Peer review of: Arsenic contamination of Bangladesh aquifers exacerbated by clay layers (2020)

    

 

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
The authors have robustly and reasonably defended their ms against the comments made by 3 
reviewers. They have certainly made some changes in their revised manuscript that have improved 
the robustness and accuracy of the manuscript however there are a number places where I retain 
some reservations, notably: 
 
[1] In terms of novelty, the idea of intercalated clay layers providing a critical source of organic 
carbon with the potential to drive arsenic mobilisation has been published before. 
 
I do agree, though, with the authors, in their rebuttal, that the current ms is novel in that they have 
uniquely put an outstanding combined geological, geochemical and isotopic database together with the 
aim of support this idea, particularly in relation to groundwater abstraction driven processes at their 
study site. 
 
Its clearly an Editorial decision with regard to whether the critical aspect of novelty is related to the 
idea, the particular driver of a known process or the nature of the dataset, but in my view, it's a very 
interesting dataset (particularly including the groundwater arsenic time series) in its own right, so I 
would err on the side of recommending publication, perhaps encouraging the authors to more 
explicitly state the method-related and driver-related novelty of their work. 
 
[2] The speculative nature of the assertion (albeit tempered down by the use of the word "probably") 
regarding the orange colour of the Pleistocene sediments at the time of deposition and/or early burial 
remains. Given the massive importance previously widely attached by many research groups to this 
orange/grey colour distinction being largely causally associated with low As/high As groundwater 
respectively, it seems rather odd to claim in the rebuttal that the timing (e.g. thousands of years ago 
OR last few decades OR during groundwater pumping) of this colour change was not important to 
arsenic mobilisation in the field area. However, if this IS the case, then the point is rather mute. 
 
[3] My major concern remains the timescales of the observed changes compared to plausible and 
actual timescales of the arsenic mobilising processes discussed - these concerns are important 
because they inform what are the major plausible processes leading to the observed changes in 
groundwater arsenic - particularly noting that the observed changes (Figure 1) are, in various wells 
and particular time periods, any of (i) increasing; (ii) decreasing and (iii) no trend with time. 
 
In the rebuttal, the authors claim that the increased arsenic in the 41 m-screened well is clearly 
associated with massive drinking water pumping around Dhaka. That is a very reasonable first-glance 
hypothesis to test - but the association (let alone any causal link) is NOT clearly demonstrated in this 
manuscript - not least of all because: 
 
(i) the period of massive pumping in and around Dhaka is not clearly indicated in the manuscript (my 
apologies if I have missed that); 
 
(ii) the increase in groundwater arsenic in the 41 m well (Figure 1) seems to be largely restricted to 
the years 2014-2016 - before that the groundwater arsenic time series seems pretty flat, and after 
2016 the groundwater arsenic time series also seems pretty flat. Presumably massive groundwater 
pumping in and around Dhaka was taking place across ALL of these time periods (i.e. 2011-2014, 
2014-2016, 2016-2018) ... so its difficult to be convinced of an association between this groundwater 
pumping and increase in groundwater arsenic from the data presented (particularly in the absence of 

the appropriate groundwater abstraction data and any explicit appropriate statistical test/analysis to 
demonstrate the claimed association) 
 
(iii) over the same period of time, groundwater arsenic in the 51 m well increases between 2011 and 
2014 and then there is a pretty flat time series after that; given that groundwater arsenic at 51 m is 
massively higher than the pre-2014 groundwater arsenic at 41 m - is it not equally or more plausible 
that the changes observed in groundwater arsenic at the 41 m well rather reflect the movement of 
higher arsenic groundwaters from elsewhere within the aquifer ? - with this movement accelerated by 
the increased hydraulic gradients caused by massive groundwater exploitation ? This is obviously a 
quite different process from that proposed in the manuscript. 
 
(iv) interestingly, as the authors state, in the 64 m well, they observe a DECREASE in groundwater 
arsenic with time - this could also be readily explained by the same process as in (iii) above - with 
different directions of groundwater arsenic vs time trends consistent with the well known considerable 
heterogeneity of aquifer sediments in this region. 
 
It would be useful to have point [3] in particular satisfactorily addressed. 
 
 
 
 
 
 
 
 
Reviewer #2 (Remarks to the Author): 
 
Review of revised manuscript 
 
Dear Editor, dear authors 
 
In this 2nd round, I first read the authors complete ’rebuttal’ document and then I read the 
manuscript again. Although the authors have only made very tiny changes to the original submission, 
I must say their text is clearer to me after having read the rebuttal document than before. This I think 
shows that the authors need to improve their text so that any other first-time-reader will not struggle 
with concerns, popping up while reading, about the validity of the arguments. By having read the 
rebuttal I was able to ignore these concerns in my second reading. 
 
To mention an example of the tiny degree of revision: The proposed use of d-excess was agreed with 
in the rebuttal document, and d-excess has replaced 2H in E.D. Fig. 3. But the main text (lines 189-
190) and the Supplementary Discussion still refer to the delta-value of deuterium and not to d-excess. 
A revision ought to be much more thoroughly carried out than that. 
 
I assume the authors acknowledge that the review process is not mainly intended to be a debate 
between the authors and the few reviewers. Rather, the reviewers are a ‘test-audience’ and the 
authors should welcome any critique and use it to improve their manuscript finally resulting in the 
best possible published paper. 
 
I would like to point attention to two of my ‘major issues’ from the 1st round that I think were not 
satisfactory addressed in the revised manuscript. Perhaps in this second round I am able to be more 
specific with my concerns. I maintain that I strongly recommend publication in Nat. Comm., but I also 

suggest certain improvements prior to final acceptance and publication; with reference to the major 
issues of the 1st round: 
 
Re. ‘Major issue 1’: 
In the revised manuscript the distinction of deep vs. not-deep(?) Pleistocene aquifer(s) has been come 
more unclear. Now, in the revised ms Pleistocene aquifers are referred to as ‘deep’ as long as they 
predate Holocene (<12 ka; first sentence in Abstract). At the same time, the studied Pleistocene 
aquifer is referred to as ‘intermediate’ (L46). Lines 42-46 directly suggest that the actually studied 
Pleistocene aquifer at mainly 40-75 m depth (e.g., Fig. 2) is just a ‘more accessible’ version of the 
Pleistocene aquifer at >150 m. Lines 79-80 suggst that we are in fact dealing with a risk for shallow 
(‘upper’) Pleistocene aquifers. This confusion is unnecessary. I would say that the Pleistocene aquifer 
directly underneath Holocene sediments must be referred to as ‘shallow Pleistocene’, not deep, nor 
intermediate. 
 
In the rebuttal the authors state that they see no fundamental difference between Pleistocene aquifer 
materials of different depth. By ‘fundamental difference’, I assume they refer to the hydrogeology, 
geochemistry and reactivity related to the potential As release and mobility. To me, an obvious 
fundamental difference in the context of the study is that the shallow Pleistocene aquifer is capped by 
(young) Holocene clay (=source of carbon). This is of course not the case for deeper Pleistocene 
aquifers. Moreover, the studied aquifer sediments are young because they are post-LGM deposits, 
while the >150 m sediments are much older because they are pre-LGM deposits. For that reason 
alone I would assume it is very likely that some fundamental differences occur, even if it doesn’t 
shown in the variables measured by in the study. 
 
I suggest that the authors revise the manuscript to reflect specifically that their data and conclusions 
are for a Pleistocene aquifer capped by Holocene clay. Their finding does not need to apply to >150 m 
deep pleist. aqf. conditions; the conclusions are important enough to show this well for the uppermost 
Pleistocene aquifer. 
 
Re. ‘Major issue 3’: 
The smoking gun is the current increase in As just below the capping clay, in water without a trace of 
tritium, etc. as opposed to low As water in modern recharge. This IS the story. But the ms deals a lot 
with whether or not the grey sediment was oxidized from the start or not. They end up concluding that 
this actually doesn’t matter, but on the way at least two speculative statements are presented which 
could be deleted if the discussion was organized differently: 
 
Statement 1: 1/3 of the water in the uppermost part of the Pleistocene aquifer the stem from the 
Holocene capping clay. 
 
The statement is based on Cl. However, the delta-value of 18O for the uppermost samples in the 
aquifer sand (41 m depth) are similar to or perhaps even more negative than in the pore water of the 
capping Holocene clay and more negative than deeper in the aquifer. These data hence do not support 
any mixing between the clay and the aquifer, although likely just because no samples of clay pore 
water were obtained from the very lowest part of the capping clay unit. Accordingly, statement 1 is 
shown to be wrong by a very good tracer, and therefore the statement is so weakly founded that is 
should be omitted. 
 
Statement 2: Even though the combined advective and diffusive downwards carbon flux can only bring 
about 1 meter of sediment from a Fe(II)/Fe ratio of 0.3 to the 0.5 threshold for As release, the flow 
situation due to the variable elevation of the clay/aquifer interface will create a much deeper reduction 
into the underlying aquifer. 

 
I disagree. If we assume that the advective leakage from 50 yr pumping + 5000 yr diffusive flux from 
the bottom of the clay results in a combined carbon flux of 12 mol/m2 then vertical dispersive mixing 
or other types of flow beneath the clay do create more electrons than 12*4 for Fe reduction. 
Statement 2 in my view should be deleted. 
 
The authors have the smoking gun. They don’t need to explain everything else around it. 
 
Nevertheless: I suggest that the authors try to calculate how much arsenic may be released by the 
reductive dissolution of 12*4 moles Fe-oxides. I get that the As released will be enough to 
contaminate lots water to several hundred µg/L: 
 
In one of my previous studies, let’s assume that there’s just ca. 0.01 µmol As/g sediment (Fig 5, 
Jessen et al. 2012, GCA) of quite loosely bound As. In 1 m3 of aquifer sand (porosity 0.3) this 
amounts to 1800 kg/m3 * 1000g/kg * 0.01 µmol/g * 75 µg/µmol = 1,800,000 µg As. If this As is 
dissolved in the 1 m3’s 300 L of porewater the concentration will be 6000 µg/L. The loosely bound As 
content is hence enough to contaminate 30 pore volumes of the 1 m3 to 200 µg As/L. Both the high 
flux of 12 mol C/m2 and the many pore volumes would require a high leakage = recent phenomenon 
caused by Dhaka pumping. 
 
The sediment was probably already reduced from the beginning. 
The sudden extra input of carbon is what triggered an addition quick As release which now shows up 
in the wells. 
 
Best regards 
Soren Jessen 
 
 
Reviewer #3 (Remarks to the Author): 
 
I feel the authors have adequate addressed the review comments raised. Although several of the 
points are actually still unknown (not only within this study but in the field in general), the authors' 
statements seem reasonably well-justified and the revisions help to address the concerns raised by all 
reviewers.