Heavy Metals Contamination of Urban Soils—A Decade Study in the City of Lisbon, Portugal

Round 1

Reviewer 1 Report

In this study, the Authors performed long-term soil analysis to evaluate soil pollution with selected heavy metal for Lisbone city.  The total number of samples that were analyzed and the repetitions of such analyses should be appreciated because the obtained results are of good quality. As a result, this paper contains a lot of data that can be an useful database on soil contamination with heavy metals. This study is well structured and well presented. Thus, my reccomendation for this paper is minor revision. 
The specific comments are listed below:

• L23 'mean concentrations' instead of the 'mean values' seems to be better
• L78 Please give full description for 'GFAAS' that is used here first time
• L92-104 Are you able to give some values for traffic intesity, population density in Lisbona's region. Such data could be valuable, if we would like to compare (similarities or differences) the metal contamination in soil from different parts of Lisbona city. Saying 'high' traffic, do you mean the same traffic intensity in different parts of Lisbona?
• L152 sth is missing here, probably '+-'?
• L194 using 'classification' instead of 'terminology' would be better here
• L211 'Er' should be with subscript
• L251 The title of the section 3 should be rather 'Results and Discussion'
• L267-268 how do you know about such reason/explanation? on what basis?
• the font in Figs. 2, 4, 5, 6 should be slightly enlarged
• L380-381 please rephrase 'situation'
• L435 Please change the title of subsection 3.4. The 'statistical analysis' is too general and it was already indicated in Materials and methods
• L468 In my opinion the title 'Discussion and Conclusions' is not appropriate. This is because only one reference was used here. Instead of that 'Summary' seems to be better. The reference Leitao et al. 2018 should be transferred to Results and Discussion section. 

Author Response

 

The authors would like to thank the reviewers for their time dedicated to reviewing this article.  Additionally, we are very grateful for the valuable suggestions that improve the article and make it more useful for the scientific community. It is good to receive suggestions from experts who really demonstrate to know the subject.

As requested, a revision of the manuscript was carried out being our goal, as authors, to accept all suggestions of improvement. However, due to the different opinions of two or more reviewers some of them were not fully implemented. We ask the reviewers to understand these limitations.

The major changes to the original manuscript are highlighted in yellow in this new version.

We answer below to each of the reviewers' suggestions.

 

Reviewer 1

Comments and Suggestions for Authors

In this study, the Authors performed long-term soil analysis to evaluate soil pollution with selected heavy metal for Lisbone city.  The total number of samples that were analyzed and the repetitions of such analyses should be appreciated because the obtained results are of good quality. As a result, this paper contains a lot of data that can be an useful database on soil contamination with heavy metals. This study is well structured and well presented. Thus, my reccomendation for this paper is minor revision. 
The specific comments are listed below:

 

L23 'mean concentrations' instead of the 'mean values' seems to be better

 

Corrected.

 

L78 Please give full description for 'GFAAS' that is used here first time

 

Corrected.

 

L92-104 Are you able to give some values for traffic intensity, population density in Lisbona's region. Such data could be valuable, if we would like to compare (similarities or differences) the metal contamination in soil from different parts of Lisbona city. Saying 'high' traffic, do you mean the same traffic intensity in different parts of Lisbona?

 

The traffic intensity was the one observed during the years of study and based on reports from the municipality. But the existing data is only quantitative.

 

L152 sth is missing here, probably '+-'?

 

Corrected.

 

L194 using 'classification' instead of 'terminology' would be better here

 

Corrected.

 

L211 'Er' should be with subscript

 

Corrected

 

L251 The title of the section 3 should be rather 'Results and Discussion'

 

Corrected.

 

L267-268 how do you know about such reason/explanation? on what basis?

The paragraph was modified and simplified. A new table was introduced with the systematic presentation of the results.

 

the font in Figs. 2, 4, 5, 6 should be slightly enlarged

 

The graphics were redone in order to be able to increase the size of the fonts.

 

L380-381 please rephrase 'situation'

 

Corrected.

 

L435 Please change the title of subsection 3.4. The 'statistical analysis' is too general and it was already indicated in Materials and methods

 

Corrected. Subsection 3.4 was renamed to Data Correlation and principal component analysis

 

L468 In my opinion the title 'Discussion and Conclusions' is not appropriate. This is because only one reference was used here. Instead of that 'Summary' seems to be better. The reference Leitao et al. 2018 should be transferred to Results and Discussion section. 

 

The section was renamed for Conclusions, only. The reference was transferred to the Results and Discussion section.

Reviewer 2 Report

Dear authors,

In your article you presented a very interesting issue such as soil contamination with heavy metals. You have determined the Geo-Accumulation index, Pollution Factor, Degree of contamination, Pollution Load Index and Ecological risk factor. You presented the research results in a clear and interesting form. Both the sampling and the chemical analysis have been described in detail. The work deserves to be published after minor corrections have been made.

Please correct:

1. The method of citing the literature in accordance with the MDPI requirements (in the text square brackets with the number of the cited item, in the list of bibliographies - the individual items of the literature should be given in the order of citation).
2. I propose to transform the work layout regarding results, discussions and conclusions. Please leave the results without references to the literature. In the discussion, refer to the results of other researchers, and in the conclusions write some of the most important observations from the research.
3. Where did such high Ni contents come from? And, compared to world studies, for example from Poland, low Pb content was found. Recently, a work on the content of metals in soils of protected areas has been published, in which, similarly to this article, individual pollution coefficients were determined. Perhaps it is worth referring to her? Korzeniowska, J .; Krąż, P. Heavy Metals Content in the Soils of the Tatra National Park Near Lake Morskie Oko and Kasprowy Wierch — A Case Study (Tatra Mts, Central Europe). Minerals 2020, 10, 1120. https://doi.org/10.3390/min10121120

With best regards,

Reviewer

Author Response

 

The authors would like to thank the reviewers for their time dedicated to reviewing this article.  Additionally, we are very grateful for the valuable suggestions that improve the article and make it more useful for the scientific community. It is good to receive suggestions from experts who really demonstrate to know the subject.

As requested, a revision of the manuscript was carried out being our goal, as authors, to accept all suggestions of improvement. However, due to the different opinions of two or more reviewers some of them were not fully implemented. We ask the reviewers to understand these limitations.

The major changes to the original manuscript are highlighted in yellow in this new version.

We answer below to each of the reviewers' suggestions.

 

Reviewer 1

Comments and Suggestions for Authors

In this study, the Authors performed long-term soil analysis to evaluate soil pollution with selected heavy metal for Lisbone city.  The total number of samples that were analyzed and the repetitions of such analyses should be appreciated because the obtained results are of good quality. As a result, this paper contains a lot of data that can be an useful database on soil contamination with heavy metals. This study is well structured and well presented. Thus, my reccomendation for this paper is minor revision. 
The specific comments are listed below:

 

Please correct:

The method of citing the literature in accordance with the MDPI requirements (in the text square brackets with the number of the cited item, in the list of bibliographies - the individual items of the literature should be given in the order of citation).

Corrected.

 

I propose to transform the work layout regarding results, discussions and conclusions. Please leave the results without references to the literature. In the discussion, refer to the results of other researchers, and in the conclusions write some of the most important observations from the research.

Corrected. The structure of the article was revised, and the sections are now:

Introduction

Materials and Methods

Results and Discussion

Conclusions

 

Where did such high Ni contents come from? And, compared to world studies, for example from Poland, low Pb content was found. Recently, a work on the content of metals in soils of protected areas has been published, in which, similarly to this article, individual pollution coefficients were determined. Perhaps it is worth referring to her? Korzeniowska, J .; Krąż, P. Heavy Metals Content in the Soils of the Tatra National Park Near Lake Morskie Oko and Kasprowy Wierch — A Case Study (Tatra Mts, Central Europe). Minerals 2020, 10, 1120. https://doi.org/10.3390/min10121120

Corrected. Thanks for the suggestion of this interesting work. This reference was included in the “Results and Discussion”, the data and opinions of the authors of the article were considered.

 

 

Reviewer 3 Report

The manuscript reports a ten-year study of trace element concentrations in urban soils in Lisbon. The study is well designed and the analytical methodology is explained in much detail, although further statistical analyses are needed. Overall, the paper is well written and correctly organized and the results are clearly exposed and dicussed. Only the section where the edaphic properties of the soils are presented needs considerable improvement. For the rest, the manuscript is fine.

Specific comments

Line 28 Please remove "then" as that sentence is not a consequence of a previous one

Line 40 I would suggest replacing "areas" with "processes" or similar

Lines 71-74 This paragraph does not fit well in the structure of the introduction, I would suggest moving it to the M&M section

Lines 76-79 The motivation for the study and the selected approach should be better justified in term of objectives

Line 93 Adding a schema with the topographic location of the six points would help understand this paragraph and the differences between the areas studied

Lines 259-260 Differences in rainfall in a short period cannot explain OM contents in a soil; since presenting the moisture of the soils at the moment of sampling is not very relevant either, I would suggest to remove the whole paragraph and at least briefly comment if there are any differences among sites

Lines 262-265 This sentence is very confuse. First, it seems that there is no difference in EC at the two samplings. Second, salinity is not decisively affected by short-term rainfall or by soil OM contents. Third, we do not know to which "increased growth of plants" or "microbial growth" do authors refer, since these data are not presented.

Lines 266-267 Seeing the values I do not think there is really a decrease in pH, given the typical heterogeneity of urban soils and the normal variation of soil analytical techniques. In this sense, have statistical analyses performed to determine the significance of the difference?

Lines 267-270 Here as in the previous paragraphs, the authors try to attribute differences in pH and other properties to soil moisture at the moment of sampling, which is very hard to justify. Also, some basic processes in soil chemistry are incorrectly considered: soil acidification is primarily due to leaching of basic cations from minerals; dissolution of humic acids should not have an impact on pH and in any case, it is not justified speaking of these processes in soils that are neutral to alkaline; soil moisture per se does not influence element leaching, it is the rainfall regime and soil pH and permeability, among other factors, that determine the mobility of metals in soils; finally, the last sentence implies that soil acidification to increase metal leaching can be considered a positive process that promotes remediation when in fact it is the opposite: increasing metal mobility increases bioavailability and therefore the risk of transference of poillutants to living organisms and the food chain, as well as the risk of transference to water bodies, which is obviously an undesirable situation.

In view of the inaccuracies in the whole section 3.1, I would suggest the authors to present soil properties in a table, maybe presenting the data separated for the six areas, then perform statistics to see if significant differences exist between the two times of sampling and the six points, and briefly comment the results without going too much into detail.

Line 276 A statistical test should be run to determine the significance of these differences

Table 1-Figure 2 Measures of deviation of the mean should be added. Also, are the two decimals meaningful in Cr, Ni and Pb data?

Line 430 It would be interesting to justify why other metals such as Cu and Zn, which are very typical pollutants in urban soils, have not been considered in this work.

Lines 436-437 This sentence could be removed, as it has been already said in the methods section

Lines 502-503 Although it is clear in view of the results that Cr and Ni have a common origin, it is not so obvious that this origin should be only anthropogenic. Cr and Ni are both present in high concentrations in soils derived from some mafic rocks so the contribution of a natural origin cannot be discarded, as it has been observed in cities developed over these geological materials, there are several examples in the review by Ajmone-Marsan that you cited.

Lines 510-511 The risk of metal transference from soil to plants and the food chain is not only related to total concentrations in soil, it depends on factors such as soil pH and OM content. I would suggest adding a sentence acknowledging that, although low metal levels is a good result in this sense, bioavailibility issues should also be considered in the future in order to assess the environmental risk related to urban agriculture.

Author Response

The authors would like to thank the reviewers for their time dedicated to reviewing this article.  Additionally, we are very grateful for the valuable suggestions that improve the article and make it more useful for the scientific community. It is good to receive suggestions from experts who really demonstrate to know the subject.

As requested, a revision of the manuscript was carried out being our goal, as authors, to accept all suggestions of improvement. However, due to the different opinions of two or more reviewers some of them were not fully implemented. We ask the reviewers to understand these limitations.

The major changes to the original manuscript are highlighted in yellow in this new version.

We answer below to each of the reviewers' suggestions.

Line 28 Please remove "then" as that sentence is not a consequence of a previous one

Corrected.

 

Line 40 I would suggest replacing "areas" with "processes" or similar

Corrected.

 

Lines 71-74 This paragraph does not fit well in the structure of the introduction, I would suggest moving it to the M&M section

Moved as suggested..

 

Lines 76-79 The motivation for the study and the selected approach should be better justified in term of objectives

Corrected. Please see the last paragraph of the Introduction section.

 

Line 93 Adding a schema with the topographic location of the six points would help understand this paragraph and the differences between the areas studied.

To help to find the exact locations of the sampling sites, the GPS coordinates were added to each location.

 

Lines 259-260 Differences in rainfall in a short period cannot explain OM contents in a soil; since presenting the moisture of the soils at the moment of sampling is not very relevant either, I would suggest to remove the whole paragraph and at least briefly comment if there are any differences among sites

Corrected. A table with the soil characterization data was introduced as suggested bellow.

 

Lines 262-265 This sentence is very confuse. First, it seems that there is no difference in EC at the two samplings. Second, salinity is not decisively affected by short-term rainfall or by soil OM contents. Third, we do not know to which "increased growth of plants" or "microbial growth" do authors refer, since these data are not presented.

Corrected.

 

Lines 266-267 Seeing the values I do not think there is really a decrease in pH, given the typical heterogeneity of urban soils and the normal variation of soil analytical techniques. In this sense, have statistical analyses performed to determine the significance of the difference?

Corrected.

 

Lines 267-270 Here as in the previous paragraphs, the authors try to attribute differences in pH and other properties to soil moisture at the moment of sampling, which is very hard to justify. Also, some basic processes in soil chemistry are incorrectly considered: soil acidification is primarily due to leaching of basic cations from minerals; dissolution of humic acids should not have an impact on pH and in any case, it is not justified speaking of these processes in soils that are neutral to alkaline; soil moisture per se does not influence element leaching, it is the rainfall regime and soil pH and permeability, among other factors, that determine the mobility of metals in soils; finally, the last sentence implies that soil acidification to increase metal leaching can be considered a positive process that promotes remediation when in fact it is the opposite: increasing metal mobility increases bioavailability and therefore the risk of transference of pollutants to living organisms and the food chain, as well as the risk of transference to water bodies, which is obviously an undesirable situation.

In view of the inaccuracies in the whole section 3.1, I would suggest the authors to present soil properties in a table, maybe presenting the data separated for the six areas, then perform statistics to see if significant differences exist between the two times of sampling and the six points, and briefly comment the results without going too much into detail.

Corrected. A table with the data was introduced with a brief commentary on the results.

 

Line 276 A statistical test should be run to determine the significance of these differences

Corrected. A t-test was carried out in the results.

 

Table 1-Figure 2 Measures of deviation of the mean should be added. Also, are the two decimals meaningful in Cr, Ni and Pb data?

Corrected. The decimals of the Cr, Ni and Pb data were reduced. A column with the RSD in % has been added to make it comparable between the different metals. However, this RSD refers to values with a temporal evolution, so the authors accept their inclusion but consider it debatable. The expanded uncertainty (95%, k = 2) associated with the analytical determinations in the last column was maintained. The authors think that only the uncertainty of the results should be maintained and the RSD should not be included because it does not refer to repeatable values, but rather to values of a temporal evolution.

 

Line 430 It would be interesting to justify why other metals such as Cu and Zn, which are very typical pollutants in urban soils, have not been considered in this work.

Corrected. When choosing metals, their toxicity was considered and zinc and copper are less toxic metals than those considered in this study (Cr, Cd, Ni, Pb). Mercury and arsenic were not considered because we do not have analytical capacity in the laboratory for the analysis of these metals.

 

Lines 436-437 This sentence could be removed, as it has been already said in the methods section

Removed.

 

Lines 502-503 Although it is clear in view of the results that Cr and Ni have a common origin, it is not so obvious that this origin should be only anthropogenic. Cr and Ni are both present in high concentrations in soils derived from some mafic rocks so the contribution of a natural origin cannot be discarded, as it has been observed in cities developed over these geological materials, there are several examples in the review by Ajmone-Marsan that you cited.

Corrected. The considerations of the aforementioned authors were introduced.

 

Lines 510-511 The risk of metal transference from soil to plants and the food chain is not only related to total concentrations in soil, it depends on factors such as soil pH and OM content. I would suggest adding a sentence acknowledging that, although low metal levels is a good result in this sense, bioavailibility issues should also be considered in the future in order to assess the environmental risk related to urban agriculture.

Corrected. The suggested reflection was introduced through the penultimate paragraph in the conclusions section.

 

Round 2

Reviewer 3 Report

The authors have considered the remarks by the reviewers and modified the manuscript accordingly. In my opinion, my previous objections have been adequately addressed and the manuscript could now be accepted for publication.