The Long-Term Effects of Dredging on Chromium Pollution in the Sediment of Meiliang Bay, Lake Taihu, China

Round 1

Reviewer 1 Report

Line 23 (and Lines 293-294) – you may want to qualify this statement since I don’t think you presented any data that prove this to be true.

Lines 25-26 – Is the “DGT Induced Fluxes Model” sufficiently ubiquitous that readers will be familiar with it? If not, you might consider using a more generic statement in the abstract, e.g. “Modeling of DGT induced fluxes from sediment into overlying water showed…”

Line 27 – “can” instead of “could”? or may be just “…dredging decreased Cr…”

Line 28 – should “decreases” be “decreased”?

Line 29 – So, it reduced the risk of Cr contamination in winter. What about other seasons?

Line 152 – Tc and k1 were defined it the abstract, but not in the body of the paper. Just a note for the editors; I am not sure of this journal’s protocols.

Line 250 – the term “dredged sediments concentrations” is confusing.  I assume this refers to sediment in the previously dredged area. But it could refer to the actual sediment dredged in 2011. Suggest clarifying.

Line 255 – suggest “…Cr, increased after dredging…” reasons: “was obviously” is unnecessary.

Table 2 – I find this table a bit difficult to read, probably because the left justification and varied column widths necessary to fit it across the page. I realize that adjustments within the formatting guidelines may be limited.

Lines 285-6 – I am not sure what you are saying. Are you referring to the fact that the Tc values increase in Table 2 for every season except summer and those remained constant? I suggest being more specific.  For editorial consideration:

• Delete “All of” and “solid”.
• Not sure what “overall 6 years of dredging” means.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

No comment

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

There are many questions and suggestions which I have shown directly on the submitted manuscript, which is attached.

General Comments:

1. The manuscript needs a very careful reading by someone whose first language is English because there are many examples of imprecise language that make it difficult to see what the manuscript is intended to say.
2. It was annoying to have considerable amounts of text in red font, presumably entered after the manuscript was originally written, but not "accepted".
3. Figures 1 and 2 would be easier to read if thin gridlines were placed at 20 mm divisions so that comparisons across the four panels were easier. They are nice graphics.
4. I recommend using "total dissolved Cr" be used where the samples were water and "total sediment Cr" be used for the total (of all extractable and residual species) concentration in sediment.
5. There is a lot of material that needs to be included within the manuscript, or at least in the SI section including. (a) location map for the lake, location of discharge pipes, location of dredged sites and un-dredged sites (large scale with seasonal collections differentiated).

Major problems:

1. I do not understand how the dissolved Cr can change so much in a 10 cm core (cores from dredged sites or un-dredged sites) from season to season, because the water has the same total dissolved Cr concentration above and below the sediment water interface. Diffusion is not indicated because of the vertical constancy of the Cr concentration.
2.  Problems 1 and 2 suggest to me that the sediment cores collected sediment that was commonly resuspended and redeposited, providing a generally homogeneous upper 10 cm of sediment, at least. No evidence was provided that there has been continuous sedimentation that retains any record of actual input of polluted sediment or polluted water with material that was adsorbed. Resuspension of sediment should be very common in such a shallow lake. Although wave height would be restricted by the water depth, resuspension would be common with any persistent winds exceeding 10 mph, I think. Is there any real stratigraphic evidence that the sediment column actually archives the history of pollution in the upper 10 cm? ²¹⁰Pb, stable Pb, other metals with a known history of pollution? If your chemical data are fine (no data presented for QA/QC), the rapid changes from season to season can only be accomplished by altering the sediment on a sub-seasonal basis. Easy to do with resuspension.

Comments for author File: Comments.pdf

Author Response

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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

The authors describe Cr concentrations in sediment porewater 6 years after dredging and compare it to an area where dredging did not occur. This is an interesting paper and the results could be useful. The authors are encouraged to revise the paper to address the concerns below before resubmission.

The paper provides some quite interesting results. Unfortunately, the authors did not provide enough context to evaluate those results and draw any defensible conclusions. One specific area that needs bolstering is contextual information about the dredging project goals and objectives. What did the dredging attempt to accomplish? Did it remove high concentration sediments and attempt to leave a clean surface? Or, was the area deepened solely for navigation purposes and, thus, exposed sediments with higher Cr concentrations? Without this background information, no defensible conclusions can be drawn about dredging and its efficacy.

A second major area of weakness is the lack of information on the two sites compared throughout the manuscript. How did sediment concentrations in the two areas compare, both before and after dredging? What about the physical characteristics, e.g. water depth, velocity, etc.; where they similar or different? This reviewer assumes the authors selected the areas because of their similarities; but, that needs to be communicated in quantifiable terms so the reader can draw their own conclusions about the validity of the comparisons.

Some specific notes on the manuscript itself:

1. Where did the overlying water samples come from? Was it collected at the time of the sediment samples or taken from settling of samples? There is no mention in Section 2 of water sample collection.
2. Make all Cr references consistent. Specifically, the sentence starting on line 58 (“The zirconium oxide…”) uses 2 references different from the remainder of the paper.
3. Line 77 – correct superscript for km². (Post-review note: superscripts and subscripts are written on the same line as the preceding characters a number of places in the manuscript).
4. Line 81 states “…was dredged 6 years ago, …” Replace with a date.
5. Line 82 - Specify the pre- and post-dredge depths in the dredging area and provide the water depth in the non-dredging area.
6. Lines 86-87 – The parenthetical wording needs clarification.
7. Line 94 – I assume Cr concentrations were determined only on the fine fraction that passed a #200 sieve? If so, state it clearly.
8. Lines 123-128 – These sentences need clarification.
   1. The last sentence refers to equations 3 and 4 in the reverse order.
   2. Need to explain more thoroughly how Tc was computed.
   3. Add units for all variables
9. Line 134 – do you mean “sediment porewater”?
10. Line 135 – “…an average of different depths…” which different depths? Be specific.
11. 30 cm cores were collected, but the data are for only the upper 10 cm. Explain why.
12. Depth figures – What is the depth datum? I would normally assume that they are relative to the sediment surface. But, all of them show both depths > 0 indicating a different datum.
13. Lines 138, 143 – Figure caption “…in sediment porewater and overlying water…”?
14. Doesn’t Figure 1a (and other “a” figures) show annual averages and ranges for the 4 samples over depth? If so, please state. If not, explain what they show.
15. Doesn’t Figure 1b (and other “b” figures) show “depth averaged” concentrations for the pore-water data? Is the overlying water data a single value? Please explain.
16. Line 185 – explain or remove (F1), (F2), (F3), and (F4).
17. Figure 3 – Vertical axis on right side needs title and units.
18. Line 202 – “The” should not be capitalized.
19. Lines 218-220 – parentheticals need to be placed after “dredged site” and “non-dredged site” respectively.
20. I did not find sediment concentrations on a mass/mass basis in the paper. It might be useful to compare these for the dredged and non-dredged sites. It might explain a lot of the results.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

The paper does not describe the characteristics of the procedures used for the determination of total chromium and chromium(VI). The Authors wrote: "The zirconium oxide based DGT has been found to exhibit high selectivity for Cr(VI) [14,15], with studies showing that DGT-labile Cr(VI) extracted was mainly composed of hexavalent Cr, ..." - it is non-adequate description of the selectivity. Selective methods for the determination of the speciation form of chromium (e.g. Cr(VI)) require the control of the sampling conditions and its preservation at the collection site (including pH). Also, no validation parameters of the procedures used were described. As the procedures involve sampling, isolation / concentration /preparation /extraction / etc. and determination of the analytes (Cr and Cr (VI)), they are probably characterized by high standard uncertainty (in the case of Cr(VI) certainly higher than 35%). In the case of ICP-MS detection technique, there is no infromation concerning the isotope applied to measurement and if collision cell /dynamic reaction cell technology was applied (and type of CCT /DRC gas/gases).

The authors did not provide sufficient evidence to show that the deepening of the sediments may contribute to the reduction of total chromium and chromium (VI) concentrations in the overlying water and sediment pore water. The more so that the concentration reduction occurred only during the summer and winter periods, while at the same time other results were obtained, showing an opposite trend during spring and autumn. It seems that other factors (probably characterized by seasonal variability) may determine the presence of chromium and its speciation forms in the investigated environment of the lake (under anthropogenic pressure). The deepening of a certain part of the lake carried out 6 years ago probably had little impact on end effect observed. Authors underlined the results achieved during summer and winter (the Abstract and Conclusions), but significant difference (p<0.05) between the Cr concentrations of non-dredged and dredged sediments were indicated only during spring and autumn. The authors highlight a decrease of 17% (summer) and 38% (winter), but they do not mention in the abstract and conclusions an increase of 84% (spring) - 125% (autumn).

Taking into account the trueness and precision of the determinations, the number of significant figures in the results should be reduced, e.g. 115.3 ±29.93 μg/L.

Line 77: should be "km²".

Line 128-129: there are no correlations between k₁ and k_-1 and Eq. 3 and Eq. 4.

Fig. 1 and Fig. 2: spaces should be between words: "changes" and "(a)" /"average" and "(b)".

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Thanks for considering my input. I have just a few more minor notes for your consideration. 

1. Newly added text on line 23 states that dissolved Cr concentrations were "far below" the fisheries limit. "Far" is a bit nebulous. Consider removing "far", or using "well below", "significantly below"; yes, I realize that these are only slightly less nebulous!
2. Line 76 - it seems like " the Notice..." should be the "Notice..." 
3. Line 77 - Consider this minor rewrite..."From 2009 to 2011, the dredging engineering was performed in the Meiliang Bay of Lake Taihu, and averaged 30 cm depth of surface sediments were removaled from the lake."
4. Line 90 - The minimum pre- and post-dredging depths are the same, 2.8 m. Were some areas not dredged?

Reviewer 2 Report

1. The Authors wrote (lines 255-257):

"All of these results indicate that the risk of Cr release from solid sediments decreased overall 6 years after dredging, though the capability of dredging to control Cr leaching fluctuated with season and depth."

However, in the cases of data established during the summer presented in Table 2, values of R, K_d, T_c, K₁ and K_-1 are practically reach the same levels. It can be concluded that during the summer, no influence of the applied dredging process (carried out 6 years ago) on the observed chromium concentrations was observed. In fact dissolved chromium decreased only by 5% in sediment pore water in the dredged site in this season, which, as the Authors pointed out, does not significant from a statistical point of view. Given the additional statement of the Authors from the lines 183-185: "The dissolved Cr concentration in sediments of the dredged site increased by 7% in spring and by 63% in autumn, while decreasing by 5% in summer and 20% in winter, compared with the non-dredged sediments." it can be concluded that the described beneficial effect of dredging can be observed after 6 years - only during winter (which is not statistically significant anyway).

The Authors wrote (Abstract):

"...model analysis showed a higher response time (T_c), lower adsorption rate (k₁), and desorption rate 27 (k₋₁) in the dredged site, indicating that sediment dredging could decrease Cr mobility in sediments."

nevertheless, if this is the case for these parameters determined during spring and autumn (Table 2), why was the opposite effect observed during these seasons - an increase in dissolved chromium concentrations in sediments of the dredged site?

2. It should be emphasized that only the observed increases in chromium concentrations in water during spring and autumn were statistically significant, contrary to the falls during summer and winter (The Authors wrote: "In the overlying water, compared with the non-dredged site, the average amount of dissolved Cr in the dredged site increased by 84% in spring and by 125% in autumn..."). The fact that the above-mentioned increases did not exceed the standards is not a reason to take into account only the decreases that occurred during summer and winter, and the attempts to correlate this situation with the applied process of dredging the bottom of the lake are rather debatable. Probably other factors determine the seasonal variability of the concentration of chromium in the water - intense rainfall and sewage discharge into the lake, which can be easily checked by conducting a simplified monitoring of this metal concentration during subsequent next seasons.

3. Additionally, in the case of Table 2, it is not clear why there are such large differences of up to 9 orders of magnitude between the values of T_c (dredged/non-dredged: 0.01 - 1.00×10⁷). The same type of differences exist in cases of K₁ and K_-1.

4. For the determination of total chromium, the Authors applied an inductively coupled plasma mass spectrometry. They wrote that:

"In the case of ICP-MS detection technique, there is no isotope applied to the measurement,..."

Which isotope was applied to the detection in quadropule ICP-MS (m/z 52 or 53)?

5. Line 151-153: should be "T_c" (not Tc).

Line 220: "1.93 to mg/kg" should be erased.

Table 2:

• should be [s] and [/s] (not "S" and "/S").

Fig. 3:

• should be "dredged" (not dreged).