Understanding Complex Hydraulic Heterogeneities in Crystalline Basement Aquifers Used as Drinking Water Sources
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsLine 106. Basic references for traditional methods to characterize aquifers (tracer tests, pumping tests, geophysics…) should be stated with basic manuals or handbooks. I agree with references 32 and 33, but 34, 1 and 50 are not suitable. Include basic references for pumping and tracer tests.
Line 110. It depends on groundwater velocity (see, for example, Non-Stationary Contaminant Plumes in the Advective-Diffusive Regime). It should be included some words to state that, for example: …is typically governed, depending on groundwater velocity, by advection…
Line 113. The traditional use of Peclet number is limited to porous media. The dimensional inconsistency of the number has been discussed in some papers (discriminated dimensional analysis. See, for example The dimensional character of permeability: Dimensionless groups that govern Darcy’s flow in anisotropic porous media), so anisotropic media like fractured aquifers are even far from its applicability. Delete any reference to Peclet number in this document (lines 113-116), since it is not necessary.
Line 132. “Which is essential for sustainable water resource management.” At this point, there is no evidence for the reader about the contribution of your work to sustainability, since there are no comments regarding a real case study (section 2). Please, include some lines in Section 1.
Line 140. What do you mean by “architecture”?
Line 168. Include clay and sand in the legend.
Line 216. Eliminate duplicated reference 40.
Line 222. Authors should mention every time they are assuming porous media analytical methods to obtain results in fractured aquifers.
Line 274. What is the EC baseline of the groundwater in Southwestern Nigeria’s Precambrian basement complex CBAs. It should be mentioned in the manuscript.
Line 316: Table 1. Specify if m relates to m above sea level, m below surface…
Line 440: Include a legend in Figure 10.
Line 526. Change Sustainable use by sustainable yield.
Line 530-532. electrical re-531 sensitivity tomography and ground penetrating radar have not been used in this work. Eliminate the paragraph.
Line 536. In paragraph 121 to 134, authors mentioned “By integrating pumping and tracer test data, this study provides an assessment of groundwater flow and transport processes in a crystalline basement aquifer”. Conclusions should support this, so the interdisciplinary contribution must be clarified. If not the case, delete or rewrite paragraph from lines 121 to 134.
Author Response
Comment 1: Line 106. Basic references for traditional methods to characterize aquifers (tracer tests, pumping tests, geophysics…) should be stated with basic manuals or handbooks. I agree with references 32 and 33, but 34, 1 and 50 are not suitable. Include basic references for pumping and tracer tests.
Response: We thank the reviewer for pointing this out. We added two basic references for tracer and pumping tests. We updated reference [50] and added a new reference – reference [51]. Updated and new references are:
Leibundgut, C.; Maloszewski, P,; Külls, C. Tracers in hydrology. John Wiley & Sons; West Sussex, UK, 2011; pp 434
Weight, W.D. Hydrogeology Field Manual, Second Edition, The McGraw-Hill Companies, Inc., New York, USA, 2008; pp 741
Comment 2: Line 110. It depends on groundwater velocity (see, for example, Non-Stationary Contaminant Plumes in the Advective-Diffusive Regime). It should be included some words to state that, for example: …is typically governed, depending on groundwater velocity, by advection…
Response: We accept the reviewer’s suggestion and added the phrase “depending on groundwater velocity” as suggested.
Comment 3: Line 113. The traditional use of Peclet number is limited to porous media. The dimensional inconsistency of the number has been discussed in some papers (discriminated dimensional analysis. See, for example The dimensional character of permeability: Dimensionless groups that govern Darcy’s flow in anisotropic porous media), so anisotropic media like fractured aquifers are even far from its applicability. Delete any reference to Peclet number in this document (lines 113-116), since it is not necessary.
Response: We appreciate the reviewer’s suggestion and acknowledge that the definition of Peclet number varies in the literature – see:
Huysmans, M. and Dassargues, A., 2005. Review of the use of Péclet numbers to determine the relative importance of advection and diffusion in low permeability environments. Hydrogeology Journal, 13, pp.895-904.
However, we would like to point out that the term porous media does not exclude fractured media, hence, our study considers the crystalline basement rocks as fractured porous media. While we agree that flow in fractured porous media is highly heterogeneous and complex, the calculated Peclet numbers in this study provide a first level assessment of flow and transport in the system. Although a detailed assessment of the relevance of Peclet number in crystalline basement aquifers considered as fractured porous media is beyond the scope of the study, we follow assumptions discussed in Russell et al, 2003; Huysmans and Dssargues, 2004; Nissan and Berkowitz, 2019 and Masciopinto et al., 2021 in applying Peclet number calculations to the studied aquifers.
No edits were made to the manuscript based on this.
Detwiler, R.L., Glass, R.J. and Bourcier, W.L., 2003. Experimental observations of fracture dissolution: The role of Peclet number on evolving aperture variability. Geophysical Research Letters, 30(12).
Nissan, A. and Berkowitz, B., 2019. Reactive transport in heterogeneous porous media under different peclet numbers. Water Resources Research, 55(12), pp.10119-10129.
Masciopinto, C., Passarella, G., Caputo, M.C., Masciale, R. and De Carlo, L., 2021. Hydrogeological models of water flow and pollutant transport in karstic and fractured reservoirs. Water Resources Research, 57(8), p.e2021WR029969.
Comment 4: Line 132. “Which is essential for sustainable water resource management.” At this point, there is no evidence for the reader about the contribution of your work to sustainability, since there are no comments regarding a real case study (section 2). Please, include some lines in Section 1.
Response: We appreciate the suggestion by the reviewer, and we edited lines 131 – 134 to read: “By integrating pumping and tracer test data, this study provides an assessment of groundwater flow and transport processes in a crystalline basement aquifer. Knowledge gained from this assessment would be important, which is essential for sustainable water resource management”
Comment 5: Line 140. What do you mean by “architecture”?
Response: By “architecture” we meant the lithology and structure of the aquifer system. To ensure that this is clearer, we edited lines 141-142 by replacing “architecture” with “lithology and structure”
Comment 6: Line 168. Include clay and sand in the legend.
Response: Following the reviewer’s suggestion, we added the “sandy clay” and clayey sand” to show the coarsening downward nature of the overburden material
Comment 7: Line 216. Eliminate duplicated reference 40.
Response: The duplicated reference was deleted.
Comment 8: Line 222. Authors should mention every time they are assuming porous media analytical methods to obtain results in fractured aquifers.
Response: We appreciate the reviewer’s suggestion and included the phrase “assuming a porous media framework to line 225.”
Comment 9: Line 274. What is the EC baseline of the groundwater in Southwestern Nigeria’s Precambrian basement complex CBAs. It should be mentioned in the manuscript.
Response: As suggested by the reviewer, we included the information on the baseline EC of 0.52 mS/cm and edited lines 273 -275 accordingly.
Comment 10: Line 316: Table 1. Specify if m relates to m above sea level, m below surface…
Response: The depths are with reference to the ground surface. We updated the Table caption to include this.
Comment 11: Line 440: Include a legend in Figure 10.
Response: We included the legend to Figure 10 as suggested by the reviewer
Comment 12: Line 526. Change Sustainable use by sustainable yield.
Response: Done. We appreciate the reviewer for drawing our attention to this.
Comment 13: Line 530-532. electrical re-531 sensitivity tomography and ground penetrating radar have not been used in this work. Eliminate the paragraph.
Response: We agree with the reviewer and deleted the details on the geophysical methods.
Comment 14: Line 536. In paragraph 121 to 134, authors mentioned “By integrating pumping and tracer test data, this study provides an assessment of groundwater flow and transport processes in a crystalline basement aquifer”. Conclusions should support this, so the interdisciplinary contribution must be clarified. If not the case, delete or rewrite paragraphs from lines 121 to 134.
Response: We revised the conclusion to reflect the objective of the study to combine pumping and tracer tests for assessing groundwater flow and solute transport in a crystalline basement aquifer. Lines 559 - 562 were edited accordingly.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript is well written and structured. The English is clear and fluent. However, some minor revisions are necessary before publication.
- In lines 301–305, it is mentioned that the drawdowns in the observation wells range from 1.63 to 3.63 m during the 9-hour pumping test. However, in Table 1, a value of 1.28 m is reported for well D while pumping in C. This value needs to be corrected. It is also mentioned that “the minimum drawdown in the observation well was observed in well D while pumping at well A”. However, in the same Table 1, it can be observed that the minimum value is the one mentioned previously (in well D while pumping in C).
- There appear to be some errors in the table. In the “Status” column for the third set of pumping tests (starting from row 11), it seems to indicate that there are two pumping wells and two observation wells, while in the last set, all wells are listed as observation wells. Shouldn’t the correct interpretation be that in the third set, pumping occurs at well C (distance 0), and in the fourth set, pumping occurs at well D (distance 0)?
- Figure 3 shows the drawdown curves for each of the wells when pumping occurs in another well. However, all the graphs are synchronized, meaning they start at time zero for each graph. What about the time lag between the moment one well starts pumping and when measurements begin in another well? This time lag can provide valuable information about the aquifer, e.g. aquifer transmissivity, storage coefficient, or hydraulic connection between wells.
- In the same Figure 3, it would be more appropriate to plot the derivative of the drawdown rather than the drawdown itself, as it provides more informative insights. It has been reported that entirely different models can produce identical drawdown responses and nearly identical derivative curves, although with slight differences at transition stages. See, for example, the figure 8 of the work by Vivas-Cruz et al. 2020 regarding the equivalence of double porosity models (DOI: 10.1615/JPorMedia.2020033939; https://doi.org/10.1016/j.petrol.2014.06.030). It is not necessary to include the reference.
- In Section 3.1 or alternatively, in Section 4.1, it is important to clarify the difference between pumping tests and interference tests. Although the analysis is similar, there are key distinctions between the two. In a pumping test—particularly in the derivation of analytical solutions—it is typically assumed that the location where pumping occurs is the same as where the drawdown is measured. This is not the case in an interference test, where the pumping location does not necessarily coincide with the observation point. When fitting an analytical model to the latter, it is important to account for the distance between the pumping well and the observation well. Similarly, in an interference test, one could attempt to fit the model by considering one pumping well and three observation wells, in such a way that the fitting yields a single set of parameters. Alternatively, each observation well could be fitted individually, resulting in a distinct set of parameters for each observation well (three sets in the case presented, as shown in Table 2).
- Table 2 shows that in almost all the pumping tests, a value of n = 2 was obtained for Baker’s GRF model. It would be important for the reader to include a brief explanation of the (somewhat complex) meaning of this parameter and the significance of the value 2.
- For greater clarity, include the letters (a), (b), ..., within each subfigure in Figures 3 to 9.
- In lines 385–387, it is mentioned that the pumping well is A, referring to Figure 7. However, in the caption of Figure 7, it is stated that the pumping wells are C and D.
- Improve the resolution of some of the figures, especially Figures 4 and 5.
Author Response
General Comment: The manuscript is well written and structured. The English is clear and fluent. However, some minor revisions are necessary before publication.
Response: We appreciate the positive feedback from the reviewer and have considered the suggestion provided in revising the manuscript.
Comment 1: In lines 301–305, it is mentioned that the drawdowns in the observation wells range from 1.63 to 3.63 m during the 9-hour pumping test. However, in Table 1, a value of 1.28 m is reported for well D while pumping in C. This value needs to be corrected. It is also mentioned that “the minimum drawdown in the observation well was observed in well D while pumping at well A”. However, in the same Table 1, it can be observed that the minimum value is the one mentioned previously (in well D while pumping in C).
Response: We thank the reviewer for spotting these errors which we have corrected accordingly. The minimum drawdown has been corrected to 1.28 m as observed in D while pumping at well C.
Comment 2: There appear to be some errors in the table. In the “Status” column for the third set of pumping tests (starting from row 11), it seems to indicate that there are two pumping wells and two observation wells, while in the last set, all wells are listed as observation wells. Shouldn’t the correct interpretation be that in the third set, pumping occurs at well C (distance 0), and in the fourth set, pumping occurs at well D (distance 0)?
Response: We thank the reviewer for spotting the inconsistencies in Table 1. We have corrected the table accordingly by updating the column with the well status.
Comment 3: Figure 3 shows the drawdown curves for each of the wells when pumping occurs in another well. However, all the graphs are synchronized, meaning they start at time zero for each graph. What about the time lag between the moment one well starts pumping and when measurements begin in another well? This time lag can provide valuable information about the aquifer, e.g. aquifer transmissivity, storage coefficient, or hydraulic connection between wells.
Response: We appreciate the reviewer’s perspective. The time lag in drawdown is hardly visible but can be seen in the second column of the log-linear plot. Also, a plot of the hydraulic head with time also shows this. We added the plot of head with time to the supplementary file (Figure S1) and referred to it in the text.
Comment 4: In the same Figure 3, it would be more appropriate to plot the derivative of the drawdown rather than the drawdown itself, as it provides more informative insights. It has been reported that entirely different models can produce identical drawdown responses and nearly identical derivative curves, although with slight differences at transition stages. See, for example, figure 8 of the work by Vivas-Cruz et al. 2020 regarding the equivalence of double porosity models (DOI: 10.1615/JPorMedia.2020033939; https://doi.org/10.1016/j.petrol.2014.06.030). It is not necessary to include the reference.
Response: We appreciate the reviewer’s suggestion. We show the derivative plots for selected tests in Figures 5 and 6 and used them as well to analyze the well behavior. Following the reviewer’s comments, we plotted the drawdown derivative plots for the other test and provided all plots in the supplementary information document in Figures S2 to S5.
Comment 5: In Section 3.1 or alternatively, in Section 4.1, it is important to clarify the difference between pumping tests and interference tests. Although the analysis is similar, there are key distinctions between the two. In a pumping test—particularly in the derivation of analytical solutions—it is typically assumed that the location where pumping occurs is the same as where the drawdown is measured. This is not the case in an interference test, where the pumping location does not necessarily coincide with the observation point. When fitting an analytical model to the latter, it is important to account for the distance between the pumping well and the observation well. Similarly, in an interference test, one could attempt to fit the model by considering one pumping well and three observation wells, in such a way that the fitting yields a single set of parameters. Alternatively, each observation well could be fitted individually, resulting in a distinct set of parameters for each observation well (three sets in the case presented, as shown in Table 2).
Response: We appreciate the reviewer’s perspective on the need to clarify the test conducted in this study - if it was a pumping or interference test. First, we would like to point out that while both are similar in theory, the term pumping test is commonly used in hydrogeological studies for water extraction from a well and response monitored in the pumped and or sounding wells to estimate the aquifer’s characteristics such as transmissivity (the product of hydraulic conductivity and the aquifer thickness) and storativity. The term interference test is, however, more commonly used in reservoir engineering to assess the effect of a production well on the surrounding observation well to assess connectivity. For this study, we retained the term pumping test as it is more commonly used in the hydrogeological community.
No additional changes were made to the manuscript.
Comment 6: Table 2 shows that in almost all the pumping tests, a value of n = 2 was obtained for Baker’s GRF model. It would be important for the reader to include a brief explanation of the (somewhat complex) meaning of this parameter and the significance of the value 2.
Response: The range of n values (0.90 to 2.60) obtained from the Baker’s GRF model show variation in the aquifer structure typical of fractured and rocks dual porosity systems (n = 1 – 2 or < 1), homogeneous and isotropic system (where n = ~2) and connected fractures (n = 2 – 3+). This further highlights the high level of heterogeneity and complex structure typical of crystalline basement aquifers.
As suggested by the reviewer, we added these details to lines 372 – 376.
Comments 7: For greater clarity, include the letters (a), (b), ..., within each subfigure in Figures 3 to 9.
Response: We appreciate this suggestion and have added letters to Figures 3 - 9
Comments 8: In lines 385–387, it is mentioned that the pumping well is A, referring to Figure 7. However, in the caption of Figure 7, it is stated that the pumping wells are C and D.
Response: We thank the reviewer for drawing our attention to this. The step-drawdown tests were actually conducted by pumping in wells C and D. We have corrected the errors accordingly in lines 400 - 404.
Comment 9: Improve the resolution of some of the figures, especially Figures 4 and 5.
Response: Figure edited as requested to improve the resolution.
Reviewer 3 Report
Comments and Suggestions for Authors This study systematically evaluated the hydraulic properties, connectivity, and sustainable exploitation potential of the crystalline basement aquifers (CBAs) in southwestern Nigeria through pumping tests and tracer tests. The research question holds important practical importance. However, there is still room for improvement in the paper in terms of the depth of result interpretation and the connection with practical applications. 1. Please unify the expression of CBA or CBAs. 2. Please check that the numerical expressions in this study are consistent with the corresponding units, such as maintaining a consistent number of decimal places. 3. Add a comparative analysis in the introduction. For example, "Compared with traditional models, the advantages of the model used in this study." And emphasize the synergistic effect of tracer tests and pumping tests. 4. In Section 3.1, figures can be used to assist the textual description to make the expression more concise. 5. Please improve the clarity of Figures 3 - 7. Some figures lack axis units or legend descriptions. 6. In Section 5.1, the hydraulic conductivity was compared with other literature, but its practical significance was not discussed. It is recommended to propose specific management suggestions based on the results of this study.Author Response
General comment: This study systematically evaluated the hydraulic properties, connectivity, and sustainable exploitation potential of the crystalline basement aquifers (CBAs) in southwestern Nigeria through pumping tests and tracer tests. The research question holds important practical importance. However, there is still room for improvement in the paper in terms of the depth of result interpretation and the connection with practical applications.
Response: We appreciate the positive feedback from the reviewer and took all feedback into consideration in revising the manuscript accordingly.
Comment 1: Please unify the expression of CBA or CBAs.
Response: The addition of “s” to the acronym CBA was to indicate plurality. However, for consistency we have edited it to CBA.
Comment 2: Please check that the numerical expressions in this study are consistent with the corresponding units, such as maintaining a consistent number of decimal places.
Response: We appreciate the reviewer for noticing these inconsistencies. We have reviewed the entire manuscript to ensure that all numerical values are expressed with a consistent number of decimal places and all units are correct.
Comment 3: Add a comparative analysis in the introduction. For example, "Compared with traditional models, the advantages of the model used in this study." And emphasize the synergistic effect of tracer tests and pumping tests.
Response: As suggested by the reviewer, we edited lines 127 – 144 in the introduction to highlight the advantage of our study combining multiple pumping and tracer tests in the same wells and aquifer system
Comment 4: In Section 3.1, figures can be used to assist the textual description to make the expression more concise.
Response: We appreciate the suggestion by the reviewer. However, since pumping and tracer testing methods are well established in the hydrogeological literature, providing a figure that show pumping or tracer test will be redundant for most readers. We, however, referred the readers to multiple literature where the basic theory of pumping and tracer tests are documented.
No further edit was made to the manuscript.
Comment 5: Please improve the clarity of Figures 3 - 7. Some figures lack axis units or legend descriptions.
Response: We updated Figures 3 – 9 in the revised manuscript. The figure captions were also updated accordingly.
Comment 6: In Section 5.1, the hydraulic conductivity was compared with other literature, but its practical significance was not discussed. It is recommended to propose specific management suggestions based on the results of this study.
Response: We appreciate the reviewer’s suggestion and provided a practical significance of the results of this study to section 5.1. The flowing sentence was added to lines 505 – 526:
“The variation in estimated hydraulic conductivity over two orders of magnitude (7.85 × 10-7 m/s – 1.45 × 10-5 m/s) within an area < 400 m2 has implication for exploration and management of the aquifer system in the region. This shows that groundwater flow is highly non-uniform with high volumetric flux at areas with high hydraulic conductivity. Groundwater in this area would follow discontinuous and anisotropic flow paths. This implies that within a small area groundwater availability and flow would vary significantly. Such non-uniform flow field could limit pumping and remediation in cases of contamination with recharge and potential pollutant migration being highly non-uniform. Localized high hydraulic conductivity areas would yield significant amounts of water, hence, are suitable for productive boreholes while adjacent areas with low hydraulic conductivity may lead to boreholes that dry out over time. This is consistent with dry boreholes observed within the Ibadan metropolis in Nigeria. Hence, we recommend site specific geophysical and hydrogeological characterization prior to drilling groundwater wells as boreholes drilled a few meters apart could produce significantly different results.
Also, the upper (1.45 × 10-5 m/s) and lower (7.85 × 10-7 m/s) bounds of estimated hydraulic conductivity at the site could sustain groundwater wells with moderate and marginal to unproductive wells. This further justifies the need for detailed site-specific investigations as well performance cannot be generalized from one location to surrounding areas. We recommend using these results to guide extraction rates from these aquifers to avoid over-exploitation at areas with low hydraulic conductivity. The storativity values observed in this study also suggest a relatively limited storage capacity, reinforcing the need for careful water balance assessments to ensure sustainable groundwater use.”
