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ARTICLE
Arsenic contamination of Bangladesh aquifers
exacerbated by clay layers
Ivan Mihajlov 1,2, M. Rajib H. Mozumder 1,3,7, Benjamín C. Bostick 3, Martin Stute3,4, Brian J. Mailloux 4,
Peter S. K. Knappett 5, Imtiaz Choudhury6, Kazi Matin Ahmed6, Peter Schlosser 1,3,8 &
Alexander van Geen 3✉
Confining clay layers typically protect groundwater aquifers against downward intrusion of
contaminants. In the context of groundwater arsenic in Bangladesh, we challenge this notion
here by showing that organic carbon drawn from a clay layer into a low-arsenic pre-Holocene
(>12 kyr-old) aquifer promotes the reductive dissolution of iron oxides and the release of
arsenic. The finding explains a steady rise in arsenic concentrations in a pre-Holocene aquifer
below such a clay layer and the repeated failure of a structurally sound community well.
Tritium measurements indicate that groundwater from the affected depth interval (40–50 m)
was recharged >60 years ago. Deeper (55–65 m) groundwater in the same pre-Holocene
aquifer was recharged only 10–50 years ago but is still low in arsenic. Proximity to a confining
clay layer that expels organic carbon as an indirect response to groundwater pumping, rather
than directly accelerated recharge, caused arsenic contamination of this pre-Holocene
aquifer.
https://doi.org/10.1038/s41467-020-16104-z OPEN
1Department of Earth and Environmental Sciences, Columbia University, New York, NY 10025, USA. 2Geosyntec Consultants, Huntington Beach, CA 92648,
USA. 3Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA. 4Environmental Sciences, Barnard College, New York, NY
10025, USA. 5Department of Geology & Geophysics, Texas A&M University, College Station, TX 77843, USA. 6Department of Geology, University of Dhaka,
Dhaka, Bangladesh.
7
Present address: Gradient, Cambridge, MA 02138, USA.
8
Present address: School of Sustainability, Arizona State University, Tempe, AZ
85281, USA. ✉email: avangeen@ldeo.columbia.edu
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Most of the rural populations of Bangladesh and several
neighboring countries obtain their drinking water from
shallow tubewells that often do not meet the World
Health Organization guideline of 10 µg/L arsenic (As). Chronic
exposure to As above this level has been linked to increased infant
and adult mortality, inhibited intellectual and motor function in
children, and significantly reduced household earnings1–4.Inan
effort to reduce As exposure, government and non-governmental
organizations in Bangladesh have installed several hundred
thousand deep (>150 m) community wells that are often,
although not always, low in As5–11. Impermeable clay layers
capping these deep low-As aquifers were deposited before the
onset of the current Holocene epoch ~12 kyr ago and are widely
seen as protective because they inhibit the downward flow of
overlying high-As groundwater12,13. The present study of a more
accessible pre-Holocene aquifer in an intermediate (40–90 m)
depth range challenges this notion by considering biogeochemical
reactions initiated by clay layers that could trigger the release of
As to underlying aquifers14–17.
Microbially-mediated reduction of iron (Fe) oxides coupled to
organic carbon oxidation is held responsible for the widespread
release of As from Holocene sediments to groundwater in the
Bengal basin7,18–23. A similar process can occur in pre-Holocene
sands where it is made apparent by the conversion of orange
Fe(III) to gray Fe(II) oxides in response to a supply of organic
carbon24–27. The sources of this organic carbon could be
immobile plant matter co-deposited with aquifer materials or
mobile reactive dissolved organic carbon (DOC) advected by
groundwater flow. The relative importance of these two pathways
for As release to groundwater is still debated7,18,21,28–34.
Our detailed study of a site in Bangladesh illustrates that
organic carbon within clay layers, defined here to include both
clay and silt and are also referred to as mud35, is a third source of
reactive carbon that can be mobilized by distant pumping and
result in the contamination with As of a pre-Holocene aquifer.
The new data show that reactive carbon released by clay layers
can instead drive chemistry changes in aquifers14 and trigger the
release of As to underlying groundwater. The new results offer the
most direct evidence yet of a new mechanism for groundwater
contamination with As triggered by pumping, which was inferred
from observations elsewhere in Bangladesh, the Mekong delta of
Vietnam, and the Central Valley of California15–17.
Results
Failures of a community well. The study was motivated by a
sudden increase in As concentrations from <10 to 60 µg/L in
groundwater from a hand-pumped community well (CW12) in
200536, i.e., 18 months after installing a 3-m long screen in orange
sands of a pre-Holocene aquifer at 60 m depth in a village 20 km
east of Dhaka (Fig. 1a). Local drillers guided by the orange color
of sands commonly install household wells in the 30–90 m depth
range in the study area (Fig. 1a) and elsewhere in the Bangladesh
basin9,25,36–39. Leaks of high-As groundwater40 that could have
caused the increase were not detected by pumping sections of the
well with an inflatable packer36. The second installation of a
community well screened within a few meters of the initial well
confirmed that the orange sands are capped by a 10-m thick layer
of clay at the site but this well also failed after producing low-As
water for several months (the failed well was replaced with a
deeper low-As well at 90-m depth soon after the problem was
detected for the second time). The failure of two separate wells,
manually pumped at modest flow rates, indicates that leakage of
shallow groundwater along the well annulus is unlikely to have
been the cause of the increase in As concentrations. The concern
that such failures could instead be an early sign of broader
contamination of pre-Holocene aquifers within the cone of
depression in groundwater levels due to massive deep pumping
for the municipal water supply of Dhaka41,42 led to further study
and the monitoring reported here.
Drilling and monitoring of the impacted aquifer. Drill cuttings
collected in 2010–11 while installing four nests of monitoring wells
within a radius of 100 m of the failed community well confirm the
presence throughout the area (Site M) of a 6–13-m thick clay layer
separating a shallow aquifer composed solely of gray sands from a
deeper aquifer containing both orange and gray sands (Fig. 2c).
Lithologs also show that the layer of orange sand tapped by the
failed community wells is at least 9-m thick throughout the area and
intercalated between gray sands above and below (Supplementary
Fig. 1). Concentrations of As in the four monitoring wells installed
in the orange sand were <10 µg/L (Fig. 3e), including the mon-
itoring well location (M-Middle) installed within <10 m of the failed
community well. Monitoring wells in the gray sands overlying the
orange layer in the same aquifer initially contained concentrations
of As ranging from 20 to 250 µg/L, whereas concentrations in the
layer of gray sand below were <5 µg/L. Similarly, concentrations of
dissolved Fe were >5 mg/L in gray intervals overlying the orange
sand layer, <1 mg/L in most orange intervals, and generally low also
in the gray sand below the orange interval (Fig. 3f). Between 2011
and 2018, concentrations of As at the central nest of monitoring
wells (M-Middle) remained low at 61 and 64 m, but two shallower
monitoring wells at 41 and 51-m depth at the same location show
worrisome increases in As concentrations from 50 to 150 µg/L and
from 250 to 400 µg/L, respectively, over the same period (Fig. 1b).
Both of these monitoring wells were installed in gray sand below
the thick clay layer separating the aquifer from the shallower
Holocene aquifer, which is consistently elevated in As (Fig. 3eand
Supplementary Fig. 1). Simple linear extrapolation of the time series
suggests As concentrations could have started to rise above the 5 µg/
L level typical of uncontaminated pre-Holocene aquifers around
2009 and 2003 at 41 and 51-m depth, respectively.
Depositional history of impacted aquifer. Radiocarbon (14C)
dating and other sediment characteristics document the deposi-
tional history of the aquifer tapped by the failed community well.
Within several interspersed thin layers of clay in the sandy
37–39-m depth interval below the main clay layer, plant leaves
and pieces of charcoal and wood were recovered. In all but one
case, 14C ages of this material were within a few decades of
the bulk organic carbon ages of the associated clay, indicating that
bulk clay reflects the 14C content of the atmosphere and can be
used without reservoir correction (Supplementary Table 1). The
depth profile of radiocarbon ages indicates steady accumulation
of 40 m of sediment from 17 to 5 14C kyr ago (Fig. 2a). The data
indicate that the upper portion of the pre-Holocene aquifer was
deposited ≥12 14C kyr ago (Supplementary Table 1; Fig. 2a),
which in calendar years corresponds to ≥14 ka when correcting
for changes in the 14C content of the atmosphere43, and corre-
sponds to a period when sea level was still below its current level.
A radiocarbon age of 35–38 14C kyr for the clay layer at the
bottom of the aquifer at 79-m depth indicates that the period of
rapid sediment accumulation at the Late Pleistocene/Holocene
transition, paced by the rise in sea level, was preceded by slower
accumulation or perhaps even erosion35,44. The data confirm that
deeper pre-Holocene sands can be gray45 and yet be associated
with low-As concentrations in groundwater, as reported else-
where in the Bengal basin7.
In spite of differences in color, the drill cuttings indicate that the
entire pre-Holocene aquifer in the 40–70-m depth range, composed
today of both gray and orange sands, was probably deposited under
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23.760
23.765
23.770
23.775
23.780
23.785
23.790
90.615 90.620 90.625 90.630 90.635 90.640 90.645 90.650
Longitude (degrees east)
0–10 μg/L As <0.1 TU Failed well - CW12
25–50 μg/L As 0.1–1.0 TU
100+ μg/L As 1–6 TU
.
Bangladesh
Araihazar
Arsenic in
2012-13 survey wells
45-90 m deep:
3H in clay-capped
community wells
<90 m deep:
M-Middle well depth,
symbols color-coded as in (a)
1 km 100 m
M-South
M-North
M-West
M-Middle
New
CW12
M-Core
Site M 100 m
2011/12 Average (as in Figure 3e)
0
50
100
150
200
250
300
350
400
450
Jan-11 Jan-13 Jan-15 Jan-17 Jan-19
Groundwater As (
μ
g/L)
41 m 51 m 61 m 64 m
for initial As content:
b
Latitude (degrees north)
a
Fig. 1 Regional and site map with tritium (3H) and arsenic (As) distribution. a Map of the field area and the focus site M. The small symbols denote the
wells 45–90 m deep, surveyed in 2012–13 and color corded according to their As concentration measured using the ITS Arsenic Econo-Quick kit37,70. The
large symbols denote the surrounding (2 km radius) community wells installed <90 m deep within a clay-capped low-As aquifer36 and are color coded to
reflect the highest measured 3H concentrations in tritium units (TU), as reported in Mihajlov et al.48. The enlarged inset map of site M displays sampled
multi-level well nest locations, an additional coring location, and the location of community well (CW12) where arsenic concentrations rose twice prior to
reinstallation at a greater depth. bTime-series of As concentrations in groundwater at the well cluster M-Middle from 2011 to 2017 (41 m) or from 2011 to
2016 (other depths); 2011/2012 average As concentrations plotted on Fig. 3e are also shown.
0.00.5 1.0
ΔR at 520 nm
04812
Total Ca (g/kg)
0.0 0.5 1.0
HCl leach. Fe(II)/Fe
Clay
Gr. sand
Or. sand
0
10
20
30
40
50
60
70
80
010203040
Depth (m b.g.l.)
14C age (kyr)
M clay TOC
M fossil
abc d
Fig. 2 Site sediment vertical profiles. a Conventional radiocarbon (14C) ages expressed in thousands of years (kyr) measured on fossil plant material
embedded in the sediment and/or total organic carbon of the clay layers. bTotal calcium (Ca) content determined by X-ray fluorescence. cDiffuse spectral
reflectance between 530 and 520 nm (ΔR). dPercentage of Fe(II) within the total Fe extractable by 1 N hot HCl. Sand color, quantified by ΔR and dictated
by Fe speciation, is explicitly displayed to visualize orange and gray sand distribution. Results from the four multi-level well nest boreholes and the
additional coring location are combined in the graphics and were used to prepare the generic site lithology displayed on the right.
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similarconditionsuntil14ka,whensealevelwasstillquite
low. There is no clear difference in grain size or exchangeable As
content of gray and orange sands within the pre-Holocene layer
(Supplementary Fig. 2). The calcium (Ca) content of cuttings
from the entire pre-Holocene aquifer averages 3 ± 2 g/kg (1-sigma)
between 40 and 70 m. The lower portion of the overlying clay layer
to ~35-m depth contains even less Ca at ~1 g/kg (Fig. 2b). Above
this interval, Ca concentrations sharply shift upwards and remain
elevated at 7 ± 2 g/kg throughout the shallow Holocene aquifer.
This contrast has been observed elsewhere and attributed to the
combination of authigenic precipitation of calcium carbonate in
Holocene sands and extensive weathering of pre-Holocene sands
while sea level was lower than today26,46. The key question is why
concentrations of As in pre-Holocene sands below the clay layer
have been rising since at least 2011 as this likely played a role in the
repeated failure of the local community well.
3H (TU)
01 2 3 020406080
3H/3He age (yr)
0
10
20
30
40
50
60
70
80
2.5 3.5 4.5
Depth (m b.g.l.)
Water level (m a.s.l.)
–5
δ18O (‰)
–4 –3 –2 –1
0
10
20
30
40
50
60
70
80
1 10 100 1,000
Depth (m b.g.l.)
As (μg/L)
0 5 10 15
Fe (mg/L)
a b c d
e g h
0 204060
Cl (mg/L)
0 5 10 15 20 25
DOC (mg/L C)
M-Middle M-South M-West M-North
Groundwater
(sediment color)
M-Middle clay
pore water
f
Fig. 3 Vertical profiles of groundwater and clay pore water properties. A generic site litholog is displayed on the right with shading in the panels
indicating the extent of major clay/silt layers encountered. aWater levels are the annual average (December 2012–November 2013) groundwater
elevations in meters above sea level. bTritium (3H) concentrations and dOxygen-18 isotopic composition in water (δ18O) are one-time measurements
with analytical error bars smaller than the symbol size. cTritium-helium (3H/3He) ages were corrected for radiogenic He contribution and degassing at the
time of sampling, where necessary; error bars indicate propagated analytical errors or standard deviations of the ages determined under different
assumptions, whichever error is greater. eArsenic, firon, and hchloride concentrations in groundwater were averaged from discrete samples collected in
2011–2012 (arsenic data through 2016/17 from well nest M-Middle are shown in Fig. 1b); at depths where >3 samples were measured, standard deviations
are also shown (As and Fe); Cl standard deviations are smaller than the symbol size. gDissolved organic carbon concentrations.
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Contamination with arsenic by advection from shallow aquifer.
The shallow Holocene aquifer is one potential source of either As
or organic carbon that could have triggered the local release of As
through potential lateral discontinuities in the clay layer47. This
pathway is a possibility given that, throughout the year, the water
level in the aquifer tapped by the failed community well is at least
1 m below the water level in the shallow Holocene aquifer (Fig. 3a
and Supplementary Fig. 4). The difference in hydraulic head is
not driven by irrigation pumping, which only draws water from
the shallow aquifer during winter months (Supplementary Fig. 4),
but rather by massive pumping from the deep aquifer for the
municipal water supply of Dhaka at a distance of 20–30 km to the
west41,42. A steady intensification of this vertical hydraulic gra-
dient over two decades has been documented at a site 2 km closer
to Dhaka, where the difference is even larger than at the study
site48 (Supplementary Fig. 5).
Regional downward flow induced by massive pumping in
Dhaka is confirmed by the penetration of the radioisotope tritium
(3H) produced by atmospheric testing of nuclear weapons in the
1950s and 1960s in portions of the pre-Holocene aquifer (Fig. 1a).
Levels >1 tritium unit (TU) were detected within clay-capped
orange sand in the 35–90 m depth range in four out of 18
community wells within a 2-km radius of the failed community
well36,48. Concentrations of As were not elevated in these
community wells, however, which is consistent with findings at
the study site. Elevated levels of 3H (>0.1 TU) were detected in a
total of nine monitoring wells tapping the pre-Holocene aquifer
in the 50–70-m depth range at Site M (Fig. 3b). The contribution
of recent recharge is the largest in the orange sands at 50–60-m
depth where groundwater As concentrations are low (i.e., in the
middle of the pre-Holocene aquifer) and much lower to
undetectable (≤0.1 TU) near the bottom of the clay layer capping
the pre-Holocene aquifer.
One possible entry point for 3H-containing water to the 55–65-
m depth range may be an area 500 m to the south of the study site
where a thick clay layer capping the pre-Holocene aquifer is
missing49, but there may be other entry points. Profiles of
groundwater ages based on the tritium-helium method show that
the youngest ages of 10–40 years are focused in the orange sands
at this site (Fig. 3c and Supplementary Note 1). Transport to these
orange sands must be rapid since the 3H–3He ages bracket the age
of groundwater in the shallow Holocene aquifer measured in the
area (Fig. 3c)50. Adsorption to aquifer sediments has evidently
been sufficient to delay any detectable influx of As or reactive
DOC from the shallow aquifer to this portion of the pre-
Holocene aquifer26,46,51.
Alternative mechanism for aquifer contamination with arsenic.
Contamination with As of the pre-Holocene aquifer is con-
centrated within a shallower and more reduced portion where
there is little to no indication of recent recharge. The one
exception is the well monitored at 51 m at nest M-Middle, which
contains 3H but, based on the observations from all the other
wells, even the rise in As concentrations in this well is more likely
to be driven by a process that is disconnected from recent
recharge (Fig. 3b and Supplementary Note 1). If rapid advection
from the shallow Holocene aquifer is not responsible, an alter-
native source of reactive DOC is required to explain the release of
As within pre-Holocene sands. The pore water chemistry and
tracers indicate that the thick clay layer could be this alternative
source. In addition to the solid phase organic carbon content
reaching 7% (Supplementary Fig. 2), the clay separating the
Holocene and pre-Holocene aquifers contains pore water with
DOC concentrations of up to 23 mg/L (Supplementary Note 2),
i.e., one order of magnitude higher than in most of the
groundwater sampled by the monitoring wells (Fig. 3g). Generally
unreactive tracers, such as chloride (Supplementary Note 3),
sodium, and the stable isotopes of water (Supplementary Note 4),
provide evidence of a flux of clay pore water across the interface
separating the two units that is distinct from that of groundwater
within the orange sands (Fig. 3d, h and Supplementary Fig. 3;
Supplementary Table 2). Only in the case of chloride, however,
was enough pore water extracted from the two clay intervals
closest to the interface for analysis. The 20% contribution of clay
water to the upper portion of the pre-Holocene aquifer estimated
from chloride implies that advection of As contained in pore
water from the clay alone cannot explain elevated levels and the
rise of As concentrations in the monitoring wells.
Discussion
Advection or diffusion out of the clays are two mechanisms
through which DOC could be supplied from the clay to the
underlying aquifer. In the case of advection, using a vertical dif-
ference of 1 m in hydraulic head across the 10-m thick clay layer
(Fig. 3a) and a plausible range of vertical hydraulic conductivities
for the clay of 10−9–10−7m/sec39, the Darcy velocity of clay pore
water into the pre-Holocene aquifer is on the order of 0.3–30 cm
per year, i.e., 3–300 L/m2per year (Supplementary Note 5). This
corresponds to a total organic carbon flux of 5–500 × 10−3mol
C/m2per year for a concentration of DOC in clay water of 20 mg/L
(Fig. 3g). Assuming this flux magnitude over the past 20 years to
reflect the trend in deep pumping and the development of the
vertical hydraulic head difference (Supplementary Fig. 5), this
corresponds to a total input of 0.1–10 mol C/m2into the upper
portion of the pre-Holocene aquifer.
Fick’sfirst law can be used to calculate the flux from diffusion.
A much lower diffusive flux of DOC of 0.2–1×10
−3mol C/m2
per year is calculated based on the difference in concentration of
17 mg/L spanning half of the thickness of the 10 m clay layer, the
diffusivity constant of 9 × 10−3m2/yr for acetate, and an effective
porosity range of 0.1–0.5 (Supplementary Note 5). If the diffusion
gradient was maintained over 5000 years by the continuous
release of DOC from buried plant matter in the clay, the inte-
grated flux of reactive carbon over this longer period corresponds
to a total input of 1–5 mol C/m2. The estimated flux of organic
carbon estimated for advection over 20 years is therefore within
the range of estimates for the diffusive flux over 5000 years.
Radiocarbon provides further evidence of the capping clay as a
source of reactive DOC. The radiocarbon age of DOC of 4 kyr in
the clay layer is comparable to that of DOC in the gray portion of
the pre-Holocene aquifer and is unaffected by bomb radiocarbon
input (Supplementary Fig. 3). In contrast, the DOC within the
orange portion of the pre-Holocene aquifer is younger. Radio-
carbon ages of DOC therefore point to the clay as the source but
cannot differentiate between recent advection or long-term dif-
fusion. Clay compaction linked to land subsidence caused by
municipal pumping in Dhaka52 could also be contributing by
expelling reactive DOC. Such a mechanism has been invoked
largely indirectly from broad-scale patterns to explain As con-
tamination of groundwater in the Mekong Delta of Vietnam15
and the Central Valley of California17.
How does the magnitude of the DOC flux leaving the clay
layer, by advection or diffusion, compare to what would be
required to convert a 10–15-m thick layer of sand from orange to
gray? This simple calculation constitutes an upper limit to what
would be required to release As from pre-Holocene sands below
the clay. If this layer never was as oxidized as the orange sand
layer below it, then less DOC would be required. Sediment pro-
files from the site show that a shift in the acid-leachable Fe(II)/Fe
ratio from 0.3 to 0.5 accompanies the change in sand color from
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orange to gray (Fig. 2d)19. Given the average measured HCl-
leachable Fe concentration of 5.6 g/kg for orange and gray sands
in the pre-Holocene aquifer (about half the total Fe measured by
X-ray fluorescence, Supplementary Fig. 2), ~40 moles of Fe would
have to be reduced to change the color of 1 m3of sand from
orange to gray (Supplementary Note 5). Using a stoichiometric
Fe/C ratio of 4 for reductive dissolution of Fe oxides53, this means
that an input of 1 mol C/m2would be able to change the color of
only a ~0.1 m layer of aquifer sand. The flux of DOC, whether
advected over the past 20 years or diffusing over 5000 years, is
therefore insufficient by two orders of magnitude to reduce
orange Fe oxides over the entire gray layer.
We offer two possible explanations for the apparent dis-
crepancy. The first is that the 10-m thick upper layer of gray pre-
Holocene sands below the clay layer may never have been oxi-
dized completely, and thus would not have required as much
reduction to release As into groundwater. This is a possibility
because the relationship between the extent of reduction of Fe
oxides in aquifer sands and As concentrations in groundwater is
far from linear (Supplementary Note 6). The threshold of
reduction associated with marked increase in groundwater As
concentration is only reached when about a half of the sedi-
mentary Fe oxides have already been reduced and the char-
acteristic orange color of Fe(III) oxides has been lost19,33. The
long-term diffusive flux of DOC could have contributed to
approaching this threshold over several thousand years, with
more recent Dhaka pumping providing the additional advective
flux of DOC (Supplementary Note 7) to cross this threshold and
cause the observed rise in groundwater As concentrations below
the clay layer.
The second explanation relies on the observation that the
bottom of the clay surface at the interface with the upper portion
of the pre-Holocene aquifer varies in depth by as much as 5–10 m
within 20–100 m of the failed community well after correcting for
elevation differences at the surface (Supplementary Fig. 2).
Combined with lateral flow, the diffusive input of reactive DOC
from the bottom of a clay over time that varies in depth could
have converted pre-Holocene sand from orange to gray in dis-
crete intervals over a considerably wider depth range. We suggest
this poised the aquifer for further reduction by DOC released
from the clay layer and a rise in groundwater As concentrations
around the time when local groundwater elevations started to
show the impact of Dhaka pumping (Supplementary Fig. 5). Such
additional reduction of even discrete intervals of the aquifer
tapped by a long-screened well would be sufficient to contaminate
with As the water drawn at the pump.
In summary, groundwater-As-concentrations rose over the past
decade in a pre-Holocene aquifer capped by a clay layer. Using
multiple lines of evidence, such a rise is attributed here for the first
time to the reduction of Fe oxides driven by a flux of reactive
carbon originating from a clay layer linked in turn to deep
pumping at a considerable distance. The stoichiometry of Fe
reduction by organic carbon suggests that the upper portion of the
pre-Holocene aquifer either was fully oxidized and/or that DOC
was released by neighboring clay layers over a wider depth range.
In this particular area where the hydrogeology is clearly affected
by Dhaka pumping, direct downward advection of As from the
shallow aquifer is evidently not the cause of contamination of the
pre-Holocene aquifer below the clay layer26,46,51.
Our findings are of concern locally because many households
within the Dhaka cone of depression are privately re-installing
their wells to relatively shallow pre-Holocene aquifers37. Even in
the absence of deep pumping, long-term diffusion of DOC from
clay layers could explain why private wells screened just below a
clay layer in other sedimentary aquifers are more likely to be
contaminated with As than deeper wells with longer screens54.
With groundwater pumping from sedimentary aquifers expected
to continue throughout the world, more attention should be paid
to potential contamination of groundwater with As by com-
pacting clay layers15,17.
Methods
Site description and installation. The study site (23.7760°N and 90.6325°E,
Fig. 1a), referred to throughout as site M, is located in Araihazar upazila, a sub-
district of Bangladesh located ~25 km east of the capital, Dhaka. The site is ~750 m
southwest of the village Baylakandi/site B (Fig. 1a); both this region and site B, in
particular, were described in detail by van Geen et al.55, Zheng et al.39, and Dhar
et al.56. Four multi-level observation well nests were installed at site M in the winter
of 2010/11, with 1.5 m long well screens strategically placed to monitor all major
depth zones of the pre-Holocene aquifer (bottom of well reported as well depth). In
the shallow aquifer, observation wells either had long screens permeating the entire
aquifer (nests -Middle and -North, middle of screen reported as well depth), or
1.5 m long screens installed approximately mid-depth through the shallow aquifer
(nests -South and -West, bottom of well reported as well depth). An additional
location (M-Core) was drilled for the collection of sediment cuttings. The eleva-
tions of top of well casings relative to the reference well (shallow well at nest M-
Middle: M-M.1) were determined visually within ±1 mm by leveling with a
transparent, flexible U-tube filled with water. All well depths reported, thus, are
relative to the M-M.1 top of casing. Well M-M.1 was, in turn, leveled by the same
method to top of casing of well BayP7 at site B, for which the absolute elevation
above sea level is known39. Thus, measured hydraulic head elevations could be
referenced to the absolute elevation above sea level (m asl).
Sampling and analyses of solid materials. Sediment cuttings were collected at
0.6 m (2 ft) or 1.5 m (5 ft) intervals while drilling by the traditional hand-flapper or
sludger method19,57 to install the wells. This method biases samples slightly
towards the coarser fraction, especially when sand and silt are mixed. Cuttings were
described by grain size (clay, silty clay, or sand) and by sediment color (gray or
orange) to construct lithologs. On the day of collection, diffuse spectral reflectance
between 530 and 520 nm was measured on the cuttings wrapped in Saran wrap to
indicate the Fe speciation in the solid phase19. The cuttings were also analyzed by
X-ray fluorescence (XRF) using a portable InnovX Delta instrument for total ele-
mental concentrations of Ca and Fe contained within the sediment. Samples were
run without drying or grinding to powder, and the internal calibration of the
instrument was checked before and after each run by NIST reference materials
SRM 2709, 2710, and 2711. A subset of ~26 and 21 cuttings from representative
depths at well nests M-Middle and M-West, respectively, were additionally sub-
jected to same-day extractions by a hot 10% (1.2 M) HCl leach for 30 min to release
Fe from amorphous Fe minerals19. The acid leachates were analyzed immediately
for Fe(II) and total Fe concentrations by ferrozine colorimetry58. Separate samples
from the same set of cuttings were also subjected to a same-day extraction in a N
2
-
purged 1 M NaH
2
PO
4
solution (pH~5) at room temperature for 24 h59. The
phosphate extracts were analyzed for As by high-resolution inductively coupled
plasma-mass spectrometry (HR ICP-MS).
While drilling through clay and silt layers, various leaf fragments, pieces of
wood, a piece of charcoal, and select samples of clay itself (for bulk organic carbon)
were preserved in zip-lock bags for 14C dating and 13C isotopic analysis. 14C/12C
and 13C/12C analyses were performed at National Ocean Science Accelerator Mass-
Spectrometer (NOSAMS) facility of Woods Hole Oceanographic Institution
following standard protocols60. Radiocarbon data were reported as fraction modern
(FM) 14C, with measurement errors listed in Supplementary Table 1. The values of
13C/12C were calculated as deviations in per mil (‰) from the Vienna Pee Dee
Belemnite standard (δ13C
VPDB
), with analytical errors typically <0.1‰.
Radiocarbon ages were calculated using 5568 years as half-life of 14C61 and no
reservoir corrections or calibration to calendar years were made.
Clay samples on which 14C and 13C analyses of bulk organic C were performed,
as well as 17 other representative sand and clay samples from various depths at site
M, were refrigerated and analyzed ~2 years later for C content in the sediment.
Total C (TC) and inorganic C (IC) in the sediment were measured on the solid
analysis unit of a Shimadzu carbon analyzer, and the difference between the two
measurements was reported at total organic C (TOC) percentage in the sediment.
Quantification limits for TC were 0.06% and 0.03% (% of total sediment) in clay
and sand samples, respectively, while the respective limits of IC analyses in clay and
sand were 0.02% and 0.01%.
Hydraulic head measurements. Variations in hydraulic heads relative to the top
of the well casing were manually measured on a monthly basis using a Solinst
Model 101 meter. Monitoring in some wells started in January 2011, but monthly
readings were taken in all M wells simultaneously starting in July 2011. The
reported annual average water levels (Fig. 3a) include readings from December
2012 to November 2013. Submersed pressure transducers with data loggers
(Levelogger, Solinst) were used to record long-term water levels and barometric
pressure at 20-min intervals in select wells at M-Middle nest starting in February
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2011. All water level elevations are reported in meters above sea level (m asl; see
above for elevation leveling).
Chemical measurements in the field. Groundwater was sampled in April 2011 for
pH, oxidation/reduction potential (ORP), temperature and conductivity in a tight
flow-through chamber (MP 556 from YSI, Inc.) equipped with appropriate probes
until the readings were stable. At the same time, dissolved oxygen was measured with
a CHEMet kit and alkalinity samples were obtained by Gran titration62.Dissolved
inorganic carbon (DIC) values were then calculated from the concurrently measured
pH values and alkalinity. Ammonia was measured in select M nest wells using a NH
3
electrode (AmmoLytPlus 700 IQ from YSI, Inc.) in May 2012.
Clay pore water collection. Clay pore water samples were collected in May 2012
by squeezing clay cuttings from a borehole drilled near well nest M-Middle.
Immediately upon the clay cutting collection and the squeezing of 2–20 mL of pore
water, the pore water samples were filtered through 0.45 µm syringe filters
(Whatman 6753-2504) and processed for various analyses, described below, in the
same way as groundwater samples.
Incubation experiments and DOC and DIC analyses. Dissolved organic carbon
(DOC) samples were collected in May 2012, immediately filtered through the 0.45 µm
syringe filters into glass vials, and acidified to 1% HCl. Some of the samples were
purposefully left unacidified in tightly capped vials filled without a headspace of air,
then analyzed for DOC one month later. The DOC that decayed in unacidified
samples was calculated by subtracting DOC levels of unacidified samples from those of
acidified samples and expressed as % reactive DOC. DOC (from all M samples) and
DIC (clay pore water only, unacidified) were measured in triplicates on a Shimadzu
carbon analyzer to a precision of <5% for most samples, and the average was reported.
Sampling and groundwater analysis. Groundwater samples for major and trace
elemental analysis by HR ICP-MS were collected on a monthly basis from July
2011 to June or August 2012 from certain wells, for which a time-series average and
standard deviation is reported; for other wells, 1–5 samples were collected over a
period between April 2011 and December 2012, and their time-series average is
reported without standard deviations, unless >3 samples were analyzed. Addi-
tionally, monthly samples for As analysis were collected from the wells at nest M-
Middle from February 2013 to December 2017 (well screened at 41 m depth) or
March 2016 (the remaining wells at the nest). All samples were acidified to 1% HCl
in the laboratory at least one week prior to the analyses of Na, K, Ca, Mg, P, As, Fe,
Mn, Sr, and Ba using HR ICP-MS62,63 to a precision of <10% and accuracy of <10%
when compared to internal laboratory reference standards. Groundwater samples
for anion analysis were collected at the same time as the HR ICP-MS samples, but
were not acidified, and only a subset of 1–8 samples per well were analyzed for the
period of April 2011–July 2012. Anion samples were analyzed for Cl, SO
4
, and F
using ion chromatography, with a precision of <5% for Cl and 5–15% for SO
4
and
F. The anion results are reported as averages of time series, with time-series
standard deviations reported only where >3 monthly samples were analyzed.
Stable isotopes (δ2H and δ18O) in water. Samples for stable isotope (2H and
18O) measurements were collected in 60 mL glass bottles with polyseal-lined caps
in April 2011 (site M groundwater) and May 2012 (site M clay pore water). They
were analyzed on a Picarro Isotopic Water Analyzer at Lamont-Doherty Earth
Observatory with a precision of ±0.01–0.07‰(δ18O) and ±0.01–0.24‰(δ2H)
(Supplementary Table 2). The values were reported in per mil (‰) differences from
the Vienna Standard Mean Ocean Water values (VSMOW).
Radiocarbon (14C) and 13C of DIC and DOC. Samples for the analysis of 14C and
13C were collected in 125 mL (DIC) or 250 mL (DOC) glass bottles with Polyseal-
lined caps in April 2011 (site M DIC) and October 2012 (site M DOC). They were
preserved with mercury chloride (DIC) or acid (1% HCl final concentration, DOC)
to arrest potential biological processes after collection. The three clay pore water
samples for 14C and 13C in DOC were much smaller (5–10 mL) and collected in
May 2011. All radiocarbon and 13C analyses were performed, and the results
reported (Supplementary Table 3), as described above for sediment samples.
Tritium (3H) and noble gas sampling. The atmospheric testing of nuclear
weapons released 3H, a radioactive isotope of H that peaked in the early 1960s,
which made it possible to date groundwater recharged since the onset of tests by
the 3H/3He technique64–66. Samples for 3H/1H measurements were collected in
125 mL glass bottles with polyseal-lined caps and analyzed at Lamont-Doherty
Earth Observatory’s Noble Gas Laboratory using the 3He ingrowth technique67,68.
The analytical precision and detection limit of the 3H measurements were
±0.03–0.06 TU (Supplementary Table 4) and 0.05–0.10 TU, respectively, (3H/1H
ratio of 10−18 corresponds to 1 TU). Samples for He and Ne isotopic measure-
ments were collected in ~1 cm outer diameter soft copper tubes that hold ~19 cm3
of groundwater. Concentrations of He, Ne, and 3He/4He were measured by mass
spectrometry69 at Lamont-Doherty Earth Observatory’s Noble Gas Laboratory,
with typical analytical precisions of ±0.05–0.10% for He and Ne concentrations and
±0.6–0.7% for 3He/4He ratio.
Data availability
Data that support the findings of this study that are not already included as tables in the
paper will be deposited upon acceptance at https://www.hydroshare.org/
Received: 3 August 2019; Accepted: 8 April 2020;
References
1. Argos, M. et al. Arsenic exposure from drinking water, and all-cause and
chronic-disease mortalities in Bangladesh (HEALS): a prospective cohort
study. Lancet 376, 252–258 (2010).
2. Quansah, R. et al. Association of arsenic with adverse pregnancy outcomes/
infant mortality: a systematic review and meta-analysis. Environ. Health
Perspect. 123, 412–421 (2015).
3. Rosenzweig, M., Pitt, M. & Nazmul Hassan, N. Identifying the costs of a public
health success: arsenic well water contamination and productivity in
Bangladesh. Rev. Econ. Studies http://ibread.org/bread/system/files/
bread_wpapers/546.pdf (2020).
4. Wasserman, G. A. et al. Water arsenic exposure and children’s intellectual
function in Araihazar, Bangladesh. Environ. Health Perspect. 112, 1329–1333
(2004).
5. Aggarwal, P. K. et al. Isotope hydrology of groundwater in Bangladesh:
Implications for characterization and mitigation of arsenic in groundwater,
IAEA-TC Project (BGD/8/016). Report No. IAEA-TC Project (BGD/8/016),
(International Atomic Energy Agency, 2000).
6. Ahmed, M. F. et al. Epidemiology—ensuring safe drinking water in
Bangladesh. Science 314, 1687–1688 (2006).
7. BGS/DPHE. Arsenic contamination of groundwater in Bangladesh, BGS
echnical Report WC/00/19 (Catalogue No. BGS Technical Report WC/00/19,
British Geological Survey and Department of Public Health Engineering,
Keyworth, UK, 2001).
8. DPHE/JICA. Report on Situation Analysis of Arsenic Mitigation, 2009 (Dept.
of Public Health Engineering, Local Government Division, Government of
Bangladesh and Japan International Cooperation Agency, 2010).
9. Ravenscroft, P., Burgess, W. G., Ahmed, K. M., Burren, M. & Perrin, J. Arsenic
in groundwater of the Bengal Basin, Bangladesh: distribution, field relations,
and hydrogeological setting. Hydrogeol. J. 13, 727–751 (2005).
10. Ravenscroft, P. et al. Effectiveness of public rural waterpoints in Bangladesh
with special reference to arsenic mitigation. J. Water Sanitation Hyg. Dev. 4,
545–562 (2014).
11. Ravenscroft, P., McArthur, J. M. & Hoque, M. A. Stable groundwater quality
in deep aquifers of Southern Bangladesh: The case against sustainable
abstraction. Sci. Total Environ. 454, 627–638 (2013).
12. Ahmed, F. & Ahmed, T. in Comprehensive Water Quality and Purification.
(ed. S. Ahuja) Vol. 1, 104-121 (Elsevier, 2014).
13. Michael, H. A. & Voss, C. I. Evaluation of the sustainability of deep
groundwater as an arsenic-safe resource in the Bengal Basin. Proc. Natl Acad.
Sci. USA 105, 8531–8536 (2008).
14. McMahon, P. B. & Chapelle, F. H. Microbial production of organic acids in
aquitard sediments and its role in aquifer geochemistry. Nature 349, 233–235
(1991).
15. Erban, L. E., Gorelick, S. M., Zebker, H. A. & Fendorf, S. Release of arsenic to
deep groundwater in the Mekong Delta, Vietnam, linked to pumping-induced
land subsidence. Proc. Natl Acad. Sci. USA 110, 13751–13756 (2013).
16. Planer-Friedrich, B. et al. Organic carbon mobilization in a Bangladesh aquifer
explained by seasonal monsoon-driven storativity changes. Appl. Geochem. 27,
2324–2334 (2012).
17. Smith, R., Knight, R. & Fendorf, S. Overpumping leads to California
groundwater arsenic threat. Nat. Commun. https://doi.org/10.1038/s41467-
018-04475-3 (2018).
18. Fendorf, S., Michael, H. A. & van Geen, A. Spatial and temporal variations of
groundwater arsenic in South and Southeast Asia. Science 328, 1123–1127 (2010).
19. Horneman, A. et al. Decoupling of As and Fe release to Bangladesh
groundwater under reducing conditions. Part 1: evidence from sediment
profiles. Geochim. Cosmochim. Acta 68, 3459–3473 (2004).
20. Islam, F. S. et al. Role of metal-reducing bacteria in arsenic release from Bengal
delta sediments. Nature 430,68–71 (2004).
21. McArthur, J. M. et al. Natural organic matter in sedimentary basins and its
relation to arsenic in anoxic ground water: the example of West Bengal and its
worldwide implications. Appl. Geochem. https://doi.org/10.1016/j.
apgeochem.2004.02.001 (2004).
NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-16104-z ARTICLE
NATURE COMMUNICATIONS | (2020) 11:2244 | https://doi.org/10.1038/s41467-020-16104-z | www.nature.com/naturecommunications 7
Content courtesy of Springer Nature, terms of use apply. Rights reserved
22. Nickson, R. et al. Arsenic poisoning of Bangladesh groundwater. Nature 395,
338–338 (1998).
23. Nickson, R. T., McArthur, J. M., Ravenscroft, P., Burgess, W. G. & Ahmed, K.
M. Mechanism of arsenic release to groundwater, Bangladesh and West
Bengal. Appl. Geochem. 15, 403–413 (2000).
24. Dhar, R. K. et al. Microbes enhance mobility of arsenic in pleistocene aquifer
sand from Bangladesh. Environ. Sci. Technol. 45, 2648–2654 (2011).
25. McArthur, J. M. et al. How paleosols influence groundwater flow and arsenic
pollution: a model from the Bengal Basin and its worldwide implication.
Water Resour. Res.https://doi.org/10.1029/2007wr006552 (2008).
26. van Geen, A. et al. Retardation of arsenic transport through a Pleistocene
aquifer. Nature 501, 204–207 (2013).
27. van Geen, A. et al. Decoupling of As and Fe release to Bangladesh
groundwater under reducing conditions. Part II: evidence from sediment
incubations. Geochim. Cosmochim. Acta 68, 3475–3486 (2004).
28. Harvey, C. F. et al. Arsenic mobility and groundwater extraction in
Bangladesh. Science 298, 1602–1606 (2002).
29. Klump, S. et al. Groundwater dynamics and arsenic mobilization in
Bangladesh assessed using noble gases and tritium. Environ. Sci. Technol. 40,
243–250 (2006).
30. Neumann, R. B. et al. Anthropogenic influences on groundwater arsenic
concentrations in Bangladesh. Nat. Geosci. 3,46–52 (2010).
31. Polizzotto, M. L., Kocar, B. D., Benner, S. G., Sampson, M. & Fendorf, S. Near-
surface wetland sediments as a source of arsenic release to ground water in
Asia. Nature 454, 505–U505 (2008).
32. Sengupta, S. et al. Do ponds cause arsenic-pollution of groundwater in the
Bengal Basin? An answer from West Bengal. Environ. Sci. Technol. 42,
5156–5164 (2008).
33. van Geen, A. et al. Flushing history as a hydrogeological control on the
regional distribution of arsenic in shallow groundwater of the Bengal Basin.
Environ. Sci. Technol. 42, 2283–2288 (2008).
34. Mailloux, B. J. et al. Advection of surface-derived organic carbon fuels
microbial reduction in Bangladesh groundwater. Proc. Natl Acad. Sci. USA
110, 5331–5335 (2013).
35. Goodbred, S. L. & Kuehl, S. A. The significance of large sediment supply,
active tectonism, and eustasy on margin sequence development: Late
Quaternary stratigraphy and evolution of the Ganges-Brahmaputra delta.
Sediment. Geol. 133, 227–248 (2000).
36. van Geen, A. et al. Monitoring 51 community wells in Araihazar, Bangladesh,
for up to 5 years: implications for arsenic mitigation. J. Environ. Sci. Health
Part A Toxic./Hazard. Substances Environ. Eng. 42, 1729–1740 (2007).
37. Jamil, N. B. et al. Effectiveness of different approaches to arsenic mitigation
over 18 years in Araihazar, Bangladesh: implications for National Policy.
Environ. Sci. Technol. 53, 5596–5604 (2019).
38. von Brömssen, M. et al. Targeting low-arsenic aquifers in Matlab Upazila,
Southeastern Bangladesh. Sci. Total Environ. 379, 121–132 (2007).
39. Zheng, Y. et al. Geochemical and hydrogeological contrasts between shallow
and deeper aquifers in two villages of Araihazar, Bangladesh: implications for
deeper aquifers as drinking water sources. Geochim. Cosmochim. Acta 69,
5203–5218 (2005).
40. Choudhury, I. et al. Evidence for elevated levels of arsenic in public wells of
Bangladesh due to improper installation. Ground Water 54, 871–877 (2016).
41. Khan, M. R. et al. Megacity pumping and preferential flow threaten
groundwater quality. Nat. Commun. 7, 12833 (2016).
42. Knappett, P. S. K. et al. Vulnerability of low-arsenic aquifers to municipal
pumping in Bangladesh. J. Hydrol. 539, 674–686 (2016).
43. Bard, E. et al. Calibration of the 14C timescale over the past 30,000 years using
mass spectrometric U–Th ages from Barbados corals. Nature 345, 405–410
(1990).
44. Goodbred, S. L. & Kuehl, S. A. Enormous Ganges-Brahmaputra sediment
discharge during strengthened early Holocene monsoon. Geology 28,
1083–1086 (2000).
45. Hoque, M. A., McArthur, J. M. & Sikdar, P. K. Sources of low-arsenic
groundwater in the Bengal Basin: investigating the influence of the last glacial
maximum palaeosol using a 115-km traverse across Bangladesh. Hydrogeol. J.
22, 1535–1547 (2014).
46. McArthur, J. M. et al. Migration of As, and H-3/(3) He ages, in groundwater
from West Bengal: implications for monitoring. Water Res. 44, 4171–4185
(2010).
47. Hoque, M., Burgess, W. & Ahmed, K. M. Integration of aquifer geology,
groundwater flow and arsenic distribution in deltaic aquifers—A unifying
concept. Hydrol. Process. 31, 2095–2109 (2017).
48. Mihajlov, I. et al. Recharge of low-arsenic aquifers tapped by community wells
in Araihazar, Bangladesh, inferred from environmental isotopes. Water
Resour. Res 52, 3324–3349 (2016).
49. Mihajlov, I. The vulnerability of low-arsenic aquifers in Bangladesh: a multi-
scale geochemical and hydrologic approach PhD Thesis, Columbia University,
(2014).
50. Stute, M. et al. Hydrological control of As concentrations in Bangladesh
groundwater. Water Resou. Res.43, W09417 https://doi.org/10.1029/
2005WR004499 (2007).
51. Radloff, K. A. et al. Arsenic migration to deep groundwater in Bangladesh
influenced by adsorption and water demand. Nat. Geosci. 4, 793–798 (2011).
52. Steckler, M. S. et al. Modeling Earth deformation from monsoonal flooding in
Bangladesh using hydrographic, GPS, and Gravity Recovery and Climate
Experiment (GRACE) data. J. Geophys. Res. Solid Earth https://doi.org/
10.1029/2009jb007018 (2010).
53. Postma, D. et al. Arsenic in groundwater of the Red River floodplain, Vietnam:
controlling geochemical processes and reactive transport modeling. Geochim.
Cosmochim. Acta 71,5054–5071, https://doi.org/10.1016/j.gca.2007.08.020 (2007).
54. Erickson, M. L. & Barnes, R. J. Well characteristics influencing arsenic
concentrations in ground water. Water Res. 39, 4029–4039 (2005).
55. van Geen, A. et al. Spatial variability of arsenic in 6000 tube wells in a 25 km
(2) area of Bangladesh. Water Resour. Res.https://doi.org/10.1029/
2002wr001617 (2003).
56. Dhar, R. K. et al. Temporal variability of groundwater chemistry in shallow
and deep aquifers of Araihazar, Bangladesh. J. Contaminant Hydrol. 99,
97–111 (2008).
57. Ali, M. in Groundwater Resources and Development in Bangladesh (eds A. A.
Rahman & P. Ravenscroft) 197–219 (The University Press Limited, 2003).
58. Stookey, L. L. Ferrozine—a new spectrophotometric reagent for iron. Anal.
Chem. 42, 779–781 (1970).
59. Keon, N. E., Swartz, C. H., Brabander, D. J., Harvey, C. F. & Hemond, H. F.
Validation of an arsenic sequential extraction method for evaluating mobility
in sediments. Environ. Sci. Technology 35, 2778–2784 (2001).
60. Elder, K. L., McNichol, A. P. & Gagnon, A. R. in Proc. 16th International
Radiocarbon Conference. 223–230 (Radiocarbon 40, 1997).
61. Stuiver, M. & Polach, H. A. Reporting of C-14 data—discussion. Radiocarbon
19, 355–363 (1977).
62. Gran, G. Determination of the equivalence point in potentiometric titrations
.2. Analyst 77, 661–671 (1952).
63. Cheng, Z., Zheng, Y., Mortlock, R. & van Geen, A. Rapid multi-element
analysis of groundwater by high-resolution inductively coupled plasma mass
spectrometry. Anal. Bioanal. Chem.https://doi.org/10.1007/s00216-004-2618-x
(2004).
64. Poreda, R. J., Cerling, T. E. & Salomon, D. K. Tritium and helium isotopes as
hydrologic tracers in a shallow unconfined aquifer. J. Hydrol. 103,1–9 (1988).
65. Schlosser, P., Stute, M., Sonntag, C. & Munnich, K. O. Tritiogenic He-3 in
shallow groundwater. Earth Planet. Sci. Lett. 94, 245–256 (1989).
66. Tolstikhin, I. N. & Kamenski, I. L. Determination of ground-water ages by T-
He-3 method. Geochem. Int. 6, 810–811 (1969).
67. Bayer, R. et al. Performance and blank components of a mass spectrometric
system for routine measurement of helium isotopes and tritium by the 3He
ingrowth method. 241–279 (Sitzungsberichte (5) der Heidelberger Akademie
der Wissenschaften, Mathematisch-naturwissenschaftliche Klasse, Jahrgang,
1989).
68. Clarke, W. B., Jenkins, W. J. & Top, Z. Determination of tritium by mass-
spectrometric measurement of He-3. Int. J. Appl. Radiat. Isotopes 27, 515–522
(1976).
69. Ludin, A. R., Weppernig, R., Boenisch, G. & Schlosser, P. Mass Spectrometric
Measurement of Helium Isotopes and Tritium, Internal Report. (Lamont-
Doherty Earth Observatory, Palisades, 1997).
70. van Geen, A. et al. Comparison of two blanket surveys of arsenic in tubewells
conducted 12years apart in a 25km2 area of Bangladesh. Sci. Total Environ.
488–489, 484–492 (2014).
Acknowledgements
Columbia University and the University of Dhaka’s research in Araihazar, Bangladesh
has been supported since 2000 by NIEHS Superfund Research Program grant P42
ES010349. NSF Coupled Natural and Human Systems Dynamics grant ICER 1414131
provided additional support. We thank M. S. Shahud, M. M. Hosain, and the villagers at
site M for their help in the field, L. Baker, R. Friedrich, and R. Newton for data acqui-
sition help, and Y. Zheng, H. Michael, and C. F. Harvey for their ideas and comments.
This is Lamont-Doherty Earth Observatory contribution number 8396.
Author contributions
I.M., B.C.B., M.S., and A.v.G. designed the study and conducted the fieldwork. B.J.M.,
P.S.K.K., M.R.H.M., and I.C. provided field and laboratory assistance. B.C.B., M.S., K.M.A.,
P.S., and A.v.G. advised and supported the work of I.M. I.M., and M.R.H.M analyzed the
data and, with A.v.G., wrote the manuscript, which was then edited by all co-authors.
Competing interests
The authors declare no competing interests.
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Additional information
Supplementary information is available for this paper at https://doi.org/10.1038/s41467-
020-16104-z.
Correspondence and requests for materials should be addressed to A.v.G.
Peer review information Nature Communications thanks Søren Jessen, Dave Polya and
the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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