POLLUTEv10 Example 5: Hydraulic Trap (Upward Flow into the Landfill)

Introduction

Example 5 demonstrates a fundamentally different hydrogeological condition compared to previous cases: a hydraulic trap, where groundwater flow is directed upward into the landfill rather than downward into the aquifer.

This scenario is critical in environmental modeling because it represents conditions where contaminant migration is naturally limited or even suppressed due to opposing hydraulic gradients. The example builds on Example 4 (finite mass source with leachate collection) but modifies flow conditions and aquifer properties to simulate this protective mechanism.

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What is a Hydraulic Trap?

A hydraulic trap occurs when:

• The vertical hydraulic gradient is upward
• Groundwater flows into the landfill base
• Downward contaminant migration is restricted or reversed

In modeling terms, this is represented by a negative Darcy velocity.

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Conceptual Model

The system consists of:

• A finite mass landfill source at the surface
• A 4 m thick aquitard
• A 1 m thick aquifer beneath
• A low permeability layer below the aquifer
• A hydraulic trap condition (upward flow)

Key Differences from Example 4:

• Upward flow instead of downward infiltration into aquifer
• Thinner aquifer (1 m vs 3 m)
• Slightly higher aquifer porosity (0.35)
• Landfill width simplified to W = 1 m (2D strip model)

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Finite Mass Source (Same as Example 4)

The source term remains unchanged:

• Reference Height of Leachate: Hr = 7.5 m
• Source Concentration: 1000 mg/L
• Rate of Increase: Cr = 0

This ensures comparability between Example 4 and Example 5.

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Hydraulic Conditions

Vertical Darcy Velocity (Key Change)

$v_a = -0.001 \, \text{m/a}$

• The negative sign indicates upward flow
• This is the defining characteristic of the hydraulic trap

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Leachate Collection System

Because upward flow limits infiltration into the subsurface:

$Q_c = q_o$

Where:

• $q_o = 0.3 \, \text{m/a}$

Result:

• $Q_c = 0.3 \, \text{m/a}$

This means:

• Nearly all infiltrating water is captured
• Minimal contaminant mass enters the subsurface

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Groundwater Flow Balance

Upgradient Inflow:

• $v_{in} = 4 \, \text{m/a}$

Downgradient Outflow:

$v_b = v_{in} + \frac{v_a L}{h}$

Substituting values:

• $v_b = 4 – \frac{200 \times 0.001}{1} = 3.8 \, \text{m/a}$

Interpretation:

• Outflow is reduced due to upward flow
• The aquifer receives less contaminant loading

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Model Parameters

Property	Symbol	Value	Units
Darcy Velocity	va	-0.001	m/a
Diffusion Coefficient	D	0.01	m²/a
Distribution Coefficient	Kd	0	cm³/g
Soil Porosity	n	0.4	–
Dry Density	ρd	1.5	g/cm³
Soil Thickness	H	4	m
Sub-layers	–	4	–
Source Concentration	co	1000	mg/L
Rate of Increase	cr	0	mg/L/a
Reference Height	Hr	7.5	m
Leachate Collected	Qc	0.3	m/a
Landfill Length	L	200	m
Landfill Width	W	1	m
Aquifer Thickness	h	1	m
Aquifer Porosity	nb	0.35	–
Base Outflow Velocity	vb	3.8	m/a

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Graphical Output: Depth vs Concentration

PDF Report

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Interpretation of Results

1. Upward Flow Suppresses Contamination

The hydraulic trap:

• Prevents downward contaminant migration
• Reduces plume formation in the aquifer
• Acts as a natural containment mechanism

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2. Reduced Aquifer Impact

Compared to Example 4:

• Plume size is significantly smaller
• Concentrations are lower
• Transport is diffusion-dominated rather than advective

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3. Importance of Aquifer Thickness

With only 1 m thickness:

• Less storage capacity
• Faster response to hydraulic changes
• Greater sensitivity to vertical gradients

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4. Engineering Implications

Hydraulic traps can be:

• Naturally occurring
• Engineered using pumping systems

They are often used in:

• Containment strategies
• Remediation design
• Groundwater protection systems

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Practical Applications

This modeling scenario is highly relevant for:

• Landfill sites with upward gradients
• Confined or semi-confined aquifers
• Sites underlain by low permeability layers
• Remediation systems using hydraulic control
• Advanced Phase II ESA and risk assessments

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Comparison: Example 4 vs Example 5

Feature	Example 4	Example 5
Flow Direction	Downward	Upward
Darcy Velocity	+0.03 m/a	-0.001 m/a
Aquifer Thickness	3 m	1 m
Leachate Collection	Partial	Near complete
Plume Development	Significant	Minimal
Risk Level	Higher	Lower

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Key Takeaways

• Hydraulic traps are a powerful natural or engineered control on contaminant migration
• Upward gradients can significantly reduce environmental risk
• POLLUTEv10 effectively models complex flow reversals
• Comparing scenarios helps inform design and regulatory decisions

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