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 Engineering Geology, Lund University, S-211 00 Lund, Sweden.

2 Water Resources Engineering, Lund University, S-211 00 Lund, Sweden.

3 Leibniz Institute for Applied Geophysics, Stilleweg 2, 30655 Hannover, Germany

SGU Project 60-1658/2009

Outline

● Aim of this study

● Introduction

● Methods

● Experiments 1- 2(5)

● Conclusion

Aim of the study

● To analyse the movement of water and solutes across the capillary fringe

● To compare image analysis and resistivity imaging

Capillary Fringe

● Zone immediately above the ground water table

● Between saturated and unsaturated zone

● Caused by capillary action

● (Adhesion and Cohesion)

● Soil pores almost completely filled with water

● Thickness depends on the grain size of the soil

● Flow?

(Kasenow, 2001)

Tracer Application

The ideal tracer for water tracking should be/have:

● Easily visible

● Same or similar transport properties as water

● Not toxic!

→ Brilliant Blue FCF ☑

Brilliant Blue FCF

● Synthetic dye

● Used to colour food and hygiene products

● The flow paths are stained and easily separated from non-stained soil

● Lowest detectable dye concentration = 0.1 g/l

● High solubility in water

● Low toxicity

● Conductive

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

Image Analysis

● Distance to the aquarium: 3 m

● Time interval: 3 minutes

● Every 10th picture analysed (30min Time Steps)

Experiment 1: Setup

Configuration:

● Difference in the hydraulic head: 2 cm

● Angle: 0.93 degree

● Infiltration rate: 2 l/h

● Circulating flow: 4 l/h

Experiment 1: Image Analysis

Experiment 1: Image Analysis

Measured Velocity: 6.32 cm/h

Experiment 1: Resistivity Model

Experiment 1: Change in Resistivity

Experiment 1: Vertical slice from 3D model

After 1 hour After 2 hours

After 3 hours

Experiment 1: Comparison

Experiment 2: Setup

● Difference in the hydraulic head: 4 cm

● Angle: 1.88 degree

● Infiltration rate: 2 l/h

● Circulating flow: 8 l/h

Experiment 2: Image Analysis

Measured Velocity: 9.47 cm/h

Experiment 2: Resistivity Model

Experiment 2: Change in Resistivity

Experiment 2: Comparison

Conclusions● 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!

Thanks for your attention!

How to repeat this under field conditions?

SGU Project 60-1658/2009