EGU General Assembly 2020
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the Creative Commons Attribution 4.0 License.

Macropore-matrix mass transfer: reactive solute transport as quantified with Fluorescence imaging

Christoph Haas, Ruth Ellerbrock, and Horst H. Gerke
Christoph Haas et al.
  • Working Group “Hydropedology”, Research Area 1 “Landscape Functioning”, Leibniz Centre for Agricultural Landscape Research (ZALF) Müncheberg, Germany,

Preferential flow paths in soils play a major role for transport processes of heat, gas, water, and solutes and are important adsorption sites. For mass-exchange processes and water storage in soils, small-scaled soil properties, like the spatial distribution of adsorption sites and their accessibility, and the permeability are crucial. Interfaces between macropores (i.e., earthworm burrows, cracks, and root channels) and the soil matrix control the mass exchange. Water and solute transfer through the interface between bio-pores, aggregate or crack surfaces and the matrix was traced at the scale of small soil blocks (≤45 mm edge length) with Fluorescein (i.e., a reactive, fluorescent dye). The objectives were to visualize and quantify hydraulic transport, and sorption characteristics of earthworm-, root- and shrinkage-induced interfaces. Batch experiments were performed to calibrate the Na-Fluorescein tracer concentration versus fluorescence-intensity relationship and to derive parameters for two kinetic sorption models (i.e., Freundlich vs. Langmuir). Fluorescence imaging in the laboratory of small soil blocks was applied with a self-constructed spraying device, and with the help of the calibration, small-scaled dye-concentration maps were derived. Time- and interface-dependent positions of the wetting fronts in vertical direction were estimated with the help of the cumulative infiltration. Assuming equilibrated conditions between Na-Fluorescein in solution (calculated by multiplying the locale dye-concentration and the local water content) and Na-Fluorescein sorbed to soil, the total mass transfers as a function of macropore-type and spraying time were determined. The results of the mass transfer for water and reactive solutes were characteristic for the soil structure type and depending on the composition of the macropore-matrix interface. Differences were explained by alterations in soil structure and chemical composition of the coatings. Results suggest relations between mass exchange and observable soil properties. This can be helpful for improving the numerical simulation of macropore-matrix mass transfer and inverse simulations of small-scaled hydraulic, transport, and sorption characteristics of macropore walls.

How to cite: Haas, C., Ellerbrock, R., and Gerke, H. H.: Macropore-matrix mass transfer: reactive solute transport as quantified with Fluorescence imaging, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14230,, 2020


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displays version 1 – uploaded on 04 May 2020
  • CC1: Comment on EGU2020-14230, Paul Hallett, 05 May 2020

    Very interesting Christoph!  With the crack vs. biological coatings what do you think is driving the difference in dye movement from the pore/crack surface?  Compression, microcracking in crack, mucilage....all of the above?   Thanks.  Paul Hallett.

    • AC1: Reply to CC1, Christoph Haas, 05 May 2020

      Thank you Paul!

      • AC3: Reply to AC1, Christoph Haas, 05 May 2020

        All processes you mentioned can alter the water retention curve and the (un-)saturated conductivity, and consequently, the water and dye movement through the crack surfaces. Additionally, exudation of organic compounds may clock pores and alter the wettability. The accessibility of adsorption sites is also influenced by all these processes, which impacts the movement of the dye, too.

  • CC2: Comment on EGU2020-14230, Dani Or, 05 May 2020

    Nice work Christoph!

    I can see the importance of biopore wall properties for mass exchange, yet to place this in the context of the motiavting example in slide 3 (and the conceptual picture slide 1) - what is the importance of differentiating this relative to biopore topology (dead end, connectedness to the surface, etc.) 

    (I am wondering if you could imagine testing this under stationary and active flow conditions...)


    • AC5: Reply to CC2, Christoph Haas, 05 May 2020

      Thank you Dani,
      the connectivity to the surface is one factor that controls the mass of e.g. water, available for preferential flow. Thus, if the biopore is not connected to the surface, no flow would occur. While dead ends minder the drainage of the biopores. The horizontal fluxes are still controlled by the macropore surface properties. However, this was just a first try in the lab. We will use soil columns for a tracer experiment in combination with CT to link fluxes with soil pore properties like dead ends.   

      • CC3: Reply to AC5, Dani Or, 05 May 2020

        hmm... imagine a fully saturated soil and a vertical biopore not connected to the surface... do you think that there will be no flow into the empty biopore (even by gravity differences)? I think that activation of macri/bio pores is not as simple as we tend to invision - but indeed it require extreme wetness conditions if not connected to the surface. 

        • AC6: Reply to CC3, Christoph Haas, 05 May 2020

          The connectivity to the surface is not an on/off switch for PF, just one factor. Surely, if the soil is fully saturated water moves in the biopore   

  • AC4: Comment on EGU2020-14230, Horst H. Gerke, 05 May 2020


    The plan is to carry  out similar experiments in a Neutron Beam facility (e.g.,PSI).