Simulation of reactive solute transport in the critical zone: a Lagrangian model for transient flow and preferential transport

Sternagel, Alexander; Loritz, Ralf; Klaus, Julian; Berkowitz, Brian; Zehe, Erwin

doi:https://doi.org/10.5194/hess-25-1483-2021

Articles | Volume 25, issue 3

https://doi.org/10.5194/hess-25-1483-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/hess-25-1483-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 25, issue 3

Research article

|

25 Mar 2021

Research article |

| 25 Mar 2021

Simulation of reactive solute transport in the critical zone: a Lagrangian model for transient flow and preferential transport

Alexander Sternagel, Ralf Loritz, Julian Klaus, Brian Berkowitz, and Erwin Zehe

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Final revised paper (published on 25 Mar 2021)
Preprint (discussion started on 21 Oct 2020)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Review comments on the manuscript "Simulation of reactive solute transport in the critical zone: A Lagrangian model for transient flow and preferential transport" by Alexander Sternagel, Ralf Loritz, Julian Klaus, Brian Berkowitz, and Erwin Zehe.', Anonymous Referee #1, 12 Nov 2020
- AC1: 'Response to comments of Anonymous Referee #1', Alexander Sternagel, 23 Nov 2020
RC2: 'comments to both authors and Associate Editor', Anonymous Referee #2, 30 Nov 2020
- AC2: 'Response to comments of Anonymous Referee #2', Alexander Sternagel, 08 Dec 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (further review by editor and referees) (23 Dec 2020) by Alberto Guadagnini

AR by Alexander Sternagel on behalf of the Authors (11 Feb 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (13 Feb 2021) by Alberto Guadagnini

RR by Anonymous Referee #3 (14 Feb 2021)

Suggestions for revision or reasons for rejection

The manuscript proposes and extension of the Lagrangian approach that
was developed by the authors and others (Sternagel, Zehe, Jackisch)
for conservative solutes in previous publications to reactive transport.
I am reviewing a revised version of the manuscript, and take into account
also previous reviewer comments and replies by the authors.

Section 2 of the paper summarizes the Lagrangian flow and transport methodology
for conservative solutes from previous publications, Section 3 the
implementation of sorption/desorption and degradation, the central part of the
manuscript. The model is used for the interpretation and
modeling of vertical concentration data from 2 study sites, and compared
to the corresponding modeling results using Hydrus. Section 4 describes the
experimental data and parameters as well as the model setup. Section 5
reports on the modeling results, and the interpretation of the
experimental data. Section 6 provides a discussion of the results, and
Section 7 conclusions and outlook.

To my reading the authors have addressed the reviewer comments satisfyingly. The paper
is interesting because it provides an effective 1D Darcy-scale modeling approach for
reactive solute transport under unsaturated flow conditions. I only have two comments
detailed in the following.

Comments:

- Section 4 is termed "Model Benchmarking". I find this confusing on several accounts. Firstly, the
"benchmarking" results are reported in Section 5 (Results) while Section 4 reports on simulation
setup, and experimental data. Secondly, and more importantly, the comparison of the numerical
model results with experimental data is not a benchmarking of the model. I would expect that
the model be benchmarked against and/or (i) analytical solutions (if available in simplified setup),
(ii) numerical results from other well established codes (simulating exactly the same scenario),
(iii) fully controlled analogue experiments (for controllably the same scenario as the numerical model scenario).
The provided benchmarking does not meet any of these criteria, but applies the (unbenchmarked)
model for the interpretation of concentration data from field experiments in heterogeneous soils where
other processes than the ones represented in the numerical model may also be important. Thus, I would
recommend benchmarking the numerical model against known solutions (analytical or numerical), and
rename Sections 4 and 5 with a title that reflects that the model is used for the interpretation
of experimental data.

- Section 6.4: "Instead, they use a co-location probability approach, which describes solute
particle interactions like mass transfer based on a reaction probability dependent on the
distance between a pair of particles." This passage refers to the reactive random walk particle
tracking approaches of Benson and Meerschaert and others. The feature mentioned by the authors is
relevant for chemical reactions between different mobile chemical species. The approach described by the authors
only considers reactions for a single mobile species (sorption/desorption,
first-order degradation). Thus, it is not clear what the authors want to say with this
discussion. Do the authors refer to the implementation of diffusive mass transfer between
fluid particles or chemical reactions?

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ED: Publish subject to minor revisions (review by editor) (15 Feb 2021) by Alberto Guadagnini

AR by Alexander Sternagel on behalf of the Authors (22 Feb 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (23 Feb 2021) by Alberto Guadagnini

AR by Alexander Sternagel on behalf of the Authors (23 Feb 2021)

Short summary

The key innovation of the study is a method to simulate reactive solute transport in the vadose zone within a Lagrangian framework. We extend the LAST-Model with a method to account for non-linear sorption and first-order degradation processes during unsaturated transport of reactive substances in the matrix and macropores. Model evaluations using bromide and pesticide data from irrigation experiments under different flow conditions on various timescales show the feasibility of the method.