Groundwater Circulation in the Shallow Crystalline Aquifer of Tharisa Mine, South Africa: Evidence from Environmental Isotopes and Near-Surface Geophysics

Round 1

Reviewer 1 Report

Jureya Dildar, Musa Manzi, Tamiru Abiye, Sikelela Gomo, Moyagabo Kenneth Rapetsoa, Gillian Drennan

An Integrated Approach to Characterise the Dynamics of Groundwater Using Environmental Isotopes and Near Surface Geophysical Methods, Tharisa Mine, South Africa

 

The authors set the goal of "Characterize the dynamics of groundwater". However, the hydrogeological conditions are not fully characterized both in the text and on graphic materials. There is no characterization of the filtration properties of aquifers, their thickness, well sampling intervals.

It is not clear what these quarries are, how deep they are, what kind of water is in the quarries. Table 2 - "Physiochemical and isotopic parameters" is almost illegible. The purpose of water sampling from tailings is unclear. It is not clear when the studies were carried out.

All this makes it impossible to evaluate the obtained results.

 

Some remarks

 

Introduction

 

Lines 30-32

Reference [1] is best used at the end of the second sentence.

 

Lines 36-37

You write: Typically, conventional hydrological tools and flow modelling cannot characterise groundwater flow in mining conditions owing to the complexity of groundwater flow

 

It is necessary to clarify the hydrogeological conditions under which this statement is true. For example, in sedimentary basins, it is sufficient to carry out a complex of experimental filtration works and modeling (https://doi.org/10.1007/s11356-018-3308-0). Further, it is also advisable to clarify the features of the hydrogeological conditions in the area of the Tarisa mine, South Africa, and the research methods that were used here to assess water inflows by previous researchers. What are these estimates in quantitative terms (m³/s)? Can your new methods improve the reliability of these water inflow estimates?

 

Lines 41-44

You write: The mine plans to move to underground mining in future; however, water infiltrating the surface may impede future mining activities. Therefore, in addition to geophysical studies, we sampled the water for geochemical analysis to determine the relationship between groundwater chemistry and the water source.

 

Do you mean "water seeping into underground mines/adits"? "hydrogeochemical analysis"? "to determine the water sources"?

 

1.1.            Study Area

Figure 1.

There is no reference to "Taylor et al., 2009" in the References section.

At the link:

https://upload.wikimedia.org/wikipedia/commons/b/bb/Bushveld_Igneous_Complex.png this figure can be found on the Internet, but it is presented there more correctly, since the position of the Bushveld Igneous Complex is shown on the South Africa diagram, as and should be shown in a scientific paper.

 

Line 82

 

Give an explanation of the abbreviation PGM

 

Figure 2.

There is no reference to "Zientek et al., 2010" in the References section.

You give geological characteristics, but you need to show a conceptual schematic cross section of the groundwater system. It should show all aquifers and quarry. The text should describe the filtration properties of aquifers, in particular the thickness of a fractured and unweathered aquifer located below the weathering zone. Also on the cross-section it is necessary to show the intervals of all groundwater samples taken and the isolines of groundwater TDS (https://doi.org/10.1007/s11356-018-3308-0).

 

Line 106

Give the decoding of the abbreviation BIC

 

Figure 2.

There is no reference to "Lomberg et al., 2016" in the References section.

What is the area of the developed deposit and the Tharisa Mine? Is it a quarry? What are its dimensions, depth, water inflows?

 

2.                  Materials and Methods

This section should first describe the concept of water sampling. All samples must be divided into separate clusters: 1) River waters. 2) Quarry water. It is necessary to indicate the sources of their occurrence in the quarry at the time of sampling. 3) Groundwater. Water samples from boreholes should be characterized by the dates of sampling and confinement to certain aquifers. 4) ... whether sampling was carried out from somewhere else and for what purposes.

 

Lines 185-186

You write : value of 5.6 T.U [37] was used as the input tritium concentration (??₀) from rainfall in the area.

 

What year does this value for the field of study date from?

 

Figure 6

Why are you repeating Figure 5 as Figure 6a?

 

3. Results

3.1. Hydrogeological Results  

 

Lines 302-304

You write: Drinking water has TDS ranging from 500 – 1000 mg∙l^-1 and electrical conductivity values below 1500 μS∙cm^-1. Natural water has EC values of 500 to 3000 μS∙cm^-1 [52].

 

However, natural water has TDS ranging from ~0.1 to ~528,000 mg∙l^-1

 

Lines 305-307

You write: These values lie within the healthy range for water; therefore, the water is not saline. However, some areas may be more saline as they may be filled by water that has travelled much further, and streams become more saline with distance (e.g., sample point T4).

 

These phrases need semantic editing

 

Table 2.

Missing sampling dates. What is "Point", sometimes the same as "Sample"?

 

It is not always clear what the names given in the "Description" and "Type" columns mean, for example, DYKE, SAMANCOR, 1416, etc.

Why do you specify ‰ for tritium?

 

Lines 311-325

 

You write: The stable isotope plot (Figure ?) demonstrates several possible source interpretations. The environmental stable isotope data of δ²H range from most depleted -24 ‰ in a borehole that taps a deeper aquifer to 4.7 ‰ in surface water from a tailings dam, whereas ?18? values range from -4.63 ‰ to 1.5 ‰. A plot of the data against the Pretoria Local Meteoric Water Line (PLMWL) displays several types of water: surface water and seepage water, revealing a highly enriched isotopic signal caused by fractionation during evaporation. The shallow waters are more enriched than the deeper waters and may indicate a substantial mixture of surface waters with recent recharge infiltrating into relatively deep water system besides possible water recycling. Isotopically depleted waters signify the third type of water, hence deeper water systems. However, borehole T20 (Far West Pit borehole) is particularly depleted isotopically, proposing a different flow/aquifer system. There is also evidence of mixing in the tailings dam (e.g. T5 from TSF 2), and consequently, tailings dam seepage as a tailings sample (T6, from TSF 1) has a similar isotope signature to T29 (TMGW COMM 02). Clustering the stable isotope data reveals the similarity in the water source that may not have occurred  simultaneously.

 

However, you do not have a well sampling chart with sampling intervals, so it is not clear which aquifers were sampled in which wells. It is also not clear what surface water is in the tailings, but since their TDS is higher than in groundwater, it is likely groundwater that has undergone evaporation. How do they relate to achieving the goal of your article "Characterize groundwater dynamics"?

It's more of an environmental problem.

 

Figures are numbered from 1 to 13, then from 6 to 10.

Comments for author File: Comments.docx

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Dear authors,

I read the article carefully. This article can be useful if its structure is rearranged. But there are fundamental problems in arranging images and presenting a groundwater conceptual model that can be fixed. My suggestion is suitable and I expect the authors to carefully edit the article.

1-      The title of the article is long and does not fit the work done, which aims to evaluate the conceptual model of groundwater flow in the mine. Please make the title shorter and more accurate.

2-      In the introduction, more details about groundwater leakage into open pit mines and its destructive effects should be given. Some similar international works should also be mentioned.

3-      In the introduction, hydrogeochemical methods are mentioned in understanding the path of groundwater flow and its hazards. Please, provide more explanations about the importance of hydrogeochemical methods in groundwater modeling, environmental hazards such as karst and rock solubility. You can also see these articles and mention geophysical and geochemical methods more comprehensively in the introduction:

 

https://link.springer.com/article/10.1007/s12665-021-09763-8

https://link.springer.com/article/10.1007/s10064-015-0744-7

 

4-      In the study area section, figures 1 to 3 and figure 5 should be merged together. First, figure 5 above should be presented more clearly, and then the geological map along with the cross-section and stratigraphic column should be displayed. Image resolution 3 is not suitable.

5-      Please provide a more detailed description of the problems caused by groundwater leakage along with the hydrogeological map of the area in the study area section.

6-      Please show the tailing storage facility in photo 4 with better quality and from a further view.

7-      Figures 8 and 10 do not help much to understand. Please compare standard charts.

8-      Are you sure that figure 13 helps to understand what was said? Is this form useful to the reader? I did not understand much of it!

9-      Please show the conceptual model of groundwater flow and resident time and its possible effects on the mine based on all the obtained results. Please provide a conceptual model based on mining conditions and the interaction of water resources and geophysical-hydrogeochemical and geological evidence.

10-   Please provide more objective evidence of the usefulness of this study to better understand the mechanism of underground water in this mine in the discussion and conclusion.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

1. In the abstract: added some isotopic values

2. Minimize the number of the keywords

3. You calculate the residence time using 3H (the vertical movement) but what about the lateral movement); V=d/Time "A14C between 2 points A and B)

4. Added these references (Hamed et al. 2008, 2010, 2014)

5. Try to explain and do the graph (Residence time vs samples vs 3H vs depth) in the same graph

6. Sometimes, you forget to signal the number of the figure. Example: Figure illustrates a surface contour map joining (page 15 line 403)

7. What about the relation between the TDS vs depth vs 3H vs 18O/2H

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors somewhat clarified the situation, but questions remained.

 

1.1. Study Area

 

You write: The mine is an open pit and is ~4km wide and ~9km long with a maximum mining depth of 75m. Water inflow values are not yet known.

1) Why can't you say how much water is pumped out of the pit to drain it, which is necessary for mining operations (Water inflow values are not yet known)? Or is the quarry completely dry? Then what is the problem you are working on to solve?

2) Figures 1d and 2 do not show the quarries and the river in the same way.

First, judging by the scale bar, the size of the quarry is clearly less than 4x2 km.

Secondly, in Fig. 1d the river flows between the quarries, and in Fig. 2 - crosses the quarry. Please specify that the situation is identical in both figures. Also clarify what kind of river it is, what its flow rate is, whether it is diverted from the quarry or is completely pumped out of the quarry.

 

2. Materials and Methods

 

3) You write that the input of tritium with a concentration of 5.6 T.U. (??0) from rainfall into the aquifer occurred in 2015. In this case, in 2023, the concentration of tritium in groundwater due to its decay should be 3.57 T.U, and the age of this water is 8 years (2023 – 2015 = 8).

However, in Figure 9 you show the ages of groundwater from 8 to 55 years.

Try to find a more correct scientific method for tritium dating, or limit yourself to tritium concentrations without dating

Comments for author File: Comments.docx

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

No comments.

Author Response

Thank you for your comments and recommendations.

Round 3

Reviewer 1 Report

Responses to the first and second comments can be accepted.

However, the answer to the third remark is unacceptable:

Comment:

3) You write that the input of tritium with a concentration of 5.6 T.U. (??0) from rainfall into the aquifer occurred in 2015. In this case, in 2023, the concentration of tritium in groundwater due to its decay should be 3.57 T.U, and the age of this water is 8 years (2023 – 2015 = 8).

However, in Figure 9 you show the ages of groundwater from 8 to 55 years.

Try to find a more correct scientific method for tritium dating, or limit yourself to tritium concentrations without dating

Authors response:

Apologies for the miscommunication; the 5.6 TU was the average values from over 40 years of rainfall reported in 2015. In the tritium interpretation, it is important to consider the average values in rainfall. See inserted explanation on page 7.

Comment:

The average values from over 40 years of rainfall reported in 2015 cannot be used to estimate the age of groundwater. Try to find a more correct scientific method for tritium dating, or limit yourself to tritium concentrations without dating

See for example:

 

Kralik, M., 2015.  How to Estimate Mean Residence Times of Groundwater.  Procedia Earth and Planetary Science   13, 301–306. https://doi.org/10.1016/j.proeps.2015.07.070

Malov, A.I. Tritium records to trace groundwater recharge and mixing in the western Russian Arctic. Environ Earth Sci 80, 583 (2021). https://doi.org/10.1007/s12665-021-09893-z

 

Comments for author File: Comments.docx

Author Response

Please see the attachment.

Author Response File: Author Response.pdf