The Effect of Gravel and Sand Mining on Groundwater and Surface Water Regimes—A Case Study of the Velika Morava River, Serbia

Round 1

Reviewer 1 Report

The manuscript entitled The effect of gravel and sand mining on groundwater and surface water regimes – A case study of the Velika Morava River, Serbia raises an interesting topic and concerns an interesting area of research. The work may be of interest to a wide range of readers of Water magazine. However, it needs some adjustments at this stage. Some parts of the text require additional commentary and/or supplementation. In some parts, the work is written in a way that is not very understandable, I suggest carrying out language correction by a native speaker in the field of natural sciences. I also have substantive comments regarding the presented concepts. Detailed comments are given below.

Detailed comments:

The Introduction chapter is interesting, but in its current form a bit too extensive and a significant part of the information presented there is not used in any way in the further part of the work, so I suggest removing all this type of redundant information. There is also a lot of repetition of the same or very similar information in different parts of the text.

Lines 66-69: This statement is more suited to the discussion/conclusions chapter.

Lines 84-86: "As a..." - this sentence seems to be out of context for this part of the text.

Lines 94 and further: The interaction between groundwater and river water is very complex. The impact of groundwater and surface waters is generally studied on a river scale and is very strongly dependent on the variability of local conditions. The main factors controlling the sw/gw interactions are: varying thickness of the clogging layer, riverbed and differences in location between the groundwater table and the water level in the river. At the same time, changes in the hydraulic properties of the riverbed can have very different temporal and spatial dynamics. In most cases, both the thickness of the plug layer and its hydraulic permeability independently control the state of disconnection between the river and the aquifer. For example, studies on the dynamics of groundwater/river water interactions conducted on a range scale (several tens of meters) allow us to understand the diverse nature of water mixing and inflow to individual wells of the infiltration intake (Kotowski et al., 2023, and many other works), which allows for better managing the operation of the entire intake. On a smaller and/or larger scale, this phenomenon will be completely unnoticeable. Examples of studies of surface-groundwater interactions conducted at various scales, aspects and environmental conditions are presented, among others, by: Treese et al., 2009: Frei et al., 2009; Brunner et al., 2011; Tang et al., 2019 and many, many more.

Line 118: Vericat et al. [22] how does this article relate to the content presented in the manuscript?

Line 133: I propose to change the order and place the chapter on the area of research (3) before the chapter on research methods (currently No. 2). Usually in scientific works there is this order (Study area/Material&Methods), which is partly due to the fact that sometimes the conditions of the research area (object) determine the adoption of a specific research methodology.

Line 136: „… water mirror levels” ?– unclear wording. What exactly do the Authors want to convey to the readers here? By the way, he also suggests considering the term "altitude" instead of "absolute elevations".

Lines 138 and 144: Most of this information is common knowledge and it is enough to refer to the relevant articles discussing the correlation coefficient and its application in scientific research. There is no information on the size of the analyzed sets, although the figures clear show that the analyzed data sets are very numerous. It would also be important to know about the types of distribution of the analyzed data, whether they are normal distributions. Both of these pieces of information determine the selection of the appropriate group of methods (parametric or non-parametric) to conduct statistical research. It is also not known in which research scheme the measurement data was used, whether as independent variables or as dependent variables. I also have doubts about the use by the Authors of the Student's t-test and Fisher's test. Firstly, in order to use these tests, the analyzed data should be normally or nearly normal distributed, otherwise appropriate non-parametric versions of these tests are used. Secondly, the Student's t-test should basically be used for TWO groups of variables, and the Authors consider three groups of data (Bagrdan, Žabarski Most and Ljubičevski Most). Perhaps this is to some extent compensated for by the huge amount of data, but it may cause some methodological and/or interpretation problems, and therefore I suggest in future research to choose statistical techniques for data interpretation more carefully. In such a situation (three groups of data) I can suggest the use of one-way (or even factorial) analysis of variance (ANOVA) or its non-parametric equivalent, the Kruskal-Walis test and appropriate post-hoc tests.

The next issue is that the diversity of variables is rather polynomial than linear, which is also worth considering in future research with this type of variables.

Finally, it is not clear to me how the geometry/depth of the riverbed was measured in the individual research profiles?

Lines 202-206: This text fragment is more suitable for the research methods section.

Lines 204-205: „…so the data from the new observation wells was not relevant” – On what basis do the Authors base this statement? The high frequency of measurements can help to better understand the nature of groundwater and river water interactions in the study area. Generally, this section lacks a description of the environmental conditions. The description of hydrogeological settings is too simplistic. The current nature of the flow between surface waters and groundwater systems is not clear – one sentence is not enough. It is also not known where the groundwater recharge areas are located. For a better understanding of the problem, at least one hydrogeological cross-section and a map of the groundwater table would be useful.

Lines 233-234: „The river channel  was stable…” – on what basis was this observation made? What about the natural changes (flood waves etc.) of the riverbed during this time?

Lines 244-245: „… resulting in the re-alignment and deepening of the Velika Morava River at Bagrdan.” Again. On what basis was this observation made?

Line 254: It is worth specifying the value of R here or simply refer to the figure.

Lines 275-286: This description of the dynamic changes in the direction of water circulation is a bit unclear, maybe it is worth making some kind of synthetic scheme/diagram showing the temporal variability of the nature of the flows in this area? In my opinion, the most interesting and authoritative is the nature of sw/gw interactions at low river water flow/levels.

Line 292: „Based on the above comparative plots of absolute elevations…” – unclear. On what exact basis (any research result or test) was the conclusion regarding the change in the flow regime formulated?

Lines 306-307: On what basis was this observation made? The value of R does not distinguish between anthropogenic impacts and impacts caused by natural factors.

Line 314-329: The question arises here what does "best water level correlations ..." actually mean in the context of the whole work. In some situations, even such a high value of R does not allow for an unambiguous interpretation. I think that here some version of the analysis of variance (ANOVA/MANOVA) in the appropriate research schemes together with appropriate post hoc tests would allow for a more precise and extensive interpretation. An example of the use of one-way analysis of variance (parametric and non-parametric versions) for the analysis of large data populations was presented by Kotowski et al., (2020) and many others.

 Line 338: „ … river reach..” – unclear. Please rephrase this sentence.

Line 344: Why is human activity indicated as the cause of this phenomenon? Any value of R not allows this type of distinction.

Lines 357-358: I do not understand this sentence. For what purpose are the Authors presenting the basic descriptive statistics of the analyzed data at this stage of the work? What matters is the homogeneity of the data, or lack of it. However, perhaps this is due to the vague and confusing style of this sentence.

Lines 393-397: ANOVA again, see my comment on lines 314-329.

Line 421: „…reduced river stages resulted in the reduced groundwater levels.” – are you sure that's what happened? effect/cause, cause/effect ;)

Lines 460 and further: The Conclusions chapter is currently too extensive and these are not entirely conclusions from the research conducted by the Authors, it looks rather like a summary of the work. In the Conclusions chapter, the Authors present a number of statements that were not the main subject of the research and their relationship with the results of direct calculations and interpretation of the analyzed data is rather weak. This applies to conclusions: 1, 2, 3 (almost all) and 5. I strongly suggest that you rethink your research findings and re-state your conclusions.

References:

Brunner, P., Cook, P.G. and Simmons, C.T., 2011. Disconnected surface water and groundwater: From theory to practice, Ground Water , 49(4), 460-467.

Frei S., Fleckenstein J.H., Kollet S., Maxwell R.M., 2009. Patterns and dynamics of river–aquifer exchange with variably-saturated flow using a fully-coupled model. Journal of Hydrology, 375: 383–393.

Kotowski T., Najman J., Nowobilska-Luberda A., Bergel T. and Kaczor G., 2023. Analysis of the interaction between surface water and groundwater using gaseous tracers in a dynamic test at a riverbank filtration intake. Hydrological Processes, 37(4),  e14862, doi.org/10.1002/hyp.14862

Tang, Q., Kurtz, W., Brunner, P., Vereecken, H., Hendricks Franssen, H.-J., 2015. Characterizations of river-aquifer exchange fluxes: The role of spatial patterns of riverbed hydraulic conductivities, J. Hydrol., 531, 111-123.

Kotowski T., Motyka J., Knap W., Bielewski J., 2020 - 17-year study on the chemical composition of rain, snow and sleet in very dusty air (Krakow, Poland). Journal of Hydrology, 582: 124543, doi.org/10.1016/j.jhydrol.2020.124543

Treese, S., Meixner, T. and Hogan, J.F., 2009.  Clogging of an effluent dominated semiarid river: A conceptual model of stream–aquifer interactions (1), American Water Resources Association, 45(4), 1047–1062.

In some parts, the work is written in a way that is not very understandable, I suggest carrying out language correction by a native speaker in the field of natural sciences.

Author Response

Please see the attachment

Author Response File: Author Response.doc

Reviewer 2 Report

In fact, I could not clearly get the effect of gravel and sand mining on groundwater and surface water regimes. In other words, how did the mining impose threat on water regimes? the mechanism or its process should be presented or illustrated more clearly.

Author Response

Please see the attachment

Author Response File: Author Response.doc

Round 2

Reviewer 1 Report

At the outset, I would like to say that I appreciate the effort put in by the Authors to improve this manuscript. At the same time, I am glad that my comments helped the Authors to improve this interesting article. I have no further comments, good luck with your further research.