On the Understanding of the Hydrodynamics and the Causes of Saltwater Intrusion on Lagoon Tidal Springs

Round 1

Reviewer 1 Report

Using a study case (La Carbonera lagoon, Yucatan Peninsula, Mexico), the manuscript deals with the understanding of the driving forces controlling the saltwater intrusion in submerged spring located in karst lagoons. The manuscript is apparently intended to check previous interpretations.

From line 35 to line 116 the authors carry out a sort of review of knowledge in which many features on karst spring are provided, not all of which are useful for the manuscript purpose and burdening the reading. Here are reported the discharges of springs in Turkey and Florida, without understanding the reason. Unexpectedly, instead, the freshwater discharge of the studied spring into the lagoon is not reported in the manuscript. Or it eluded me; in this case, sorry, where is it?

The lack of the spring flow rate data is not the only shortcoming. The spring is not sufficiently described and documented from a geomorphological and hydrogeological point of view. As an example: where does the spring open? Maybe it opens at the bottom of the pool? Without enough information, a reader may ask himself various questions, such as the following. Could the "pool of roughly 60 m² and an average depth of 2 m" [lines 181-182] be a sinkhole where the water table simply emerges? On the other hand, the water levels fluctuations at SE and SP [Figures 3a, 3d] appear to be in perfect phase, as well as for SE, P5, B9 (together with SP itself) in Figure 2c. I was not able to see (in the text and figures) sufficient evidence regarding the spring function of SP.

As the manuscript intends to prove or refute previous interpretations of the saline intrusion process, particular attention was given to data acquisation and analysis, tide component extraction, and cross correlation. However, the "sampling campaign was carried out between November 2017 and January 2018" [lines 195-196]. If the campaign was carried out in another period and in other year, how could the results have been affected? Do the authors feel they need to carry out further campaigns to verify the results presented in the manuscript?

The major finding is shown in lines 283-290 and consists of an inversion of the flow at the outlet of a canal that alternatively feeds a pool, on the opposite side of which the studied spring is located. One can guess the logistical difficulties of instrumenting at best the site, however it is logical to ask why no flow measurements were made at the spring itself. Again, is the slope of the Peten channel known? Is it possible to exclude the existence of other springs along this channel or in the immediate surroundings?

The authors state "the saltier lagoon water gets into the channel and in some instances up to the spring during spring low tides (i.e., the lowest tidal level) due to flow reversal as reported by Febles and Batllori [28] and Rey [30], transforming the spring from a source of fresh water to a sink of brackish water. In that sense, this lagoon tidal spring behaves like an estavelle" [lines 320-324]. However, in Figure 2b, the water level of SP is always greter than C3. If the Peten channel feeds the pool and this latter the supposed estavelle, one should wait to see the water level of SP as the lowest one. The C3 water level must be placed also in the zoom of Figure 3a,d, in order to better appreciate the differences in height. Again, it would be useful to have SP data in Figure S1 (Hydraulic gradient between SE, C1 ... ...).

The authors conclude "The intrusion events occur during spring low tides, opposite to inversion events in nearby submarine springs on the coastal ocean floor" [lines 500-501]. They define the spring low tides: "the lowest tidal level" [lines 321-322]. It is necessary to clarify because the lowest tidal level can be confused with the "lowest astronomical tide" which has a time interval of 18 and half years. The authors, perhaps,  want to refer to the monthly water level of full or new moon (equinoctial tides). Or, again, to the low tides that should occur during the dry season, since "spring high tides occurring during the rainy season" [lines 170-171]?

In the Discussion so many arguments are touched upon that the logical thread is lost. It would be advisable to summarize the supported hypotheses before Conclusions.

Minor comments:

Keywords list must be increased, adding somethings about the karst and the study site. Possibly, "Peten" may also appear on the list.

"Recent studies in coastal lagoons have reported an opposite behavior in lagoon springs, in which the flow reversal occurs during spring low tide, a behavior apparently counterintuitive" [lines 30-32]: these recent studies should be quoted here, even if are then quoted after. Alternatively, I suggest to add something like "see below" in the sentence.

"Batllori [28]" [line 111] is wrong; the right quote is "Febles and Batllori [28]".

"interface on P5 is ..." [line 132], please change as follows "interface on well P5 is ..."

The caption of Figure 1 is sometimes confusing. Please check.

In References specify which works are not in English by adding the language in brackets to the end of the citations.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper present an analysis of coastal dynamics where a freshwater discharge via a karst aquifer interacts with seawater intrusion in the vicinity of a coastal lagoon. The paper presents a thorough assessment of all feasible constraints on the system and examines potential dynamics between the different driving mechanisms. The paper is well-thought out and logically presented with adequate, suitable and clear charts and maps.

Data is adequate for the purpose and clearly described. The paper presents a useful addition to coastal process understanding and assessment of freshwater-seawater interactions.

There are a number of minor typographical errors highlighted in the attached document.

My only criticism is that the relative contribution of the different drivers are not more clearly composed in the final conclusions. That is, the flow dynamics described in section 3.1 are not combined adequately with the mechanisms described in section 3.2 to generate a complete picture of the combination of forces and resultant responses that are observed in the study. Instead and very brief conclusion re-states the initial premise of complexity, but does not assign any priority, temporal cohesion or mechanism importance for the observed salinity and pressure changes. I see this as a relatively simple exercise that would round out the paper and make it more universally applicable.

I would also have liked to see a section on additional studies that should be carried out to further understand the dynamics. Some are alluded to (e.g. local weather effects), but these are largely studies of secondary (and likely unimportant) drivers and not methods to validate the proposed mechanisms and the evolution in terms of strength and interactions. For example, the study mentions the sampling campaigns used to look in detail at two periods of changing conditions, but the sampling does not appear to include sampling for hydrochemical analysis. Hydrochemistry would provide a strong (and quantitative) approach to mixing of freshwater with seawater and confirm the importance of different spatial drivers. Only salinity measurements were taken and these were not compared to major ion chemistry. The ratio of mono- and di-valent ions, for example can clearly delimit the relative contributions of fresh and seawater. Discharge from carbonate aquifers can also introduce distinct anion signatures that can be modelled for quantitative exchange rates and mixing dynamics. Hopefully these studies will be undertaken I the future and reported to support the findings of this physical parameter assessment.

In summary, this paper presents a generally well-written, concise, succinct, logical and precise analysis of the interaction of seawater with a coastal lagoon where freshwater discharge is occurring from a karst aquifer. The paper is worthy of publication, following consideration of the comments above.

Comments for author File: Comments.pdf

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Various information is given by the authors in response to my comments. However, they should be incorporated extensively into the text. Anyway, the manuscript is almost ready to be published. Please, see attached file.

Comments for author File: Comments.pdf

Author Response

We thank the reviewer for the comments. For this last submission we can highlight the following changes to the manuscript:

1. A description of the spring and the Peten channel was added to the study area section.
2. A plot of the Peten channel bottom was added to the supplementary material as Figure S1. Therefore, the final work contains two supplementary figures.
3. A discussion about the influence of the flow transition at the expansion of the Peten channel to the spring pool was added to section 3.2. Specifically, to the subsection now called “Spring pool and Peten Channel”. The corresponding reference was added to the manuscript as well.
4. Figure 3 was updated to a new version showing the water levels at C3, as proposed by the reviewer 1.
5. Finally, we added a couple of lines at the conclusion to support our findings.

The changes are reflected in the final manuscript, and highlighted in the cover letter as well.