Constraining the Water Cycle Model of an Important Karstic Catchment in Southeast Tibetan Plateau Using Isotopic Tracers (²H, ¹⁸O, ³H, ²²²Rn)

Round 1

Reviewer 1 Report

Dear authors,

I really enjoyed reading your article, which I found very interesting and scientifically sound. Especially figures 4 and 5 with the Meteoric lines and pathways are very important as well as, later on, figures 8 and 9. The presentation and quality of the figures was exceptional. In the attached pdf are some minor comments and suggestions and in my opinion, in figure 4 the GMWL should also be provided just for comparison. Based on the above I recommend publication.

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

MDPI - Water

Manuscript Number: water-974366

Title: Constraining the Water Cycle Model of an Important Karstic Catchment in Southeast Tibetan Plateau using Isotopic Tracers (²H, ¹⁸O, ³H, ²²²Rn)

Article Type: Research Paper

Keywords: hydraulic connection; end-member mixing model; ²²²Rn; groundwater-surface water interactions; Jiuzhaigou; Changhai lake; water cycle model

The authors used environmental isotope tracers (²H, ¹⁸O ³H, and ²²²Rn) to constrain the water cycle model in the Jiuzhaigou catchments. The authors emphasize the importance of the isotope approach in basin-scale hydrogeological and geochemical studies.

The manuscript water-974366 is well written and represents a valuable contribution to the literature. I believe this manuscript should be published after a minor revision. The revision should include a better geological description.

Comments (P = page/ R = row):

P2/Introduction: Please improve references with the Italian studies by Apollaro and Vespasiano et al. that provided detailed data about natural heat tracing, reconstruction of LMWL, residence time and environmental tracer methods:

P5/Figure 2: Figure2c: In the section should be reported the horizontal scale and the kinematics of the faults represented. Figure 2a: In the figure should be reported the scale bar and a detailed legend of the geological setting. For a better comprehension, geology of the study area must be described in the section “study area”. Figure2b: also, in this portion must be reported the scale bar.

 P6/ Water Sampling and Analysis Method: I suggest reporting calibration procedures followed for the portable probes.

References:

Vespasiano G., Cianflone G., Cannata C.B., Apollaro C., Dominici R. & De Rosa R. (2016): Analysis of groundwater pollution in the Sant’Eufemia Plain (Calabria – South Italy). Italian Journal of Engineering Geology and Environment. DOI: 10.4408/IJEGE.2016-02.O-01

Apollaro C., Vespasiano G., De Rosa R. & Marini L. (2015): Use of mean residence time and flowrate of thermal waters to evaluate the volume of reservoir water contributing to the natural discharge and the related geothermal reservoir volume. Application to Northern Thailand hot springs. Geothermics 58 (2015) 62–74

Vespasiano G., Apollaro C., De Rosa R., Muto F., Larosa S., Fiebig J., Mulch A., Marini L. (2015): The Small Spring Method (SSM) for the definition of stable isotope - elevation relationships in Northern Calabria (Southern Italy). Applied Geochemistry. 63, 333-346.

Apollaro C., Vespasiano G., Muto F., De Rosa R., Barca D. & Marini L. (2016): Use of mean residence time of water, flowrate, and equilibrium temperature indicated by water geothermometers to rank geothermal resources. Application to the thermal water circuits of Northern Calabria. Journal of Volcanology and Geothermal Research.  328, 147–158

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

This paper is based on the premise that cross-valley groundwater flow occurs between two mountain valleys in the Sichuan Province.  The idea is that groundwater and lake water from the Zechawa Valley to the east moves beneath a 4200 meter mountain ridge to discharge in the Rize Valley to the west.  The proposed flow pathway is through cross-mountain faults possibly re-activated by an earthquake in 2017.  Following this earthquake water levels and streamflow increased in the western valley and decreased in the eastern valley.  The authors use water chemistry and isotopic indicators to calculated proportions of water from different sources and to develop a conceptual hydrologic model for this region.

While the overall approach to using isotopes as tracers here is reasonable as part of a hypothesis-testing program it is not sufficient, in my opinion, for confirming the overall premise of the paper.   The authors seem to assume that their hypothesis of the cross-basin connection is true and correct, (example:  “…to the fact that there is a hydraulic connection underground across the water divide that controls the distribution of surface flow in the two branches…” (line 432))  but offer no real evidence that this is true.  The isotopic tracer technique is at best an indirect method because it relies on many assumptions about the system that the authors do not state.

A thorough hydrogeologic analysis of this problem would have to start with measurements of the discharge of the various surface-water features discussed, but the paper does not present any discharge data at all other than to say that discharge increases downstream.  Understanding the discharge volume is critical if the authors want to develop a mixing or dilution model to explain downstream changes in water chemistry in the river.  Likewise, there is no discussion of hydrogeology here.  Does a large exchange of water between the valleys make physical sense, based on what is known about hydraulic gradients, the structure of the faults, and the regional geology?  It might, but this is not presented.   For example, what would the transmissivity of the faults need to be to account for the assumed volume of discharge?  Is that a reasonable number based on what we know about such faults?  Is the assumed amount of recharge reasonable for explaining the groundwater discharge?

So in summary, I cannot recommend publication of this work unless the authors can present a convincing summary of the physical hydrogeology of the area and document the actual discharges, or change in discharges, in the eastern and western valleys.  The isotope data would then be useful as confirmatory analyses but by themselves they are not enough to develop a circulation model.

Here are a few detailed comments:

1. This manuscript needs careful editing for correct English and wording. Typical examples: “…upper impermeable layers prevent modern water downward to occur mixing with deep aquifer…” (line 62) and “ The isotope of SZ samples behaves mixing isotope features between RZ and ZCW.  As the SZ is composed of ZCW and RZ, the contribution of ZCW and RZ are…” (line 246).
2. Overall organization could be improved. For example at line 137 it would be better to begin with an overview of the approach used rather than jump right into the details of sampling.
3. A few suggestions for the figures:
   1. Figure 2 (c) needs to show the geology below the carbonate rocks. There is no discussion of what might be the major aquifer below the mountain ranges.
   2. Figure 3 should show units (I assume milliequivalents, but maybe not).
   3. Figure 4 – something wrong with plot – see below.
   4. Figure 6 – there is not enough data shown for the kriging interpolation to be valid.
   5. Figure 11 – is reversed from figure 2, and north is at the bottom, which is confusing.
4. There is something wrong with the LMWL plot in figure 4. The line plotted as LMWL (blue dashes) is clearly not the correct line and is different than the line in the small inset box.  Based on the equation stated in the text (I verified that the equation is correct by plotting the data myself) the line should cross the Y axis at -86.7 (not -91 as shown on the plot) and at 18O= -12 the D should be -76.9 (not -80 as plotted).  So it appears that the entire blow-up of the LMWL has been shifted to be more negative, which impacts all the mixing calculations.
5. Why does Fig 4 should multiple values for L1 and L4 when apparently only one sample was collected for each?
6. The tritium analyses should include a history of atmospheric tritium (precipitation samples) in the region. Basing the analyses on apparently one sample (line 282) is not sufficient.

The analyses rely heavily on samples from springs to represent regional groundwater quality but provide no detail at all on these springs or how they were sampled.  What was the discharge of these springs?  Depending on their configuration, springs can represent very shallow, local groundwater, or very deep, old systems.  The authors need to discuss the springs in much more detail and explain why they are representative of regional groundwater chemistry.  Also, the sampling method can be critical – was the water allowed to flow into a spring pool where it might degas or equilibrate with the atmosphere, or were samples taken directly from a spring vent.  Ideally the spring composition would be compared with water samples from deep wells to confirm that the spring water is representative of deeper groundwater.  Perhaps no such wells were available, but this is not discussed.

Author Response

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Author Response File: Author Response.pdf

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.