Spatial and Seasonal Patterns of Mercury Accumulation in Paddy Soil around Nam Son Landfill, Hanoi, Vietnam

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

A brief summary 

The manuscript “Spatial and seasonal patterns of mercury accumulation in 2 paddy soil around Nam Son landfill, Hanoi, Vietnam” accesses the concentration of Hg in soils surrounding a landfill. I also agree that landfills can be a significant source of mercury (Hg) pollution, as the authors propose. However, I have some doubts and suggestions that could improve the quality of this manuscript.

General concept comments

a)       - According to the author, this work seeks to identify the factors that affect the distribution of mercury in the soil, including geochemical properties, distance from the landfill, climatic conditions and wind direction. For me, the weather conditions and influence of wind direction were not highlighted throughout the study. The methodology used does not allow us to conclude about the atmospheric transport of Hg from the landfill to the ground. The landfill is possibly an important source of Hg to the atmosphere, but it should not be the only source of Hg in the studied region. The authors should provide a more detailed explanation why the other sources of Hg pollution are not confounding factors in this study.

b)       I think that the present study, in its current form, is very descriptive and has little novelty, despite presenting valuable information on the concentration of Hg in soils and indicating landfills as an important source of total Hg pollution.

c)       It was not clear from the text why the Hg concentration at the REF sampling point could be considered as a background value.

Specific comments 

Introduction ...  Line 63 – 65 … Use the unit μg/g or ng/g, but not at the same time.

2. Materials and methods

L. 91 – 93 … the sum does not correspond to 100%.

L. 115 and l.132. Do not use “milli Q water” instead use “ultrapure water (Milli-Q® Millipore, USA)”

1 - Figure 1 should present Vietnam and neighboring countries. Additionally, authors must include coastline. Use a more detailed map (Suggestion: Use a representation scale of 1:200,000 instead of 1:1,000,000).

Statement in line (l 116-118) “Two different layers of paddy soil, 0 – 5 cm (surface layer), and 20 - 25 cm (bottom layer) were sampled. Each soil sample was labeled, stored in a polyethylene zip locked bags and kept at 4 oC for transportation to the laboratory.”.

In general, soil sampling is carried out according to the soil horizons. Then, include an explanation justifying why use these depths for soil sampling.

2.3.1. Hg analysis

Lines (124 – 128) Please include a more detailed description of the technique used to analyze total mercury (THg). I believe that the technique used by the authors was "Direct Thermal Decomposition".

Line 173 - 175 - The significance levels were determined based on p-values, where values less than 0.05 were considered “statistically significant”, values more than 0.05 were considered “no significant”.

SUGGESTION: Significance levels were determined based on p values, where values lower than 0.05 were considered statistically significant.

Results and discussion

Line  180 – 230 …  The authors do not inform the data presented in this section refer to soils collected in the 0-5 cm or 5-25 cm layer. I noticed the same problem in figures 2 and 3, the authors should make this clearer in the text and in the figure titles. The title of figure 2 e 4 is very short. Captions should be concise but comprehensive.

Line 274-279. The authors mentioned the Pearson correlation test. However, in this case, linear regression analysis seems to me to be the best statistical method. What is the reason for not performing linear regression?

Conclusions

Lines 298 – 304 … At the beginning of the conclusion, the author discussed the results rather than stating a conclusion of his study. Suggestion: Start completion from line 301.

Author Response

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections highlighted/in track changes in the re-submitted files.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This article is on an interesting topic and may be of broad interest to Soil System readers. However, I have major concerns that should be clarified before recommending the article for publication.

 

General

Figures must be identified in the text.

 

Introduction

Lines 77-79: Can the authors provide weather information and wind direction from both collection periods? Perhaps this data can be obtained from a local weather station or from global databases. Despite being an objective of the study, no hard data on these variables was presented, only geochemical properties and distance to the landfill.

 

2. Materials and methods

2.2. Sample collection

Line 111: The two study areas (Nam Son and Bac Son) are located in two opposite directions to the incineration area. Depending on the wind direction, the study areas can be impacted by emissions in different ways. The authors should provide the prevailing wind direction in the region in the description of the study area (Section 2.1.). In addition, it appears from the image that the sample units have different land uses. Apart from the landfill, are there other sources of Hg that may be different between Nam Son and Bac Son?

 

2.3. Sample analysis

Line 124: Inform the method used by the equipment to quantify THg.

Line 126: The authors state in this topic that the samples were weighed in triplicate, but in topic 2.4 (Line 163) they state that they were analyzed in duplicate. The authors should clarify how many replicates were used in each analysis, including determination of pH and organic matter.

Line 143 - Title of section 2.3.4.: I suggest that this be a separate topic after QA/QC. The calculation of Igeo is a procedure subsequent to laboratory analysis.Linha 150: Informar o valor de BHg utilizado para o cálculo do Igeo.

 

2.5. Statistical analysis

Seasonality is a major factor for this article. Why were THg concentrations between seasonal periods not statistically compared? I suggest that the authors carry out this comparison and broaden the discussion to explore the effects of seasonality, regardless of significant differences. These results indicated the seasonal patterns proposed by the title and objectives of the article. Also, why wasn't the difference between the sampling units tested?

Lines 170-171: The authors should clarify the meaning of "non-correlation" test. In addition, I suggest that the authors replace R2 with r to denote Pearson's correlation coefficient. R2 is usually used to represent the coefficient of determination.

Linhas 171-173: The correlation of Hg with pH and SOM was also evaluated by Pearson's correlation? The authors should highlight which variables are used in each analysis, as well as the sample size considered. For example, were samples from both soil layers considered in the analyses? This is important information that should be added to the text.

 

3. Results and discussion

The authors should expand the literature review to include international comparisons and explain the mechanisms that govern the distribution of Hg.

 

3.1. Hg concentration in soil

Lines 179-195: The Hg concentrations between the seasonal stations and study areas were only evaluated descriptively. No inferential tests were carried out, as well as no comparisons with other study areas, substantially limiting the discussion.

Lines 198-204: The figure should be positioned right after the text, not before. Report exact p-values in the figure and in the text (along with the correlation coefficient).

Lines 205-206: The study of contamination by PTEs in landfills, including Hg, is a very broad field. I believe it would be better to focus the discussion on studies carried out in study areas with similar characteristics, regardless of geographical location. For example, chemical industries and landfills should have different Hg emission patterns. If there really are no landfill studies, comparison with other emission sources is appropriate.

 

3.2. Vertical distribution of Hg concentration

The difference in Hg concentrations, organic matter content and pH between soil layers should be tested statistically. These results will lead to more assertive conclusions about the vertical distribution of Hg in the soil and the influence of physico-chemical parameters.

Lines 245-256: Can the authors explain the dry and wet deposition processes?

 

3.3. Effect of pH and SOM on Hg concentration in soils

Were the correlations carried out considering samples from both soil layers? This information is not clear. The sample size used in each analysis should be made clear in the text.

The authors only indicated the pH and MO values. The discussion should be expanded to explain how the availability and mobility of Hg in soils is influenced by pH and, above all, by SOM.

Lines 274-278: Authors should identify the figures corresponding to the results in the text. In addition, the figures should be positioned after the text to which they refer.

Line 276: Why? Explain binding mechanisms between SOM and Hg.

Line 278: Inform where the study was carried out. Does the study area have similar physical characteristics and sources of Hg?

Line 283: Why was Igeo only calculated for the 2022 collection? Calculating Igeo for both collections would provide information on seasonal pollution patterns.

 

3.4. Geo-accumulation (Igeo) in soil

Lines 286-287: Initially, I had understood that the Ref sample was used to calculate Igeo. However, apparently Ref was also included in the pollution assessment. What value was adopted as the background for calculating Igeo? This value should be highlighted in the text.

Figure 5: Separate into two frames to better visualize the distances at each collection site. This way it looks a bit confusing. a) Ref + NS. b) Ref + BS.

 

Conclusions

Only the conclusions about spatial distribution and the influence of organic matter were adequately supported by the data. The distribution patterns between study areas and seasonal periods were not well evaluated. The same applies to the vertical distribution of Hg in the soil and its relationship with physico-chemical parameters. These data were only evaluated descriptively. As previously suggested, the authors should use statistical analyses to make inferences about the distribution of Hg between study areas, seasonal periods and soil layers.

Author Response

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections highlighted/in track changes in the re-submitted files.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This paper conducts an evaluative analysis of the Nam Son landfill, emphasizing the severity of the mercury (Hg) issue. Due to the gravity of this problem, authors must exercise caution in elucidating the methods. There is a need to improve the methodology, accompanied by a thorough refinement of the results and discussion sections. Besides this, here are some of the points to address too.

Correct Line 24-25; with the higher value  is observed in the surface layer (0 - 5 cm) and the lower in bottom layer (20 - 25 cm) by Hg.

Correct line 29; assessing the risks of Hg-containing waste. 

Figure 1 is not highlighted in the text under 2.1 section for the sampling sites. I also suggest putting coordinates on the map, though you mentioned in the text.

Could authors explain how line 118- 120 was achieved because at room temperature, mercury can evaporate to become an invisible, odorless toxic vapor?

Line 136- When conducting LOI, first water content or soil moisture has to be removed then you can measure, why was this step excluded in your analysis.

Results and discussion

Line 179, Which samples were used for Hg concentration in soil for rainy and dry season, was there other samples other than 0-5 and 20- 25 cm, this section does say any of the aforementioned depths.

How do you justify direct seepage with volatility of mercury, 300 meters is outside the landfill, did you consider the wind directions in the two seasons?

Line 186 (RDD), Line 209, RRD was already introduced, such repetition might be confusing, Line 254 Red River delta, once abbreviations are introduced, then keep consistency.

Line 237, what is sample CT? not on the map.

Line 134 SOM was already introduced, check line 270.

Line 267 to 272, soil pH varied from 5.05 to 8.08 for which samples? because you collected at different depths same applies for your SOM calculation.

Line 296 Caption for Igeo…indicate that the figures below sampling sites represents distance from the landfill.

Author Response

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections highlighted/in track changes in the re-submitted files.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Considering that the authors made the suggested changes and even improved the manuscript in general. The article can be accepted in its current form.

Author Response

Thank you very much for taking the time to review this manuscript. 

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have carried out an extensive revision considering the reviewers' suggestions, however, for continuous improvement of the manuscript, I have minor comments that should be addressed before recommending the article for publication:

·        L77-82: Transfer the description of the climate and wind direction to section 2.1.

·        Section 3.3: The linear regressions were carried out considering samples from both soil layers. Therefore, the samples are not dependent? The authors should diagnose the residuals of the linear regression to ensure that there is no autocorrelation. See Moran and Durbin-Watson tests.

·        L270-271: The authors explained the difference between the dry and wet deposition processes of Hg in their response to the reviewers, but did not include it in the text. I suggest that the authors explain it briefly in the text of the manuscript.

·        Section 3.4: If the authors considered the concentration of the reference area as the background value for calculating the geoaccumulation index, it makes no sense for the reference area to be included in the pollution assessment. Considering that the Igeo formula consists of the ratio of the concentration obtained in the sampling site to the background concentration, the only variation in the Igeo is attributed to multiplying the background value by a factor of 1.5. Therefore, I suggest that Igeo should not be calculated for the reference area, only for the other sampling sites.

Author Response

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections highlighted/in track changes in the re-submitted files.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

1) Line 291.. better to write it as…The SOM and pH analysis were conducted due to their significance as key soil factors that might affect the availability and mobility of Hg in soils (ref), because the content…

2) You have not highlighted Figure 1 in the text on lines 83 to 84

3) Line 126 Ziplock is better than ziplocked.

4) Figure 5.1 The colors green, blue, and red on the diagram are not understandable for value and class, level of pollution removed. Single color is more appropriate e.g …black

 

 

 

 

 

 

Author Response

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections highlighted/in track changes in the re-submitted files.

Author Response File: Author Response.pdf