An Assessment of Self-Purification in Streams

Round 1

Reviewer 1 Report

In the whole text please use the notation in ionic form.

Complete the title of table 2 (line 215).

175-176; cited standards, please place in the reference list. 188; please quote where the formula for streams self-purification came from.

The text would be more readable if the authors applied figures showing nitrate and phosphate concentrations for all streams.

Please describe in the chapter the methods that were used statistical tools  and software to assess the differences in the level of self- purification in the studied streams.

Please illustrate (as figures) the self-purification coefficient for all streams.

Author Response

Thanks to the reviewer for helpful Comments. Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

The paper presents a comparison of self purification of natural parts and artificial parts of rivers in Lithuania. The comparison includes nitrates and orthophosphates for monthly surveys. It could have been interesting. Unfortunately, the methodology and assessment suffer from strong limitations. You’ll find here-attached my main comments:

Comment 1: Global comment: some English wording must be revised. Comment 2: Line 50: Practically, you cannot argue of excess moisture only based from a mean precipitation exceeding the mean evaporation. You must consider the whole water cycle for concluding. For instance, you could have a huge infiltration to groundwaters, meaning no excess soil moisture. Moreover, what about ETP that is different than EP. Be so kind to rewrite this sentence. Comment 3: Line 93: Take care of your presentation. The retention is totally dependent of contextual parameters. For that reason, you shouldn't be too precise, and certainly not at a 0,1%. Moreover, your 4% uncertainty is probably linked with a specific set of data, not a real range value for the above-mentioned reason. So be so kind to revise your sentence accordingly. Comment 4: Line 96: What do you mean by width of zone? Do you mean the zone between the cultivated land and the river border? Comment 5: Line 97-98: Here again, take care of the contextual data. Phosphorus retention is strongly dependent of parameters such as topography, soil characteristics, rainfalls... You cannot indicate such a 40% value as if it was universal. Moreover, be so kind to add the corresponding references, and mention that it is only indicative value. Comment 6: Line 101: Here again it is site dependent. Comment 7: Line 104: What do you mean by quick phosphorus? Is it SRP? Comment 8 : Line 110-112 : It is exactly what I meant. Comment 9: Lines 124-125: I thought you were saying that in Lithuania you didn't need irrigation due to excess moisture in the soils. Could you clarify that? Comment 10: Lines 128-129: Practically, you can add that it doesn't suppress the impact on water tables and water level in rivers. Comment 11: Line 129: What is a “meliorated land”? Comment 12: Lines 135-136: It is amazing because we have key improvements in some French rivers. Comment 13: Lines 143-145: And other key parameters Comment 14: Lines 146-148: English to revise. You need a conjugated verb in your sentence. Comment 15: Lines 160-172: Maybe, it should be better to put that lines 160-172 lines in a table. Comment 16: Line 183: Practically, using a mean annual value is a strong loss of information. It is not the same to have stable or strongly variable values with time. Comment 17: Line 186: Be so kind to indicate the exact unit of your results. Are they molar or atomic values? Comment 18: Lines 187-191: Your approach could be effective at the unique condition that your river receive not nutrients between the start and end of the considered straight. How could you be sure and demonstrate it is the case? It is a strong limitation of your approach. Comment 19: Lines 194-198: What about aquifers? Comment 20: Line 215: Replace « utilized” by « used ». Comment 21: Lines 218-221: Do you know the level of used fertilizers per Ha for each of the watershed? Comment 22: Line 225: This kind of classification is an ecological non-sense. Ecological status is not exclusively dependent of the mean level of PO4. Comment 23: lines 232-234: What is the residence time in the corresponding rivers? Comment 24: Lines 245-247: Yes, but practically, you cannot conclude because you only look at the PO4 sub component. What about organic matter? If you have a lot of organic matter being mineralized it could be argued that the river self purifies the water while PO4 level increase. So, it is rather difficult to conclude with your partial data. Comment 25: Lines 245 -256: Be so kind to add the units in this text, for your self-purification coefficients. Comment 26: Lines 250-151: Do you mean that the similar decrease of PO4 for different self-purification coefficient is due to different length of rivers? Comment 27: Lines 258-259: Exactly. It means that your methodology is wrong, as you cannot conclude without considering the lateral fluxes of N and P in your rivers. Comment 28: Lines 278-279: What is the retention time in this dam. It also impacts the water balance. Comment 29 : Lines 282 – 283 : As you only measured dissolved PO4, it also comfort my conclusion that you lack a global assessment of nutrients fluxes, that make it impossible for you to conclude for sure. I suggest you at least indicate the limitation of your methodology in your discussion step, and suggest future adding’s. Comment 30: Lines 295-299: How could you argue that twice the self-purification coefficient is not an essential difference? Comment 31: Lines 303-305: This demonstrate that the whole methodology is not sufficient to conclude. Indeed, even where you have positive coefficient the value could/should be totally different due to the lateral fluxes that are not considered in your rivers assessments. Comment 32: lines 312-315: But you also need flooding expansion areas. Comment 33: Lines 321-325: Could you separate your rivers in these three classes of hydrological regions? Comment 34: Lines 332-334: It is, but I imagine that you refer to sites that were poorly restored. The restoration processes as you referred to are only partial and don't take into account the full set of data that should be included. It can explain the limited results. Comment 35: Lines 348-351: Yes, but it also impacts the full set of your results, not only where you have an increase of NO3 level.

Author Response

Thanks to the reviewer for helpful Comments. Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Please again support the results with figures for all streams.  The results of the research are described but not supported by evidence. Only selected streams are shown in detail. Please complete the results.

Author Response

Response: We have included the distribution of nitrate and phosphate concentrations in three streams that belong to different hydrological regions. The values of decontamination coefficients for all streams are given in Table 4, so we did not want to repeat.

Thanks to the reviewer for helpful Comments .

Reviewer 2 Report

The paper has been improved. Meanwhile there are still some issues to solve. You’ll find here-attached my main comments (I refer to my preceding comments number for easiness):

Comment 4: So, be so kind to precise that in your text to avoid any misunderstanding. Comment 6: You should indicate that in your paper. Comment 12: I still suggest you amend this sentence, precising that it is site dependent, and that in some case, the improvement has been demonstrated. Comment 13: Could you also add: “sediment characteristics “. Comment 16: Yes, but what I meant is that it is generally not sufficient to conclude. It was just a comment. Comment 17: Thanks for the answer. I know the difference, but I think you have not understood my request. My concern is that you have still not indicated the exact unit in your paper. You must do it each time you present a phosphorus or a nitrogen value. So, as it is molar results, be so kind to replace phosphorus results in mg/l by mg PO4/l and nitrates by mg NO3/l. It would then be possible for every reader to have this key information that impacts the meaning of the values (there are a 3 fold ratio for phosphorus and 4 fold ratio for nitrogen between molar and atomic values). Comment 18: What do you mean by surface as well as ground water “more or less equal”? Do you mean that there are few inputs from surrounding groundwaters? If it is not the case, your fluxes evolve between the entrance and the output of your straights. Comment 19: You should address that issue in your discussion step, presenting that as a future assessment to perform. Comment 20: Not really. It is still in the text (lines 63, 201, 203, 205, 212, 226, 276) Comment 21: Could you add that rough estimation in the paper? Comment 22: May be, you could add that information in your paper. Comment 23: Thanks for the information. The sampling frequency is not an indication of residence time in the rivers. Could you clarify that issue ? Comment 24: You should discuss that issue in the discussion part of your paper, and propose that assessment as a future work to perform. Comment 27: I am still concerned about potential inputs from underground waters, and you have no data on that. So I suggest you address that issue in your paper as potential impacting factor that must be further studied in future works. Comment 28: As you mentioned, it can have a strong impact on water quality, then on rivers self-purification. You should address that issue as potential impacting in your discussion step of your paper. Comment 29 : May be you could be more precise including “(for instance organic fraction of nutrients)“ in lines 333-337 as an example. Comment 30: May be it could be useful to add this 95% reliability in the paper for clarification. Comment 31: OK but without any clear pattern on aquifer inputs, more especially for nitrates. Comment 32: In practice, suppressing flooded areas is counterproductive for long term quality of agricultural soils, and for ecological protection/development, more especially for biodiversity protection. Comment 33: I don‘t understand where are the class in this table. Be so kind to clarify.

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 3

Reviewer 1 Report

Please check the typos.

Author Response

We checked the typos. Thanks.

Reviewer 2 Report

Thanks for the amended paper. I have one remaining comment at that stage:

Regarding your answer on my comment 27 on the lateral inputs from aquifers, it is not sufficient to look at groundwater quality. There is also a need to assess underground water fluxes, in order to be able to quantify the nutrient fluxes to the rivers. Be so kind to include that in your sentence.

Author Response

Response 27: Corrected according to notes. 

The distribution of nitrate and phosphate concentrations in rivers is also influenced by the underground water, which we consider relatively small and similar, unfortunately, it is not analysed in detail in our research. However, in the future, in order to get a more detailed analysis of nitrate and phosphate changes, a more detailed assessment of groundwater (underground water fluxes, quantify the nutrient fluxes) should be carried out.