Surface Water Monitoring with Sedimentation Boxes: Assessing the Sampling Performance and Its Effect on Microplastic Concentration

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The work is interesting. Only minor comments areas below:

1. Figure 5: how many times the data was taken? please include error bars.
2. Did the author quantify each sample if it is laminar or non-laminar flow?
3. Why the rentention rates of PS is higher compare to others? 
4. Why the authors didnt used FTIR to analyze the presence of MP?

 

Author Response

1. Figure 5: how many times the data was taken? please include error bars.

Figure 5 shows the mean values of 6 samples, as specified in the caption. Error bars indicating the standard error are now included.

2. Did the author quantify each sample if it is laminar or non-laminar flow?

When operated with a pump, the flow inside the sedimentation boxes was laminar in all experiments. However, as simulated and demonstrated in Figure 2, there are some areas in the box (e.g. in the corners), where the flow becomes turbulent. For in-situ application, the flow velocity inside the box remains unknown, therefore it cannot be determined whether the flow is laminar.

3. Why the rentention rates of PS is higher compare to others?

The retention rates for PE and PS were very similar in the laboratory experiments as stated in paragraph 3.1 and shown in Figure 4. This similarity is probably due to the similar density difference to water. Retention rates for polymers with different densities will probably differ from those calculated in Figure 4.

4. Why the authors didnt used FTIR to analyze the presence of MP?

We chose to use TED-GC/MS as this method is suitable to directly determine the total MP mass in the samples. This way, results can be provided as MP mass concentrations. In our case, this method is superior over FTIR analysis, which usually provides MP concentrations as number of particles per volume. MP particles degrade and break up with time, making number of particles an unreliable value to assess MP pollution. Furthermore, FTIR analysis typically involves a previous optical selection of putative MP particles, making it time-consuming and error-prone.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Manuscript: Water-3536498

Title: “Surface Water Monitoring with Sedimentation Boxes: Assessing the Sampling Performance and Its Effect on the Microplastic Concentration”

Authors: Saravia CJ., et al.

 

General comment.

This manuscript is generally well written and structured and deals with an undoubtedly interesting topic. I personally believe that the proposed methodology is worthy of investigation, however I think that the authors must necessarily pay more attention to some aspects related to the determination of the uncertainty and variability of the data provided, which at the moment is completely unclear, to the point that it is not clear whether the proposed methodology is actually competitive with respect to different approaches. The authors are therefore invited to pay attention to these aspects, some specific considerations are provided below.

 

Specific comments.

Introduction in general: Personally, I would avoid dividing the introduction into paragraphs, a single and homogeneous text would be preferred, in order to make the description more fluid. I think that the authors should in fact provide a more streamlined Introduction, since many parts could be summarized, while on the contrary they should better describe the main scope of the investigation, underlining the innovative aspects that they propose.

Lines 32-38: The introduction is clear, but probably too straight to the point. Authors should first provide a brief description of the context and relevance of MPs contamination in water bodies, including some key references.

Paragraph 1.1.: As already commented, I would avoid the division into paragraphs; in any case, this part should be drastically summarized, since it is too verbose, sometimes repetitive and not entirely in line with the main focus of the manuscript.

Line 159: “Over a period of several years…” please specify.

Line 159-160: “we have applied sedimentation boxes in various experiments to sample MP from different rivers in Central and Eastern Europe (partly published in Kittner et al. [41]).” First of all, the authors should specify in detail which areas and rivers are being investigated. In addition, they should better specify what they mean by "partly published" elsewhere, does it mean that the proposed data are a part of a larger investigation? What has been investigated previously? Why has the investigation been divided into different parts? The authors should provide these information, as the text currently does not appear to be completely clear.

Table 1 and related description: “Mass share, TOC and polymer concentrations are mean values” since these are average values, what about the variability (standard deviation or standard error)? furthermore, what is the uncertainty of the data? how was the precision and accuracy of the measurements estimated? this is currently unclear.

Figure 5 and related description: what about the variability end uncertainty of the proposed data?

Lines 433-434: “Hence, sampling with the sedimentation box is a relatively robust method resulting in a sampling error of around 15%.” how was this error estimated? this is unclear. In addition, can the authors provide a comparison to conventional methods, describing whether the error is comparable or not?

Line 435-436: “However, the method is sensitive towards the sampling conditions and alters the sample composition compared to the input material.” How did the authors estimate the sensitivity of the method? What is the benchmark to consider the sensitivity good? This is not clear.

Lines 496-497: “In view of the many factors which may impact the sampling performance of sedimentation boxes, retention rates will remain subject to a high degree of uncertainty.” Can the authors provide an estimation of such uncertainty? How does this uncertainty affect those of the data and the intrinsic error of the method? It seems clear to me that the variability of the retention rate strongly affects the entire process, however what is the sensitivity limit beyond which such uncertainty becomes decisive? Reading the present text seems to me that what is missing from the treatment of the results is the fact that it is not easy to have control of the variables and associated errors. A more in-depth description seems necessary.

Lines 503-508: “On one hand, these additional factors need to be assessed in further laboratory experiments. On the other hand, more information on the actual retained plastics (e.g. size, density, etc.) is necessary to assign polymer specific retention rates. Up to date the size distribution is only roughly approximated with the fractionated filtration and information on the shape of the MP (e.g. fibers, films, spheres, etc.) is lost due to the detection method based on combustion” First of all, these sentences are partly repetitive and confusing. In any case, it seems that the lack of such information makes the present study partially incomplete: there are no convincing elements that ascertain the variability and sensitivity associated with the provided data. A more in-depth description would be necessary.

Author Response

1. Introduction in general: Personally, I would avoid dividing the introduction into paragraphs, a single and homogeneous text would be preferred, in order to make the description more fluid. I think that the authors should in fact provide a more streamlined Introduction, since many parts could be summarized, while on the contrary they should better describe the main scope of the investigation, underlining the innovative aspects that they propose.

The introduction is no longer divided into paragraphs. We shortened the introduction and have described the main scope of the investigation in more detail.

2. Lines 32-38: The introduction is clear, but probably too straight to the point. Authors should first provide a brief description of the context and relevance of MPs contamination in water bodies, including some key references.

We slightly extended the first introduction paragraph avoiding general remarks about e.g. MP sources, occurrence, and impact which are described in the following text.

3. Paragraph 1.1.: As already commented, I would avoid the division into paragraphs; in any case, this part should be drastically summarized, since it is too verbose, sometimes repetitive and not entirely in line with the main focus of the manuscript.

We shortened the introduction and summarized the mentioned paragraph.

4. Line 159: “Over a period of several years…” please specify.

Is specified.

5. Line 159-160: “we have applied sedimentation boxes in various experiments to sample MP from different rivers in Central and Eastern Europe (partly published in Kittner et al. [41]).” First of all, the authors should specify in detail which areas and rivers are being investigated. In addition, they should better specify what they mean by "partly published" elsewhere, does it mean that the proposed data are a part of a larger investigation? What has been investigated previously? Why has the investigation been divided into different parts? The authors should provide these information, as the text currently does not appear to be completely clear.

The mentioned lines have been adapted. In Kittner et al., sedimentation boxes have been applied to observe the development of MP sedimentation box sample concentrations along the river Danube and its tributaries. However, the sampling performance of the sedimentation boxes has not been investigated and the properties of the sampled rivers have not been considered.

6. Table 1 and related description: “Mass share, TOC and polymer concentrations are mean values” since these are average values, what about the variability (standard deviation or standard error)? furthermore, what is the uncertainty of the data? how was the precision and accuracy of the measurements estimated? this is currently unclear.

We added the standard deviation to samples with replicate measurements. For estimation of variability and uncertainty, see below.

7. Figure 5 and related description: what about the variability end uncertainty of the proposed data?

We added the standard deviation as error bars. The uncertainty of these data is described below.

8. Lines 433-434: “Hence, sampling with the sedimentation box is a relatively robust method resulting in a sampling error of around 15%.” how was this error estimated? this is unclear. In addition, can the authors provide a comparison to conventional methods, describing whether the error is comparable or not?

We deleted this sentence to provide more detailed information on the reproducibility of sedimentation box sampling. The reproducibility of sedimentation box sampling is specified further in sections 3.1 and 3.2. The comparison to other methods is included in the discussion.

9. Line 435-436: “However, the method is sensitive towards the sampling conditions and alters the sample composition compared to the input material.” How did the authors estimate the sensitivity of the method? What is the benchmark to consider the sensitivity good? This is not clear.
10. Lines 496-497: “In view of the many factors which may impact the sampling performance of sedimentation boxes, retention rates will remain subject to a high degree of uncertainty.” Can the authors provide an estimation of such uncertainty? How does this uncertainty affect those of the data and the intrinsic error of the method? It seems clear to me that the variability of the retention rate strongly affects the entire process, however what is the sensitivity limit beyond which such uncertainty becomes decisive? Reading the present text seems to me that what is missing from the treatment of the results is the fact that it is not easy to have control of the variables and associated errors. A more in-depth description seems necessary.
11. Lines 503-508: “On one hand, these additional factors need to be assessed in further laboratory experiments. On the other hand, more information on the actual retained plastics (e.g. size, density, etc.) is necessary to assign polymer specific retention rates. Up to date the size distribution is only roughly approximated with the fractionated filtration and information on the shape of the MP (e.g. fibers, films, spheres, etc.) is lost due to the detection method based on combustion” First of all, these sentences are partly repetitive and confusing. In any case, it seems that the lack of such information makes the present study partially incomplete: there are no convincing elements that ascertain the variability and sensitivity associated with the provided data. A more in-depth description would be necessary.

Comment 9-11: We revised the entire discussion part and set a larger focus on the uncertainty of the presented data and the respective sampling process. Lines 422-432 summarize the process of determining MP concentrations in rivers and the respective uncertainties. The sensitivity of the method cannot be fully determined, due to the unknown influence of some parameters such as weathering, interaction with natural SPM or advection to the devices’ walls. The limit beyond which sensitivity becomes decisive is highly dependent on the scope of the respective monitoring program.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors, I personally believe that the applied changes to the new version of the manuscript are adequate to the requests previously suggested, therefore I have no new comments to propose and I believe that the text can be now considered acceptable. Best regards.