Towards More Accurate Risk Assessment of Sediment Trace Metals: Integrating Sedimentary Background Determination and Probabilistic Evaluation in Chaohu Lake, China

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

After reading the manuscript, I found it well conducted and written. This work, however, needs clarifications and revisions that will significantly enhance its content. Therefore, I recommend the following six major amendments:

1. Elucidate the methodology used to ascertain metal fractionation factors (f) and indicate whether a sensitivity analysis was conducted.
2. In section 2.2, what was the range of the standard solutions used for calibration?
3. Could you please provide additional mathematical details of the CIC model? Could you specifically provide the formula for calculating the average sedimentation rate?
4. Offer more justification for selecting 1960 as the reference year for sediment background values. 
5. Please evaluate the manuscript for grammatical accuracy and enhance sentence clarity in sections 2.3, 3.2, 4.1, and 4.2.
6. Define all acronyms upon their first use (e.g., CIC, TEL, PEL, ∑TU). They need to be defined and understood by the journal's audience. I think it's wrong that the TEL and PEL definitions are below the table and in the same font size (main text).
7. Please indicate the software used for plotting maps and other graphics.
8. Propose the incorporation of a schematic or graphical abstract that encapsulates the study's procedure and results.
9. In line 341, explain the statistical methods used to obtain Figure 7.
10. Some plots (e.g. 4,7) are blurred, and they need to improve the resolution.
11. The discussion is limited to five references; please add more current literature that supports your findings. Improve the discussion by adding more substantial text that explains your results.
12. In Figure 3, define H in the caption. What does the horizontal dashed line indicate?
13. By looking at Figure 6, I do not understand the green, black, and red circle lines. What is the meaning of the arrows? Explain that in the caption. What are PC1 and PC2?
14. The conclusions would be more beneficial and clear by giving more quantitative results.

Author Response

Reviewer1# After reading the manuscript, I found it well conducted and written. This work, however, needs clarifications and revisions that will significantly enhance its content. Therefore, I recommend the following six major amendments:

1、Elucidate the methodology used to ascertain metal fractionation factors (f) and indicate whether a sensitivity analysis was conducted.

Rely： Thank you very much for your constructive comment. In response, we have revised the manuscript to elucidate the methodology used to ascertain metal fractionation factors (f). Specifically, we employed a sequential extraction procedure based on the modified Community Bureau of Reference (BCR) method, which categorizes metals into four operationally defined fractions (exchangeable, reducible, oxidizable, and residual). The fractionation factor (f) was calculated as the proportion of each metal in a specific fraction relative to its total concentration. In addition, we conducted a sensitivity analysis by adjusting extraction parameters (reagent concentration and extraction time) within controlled ranges to evaluate the robustness of the fractionation outcomes. The detailed information has been added to the revised manuscript (Line110-118).

 

 2、In section 2.2, what was the range of the standard solutions used for calibration?

Rely：Thank you for your insightful comment. In response, we have supplemented the manuscript with information regarding the concentration range of the standard solutions used for instrument calibration. Specifically, multi-element standard solutions ranging from 0.1 μg/L to 100 μg/L were utilized for calibration prior to each batch of analysis. This information has been added to Section 2.2 (Lines 144–145).

3、Could you please provide additional mathematical details of the CIC model? Could you specifically provide the formula for calculating the average sedimentation rate?

Rely: Thank you for your valuable suggestion. In response to your comment, we have added additional mathematical details of the Constant Initial Concentration (CIC) model, including the formula used to calculate the average sedimentation rate based on the ²¹⁰Pbex activity distribution. Specifically, the relationship between activity and depth is now clearly described, and the method for deriving the sedimentation rate from the slope of the linear regression is provided (Lines 170–175).

 

4、Offer more justification for selecting 1960 as the reference year for sediment background values.

Rely: Thank you for your insightful comment. We have supplemented a more detailed scientific explanation regarding the selection of 1960 as the reference year for sediment background values, based on historical hydrological and environmental data of Chaohu Lake (Lines 264–272).

 

5、Please evaluate the manuscript for grammatical accuracy and enhance sentence clarity in sections 2.3, 3.2, 4.1, and 4.2.

Rely: Thank you for your valuable suggestion. We have carefully re-evaluated the manuscript for grammatical accuracy and have thoroughly revised sections 2.3, 3.2, 4.1, and 4.2 to enhance sentence clarity and improve the overall language quality. All suggested sections have now been rephrased for better readability and professional presentation.

 

6、Define all acronyms upon their first use (e.g., CIC, TEL, PEL, ∑TU). They need to be defined and understood by the journal's audience. I think it's wrong that the TEL and PEL definitions are below the table and in the same font size (main text).

Rely: Thank you for your careful review. In our revised manuscript, the acronyms CIC (Line 168) and ∑TU (Line 243-244) have already been defined upon their first use. For TEL and PEL, we have adjusted the font size and formatting in Table 2 as suggested, and we have now provided the definitions at the first appearance of PEL (Line 239-243) in the main text. We hope this clarification addresses your concerns.

 

7、Please indicate the software used for plotting maps and other graphics.

Rely: Thank you for your valuable suggestion. We will pay attention to this point and clearly indicate the software used for plotting maps and graphics in the revised manuscript.

 

8、Propose the incorporation of a schematic or graphical abstract that encapsulates the study's procedure and results.

Rely: Thank you for the suggestion. We have created and incorporated a graphical abstract that effectively summarizes the study’s procedure and results, as requested. This visual representation provides a clear and concise overview of the research, enhancing the reader's understanding of the study’s key findings.

 

9、In line 341, explain the statistical methods used to obtain Figure 7.

Rely: Thank you for your comment. We have added an explanation of the statistical methods used to obtain Figure 7. Specifically, the probabilistic risk assessment results shown in Figure 7 were based on the calculated RI and ∑TU values for each sampling point (as presented in Figure 5). Using the risk classification thresholds for RI and ∑TU (defining low, moderate, and high-risk levels), we categorized each sampling point accordingly. Finally, we statistically calculated the proportion of sampling points falling into each risk category to obtain the cumulative probability distributions presented in Figure 7（line387-399）.

 

10、Some plots (e.g. 4,7) are blurred, and they need to improve the resolution.

Reply: Thank you for your comment. We have improved the resolution of the relevant figures (e.g., Figures 4 and 7) in the revised manuscript. If the current quality is still not sufficient, we are happy to provide the original high-resolution images as needed.

 

11、The discussion is limited to five references; please add more current literature that supports your findings. Improve the discussion by adding more substantial text that explains your results.

Reply: Thank you very much for your valuable comments. In the revised manuscript, we have added a new Section 4.4 (Broader Contextualization of Ecological Risk Assessment Results) to enrich the discussion and deepen the interpretation of our findings. Furthermore, we have incorporated six additional recent references (References 39–45) to provide stronger support and broader context for our results.

 

12、In Figure 3, define H in the caption. What does the horizontal dashed line indicate?

Reply: Thank you for your comment. We have defined "H" in the main text as the sediment depth ("The sediment depth (H) in the west is around 36.60 cm, the central is about 21.35 cm, and the east is around 47.58 cm."). We would like to clarify that in the current version, "H" (sediment depth) and the meaning of the horizontal dashed line have already been defined in the caption of Figure 3 and explained in the main text (Line274).

 

13、By looking at Figure 6, I do not understand the green, black, and red circle lines. What is the meaning of the arrows? Explain that in the caption. What are PC1 and PC2?

Reply: Thank you for your valuable suggestion. In the revised manuscript, we have explained the meaning of PC1 and PC2 in the main text, clarified that the arrows represent the loading directions and strengths of trace metals, and stated that the green, black, and red circles are used to highlight trace metals with similar distribution patterns and sources (Line 360-365).

 

14、The conclusions would be more beneficial and clear by giving more quantitative results.

Reply: Thank you for your suggestion. We have revised the conclusion section by incorporating more quantitative results, including the contamination levels (I_geo values) of key trace metals, the proportion of low and moderate ecological risks based on RI and ∑TU values, and the primary metals contributing to the overall ecological risk. We believe these revisions make the conclusions clearer, more informative, and better reflect the study's key findings (Line 483-500).

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

NIce and well structured paper reporting a dense field work, showing that probabilistic estimators are better than statistical ones.

Author Response

Thank you to the reviewers for your approval of this manuscript. We will continue to improve and enhance the quality of publication and form a printable paper in the best form.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors study trace element concentrations in 120-135 cm-thick sediment layers in three areas of Chaohu Lake (China) and assess the respective sedimentation rates, geo-accumulation and ecological risk indices, and total toxicity units based on the element concentrations.
Although the study is multi-faceted and informative enough, it much resembles the article by Fang et al. "Trace metals in sediment from Chaohu Lake in China: Bioavailability and probabilistic risk assessment" (Sci. Total Environ. 2022, 849, 157862) in terms of the object under research (Chaohu Lake sediments), methodology applied (including the same suite of elements under consideration) and flow of presentation, although the authors do not even cite this paper in the manuscript. Moreover, this is not the only one uncited recent publication on Chaohu Lake sediments, e.g. see Cai et al. 2023, https://doi.org/10.34133/ehs.0040. Thus, I am afraid there is not enough conceptual novelty in the manuscript. It would be imperative that the authors at least compare their own results with those from the mentioned studies.
Another conspicious deficiency in the manuscript is the extermely poor presentation of the methodology used in this work. This flaw disables the reader's proper comprehension and analysis of the results and reproduction of methods used to obtain them. More specific comments on materials and methods are given below.
The discussion is also very poor. The section the authors head as "Discussion" contains largely results rather than discussion, including element correlations, PCA and risk assessment. The authors do not attempt to place their results in a broader global or even regional context although studies employing similar methodology are readily available from literature.
Apart from the above, there are more specific additional comments to be addressed.
L.124-126: The trace element analysis description is not satisfactory. More details are needed, including a detailed information on sample  preparation, analytical procedure, instrument regime/settings etc.
What granulometric sediment fraction was used for the analysis?
L.134: The subtitle needs to be corrected.
L.143: How is ²¹⁰Pb_sup subtracted from the total ²¹⁰Pb?
How was the ²¹⁰Pb activity measured?
L.152: What does the "cm/a" unit mean?
L.187: What is f? where is it extracted from?
L.203: What is PEL? Abbreviations should be explained at first mention.
How would the authors explain the baseline concentrations of some elements (Hg, As) being higher in the eastern part of the lake  (Table 1)?
L.284: What is "moderately moderate grade"?
L.287-290: "It can be seen that the ecological risk of heavy metal pollution, as indicated by the index Hg > Zn > As > Cd > Pb = Cr > Cu =  Ni, is greatest in the western region, followed by the central region and then the eastern region." Sorry, where is this evident from? In  Fig. 4, there are only percentages and no integral assessment of the geo-accumulation index.
Fig. 5 and description (L.298-306): Where do the delimiting values (150 and 300 for risk index and 2, 4 and 6 for toxicity units) appear from?  This should have been indicated in the M&M section.
Fig. 6: What correlation coefficient has been used? Which correlations are significant (p<0.05)? The authors seem to have used pooled data  from the three areas; this should be stated explicitly in the text.
L.343: The authors provide the reader with no clue how to calculate the index probabilities, which should have been indicated in the M&M  section.
In Fig.8 caption, there should be an indication of the sediment layer involved in the mapping. Additionally, in Materials and Metods, a  software used for mapping and the gridding algorithm should be mentioned.
The paragraph in L.376-394 is unclear. (1) The authors did not mention "chemical extraction" in the methodology applied. (2) Where does "the 24.8% probability of toxic effects on aquatic organisms due to trace metals" emerge from?
"enhancements enhance" is a tautology.
L.401: It is not clear what the author mean in the statement: "while some risk from trace metals exists, much of it stems from sediment release."

Author Response

Reviewer3# The authors study trace element concentrations in 120-135 cm-thick sediment layers in three areas of Chaohu Lake (China) and assess the respective sedimentation rates, geo-accumulation and ecological risk indices, and total toxicity units based on the element concentrations.

Although the study is multi-faceted and informative enough, it much resembles the article by Fang et al. "Trace metals in sediment from Chaohu Lake in China: Bioavailability and probabilistic risk assessment" (Sci. Total Environ. 2022, 849, 157862) in terms of the object under research (Chaohu Lake sediments), methodology applied (including the same suite of elements under consideration) and flow of presentation, although the authors do not even cite this paper in the manuscript. Moreover, this is not the only one uncited recent publication on Chaohu Lake sediments, e.g. see Cai et al. 2023, https://doi.org/10.34133/ehs.0040. Thus, I am afraid there is not enough conceptual novelty in the manuscript. It would be imperative that the authors at least compare their own results with those from the mentioned studies.

Reply: Thank you very much for your valuable comments and suggestions. We fully acknowledge your concern regarding the similarity of our study to previous works, such as Fang et al. (2022) and Cai et al. (2023), which focused on Chaohu Lake sediments. Following your suggestion, we have carefully revised the manuscript. Specifically, we have:

• Added citations to Fang et al. (2022) and Cai et al. (2023) in the revised manuscript, and compared our results with theirs, particularly in Table 2 and related discussions. This helps situate our findings within the context of recent research.
• We would also like to emphasize that our study has distinct methodological innovations compared to previous research. ①In particular, the background values for trace metals in our study were derived from vertical profile analysis of sediment cores through self-deposition historical sediment layers. ② This approach differs significantly from previous studies, where background values were generally referenced from large regional or generalized datasets, which may not accurately reflect the historical geochemical baseline of Chaohu Lake itself. ③We believe that deriving background values from site-specific sediment cores enhances the precision and site-specific relevance of the ecological risk assessment, and this constitutes a key originality and strength of our study. Once again, we sincerely appreciate your constructive feedback, which helped us further improve the clarity and scientific depth of our manuscript.

Another conspicious deficiency in the manuscript is the extermely poor presentation of the methodology used in this work. This flaw disables the reader's proper comprehension and analysis of the results and reproduction of methods used to obtain them. More specific comments on materials and methods are given below.

Reply: Thank you very much for your detailed and constructive comments regarding the presentation of the methodology section. We fully acknowledge that a clear and comprehensive description of methods is critical for proper understanding, analysis, and reproducibility. In response, we will carefully revise the Materials and Methods section according to your specific suggestions, ensuring that all procedures, parameters, and calculations are described with greater clarity and detail.

The discussion is also very poor. The section the authors head as "Discussion" contains largely results rather than discussion, including element correlations, PCA and risk assessment. The authors do not attempt to place their results in a broader global or even regional context although studies employing similar methodology are readily available from literature.

Reply: Thank you very much for your valuable comments. In the revised manuscript, we have added Section 4.4 (Broader Contextualization of Ecological Risk Assessment Results) to place our findings in a wider regional and global context. We have also streamlined the Discussion section to avoid repetition and improve scientific depth.

 

Apart from the above, there are more specific additional comments to be addressed.

1、L124-126: The trace element analysis description is not satisfactory. More details are needed, including a detailed information on sample preparation, analytical procedure, instrument regime/settings etc. What granulometric sediment fraction was used for the analysis?

Reply: Thank you very much for your constructive suggestion. In the revised manuscript, we have substantially expanded the description of trace element analysis (Lines 131–142), including detailed information on sample preparation, analytical procedures, instrument models and settings, as well as the granulometric fraction used (<63 μm). This revision aims to enhance the transparency, reproducibility, and scientific rigor of the methodology.

 

2、L134: The subtitle needs to be corrected.

Reply: Thank you very much for your valuable suggestion. In the revised manuscript, we have corrected the subtitle to "Determination of sediment accumulation rates based on ²¹⁰Pb_ex dating" to more accurately reflect the methodology and purpose of this section.

 

3、L143: How is ²¹⁰Pb_sup subtracted from the total ²¹⁰Pb? How was the ²¹⁰Pb activity measured?

Reply: Thank you for your valuable comment. In the revised manuscript, we have added details on how the total ²¹⁰Pb activity was measured and how ²¹⁰Pb_sup was subtracted. Specifically, total ²¹⁰Pb activity was determined by alpha spectrometry following radiochemical separation procedures. ²¹⁰Pb_sup was estimated based on the activity of ²²⁶Ra measured via its daughter isotopes under secular equilibrium conditions using gamma spectrometry, and ²¹⁰Pb_ex was obtained by subtracting ²¹⁰Pb_sup from the total ²¹⁰Pb.

 

4、L152: What does the "cm/a" unit mean?

Reply: Thank you for your comment. We have kept the original unit format (cm/a) in the text and added an explanation that "cm/a" represents the thickness of sediment accumulated per year.

 

5、L187: What is f? where is it extracted from?

Reply: Thank you for your comment. We have clarified that "f" represents the proportion of bioavailable or potentially mobile fractions relative to the total content, and it was derived based on the method described in reference [17].

 

6、L203: What is PEL? Abbreviations should be explained at first mention. How would the authors explain the baseline concentrations of some elements (Hg, As) being higher in the eastern part of the lake (Table 1)?

Reply: Thank you for your comments. We have added explanations for the abbreviations PEL and TEL at their first mention. Upon careful examination of Table 1, we found that the average concentrations of Hg and As in the eastern part of the lake (Hg: 0.056 mg/kg, As: 4.43 mg/kg) are slightly higher than those in the central (Hg: 0.049 mg/kg, As: 3.55 mg/kg) and western regions (Hg: 0.041 mg/kg, As: 3.79 mg/kg), but remain within the same order of magnitude. These variations are considered normal spatial fluctuations due to differences in sedimentation dynamics, local hydrological conditions, and possible minor anthropogenic influences. No significant abnormal enrichment was observed.

 

7、L284: What is "moderately moderate grade"?

Reply: Thank you for your helpful comment. We have corrected the description of the Igeo pollution grades. In the revised manuscript, the terms "moderate pollution" (1 < Igeo ≤ 2) and "moderate to heavy pollution" (2 < Igeo ≤ 3) are used to accurately describe the pollution levels according to standard classification.

 

8、L287-290: "It can be seen that the ecological risk of heavy metal pollution, as indicated by the index Hg > Zn > As > Cd > Pb = Cr > Cu = Ni, is greatest in the western region, followed by the central region and then the eastern region." Sorry, where is this evident from? In Fig. 4, there are only percentages and no integral assessment of the geo-accumulation index.

Reply: Thank you for your careful review. We have revised the description to clarify that the ecological risk trend is inferred based on the distribution proportions of pollution grades for each element shown in Fig. 4, rather than directly observed from an integrated I_geo assessment. The wording has been adjusted accordingly to ensure a more accurate interpretation.

 

9、Fig. 5 and description (L.298-306): Where do the delimiting values (150 and 300 for risk index and 2, 4 and 6 for toxicity units) appear from? This should have been indicated in the M&M section.

Reply: Thank you for the valuable comment. We have added a detailed explanation of the delimiting values for the potential ecological risk index (RI) in the Materials and Methods section. Specifically, we clarified that the thresholds (RI < 150, 150 ≤ RI < 300, RI ≥ 300) are based on the classification criteria proposed by Hakanson (Reference [27]). Regarding the toxicity units, the delimiting values (2, 4, and 6) were already described at the end of Section 2.3. Revisions have been made accordingly (L.216–219).

 

10、Fig. 6: What correlation coefficient has been used? Which correlations are significant (p<0.05)? The authors seem to have used pooled data from the three areas; this should be stated explicitly in the text.

Reply: Thank you for your valuable comment. In Fig. 6, Pearson correlation coefficients were used to evaluate the relationships among the Igeo values of the eight trace metals. The correlations were calculated using pooled data from the three regions, as specified in the revised text. Correlations with p-values < 0.05 were considered statistically significant and are represented in the correlation matrix. We have updated the manuscript accordingly (L.349–357).

 

11、L343: The authors provide the reader with no clue how to calculate the index probabilities, which should have been indicated in the M&M section.

Reply: Thank you for your insightful comment. In the Materials and Methods section, we have now included a detailed explanation of how the probabilistic risk (RI and ∑TU) for trace metals was calculated. Specifically, we clarified that the calculation was based on statistical methods using total and bioavailable concentrations [35]. The points were then classified into risk categories (low, moderate, high) according to predefined thresholds, and the cumulative probability distributions were generated for each region. We have made these updates in the manuscript to provide a clearer understanding of the method (L.384–391).

 

12、In Fig.8 caption, there should be an indication of the sediment layer involved in the mapping. Additionally, in Materials and Metods, a software used for mapping and the gridding algorithm should be mentioned.

Reply: Thank you for your helpful comment. We have updated the Figure 8 caption to clarify that the mapping was based on surface sediment samples. Furthermore, in the Materials and Methods section, we have added details about the software and methodology used for spatial interpolation. Specifically, the RI values were interpolated using the Inverse Distance Weighting (IDW) method in Origin 2023, in conjunction with the boundary vector map of Chaohu Lake. These changes have been made to ensure clarity (L.222–224).

 

13、The paragraph in L.376-394 is unclear. (1) The authors did not mention "chemical extraction" in the methodology applied. (2) Where does "the 24.8% probability of toxic effects on aquatic organisms due to trace metals" emerge from?" enhancements enhance" is a tautology.

Reply: Thank you for your helpful comments. We have made the following revisions to address your concerns: (1) As the metal fractionation method and its associated toxicity factors (f) were directly referenced from previous literature, we have decided to remove the mention of "chemical extraction" from the text to avoid confusion. This method and the specific factors used are clearly outlined in the referenced studies, and we have provided proper citations to ensure the methodology is transparent. (2) The 24.8% probability of toxic effects refers to findings in earlier studies on the toxic impact of trace metals in aquatic environments, which we have clarified in the revised text. We have also removed the tautological phrase "enhancements enhance" for improved clarity and precision. (L.423–440).

 

14、L401: It is not clear what the author mean in the statement: "while some risk from trace metals exists, much of it stems from sediment release."

Reply: Thank you for your valuable feedback. We have carefully considered your comment and have rewritten the conclusion section for greater clarity. As part of this revision, we have removed the sentence in question ("while some risk from trace metals exists, much of it stems from sediment release"), as it may have caused confusion.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

After revising the response letter, I believe the manuscript is worthy of publication in Water.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have answered the questions touched upon in my previous review.