Quantifying Baseflow with Radon, H and O Isotopes and Field Parameters in the Urbanized Catchment of the Little Jukskei River, Johannesburg

Round 1

Reviewer 1 Report (New Reviewer)

Comments and Suggestions for Authors

The authors have used multiple physical techniques to understand surface water ground water interaction in a complex geological setting in an urban environment. The study points to the usefulness of multi-pronged approach to understand this problem of practical value. The manuscript is publishable but can be improved with some minor modifications as suggested below.

 

Abstract: Can be slightly shortened given that it is 302 words long and the submission requirements at 200 words. I won’t take it against the authors since 200 words are often not enough.

1’           Line 52 Replace trace evaporation processes with evaporation processes.

2. Line 92. Check the referencing format for ref [14]
3. Equation 1 seems to be a standard equation used in such studies. My question is that many mentioned parameters can be measured and are measured values. How was I, Ground water inflow rate measured?

4            Line 105 and the para that follows. The authors should specify that precipitation is the main source of surface and ground water and its isotope ratios eventually determine the final ratios of various water bodies. Fractionation actually happens when precipitation undergoes evaporation, mixing etc. There are many papers that deal with this.

5            Lines 227-229 can be deleted. The readers are aware of this.

6. The captions for Table 2 and 3 can be slightly altered. Water chemistry essentially means chemical parameters (alkalinity, cations, anions etc.). Just say stable isotope and physical parameters.

7            Line 309. Where is the acidic rainwater coming from?

8            Line 323. What is a Local Evaporation Line?  Was it derived from Ground Water data?

9            Fig 8. Are the radon concentrations measured or modeled ones? It would be a good idea to see the correlation between measured radon concentrations and measured/calculated ground water flow rates.

10          It would be worthwhile doing a correlation between radon concentrations and either oxygen or hydrogen isotope ratios (Fig 3), preferably hydrogen. From the figure it looks like there might be a decent correlation. Since the authors claim that stable isotope ratios are directly connected with the source of recharge waters, this exercise will strengthen their argument in using radon as a valuable tracer in this area.

11          A curious question for the authors. The  FINIFLUX model, as suggested by the authors, is applicable using any tracer. Indeed they have used radon. Is it worthwhile trying this on the stable isotope values? If so, it will strengthen the paper should there be a good agreement between the findings based on radon and stable isotopes.

Author Response

MDPI Hydrology

3737783 – 18 July 2025

Quantifying baseflow with radon, H and O isotopes and field parameters in the urbanized catchment of the Little Jukskei River, Johannesburg

Khutjo Diphofe, Roger Diamond and Francois Kotze

 

 

Comment – Response Letter

Reviewer 1 Comments

The authors have used multiple physical techniques to understand surface water ground water interaction in a complex geological setting in an urban environment. The study points to the usefulness of multi-pronged approach to understand this problem of practical value. The manuscript is publishable but can be improved with some minor modifications as suggested below.

Thank you.  Please see the changes made for each comment below.

 

Abstract: Can be slightly shortened given that it is 302 words long and the submission requirements at 200 words. I won’t take it against the authors since 200 words are often not enough.

Done. The abstract has been shortened to 208 words.

1. Line 52 Replace trace evaporation processes with evaporation processes.

Not done.  The word “trace” here is a verb, as the preceding items in the list each have their own verb (distinguish and identify), so removing trace would leave the last item without a verb.

2. Line 92. Check the referencing format for ref [14]

Done.  The citation and reference seem to be correct and match the style of others: in text references are “name, date [number]”.

3. Equation 1 seems to be a standard equation used in such studies. My question is that many mentioned parameters can be measured and are measured values. How was I, Ground water inflow rate measured?

Done.  A column has been added to Table 1, telling the reader the source of the parameters.  The following change to line 84 also makes this more obvious: “fluxes” was deleted and replaced with “inflow (I)”.

4. Line 105 and the para that follows. The authors should specify that precipitation is the main source of surface and ground water and its isotope ratios eventually determine the final ratios of various water bodies. Fractionation actually happens when precipitation undergoes evaporation, mixing etc. There are many papers that deal with this.

Done.  A sentence has been added to the end of that paragraph: “In many cases, especially where surface water and groundwater have not travelled far since their origin as precipitation, the main source of isotope variations is inherited from precipitation factors, such as latitude, altitude, temperature, cloud height and evaporation during rainfall.”

5. Lines 227-229 can be deleted. The readers are aware of this.

Done.  Deleted “Positive δ values indicate more ²H and ¹⁸O in the sample than in the standard, whereas negative δ values indicate less ²H in the sample than in the standard. In other words, the more positive a δ value is, the more enriched the sample is in the heavy isotope.”

6. The captions for Table 2 and 3 can be slightly altered. Water chemistry essentially means chemical parameters (alkalinity, cations, anions etc.). Just say stable isotope and physical parameters.

Done.  “Chemistry” deleted and replaced with “field parameters” in Table 2 and 3 captions.

7. Line 309. Where is the acidic rainwater coming from?

Done.  The sentence has been rewritten as follows: “Post-rainfall decreases in groundwater pH, linked to acidic rainwater (pH 3.0-6.56) caused by natural CO₂ and anthropogenic pollutants (SOx, NOx, etc.),) demonstrate …”.

8. Line 323. What is a Local Evaporation Line? Was it derived from Ground Water data?

Done.  The first part of that paragraph has been rewritten as follows: “Surface water isotopic signatures were more tightly clustered and aligned along a local evaporation line (LEL). The true LEL for a region will be plotted with points from water affected only by evaporation. In this case, the surface water line (SWL in graph) data is more clustered close to the local meteoric water line, reflecting some evaporation but also consistent groundwater contributions [24, 25] (see Figure 7).”.

9            Fig 8. Are the radon concentrations measured or modeled ones? It would be a good idea to see the correlation between measured radon concentrations and measured/calculated ground water flow rates.

Partly done.  The word “Measured…” has been added to the start of the caption. These graphs are exactly that – they show the measured radon concentrations in the river and the calculated groundwater inflow rates.

10          It would be worthwhile doing a correlation between radon concentrations and either oxygen or hydrogen isotope ratios (Fig 3), preferably hydrogen. From the figure it looks like there might be a decent correlation. Since the authors claim that stable isotope ratios are directly connected with the source of recharge waters, this exercise will strengthen their argument in using radon as a valuable tracer in this area.

Done.  There is in fact no correlation between δ²H or δ¹⁸O and Rn.  The Pearson's r correlation coefficient values for these are as follows:

surface water: δ²H x Rn, r = 0.15; δ¹⁸O x Rn, r = -0.09

groundwater: δ²H x Rn, r = -0.16; δ¹⁸O x Rn: r = -0.17.

However, in order to make this clear to the reader, the following sentences were added to line 326: "There is no correlation between Rn and δ²H (Pearson's r = -0.16) or Rn and δ¹⁸O (r = -0.17)." and line 366: "As for groundwater, there is no correlation in the surface water between Rn and δ²H (r = 0.15) or Rn and δ¹⁸O (r = -0.09)."

11          A curious question for the authors. The  FINIFLUX model, as suggested by the authors, is applicable using any tracer. Indeed they have used radon. Is it worthwhile trying this on the stable isotope values? If so, it will strengthen the paper should there be a good agreement between the findings based on radon and stable isotopes.

Partly done.  This is an interesting point, however there is less difference between groundwater and surface water values of δ²H or δ¹⁸O than the Rn, so these tracers will be less sensitive.  In addition, as Rn degasses from the surface water, new input of groundwater is noticeable using Rn measurements, whereas the limited input of groundwater of very slightly different δ²H or δ¹⁸O value, will mean these are relatively poor tracers of baseflow.

As such, a sentence was added at the end of the Methods section, line 270: "The Rn data was used for calculation of baseflow, whereas the δ²H and δ¹⁸O data was not. This is because these stable isotopes varied less than the Rn between groundwater and surface water, partly because the original values are less different, but mainly be-cause Rn degasses from surface water and therefore groundwater inputs are more noticeable than for δ²H or δ¹⁸O."

Author Response File: Author Response.pdf

Reviewer 2 Report (New Reviewer)

Comments and Suggestions for Authors

Review of the manuscript “Quantifying baseflow with radon, H and O isotopes and field parameters in the urbanized catchment of the Little Jukskei River, Johannesburg”.

 

This research is interesting, and well organized and if the authors follow the minor comments, it will be better.

 

 This manuscript needs some improvement as Minor Revision.

 

All the comments are in the attached paper (PDF) format.

 

This manuscript is important and could be accepted for publication after Minor Revisions.

 

 

Comments for author File: Comments.pdf

Author Response

MDPI Hydrology

3737783 – 18 July 2025

Quantifying baseflow with radon, H and O isotopes and field parameters in the urbanized catchment of the Little Jukskei River, Johannesburg

Khutjo Diphofe, Roger Diamond and Francois Kotze

 

 

Comment – Response Letter

Reviewer 2 Comments

This research is interesting, and well organized and if the authors follow the minor comments, it will be better.

 This manuscript needs some improvement as Minor Revision.

All the comments are in the attached paper (PDF) format.

 

Exported comments from .pdf document.

1. Should be write as (δ2H and δ18O).

Done.  Changed to “…δ²H, δ¹⁸O and…”.

 

2. Please write first the full expression as Electrical conductivity (EC).

Done. Now written as “... of electrical conductivity (EC) were …”.

 

3. Please write more explanation about the hydrological meaning for this figure.

Done.  The following text was added to the caption: “Being a noble gas, radon is ejected from the aquifer matrix and dissolves into the groundwater, but upon exposure to air, the radon is soon lost, so it acts as a useful tracer of recent groundwater discharge into surface waters.”

 

4. Please try to write the coordinates Long and Lat.

Done.  Latitude and longitude have been added.

5. Also please try to write the geographic coordinates.

Done.  Latitude and longitude have been added.

 

6. This figure needs more explanations to show the hydrological meaning and impact on the water.

Done.  The following text was added to the caption: “The radon data show the clear difference between higher values for groundwater than the river, enabling the use of radon to estimate baseflow. For δ²H and δ¹⁸O, the differences are less, but suggest evaporative enrichment in the river.”

 

7. Please try to revise the reference in the manuscript and in the List of reference, because for example I didn’t find the reference No. 54 in the text, but you added it in the list.

Needs to be done.  There seem to be MSWord/Latex/format issues.  Will the MDPI team please add 54 to the same point as 55 and 56 - i.e. at line 455. My version of MSWord is threatening to totally reformat the entire reference style, so I do not want to insert this extra reference. Thank you.

 

Author Response File: Author Response.pdf

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

While the study presents valuable data on urban groundwater interactions, the follow major revisions are required to address.

1. Strengthen the novelty: While the introduction contextualizes radon applications, the manuscript should explicitly articulate the unique advantages of radon tracing in this specific geographic/geologic setting and selection rationale for the radon isotope used in the study.
2. FINFLUX model validation: Provide quantitative validation of FINIFLUX outputs against field-measured radon concentrations and Discuss discrepancies between modeled and observed data in relation to urban hydrological processes.

Minor Revisions:

1. Line 42-43: “has seen” should be replaced by “has been seen” or “has been widely adopted”
2. Equation 1: There is unit inconsistency between the last two items in the right of the equation. Verify dimensional homogeneity.
3. Line 87: Complete the
4. Line 99: “Figure 3” should be replaced by “(Figure 3)”
5. Section 3.4: Justify FINFLUX selection by comparing against alternatives or citing its validation in similar hydrogeologic studies.
6. Table 2: define the meaning of d-excess upon first mention in the text, noting its utility in identifying evaporation effects.
7. Chart/Graph optimization: Figure 6: “EC = 17 ×km + 57” should be replaced by “EC=17*distance+57”； Figure 7: specify the point BH006.
8. Figure 8: Add validation metrics between modeled and sampled Rn.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

If possible, please improve the result graphics to better illustrate the river and groundwater districts—particularly in Figures 6 and 7. The sample numbers should be made clearer for the reader, as some correlations are currently difficult to follow.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The title of the manuscript and the results obtained are exciting, but the manuscript lacks a significant quantification component. Please try to resolve the issue along with the overall suggestions and comments provided as follows;

1. Abstract needs to be restructured. It is suggested that the background be rearranged first, followed by the methodology, key results, and implications.
2. Background information is found to be dense. It is suggested that the background information be shortened and results added along with a clear study objective.
3. Line no. 8 needs to be rearranged. Start from “The averaged EC value for groundwater and surface….”
4. Line no 13. What does the “groundwater dH and d18O values are spread widely” mean? Specify the value that comes directly into the conclusion without adding the information of the stable isotope values of H and O from rainfall.
5. The values might reflect the recharge altitude since the groundwater might have been recharged from various depths. Provide more evidence to shed light before pointing to only one angle of evidence.
6. What is the depth range from which the groundwater samples were collected, and what is the type of groundwater source? If it’s a borehole, try adding the information on the number of screens used.
7. Replace the word infield in Line no. 149.
8. Enlarge the indicators of the sampling points in Figure 3, as it is difficult to locate at first glance.
9. Also, since the discussions made by the authors are directed towards faults, lineaments, and shear zones, mark that secondary structure in the map for better visualization and clearer discussion.
10. Line 238-239. The direct jump to the conclusion being, groundwater tapping municipal tap water leak seems irradical. The reference being used is from 2014, while the samplings were done from 2020/2021. The reason might be true, however, I am looking at the distance between BH005 and BH006, it seems the reason might not be the same. To provide strong reasons for your assumption, better locate the municipal tap water supply pipelines, the groundwater flow direction, and the land use.
11. In contrast to the detailed description of the isotopic distribution provided to the stream, there is a lack of a detailed description of springs and groundwater. Please add a description to all the components. Without the details, the study seems biased.
12. Please indicate how this Pearson correlation coefficient was calculated for the line. Is it that common and useful to use Pearson for the line in proving your hypothesis?
13. Line 266 “interpretation of stable isotope is further complicated”. Using such sentences won’t justify the research you are doing. I’d better suggest additional discussion to simplify the complication you are facing. Since the author added d-excess in the table, please use d-excess, as it displays a lot more information in the study area.
14. The manuscript depicts a report version rather than a scientific manuscript. The discussion lacks evidence to prove the hypothesis in most parts. Thus, the discussion needs rewriting.
15. Although the manuscript titles quantify, quantification of the base flow in the results, discussion, and conclusion is missing. Better change the title of the manuscript.

Comments on the Quality of English Language

Needs a thorough grammar check and correction.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

I have reviewed with interest the manuscript titled "Quantifying baseflow with radon, stable isotopes and hydrochemistry in the urbanized catchment of the Little Jukskei River, Johannesburg". The Authors based on measurement of EC, stable isotopes of oxygen, hydrogen and measurement of radon in surface and groundwater and by using the FINIFLUX model confirmed seasonal groundwater level fluctuations and showed that baseflow is highest at the end of the rainy season (March) and lowest at the end of the dry season (September).

The material is presented appropriately and clearly, the data contained in figures and tables represent understandable documentation of the research. The references are appropriate. In the "Results and discussion" section, the Authors described the obtained research results and interpreted them accordingly. The conclusions are consistent with the presented results and arguments. The results presented in the manuscript are valuable and will interest not only a local reader but all of the readers interested in hydrogeology.

I recommend the publication after considering the suggestions below and after possible corrections:

 

1. It is worth considering the title of the article. It is worth adding in the title what stable isotopes were tested (because O and H are just two of many others). In addition, the title includes "hydrochemistry", while only EC was measured.
2. Fig 1. It is attached in the wrong place (too early compared to the information in the text). It is worth considering removing the Figure and Table from the "Introduction" chapter and placing them somewhere else.
3. Line "49 - The Authors wrote: "Radon's ubiquity makes it a useful environmental tracer." This sentence refers to the geological conditions at the site studied. Radon is not a common isotope in all geological conditions, as is the case with H and O. It is worth stating this more clearly.
4. At the end of the „Introduction” chapter it is worth clearly defining the purpose of the research.
5. Line 86-87 – The Authors wrote “The study site is located in the Jukskei River catchment (JRC), within the area of ​​greater Johannesburg in the centre of Gauteng Province (Figure 2.)” – however, Gauteng Province is not located in Figure 2. It is worth checking the article and adding all the names in the figures that are in the text.
6. Fig 4 i Fig 5 – please add where the data comes from, currently it is "Data from ?"
7. Line 149 – The Authors wrote „In field pH, EC (electrical conductivity) and temperature were measured…”, while in Tables 2 and 3 there is no information about pH and temperature. The description and interpretation apply only to EC.
8. It is worth illustrating the hydrogeological conditions with a cross-section and adding from what depth the samples were taken (boreholes) and at what depth the groundwater is located in the studied area.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf