Large Scale Trials of Waste Mine Burden Backfilling in Pit Lakes: Impact on Sulphate Content and Suspended Solids in Water
Round 1
Reviewer 1 Report
The manuscript Oggeri et al. “Disposal of waste mine burden in pit lakes: large scale trials for the assessment of the impacts of backfilling on water quality” reports on a pilot-scale test of release of turbidity and sulphate from overburden dumped into water. This test was intended to provide insights into the consequences of dumping overburden into pit lakes. This is an interesting issue and the fact that there have not been such publications in the last decades and that there are generally only very few publications on pit lakes in former lime stone quarries makes the manuscript basically a valuable one. However, in its current state, the manuscript cannot be accepted for publications. Too many issues are unclear.
The shortcomings of the manuscript are as follows:
The Introduction is mentioning many general aspects of pit lakes and their water quality. However, much of that is not relevant for dumping the overburden of lime stone quarries in mined out neighbouring lime stone quarry pit lakes. The authors should focus much more on what they really deal with and provide at the end. And they also should make the title a bit more specific.
I miss the consideration of existing knowledge from designing and operating settling ponds which are common e.g. in waste water treatment or for storage of tailings in hard coal and metal mining. There are text books on that and also design and operation guidelines.
It remains fully open why sulphate is an issue in the given context. Where does the sulphate come from in the overburden (which minerals and why present there?) and also in the water used for the test? The initial concentrations measured in the morning of Day 1 were quite high (441/448 mg/L) and the increase due to the dumping of overburden (ca. 50 mg/L) was rather small compared to that. That means that rather the background concentrations are a problem and not the dumping of overburden. However, the authors need to explain why this was unknown before the experiment and why the experiment is interesting for others.
Where is the investigated quarry located? Why do the authors not provide the location of the test quarry?
To which extent shall the pit be filled with overburden? Will a pit lake remain at the end?
The authors inform that the ground water level in the investigated region is at -45 m asl. This is very deep and I wonder how this is possible? How could the ground water level get below the sea level although the surface of the landscape is at about 35 m asl (according to the data provided in lines 111-112)?
In Table 1, the meaning of several columns remains unclear. This applies for the grain size (no unit) and for the consistency indices and the classification (the meaning of the used abbreviations is needs to be explained).
The used font size in Fig. 5 is too small for reading and no unit for the given lengths is provided (probably meter).
The font size of the legend of Fig. 6 is also much too small for convenient reading. Furthermore, the meaning of E, E’ and E’’ remains unclear although E seems to be the source of the used water. Completion of the explanations is needed.
The analytical method for sulphate analysis is not explained. This has to be added.
In lines 282 to 285, the authors say that they found a good relationship between turbidity and mass of suspended matter. These results need to be shown.
In the formulas, the authors often use variable names or indices that are not very specific or even hard to understand. Examples are “S” in “CS” for the indication of fine particles, “1” for the end of the day, “u” for outflow. The meaning of “i” is not explained (probably the number of the day).
Using “Vu/2” in formula 2 is a simplification. The authors need to provide a reason that justifies this simplification.
For me, it is not clear why “K” is needed in formula 4. It would make sense only if the conclusion drawn in lines 553/554 would be correct. However, no results were provided to justify this conclusion, i.e. that the clay does not contribute.
The design of Fig. 7 and 8 is not optimal from my point of view. Why do the authors not prepare one diagram showing the water flow and overburden addition and a second one showing all results for the suspended matter. The first diagram should be placed directly above the second one. Eventually, this would result in an easier to read combination of Fig. 7 and 8. Furthermore, I do not understand the meaning of the brownish line (soil filling) at the level of 56 m³ for the time period ca. 15 h to ca. 220 h. Finally, the used unit for the volume axis and also the name volume are not fully correct. While the presented data for the added soil is really a volume (shown as cumulative volume) the presented water flow is volume per time unit, i.e. a flow rate. The diagram needs to be revised at least for that.
The data provided in Tab. 4 require some basic comments. Such comments are missing in the Results section and also in the section Discussion. Usually, water analyses have uncertainties in the range of 5-10%, at least when done by routine labs. This means, that there are hardly any relevant differences between the sampling sites. And even the changes over time remain over a wide extent within the 10% range of the measured background concentrations (results for Day 1, 9:00). This makes the interpretation and evaluation of the sulphate results complicated. However, the fact that the authors do not make use of the results of the Day 1 soil addition for estimating the release rate of sulphate is hard to understand. After peaking to ca. 480 mg/L, there was a decrease. Where did the initially dissolved sulphate go? Has this to be attributed to gypsum precipitation (rather not very likely) or is the decrease the result of adsorption of sulphate to the clay minerals? This is a very important issue since it is highly relevant for long-term consequences and potential contamination of down-gradient groundwater. The authors need to address this issue and not only the short-term measurements from Days 7-14. On the other hand, the last days of the test did not show decreasing concentrations although no soil was added and assuming the background concentrations from Day 1 (9:00) as inflow concentration suggests at least a bit dilution, except, the release of sulphate from the dumped material was going on. One more time: Many open questions regarding key issues. The authors need to discuss the processes behind their findings.
The unit used in Fig. 9 for the sedimentation velocity axis should be mm/s. The factor provided at the upper end of the axis tells exactly the same but can easily be overlooked and makes the reading unnecessarily complicated.
In Tab. 7 and 8, the authors present results of calculations which use two different Ci. For me, it remains unclear why these different Ci were used and why exactly these values were chosen. Real measured concentrations are mixed with not explained and justified assumptions. Using directly the measured results for estimating the sulphate release rates would make much more sense from my point of view. Adding later a theoretical estimation on the influence of different Ci might make sense but not starting with something speculative before evaluating the measured results. Furthermore, I wonder why the results presented in the last two columns of both tables for the Ci 450 mg/L are differing in the order of magnitude. Check and correct where needed. Finally, according to the definitions given for formula 2, Mlix is a mass. The unit “mg/l m³” does not fit for a mass. The authors need to pay more attention of providing correct names and units.
In line 490, the authors present the assumed bulk density of the soil mixture. How was that value estimated and why? The bulk densities of all components are higher according to Tab. 1.
Table 10 is one more example for missing exactness: according to the headline, rates shall be presented while the data show volumes and mass.
The missing justification for the lines 553 and 554 has been mentioned already above.
The authors mention that the transport of turbidity was caused by wave action and not by water exchange because of inflow and outflow. The flow rate provided in Tab. 5 fits to that. However, I wonder how far the test was simulating the conditions in a pit lake. The dimensions of the quarry as provided in lines 106/107 are very different from the dimensions of the test basin. From my point of view, the test very well represents the area where overburden is dumped into the water of a pit lake. And given the dimension of the quarry, a quite rapid mixing of dissolved substances over the entire lake can be assumed as long as the density of the forming suspension does not hinder the mixing with clear water, particularly during thermal summer stratification. Regarding turbidity, the conditions in a full pit lake would be probably quite different from the test basin due to the much greater length of the pit lake. I.e., probably there would be enough space for sedimentation and clarification. Therefore, it might be a relevant option to dump the overburden at one end of the quarry while extracting water at the opposite end in order to get as clear water as possible, if needed.
In summary, the manuscript needs basic revision. Based on the outcome, it can be decided if enough is remaining for a paper in Sustainability or not. I hope that the authors are able to improve to a satisfying level because, as stated at the beginning, there are not many studies on quarry lakes and dumping of overburden in pit lakes.
Author Response
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Reviewer 2 Report
References on pit lakes are sparse; especially for waste placed in pit lakes. I recommend the following texts be read and cited:, especially in the introduction to leading practice and the state-of-the-science that the research intends to contribute to
McCullough, C. D.; Marchand, G. & Unseld, J. (2013). Mine closure of pit lakes as terminal sinks: best available practice when options are limited? Mine Water and the Environment. 32: 302-313.
Puhalovich, A. A. & Coghill, M. (2011). Management of mine wastes using pit/underground void backfilling methods: current issues and approaches. In, Mine Pit Lakes: Closure and Management, McCullough, C. D. Australian Centre for Geomechanics, Perth, Australia, 3-14pp.
Schultze, M.; Boehrer, B.; Friese, K.; Koschorreck, M.; Stasik, S. & Wendt-Potthoff, K. (2011). Disposal of waste materials at the bottom of pit lakes. Mine Closure 2011: Proceedings of the Sixth International Conference on Mine Closure. Lake Louise, Canada. Fourie, A. B.; Tibbett, M. & Beersing, A. (eds.), Australian Centre for Geomechanics (ACG), Perth, Australia, 555-564pp.
There are no such things as "sulphates". Sulphate is the cation solute as SO4.
"Pit lakes are generally expected to be managed as closed-circuit waterbodies, until 62 the water quality is good enough to be reconnected to the receiving environment without 63 causing adverse effects to aquatic life."
I disagree; pit lakes are typically excised from natural catchments. See for discussion: McCullough, C. D. & Schultze, M. (2018). Engineered river flow-through to improve mine pit lake and river water values. Science of The Total Environment. 640-641: 217-231.
"Inflow of acid water or dissolution of heavy metals from burden 66 or waste rock can be a serious challenge". Do not forget salinity and non-metals (e.g., Se) and metalloids e.g., As. It is a dangerous bias only to concern water quality issues with acidity and metals. See: Vandenberg, J.; Schultze, M.; McCullough, C. D. & Castendyk, D. (2022). The future direction of pit lakes: part 2, corporate and regulatory needs to improve management. Mine Water and the Environment. 41: 544–556.
"The bottom of the open-pit is located at depth much lower 79 than the groundwater level, and therefore an artificial lake is forming" Naive. Also requires a groundwater connection and net positive water balance.
"suspended solid" always plural as "solids"
Author Response
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Reviewer 3 Report
General suggestions:
More detailed information on the hydrogeological conditions around the quarry would be desirable (groundwater table map and/or a cross section instead of figure 1).
It should be clearly stated what the authors consider as a main source of sulphates. Whether the sources of sulphates are sulphides subjected to oxidation (e.g. dispersed pyrite) or yet existing sulphates (e.g. gypsum). This is an important factor in forecasting of long-term environmental impacts.
Detailed remarks:
Lines 13-14 – it should be checked and rephrased perhaps.
Line 420-421 – I am not quite convinced, that value of 5 µS/cm could be considered as ‘typical value’ for drinking water. It is rather typical for distilled or demineralized water or rainwater perhaps. Are there any examples of such low values available in literature? In my opinion 50 µS/cm is more probable.
Figure 9 – it would be interesting seeing the correlation parameters (e.g. coefficient of determination), since graphically, the fit looks astonishingly accurate.
Author Response
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Round 2
Reviewer 1 Report
The authors of Oggeri et al. “Large scale trials of waste mine burden backfilling in pit lakes: impact on sulphate content and suspended solids in water” (revised title) revised their manuscript to a wide extent. They addressed all my comments appropriately. The consideration of the comments of other reviewers additionally contributed to the reached comprehensive improvement. Now, the manuscript constitutes a very valuable contribution. Therefore, I recommend acceptance after minor revisions.
The minor revisions needed from my point of view:
Line 143: I recommend inserting the following text after “Therefore”: “and because long-term continuation of quarrying and, thus, pumping has to be expected”. This text indicates that “long-term” does not mean “natural” conditions after ceasing quarrying and artificial lowering of the groundwater level. Usually, “long-term” means “after mine closure and remediation” in the context of pit lakes since pit lakes are usually the result of mine closure.
Line 210: Maybe, “consisting” is the better term compared to “made”.
Figure 5: The characters and numbers in the figure are of low quality, contracting to the legend. The authors should improve that.
Lines 514/515: The provided typical range of the electrical conductivity of drinking water is well estimated but needs a reference.
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