A multi-tracer approach to constraining artesian groundwater discharge into an alluvial aquifer

Iverach, Charlotte P.; Cendón, Dioni I.; Meredith, Karina T.; Wilcken, Klaus M.; Hankin, Stuart I.; Andersen, Martin S.; Kelly, Bryce F. J.

doi:https://doi.org/10.5194/hess-21-5953-2017

Articles | Volume 21, issue 11

https://doi.org/10.5194/hess-21-5953-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/hess-21-5953-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 21, issue 11

Research article

|

28 Nov 2017

Research article |

| 28 Nov 2017

A multi-tracer approach to constraining artesian groundwater discharge into an alluvial aquifer

Charlotte P. Iverach, Dioni I. Cendón, Karina T. Meredith, Klaus M. Wilcken, Stuart I. Hankin, Martin S. Andersen, and Bryce F. J. Kelly

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Final revised paper (published on 28 Nov 2017)
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Preprint (discussion started on 21 Jun 2017)
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Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review of "A multi-tracer approach to constraining artesian groundwater discharge into an alluvial aquifer"', Anonymous Referee #1, 14 Jul 2017
- AC1: 'Response to reviewer 1', Charlotte P. Iverach, 13 Sep 2017
RC2: 'Referee Comments on "A multi-tracer approach to constraining artesian groundwater discharge into an alluvial aquifer" by C.P. Ivervach et al.', Anonymous Referee #2, 24 Jul 2017
- AC2: 'Response to reviewer 2', Charlotte P. Iverach, 13 Sep 2017
RC3: 'Comments to A multi-tracer approach to constraining artesian groundwater discharge into an alluvial aquifer', Anonymous Referee #3, 02 Aug 2017
- AC3: 'Response to reviewer 3', Charlotte P. Iverach, 13 Sep 2017

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Publish subject to revisions (further review by Editor and Referees) (14 Sep 2017) by Markus Hrachowitz

AR by Charlotte P. Iverach on behalf of the Authors (09 Oct 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (09 Oct 2017) by Markus Hrachowitz

RR by Anonymous Referee #1 (13 Oct 2017)

Suggestions for revision or reasons for rejection

Review of “A multi-tracer approach to constraining artesian groundwater discharge into an alluvial aquifer”

This is a much better paper than the original version and I consider that it is suitable for publication in HESS following moderate revisions. I have made several specific comments below that are mainly concerned with the details of the paper. The overall methodology and conclusions are sound and the aims and objectives are generally clear.

The one major concern that I have is with the calculation of 36Cl residence times. The interpretation of 36Cl is difficult in the best of circumstances due to problems with defining the 36Cl input function over time (given that the rainfall patterns, distance from the coast, input of dust etc is likely very different several hundred thousand years ago to today). Additionally, in many studies (including this one), the in situ production rate of 36Cl is assumed rather than measured and the definition of residence times via Eq. (1) implicitly assumes piston flow which is also unlikely. In groundwater such as that discussed in this study, there is also the problem of mixing which means than the calculations can only be carried out in waters that can be guaranteed to be the pre-mixing endmembers (which given the extent of mixing may be difficult). The fact that the GAB groundwater has long (several 100 ka) residence times is well established in previous studies and this paper does not add to that understanding. It would be sufficient to note that the low 36Cl activities are consistent with the old GAB groundwater and focus on the mixing.

Specific Comments

Lines 50-55. Strictly recharge is infiltration of rainfall, what you are discussing here is mixing between recently recharged waters and older upward flowing groundwater. Since your main story is mixing and the water balance then I would frame the first sentences around that topic.

Lines 62-75. Similar comments apply. The Scanlon study is specific to recharge but some of the others deal with mixing in aquifers.

Lines 79-83. Not very clearly expressed. It would be good to reference Jascheko (2016, Chemical Geology, 427, 35-42) who deals with this in some detail.

Lines 89-93. The interpretation of 36Cl is also hampered by uncertainties in the input function. Because rainfall R36Cl values vary temporally due to climate variations the input function at any time in the past may not be the same as it is today (e.g., Phillips et al., 2000. In: Cook & Herczeg, Environmental Tracers in Subsurface Hydrology. pp. 299-348).

The introduction could be clearer and framed more specifically around understanding mixing between groundwater from different sources in aquifers. As it is you use recharge and discharge alternately to describe the input of the deeper waters in the alluvials. Also it would be good to note that geochemistry allows us to understand long-term patterns of mixing and groundwater flow whereas using hydraulic heads (especially in systems perturbed by land clearing or water abstraction) only informs present day groundwater flow.

While the choice of references is always personal, there has been more done in this field than is apparent from the introduction. For example, the numerous references by Edmunds (especially the review in Applied Geochemistry, 24, 1058-1073). These are also several Australian examples of the use of major ions, stable isotopes, and radioactive isotopes by CSIRO (Herczeg and co-workers) and the Monash (Cartwright and co-workers) groups.

Lines 109-115. This is a rather convoluted way to say that you use the radioisotopes to determine mixing between waters with long and short residence times. It is obvious that determining residence times in mixed waters is nigh on impossible and I don’t think that you need this as your explanation above is clear.

Line 134 (and elsewhere) ML is a commonly used unit in Australia but less so elsewhere. Suggest using m3 (which is the conventional SI unit).

Section 2 is much more relevant and informative than in the previous version of the paper. Adding the groundwater flow directions to one of the maps and the cross-sections would be good.

Lines 245-246. Can you give typical screen intervals (it is important as interpretation of geochemistry from short-screened wells is much easier than from those with long screens)?

Lines 268-275. A bit of a longwinded way of saying that you used existing data to characterise the GAB groundwater.

Lines 278-283. This point about the aquifer structure was made earlier. While it might be of local interest, I am not sure that it is worth repeating through the paper (and in many ways it is misplaced as you are stating a conclusion in the methods section). It is a common observation globally that geological formations (which might be determined on ages) do not always form separate aquifers. Your explanation on lines 178-183 explains it well enough.

Lines 290-291. That is precision not accuracy.

Lines 335-339. You seem to be describing the procedure for getting the CBE correct. If you determined NH4 and the subsequent CBE was OK, then just make sure that the NH4 concentrations are in the table and report your final CBEs.

Lines 342-348. These details are getting beyond strict methodology and it would be better to discuss the exact compositions of the endmembers when you present the results of the calculations.

Lines 349-359. This section is fine. However, as noted elsewhere these calculations do not add much to the paper and are highly speculative given that mixing has occurred.

Lines 363-403. This section describing the geochemistry is far better than in the first version of the paper. The only thing that I would suggest adding is the standard deviation to the average concentrations.

Fig. 3a. It is not clear what the calcite saturation line is (explain in caption or text).

Lines 406-414 (and elsewhere). Given the quoted precision your δ2H values should not have decimal places and δ18O values should only have 1 decimal place.

Lines 410-412. Add a reference for the evaporation trend and perhaps rephrase it as “define a trend to the right of the meteoric water line with a slope of XX that is consistent with evaporation (REF)”.

Lines 409-410. Are the local sites close? Given that your samples seem to lie on the GMWL, I’m not sure whether the LMWLs are relevant (?)

Lines 422-433. Here and elsewhere I would specify “<0.04 TU” rather than “detection limit” in the text as it is more specific.

Lines 423-425. The statement that you have modern recharge is an interpretation. Try to rephrase it so you explain to the reader why you said this (eg “The highest 3H activities of XX are near the river… these are similar to those of modern rainfall(?)… this implies…”). On that point I presume that you can infer a modern rainfall 3H activity from the Tadros (2014) compilation and that would be worth doing.

Lines 426-429. This is true but the explanation is a little brief for anyone not familiar with southern hemisphere 3H. A couple more sentences would be helpful.

Fig. 5. Suggest plotting the samples that are below detection on the left hand axis as the 3H values have no actual meaning. The same is true in your tables (ie designate these as “<0.04” or “bd” rather than quoting actual values).

Lines 492 to 497. This section is not very clearly expressed. You could add a few more details (eg where in the region these observations come from and whether the data are from the same wells). Also you should stress that both observations (the high initial 3H and the subsequent increase in 3H) imply recharge from the surface.

Line 492. Where does the estimate of <70 years come from?

Lines 503 to 505. Does this interpretation agree with the major ion geochemistry? For example if your deeper water is 14C-free and the surface water has a 14C activity close to 100 pMC, then given the relative HCO3 concentrations how much old water do you need to add to reduce the 14C down to the observed values? What then would the predicted 3H activities of the mixed water be? The interpretation is plausible but it would be good to see it confirmed.

Fig. 6. I found the reversed axis for 14C confusing (is there any reason why 0 is not at the left especially since 3H is plotted conventionally). Also due to the waters containing different DIC concentrations, mixing lines are curved (if you know the relative DIC concentrations, you should be able to predict whether the curvature is concave or convex).

Lines 531-562. As outlined above, I think that estimating residence times using 36Cl in waters that have undergone mixing is not very convincing. It is clear from the available literature that the GAB waters have residence times of several hundred thousand years and this study does not add to that understanding. I strongly suggest that this section is reframed to discuss the mixing with a statement referenced to previous works that the low 36Cl in GAB groundwater is a consequence of the (already established) long residence times.

Lines 546-547. The Cl/Br ratios are a better indication of the lack of halite dissolution than the Cl concentrations.

Lines 593-597. As in the introduction, it is worth stressing here that the geochemistry provides an indication of the long-term groundwater flow and mixing whereas hydraulics in perturbed systems show how the system is currently functioning.

Section 5.3. This section appears almost as an afterthought (it has its own introduction, data description, and interpretations) and is speculative. While the data is interesting they are not that well integrated with the main part of the paper. I suggest shortening the material and noting that the 14C contents in some of the groundwater has changed over time implying a dynamic system without trying to over interpret it. As it is, the data seems to have been collected without a specific hypothesis in mind (eg collecting from an area before and after groundwater extraction was established) and so are not that easy to interpret with any confidence.

Conclusions. These are a summary of the paper. Given HESS is an international journal, it would be good to emphasise what is of broader interest. Some of that material (changes over time, general utility of tracers) appears in Section 5 but probably would be better here to round off the study.

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ED: Publish subject to minor revisions (further review by Editor) (13 Oct 2017) by Markus Hrachowitz

AR by Charlotte P. Iverach on behalf of the Authors (22 Oct 2017) Author's response Manuscript

ED: Publish as is (23 Oct 2017) by Markus Hrachowitz

AR by Charlotte P. Iverach on behalf of the Authors (24 Oct 2017)

Short summary

This study uses a multi-tracer geochemical approach to determine the extent of artesian groundwater discharge into an economically important alluvial aquifer. We compare estimates for artesian discharge into the alluvial aquifer derived from water balance modelling and geochemical data to show that there is considerable divergence in the results. The implications of this work involve highlighting that geochemical data should be used as a critical component of water budget assessments.