Analyzing Particle-Associated Pollutant Transport to Identify In-Stream Sediment Processes during a High Flow Event

Round 1

Reviewer 1 Report

The presented work is interesting, and has a great potential for further development. However I have some recommendations for the authors of the manuscript.

1. In 3. Field Instrumentation, please provide more information on the turbidity sampling methodology taking into account the depth of the investigated river. This is important for understanding the correctness of your research.
2. What was the interaction between the river and its floodplain at the time of your work, from July 27 to July 28, 2019? Was any flooding within the river floodplain observed during analyzed flood events? Or were these events only within the riverbed? As you know, river floodplains are one of the most important accumulators (short-term and long-term) of suspended sediment and sediment-associated contaminants. If such a connection with the floodplain was not noted at the time of your research, then how representative will your findings be in cases of larger floods, including those accompanied by flooding of the floodplain of the river studied? I would like this issue to be briefly considered in your manuscript.
3. In Conclusions, please provide brief information on the practical significance of your findings, including for the study region.

Author Response

The presented work is interesting, and has a great potential for further development. However I have some recommendations for the authors of the manuscript.

1. In 3. Field Instrumentation, please provide more information on the turbidity sampling methodology taking into account the depth of the investigated river. This is important for understanding the correctness of your research.

• We agree that important information on the turbidity measurement is missing so far. We therefore included more aspects on the on-line turbidity probes, and the samples that were used for the laboratory measurements (now line 121 - 125).

2. What was the interaction between the river and its floodplain at the time of your work, from July 27 to July 28, 2019? Was any flooding within the river floodplain observed during analyzed flood events? Or were these events only within the riverbed? As you know, river floodplains are one of the most important accumulators (short-term and long-term) of suspended sediment and sediment-associated contaminants. If such a connection with the floodplain was not noted at the time of your research, then how representative will your findings be in cases of larger floods, including those accompanied by flooding of the floodplain of the river studied? I would like this issue to be briefly considered in your manuscript.

• There does not exist a “classical” connection between floodplain and river. Overbank flows typically do not occur. This is, on the one hand, because the discharge maxima are in general not very high, due to the catchment geology and morphology. On the other hand, the Ammer River is strongly modified and the channel is deeply incised into the flood plain. This is now mentioned in the text (now line 270/271).

3. In Conclusions, please provide brief information on the practical significance of your findings, including for the study region.

• We included a short section (now line 396 - 399) that shows the practical significance of this study.

Author Response File: Author Response.DOCX

Reviewer 2 Report

In this study, authors evaluated that the riverbed sediment act as intermediate storage for contaminated particles from urban runoff by correlating TSS and PAH15. This would extend our understanding on the delayed transport of hydrophobic organic contaminants such as PAHs during the heavy precipitation with sediment transport. The manuscript is neatly written and is acceptable for publication after revisions. Specific comments/suggestions are as follows:

Line 47: log Kd is not defined.

Line 143: “DIN” was not defined.

In Equations 1-4, units of Kd is L/kg but those for TSS is kg/m3. The units are inconsistent.

Line 182: Units of Kd equlas to those of inverse of TSS, not values.

Line 186: Kd values vary significantly among different sorbents and different PAHs. Although 10^5 represents typical order of magnitude, there is huge uncertainties.

Figure 5: “urban wave” is used in the figure, whereas “sewer wave” is used in the caption.

Figure 6 caption: ‘average’ particle size? It needs to be clarified.

Section 4.3 is the most important section of the manuscript. It will be helpful to add a paragraph how the findings improve the urban runoff management with an emphasis on pollutant transport.

Author Response

In this study, authors evaluated that the riverbed sediment act as intermediate storage for contaminated particles from urban runoff by correlating TSS and PAH15. This would extend our understanding on the delayed transport of hydrophobic organic contaminants such as PAHs during the heavy precipitation with sediment transport. The manuscript is neatly written and is acceptable for publication after revisions. Specific comments/suggestions are as follows:

1. Line 47: log Kd is not defined.

• Thank you for this hint. The log K_d represents the distribution coefficient which is now defined in the text (now line 47).

2. Line 143: “DIN” was not defined.

• We took up this aspect and now define the term “DIN” by: …according to a standardized procedure from the “Deutsche Institut für Normung e.V.” entitled DIN…” (now line 151).

3. In Equations 1-4, units of Kd is L/kg but those for TSS is kg/m3. The units are inconsistent.

• We agree that there are inconsistencies regarding the units of K_d and TSS. We therefore include the dimension symbol for all parameters in all equations.

4. Line 182: Units of Kd equlas to those of inverse of TSS, not values.

• We rearranged this sentence and hope that the meaning is clearer now (now line 189).

5. Line 186: Kd values vary significantly among different sorbents and different PAHs. Although 10^5 represents typical order of magnitude, there is huge uncertainties.

• We are aware of this uncertainty and agree that there could be differences depending on the sorbents and the PAHs. The “true” K_d for single PAHs is difficult to determine, especially for field conditions. Since the K_d value we refer to (Schwientek et al. 2013) was obtained for fluoranthene which is known to be representative for all PAHs, we are convinced that this value obtained for PAH₁₅ is representative for calculating the fraction of pollutants associated to particles. Even if the actual K_d value is not equal to 10^5, the mean value is assumed to fluctuate around this value. Additionally, we used the K_d only for the evaluation of Figure 6, and not for discussing distribution processes.

6. Figure 5: “urban wave” is used in the figure, whereas “sewer wave” is used in the caption.

• Thanks for this hint! We adapted the term in the figure caption in “urban wave” according to the figure itself.

7. Figure 6 caption: ‘average’ particle size? It needs to be clarified.

• We agree that this was not clear so far. We show median particle sizes and have specified this in the figure caption.

8. Section 4.3 is the most important section of the manuscript. It will be helpful to add a paragraph how the findings improve the urban runoff management with an emphasis on pollutant transport.

• Thank you for this suggestion. We added a section at the end of 4.3 that provides a more integrative view on the consequences of the described processes, including urban water management (now line 364 - 379).

Round 2

Reviewer 1 Report

I no longer have additional comments on the manuscript.