How to Implement User-Friendly BLMs in the Absence of DOC Monitoring Data: A Case Study on Bulgarian Surface Waters

Round 1

Reviewer 1 Report

 I thank the effort of the authors in this study. The authors included many physicochemical parameters and comparisons in this work. The aim of the present study is to propose to the national environmental bodies an approach for dealing with missing DOC data for the implementation of BLM in compliance assessment. But the algorithm dealing with the missing data is  lacks novelty.  Some specific aspects that are not clear to me include, for example：

• Alternative approaches that are possible to deal with missing inputs include only the use of historical monitoring data and the use of substituting data. Would you please explain what is your contribution and innovation?
• “Substituting data approach revealed a poor correlation between Fe_dissolved concentrations and DOC concentrations”. Please explain the contribution of this conclusion in this paper?
• The method should include the comparison of existing methods for dealing with missing data in the introduction.
• Would you please give a detailed description and indicate the intension of figure that are mentioned in Section 3.4 . How to compare?
• Lack of literature comparison in Results.
• There is a grammatical problem in some parts of paper and the format of the reference is problematic.

Author Response

Reviewer 1:

I thank the effort of the authors in this study. The authors included many physicochemical parameters and comparisons in this work. The aim of the present study is to propose to the national environmental bodies an approach for dealing with missing DOC data for the implementation of BLM in compliance assessment. But the algorithm dealing with the missing data is lacks novelty. Some specific aspects that are not clear to me include, for example：

• Alternative approaches that are possible to deal with missing inputs include only the use of historical monitoring data and the use of substituting data. Would you please explain what is your contribution and innovation?

Dear Reviewer 1,

Thank you for your time and efforts in reviewing our paper. We revised the paper in order to emphasise the novel approach for substituting the missing DOC data. Firstly, we included a data availability map of Bulgaria, illustrating the need of such an approach. Furthermore, we included linear regression comparisons of the RCRs calculated by the BLMs (BIO-MET, M-BAT and PNEC Pro) using the i) “matching” data, ii) the DOC concentration, calculated by the TOC data, iii) the use of the 25^th percentile of the available DOC data for 2020, and iv) the use of the 25^th percentile of the calculated DOC concentration from the TOC data for the respective water body for 2020. The only possible way to confirm the applicability of all the approaches was to use the validated BLMs. To the best of our knowledge such comparison was not previously performed. It gives opportunity to find the most suitable pair of substitution approach and user-friendly BLM for the region under investigation. References were included in the text to support the difference in the approach contrary to the UK and France in Europe and the USA.

• “Substituting data approach revealed a poor correlation between Fe_dissolved concentrations and DOC concentrations”. Please explain the contribution of this conclusion in this paper?

One of the recognised approaches for missing DOC substitution is to use the historical data for Fe_dissolved. The applicability of such an approach was demonstrated in UK, but the obtained poor linear regression for the Bulgarian data renders such an approach useless. Therefore, we moved to the use of the DOC, calculated from the available matching TOC, and the use of the 25^th percentiles.

• The method should include the comparison of existing methods for dealing with missing data in the introduction.

Thank you for your comment. In the reorganised MS, we have moved this paragraph and the relevant references in the Introduction.

• Would you please give a detailed description and indicate the intension of figure that are mentioned in Section 3. 4. How to compare?

Thank you for the comment. For the clarity of the results and their explanation, we deleted the figure 3 in the original MS, so the focus of our findings is not moved away to the explanation of the methodology of the compliance assessment, which is published elsewhere. The newly introduced figures and the supporting figures (now moved to the supplement) are fully described in the Results and in the Discussion.

• Lack of literature comparison in Results.

Thank you for the remark, it will certainly add value to the MS if the literature comparison is added to the Discussion. In the revised MS we added such comparisons, so they illustrate better our data and compare them to other countries around the world.

• There is a grammatical problem in some parts of paper and the format of the reference is problematic.

Thank you for the sharp eye. We made a lot of changes, including grammar and references in the revised version.

Author Response File: Author Response.pdf

Reviewer 2 Report

Nothing to report. The paper is interesting and well described.

Author Response

Reviewer 2:

Nothing to report. The paper is interesting and well described.

Dear Reviewer 2,

Thank you for your time and efforts in reviewing our paper. We are glad for your high appraisal of our work.

Nevertheless, we revised the paper completely based on the other reviewers’ comments and should you feel necessary, please consult the revised MS.

Best regards,

The authors

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper „ Dealing with the missing monitoring DOC data for the implementation of user-friendly BLMs to Bulgarian surface waters in 3 compliance assessment” presents rather simple approach to  data substituting.

TOC is a key component of calculating DOC, and the Authors used correlation coefficient to calculate missing data. This is only one (out of many recognized methods),  to fill the gaps in the data set.

There is no comparison with other recognized approaches in the paper. Is correlation better or just simpler than other methods?

The paper presents no novelty. What is more, the Authors provided no explanation of necessity of presented calculations.

There are no scientific hypothesis stated. Presented calculations have very little scientific value.

Author Response

Reviewer 3:

The paper „ Dealing with the missing monitoring DOC data for the implementation of user-friendly BLMs to Bulgarian surface waters in compliance assessment” presents rather simple approach to data substituting.

TOC is a key component of calculating DOC, and the Authors used correlation coefficient to calculate missing data. This is only one (out of many recognized methods), to fill the gaps in the data set.

There is no comparison with other recognized approaches in the paper. Is correlation better or just simpler than other methods?

The paper presents no novelty. What is more, the Authors provided no explanation of necessity of presented calculations.

There are no scientific hypothesis stated. Presented calculations have very little scientific value.

Dear Reviewer 3,

Thank you for your time and efforts in reviewing our paper. We revised the paper in order to emphasise the novel approach for substituting the missing DOC data. Firstly, we included a data availability map of Bulgaria, illustrating the need of such an approach. Furthermore, we included linear regression comparisons of the RCRs calculated by the BLMs (BIO-MET, M-BAT and PNEC Pro) using the i) “matching” data, ii) the DOC concentration, calculated by the TOC data, iii) the use of the 25^th percentile of the available DOC data for 2020, and iv) the use of the 25^th percentile of the calculated DOC concentration from the TOC data for the respective water body for 2020. The only possible way to confirm the applicability of all the approaches was to use the validated BLMs. To the best of our knowledge such comparison was not previously performed. References were included in the text to support the difference in the approach contrary to the UK and France in Europe and the USA.

Additionally, to the correlation coefficient, obtained for the available matching TOC data, we used the 25^th percentile of the whole dataset for DOC in 2020 and the 25^th percentile of the matching TOC, converted to DOC by the already calculated correlation coefficient in order to allow significant broadening of the BLMs implementation inside Bulgarian monitoring network. In newly added figure of the data availability, we show that such approaches are necessary, as limited data is available country-wide. Furthermore, the availability of data for the other recognised approaches for DOC substitution, is even scarcer. In the revised MS the working scientific hypothesis is stated as follows: “The described methodology allows the environmental authorities to estimate the bioavailability of a certain metal choosing the best possible substitution approach by the implementation of the most appropriate BLM covering as much as possible SWBs without matching data available.”

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The paper has been revised according to the reviewer's suggestions. It has improved significantly and may be published in its present form.