correspondence
Aquifer arsenic source
a
–6
0
δ18O (Ê)
–5
–4
arsenic at 36.6 mbgl, where δ18O is −3.5‰.
The minimum δ18O value of −6.5‰ occurs
24 mbgl, where the arsenic concentration
is 50% lower (Fig. 1a, where we laterally
juxtapose Figs 1a and 1g of Neumann et al.).
Their model outputs, arbitrarily doubled
to make them approach reality 1, show that
70% of the arsenic pollution resides in
isotopically heavy water that was recharged
before irrigation pumping began (Fig. 1a
inset). Vertical profiles through the aquifer
at Bejgaon (Fig. 1c), 800 m northwest of
Bashailbhog, also show that appreciable
arsenic resides in deeper, isotopically heavy
groundwater. These findings suggest that
isotopically light recharge is not driving
arsenic pollution.
Our view — that isotopically light
water is flushing arsenic from the aquifer
in Munshiganj — is supported by the fact
–3
Unmineralized
recharge
10
b
As (mg l–1)
To the Editor — Neumann et al.1 claim
that dissolved organic carbon (DOC) in
isotopically light recharge leaking from
ponds in Bangladesh causes arsenic
pollution of groundwater. They thereby
link human activity to arsenic pollution of
groundwater that adversely affects millions
of people2. We suggest that the data of
Neumann et al. show that isotopically
light recharge is flushing arsenic from
the aquifer at their two field sites in
Munshiganj, Bangladesh.
Relying on Fig. 1 in their paper,
Neumann et al. state that the minimum δ18O
value in groundwater coincides with the
depth of maximum arsenic at 30 m below
ground level (mbgl), and so conclude that
DOC in isotopically light ‘pond recharge’
is driving arsenic pollution. Their Fig. 1
actually shows peak concentrations of
20
200
0
5
10
DOC (mg l–1)
15
70
0
250
500
200
0
5
10
Ca (mg l–1)
Post-development
recharge
15
500 1,000
As (mg l–1)
750
1,000
0
Isotopically
light recharge
40
50
60
600
Predevelopment
recharge
δ18O (Ê)
–2
–1
30
400
0
% water types
50
100
–3
20
Depth (m)
As (mg l–1)
Depth (mbgl)
60
0
0
10
20
30
40
50
60
70
80
0
As (mg l–1)
Depth (m)
50
Isotopically
normal
recharge
10
600
40
–4
0
400
Isotopically
light recharge
30
80
c
600
0
that, through a flushing zone 0–30 mbgl
at Bejgaon (data for Bashailbhog are not
available), concentrations of dissolved
species plot along linear mixing lines
between predevelopment water and
fresher recharge (Fig. 1b). Finally, if
DOC in isotopically light recharge was
driving arsenic release, as proposed by
Neumann et al., the concentration of
DOC would decrease along the flow path
as it reacts to reduce sedimentary iron
oxyhydroxides and so release arsenic; in
reality, concentrations increase along the
flow path and with depth to 30 mbgl (ref. 3).
Our proposal on flushing agrees with
previous indications that ponds do not
cause arsenic pollution in the Bengal
basin4, and that the upper part of the
shallow aquifer at Munshiganj is being
flushed of arsenic5,6.
❐
400
70
200
0
0
As (μg l–1)
5
10 15 20 25
Cl (mg l–1)
80
0
250
500
As (μg l–1)
750
Figure 1 | a, Aquifer profiles from Neumann et al. of arsenic and δ18O (Figs 1a and 1g in their paper). Yellow areas denote arsenic-rich water in the aquifer
at Bashailbhog. Dark blue area is water identified by Neumann et al. as having δ18O < −4.75‰ and so is, in their interpretation, derived from ponds. In our
interpretation, areas of both shades of blue indicate water that is flushing the aquifer of arsenic. Inset shows model outputs of Neumann et al. predicting
the proportion of predevelopment water with depth (their Fig. 1e), overlain by the distribution of arsenic in yellow. b, Cross-plots of DOC, dissolved
calcium, chloride and arsenic in the Munshiganj aquifer at Bejgaon, 800 m northwest of Bashailbhog (data from ref. 7; similar data are not available for
Bashailbhog) for depths to 30 mbgl — the depth of penetration of water that is isotopically light — and so, according to Neumann et al., derives from
ponds. We do not plot deeper waters from beneath the zone of flushing as they are of different derivation and age. The linear trends show mixing between
a mineralized groundwater around 30 mbgl and less-mineralized shallower recharge. c, Aquifer profiles of arsenic and δ18O in the aquifer at Bejgaon; data
for arsenic from ref. 7; data for δ18O from ref. 8, includes samples 30(2), 30(3) and 61(2).
NATURE GEOSCIENCE | VOL 4 | OCTOBER 2011 | www.nature.com/naturegeoscience
© 2011 Macmillan Publishers Limited. All rights reserved
655
correspondence
References
Neumann, R. B. et al. Nature Geosci. 3, 46–52 (2010).
Argos, M. et al. Lancet 376, 252–258 (2010).
Harvey, C. F. et al. Science 298, 1602–606 (2002).
Sengupta, S., McArthur, J. M., Sarkar, A. & Leng, M. J.
Environ. Sci. Technol. 42, 5156–5164 (2008).
5. McArthur J. M. et al. Appl. Geochem. 19, 1255–1293 (2004).
1.
2.
3.
4.
Authors’ reply — McArthur et al. argue
that a single process, termed aquifer
flushing, explains the pattern of dissolved
arsenic concentrations we observe in
groundwater in Bangladesh. We concur
that rice-field recharge has the potential
to flush out arsenic-contaminated
groundwater at shallow depths; we have
shown that rice-field recharge carries little
arsenic or biologically available organic
carbon that could mobilize arsenic from
the aquifer1–3. However, we contend that
their interpretation explains neither the
origin of the high arsenic concentrations
in groundwater located at intermediate
depths, nor why concentrations decline at
greater depths.
We argue that McArthur et al.’s
interpretation is inconsistent with the
physics that force groundwater layering.
Pond recharge must flow horizontally
beneath low-arsenic recharge from rice
fields, to reach the irrigation wells and river
channels where it discharges. Thus pond
recharge should predominately occupy
arsenic-contaminated intermediate depths2,4.
In their Fig. 1a, McArthur et al. compare
water isotope data from one location with
arsenic concentration data from multiple
locations, where flow patterns differ. In
fact, the δ18O minimum does align with the
arsenic peak, within the resolution of the
data, when data from the same wells are
compared, contradicting their assertion of a
656
6. Klump, S. et al. Environ. Sci. Technol. 40, 243–250 (2006).
7. Swartz, C. H. et al. Geochim. Cosmochim. Acta 68, 4539–4557 (2004).
8. Harvey, C. F. et al. Chem. Geol. 228, 112–136 (2006).
J. M. McArthur1*, P. Ravenscroft2 and O. Sracek3
1
Earth Sciences, University College London,
mismatch (see Supplementary Information
and their Fig. 1c). Our interpretation,
however, does not rely on this alignment.
Plumes of different solutes that
originate from the same source are often
not collocated. Solutes follow different
patterns because surface sorption retards
transport to varying degrees and many
solutes, including arsenic, are mobilized
from the aquifer. Reactive transport
of most solutes in groundwater is not
explained by ‘endmember’ mixing, as
suggested by McArthur et al. Indeed,
according to their proposed model of linear
mixing, the consistent decline in arsenic
concentrations with depth, below its peak,
implies more flushing towards the bottom
of the aquifer, an implication that we feel is
physically implausible.
In their Fig. 1b, McArthur et al. do not
present our measurements5 from 30 m and
below, arguing that they are of different
derivation and age. However, the origin
of the contaminated groundwater is the
question being pursued; all samples have
different ages; and ponds existed before
the advent of irrigation pumping. When
the complete data set is plotted, the trends
apparent in their Fig. 1b are no longer
evident (see Supplementary Information).
Our interpretation — that water within
the intermediate contaminated zone
originates from pond recharge — is also
supported by chemical analysis showing
Gower Street, London WC1E 6BT, UK,
2
AMEC Entec, 17 Angel Gate, City Road, London
EC1V 2SH, 3Department of Geology, Faculty of
Science, Palacký University, 17. listopadu 12, 771
46 Olomouc, Czech Republic.
*e-mail: j.mcarthur@ucl.ac.uk
that ponds provide dissolved organic
carbon that is biologically available;
carbon-dating analysis suggesting
that organic carbon concentrations
are maintained by old organic carbon
released from the aquifer; and the
observation that only pond water from
early in the dry season can provide the
isotopically light recharge matching
contaminated groundwater.
❐
References
1.
2.
3.
4.
5.
Harvey, C. F. et al. Science 300, 584D–U3 (2003).
Neumann, R. B. et al. Nature Geosci. 3, 46–52 (2010).
Neumann, R. B. et al. Environ. Sci. Technol. 45, 2072–2078 (2011).
Harvey, C. F. Nature 454, 415–416 (2008).
Swartz, C. H. et al. Geochim. Cosmochim. Acta
68, 4539–4557 (2004).
Additional information
Supplementary information accompanies this paper on
www.nature.com/naturegeoscience.
Rebecca B. Neumann1,2, Khandaker N.
Ashfaque1, A. B. M. Badruzzaman3,
M. Ashraf Ali3, Julie K. Shoemaker4 and
Charles F. Harvey1*
1
Massachussetts Institute of Technology,
Cambridge, Massachusetts 02139, USA,
2
University of Washington, Seattle,
Washington 98195, USA, 3Bangladesh
University of Engineering and Technology,
Dhaka 1000, Bangladesh, 4Harvard University,
Cambridge, Massachusetts 02138, USA.
*e-mail: charvey@mit.edu
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© 2011 Macmillan Publishers Limited. All rights reserved
SUPPLEMENTARY INFORMATION
DOI: 10.1038/NGEO1278
Supplement for Reply
Rebecca B. Neumann1, Khandaker N. Ashfaque2, A. B. M. Badruzzaman3,
M. Ashraf Ali3, Julie K. Shoemaker4 and Charles F. Harvey2
1 University of Washington, Seattle, Washington 98195, USA
2 MIT, Cambridge, Massachusetts 02139, USA
3 BUET, Dhaka 1000, Bangladesh
4 Harvard University, Cambridge, Massachusetts 02138, USA
Figure S1.
1
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(a–c) Groundwater solute profiles from the Bejgaon well cluster and solutes measured
in pond and rice field recharge water (see Figure S2 and S3 for locations, and Table
S1 – S3 for raw data). Aquifer As, Cl, Ca and DOC are from Swartz et al.1; oxygen18 and all recharge data, except Ca, are from Neumann et al.2. Calcium recharge
measurements were not previously published, but are presented in Table S2. Data are
presented only for wells from which all five solutes were measured (i.e., all shallow
aquifer wells in Swartz et al. 1 except 30(2), 30(3) and 61(2) where oxygen-18 was not
measured in 2005, see Table S1). Colored regions highlight depths in which
maximum or minimum solute concentrations could exist given the measurement
depths and assuming a single peak. The overlap of these regions demonstrates that,
within the resolution of the data, the peak arsenic concentration is aligned with
minimum oxygen-18 and Cl concentrations and elevated Ca and DOC concentrations.
Pond recharge concentrations for the conservative solutes (Cl and oxygen-18) match
those measured within the aquifer depth at which maximum arsenic concentrations
occur. However, pond recharge concentrations for reactive solutes (As, Ca, DOC) do
not necessarily match those measured within this aquifer depth, and this concentration
difference provides information about the reactions that occur as the pond recharge
water evolves into groundwater2. (d-f) Cross plots of solutes to a depth of 80 m from
Swartz et al. 1, and including 30(2), 30(3) and 61(2). In contrast to McArthur et al.’s
Figure 1B, there is no clear correlation between arsenic and either Cl or DOC because
the depth profiles are slightly offset (see a-c), as is expected for reactive solutes (e.g.,
As). The geochemical processes that couple the concentrations of arsenic, calcium
and DOC are discussed in Neumann et al.2.
2
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Figure S2.
Field site overview with sampling locations and boundary for model in Neumann et
al.2. See Tables S1 – S3 for data obtained from each sampling location.
3
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Figure S3.
ed
studi
field
“old pond”
well profiles
Zoomed in view of the Bashailbog section of the field site. See Table S1 – S3 for data
obtained from each sampling location.
4
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Table S1: Bejgaon Well Profile
Stable water isotopes collected in 2002 were analyzed in 2002 at the National
Hydrology Research Institute (NHRI) in Saskatoon. Two of these samples were reanalyzed in 2005 at the Pacific Centre for Isotopic and Geochemical Research
(PCIGR) at the University of British Columbia when isotope samples collected in
2005 were sent to this laboratory. The two re-analyzed samples sat sealed in the fridge
between the 2002 and 2005 analysis periods. Isotopes values from the PCIGR center
are lighter than those from the NHRI. Based on the high level of quality control
reported by the PCIGR center, we believe the lighter 2005 values are correct.
Therefore, we used the 2005 data for our analyses in Neumann et al.2 and above in
Figure S1.
Table S2: Recharge Data
!"#$%&'
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>8?70@87./0AB3/012;70<=:7;
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;&<70
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5
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Table S3: Bashailbog Well Profiles
!"#$%&'
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,$
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C1-65
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C1-120
C1-170
C1-230
C3-17
C3-30
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S1-17
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Table continues next page…
6
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Table S3 continued.
C4-17
C4-55
C4-60
C4-90
C4-105
C4-120
C5-17
C5-60
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C5-105
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References:
1. Swartz, C. H. et al. Geochim. Cosmochim. Acta., 68(22), 4539-4557 (2004).
2. Neumann, R. B. et al. Nature Geosci. 3, 46-52 (2010).
3. Harvey, C. F. et al. Chem. Geol., 228, 112-136 (2008).
4. Harvey, C. F. et al. Science, 298, 1602-1606 (2002).
7
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