EGU2020-3112
https://doi.org/10.5194/egusphere-egu2020-3112
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Preferential fluid flow and chemical transport in saturated fractured porous media and in heterogeneous vadose zones: Two sides of the same coin

Brian Berkowitz
Brian Berkowitz
  • Weizmann Institute of Science, Department of Earth and Planetary Sciences, Rehovot, Israel (brian.berkowitz@weizmann.ac.il)

Preferential fluid flow and chemical transport occur on scales ranging from pores to aquifers and catchments, in both fully and partially water-saturated geological formations. Preferential flows can be considered, in a general sense, manifestations of self-organization that hinders perfect mixing within a system, and leads to faster throughput of water and chemicals. However, unified concepts for the onset, spatiotemporal patterning, and magnitude of such preferential flows are generally difficult to define; and this is compounded by the difficulty – or practical impossibility – of obtaining detailed measurements of the structure and hydraulic functioning of vadose zones, catchments, and aquifers. We propose that conceptualizations and quantitative characterizations of preferential fluid flow and chemical transport in all of these systems can be unified in terms of tools that connect them in a dynamic framework. Here, we discuss key, shared features of fluid flow and chemical transport dynamics in each of these two systems, based on both laboratory and field measurements, and numerical simulations. We show how even well-connected fracture networks can display highly non-uniform preferential paths for fluid and chemicals. We then recognize that this behavior is similar to that of rapid infiltration in soils and the vadose zone, which exhibits strongly localized preferential pathways in root channels, cracks, worm burrows or connected inter-aggregate pore networks. Moreover, both types of domains can display “memory effects”, in terms of the location and functioning of preferential paths even during perturbations in the velocity gradient and/or rates of infiltration. We argue that the ubiquity of unresolved (or uncharacterized) heterogeneity at all spatial and temporal scales necessitates the use of effective medium models that enable an accounting of a wide range of flow and transport behaviors. For chemical transport, we focus on a probabilistic modelling framework that can capture the dynamics in heterogeneous vadose zones and fractured (or otherwise heterogeneous) geological formations. We then demonstrate application of this model to interpret field-scale tracer breakthrough curves (concentration vs. time) in a highly fractured karst formation over length scales of up to more than 7 km.

How to cite: Berkowitz, B.: Preferential fluid flow and chemical transport in saturated fractured porous media and in heterogeneous vadose zones: Two sides of the same coin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3112, https://doi.org/10.5194/egusphere-egu2020-3112, 2020

Comments on the presentation

AC: Author Comment | CC: Community Comment | Report abuse

Presentation version 1 – uploaded on 01 May 2020
  • CC1: Comment on EGU2020-3112, Dani Or, 05 May 2020

    Hi Brian,

    1. Do you think that the mechanisms that generate preferential trasport pathways in these two domains similar?

    2. what do you think is the main agent for such pathways in the shallow VZ?  

    3. How important these VZ pathways are at the large scale (catchment, continent) 

    • AC1: Reply to CC1, Brian Berkowitz, 05 May 2020

      Hi Dani, 

      To answer briefly:

      1. Do you think that the mechanisms that generate preferential trasport pathways in these two domains similar?

      The main idea here is that flows - and chemical transport - in fractured formations, fractured porous formations, and at least in STEADY-STATE infiltration in partially saturated (fractured, porous) domains, are similar. The actual driving forces / mechanisms may differ, but the end result is the same. Strong preferential pathways, even in "homogeneous" domains.

      2. what do you think is the main agent for such pathways in the shallow VZ?  

      The main agent - we have shown, as have others, in various experiments (from cm scale MRI measurements to meter scale cells), even in "homogeneous" acid-washed and sieved sand - that preferential paths and long tailing (non-Fickian, non-classical advection-dispersion) still arise. For unsteady (time-dependent) infiltrations, and multiple infiltration events, we find a "memory effect" where similar or the same flow paths are formed each time. Inflowing water looks for minimum energy pathways. Once it finds such paths in a soil/sand column, then subsequent infiltrations will try to follow the same paths, unless there has been swelling, compression, earthworm/roo activity, or other mechanisms that change the geometry.

      3. How important these VZ pathways are at the large scale (catchment, continent) 

      There are (unresolved) heterogeneities at all length scales, so preferential pathways are relevant and ubiquitous at all scales. In the VZ, two initial questions might be - how thick is the VZ (to the saturated zone), and, over what areal domain are we considering? The answer will certainly influence how we treat the importance of the VZ and pathways at large scales. For chemical transport, VZ pathways may be more significant than for pure fluid flow.

      Thanks.