concentration occurs early:

• Rate of increase in concentration: $$c_r = 0$$

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Leachate Generation and Collection

The water balance in the landfill is defined as:

• Infiltration through cover: 0.3 m/a
• Exfiltration through base: 0.01 m/a

Volume of Leachate Collected

$$Q_c = q_0 – v_a = 0.3 – 0.01 = 0.29 \, \text{m/a}$$

This collected leachate is managed by the primary system, while the secondary system is modeled using a passive sink.

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Hydrogeologic Stratigraphy

The subsurface consists of:

Layer	Description	Thickness
Layer 1	Clay layer	1 m
Layer 2	Granular layer (secondary LCS)	0.3 m
Layer 3	Aquitard	2 m
Aquifer	Flowing groundwater zone	1 m

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Flow Conditions

Vertical Flow

• From landfill to secondary system: $$v_a = 0.01 \, \text{m/a}$$
• From secondary system to aquifer: $$v_a = 0.0 \, \text{m/a}$$

This implies the water table is located at the base of the secondary system.

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Horizontal Flow in Passive Sink Layer

The passive sink (granular layer) induces lateral flow:$$v_s = \frac{(v_{a1} – v_{a2}) \times L}{h}$$$$v_s = \frac{(0.01 – 0.0) \times 200}{0.3} = 6.67 \, \text{m/a}$$

This represents efficient lateral drainage within the secondary leachate collection system.

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Aquifer Flow

• Upgradient inflow: 4 m/a
• Down-gradient outflow:

$$v_b = 4 + \frac{200}{1} \times 0.0 = 4 \, \text{m/a}$$

Thus, aquifer flow remains constant beneath the landfill.

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Passive Sink Layer Configuration

The Passive Sink option divides the system into three computational layers:

1. Layer 1 (Clay Layer)
   • Vertical flow only: 0.01 m/a downward
   • No horizontal flow
2. Layer 2 (Granular Secondary LCS)
   • Horizontal flow: 6.67 m/a
   • Represents leachate collection
3. Layer 3 (Aquitard)
   • No advective flow
   • Diffusion dominates transport

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Model Parameters

Property	Symbol	Value	Units
Darcy Velocity	va	Variable	m/a
Sink Velocity	vs	Variable	m/a
Diffusion Coefficient	D	Variable	m²/a
Distribution Coefficient	Kd	0	cm³/g
Soil Porosity	n	0.4	–
Granular Porosity	n	0.3	–
Dry Density	ρd	1.5	g/cm³
Source Concentration	c₀	1000	mg/L
Ref. Height	Hr	7.5	m
Leachate Collected	Qc	0.29	m/a
Landfill Length	L	200	m
Aquifer Velocity	vb	4	m/a

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Graphical Output: Concentration vs Time

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PDF Report

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Key Insights

• The secondary leachate collection system significantly reduces vertical contaminant migration.
• The passive sink approach effectively simulates engineered drainage layers.
• With zero vertical flow below the secondary system, contaminants are largely contained above the aquifer.
• Diffusion becomes the dominant transport mechanism in deeper layers.

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Important Modeling Considerations

⚠️ This example is illustrative only and should not be used as a design standard.

Each landfill system is unique, and accurate modeling requires:

• Site-specific hydrogeologic data
• Proper calibration of transport parameters
• Expertise in contaminant transport processes

The Passive Sink feature should only be used by professionals with a strong hydrogeotechnical background.

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Conclusion

POLLUTEv10 Example 14 highlights the importance of incorporating engineered systems—such as secondary leachate collection layers—into contaminant transport models. By using the Passive Sink feature, this example provides a realistic framework for simulating leachate interception and lateral drainage, improving our understanding of landfill performance and environmental protection.

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