Evaporite Dissolution Rate through an on-site Experiment into Piezometric Tubes Applied to the Real Case-Study of Quinis (NE Italy)

Round 1

Reviewer 1 Report

The manuscript tests evaporite dissolution through weight and volume loss in rock tablets insert into piezometric tubes at different depth with the aim to contribute to the knowledge of karst processes responsible for sinkhole hazard. The authors present an interesting approach in the study of this problem but the manuscript needs to be improved in several aspects, especially the discussion. A main point is that dissolution rate depends from rock composition. In the experiment design seems that the authors did not consider the mineralogical variability of the rock samples, limiting the opportunity to extend the results to sinkhole hazard analysis. The sinkhole problem is not developed in the text, and also erosion/dissolution is not well presented.

Abstract

Line 16: no information of degree of humidity and air flow were reported in the text;

Line 19: “…highlight significant changes in the water table, converting a former phreatic into vadose zone“: the meaning of this sentence is not clear;

Line 25: “…dissolution due to NaCl water admixing in a complex scenario driven by changes in the hydrodynamic regime”: this kind of scenario is not reported in the text;

Line 29: no information about erosion processes are reported in the text;

Introduction

Line 32-33: please describe the role of evaporite in the genesis and evolution of sinkhole, and the relationship with groundwater. Authors just indicate a list of references;

Line 34: check “souled”, the meaning is not clear;

Line 35: give examples of sinking morphologies;

Line 38: give quantitative information, such as the total surface worldwide respect other evaporite outcrops;

Line 40: give quantitative information of high solubility, such as g/L of gypsum and anhydrite respect to other rocks or minerals;

Line 42: Please explain which are the general mechanism of sinkhole;

Line 48-49: you mention “…flowing waters of the Tagliamento River basin alluvial deposits…” but later in the study area you focus on evapotite rocks;

Line 51: list in number order such as PZ3, PZ7, PZ8, PZ16, PZ22, PZ23, PZ25 and follow this scheme also in tables;

Line 53: “…non-theoretical numbers usable values have been obtained in the sinkhole hazard analysis” the meaning of this sentence is not clear, please rephrase it;

Study area

Line 61: “It is a small village, placed …” the subject of the first sentence is the study area, but the second one starts describing the Quinis village. Please rephrase. A geomorphological and hydrological description of the area should be included, such as surface features or the presence of a small stream (Rio Quinis) tributary of the main surface drainage (Tagliamento river), ect.;

Line 62-63: “…as well as the critical issues, can also be recognized in several places of the whole valley”: please specify these lithological and morphological features;

Line 64: again the authors refer to genesis and evolution of sinkholes but no description of this geomorphological structure has been reported;

Line 69: the geological map with the main tectonic lineaments is missing. It should be better to insert also a cross section with the three main member of Raibl formation. Please, describe the lithology reported in figure 3b; I would suggest to improve the general geological overview to a larger area to justify the H2S gas and the pyrite oxidation;

Line 75: what does mean “won” in this sentence? Transgression? Please rephrase it;

Line 81: Please, indicate caves in Fig 3a;

Line 84: “…characterized by gray dolostones often vacuolar and cataclastic, marls and multicolor clays cover evaporites” the meaning is not clear, please rephrase it;

Line 96: please the different aquifers should be indicated in Fig 3;

Line 98: “2,500” please use comma for thousands;

Lines 101-106: please show in figure 1 a picture of a typical sinkhole in the study area;

Line 107: please indicate which kind of sinkholes are found in the study area. The authors just mention that are 2 types;

Line 109 and in the follow lines: the first time you use an abbreviation, please detail it. b.g.l.: I’ve supposed is below ground level, please specify here for the rest of the manuscript;

Line 114: “small cave present along one of the Rio Quinis tributaries” check English.

Material and Methods

Line 118: no information is reported about the effective causes of solution/erosion processes.

Line 119: which are the different hydrogeological conditions in the area?

Line 120: which size and shape have the rock samples? Are all samples equal also in composition? For a correct experimental design, the whole 42 evaporite rock samples should be of the composition, size and shape, and possibly of the same weight. This is a specific requirement for the further speculations on dissolution rate;

Line 127: figure 3a Indicate the piezometer with the rock samples with a different colour. Please label in this map also the piezometers reported in figure 3b;

Line 130: specify in the legend which are the mantling deposits.

Line 131: the cross section was not drawn on Tagliamento river;

Line 133: It should more readable if the stratigraphic columns would be listed in number order or as in figure 3b. In PZ3, please indicate the waterfall height;

Line 149: The data-logger are 4;

Line 153: water level in PZ24 is not reported in figure 4 and data from PZ8 and PZ16 are missing;

Line 157: describe how groundwater sampling at different depth were performed. You mentioned that water samples were collected on August 2018 but you don’t give information when the experiment started. At the beginning of at the end of the dissolutional period of the rocks? it would have been better if the experiment had foreseen more withdrawals during this period;

Line 166: please report ion balance in table 3;

Line 173: please specify which kind of special calculation you performed with this software;

Results and discussion

Line 179: you mentioned here that composition is different among sample rock. Which kind of difference? Again, if the sample composition is different, it is not possible to discuss in detail about dissolution rate, because different results can be related to different mineralogical composition. Please, insert in the result a table with XRD results of rock samples analysis;

Line 180: which kind of surface measurement were performed? Please describe it in the Methods paragraph. Specify here the cell colour as in table 2;

Line 182: Table 1. Please list the data following a number order, such as PZ3, PZ7, PZ8, PZ16, PZ22, PZ23, PZ25. Please also merge the information contained in Table 2, adding in this table 2 new column: days of immersion and % days of immersion. Please use dot to indicate decimals. The unit of grams is g, not gr, please check the table. Check also “Area base” and “Area surface”: what do they mean?

Lines 189-190: the part related to dissolution rate is not well supported by description of the results. Please improve the manuscript;

Lines 191-192: move to Methods

Lines 193-195: As mention above, explain mineral composition difference among rock samples.

Line 196: the x and y axes are not readable

Lines 204-206: All samples have this mineralogical composition? Which is the clay proportion in the samples?

Line 207: this sentence anticipates the results. Please, first describe the results, then report the relationship of weight loss.

Lines 216-217: authors state that weight loss of rock samples depends on behaviour of flowing water, mineralogical characteristics and Ca-sulphate saturation. Data about flowing water in PZ8 and PZ16 are not presented. Among the 7 piezometers, they represent the 2 end-member: the one with very low dissolution (PZ16) and one with the highest dissolution (PZ8). It should be important to show these data in Figure 6. About Ca-sulphate saturation, the authors supposed that this feature didn’t change during the 13-months monitoring period. It should be interesting to show the other results of groundwater composition, such as the pattern of temperature and EC for the overall year. If this is not possible, then the information in the Materials and Methods are redundant.

Line 225: there are not enough evidence to state that high water table level is the reason of high dissolution in PZ7. PZ8 has highest dissolution, but water level data are not shown;

Line 228: in figure 6 data about water level in PZ8 and PZ16 are not shown;

Line 244: remove table 2;

Line 248: why you use erosion and not dissolution? Please explain it;

Lines 251-253: do you detected H2S in groundwater?

Line 254, Table 3: Please list the samples in numerical order. Ion balance should be reported. Please also insert Saturation Index of the other mineral phases mentioned later in the text. Please, enter in the caption the legend of the symbol, such as ionic strength for I;

Table 4: Please list the samples in numerical order. Please report just the trace element of which results are reported and used for the discussion. Why sometime the concentration is expressed as <? Please explain it.

After table 4 the manuscript restart with line 1.

Line 16: How you can state this? Is it an instantaneous measurement or was detected by the data loggers. How?

Line 25: Symbols are not reported.

Lines 34-35: This state is new for me. Please cite reference. Normally, equilibrium state is between +/-0.5. the figure 8 shows that most of the samples are in equilibrium with gypsum dissolution, just 3 are undersaturation. You should justified it;

Line 41, figure 9: again most of the samples show stoichiometric dissolution of Ca-sulphate. You should speculated about the 6 outliers that have higher SO4. This might be related to neutralization of acid groundwater by carbonate;

Line 47: the possible source of sulphate might be represented by pyrite oxidation, but you need important ore bodies, not those related to restricted domains characterized by anaerobic conditions. Considering the geology of the area, authors should look to mineralized fluids in the area, maybe connected to volcanic activity or hydrothermal systems.

Line 53: Iron solubility has a strict pH-Eh constrain. With pH around 7-8, iron is not stable and precipitates. For better understand the origin of this element you should check its oxidation state;

Lines 65-67: authors mentioned that groundwater is supersatured with Fe-oxy-hydroxides, but they don’t explain the reason. Please, report saturation indexes of these Fe-minerals in table 3;

Line 76: you should report halite saturation index in table 3;

Line 81: this is the only part in the manuscript where authors refer to processes that contribute to sinkholes. One short sentence is not enough if the aim of the paper is to discuss evaporite sinkholes. Please improve this part of the text;

Line 87: lithium is a typical element that concentrate in evaporative brine, such as in Atacama desert where it is exploited;

Line 99: please insert data on Celestine saturation index in table 3;

Line 106: please insert Ba-mineral saturation index in table 3;

Line 112: arsenates are typical of ore bodies in hydrothermal system related to pyrite. Author should be check if in the area there are mines. I suggest to apply multivariate statistical analyses for highlighting the main components affecting this area;

Conclusion

Line 122: dissolution kinetics is not reported in the manuscript. Please improve it in the results paragraph;

Lines 139-142: this aspect was not discussed in the results;

Line 141: melting is used for snow. Please use dissolutional.

Author Response

Dear Reviewer, thanks for the given suggestions.Attached you will find a file with all our answers in red.

Author Response File: Author Response.pdf

Reviewer 2 Report

See attached file.

Comments for author File: Comments.pdf

Author Response

Dear Reviewer, thanks for the given suggestions.Attached you will find file with all our answers in red.

Author Response File: Author Response.pdf

Reviewer 3 Report

Dear Authors: Thank you for your excellent work- however, minor comments attached in word file.

Thanks

Comments for author File: Comments.pdf

Author Response

Dear Reviewer, thanks for the given suggestions.Attached you will find file with all our answers in red.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The revised manuscript presents an interesting contribute about sinkhole hazard related to karst processes. The authors has been significantly improved the manuscript as requested both inthe text, figures and tables making more clear the innovative approach applied to this research.