Transit Time index (TTi) as an adaptation of the humification index to illustrate transit time differences in karst hydrosystems: application to the karst springs of the Fontaine de Vaucluse system (southeastern France)

Serène, Leïla; Batiot-Guilhe, Christelle; Mazzilli, Naomi; Emblanch, Christophe; Babic, Milanka; Dupont, Julien; Simler, Roland; Blanc, Matthieu; Massonnat, Gérard

doi:https://doi.org/10.5194/hess-26-5035-2022

Articles | Volume 26, issue 19

https://doi.org/10.5194/hess-26-5035-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/hess-26-5035-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 26, issue 19

Research article

|

11 Oct 2022

Research article |

| 11 Oct 2022

Transit Time index (TTi) as an adaptation of the humification index to illustrate transit time differences in karst hydrosystems: application to the karst springs of the Fontaine de Vaucluse system (southeastern France)

Leïla Serène, Christelle Batiot-Guilhe, Naomi Mazzilli, Christophe Emblanch, Milanka Babic, Julien Dupont, Roland Simler, Matthieu Blanc, and Gérard Massonnat

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Final revised paper (published on 11 Oct 2022)
Preprint (discussion started on 31 Mar 2022)

Interactive discussion

Status: closed

RC1:
'Comment on hess-2022-100', Weiquan Dong, 22 Apr 2022

Title: by Serène et al. and was submitted to Hydrology and Earth System Sciences (https://doi.org/10.5194/hess-2022-100. Preprint. Discussion started: 31 March 2022)

Reviewed by

Weiquan Dong

Professional Engineering Specialist, Ph.D., P.E.

Bureau of Industrial Site Cleanup

Nevada Division of Environmental Protection

Department of Conservation and Natural Resources

375 E Warm Springs Rd, Suite 200

Las Vegas, NV. 89119

wdong@ndep.nv.gov; quan78@gmail.com

(O) (702)-668-3929; Fax (702) 486-2863

The authors presented the Transit Time index (TTi) as a potential natural transit time tracer for the groundwater residence time within the range of 0-6 months based on the data of karst springs and their catchments of Fontaine de Vaucluse system (Southeastern France). The TTi appears an advance from the previous Humification IndeX (HIX) as initially proposed by Blondel et al. (2012). The TTi has the potential to provide a cost-effective transit time tracer for the groundwater residence time within the range of 0-6 months compared to artificial tracers, which will fill the gap of the natural transit time tracers for a short range of 0-6 months. The authors presented basic spring and catchment settings, detailed how TTi was derived, and conducted principal component analysis (PCA) to determine TTi components (Tyr, P1, H1&H2) and other variables related to the transit time (electrical conductivity, discharge, magnesium, and silica contents). The authors concluded that the TTi is more sensitive and can identify freshwater arrivals in mixtures. Therefore, the TTi has the potential to be a supplemental tracer to artificial tracers and is likely useful for assessing the vulnerability of the aquifer which is the fast response to the recharge. However, besides the comments noted on the manuscript, I do have 3 suggestions for this manuscript before accepting it for publication:

1) I would like to see more detailed hydrogeology (karstification types), groundwater movement, and catchment delineation because this information is the base used to validate the TTi results.

2) Perform hydrograph separation to quantify the storm flow and base flow for the storm events in different seasons, because this information is also helpful to interpret the TTi results.

3) Double-check the positive correlation between electrical conductivity (CE) and spring discharge, which is very unusual compared to other karst springs.

Finally, I would also like to suggest that the authors conduct the artificial tracer tests for the springs investigated in this manuscript and validate the TTi with the artificial tracer test results. However, this suggestion is not required for accepting this manuscript for publication.

Citation: https://doi.org/10.5194/hess-2022-100-RC1
- AC1:
  'Reply on RC1', Leïla Serène, 23 May 2022
  
  Please find attached our responses to your suggestions and comments.
  
  Citation: https://doi.org/10.5194/hess-2022-100-AC1
  - RC3: 'Reply on AC1', Weiquan Dong, 23 May 2022
    
    My comments and remarks on the pre-print were well addressed and I don't have additional comments and the revised pre-print version is acceptable to be published.
    Thanks,
    Weiquan Dong
    
    Citation: https://doi.org/10.5194/hess-2022-100-RC3
    
    AC5: 'Reply on RC3', Leïla Serène, 28 May 2022
    
    Thank you very much for your remarks and comments, they helped us a lot to improve the clarity of the paper.
    
    Citation: https://doi.org/10.5194/hess-2022-100-AC5
RC2:
'Comment on hess-2022-100', Anonymous Referee #2, 28 Apr 2022

Please see the attach.

Citation: https://doi.org/10.5194/hess-2022-100-RC2
- AC2: 'Reply on RC2', Leïla Serène, 23 May 2022
  
  Please find attached our responses to your comments.
  
  Citation: https://doi.org/10.5194/hess-2022-100-AC2
CC1:
'Comment on hess-2022-100', Neil Terry, 30 Apr 2022

This study presents a new analysis metric (transit time index, TTi) for qualitative assessment of relative short-term transit times in karst systems. The method uses fluorescence spectroscopy of water samples. The authors perform parallel factor analysis on the excitation-emission matrix (EEM) from these samples to identify regions of the EEM that can be associated with humic (component 1) verus protein-like (components 2 – 4) substances. The TTi is then the ratio of integrated EEM intensities of humic-like to humic-like+protein-like components. More protein-like components cause the TTi to shift closer to 0 and indicate a shorter residence time. The authors present analysis that suggests the TTi may be more sensitive to short-term (< 6 months) transit times compared to other natural tracers even when significant mixing has likely occurred. This is supported by comparison to other measured constituents, specific conductance measurements, analysis of the hydrograph, and general properties of the karst systems studied.
Though I am not a user of fluorescence spectroscopy, I found the methods to be understandable and relatively clearly explained. The reasoning for the methods used is easy to follow and appears sound, although may at times be too brief. The graphics are visually pleasing and helpful for explaining the research.
As other reviewers have pointed out (RC1 and RC2), the main weakness of this research is that transit time was not actually quantified, and only indirect evidence for the effectiveness of TTi is supplied. This makes all analyses/conclusions seem somewhat tentative and the overall findings much less impactful. Nevertheless, the methods and reasoning using indirect evidence were convincing enough to be suitable for publication in my opinion. Hopefully subsequent lab/field experiments will be published to further validate and refine the method.
Therefore, I have no additional general or specific comments in addition to what other reviewers (RC1 and RC2) have already requested. Thanks for the chance to review this interesting work.

Citation: https://doi.org/10.5194/hess-2022-100-CC1
- AC3: 'Reply on CC1', Leïla Serène, 23 May 2022
  
  Please find attached our responses to your comments.
  
  Citation: https://doi.org/10.5194/hess-2022-100-AC3
AC4:
'Comment on hess-2022-100', Leïla Serène, 26 May 2022

Please, find attached the section 3.4 added to the paper, in accordance with the comments of the reviewers and the community.

Citation: https://doi.org/10.5194/hess-2022-100-AC4
- RC4:
  'Reply on AC4', Weiquan Dong, 26 May 2022
  
  This Section 3.4 3.4 (Way forward towards a transferable qualitative tracer and towards quantitative approach) answers my questions on seasonality, artificial tracers, land use, and radioisotopes. This new section shows that the authors not only looked into the questions but also address the questions other readers may have. The TTi is a new method that can be used to study the transit time for the residence time less than a year in karst aquifers. There are a few applicable methods for this short residence time, so the TTi has a good potential to fill this gap.
  
  Citation: https://doi.org/10.5194/hess-2022-100-RC4
  - AC6: 'Reply on RC4', Leïla Serène, 28 May 2022
    
    Thank you for this very positive comment, we are very glad that we were able to convince you of the potential of the TTi.
    
    Citation: https://doi.org/10.5194/hess-2022-100-AC6

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Publish subject to minor revisions (further review by editor) (31 May 2022) by Zhongbo Yu

AR by Leïla Serène on behalf of the Authors (06 Jun 2022) Author's response Author's tracked changes Manuscript

ED: Publish as is (19 Jun 2022) by Zhongbo Yu

AR by Leïla Serène on behalf of the Authors (28 Jun 2022)

Short summary

This work aims to develop the Transit Time index (TTi) as a natural tracer of karst groundwater transit time, usable in the 0–6-month range. Based on the fluorescence of organic matter, TTi shows its relevance to detect a small proportion of fast infiltration water within a mix, while other natural transit time tracers provide no or less sensitive information. Comparison of the average TTi of different karst springs also provides consistent results with the expected relative transit times.