Interactive comment on “ Impact of skin effect on single-well push-pull tests with the presence of regional groundwater flow ”

Groundwater tracer experiments are an important tool for the in-situ assessment of aquifer physical, chemical, and biological properties. Among other techniques, singlewell push-pull tests (PPTs) have received considerable attention over the past decades for in-situ assessment of aquifer characteristics. Early PPT papers dealing with the determination of regional groundwater flow velocity and porosity (Leap and Kaplan,


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Printer-friendly version Discussion paper 1988;Hall et al., 1991) mentioned a "velocity shadow" downgradient of the pumping well, which may adversely affect the estimation of these parameters.However, this issue has not been quantitatively addressed in the literature to date.
In their current manuscript, the authors attempt to fill this gap by producing numerical simulations of PPTs in the presence of a skin effect under regional groundwater flow conditions.As such, I see merit in this manuscript, as it would provide scientists and practitioners with important information on the accuracy of parameters obtained from PPTs conducted under these particular conditions.On the other hand, I see several important shortcomings in this manuscript, which need to be addressed before it may become suitable for publication.My main concerns are listed here, detailed issues are in the specific comments section below.
1.The manuscript currently lacks conciseness in writing and a careful review of the pertinent (including recent) literature (see specific comments 8 and 9).As suggested by the title, the focus should be on the effect of skin effects on PPTs, because this issue has not been addressed quantitatively before.But as is, the results of the COM-SOL simulations are presented in an excessively large number of figures.The authors should carefully consider which figures are essential to providing new insights into the skin effect during PPTs (i.e., the main objective of their paper), and consider combining these figures whenever possible.Unrelated figures (e.g., effect of aquifer effective porosity, dispersivity, etc. on PPT breakthrough curves) should be deleted or moved to a supplementary information section.
2. The simulation results are presented in "qualitative" fashion only, i.e., the reader can only visually compare the breakthrough curves and 2-d spatial concentration distributions between different simulations to judge the effect and relevance of the skin effect.To allow for a more quantitative comparison between simulations, the authors could, e.g., compute relative tracer mass recovered by the end of each PPT, or provide a moment analysis for mass distribution in the 2-d plots.In addition, the presented results are conditional with respect to the simulated scenarios.For readers to apply these re-

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Printer-friendly version Discussion paper sults in their own work, a more general (dimensionless) analysis of PPT breakthrough curves would be preferable.
3. An important deficiency of the current manuscript is that the authors never go beyond presenting PPT breakthrough curves and 2-d spatial concentration patterns as affected by skin effects.The central question, how the skin effect affects the estimation of aquifer properties such as regional groundwater flow velocity and porosity estimated from PPTs (which is why PPTs are conducted in the first place), remains unanswered.Without such information, the reader cannot judge the importance of this phenomenon on the results presented in this manuscript, and the relevance of skin effects during PPTs in general.Quantitative information on this issue could be provided, e.g., by applying the model of Hall et al. (1991) to simulation PPT breakthrough curves in an attempt to recover values for regional groundwater flow velocity and porosity, and to compare the latter with respective simulation input values.
4. The current writing style is poor and improvements need to be made both with regard to sentence/paragraph structure as well as grammar.The manuscript should be edited by a native English speaker.Specific comments: 1. l. 18-33: Abstract: I am afraid that the abstract is not very informative to a general audience, as it is full of unexplained, specific terminology that only an insider to the subject matter may understand.Examples are "dividing streamline", "skin", "positive skin", "negative skin".iments.Several lines below (l.43) they revisit this subject and state that a PPT may contain four phases (tracer injection, chaser injection, rest and pumping).Why not combine the two and say from the beginning that PPTs may consist of up to four phases?This would avoid confusion and redundancy.4. l. 44: The term "rest phase" is an unfortunate terminology in the context of this manuscript.Although I am aware that this term is used in some of the PPT literature, the PPT literature dealing with determination of groundwater flow velocity and porosity prefers the term "drift phase".The latter term much better reflects the conditions encountered under regional groundwater flow conditions.In addition, whereas the authors mention that "the rest phase is for tracer to diffuse and/or react with the aquifer (if a reactive tracer is employed)", they fail to mention here that such a drift phase is crucial for the determination of groundwater flow velocity and porosity (Leap and Kaplan, 1988;Hall et al., 1991).5. l. 73-74: In light of previous findings (e.g., Vandenbohede et al., 2008), I believe that the statement regarding determination of regional groundwater flow velocity is not really supported in recent literature.6. l. 75: Why a three-well minimum?A gradient may be obtained from two wells given that they are aligned in groundwater flow direction.A better explanation should be provided.7. l. 84: Here the authors return to explaining PPTs (see comment 3), and now mention three phases.This is confusing and redundant.Why not combine with previous sections (l.39/44)?8. l. 90: "that if the solute transport drifted over the location of dividing streamline toward downstream".First, it is unclear what is meant by "dividing streamline".Whenever new terminology is introduced, it should be explained to readers at the first instance it is used.Second, more importantly, and to the best of my knowledge, this is not what Leap and Kaplan (1988) have reported!They do not mention a dividing streamline

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Printer-friendly version Discussion paper 15. l. 189: "During the rest phase, the solute flux from the borehole into the aquifer is zero,. ..".I don't agree with this statement.Given that the borehole has a finite dimension in the authors' simulations, there should be solute mass contained in the borehole at the end of the injection phase, and thus at the beginning of the rest phase.This solute should get flushed out of the borehole by regional groundwater flow.
16. Table 1: I couldn't find the skin radius rs in Table 1.Is there a reason not to list it?17. l. 228: "..progressively refined near the well."It remains unclear how fine the mesh size actually was near the well.Readers wanting to repeat the simulations will need to know.19.l. 250: ".. one can see that there is a stagnation point (Sp) located at the dividing streamline (Ds) as shown in Fig. 4." This statement and figure are correct, but not new (see Monkmeyer and Netzer, 1993).Also, the term stagnation point is introduced without an explanation.What is the relevance of the stagnation point?20.l. 270: the effect of resting time: The results of this section are a logical consequence of results from the previous section, where the effect of regional groundwater 2. l. 22: The sentence "In this study, a new numerical model . ..was established" is misleading.The authors used/adapted the commercially available COMSOL code/model to simulate PPTs in a confined aquifer under regional groundwater flow in the presence of skin effects.They did not develop a new numerical (finite-element) model.3. l. 39: Here the authors describe PPTs as two-stage (injection/extraction) exper- 18. l. 239: Results: (1) The results of the COMSOL simulations are presented in an excessively large number of figures.The authors should carefully consider which figures are essential to providing new insights into the skin effect during PPTs (the main objective of their paper), and consider combining figures whenever possible.For example, Figs. 14 and 16 show PPT breakthrough curves affected by positive and negative skin effects.These two figures could easily be combined into a single figure.Other figures not immediately related to the main objective should be deleted or may be moved to a supplementary document.(2) The results are presented in qualitative fashion only, i.e., the reader can only visually compare the breakthrough curves between different simulations to judge tracer mass recovery.To improve this comparison the authors could, e.g., compute relative tracer mass recovered by the end of each PPT.This would allow for a more quantitative comparison.