Technologie-Transfer Service und Beratung in Sachen

20 downloads 2729 Views 1MB Size Report
Email: [email protected]. 2 Central Institute for Electronics (ZEL), ... exchange between macropores and bulk soil. between macropores and bulk soil.
Investigating preferential flow processes in soils using anisotropy in electrical resistivity

H53I-1640

Sadam Al-Hazaimay1, Johan A. Huisman1, Egon Zimmermann2, Andreas Kemna3, and Harry Vereecken1 1

Agrosphere (IBG-3), Forschungszentrum Jülich, 52425 Jülich, Germany. Email: [email protected] 2 Central Institute for Electronics (ZEL), Forschungszentrum Jülich, 52425 Jülich, Germany 3 Department of Geodynamics and Geophysics, University of Bonn, 53115 Bonn, Germany

Preferential flow processes have become an important societal concern because they can quickly transport water, solutes, and contaminants from the surface to the groundwater. In the last few decades, many efforts were made to improve understanding of macropore preferential flow processes. Unfortunately, very few measurement methods provide insights into these preferential flow processes. Therefore, the objective of this study is to evaluate whether anisotropy in electrical resistivity can be used to identify the existence of flow in macropores and perhaps even to characterize the exchange between macropores and bulk soil. between macropores and bulk soil.

Synthetic case infiltration

study

for

Matrix Macropore

Background and objectives

theta (-)

time0

time1

a

b

time10

c

d

e

Figure 1. Simulated water content distributions in HYDRUS model for both the macropore and matrix domain before, during and after water infiltration.

Macropore

θr (-)

0.1447

0.05

θs (-)

0.3487

0.4

α (m-1)

1.68

3.5

n

4.913

8

Ks (m d-1)

0.0396

38.88

F

3.306

3.207

n

1.339

2.164

ϕ (%)

0.387

0.378

Our experimental approach consists of the following steps:

Table 1. Mualem van Genuchten and Archie‘s parameters for both domains.

Figure 2. Fitted Archie's conductivity model for sand-clay mixture and pure sand to the measured conductivity using TDR.

Small positive anisotropy 1- High resistivity before infiltration starts

• Water infiltration was simulated in two domains parametrized with different soil hydraulic properties (Table 1, Figure 1a).

2- Infiltration started & macropore is activated

a

b

c

d



Develop a soil column (Figure 5) to measure the anisotropy in effective resistivity in two directions.



Select two materials with known hydraulic parameters (Table 1) for packing in the matrix and macropore domains.



Determine the petrophyical parameters materials (Table 1).



Pack the materials into the soil column to built macropore for water infiltration.



Install electrodes for current injections and potential measurements and tensiometers to monitor the matric potential during infiltration.



Perform actual measurements of electrical resistivity in two directions using the EIT tomography system.

Conclusions and outlook a

b

f

Figure 3. Simulated electrical resistivity (a) in horizontal and vertical direction obtained from the forward model calculations and the temporal development (b) of the anisotropy ratio.

g

h

i

j

Figure 4. Sensitivity of the anisotropy ratio. Parameter n for the macropore domain could not be increased by 10% (c) because of numerical issues with the HYDRUS model.



We showed that water infiltration through an artificial macropore led to an anisotropy ratio in electrical resistivity.



Experimental and modeled electrical resistivity in vertical and horizontal direction generally showed similar behavior. For both measurements and simulations, electrical resistivities strongly decreased as soon as water infiltration into the macropore started.



The anisotropy of the electrical resistivity can be used to identify the existence of flow in macropores.



Our next step is to perform measurements in 2D or 3D soil tanks to obtain the temporal development of the electrical anisotropy following the work of Moysey and Liu (2012).

E3 & E4 opposite to E6 & E5, respectively artificial macropore E8

• The simulated temporal development of the resistivity anisotropy was then calculated from the simulated resistivity in horizontal and vertical direction for the electrode configuration used in the experimental set-up using MATLAB (Figure 3a-b).

T3 E7 E5

E6

T2

E2 T1

• Sensitivity of the anisotropy ratio was analyzed by varying the hydraulic parametes for both the macropore and matrix domains (Figure 4a-j).

Acknowledgement This study has been supported by the Transregional Collaborative Research Centre 32 “Patterns in Soil-Vegetation-Atmosphere Systems: Monitoring, Modelling & Data Assimilation” from the Deutsche ForschungsGemeinschaft (DFG). We also thank SIBELCO Deutschland GmbH for providing the clay material.

of both

e

3- Infiltration stopped & water re-distributed into matrix

• Initially, water quickly moved vertically in the macropore domain. Some lateral flow into the soil matrix was also simulated (Figure 1b).

• Simulated water content distributions were converted to electrical resistivity distributions using the petrophysical relationship of Archie (Table 1).

Matrix

time11 time40

water

• The entire macropore was saturated when infiltration into the macropore stopped (after 0.0075 days), and water flowed from the saturated macropore into the matrix (Figure 1c-e).

Measurement case study for water infiltration

Domain

E1

a a

b

b

Figure 5. Experimental setup (a) and 3D computational domain (b) corresponding to the soil column. Eletrodes (E1,E8) & (E3,E6) injected current and electrodes (E2,E7) & (E4,E5) measured the potential differences. Tensiometers (T1-T3) recorded the matrix potential.

Figure 6. Measured electrical resistivity (a) in horizontal and vertical direction obtained using the Electrical Impedance Tomography (EIT) system of Zimmermann et al. (2008) and the temporal development (b) of the anisotropy ratio.

References Moysey, S. M. J., and Z. Liu, (2012), Can the Onset of Macropore Flow be detected using Electrical Resistivity Measurements? Soil Sci. Soc. Am. J., 76(1), 10-17, doi:10.2136/sssaj2010.0413 Zimmermann, E., A. Kemna, J. Berwix, W. Glaas, and H. Vereecken, (2008), EIT measurement system with high phase accuracy for the imaging of spectral induced polarization properties of soils and sediments, Meas. Sci. Technol., 19, doi:10.1088/0957-0233/19/9/094010