This study aimed to analyze water and solute movement across the capillary fringe using resistivity imaging and image analysis in laboratory experiments. Two experiments were conducted with different hydraulic gradients and flow rates. Image analysis and resistivity imaging both showed horizontal flow within the capillary fringe, with tracer velocities dependent on the hydraulic gradient. The results indicate that flow characteristics in the capillary fringe are similar to the saturated zone, and sampling only in the saturated zone may miss pollution that remains in the capillary fringe.

1. Resistivity Imaging and Image
Analysis for Estimating Water and
Solute Transport Across the
Capillary Fringe in Laboratory
Experiments
Torleif Dahlin1, Magnus Persson2, Pontus Pojmark2,
Benedict Rumpf1, Thomas Günther3
1
2
3
Engineering Geology, Lund University, S-211 00 Lund, Sweden.
Water Resources Engineering, Lund University, S-211 00 Lund, Sweden.
Leibniz Institute for Applied Geophysics, Stilleweg 2, 30655 Hannover, Germany
SGU Project 60-1658/2009
Outline
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Aim of this study
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Introduction
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Methods
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Experiments 1- 2(5)
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Conclusion

2. Aim of the study
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To analyse the movement of water and solutes
across the capillary fringe
To compare image analysis and resistivity
imaging
Capillary Fringe
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Zone immediately above the
ground water table
Between saturated and
unsaturated zone
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Caused by capillary action
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(Adhesion and Cohesion)
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Soil pores almost completely
filled with water
Thickness depends on the
grain size of the soil
Flow?
(Kasenow, 2001)

3. Tracer Application
The ideal tracer for water tracking should
be/have:
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Easily visible
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Same or similar transport properties as water
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Not toxic!
→ Brilliant Blue FCF ☑
Brilliant Blue FCF
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Synthetic dye
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Used to colour food and hygiene products
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The flow paths are stained and easily separated
from non-stained soil
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Lowest detectable dye concentration = 0.1 g/l
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High solubility in water
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Low toxicity
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Conductive

4. Description of the Tank
Geoelectrical Imaging
• 2 x 5 lines x 32 electrodes
• Electrode separation 35 mm in-line, 70 mm between lines
• ABEM Terrameter LS used for measurements
• Manual move of cables between layout pairs
• Multiple gradient array and ”cross-hole” dipole-dipole array
• BERT used for 3D inversion

5. Image Analysis
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Distance to the aquarium: 3 m
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Time interval: 3 minutes
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Every 10th picture analysed (30min Time Steps)
Experiment 1: Setup
Configuration:
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Difference in the hydraulic head: 2 cm
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Angle: 0.93 degree
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Infiltration rate: 2 l/h
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Circulating flow: 4 l/h

6. Experiment 1: Image Analysis
Experiment 1: Image Analysis
Measured Velocity: 6.32 cm/h

7. Experiment 1: Resistivity Model
Experiment 1: Change in Resistivity

8. Experiment 1: Vertical slice from 3D model
After 1 hour
After 2 hours
After 3 hours
Experiment 1: Comparison

9. Experiment 2: Setup
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Difference in the hydraulic head: 4 cm
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Angle: 1.88 degree
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Infiltration rate: 2 l/h
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Circulating flow: 8 l/h
Experiment 2: Image Analysis
Measured Velocity: 9.47 cm/h

10. Experiment 2: Resistivity Model
Experiment 2: Change in Resistivity

11. Experiment 2: Comparison
Conclusions
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Clear correlation between image analysis and
resistivity results, differences may depend on different
detection levels
Horizontal movement within the capillary fringe
Tracer velocity highly dependent on hydraulic gradient
of saturated zone and infiltration depth
Infiltration depth dependent on dye infiltration rate
Flow characteristics similar to the characteristics of the
saturated zone!
Water table never intersected by tracer =>
sampling in the saturated zone may
completely miss pollution!

12. Thanks for your attention!
How to repeat this under field conditions?
SGU Project 60-1658/2009