Distribution, Concentration, and Ecological Risk Assessment of Trace Metals in Surface Sediment of a Tropical Bangladeshi Urban River

Round 1

Reviewer 1 Report

The manuscript "Distribution, Concentration and Ecological Risk Assessment of Trace Metals in Surface Sediment of a Tropical Bangladeshi Urban River" aimed to assess

the contamination level of trace metals, their sources and ecological risk in surface sediments of Karnaphuli river- a tropical urban river of Bangladesh. I think the manuscript is very well written and the topic is very current. A detailed analysis was done and the results were very well processed. 

1. Why was Fe used to normalize element content? The paper shows that an increased content of this element was observed, which means that the first condition for the selection of the element for normalization was not met, which is that there is no increased content of the selected element selected for normalization normalization at the examined site. In the paper writes:

“Mamun et al. [54] stated that Karnaphuli River is highly polluted by Fe and Al. The high concentration of Fe at S3 which was Bazar area where wastewater directly discharged in river from this area”.

…….

“The source of Fe and Cr may possibly be the refining and steel industry, excessive use of Ferrous sulphate fertilizers and leather industry”.

Such an element is not used in this case for normalization. Also, aluminum pollution is mentioned, which indicates that this element cannot be used for normalization either. Either find an adequate element for normalization or eliminate everything related to the enrichment factor, since the conclusions cannot be adequate in this case.

2. Also, it is better to use local background content than literature data for background content, but still it is less of a mistake than using Fe for normalization at a site contaminated with this element.

Author Response

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Reviewer 2 Report

The manuscript Distribution, Concentration and Ecological Risk Assessment of Trace Metals in Surface Sediment of a Tropical Bangladeshi Urban River by Nur-E-Alam et al. it is an addition to an article already published in Sustainability in 2021 and cited at [30]. Most of the authors present in this manuscript were present in the cited article. The aim of this manuscript, which is different from the other one, is towards the goal of sustainability related to life in water (14) as it is threatened by significant concentrations of metals in the sediments of the Karnaphuli River which flows into the Bay of Bengal. The title is consistent with what is analyzed and underlines the good organization of the manuscript. Some clarifications would have been useful for a reader to understand the mobilization of metals, such as the grain size of superficial sediments (is it homogeneous in the entire stretch?), a quantification, albeit approximate, of the discharge/solid transport. The discussion of the results is correct, even if at times it seems that there are imperfections between values of the maximum concentrations and corresponding sites, but perhaps it depends on the quantity of numbers and elements correlated. Other imperfection is at the last line of page 7 S of shehzad in capital. The tables and illustrations are useful and consistent with the text. Figure 2, as it was prepared, may appear incomplete. It might be useful to prepare a new figure to highlight the different level of pollution along the river resulting from the analysis of sediment metal traces (indicate also the anthropogenic activity). The citations are up to date and support this analysis. 

Author Response

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Reviewer 3 Report

This paper present the concentration of different trace elements in riverine sediments, aiming at understanding the main source and the possible related risks to the environment. The introduction is generally well written, the main hypothesis of the study is clear and methods are generally well described. However, the main lack in this study is the presentation of the results and their discussion. Different trends are only partially described, and different interpretations should be better motivated by the authors in my opinion.

Firstly, authors should better emphasize also the potentially natural sources of potentially toxic elements. In the introduction, I suggest to mention some examples (see e.g., Ahmed et al. 2018 10.2147/RMHP.S153188 for arsenic). Similarly, I suggest to add also a description of the geological setting of the study area and add a geological map in figure 1 (see for example Binda et al., 2020 https://doi.org/10.1007/s10653-019-00405-4 ).

Moving to the results, Iron values seems extremely high in all samples  for me (table 2 and page 8) , it is almost higher than in pure mafic rocks (see e.g. Botcharnicov et al.  2005 https://doi.org/10.1016/j.gca.2005.04.023 and two order of magnitude higher than in other Bangladesh rivers (table 3). Moreover, these data are not in agreement with the graph in figure 3. Authors need to find an explanation for this anomalous value. In the current version of the manuscript, they limit to state a possible anthropic effect from water but an abundant natural concentration.

Then in Figure 2, I think that the correlation matrix, as it is, lacks of most of the correlation values. Only the 2 first rows are visible here.

Moving to section 3.4, considering the anomalous values of Fe in most of the samples, I suggest to the authors to carefully analyze the calculated enrichment factors. Moreover, I suggest to the authors to stress the fact that the different approaches are in accordance instead of list the four outputs separately. I also suggest to plot the difference indexes in the same way, for a better comparison, using the same type of graph and plotting the single sites or the whole average.

In figure 8, which is the meaning of the different colors of cluster branches? It is unclear to me. I suggest to the authors to use different colors to graphically separate the clusters instead.

Why do the authors exclude the Ph and Om variables in the PCA? They could explain some possible adsorption and deposition phenomena in sediments. moreover, they show good correlation with some elements in Figure 2.

There are then some issues to revise in my opinion:

-there are some unclear sentences which need to be rephrased (the first two sentences of the introduction, the discussion regarding pH in page 6, the sentence "This section may be divided by subheadings" to be removed in page 6); I generally suggest a revision of language style;

-in section 2.2 What unit is "gm"?

-in section 2.3 authors should state more precisely which QA/QC protocols were put in place, e.g., which blanks (matrix and analytical) and how many spiked samples were used, which standard error was reached etc.

-in figure 4 Authors should add the sampling points in the different maps.

As a final remark, I suggest to the authors to add line numbering, in order to make reviewing process easier.

Author Response

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Round 2

Reviewer 1 Report

One should be very careful with the use of elements for normalization and background content, read a lot of literature on geochemical normalization to be sure what and how to do.

Author Response

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Reviewer 3 Report

The revised version of the paper has improved in terms of overall quality and interpretation of the results. However, there are still few issues  I raised in the previous comments which were not resolved, and some minor issues which still require revision in my opinion:

-In figure 1, I still suggest to add a geological map or of the distribution of main cities and lad use, for a better interpretation of spatial trends of metals; 

-line 133: the sentence is not really clear, do the authors separate sand manually or do they sieve the samples?

-Figure 2: Please indicate also in caption that the correlation matrix is indicated for pH and Organic matter only.

-Authors mentioned in line 286 that Fe can possibly be sourced by bedrock. Then they should add a bit more detail on the geochemistry of rocks and why they could release such an abundant quantity of Fe in the study area section;

-Figure 3 is still not in accordance with the observed data in table regarding Fe.  If Fe is present at the % concentrations it should be presented in another scale compared to other elements in the mg/kg range;

-In the section in lines 495-508, it can also worth a bit of discussion to indicate that some of the clustering in the PC1-PC2 plot of the site scores is in accordance with the observed clustering by CA (e.g., sites 1,7 and 8 or sites 10, 11, 12 ,14 and 15) and others seems uncorrelated, possibly indicating a mixed natural/anthropic source of elements. 

 

Author Response

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