the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Jan 2022
Meso- and microplastic distribution and spatial connections to heavy metal contaminations in highly cultivated and urbanised floodplain soilscapes – a case study from the Nidda River (Germany)
Abstract. Floodplain soilscapes act as temporary sinks in the environment and are nowadays affected by multiple contaminant accumulations and exposures, including heavy metals and (micro-)plastics. Despite increasing knowledge of the occurrence and behaviour of (micro-)plastics at the interface between aquatic and terrestrial systems, there are still major uncertainties about the spatial distribution of plastics, their sources and deposition, as well as spatial relationships with other contaminants. Our recent case study addresses these questions, using the example of a river system ranging from rural to urban areas. Based on a geospatial sampling approach we obtained data about soil properties, heavy metal contents via ICP-MS analyses, and particle-based (171 µm–52 mm) plastic contents, analysed using sodium chloride density separation, visual fluorescence identification and ATR-FTIR analysis. We found plastic contents of 0.00–35.82 p kg−1 and heavy metal enrichment (Enrichment factor 1.1–5.9). Levels of both contaminations occur in the lower range of known concentrations and show a different spatial distribution along the river course and in the floodplain cross-section. Furthermore, we found that plastic enrichment occurs in the uppermost soil layers, while heavy metal enrichment is located at greater depths, indicating different sources and deposition periods. Finally, direct short to long-term anthropogenic impacts, like floodplain restoration or tillage may affect plastic enrichments, raising questions for future floodplain management.
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CC1: 'Comment on soil-2022-1', Zacharias Steinmetz, 18 Feb 2022
Dear authors,
Thank you for sharing this very interesting and well made contribution! While reading the manuscript, some minor issues attracted my attention and I didn’t want to keep them to myself. Feel free to consider them for a future revision.
Title
The term “heavy metals” is used less and less since the density of a metal doesn’t tell you much about its environmental relevance. I’d suggest to term them “metals of interest” instead.
Introduction
I would opt for a more concise introduction that is tailored more towards your objective and research questions. I find it rather lengthy and narrative in some parts. But this is of course a matter of style and personal perception.
Furthermore: were all findings reported in L83–87 performed in NaCl solution? Which ecosystem services did you have in mind (L98)? Why isn’t there a hypothesis for bullet 3 (L123f)?
Methods
As for the introduction, I would suggest to prune section 2.1.
Though it’s quite subjective, I always find it more intuitive to number sites and abbreviate sampling points with letters. In your case, this would also help to see right away whether the location was upstream or downstream (L185 and L191).
I wonder why you didn’t use paper bags (L198f). Any specific reasons?
I haven’t read the term “fine earth“ yet and always used “fine soil” (L215). Don’t know if one is more correct than the other.
Was there any protocol or criteria that made you only “partly“ analyze plastics under the stereomicroscope (L238ff)? If so, which were analyzed this way?
The “rim jars” described in L242 were made of glass?
“Cowger et al., 2020” should be Cowger et al., 2021 and does not appear in the reference list (L296). In addition, r2 and R2 are used inconsistently.
Please be aware that non-parametric tests like Wilcoxon rank sum and Kruskal-Wallis tests do still assume equal variances. As you say, your data showed “significant differences in variance by group” (L323f). In such cases, you may consider a Welch test instead (which again assumes normality though). I would also suggest to inspect normality and homoscedasticity of the residuals rather than the data. This can as well be done graphically via QQ and residual vs. fitted plots, which is often more accurate than Shapiro and Levene tests. I tried some things on my own using the data published on figshare but struggled to interpret it correctly. You may also consider to add your R scripts to that repository. This would make it easier to follow your data analysis steps.
Results and Discussion
Well done! I really enjoyed reading those parts, especially sections 3.3.2 and 4.3. You may consider cutting some long sentences though. Also, you sometimes put question marks behind a sentence although it’s not a direct question but a relative clause.
References
You cite more than 100 papers which is already half a literature review. You may consider reducing the number of references in favor of some review papers or one exemplary study, if possible.
Hope this helps. All the best,
ZachariasCitation: https://doi.org/10.5194/soil-2022-1-CC1 -
AC1: 'Reply on CC1', Collin J. Weber, 22 Feb 2022
Dear Zacharias,
thank you for using the interactive review option and thanks for your very helpful comments! We would be happy to include your comments in a future revision. A few comments in response here:
Title
Thanks for your suggestion about the term “metals of interest”. It is true that the term "heavy metal" is too unspecific, although it is still used so often. We will use the new term in place of the old one in our revision, including title and main text.
Introduction
It is true that the introduction is too wordy in some places. We will shorten this and adjust more directly in the revision.
Regarding your questions:
The findings reported in L83-87 were performed purely visual without density separation (Weber and Opp, 2020) and in NaCl (Weber et al. 2021).
We had general ecosystems services like provisioning (e.g., food production), regulating (e.g., water purification and flood control) and cultural (e.g., non-material benefits like archive functions) services in mind.
With regard to bullet 3 (3th issue) we had an hypothesis in mind from our previous research. Maybe we change the wording to “Spatial relationships … proving the hypothesis of spatially distinct contamination patterns due to different contamination periods”
Methods
Thanks for your suggestions. We will shorten the section and make it more precise. With regard to the sampling site naming or numbering, we think that’s a subjective point, as you already mentioned. We will keep the original here.
- 198 We used corn starch bioplastic bags instead of paper bags since in our experience they are more durable with moist samples.
- 215 The term “fine-earth” is common within the soil description according to FAO guidelines. I don't know which one is more correct, but I have always referred to the FAO guidelines.
- 238 Yes, there was a specific protocol and thanks for this reminder! Sample material with a size (sieved) between >2 mm and <5 mm was inspected via stereomicroscope, whereas sample material <5mm was inspected without. We will add these criteria in our revision.
- 242 Yes, the rim jars were made of glass. We will add “glass rim jars”.
Thanks for your comments on the correct Cowger citation as well as the consistent use of r². We will adjust these errors.
Special thanks for your statistics suggestions! We have done some graphic analysis of the residuals as well. We will check residuen again. Thanks also for the hint to share the R code. With regard to the non-parametric tests, we made sure that the variances of the comparison groups did not differ significantly, because we could not perform a Welch test due to the lack of normality.
Results and Discussion
Thanks for the commendation and glad you enjoyed reading it! We will consider your comment on the length of some sentences and shorten where possible.
References
Thanks again for this remark. Your idea of favouring some specific review papers and exemplary studies sounds fine. We will include this in the revision.
Really, big thank for your support. All the best,
Collin (on behalf of all co-authors)
Citation: https://doi.org/10.5194/soil-2022-1-AC1
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AC1: 'Reply on CC1', Collin J. Weber, 22 Feb 2022
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RC1: 'Comment on soil-2022-1', Anonymous Referee #1, 02 Mar 2022
- This was a very interesting study, and very well thought out and presented in the manuscript. It was clear and pleasant reading.
- Although the introduction was very informative it may benefit from being a little shorter.
Specific comments:
- I do not think it is necessary to refer to “(micro-)plastics” as they are in lines 11 and 12. Reference could just be made to “plastics” which would encompass plastics particles/pieces of all sizes.
- Line 37: Do “physical processes” also include the role of biota in the fragmentation of plastics? Animals intentionally or unintentionally grazing on plastics results in fragmentation.
- Line 83: “In the Lahn River” should be “in the Lahn River”
- There are several instances where “River” is not capitalised in the river name. This needs to be corrected in all instances through the manuscript.
- Line 122: “enters” should be ‘enter’.
- Spaces before degree symbols should be removed. E.g. Line 134.
- Spaces after ‘<’ or ‘>’ signs should be removed. E.g. Line 156 & 160.
- Line 167: “result” should be “results”.
- Line 173: There is something wrong with the reference.
- Line 177: “back swamp” should be “backswamp”.
- Line 199: it would be good if more information could be provided about the material the bags are made from. The definition of “bioplastic” can be variable, and misleading. For example, it can be used to describe a plastic made from a feedstock of biological origin (e.g. starch, not fossil fuel) however be a traditional polymer like PE, PVC; or alternatively it can be used to describe biodegradability. Care is taken in the methods to control for the presence of it in the sample by taking spectra, but I feel it would be good here to include a bit more information and make it clear what the polymer is.
- Line 202-3: Suggest rewording the sentence “Visible plastics fragments were collected on a 20 m2 area around the drill points by walking straight lines with two persons (four-eyes-principle),……” to “Visible plastic fragments were collected in a 20 m2 area around the drill points, by two people walking straight lines in parallel (four-eyes-principle),……”.
It would also be good to give dimensions of how the 20 m2 area was obtained to show it was an equally distributed area around the central drill point.
- Line 212: Provide detail on what containers they were in for drying, or whether they were in the open bags.
- Line 202: I am not familiar with these standards and am unable to access them, however I feel that “loss of ignition” is not the right wording. Should it be combustion at 550°C until steady weight obtained?
- Line 242: I am unfamiliar with ‘rim jars’. More detail needs to be given as to what they are made from.
- Line 254: space needed after Ø = ‘Ø 47 mm’ and ‘Ø 90 mm’
- Line 258: It is not clear what “sprayer” is. Can more detail be given or a clear description of what this is?
- Line 281: reword ‘for safety reasons’. I don’t think you mean it in the sense it reads. You are doing it to ensure that if any fragments of the bag have entered the sample you are able to account for this contamination and remove it from the analysis.
- Line 282: The use of cotton lab coats specifically avoids contamination of the air by synthetic fibres that would potentially come off synthetic lab coats. Contamination would still be still but they’d be cotton, and therefore not included in your analysis.
- Line 357: Can you please provide information about what is included in your definition of “rubber” as this is a class or group of polymers, not a single type and it includes both fully synthetic and semi-synthetic alternatives. This will also influence the EPO ages, with only the natural rubber being used in 1820s, and first synthetic rubber developed in 1910, and another alternative in the 1930s.
- Line 366: The following sentence needs rewording “The plastics contain of HDPE …” Is it means to read “The plastics consists of HDPE….” Or “The plastics found included HDPE….”?
- Line 368: As this is intended for an intentional audience it may be best to avoid colloquial naming of things. Could ‘fries fork’ be changes to ‘plastic fork’, and ‘DIY store shed’ seems like a very broad category or description. Is there something more specific that can be said?
- Line: 390: ‘, at depths below 100 cm.’ is confusing. Is it meant that they are deeper, as in more >100 cm; or shallower? Please check and confirm to ensure it is less ambiguous.
- Line 426: It is not clear why ‘Meso- and single macroplastic particles….’ Is written like this. Can this be made clear, or changed to remove the ‘single’?
- Line 448: What depths do “From 45.98% to 62.5%” refer to?
- Line 489: Change “Figure 8a and Figure 8b” to “Figure 8”.
- Line 536: Is this mislabelled? “Table T1”. Should it be S1?
- Line 555: Change “incipient alteration surface structure” to “incipient alteration of surface structure”.
- Line 589: ‘(Table)’ is this meant to be here?
- Line 682: Add “River” after “Nidda”. Other occurrences like this may occur elsewhere in the document.
FIGURE
Figure 1: Add in W (wastewater treatment plants) and ID (industrial discharge) in to the figure legend.
Figure 2: a) Both resin and rubber are broad categories of polymer. You should explain somewhere what is included within these definitions. See previous note about ‘rubber’.
The scale bars and associated distance in the photos need to be larger as they are currently impossible to read. c) It may be advantageous to add in labels of Course and Fine soil to the figure to make it clearer what the reader is looking at.
Figure 3: Adding Titles to the graphs would make it clearer what the reader is looking at. a) & b) both the y and x-axis labels need to be centred on the axes. Legend: Suggest rewording “(with transect site location and river km) “ to “(with transect site location and river length (km)). This also needs to be changed in the axes labels. Add text to “dotted boxes indicate anthropogenic influence” = “dotted boxes indicate sites exposed to anthropogenic influences”
Figure 4: y-axis label: the ‘(-)’ needs to be removed.
Figure 5: x-axes labels: the ‘(-)’ needs to be removed from EF(-) and PLI(-).
Figure 6: it is not clear why for both a) and b) why the smaller particles are also presented as an additional graph inset as they are clearly presented in the total particles graphs. By removing them from both a and b the graphs showing all sized particles can widened.
Figure 7: the ‘(-)’ needs to be removed from PLI(-). Can the circles for the floodplain positions be made larger so it is easier to see the different colours?
Figure 8: y-axis on both a and b need to be flipped around 180°. the ‘(-)’ needs to be removed from EF(-).
TABLES:
Table S1: a different foodnote symbol for ‘Shape’ needs to be used as you also use ‘a’ to refer to a picture.
Table 2: It is not immediately clear what “low, moderate and high” are referring to. If they are referring to the Indices is it possible to change the format and have “low, moderate and high” right justified in the column?
Tables A1 and A2: are referred to a lot. Could they go in the main body of the article rather than as an Appendix?
Table A1: It was not clear to me why are some of the words in bold.
Citation: https://doi.org/10.5194/soil-2022-1-RC1 -
AC2: 'Reply on RC1', Collin J. Weber, 02 Mar 2022
Dear Reviewer 1 (RC1),
thank you for your time to read and comment on our manuscript! Thanks for the basic commendation and glad you liked the manuscript. Furthermore, thanks for the close look and your mention of the minor typos, which we will certainly correct. We would like to respond to some of the comments below and clarify open questions. For comments that only need to be implemented as a correction, we have set "[will be implemented during revision]".
Thank for your support. All the best,
Collin (on behalf of all co-authors)
General comments:
Although the introduction was very informative it may benefit from being a little shorter.
- Thanks for this first general comment. We will shorten the introduction also in accordance with the community comment (CC1) to make it more precise.
Specific comments:
I do not think it is necessary to refer to “(micro-)plastics” as they are in lines 11 and 12. Reference could just be made to “plastics” which would encompass plastics particles/pieces of all sizes.
- [will be implemented during revision]
Line 37: Do “physical processes” also include the role of biota in the fragmentation of plastics? Animals intentionally or unintentionally grazing on plastics results in fragmentation.
- Thanks for this comment and a good question as well. Didn’t think about the role of biota for physical fragmentation of plastics. I think, so far this point wasn’t also mentioned in current literature and is therefore perhaps not suitable for our introduction by discussing this here.
Line 83: “In the Lahn River” should be “in the Lahn River”
- [will be implemented during revision]
There are several instances where “River” is not capitalised in the river name. This needs to be corrected in all instances through the manuscript.
- [will be implemented during revision]
Line 122: “enters” should be ‘enter’.
- [will be implemented during revision]
Spaces before degree symbols should be removed. E.g. Line 134.
- [will be implemented during revision]
Spaces after ‘<’ or ‘>’ signs should be removed. E.g. Line 156 & 160.
- [will be implemented during revision]
Line 167: “result” should be “results”.
- [will be implemented during revision]
Line 173: There is something wrong with the reference.
- Thanks for mentioning this point, but we couldn’t find an error here. The reference in l. 173 is correct with “(Weihrauch, 2019)”
Line 177: “back swamp” should be “backswamp”.
- [will be implemented during revision]
Line 199: it would be good if more information could be provided about the material the bags are made from. The definition of “bioplastic” can be variable, and misleading. For example, it can be used to describe a plastic made from a feedstock of biological origin (e.g. starch, not fossil fuel) however be a traditional polymer like PE, PVC; or alternatively it can be used to describe biodegradability. Care is taken in the methods to control for the presence of it in the sample by taking spectra, but I feel it would be good here to include a bit more information and make it clear what the polymer is.
- Special thanks for raising our attention to this point. The bags are made of corn starch and thus have a biological origin and are also “biodegradability” according EU norm (EN-13432). The exact manufacturing process is unfortunately not disclosed by the manufacturer (BioFutura B.V.,), as it is a proprietary process (see website: https://www.biofutura.com/de/rohstoffe/mater-bi)
- We will change the information here and add the biological origin and biodegradability.
Line 202-3: Suggest rewording the sentence “Visible plastics fragments were collected on a 20 m2 area around the drill points by walking straight lines with two persons (four-eyes-principle),……” to “Visible plastic fragments were collected in a 20 m2 area around the drill points, by two people walking straight lines in parallel (four-eyes-principle),……”.
- [will be implemented during revision]
It would also be good to give dimensions of how the 20 m2 area was obtained to show it was an equally distributed area around the central drill point.
- Thanks for this advice. We will add the following description within the revised version: “Surface sampling area was prepared by means of a tape measure and measuring rods, while a rectangle with the extension of 4x5 m was measured and marked around the centre (drill point)”
Line 212: Provide detail on what containers they were in for drying, or whether they were in the open bags.
- Details will be added: Samples were in open bioplastic bags within the closed drying chamber.
Line 202: I am not familiar with these standards and am unable to access them, however I feel that “loss of ignition” is not the right wording. Should it be combustion at 550°C until steady weight obtained?
- Thanks for this note. We think that “Loss of ignition” is the correct wording, as it’s the most widely used method for measuring soil organic matter (SOM) content.
Line 242: I am unfamiliar with ‘rim jars’. More detail needs to be given as to what they are made from.
- Details will be added: “… stored in rim jars made of glass with a PE-cap”
Line 254: space needed after Ø = ‘Ø 47 mm’ and ‘Ø 90 mm’
- [will be implemented during revision]
Line 258: It is not clear what “sprayer” is. Can more detail be given or a clear description of what this is?
- Thanks for this demand. May “sprayer” is a misleading wording. We used a “spray bottle” for this process and will change the wording to “spray bottle”.
Line 281: reword ‘for safety reasons’. I don’t think you mean it in the sense it reads. You are doing it to ensure that if any fragments of the bag have entered the sample you are able to account for this contamination and remove it from the analysis.
- Good advice, here. We will change the wording to “for contamination control through potential abrasion from the sample bags used”
Line 282: The use of cotton lab coats specifically avoids contamination of the air by synthetic fibres that would potentially come off synthetic lab coats. Contamination would still be still but they’d be cotton, and therefore not included in your analysis.
- Thanks for mentioning this point. We will add “… to avoid air contamination by synthetic fibres”
Line 357: Can you please provide information about what is included in your definition of “rubber” as this is a class or group of polymers, not a single type and it includes both fully synthetic and semi-synthetic alternatives. This will also influence the EPO ages, with only the natural rubber being used in 1820s, and first synthetic rubber developed in 1910, and another alternative in the 1930s.
- Thanks for raising our attention to this point. The identification class “rubber” includes both fully synthetic rubbers as well as natural rubbers, based on the entries within the OpenSpecy FTIR spectra database. As we were already aware of this, the respective EPO ages were differentiated between natural and synthetic rubbers. We will ad the respective information within the method section.
Line 366: The following sentence needs rewording “The plastics contain of HDPE …” Is it means to read “The plastics consists of HDPE….” Or “The plastics found included HDPE….”?
- Your right. The correct wording must be “The plastic consist of HDPE…” [will be implemented during revision]
Line 368: As this is intended for an intentional audience it may be best to avoid colloquial naming of things. Could ‘fries fork’ be changes to ‘plastic fork’, and ‘DIY store shed’ seems like a very broad category or description. Is there something more specific that can be said?
- Thank you for this important recommendation. We will change to “plastic fork” and “DIY store shed for flowers”.
Line: 390: ‘, at depths below 100 cm.’ is confusing. Is it meant that they are deeper, as in more >100 cm; or shallower? Please check and confirm to ensure it is less ambiguous.
- We will rephrase this part of the sentence to “deeper than 100 cm”.
Line 426: It is not clear why ‘Meso- and single macroplastic particles….’ Is written like this. Can this be made clear, or changed to remove the ‘single’?
- We will change the sentence to “Larger plastic particles within the macro- and mesoplastic size range, only occur isolated within topsoils (plough horizons) of sites..”
Line 448: What depths do “From 45.98% to 62.5%” refer to?
- Thanks for this comment. We will change to “from 45.98% in upper (0–50 cm) to 62.5% in lower (50–200 cm) soil layers.”
Line 489: Change “Figure 8a and Figure 8b” to “Figure 8”.
- [will be implemented during revision]
Line 536: Is this mislabelled? “Table T1”. Should it be S1?
- Yes it is mislabelled. [will be implemented during revision]
Line 555: Change “incipient alteration surface structure” to “incipient alteration of surface structure”.
- [will be implemented during revision]
Line 589: ‘(Table)’ is this meant to be here?
- Table 2 is meant here. We will ad the table number.
Line 682: Add “River” after “Nidda”. Other occurrences like this may occur elsewhere in the document.
- [will be implemented during revision]
Figures
Figure 1: Add in W (wastewater treatment plants) and ID (industrial discharge) in to the figure legend.
- [will be implemented during revision]
Figure 2: a) Both resin and rubber are broad categories of polymer. You should explain somewhere what is included within these definitions. See previous note about ‘rubber’.
- Thanks again for this comment. We have answered already on the “rubber” note above and will add the additional information for “resins” within the method section.
The scale bars and associated distance in the photos need to be larger as they are currently impossible to read.
- [will be implemented during revision]
- c) It may be advantageous to add in labels of Course and Fine soil to the figure to make it clearer what the reader is looking at.
- [will be implemented during revision]
Figure 3: Adding Titles to the graphs would make it clearer what the reader is looking at. a) & b) both the y and x-axis labels need to be centred on the axes. Legend: Suggest rewording “(with transect site location and river km) “ to “(with transect site location and river length (km)). This also needs to be changed in the axes labels. Add text to “dotted boxes indicate anthropogenic influence” = “dotted boxes indicate sites exposed to anthropogenic influences”
- Thank you for this kind recommendations. We will change the figure according them!
Figure 4: y-axis label: the ‘(-)’ needs to be removed.
- [will be implemented during revision]
Figure 5: x-axes labels: the ‘(-)’ needs to be removed from EF(-) and PLI(-).
- [will be implemented during revision]
Figure 6: it is not clear why for both a) and b) why the smaller particles are also presented as an additional graph inset as they are clearly presented in the total particles graphs. By removing them from both a and b the graphs showing all sized particles can widened.
- Thanks for this comment. Within Figure 6 the following problem occurred: When displaying the whole particle size, differences in the smaller size classes are hardly noticeable. Therefore the additional graphs for the smaller range. We will test the display again and revise it if reasonable.
Figure 7: the ‘(-)’ needs to be removed from PLI(-). Can the circles for the floodplain positions be made larger so it is easier to see the different colours?
- [will be implemented during revision]
Figure 8: y-axis on both a and b need to be flipped around 180°. the ‘(-)’ needs to be removed from EF(-).
- We will remove the “(-). Unfortunately, I do not understand why the y-axis should be flipped? Maybe you can give us a suggestion here?
Tables:
Table S1: a different foodnote symbol for ‘Shape’ needs to be used as you also use ‘a’ to refer to a picture.
- [will be implemented during revision]
Table 2: It is not immediately clear what “low, moderate and high” are referring to. If they are referring to the Indices is it possible to change the format and have “low, moderate and high” right justified in the column?
- Of course this will be possible. We will change the format according your advice.
Tables A1 and A2: are referred to a lot. Could they go in the main body of the article rather than as an Appendix?
- Thank you for this suggestion. We think that both tables are too large and would disturb the reading flow of the paper, as they mainly contain important details, but are not necessary for the first access. Therefore, we would like to keep them as an appendix
Table A1: It was not clear to me why are some of the words in bold.
- Names of reference soil groups according WRB 2015 are in bold. We will add an explanation in the note at the end of the table
Citation: https://doi.org/10.5194/soil-2022-1-AC2
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RC2: 'Comment on soil-2022-1', Anonymous Referee #2, 07 Mar 2022
This study quantifies plastic particle abundance and heavy metal enrichment at four floodplain sites along a river in Germany in the soil depth profile down to 2 m and along a transect of increasing distance to the stream bed. No clear correlations were found between soil parameters or heavy metal enrichment and plastic abundance. Also, a clear pattern along the transect and along the river course was not observed. As seen in a few other studies, microplastic particles showed higher mean abundances in the upper horizon and decreased rapidly with depth. Interestingly, high accumulations of microplastic particles were found in different spots along the vertical profiles.
This is a very interesting study that expands sampling into a three-dimensional space and the results are valuable for publication. The results are displayed in a very precise and informative way with a lot of attention to detail. I think there is some improvement needed before publication though, mainly to organize the thoughts and intentions into a better structure.
I am somewhat missing clear objectives of the study. Why were plastics and heavy metals analyzed in the samples? What is the hypothesis that should be tested – what kind of relationship between the two metrics did the authors assume? What patterns were expected to be found in this study, along the river course, along the transect of increasing distance from the river, and along the soil profile? And why...? I appreciate that microplastic studies often need to have an exploratory nature due to the many contradicting results, but still, I think it would improve the paper if these points were addressed in a more coherent and clear way.
Introduction is rather lengthy. I suggest to eliminate information that is rather general, refer to the many reviews that are available, and focus here on the points that lead to your research question (spatial distribution, flood plain dynamics, connection to heavy metal contamination).
The conclusions are rather general, but not referring to the results.Often the language is a little unclear/ambiguous. E.g. the term spatial distribution is used for different meanings: along the river, across the river transect, or along the soil profile. E.g., line 706: What is meant here by spatial distribution? The variation along the soil profile? Maybe it would be clearer to refer to each of the three dimensions investigated.
Specific points:
- Abstract: Try avoiding acronyms, explain enrichment factor
- Line 17: lower range of known concentrations where? In soils, floodplain soils, …? This sentence does not make sense if you haven’t read the manuscript.
- Lines 21 f: Why do we need info on spatial microplastic distribution for floodplain management?
- Line 30: correct typo “scapes”
- Line 38: check wording of this sentence
- Lines 52-59: one might expect that this paragraph leads to a hypothesis of an objective, something like: “Therefore, we wanted to find out…”.
- Lines 70-72: not sure I understand the differentiation here in small-scale inputs and spacious inputs
- Line 75: “plastic abundance” instead of “spatial distribution of plastics”?
- Lines 82-92: this should be part of the discussion: putting own findings into context with literature
- Lines 97 f: not sure this is the correct reference. Also, I don't agree with this statement. Altered soil functions have only been found in the lab so far.
- Lines 101-108: is this a goal of this study? The temporal aspect is not really investigated here. Or can the sampled soil strata be somehow related to the time the sediment has been deposited?
- Line 112: replace “clear sequence” with “gradient”
- Lines 115 f: this sounds like a weak argument for the study. is it possible to find a stronger point?
- Lines 119 ff: These aims and hypotheses are coming somewhat “out of the blue”. It would be nice if the introduction would be more funneled from the general knowledge via the knowledge gaps leading to the research goal.
- Lines 123 f: the terms spatial relationship and spatial correlation seem a bit misleading, since one would expect that spatial analyses are being conducted that are commonly defined as (statistical) analyses of georeferenced data.
- 2.1 Study area: This can be shortened to the points relevant for the aim of the study
- Lines 169 f: “geospatial approach” - this term is used here in a somewhat blurry way. Also, why does a landscape consist of soilscapes? The term soilscapes should be defined at first mention.
- Line 235: replace “as control” with “for correction”?
- Past and present tense are used inconsistently (e.g. in results section) – please correct throughout the manuscript.
- Terms are used inconsistently: plastic concentration, plastic load: I’d suggest to change these to “plastic particle abundance”, since the results are based on number and concentration is more commonly used for masses.
- The order of subsections is vice versa in the methods and results chapter.
- Line 318: replace “organic (non-polymeric) with “natural organic matter”?
- Figure 2: Please explain the box plots (what quantiles are shown?) and increase the fonts. Replace “particle type” with “particle shape”
- Line 404: Mean values averaged over what?
- Figure 3: I am not sure I understand the meaning of cumulative sums. I suppose these are the abundances shown in table 1 summed up over the three depths. But that would mean that 5, 2, and 3 samples are summed up for the 0-50 cm, 50-100 cm, and 100-200 cm, respectively, so the upper stratum is overrepresented. Wouldn’t it be more intuitive to show mean abundance across each profile to compare the sites?
- Figure 6: I like the panel a a lot. But it might be clearer if the zoomed-in plots were removed. Also, consider to only keep 6a in the main paper. Instead of 6b, I'd rather like to see soil depth plotted against EPO age. Also, the gap in the size range from 2-10 mm strikes me. Is this an artifact of the analysis method? It should be discussed.
- Lines 476 f: Please delete: “with individual increased concentrations in deeper soil layers”
- Lines 494 f: replace “association” with “correlation”
- Lines 496 f, 506 f: according to figure 5a, the relationship between soil depth and number of MP is not linear, or only linear down to approx. 60 cm and then rather constant. Could this be included in the model?
- In the discussion, where the results are put into context with literature, I am missing the point that in general, particle abundances in different locations are hard, if not impossible to compare, since size and mass is not accounted for that can vary greatly depending on the origin, time of accumulation etc. of the plastics.
- Line 533: But Piehl et al found lower abundances
- Lines 566 ff: consider citing Koutnik et al. (2021) for a review of MP abundance in different soil types
- Lines 659 f: high levels of plastics in topsoil cannot explain accumulation in topsoils if the amount of translocated plastic is not known
- Line 669: “The direction of movement of the plastic deposits must therefore originate from the river” – why? What about agricultural activities, as stated earlier?
- Lines 675 f: but what about bioturbation, translocation to deeper soil?
- Lines 682-690: this is in logical contrast to the theory of sediment dating.
- Line 698: delete “spatial or statistical”
- Lines 700 f: meaning the emission of heavy metals from plastics?
- Lines 701 ff: I’m not sure what is intended here. If heavy metal as additives in plastics likely play no role, why should it be further studied?
- Line 707: What is the meaning of “significant outliers”?
- Line 723: What is the meaning of “(functions of some plastic pieces still recognizable)”?
- Line 734: Consider replacing “plastic content in floodplain deposits” with “the microplastic abundances found in this study”
- Lines 744 ff: I am not sure I understand this. In the discussion the authors state that the MP abundance even in the upper soil layers is low compared to e.g. agricultural soils in literature. So, it seems that MP are not a prominent issue in flood plains, as other pollutants like heavy metals.
- Line 754: a risk assessment usually does not follow legislation, but legislation follows an assumed or assessed risk
- Lines 768-770: this is true and a well-known problem, but it doesn't relate to the study.
- Lines 771-773: I think this is a little far-fetched in light of the comparably low levels of plastic contamination found in this study, and the positive effects on biodiversity and flood mitigation of river bed restoration.
- Lines 774-776: Also, this statement is general and not concluded from the study results.References
Koutnik, Vera S., Jamie Leonard, Sarah Alkidim, Francesca J. DePrima, Sujith Ravi, Eric M. V. Hoek, and Sanjay K. Mohanty
2021 Distribution of microplastics in soil and freshwater environments: Global analysis and framework for transport modeling. Environmental Pollution, 274:116552.Citation: https://doi.org/10.5194/soil-2022-1-RC2 -
AC3: 'Reply on RC2', Collin J. Weber, 08 Mar 2022
Reply on RC2 (soil-2022-1)
Dear Reviewer 2 (RC2),
Thank you for your effort to read our manuscript! Thanks for your recommendations and the general commendation of our work. We would like to respond to some of the comments below and clarify open questions one after the other. The mentioned typos and other minor corrections that do not require any further comment, were indicated by "[will be implemented during revision]"
Thank for your support. Best regards,
Collin (on behalf of all co-authors)
General comments:
- Thank your for raising our attention to the important points mentioned within your general comments. Regarding the introduction of our manuscript, we will formulate a clear objective within the revised version of our manuscript and will shorten the introduction consequently. For this reason we will follow your recommendations about the introductions focus also in accordance with the comments of RC1 as well as CC1.
Specific points:
Abstract: Try avoiding acronyms, explain enrichment factor
- "[will be implemented during revision]
Line 17: lower range of known concentrations where? In soils, floodplain soils, …? This sentence does not make sense if you haven’t read the manuscript.
- We will add “lower range of known concentrations within floodplain soils, …”
Lines 21 f: Why do we need info on spatial microplastic distribution for floodplain management?
- In contrast to so far “recognized” pollutants, microplastics are not considered within measures for floodplain management. The found effects of direct anthropogenic impacts, raising questions about future floodplain management. We may add “… for future floodplain management, regarding contamination assessments” to illustrate this background.
Line 30: correct typo “scapes”
- "[will be implemented during revision]
Line 38: check wording of this sentence
- Thanks for this hint! We have checked the wording and will change it within the revision.
Lines 52-59: one might expect that this paragraph leads to a hypothesis of an objective, something like: “Therefore, we wanted to find out…”.
- You are right, that the “metal” part of this paragraph suggests an objective. Within the revised introduction, we will add an objective at the end of the shortened paragraph.
Lines 70-72: not sure I understand the differentiation here in small-scale inputs and spacious inputs
- We will change the wording here from small-scale inputs to “point or local inputs”. Thus, the difference between point inputs (for example, input on a specific field) versus input from a flood (widespread) should become clearer
Line 75: “plastic abundance” instead of “spatial distribution of plastics”?
- "[will be implemented during revision]
Lines 82-92: this should be part of the discussion: putting own findings into context with literature
- The own findings compared here with other studies clearly belong to the previous preliminary work on microplastics in floodplain soils and should therefore also be mentioned here. They are taken up again in the discussion and compared with the findings of the study presented here.
Lines 97 f: not sure this is the correct reference. Also, I don't agree with this statement. Altered soil functions have only been found in the lab so far.
- It’s the correct reference here. We will formulate the statement less strongly by referring to "may affected by plastic particles in soils" and add an additional review reference here.
Lines 101-108: is this a goal of this study? The temporal aspect is not really investigated here. Or can the sampled soil strata be somehow related to the time the sediment has been deposited?
- No, the temporal component is only considered based on EPO-ages, but is no general objective of the study. The information about the temporal aspects, will be shortened and given just as side information within the revised introduction.
Line 112: replace “clear sequence” with “gradient”
- "[will be implemented during revision]
Lines 115 f: this sounds like a weak argument for the study. is it possible to find a stronger point?
- We will change the argument to “As the sink function of floodplains and depositional dynamics are already well studied for so-far recognized contaminants like metals of interest, a combined consideration of recognized metals and the new contaminant plastics, will be performed.”
Lines 119 ff: These aims and hypotheses are coming somewhat “out of the blue”. It would be nice if the introduction would be more funneled from the general knowledge via the knowledge gaps leading to the research goal.
- Thank you for this recommendation. As stated before, we will revise the introduction accordingly and refocus on our objectives, based on the knowledge gaps.
Lines 123 f: the terms spatial relationship and spatial correlation seem a bit misleading, since one would expect that spatial analyses are being conducted that are commonly defined as (statistical) analyses of georeferenced data.
- Thanks for this suggestion. Maybe the wording “correlation” leads to the assumption of a geostatistical analysis. We will change the wording here to “… investigating spatial similarities and differences between…”
2.1 Study area: This can be shortened to the points relevant for the aim of the study- "[will be implemented during revision]
Lines 169 f: “geospatial approach” - this term is used here in a somewhat blurry way. Also, why does a landscape consist of soilscapes? The term soilscapes should be defined at first mention.
- The term “geospatial approach” with regard to (micro-)plastics research was introduced by Weber et al. (2020) https://doi.org/10.1002/ldr.3676; The core idea of this approach is a study and sampling design oriented to respective land- and/or soilscape features. We will cite the reference here and will define the term “soilscape” after its first mention within the introduction.
Line 235: replace “as control” with “for correction”?
- "[will be implemented during revision]
Past and present tense are used inconsistently (e.g. in results section) – please correct throughout the manuscript.
- "[will be implemented during revision]
Terms are used inconsistently: plastic concentration, plastic load: I’d suggest to change these to “plastic particle abundance”, since the results are based on number and concentration is more commonly used for masses.
- Thanks for this recommendation. We will consistently use the term plastic load, as the results are reported as a number per kilogram soil dry weight.
The order of subsections is vice versa in the methods and results chapter.
- The order of subsections within the methods is oriented from “known” standardized methods (soil and metal analysis) to non-standardized (micro-)plastic analyses. We would like to keep the order of subsections within the results.
Line 318: replace “organic (non-polymeric) with “natural organic matter”?
- "[will be implemented during revision]
Figure 2: Please explain the box plots (what quantiles are shown?) and increase the fonts. Replace “particle type” with “particle shape”
- We think, that a standard boxplot didn’t need an explanation of quantiles as these are generally known. We would like to keep the term “particle type” as particle shapes are often associated with properties such as edges (e.g., broken)
Line 404: Mean values averaged over what?
- We are not sure where this question is aimed? Are the proximal and distal floodplain sites meant? If yes we will add: “… mean values of proximal (near-channel) floodplain sites to mean values of distal (remote-channel)…”
Figure 3: I am not sure I understand the meaning of cumulative sums. I suppose these are the abundances shown in table 1 summed up over the three depths. But that would mean that 5, 2, and 3 samples are summed up for the 0-50 cm, 50-100 cm, and 100-200 cm, respectively, so the upper stratum is overrepresented. Wouldn’t it be more intuitive to show mean abundance across each profile to compare the sites?
- Thank you for raising this question. The cumulative sums are the abundance of particles per soil section summed up over the three depths, as you correctly stated. As the mean abundance (vertical) is already displayed within Figure 5a, we want to illustrate the complete plastic abundance over the entire soil column (0-2 m) per sampling point in Figure 3. We think that both the significantly lower contents in subsoils and the sites with the highest abundance are made clear by this type of illustration.
Figure 6: I like the panel a a lot. But it might be clearer if the zoomed-in plots were removed. Also, consider to only keep 6a in the main paper. Instead of 6b, I'd rather like to see soil depth plotted against EPO age. Also, the gap in the size range from 2-10 mm strikes me. Is this an artifact of the analysis method? It should be discussed.
- Glad you like the illustration and thanks for the suggestions. We will remove the zoomed-in plots, also in accordance with RC1 comments, even if it becomes more difficult to track the lower size range. The gap within 2-10 mm range, is not an artefact of the analysis. It is due to the fact that many macroplastic particles were introduced into topsoil at two locations. We will add a brief discussion on this point.
Lines 476 f: Please delete: “with individual increased concentrations in deeper soil layers”
- "[will be implemented during revision]
Lines 494 f: replace “association” with “correlation”
- "[will be implemented during revision]
Lines 496 f, 506 f: according to figure 5a, the relationship between soil depth and number of MP is not linear, or only linear down to approx. 60 cm and then rather constant. Could this be included in the model?
- Thanks for this suggestion. As stated within l. 490-492 we found a significant regression between plastic loads and soil depth, just within the linear regression model. As we would like to keep the model as simple as possible, we think that a linear examination is sufficient here.
In the discussion, where the results are put into context with literature, I am missing the point that in general, particle abundances in different locations are hard, if not impossible to compare, since size and mass is not accounted for that can vary greatly depending on the origin, time of accumulation etc. of the plastics.
- Special thanks to raising our opinion again to this important point. In general, we agree with the statement that comparisons of the abundance is always limited. but not impossible, as long as spatial relationships but also differences are considered in the interpretation of the obtained data. We will add an respective sentence within the discussion to state the given limitations of such spatial approaches clearly.
Line 533: But Piehl et al found lower abundances
- Yes this is correct. The reference was set incorrectly here and will be removed.
Lines 566 ff: consider citing Koutnik et al. (2021) for a review of MP abundance in different soil types
- Special thanks for this nice literature hint. As CC1 already mentioned that we should reduce our reference list, we will add this reference and argue here shortened with reference to the review.
Lines 659 f: high levels of plastics in topsoil cannot explain accumulation in topsoils if the amount of translocated plastic is not known
- Yes, this is correct. We will delete the second half of the sentence and will just argue with the studies containing data on plastics in subsoils.
Line 669: “The direction of movement of the plastic deposits must therefore originate from the river” – why? What about agricultural activities, as stated earlier?
- Thanks for mentioning this point. The statement seems to be a little one-sided here. We will change to “Beside agricultural activities contributing to plastic deposits, plastic can originate from the river and deposited via flood water since further, laminar sources are excludable”
Lines 675 f: but what about bioturbation, translocation to deeper soil?
- Yes of course, this point is important and already discussed within Weber and Lechthaler (2021). The idea of an “general marker” assumes that a significant increase in content is indicative of deposition after 1950. Of course, parts of the plastic content can be displaced, as is possible with other dating methods, e.g., Pb-Cs. However, we know only little about the mass of "mobile" plastic in soils, especially for comparatively “large” microplastics. We will add a statement on the limitations due to relocation processes with regard to particle sizes.
Lines 682-690: this is in logical contrast to the theory of sediment dating.
- We do not think, that a translocation of plastic particles within soils or sediments, is contrary to dating theory. Each environmental archive (which could be dated, regardless of the used method) is influenced by dynamic processes (e.g. vertical displacements in the pore space or bioturbation). Within the results of this study, we identified a frequent accumulation within upper soil sections and significant lower loads within deeper soils, assuming that the deeper-plastics have to be relocated. Nevertheless, if you look at the total contents or the peaks (as in radionuclide dating methods), an estimation is still possible.
Line 698: delete “spatial or statistical”
- "[will be implemented during revision]
Lines 700 f: meaning the emission of heavy metals from plastics?
- Yes, thanks for this question. We will change the wording here to make this point more comprehensible.
Lines 701 ff: I’m not sure what is intended here. If heavy metal as additives in plastics likely play no role, why should it be further studied?
- This is a misconception here. Based on out method approach we were not able to identify the influence of heavy metal emissions from plastics, may due to the comparatively slight plastic loads found or the already existing geogenic as well as anthropogenic contamination of the floodplain with metals. Therefore, further studies should focus on the geochemical interactions. We will state this point clear within lines 700–704 within the revised version.
Line 707: What is the meaning of “significant outliers”?
- We will change the wording to “high accumulations” here.
Line 723: What is the meaning of “(functions of some plastic pieces still recognizable)”?
- Thanks for this question. This term means that the origin and the function of some plastic pieces is still recognizable. We will change the wording to “origin … still recognizable”
Line 734: Consider replacing “plastic content in floodplain deposits” with “the microplastic abundances found in this study”
- "[will be implemented during revision]
Lines 744 ff: I am not sure I understand this. In the discussion the authors state that the MP abundance even in the upper soil layers is low compared to e.g. agricultural soils in literature. So, it seems that MP are not a prominent issue in flood plains, as other pollutants like heavy metals.
- We believe that at this early stage of research it is not possible to say whether microplastics are a prominent issue in floodplains. Basically, the sole occurrence is to be considered negative, regardless of the quantity. As stated within the manuscript (methods, discussion), our method approach is limited through the analysed plastic size range. As many other studies have also analysed smaller particles, the contents are difficult to compare and it can be assumed that higher contents also occur in floodplain soils if smaller particles are included in the analysis.
Line 754: a risk assessment usually does not follow legislation, but legislation follows an assumed or assessed risk
- Risk assessments by monitoring programs can only be carried out if there are limit values (by legislation). Of course, an assumed or assessed risk must be given for legislation as well, but the simple proof of risk is not provided by area-based risk assessments.
Lines 768-770: this is true and a well-known problem, but it doesn't relate to the study.
- This point will be deleted within the revised manuscript.
Lines 771-773: I think this is a little far-fetched in light of the comparably low levels of plastic contamination found in this study, and the positive effects on biodiversity and flood mitigation of river bed restoration.
- As stated before, our study does not claim to provide a complete analysis of plastic content in relation to size. However, the presence of plastic, regardless of loads, should be cause for the influence of, for example, renaturation on the deposition of plastic. Here we are not referring to restrictions on restoration, but to considerations of how to deal with it. We will revise the point and make our implications more explicit
Lines 774-776: Also, this statement is general and not concluded from the study results.
- This point will be deleted within the revised manuscript
Citation: https://doi.org/10.5194/soil-2022-1-AC3
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AC3: 'Reply on RC2', Collin J. Weber, 08 Mar 2022
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EC1: 'Comment on soil-2022-1', Maria Jesus Gutierrez Gines, 10 Mar 2022
I would like to thank the two reviewers and Zacharias Steinmetz for your valuable review of this manuscript.
Given that the reviewers did not identify any major flaws and that they consider the methodology and results appropriate, I recommend to make the major and minor revisions highlighted by the reviewers, in order to make the text more concise and to include the information that is missing.
Best wishes,
Maria Gutierrez
Topical Editor
Citation: https://doi.org/10.5194/soil-2022-1-EC1
Status: closed
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CC1: 'Comment on soil-2022-1', Zacharias Steinmetz, 18 Feb 2022
Dear authors,
Thank you for sharing this very interesting and well made contribution! While reading the manuscript, some minor issues attracted my attention and I didn’t want to keep them to myself. Feel free to consider them for a future revision.
Title
The term “heavy metals” is used less and less since the density of a metal doesn’t tell you much about its environmental relevance. I’d suggest to term them “metals of interest” instead.
Introduction
I would opt for a more concise introduction that is tailored more towards your objective and research questions. I find it rather lengthy and narrative in some parts. But this is of course a matter of style and personal perception.
Furthermore: were all findings reported in L83–87 performed in NaCl solution? Which ecosystem services did you have in mind (L98)? Why isn’t there a hypothesis for bullet 3 (L123f)?
Methods
As for the introduction, I would suggest to prune section 2.1.
Though it’s quite subjective, I always find it more intuitive to number sites and abbreviate sampling points with letters. In your case, this would also help to see right away whether the location was upstream or downstream (L185 and L191).
I wonder why you didn’t use paper bags (L198f). Any specific reasons?
I haven’t read the term “fine earth“ yet and always used “fine soil” (L215). Don’t know if one is more correct than the other.
Was there any protocol or criteria that made you only “partly“ analyze plastics under the stereomicroscope (L238ff)? If so, which were analyzed this way?
The “rim jars” described in L242 were made of glass?
“Cowger et al., 2020” should be Cowger et al., 2021 and does not appear in the reference list (L296). In addition, r2 and R2 are used inconsistently.
Please be aware that non-parametric tests like Wilcoxon rank sum and Kruskal-Wallis tests do still assume equal variances. As you say, your data showed “significant differences in variance by group” (L323f). In such cases, you may consider a Welch test instead (which again assumes normality though). I would also suggest to inspect normality and homoscedasticity of the residuals rather than the data. This can as well be done graphically via QQ and residual vs. fitted plots, which is often more accurate than Shapiro and Levene tests. I tried some things on my own using the data published on figshare but struggled to interpret it correctly. You may also consider to add your R scripts to that repository. This would make it easier to follow your data analysis steps.
Results and Discussion
Well done! I really enjoyed reading those parts, especially sections 3.3.2 and 4.3. You may consider cutting some long sentences though. Also, you sometimes put question marks behind a sentence although it’s not a direct question but a relative clause.
References
You cite more than 100 papers which is already half a literature review. You may consider reducing the number of references in favor of some review papers or one exemplary study, if possible.
Hope this helps. All the best,
ZachariasCitation: https://doi.org/10.5194/soil-2022-1-CC1 -
AC1: 'Reply on CC1', Collin J. Weber, 22 Feb 2022
Dear Zacharias,
thank you for using the interactive review option and thanks for your very helpful comments! We would be happy to include your comments in a future revision. A few comments in response here:
Title
Thanks for your suggestion about the term “metals of interest”. It is true that the term "heavy metal" is too unspecific, although it is still used so often. We will use the new term in place of the old one in our revision, including title and main text.
Introduction
It is true that the introduction is too wordy in some places. We will shorten this and adjust more directly in the revision.
Regarding your questions:
The findings reported in L83-87 were performed purely visual without density separation (Weber and Opp, 2020) and in NaCl (Weber et al. 2021).
We had general ecosystems services like provisioning (e.g., food production), regulating (e.g., water purification and flood control) and cultural (e.g., non-material benefits like archive functions) services in mind.
With regard to bullet 3 (3th issue) we had an hypothesis in mind from our previous research. Maybe we change the wording to “Spatial relationships … proving the hypothesis of spatially distinct contamination patterns due to different contamination periods”
Methods
Thanks for your suggestions. We will shorten the section and make it more precise. With regard to the sampling site naming or numbering, we think that’s a subjective point, as you already mentioned. We will keep the original here.
- 198 We used corn starch bioplastic bags instead of paper bags since in our experience they are more durable with moist samples.
- 215 The term “fine-earth” is common within the soil description according to FAO guidelines. I don't know which one is more correct, but I have always referred to the FAO guidelines.
- 238 Yes, there was a specific protocol and thanks for this reminder! Sample material with a size (sieved) between >2 mm and <5 mm was inspected via stereomicroscope, whereas sample material <5mm was inspected without. We will add these criteria in our revision.
- 242 Yes, the rim jars were made of glass. We will add “glass rim jars”.
Thanks for your comments on the correct Cowger citation as well as the consistent use of r². We will adjust these errors.
Special thanks for your statistics suggestions! We have done some graphic analysis of the residuals as well. We will check residuen again. Thanks also for the hint to share the R code. With regard to the non-parametric tests, we made sure that the variances of the comparison groups did not differ significantly, because we could not perform a Welch test due to the lack of normality.
Results and Discussion
Thanks for the commendation and glad you enjoyed reading it! We will consider your comment on the length of some sentences and shorten where possible.
References
Thanks again for this remark. Your idea of favouring some specific review papers and exemplary studies sounds fine. We will include this in the revision.
Really, big thank for your support. All the best,
Collin (on behalf of all co-authors)
Citation: https://doi.org/10.5194/soil-2022-1-AC1
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AC1: 'Reply on CC1', Collin J. Weber, 22 Feb 2022
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RC1: 'Comment on soil-2022-1', Anonymous Referee #1, 02 Mar 2022
- This was a very interesting study, and very well thought out and presented in the manuscript. It was clear and pleasant reading.
- Although the introduction was very informative it may benefit from being a little shorter.
Specific comments:
- I do not think it is necessary to refer to “(micro-)plastics” as they are in lines 11 and 12. Reference could just be made to “plastics” which would encompass plastics particles/pieces of all sizes.
- Line 37: Do “physical processes” also include the role of biota in the fragmentation of plastics? Animals intentionally or unintentionally grazing on plastics results in fragmentation.
- Line 83: “In the Lahn River” should be “in the Lahn River”
- There are several instances where “River” is not capitalised in the river name. This needs to be corrected in all instances through the manuscript.
- Line 122: “enters” should be ‘enter’.
- Spaces before degree symbols should be removed. E.g. Line 134.
- Spaces after ‘<’ or ‘>’ signs should be removed. E.g. Line 156 & 160.
- Line 167: “result” should be “results”.
- Line 173: There is something wrong with the reference.
- Line 177: “back swamp” should be “backswamp”.
- Line 199: it would be good if more information could be provided about the material the bags are made from. The definition of “bioplastic” can be variable, and misleading. For example, it can be used to describe a plastic made from a feedstock of biological origin (e.g. starch, not fossil fuel) however be a traditional polymer like PE, PVC; or alternatively it can be used to describe biodegradability. Care is taken in the methods to control for the presence of it in the sample by taking spectra, but I feel it would be good here to include a bit more information and make it clear what the polymer is.
- Line 202-3: Suggest rewording the sentence “Visible plastics fragments were collected on a 20 m2 area around the drill points by walking straight lines with two persons (four-eyes-principle),……” to “Visible plastic fragments were collected in a 20 m2 area around the drill points, by two people walking straight lines in parallel (four-eyes-principle),……”.
It would also be good to give dimensions of how the 20 m2 area was obtained to show it was an equally distributed area around the central drill point.
- Line 212: Provide detail on what containers they were in for drying, or whether they were in the open bags.
- Line 202: I am not familiar with these standards and am unable to access them, however I feel that “loss of ignition” is not the right wording. Should it be combustion at 550°C until steady weight obtained?
- Line 242: I am unfamiliar with ‘rim jars’. More detail needs to be given as to what they are made from.
- Line 254: space needed after Ø = ‘Ø 47 mm’ and ‘Ø 90 mm’
- Line 258: It is not clear what “sprayer” is. Can more detail be given or a clear description of what this is?
- Line 281: reword ‘for safety reasons’. I don’t think you mean it in the sense it reads. You are doing it to ensure that if any fragments of the bag have entered the sample you are able to account for this contamination and remove it from the analysis.
- Line 282: The use of cotton lab coats specifically avoids contamination of the air by synthetic fibres that would potentially come off synthetic lab coats. Contamination would still be still but they’d be cotton, and therefore not included in your analysis.
- Line 357: Can you please provide information about what is included in your definition of “rubber” as this is a class or group of polymers, not a single type and it includes both fully synthetic and semi-synthetic alternatives. This will also influence the EPO ages, with only the natural rubber being used in 1820s, and first synthetic rubber developed in 1910, and another alternative in the 1930s.
- Line 366: The following sentence needs rewording “The plastics contain of HDPE …” Is it means to read “The plastics consists of HDPE….” Or “The plastics found included HDPE….”?
- Line 368: As this is intended for an intentional audience it may be best to avoid colloquial naming of things. Could ‘fries fork’ be changes to ‘plastic fork’, and ‘DIY store shed’ seems like a very broad category or description. Is there something more specific that can be said?
- Line: 390: ‘, at depths below 100 cm.’ is confusing. Is it meant that they are deeper, as in more >100 cm; or shallower? Please check and confirm to ensure it is less ambiguous.
- Line 426: It is not clear why ‘Meso- and single macroplastic particles….’ Is written like this. Can this be made clear, or changed to remove the ‘single’?
- Line 448: What depths do “From 45.98% to 62.5%” refer to?
- Line 489: Change “Figure 8a and Figure 8b” to “Figure 8”.
- Line 536: Is this mislabelled? “Table T1”. Should it be S1?
- Line 555: Change “incipient alteration surface structure” to “incipient alteration of surface structure”.
- Line 589: ‘(Table)’ is this meant to be here?
- Line 682: Add “River” after “Nidda”. Other occurrences like this may occur elsewhere in the document.
FIGURE
Figure 1: Add in W (wastewater treatment plants) and ID (industrial discharge) in to the figure legend.
Figure 2: a) Both resin and rubber are broad categories of polymer. You should explain somewhere what is included within these definitions. See previous note about ‘rubber’.
The scale bars and associated distance in the photos need to be larger as they are currently impossible to read. c) It may be advantageous to add in labels of Course and Fine soil to the figure to make it clearer what the reader is looking at.
Figure 3: Adding Titles to the graphs would make it clearer what the reader is looking at. a) & b) both the y and x-axis labels need to be centred on the axes. Legend: Suggest rewording “(with transect site location and river km) “ to “(with transect site location and river length (km)). This also needs to be changed in the axes labels. Add text to “dotted boxes indicate anthropogenic influence” = “dotted boxes indicate sites exposed to anthropogenic influences”
Figure 4: y-axis label: the ‘(-)’ needs to be removed.
Figure 5: x-axes labels: the ‘(-)’ needs to be removed from EF(-) and PLI(-).
Figure 6: it is not clear why for both a) and b) why the smaller particles are also presented as an additional graph inset as they are clearly presented in the total particles graphs. By removing them from both a and b the graphs showing all sized particles can widened.
Figure 7: the ‘(-)’ needs to be removed from PLI(-). Can the circles for the floodplain positions be made larger so it is easier to see the different colours?
Figure 8: y-axis on both a and b need to be flipped around 180°. the ‘(-)’ needs to be removed from EF(-).
TABLES:
Table S1: a different foodnote symbol for ‘Shape’ needs to be used as you also use ‘a’ to refer to a picture.
Table 2: It is not immediately clear what “low, moderate and high” are referring to. If they are referring to the Indices is it possible to change the format and have “low, moderate and high” right justified in the column?
Tables A1 and A2: are referred to a lot. Could they go in the main body of the article rather than as an Appendix?
Table A1: It was not clear to me why are some of the words in bold.
Citation: https://doi.org/10.5194/soil-2022-1-RC1 -
AC2: 'Reply on RC1', Collin J. Weber, 02 Mar 2022
Dear Reviewer 1 (RC1),
thank you for your time to read and comment on our manuscript! Thanks for the basic commendation and glad you liked the manuscript. Furthermore, thanks for the close look and your mention of the minor typos, which we will certainly correct. We would like to respond to some of the comments below and clarify open questions. For comments that only need to be implemented as a correction, we have set "[will be implemented during revision]".
Thank for your support. All the best,
Collin (on behalf of all co-authors)
General comments:
Although the introduction was very informative it may benefit from being a little shorter.
- Thanks for this first general comment. We will shorten the introduction also in accordance with the community comment (CC1) to make it more precise.
Specific comments:
I do not think it is necessary to refer to “(micro-)plastics” as they are in lines 11 and 12. Reference could just be made to “plastics” which would encompass plastics particles/pieces of all sizes.
- [will be implemented during revision]
Line 37: Do “physical processes” also include the role of biota in the fragmentation of plastics? Animals intentionally or unintentionally grazing on plastics results in fragmentation.
- Thanks for this comment and a good question as well. Didn’t think about the role of biota for physical fragmentation of plastics. I think, so far this point wasn’t also mentioned in current literature and is therefore perhaps not suitable for our introduction by discussing this here.
Line 83: “In the Lahn River” should be “in the Lahn River”
- [will be implemented during revision]
There are several instances where “River” is not capitalised in the river name. This needs to be corrected in all instances through the manuscript.
- [will be implemented during revision]
Line 122: “enters” should be ‘enter’.
- [will be implemented during revision]
Spaces before degree symbols should be removed. E.g. Line 134.
- [will be implemented during revision]
Spaces after ‘<’ or ‘>’ signs should be removed. E.g. Line 156 & 160.
- [will be implemented during revision]
Line 167: “result” should be “results”.
- [will be implemented during revision]
Line 173: There is something wrong with the reference.
- Thanks for mentioning this point, but we couldn’t find an error here. The reference in l. 173 is correct with “(Weihrauch, 2019)”
Line 177: “back swamp” should be “backswamp”.
- [will be implemented during revision]
Line 199: it would be good if more information could be provided about the material the bags are made from. The definition of “bioplastic” can be variable, and misleading. For example, it can be used to describe a plastic made from a feedstock of biological origin (e.g. starch, not fossil fuel) however be a traditional polymer like PE, PVC; or alternatively it can be used to describe biodegradability. Care is taken in the methods to control for the presence of it in the sample by taking spectra, but I feel it would be good here to include a bit more information and make it clear what the polymer is.
- Special thanks for raising our attention to this point. The bags are made of corn starch and thus have a biological origin and are also “biodegradability” according EU norm (EN-13432). The exact manufacturing process is unfortunately not disclosed by the manufacturer (BioFutura B.V.,), as it is a proprietary process (see website: https://www.biofutura.com/de/rohstoffe/mater-bi)
- We will change the information here and add the biological origin and biodegradability.
Line 202-3: Suggest rewording the sentence “Visible plastics fragments were collected on a 20 m2 area around the drill points by walking straight lines with two persons (four-eyes-principle),……” to “Visible plastic fragments were collected in a 20 m2 area around the drill points, by two people walking straight lines in parallel (four-eyes-principle),……”.
- [will be implemented during revision]
It would also be good to give dimensions of how the 20 m2 area was obtained to show it was an equally distributed area around the central drill point.
- Thanks for this advice. We will add the following description within the revised version: “Surface sampling area was prepared by means of a tape measure and measuring rods, while a rectangle with the extension of 4x5 m was measured and marked around the centre (drill point)”
Line 212: Provide detail on what containers they were in for drying, or whether they were in the open bags.
- Details will be added: Samples were in open bioplastic bags within the closed drying chamber.
Line 202: I am not familiar with these standards and am unable to access them, however I feel that “loss of ignition” is not the right wording. Should it be combustion at 550°C until steady weight obtained?
- Thanks for this note. We think that “Loss of ignition” is the correct wording, as it’s the most widely used method for measuring soil organic matter (SOM) content.
Line 242: I am unfamiliar with ‘rim jars’. More detail needs to be given as to what they are made from.
- Details will be added: “… stored in rim jars made of glass with a PE-cap”
Line 254: space needed after Ø = ‘Ø 47 mm’ and ‘Ø 90 mm’
- [will be implemented during revision]
Line 258: It is not clear what “sprayer” is. Can more detail be given or a clear description of what this is?
- Thanks for this demand. May “sprayer” is a misleading wording. We used a “spray bottle” for this process and will change the wording to “spray bottle”.
Line 281: reword ‘for safety reasons’. I don’t think you mean it in the sense it reads. You are doing it to ensure that if any fragments of the bag have entered the sample you are able to account for this contamination and remove it from the analysis.
- Good advice, here. We will change the wording to “for contamination control through potential abrasion from the sample bags used”
Line 282: The use of cotton lab coats specifically avoids contamination of the air by synthetic fibres that would potentially come off synthetic lab coats. Contamination would still be still but they’d be cotton, and therefore not included in your analysis.
- Thanks for mentioning this point. We will add “… to avoid air contamination by synthetic fibres”
Line 357: Can you please provide information about what is included in your definition of “rubber” as this is a class or group of polymers, not a single type and it includes both fully synthetic and semi-synthetic alternatives. This will also influence the EPO ages, with only the natural rubber being used in 1820s, and first synthetic rubber developed in 1910, and another alternative in the 1930s.
- Thanks for raising our attention to this point. The identification class “rubber” includes both fully synthetic rubbers as well as natural rubbers, based on the entries within the OpenSpecy FTIR spectra database. As we were already aware of this, the respective EPO ages were differentiated between natural and synthetic rubbers. We will ad the respective information within the method section.
Line 366: The following sentence needs rewording “The plastics contain of HDPE …” Is it means to read “The plastics consists of HDPE….” Or “The plastics found included HDPE….”?
- Your right. The correct wording must be “The plastic consist of HDPE…” [will be implemented during revision]
Line 368: As this is intended for an intentional audience it may be best to avoid colloquial naming of things. Could ‘fries fork’ be changes to ‘plastic fork’, and ‘DIY store shed’ seems like a very broad category or description. Is there something more specific that can be said?
- Thank you for this important recommendation. We will change to “plastic fork” and “DIY store shed for flowers”.
Line: 390: ‘, at depths below 100 cm.’ is confusing. Is it meant that they are deeper, as in more >100 cm; or shallower? Please check and confirm to ensure it is less ambiguous.
- We will rephrase this part of the sentence to “deeper than 100 cm”.
Line 426: It is not clear why ‘Meso- and single macroplastic particles….’ Is written like this. Can this be made clear, or changed to remove the ‘single’?
- We will change the sentence to “Larger plastic particles within the macro- and mesoplastic size range, only occur isolated within topsoils (plough horizons) of sites..”
Line 448: What depths do “From 45.98% to 62.5%” refer to?
- Thanks for this comment. We will change to “from 45.98% in upper (0–50 cm) to 62.5% in lower (50–200 cm) soil layers.”
Line 489: Change “Figure 8a and Figure 8b” to “Figure 8”.
- [will be implemented during revision]
Line 536: Is this mislabelled? “Table T1”. Should it be S1?
- Yes it is mislabelled. [will be implemented during revision]
Line 555: Change “incipient alteration surface structure” to “incipient alteration of surface structure”.
- [will be implemented during revision]
Line 589: ‘(Table)’ is this meant to be here?
- Table 2 is meant here. We will ad the table number.
Line 682: Add “River” after “Nidda”. Other occurrences like this may occur elsewhere in the document.
- [will be implemented during revision]
Figures
Figure 1: Add in W (wastewater treatment plants) and ID (industrial discharge) in to the figure legend.
- [will be implemented during revision]
Figure 2: a) Both resin and rubber are broad categories of polymer. You should explain somewhere what is included within these definitions. See previous note about ‘rubber’.
- Thanks again for this comment. We have answered already on the “rubber” note above and will add the additional information for “resins” within the method section.
The scale bars and associated distance in the photos need to be larger as they are currently impossible to read.
- [will be implemented during revision]
- c) It may be advantageous to add in labels of Course and Fine soil to the figure to make it clearer what the reader is looking at.
- [will be implemented during revision]
Figure 3: Adding Titles to the graphs would make it clearer what the reader is looking at. a) & b) both the y and x-axis labels need to be centred on the axes. Legend: Suggest rewording “(with transect site location and river km) “ to “(with transect site location and river length (km)). This also needs to be changed in the axes labels. Add text to “dotted boxes indicate anthropogenic influence” = “dotted boxes indicate sites exposed to anthropogenic influences”
- Thank you for this kind recommendations. We will change the figure according them!
Figure 4: y-axis label: the ‘(-)’ needs to be removed.
- [will be implemented during revision]
Figure 5: x-axes labels: the ‘(-)’ needs to be removed from EF(-) and PLI(-).
- [will be implemented during revision]
Figure 6: it is not clear why for both a) and b) why the smaller particles are also presented as an additional graph inset as they are clearly presented in the total particles graphs. By removing them from both a and b the graphs showing all sized particles can widened.
- Thanks for this comment. Within Figure 6 the following problem occurred: When displaying the whole particle size, differences in the smaller size classes are hardly noticeable. Therefore the additional graphs for the smaller range. We will test the display again and revise it if reasonable.
Figure 7: the ‘(-)’ needs to be removed from PLI(-). Can the circles for the floodplain positions be made larger so it is easier to see the different colours?
- [will be implemented during revision]
Figure 8: y-axis on both a and b need to be flipped around 180°. the ‘(-)’ needs to be removed from EF(-).
- We will remove the “(-). Unfortunately, I do not understand why the y-axis should be flipped? Maybe you can give us a suggestion here?
Tables:
Table S1: a different foodnote symbol for ‘Shape’ needs to be used as you also use ‘a’ to refer to a picture.
- [will be implemented during revision]
Table 2: It is not immediately clear what “low, moderate and high” are referring to. If they are referring to the Indices is it possible to change the format and have “low, moderate and high” right justified in the column?
- Of course this will be possible. We will change the format according your advice.
Tables A1 and A2: are referred to a lot. Could they go in the main body of the article rather than as an Appendix?
- Thank you for this suggestion. We think that both tables are too large and would disturb the reading flow of the paper, as they mainly contain important details, but are not necessary for the first access. Therefore, we would like to keep them as an appendix
Table A1: It was not clear to me why are some of the words in bold.
- Names of reference soil groups according WRB 2015 are in bold. We will add an explanation in the note at the end of the table
Citation: https://doi.org/10.5194/soil-2022-1-AC2
-
RC2: 'Comment on soil-2022-1', Anonymous Referee #2, 07 Mar 2022
This study quantifies plastic particle abundance and heavy metal enrichment at four floodplain sites along a river in Germany in the soil depth profile down to 2 m and along a transect of increasing distance to the stream bed. No clear correlations were found between soil parameters or heavy metal enrichment and plastic abundance. Also, a clear pattern along the transect and along the river course was not observed. As seen in a few other studies, microplastic particles showed higher mean abundances in the upper horizon and decreased rapidly with depth. Interestingly, high accumulations of microplastic particles were found in different spots along the vertical profiles.
This is a very interesting study that expands sampling into a three-dimensional space and the results are valuable for publication. The results are displayed in a very precise and informative way with a lot of attention to detail. I think there is some improvement needed before publication though, mainly to organize the thoughts and intentions into a better structure.
I am somewhat missing clear objectives of the study. Why were plastics and heavy metals analyzed in the samples? What is the hypothesis that should be tested – what kind of relationship between the two metrics did the authors assume? What patterns were expected to be found in this study, along the river course, along the transect of increasing distance from the river, and along the soil profile? And why...? I appreciate that microplastic studies often need to have an exploratory nature due to the many contradicting results, but still, I think it would improve the paper if these points were addressed in a more coherent and clear way.
Introduction is rather lengthy. I suggest to eliminate information that is rather general, refer to the many reviews that are available, and focus here on the points that lead to your research question (spatial distribution, flood plain dynamics, connection to heavy metal contamination).
The conclusions are rather general, but not referring to the results.Often the language is a little unclear/ambiguous. E.g. the term spatial distribution is used for different meanings: along the river, across the river transect, or along the soil profile. E.g., line 706: What is meant here by spatial distribution? The variation along the soil profile? Maybe it would be clearer to refer to each of the three dimensions investigated.
Specific points:
- Abstract: Try avoiding acronyms, explain enrichment factor
- Line 17: lower range of known concentrations where? In soils, floodplain soils, …? This sentence does not make sense if you haven’t read the manuscript.
- Lines 21 f: Why do we need info on spatial microplastic distribution for floodplain management?
- Line 30: correct typo “scapes”
- Line 38: check wording of this sentence
- Lines 52-59: one might expect that this paragraph leads to a hypothesis of an objective, something like: “Therefore, we wanted to find out…”.
- Lines 70-72: not sure I understand the differentiation here in small-scale inputs and spacious inputs
- Line 75: “plastic abundance” instead of “spatial distribution of plastics”?
- Lines 82-92: this should be part of the discussion: putting own findings into context with literature
- Lines 97 f: not sure this is the correct reference. Also, I don't agree with this statement. Altered soil functions have only been found in the lab so far.
- Lines 101-108: is this a goal of this study? The temporal aspect is not really investigated here. Or can the sampled soil strata be somehow related to the time the sediment has been deposited?
- Line 112: replace “clear sequence” with “gradient”
- Lines 115 f: this sounds like a weak argument for the study. is it possible to find a stronger point?
- Lines 119 ff: These aims and hypotheses are coming somewhat “out of the blue”. It would be nice if the introduction would be more funneled from the general knowledge via the knowledge gaps leading to the research goal.
- Lines 123 f: the terms spatial relationship and spatial correlation seem a bit misleading, since one would expect that spatial analyses are being conducted that are commonly defined as (statistical) analyses of georeferenced data.
- 2.1 Study area: This can be shortened to the points relevant for the aim of the study
- Lines 169 f: “geospatial approach” - this term is used here in a somewhat blurry way. Also, why does a landscape consist of soilscapes? The term soilscapes should be defined at first mention.
- Line 235: replace “as control” with “for correction”?
- Past and present tense are used inconsistently (e.g. in results section) – please correct throughout the manuscript.
- Terms are used inconsistently: plastic concentration, plastic load: I’d suggest to change these to “plastic particle abundance”, since the results are based on number and concentration is more commonly used for masses.
- The order of subsections is vice versa in the methods and results chapter.
- Line 318: replace “organic (non-polymeric) with “natural organic matter”?
- Figure 2: Please explain the box plots (what quantiles are shown?) and increase the fonts. Replace “particle type” with “particle shape”
- Line 404: Mean values averaged over what?
- Figure 3: I am not sure I understand the meaning of cumulative sums. I suppose these are the abundances shown in table 1 summed up over the three depths. But that would mean that 5, 2, and 3 samples are summed up for the 0-50 cm, 50-100 cm, and 100-200 cm, respectively, so the upper stratum is overrepresented. Wouldn’t it be more intuitive to show mean abundance across each profile to compare the sites?
- Figure 6: I like the panel a a lot. But it might be clearer if the zoomed-in plots were removed. Also, consider to only keep 6a in the main paper. Instead of 6b, I'd rather like to see soil depth plotted against EPO age. Also, the gap in the size range from 2-10 mm strikes me. Is this an artifact of the analysis method? It should be discussed.
- Lines 476 f: Please delete: “with individual increased concentrations in deeper soil layers”
- Lines 494 f: replace “association” with “correlation”
- Lines 496 f, 506 f: according to figure 5a, the relationship between soil depth and number of MP is not linear, or only linear down to approx. 60 cm and then rather constant. Could this be included in the model?
- In the discussion, where the results are put into context with literature, I am missing the point that in general, particle abundances in different locations are hard, if not impossible to compare, since size and mass is not accounted for that can vary greatly depending on the origin, time of accumulation etc. of the plastics.
- Line 533: But Piehl et al found lower abundances
- Lines 566 ff: consider citing Koutnik et al. (2021) for a review of MP abundance in different soil types
- Lines 659 f: high levels of plastics in topsoil cannot explain accumulation in topsoils if the amount of translocated plastic is not known
- Line 669: “The direction of movement of the plastic deposits must therefore originate from the river” – why? What about agricultural activities, as stated earlier?
- Lines 675 f: but what about bioturbation, translocation to deeper soil?
- Lines 682-690: this is in logical contrast to the theory of sediment dating.
- Line 698: delete “spatial or statistical”
- Lines 700 f: meaning the emission of heavy metals from plastics?
- Lines 701 ff: I’m not sure what is intended here. If heavy metal as additives in plastics likely play no role, why should it be further studied?
- Line 707: What is the meaning of “significant outliers”?
- Line 723: What is the meaning of “(functions of some plastic pieces still recognizable)”?
- Line 734: Consider replacing “plastic content in floodplain deposits” with “the microplastic abundances found in this study”
- Lines 744 ff: I am not sure I understand this. In the discussion the authors state that the MP abundance even in the upper soil layers is low compared to e.g. agricultural soils in literature. So, it seems that MP are not a prominent issue in flood plains, as other pollutants like heavy metals.
- Line 754: a risk assessment usually does not follow legislation, but legislation follows an assumed or assessed risk
- Lines 768-770: this is true and a well-known problem, but it doesn't relate to the study.
- Lines 771-773: I think this is a little far-fetched in light of the comparably low levels of plastic contamination found in this study, and the positive effects on biodiversity and flood mitigation of river bed restoration.
- Lines 774-776: Also, this statement is general and not concluded from the study results.References
Koutnik, Vera S., Jamie Leonard, Sarah Alkidim, Francesca J. DePrima, Sujith Ravi, Eric M. V. Hoek, and Sanjay K. Mohanty
2021 Distribution of microplastics in soil and freshwater environments: Global analysis and framework for transport modeling. Environmental Pollution, 274:116552.Citation: https://doi.org/10.5194/soil-2022-1-RC2 -
AC3: 'Reply on RC2', Collin J. Weber, 08 Mar 2022
Reply on RC2 (soil-2022-1)
Dear Reviewer 2 (RC2),
Thank you for your effort to read our manuscript! Thanks for your recommendations and the general commendation of our work. We would like to respond to some of the comments below and clarify open questions one after the other. The mentioned typos and other minor corrections that do not require any further comment, were indicated by "[will be implemented during revision]"
Thank for your support. Best regards,
Collin (on behalf of all co-authors)
General comments:
- Thank your for raising our attention to the important points mentioned within your general comments. Regarding the introduction of our manuscript, we will formulate a clear objective within the revised version of our manuscript and will shorten the introduction consequently. For this reason we will follow your recommendations about the introductions focus also in accordance with the comments of RC1 as well as CC1.
Specific points:
Abstract: Try avoiding acronyms, explain enrichment factor
- "[will be implemented during revision]
Line 17: lower range of known concentrations where? In soils, floodplain soils, …? This sentence does not make sense if you haven’t read the manuscript.
- We will add “lower range of known concentrations within floodplain soils, …”
Lines 21 f: Why do we need info on spatial microplastic distribution for floodplain management?
- In contrast to so far “recognized” pollutants, microplastics are not considered within measures for floodplain management. The found effects of direct anthropogenic impacts, raising questions about future floodplain management. We may add “… for future floodplain management, regarding contamination assessments” to illustrate this background.
Line 30: correct typo “scapes”
- "[will be implemented during revision]
Line 38: check wording of this sentence
- Thanks for this hint! We have checked the wording and will change it within the revision.
Lines 52-59: one might expect that this paragraph leads to a hypothesis of an objective, something like: “Therefore, we wanted to find out…”.
- You are right, that the “metal” part of this paragraph suggests an objective. Within the revised introduction, we will add an objective at the end of the shortened paragraph.
Lines 70-72: not sure I understand the differentiation here in small-scale inputs and spacious inputs
- We will change the wording here from small-scale inputs to “point or local inputs”. Thus, the difference between point inputs (for example, input on a specific field) versus input from a flood (widespread) should become clearer
Line 75: “plastic abundance” instead of “spatial distribution of plastics”?
- "[will be implemented during revision]
Lines 82-92: this should be part of the discussion: putting own findings into context with literature
- The own findings compared here with other studies clearly belong to the previous preliminary work on microplastics in floodplain soils and should therefore also be mentioned here. They are taken up again in the discussion and compared with the findings of the study presented here.
Lines 97 f: not sure this is the correct reference. Also, I don't agree with this statement. Altered soil functions have only been found in the lab so far.
- It’s the correct reference here. We will formulate the statement less strongly by referring to "may affected by plastic particles in soils" and add an additional review reference here.
Lines 101-108: is this a goal of this study? The temporal aspect is not really investigated here. Or can the sampled soil strata be somehow related to the time the sediment has been deposited?
- No, the temporal component is only considered based on EPO-ages, but is no general objective of the study. The information about the temporal aspects, will be shortened and given just as side information within the revised introduction.
Line 112: replace “clear sequence” with “gradient”
- "[will be implemented during revision]
Lines 115 f: this sounds like a weak argument for the study. is it possible to find a stronger point?
- We will change the argument to “As the sink function of floodplains and depositional dynamics are already well studied for so-far recognized contaminants like metals of interest, a combined consideration of recognized metals and the new contaminant plastics, will be performed.”
Lines 119 ff: These aims and hypotheses are coming somewhat “out of the blue”. It would be nice if the introduction would be more funneled from the general knowledge via the knowledge gaps leading to the research goal.
- Thank you for this recommendation. As stated before, we will revise the introduction accordingly and refocus on our objectives, based on the knowledge gaps.
Lines 123 f: the terms spatial relationship and spatial correlation seem a bit misleading, since one would expect that spatial analyses are being conducted that are commonly defined as (statistical) analyses of georeferenced data.
- Thanks for this suggestion. Maybe the wording “correlation” leads to the assumption of a geostatistical analysis. We will change the wording here to “… investigating spatial similarities and differences between…”
2.1 Study area: This can be shortened to the points relevant for the aim of the study- "[will be implemented during revision]
Lines 169 f: “geospatial approach” - this term is used here in a somewhat blurry way. Also, why does a landscape consist of soilscapes? The term soilscapes should be defined at first mention.
- The term “geospatial approach” with regard to (micro-)plastics research was introduced by Weber et al. (2020) https://doi.org/10.1002/ldr.3676; The core idea of this approach is a study and sampling design oriented to respective land- and/or soilscape features. We will cite the reference here and will define the term “soilscape” after its first mention within the introduction.
Line 235: replace “as control” with “for correction”?
- "[will be implemented during revision]
Past and present tense are used inconsistently (e.g. in results section) – please correct throughout the manuscript.
- "[will be implemented during revision]
Terms are used inconsistently: plastic concentration, plastic load: I’d suggest to change these to “plastic particle abundance”, since the results are based on number and concentration is more commonly used for masses.
- Thanks for this recommendation. We will consistently use the term plastic load, as the results are reported as a number per kilogram soil dry weight.
The order of subsections is vice versa in the methods and results chapter.
- The order of subsections within the methods is oriented from “known” standardized methods (soil and metal analysis) to non-standardized (micro-)plastic analyses. We would like to keep the order of subsections within the results.
Line 318: replace “organic (non-polymeric) with “natural organic matter”?
- "[will be implemented during revision]
Figure 2: Please explain the box plots (what quantiles are shown?) and increase the fonts. Replace “particle type” with “particle shape”
- We think, that a standard boxplot didn’t need an explanation of quantiles as these are generally known. We would like to keep the term “particle type” as particle shapes are often associated with properties such as edges (e.g., broken)
Line 404: Mean values averaged over what?
- We are not sure where this question is aimed? Are the proximal and distal floodplain sites meant? If yes we will add: “… mean values of proximal (near-channel) floodplain sites to mean values of distal (remote-channel)…”
Figure 3: I am not sure I understand the meaning of cumulative sums. I suppose these are the abundances shown in table 1 summed up over the three depths. But that would mean that 5, 2, and 3 samples are summed up for the 0-50 cm, 50-100 cm, and 100-200 cm, respectively, so the upper stratum is overrepresented. Wouldn’t it be more intuitive to show mean abundance across each profile to compare the sites?
- Thank you for raising this question. The cumulative sums are the abundance of particles per soil section summed up over the three depths, as you correctly stated. As the mean abundance (vertical) is already displayed within Figure 5a, we want to illustrate the complete plastic abundance over the entire soil column (0-2 m) per sampling point in Figure 3. We think that both the significantly lower contents in subsoils and the sites with the highest abundance are made clear by this type of illustration.
Figure 6: I like the panel a a lot. But it might be clearer if the zoomed-in plots were removed. Also, consider to only keep 6a in the main paper. Instead of 6b, I'd rather like to see soil depth plotted against EPO age. Also, the gap in the size range from 2-10 mm strikes me. Is this an artifact of the analysis method? It should be discussed.
- Glad you like the illustration and thanks for the suggestions. We will remove the zoomed-in plots, also in accordance with RC1 comments, even if it becomes more difficult to track the lower size range. The gap within 2-10 mm range, is not an artefact of the analysis. It is due to the fact that many macroplastic particles were introduced into topsoil at two locations. We will add a brief discussion on this point.
Lines 476 f: Please delete: “with individual increased concentrations in deeper soil layers”
- "[will be implemented during revision]
Lines 494 f: replace “association” with “correlation”
- "[will be implemented during revision]
Lines 496 f, 506 f: according to figure 5a, the relationship between soil depth and number of MP is not linear, or only linear down to approx. 60 cm and then rather constant. Could this be included in the model?
- Thanks for this suggestion. As stated within l. 490-492 we found a significant regression between plastic loads and soil depth, just within the linear regression model. As we would like to keep the model as simple as possible, we think that a linear examination is sufficient here.
In the discussion, where the results are put into context with literature, I am missing the point that in general, particle abundances in different locations are hard, if not impossible to compare, since size and mass is not accounted for that can vary greatly depending on the origin, time of accumulation etc. of the plastics.
- Special thanks to raising our opinion again to this important point. In general, we agree with the statement that comparisons of the abundance is always limited. but not impossible, as long as spatial relationships but also differences are considered in the interpretation of the obtained data. We will add an respective sentence within the discussion to state the given limitations of such spatial approaches clearly.
Line 533: But Piehl et al found lower abundances
- Yes this is correct. The reference was set incorrectly here and will be removed.
Lines 566 ff: consider citing Koutnik et al. (2021) for a review of MP abundance in different soil types
- Special thanks for this nice literature hint. As CC1 already mentioned that we should reduce our reference list, we will add this reference and argue here shortened with reference to the review.
Lines 659 f: high levels of plastics in topsoil cannot explain accumulation in topsoils if the amount of translocated plastic is not known
- Yes, this is correct. We will delete the second half of the sentence and will just argue with the studies containing data on plastics in subsoils.
Line 669: “The direction of movement of the plastic deposits must therefore originate from the river” – why? What about agricultural activities, as stated earlier?
- Thanks for mentioning this point. The statement seems to be a little one-sided here. We will change to “Beside agricultural activities contributing to plastic deposits, plastic can originate from the river and deposited via flood water since further, laminar sources are excludable”
Lines 675 f: but what about bioturbation, translocation to deeper soil?
- Yes of course, this point is important and already discussed within Weber and Lechthaler (2021). The idea of an “general marker” assumes that a significant increase in content is indicative of deposition after 1950. Of course, parts of the plastic content can be displaced, as is possible with other dating methods, e.g., Pb-Cs. However, we know only little about the mass of "mobile" plastic in soils, especially for comparatively “large” microplastics. We will add a statement on the limitations due to relocation processes with regard to particle sizes.
Lines 682-690: this is in logical contrast to the theory of sediment dating.
- We do not think, that a translocation of plastic particles within soils or sediments, is contrary to dating theory. Each environmental archive (which could be dated, regardless of the used method) is influenced by dynamic processes (e.g. vertical displacements in the pore space or bioturbation). Within the results of this study, we identified a frequent accumulation within upper soil sections and significant lower loads within deeper soils, assuming that the deeper-plastics have to be relocated. Nevertheless, if you look at the total contents or the peaks (as in radionuclide dating methods), an estimation is still possible.
Line 698: delete “spatial or statistical”
- "[will be implemented during revision]
Lines 700 f: meaning the emission of heavy metals from plastics?
- Yes, thanks for this question. We will change the wording here to make this point more comprehensible.
Lines 701 ff: I’m not sure what is intended here. If heavy metal as additives in plastics likely play no role, why should it be further studied?
- This is a misconception here. Based on out method approach we were not able to identify the influence of heavy metal emissions from plastics, may due to the comparatively slight plastic loads found or the already existing geogenic as well as anthropogenic contamination of the floodplain with metals. Therefore, further studies should focus on the geochemical interactions. We will state this point clear within lines 700–704 within the revised version.
Line 707: What is the meaning of “significant outliers”?
- We will change the wording to “high accumulations” here.
Line 723: What is the meaning of “(functions of some plastic pieces still recognizable)”?
- Thanks for this question. This term means that the origin and the function of some plastic pieces is still recognizable. We will change the wording to “origin … still recognizable”
Line 734: Consider replacing “plastic content in floodplain deposits” with “the microplastic abundances found in this study”
- "[will be implemented during revision]
Lines 744 ff: I am not sure I understand this. In the discussion the authors state that the MP abundance even in the upper soil layers is low compared to e.g. agricultural soils in literature. So, it seems that MP are not a prominent issue in flood plains, as other pollutants like heavy metals.
- We believe that at this early stage of research it is not possible to say whether microplastics are a prominent issue in floodplains. Basically, the sole occurrence is to be considered negative, regardless of the quantity. As stated within the manuscript (methods, discussion), our method approach is limited through the analysed plastic size range. As many other studies have also analysed smaller particles, the contents are difficult to compare and it can be assumed that higher contents also occur in floodplain soils if smaller particles are included in the analysis.
Line 754: a risk assessment usually does not follow legislation, but legislation follows an assumed or assessed risk
- Risk assessments by monitoring programs can only be carried out if there are limit values (by legislation). Of course, an assumed or assessed risk must be given for legislation as well, but the simple proof of risk is not provided by area-based risk assessments.
Lines 768-770: this is true and a well-known problem, but it doesn't relate to the study.
- This point will be deleted within the revised manuscript.
Lines 771-773: I think this is a little far-fetched in light of the comparably low levels of plastic contamination found in this study, and the positive effects on biodiversity and flood mitigation of river bed restoration.
- As stated before, our study does not claim to provide a complete analysis of plastic content in relation to size. However, the presence of plastic, regardless of loads, should be cause for the influence of, for example, renaturation on the deposition of plastic. Here we are not referring to restrictions on restoration, but to considerations of how to deal with it. We will revise the point and make our implications more explicit
Lines 774-776: Also, this statement is general and not concluded from the study results.
- This point will be deleted within the revised manuscript
Citation: https://doi.org/10.5194/soil-2022-1-AC3
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AC3: 'Reply on RC2', Collin J. Weber, 08 Mar 2022
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EC1: 'Comment on soil-2022-1', Maria Jesus Gutierrez Gines, 10 Mar 2022
I would like to thank the two reviewers and Zacharias Steinmetz for your valuable review of this manuscript.
Given that the reviewers did not identify any major flaws and that they consider the methodology and results appropriate, I recommend to make the major and minor revisions highlighted by the reviewers, in order to make the text more concise and to include the information that is missing.
Best wishes,
Maria Gutierrez
Topical Editor
Citation: https://doi.org/10.5194/soil-2022-1-EC1
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