progress article
Published online: 17 January 2010 | doi: 10.1038/ngeo750
Vulnerability of deep groundwater in the bengal
aquifer system to contamination by arsenic
W. g. burgess1*, M. a. hoque1, h. a. Michael2, c. i. Voss3, g. n. breit4 and K. M. ahmed5
Shallow groundwater, the primary water source in the Bengal Basin, contains up to 100 times the World Health Organization
(WHO) drinking-water guideline of 10 μg l–1 arsenic (As), threatening the health of 70 million people. Groundwater from a depth
greater than 150 m, which almost uniformly meets the WHO guideline, has become the preferred alternative source. The vulnerability of deep wells to contamination by As is governed by the geometry of induced groundwater flow paths and the geochemical
conditions encountered between the shallow and deep regions of the aquifer. Stratification of flow separates deep groundwater
from shallow sources of As in some areas. Oxidized sediments also protect deep groundwater through the ability of ferric
oxyhydroxides to adsorb As. Basin-scale groundwater flow modelling suggests that, over large regions, deep hand-pumped wells
for domestic supply may be secure against As invasion for hundreds of years. By contrast, widespread deep irrigation pumping
might effectively eliminate deep groundwater as an As-free resource within decades. Finer-scale models, incorporating spatial
heterogeneity, are needed to investigate the security of deep municipal abstraction at specific urban locations.
T
he Bengal Basin hosts the largest case of mass poisoning in
the world1. Excessive concentration of As occurs in shallow
groundwater 2 used for domestic supply by 70 million people,
30% of the combined population of Bangladesh and West Bengal,
India. Half the shallow hand-pumped wells have As concentrations
of 10–1,000 μg l–1 (ref. 2) and most inhabitants have no alternative
water source. The use of groundwater was initiated in the 1960s;
as a result much of the adult population has been exposed to toxic
levels of As for three decades. The health impacts are potentially
catastrophic3. Arsenic-affected groundwater has also been identified in fluvio-deltaic settings elsewhere in southeast Asia1,4,5.
The enormous scale of As contamination of shallow wells
became apparent during the 1980s in West Bengal6 and the 1990s
in Bangladesh7. No solutions have since been implemented that
provide As-free water to most of the affected population. (By As-free,
we mean water containing less than 10 μg l–1 As, the WHO drinking-water guideline, rather than the regulatory limit in Bangladesh
and West Bengal, which is 50 μg l–1 As). Of the mitigation options,
installation of wells to As-free depths in the aquifer 8, usually taken
to be greater than 150 m (ref. 9), offers the most popular, practical
and economic solution1,10. In Bangladesh, more than 75,000 deep
hand-pumped wells had been installed1 by 2007. Since 2000, deep
wells yielding 2,500 m3 d–1 have been installed by local initiatives
in over 100 rural supply schemes11. Previously, the Bangladesh
Department of Public Health Engineering (DPHE) had fitted deep
wells with pumps, each capable of yielding 4,500 m3 d–1, at more
than 20 towns (DPHE, personal communication). Deep groundwater continues to be targeted, however there is concern12 that
it may be vulnerable to invasion of As from shallow depths as a
consequence of pumping.
Deep wells offer a solution to another problem: saline groundwater occurring at intermediate depth across most of the coastal
region13–16. Here, pumping ‘deep groundwater’ may induce invasion
by saline groundwater, which would be expected to precede arrival
of As at deep wells, owing to the different distributions and geochemical behaviour of As and salinity. Our purpose is to review the
deep groundwater environments of the Bengal Basin (Box 1). With
reference to recent modelling results17,18, we focus on the vulnerability
of deep groundwater to invasion by As. Any development of deep
groundwater should be accompanied by chemical monitoring and
consideration of the possible requirement for treatment to mitigate
constituents such as iron, manganese and boron (ref. 19).
distribution of arsenic
Arsenic-rich groundwater occurs in reducing 9, grey-coloured,
Holocene sediments20 at depths less than 150 m (Box 1). The As
originates in association with a ferric oxyhydroxide coating of
Himalayan-derived sediment 21. Reducing conditions, sustained by
organic carbon, favour As release to groundwater by microbially
mediated22 reductive dissolution21 of the ferric oxyhydroxide. Spatial
variability of As at the 10–20 km2 scale has been related to organiccarbon availability 23, local sedimentology 24–26 and groundwater
flow 27–30. Present uncertainties include the sources of carbon23,31,
the As sorption capacity of aquifer sediments32 and future trends in
As concentrations33.
Where reductive As release is absent, and yellowish-brown
oxidized sediments with a capacity for As sorption exist, groundwater is As-free, notably in Pleistocene and older deposits deep
beneath reduced Holocene sediments34,35 and at shallow depth
in the vicinity of Pleistocene inliers2,36 (Box 1). Arsenic-free
conditions also occur in grey, reduced Pleistocene sediments
at depth37. In a national survey of Bangladesh2, of the wells at a
depth greater than 150 m (which were principally in the coastal
region), fewer than 1% exceeded 50 μg l–1 and 95% had less than
10 μg l–1. Arsenic concentration was generally negligible at depths
greater than 200 m, attributed to the geochemical context 31,37, the
refractory nature of sedimentary organic matter 27 and/or history
of groundwater flushing 2,38. A compilation of surveys2, 12,19,39 giving
a broader coverage of the basin (Fig.1) indicates that As exceeds
10 μg l–1 in 18% of deep hand-pumped wells sampled, but whether
this is a result of breached well casings or hydrological response to
pumping remains uncertain.
Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK, 2Department of Geological Sciences, University of
Delaware, Newark, Delaware 19716, USA, 3US Geological Survey, 431 National Center, Reston, Virginia 20192, USA, 4US Geological Survey, Box 25046 MS
964D, Denver, Colorado 80225, USA, 5Department of Geology, University of Dhaka, Dhaka 1000, Bangladesh. *e-mail: william.burgess@ucl.ac.uk
1
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NATure GeOScieNce doi: 10.1038/ngeo750
deep groundwater in the bengal aquifer system
coastal region14. An aquitard has been reported as regionally persistent across central40 and southeast Bangladesh16. At Khulna, groundwater has been pumped from 200 to 350 m depth for municipal
supply for over 30 years without inducing vertical flux of As or
The shallow floodplain aquifer is locally separated from deeper
groundwater by a silt-clay aquitard in places, for example in West
Bengal25,39, western Bangladesh33 and Khulna in the southwestern
Box 1 | The Bengal Aquifer System
The Bengal Basin (a) bounded by Precambrian shield and hilly areas,
internally comprises a sedimentary sequence of Late Cretaceous–
recent age, up to 20 km thick. The long history of predominantly
alluvial/fluviatile/deltaic deposition across the region, and basin
subsidence50, provide the geological basis for expectation of permeable sediments to depths of many hundreds of metres. Groundwater
is pumped from the basin sediments from a present maximum
depth of 350 m. Excessive As concentrations are largely restricted
to the uppermost 100 m across the floodplains (a). A marine clay
of basin-wide extent, the Mio-Pliocene Upper Marine Shale, probably acts as a hydraulic basement at roughly 1,200–2,000 m depth
to the aquifer system (called here the Bengal Aquifer System, BAS),
comprised of Plio-Pleistocene–Holocene sediments (b). The BAS
hosts a number of regional aquifers that are hydraulically connected
on a basin-wide scale. Plio-Pleistocene sands and silts deposited
in a braided to meandering fluvial setting44 make up the Dupi Tila
Formation which forms aquifers44 across the Madhupur and Barind
tracts. Episodes of sustained weathering during eustatic sea-level
low stands from the Early Pleistocene are reflected in the regionally
extensive oxidation of sediments of the central and northern part
of the basin. These sediments yield As-free groundwater to depths
of at least 250 m, as illustrated in c, a conceptual, bimodal sand–
clay representation of the aquifer environments. Holocene sands,
silts and silty clays beneath the active floodplains overlie Pleistocene
sediments to a depth generally of about 100 m in the south.
a
Formation of the BAS over Plio-Quaternary time (b) took place
under conditions of eustatic cyclicity, with deposition, subsidence
and erosion occurring in channels and interfluves across
the floodplain. Arsenic-rich groundwater occurs in reducing9
grey-coloured Holocene sediments20 at depths less than 150 m.
Accommodation from subsidence50 of 2 mm yr–1 broadly across
the basin allowed approximately 200 m of sediment to accumulate
over the past one million years (Myr). This time interval includes
ten eustatic cycles, each with an effective sedimentation time of
ten thousand years (kyr) (b). These sediments host ‘deep’ groundwater in the south of the basin (b,c). They include yellowishbrown, oxidized sands containing ferric-oxyhydroxides that
adsorb As and grey sediment with As largely bound to pyrite
(Supplementary Fig. S2).
Stacking of stable channel sands and adjacent interfluve
deposits produced by repeated eustatic cycles resulted in the
occurrence of belts of thick sands, and finer materials with
limited lateral extents (c) across the southern part of the basin
(Supplementary Fig. S1). However, channel migration and
dynamics of the depositional engine distributed the strata such
that nearly all boreholes intersect multiple layers of high and
low hydraulic conductivity. Groundwater flow systems ranging
from shallow to deep are developed within the BAS, driven by
topography17 and influenced by the presence of layers of low
hydraulic conductivity.
b
Himalaya
Depth (m)
<20 kyr
100
?
Fault
200
aputra
?
20 kyr to 1 Myr
1 to 3.5 Myr
300
Shillong Plateau
~1,500
Brahm
Bay of Bengal
?
Top of ‘Upper
Marine Shale’
?
>3.5 Myr
100 km
Sylhet Basin
r
ive
aR
ghn
Me
r
WEST
BENGAL
Comilla
Khulna
Kolkata
N
0
50
Plio-Pleistocene
(Early glacial cyclicity)
Pleistocene
(Glacial cyclicity)
Holocene
(Transgressive sediments)
Holocene
(High–stand deposits)
Recent
(Subaqueous delta)
Late and Early
Pleistocene boundary
Dhaka
?
tain
Con
?
Arsenic
free
Water
Tertiary
Asaffected area
?
~100 m
Pleistocene
Holocene–Late
Pleistocene boundary
Late and Early
Holocene boundary
Holocene-Early
Pleistocene boundary
Bay of Bengal
100
km
Holocene
Mio-Pliocene
(Pre-glacial high stand)
c
Sundarbans
Igneous and
metasediments
84
Hill Range
Ind
ian
Madhupur
Tract
BANGLADESH
ng
es
Riv
Dhaka
e
Ga
Indo-Burm
an
Sh
ie
ld
Barind
Tract
SSW
Madhupur Tract
NNE
0
River
NEPAL
BHUTAN
?
nic
arse
Khulna Sundarbans
Palaeo–
channel
?
Stacked
channel
Stacked
inter-fluve sand region
Arsenic
region
Arsenic
free
free
?
? Subsiding
subbasin
~50 km
Oxic sand
Oxic clay
Grey sand
Grey clay
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progress article
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defences against invasion of deep groundwater by as
Groundwater flow paths to deep pumping wells provide an element
of protection if As concentrations in recharge areas are low, or travel
times to deep wells are long. This is called the ‘flow-pattern defence’.
The potential for sediments along induced flow paths to adsorb or
otherwise trap As provides a ‘geochemical defence’. Wells should
optimally be screened in locations protected by a combination of
flow pattern and geochemistry, but where deep groundwater is ultimately vulnerable, the elapsed time before As arrival at pumping
wells is critical. Geological interpretation can define the contexts of
deep groundwater vulnerability, and the timescale of As arrival may
be estimated using groundwater models and geochemical analyses,
but acceptable timescales for security of supply are an economic and
political consideration1.
Flow-pattern defence of deep groundwater. Groundwater flow
paths to wells are controlled by spatial patterns of aquifer properties, hydrological surface conditions, aquifer geometry and the
distribution of pumping. Flow patterns in the BAS have recently
been evaluated by groundwater model analysis17,18 using a single,
basin-scale, vertically anisotropic, homogeneous aquifer representation41, calibrated against groundwater heads and ages17. Conditions
at the coast were represented by prescribing the equivalent freshwater head of water with a density of 1.025 kg l–1 and depth determined
by bathymetry over an extensive offshore region, with a no-flow
boundary at the southern limit of the model. Model robustness was
demonstrated across a variety of boundary conditions and a range
of parameter values17. The recharge provenance of ‘deep’ groundwater relative to shallow As sources, and the travel time from recharge
to deep wells, were evaluated for basin-wide groundwater development and a range of development scenarios17,18 (Fig. 2). Deep
pumping for domestic supply, with and without shallow irrigation
pumping, was found to minimally perturb the subhorizontal
flow paths from distant recharge zones. The flow-pattern defence
protected groundwater at depths greater than 150 m across more
than 90% of the As-affected area (Fig. 2c) indefinitely (modelled
as more than 1,000 years18) if deep groundwater abstraction was
limited to domestic supply and distributed among hand-pumped
wells18. A south-central subregion stands out as more vulnerable
to vertical flow on account of basin geometry, consistent with the
south-central As anomaly (Fig. 1).
Suggestions that high-As irrigation water leads to accumulation
of As in rice grains42, human exposure, and threats to sustainable
agriculture43, might prompt widespread use of deep As-free groundwater for irrigation. However, the rate of irrigation pumping is about
27° N
Himalaya
NEPAL
Igneous and
metasediments
Shillong Plateau
aR
N
22° N
0
87° E
Me
ghn
Kolkata
Khulna
Hill Range
ian
Ind
WEST
BENGAL
Indo-Burm
an
Madhupur
Tract
ng BANGLADESH
es
Riv
Dhaka
er
Ga
23° N
Pleistocene
Sylhet Basin
ive
r
Sh
iel
d
Brahm
aputra
Rive
Barind
Tract
25° N
24° N
BHUTAN
r
26° N
Latitude
chloride-rich water from shallower levels14; similar experiences13 at
other coastal towns have encouraged the view that a ‘deep aquifer’
might be developed more widely 1,13.
Data from more than 2,000 deep boreholes12 have recently
allowed a better-constrained interpretation of prevalent but laterally
discontinuous aquitards (see Supplementary Fig. S1), with multiple
layering leading to an effective large-scale vertical anisotropy 41 in
hydraulic conductivity. The resulting isolation of deep groundwater
flow from shallow flow, and maintenance of vertical differences in
hydraulic head, is similar to the effects of extensive aquitards. The
distinction is that discontinuous aquitards could locally focus vertical flow 25, providing pathways for invasion of deeper sediments
by shallow groundwater where deep pumping imposes a downward
hydraulic gradient. At issue is the conceptualization of lithological
heterogeneity and its representation in models. The effective anisotropy provided by multiple discontinuous layers of silty-clay has been
applied to describe a single, anisotropic aquifer at the scale of the
entire Bengal Basin18. We summarize the As-free, deep groundwater
environments of the Bengal Aquifer System (BAS) in the context of
the basin’s geological evolution (Box 1).
Tertiary
Holocene
Sundarbans
50
100
km
88° E
As concentration:
Water
Bay of Bengal
89° E
90° E
91° E
Longitude
< 10 µg l–1 10–50 µg l–1
92° E
> 50 µg l–1
Figure 1 | Arsenic concentration at hand-pumped wells in the Bengal Basin,
depths greater than 150 m. Data (1,165 records) compiled from refs 2, 12, 19
and 39 are all reported as laboratory analyses. Generalized geology and
structural elements are indicated.
ten times that of domestic pumping 44. Simulations17,18 show that
deep irrigation pumping would amplify downward flow, considerably shortening travel times to deep wells, to as little as 30 years,
and would create large drawdowns in water level (for example, of
20 m to 40 m at pumping depth), disabling deep hand-pumps and
rendering some powered pumping uneconomic. Deep irrigation
pumping thereby risks eliminating deep As-free groundwater as a
source of domestic supply (Fig. 2b). In contrast, shallow irrigation
pumping does not compromise the flow-pattern defence of deep
groundwater (Fig. 2c), but provides extra protection by creating a
hydraulic barrier against downward As migration.
Basin-scale analysis captures large-scale flow processes, but
geological heterogeneity might locally allow more rapid penetration of As to deep groundwater. A zone of excessive As in deep
groundwater of west-central Bangladesh, where more than 10% of
wells at depths greater than 200 m have concentrations of As higher
than 50 μg l–1, is attributed to the presence of thick Pleistocene palaeochannel sands of the proto-Ganges allowing migration of As
to depth1. Future modelling analysis at subbasinal scale should be
applied within the larger-scale framework and incorporate spatial
heterogeneity, particularly where layers of low hydraulic conductivity are rare and earlier As breakthrough might occur. While
spatial heterogeneity continues to generate uncertainty, factors of
safety are desirable, such as the 1,000-year timeframe of the basinscale models.
Geochemical defence of deep groundwater. The geochemical
defence is restricted neither by depth nor stratigraphy but by reactivity within the sediment. The hydraulic characteristics of grey
and yellowish-brown BAS sediments are similar, but their distinct
chemical characteristics, evident in contrasting groundwater compositions25,35, reflect the geochemical processes that may retard As in the
groundwater flow field.
Ferric oxyhydroxides have a large capacity for adsorbing dissolved
As (refs 45,46) (Fig. 3a) as demonstrated for oxidized, Pleistocene
sediments west of Dhaka35 and in central Bangladesh32. Adsorption
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a
NATure GeOScieNce doi: 10.1038/ngeo750
b
c
km
N
0 40 80
160
240
India
Bangladesh
Bangladesh
India
Figure 2 | Simulated regional outcomes of strategies for pumping deep groundwater from the BAS, based on the flow-pattern defence. a–c, Landsurface elevation is shown in grey-scale. The dashed black contour encloses the high-As region (As concentration greater than 50 μg l–1 in shallow
groundwater). The black contour represents the Bangladesh border. The high-As region is coloured red (a), and the blue contour indicates the model
boundary. b, Deep pumping for domestic supply and irrigation: regions with As-free recharge areas or travel times longer than 1,000 years to deep wells
are coloured green; regions with travel times shorter than 1,000 years from high-As recharge areas are coloured red; hatching indicates regions where
domestic well lift would be more than 8 m. c, Deep pumping for domestic supply, with shallow pumping for irrigation: colours and symbols as in b. Figures
reproduced with permission from ref. 18: a–c, © 2008 NAS.
a
As is bound principally in authigenic pyrite37 (Fig. 3b). Arsenian
pyrite has also been identified, at lower abundance, in deep sands,
consistent with a strong correlation of As and sulphur (ref. 37).
Accumulation of As in pyrite is a progressive diagenetic change
constituting a refractory As sink in reducing environments. The
effectiveness of As sequestration by actively forming pyrite within
the deep sands is uncertain however, as is the adsorption capacity of
framework grains within grey sediment. The contribution of these
processes to the geochemical defence of deep groundwater requires
further evaluation.
b
50 µm
conclusions and uncertainties
10 µm
Figure 3 | Arsenic-enriched phases from the BAS. a, Scanning electron
micrograph of botryoidal ferric oxyhydroxide in oxidized sediment (at a
depth of 1.6 m near Brahmanbaria, 70 km east of Dhaka, Bangladesh).
Phase accumulated approximately 0.3 wt% arsenic, by oxidation and
adsorption at the top of the saturated zone (ref. 45). b, Distribution of As in
pyrite from a depth of 260 m (Rajoir, 60 km south of Dhaka, Bangladesh).
The brightest colours indicate As contents close to 0.5 wt% whereas dark
areas contain less than 0.05 wt% of As. The circular areas are early formed
framboids, which tend to contain less As than later, massive, infilling pyrite
(from ref. 37).
capacity depends on sediment composition and history of exposure to As-rich, reducing water. Oxidized sediments proximal to
the Pleistocene inliers and boundary hills are likely to have high
adsorption capacity because of sustained oxic recharge; laboratory
experiments35 confirm this, and indicate that adsorption is enhanced
by the oxidizing potential of in situ manganese oxides. In contrast,
isolated lenses of oxidized sand are likely to have been exposed to
variable amounts of reduced groundwater, and their oxidizing and
adsorption capacities correspondingly depleted.
Grey Pleistocene sediments37 also have geochemical attributes
contributing to security against As invasion47,48. Deep, grey sediment
in southern Bangladesh contains less than 1 to 210 ppm As, where
groundwater consistently contains less than 10 μg l–1 As (ref. 47). The
highest As contents were detected in grey micaceous silts, in which
86
Studies of the hydraulics13,14,44,49 and geochemistry 14,32,37,47 of BAS
at depths greater than 150 m are few. Further lithological descriptions, measurement of hydraulic properties and groundwater head
profiles, sediment mineralogical and sorption properties, water
ages, and groundwater modelling analysis are necessary to improve
evaluations of the vulnerability of deep groundwater to invasion
by As.
Present groundwater models addressing the flow-pattern
defence are limited by the paucity of hydraulic head data available
for depths greater than 150 m, but they strongly suggest that
without invoking the geochemical defence, widespread deep
irrigation pumping might effectively eliminate deep groundwater as an As-free resource for domestic supply, possibly in less
than 100 years. Consensus in evaluating the long-term security of deep hand-pumped wells remains to be achieved, but
modelling indications are favourable. Deep municipal abstraction
may be deemed economically and socially acceptable if secure
for a more limited, but still substantial period before invasion
by As or salinity. Modelling approaches need to be refined to
elaborate the timescales of the security both of deep municipal
and hand-pumped abstraction. To maximize the security of deep
As-free groundwater, domestic wells should be screened as deep
as possible within oxidized sediments. Domestic abstraction of
shallow As-free groundwater in oxidized Pleistocene sediments
relies solely on the geochemical defence19. In relation to both
situations, the processes and limitations of As sorption need
further investigation.
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NATure GeOScieNce doi: 10.1038/ngeo750
references
1. Ravenscroft, P., Brammer, H. & Richards, K. S. Arsenic Pollution: A Global
Synthesis. 1st edn (Wiley-Blackwell, 2009).
2. Kinniburgh, D. G. & Smedley, P. L. (eds) Arsenic Contamination of Groundwater
in Bangladesh (British Geological Survey, and Department of of Public Health
Engineering, Dhaka, 2001); available at <http://www.bgs.ac.uk/arsenic/bphase2/
Reports/Vol3Atlasprelims.pdf>.
3. Das, B. et al. Groundwater arsenic contamination, its health effects and
approach for mitigation in West Bengal, India and Bangladesh. Water Qual.
Expo. Health 1, 5–21 (2009).
4. Berg, M. et al. Magnitude of arsenic pollution in the Mekong and Red River
Deltas — Cambodia and Vietnam. Sci. Total Environ. 372, 413–425 (2007).
5. Polya, D. A. et al. Arsenic hazard in shallow Cambodian groundwaters.
Mineral. Mag. 69, 807–823 (2005).
6. Mazumder, D. N. G. et al. Chronic arsenic toxicity from drinking
tubewell water in rural West Bengal. Bull. World Health Organ.
64, 499–506 (1988).
7. Groundwater Studies for Arsenic Contamination in Bangladesh. Phase I: Rapid
Investigation (British Geological Survey & Mott MacDonald Ltd, 1999).
8. Ahmed, M. F. et al. Ensuring safe drinking water in Bangladesh. Science
314, 1687–1688 (2006).
9. Bhattacharya, P., Chatterjee, D. & Jacks, G. Occurrence of arseniccontaminated groundwater in alluvial aquifers from the Delta Plains,
eastern India: Options for safe drinking water supply. Wat. Res. Dev.
13, 79–92 (1997).
10. Ahmad, J., Misra, S. & Goldar, B. Rural communities’ preferences for arsenic
mitigation options in Bangladesh. J. Wat. Health 04, 463–477 (2006).
11. Evaluation of the Performance, Village Piped Water Supply System (120 Schemes)
(Department of Public Health Engineering, Japan International Cooperation
Agency, Bangladesh, Dhaka, 2008).
12. Development of Deep Aquifer Database and Preliminary Deep Aquifer Map
(Department of Public Health Engineering, GoB and Arsenic Policy Support
Unit, Japan International Cooperation Agency, Bangladesh, Dhaka, 2006).
13. Hydrogeology Summary Report (Department of Public Health Engineering &
Danish International Development Assistance, Dhaka, 2001).
14. Groundwater Resources and Hydro-Geological Investigations in and Around
Khulna City Vol. 4 (Drilling) (Local Government Engineering Department,
Dhaka, 2005).
15. Hoque, M., Hasan, M. K. & Ravenscroft, P. in Groundwater Resources
and Development in Bangladesh - Background to the Arsenic Crisis,
Agricultural Potential and the Environment (eds Rahman, A. A. &
Ravenscroft, P.) 373–390 (Bangladesh Centre for Advanced Studies,
Univ. Press Ltd, 2003).
16. Ravenscroft, P. & McArthur, J. M. Mechanism of regional scale enrichment of
groundwater by boron: the examples of Bangladesh and Michigan, USA.
Appl. Geochem. 19, 1413–1430 (2004).
17. Michael, H. & Voss, C. Controls on groundwater flow in the Bengal Basin of
India and Bangladesh: regional modeling analysis. Hydrogeol. J.
17, 1561–1577 (2009).
18. 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).
19. Van Geen, A. et al. Monitoring 51 community wells in Araihazar, Bangladesh,
for up to 5 years: Implications for arsenic mitigation. Environ. Sci. Health,
Part A 42, 1729–1740 (2007).
20. Ravenscroft, P. et al. Arsenic in groundwater of the Bengal Basin, Bangladesh:
Distribution, field relations, and hydrogeological setting. Hydrogeol. J.
13, 727–751 (2005).
21. Nickson, R. et al. Mechanism of arsenic release to groundwater, Bangladesh
and West Bengal. Appl. Geochem. 15, 403–413 (2000).
22. Islam, F. S. et al. Role of metal-reducing bacteria in As release from Bengal
delta sediments. Nature 430, 68–71 (2004).
23. Harvey, C. F. et al. Arsenic mobility and groundwater extraction in Bangladesh.
Science 298, 1602–1606 (2002).
24. Burgess, W. G., Burren, M., Perrin, J. & Ahmed, K. M. in Sustainable
Groundwater Development, Special Publication 193 (eds Hiscock, K. M.,
Rivett, M. O. & Davison, R. M.) 145–163 (Geological Society, 2002).
25. McArthur, J. M. et al. How paleosols influence groundwater flow and arsenic
pollution: A model from the Bengal Basin and its worldwide implication.
Wat. Resour. Res. 44, W11411 (2008).
26. Weinman, B. et al. Contributions of floodplain stratigraphy and evolution to
the spatial patterns of groundwater arsenic in Araihazar, Bangladesh.
Geol. Soc. Am. Bull. 120, 1567–1580 (2008).
27. Harvey, C. F. et al. Groundwater dynamics and arsenic contamination in
Bangladesh. Chem. Geol. 228, 112–136 (2006).
28. Polizzotto, M. L. et al. Near-surface wetland sediments as a source of arsenic
release to ground water in Asia. Nature 454, 505–509 (2008).
29. 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).
30. Neumann, R. B. et al. Anthropogenic influences on groundwater arsenic
concentrations in Bangladesh. Nature Geosci. 3, 46–52 (2010).
31. McArthur, J. M. et al. Natural organic matter in sedimentary basins and its
relation to arsenic in anoxic groundwater: the example of West Bengal and its
worldwide implications. Appl. Geochem. 19, 1255–1293 (2004).
32. Swartz, C. H. et al. Mobility of arsenic in a Bangladesh aquifer: Inferences
from geochemical profiles, leaching data, and mineralogical characterization.
Geochim. Cosmochim. Acta 68, 4539–4557 (2004).
33. Burgess, W. G. et al. in Trace Metals and Other Contaminants in the
Environment (eds Bhattacharya, P. et al.) Ch. 2, 63–84 (Elsevier, 2007).
34. von Brömssen, M. et al. Geochemical characterisation of shallow aquifer
sediments of Matlab Upazila, Southeastern Bangladesh — Implications for
targeting low-As aquifers. J. Contam. Hydrol. 99, 137–149 (2008).
35. Stollenwerk, K. G. et al. Arsenic attenuation by oxidized aquifer sediments in
Bangladesh. Sci. Total Environ. 379, 133–150 (2007).
36. Zheng, Y. et al. Geochemical and hydrogeological contrasts between shallow
and deeper aquifers in the two villages of Araihazar, Bangladesh: Implications
for deeper aquifers as drinking water sources. Geochim. Cosmochim. Acta
69, 5203–5218 (2005).
37. Lowers, H. A. et al. Arsenic incorporation into authigenic pyrite, Bengal Basin
sediment, Bangladesh. Geochim. Cosmochim. Acta 71, 2699–2717 (2007).
38. Ravenscroft, P., McArthur, J. M. & Hoque, B. A. in Arsenic Exposure and Health
Effects Vol. IV (eds Chappell, W. R., Abernathy, C. O. & Calderon, R. L.)
53–77 (Elsevier, 2001).
39. Mukherjee, A. Deeper Groundwater Flow and Chemistry in the Arsenic
Affected Western Bengal Basin, West Bengal, India. PhD thesis,
Univ. Kentucky (2006).
40. Deep Tubewell II Project, Esp. Vol 2.1 Natural Resources (Mott MacDonald
International, Hunting Technical Services for the Bangladesh Agricultural
Development Corporation under the assignment to the Overseas Development
Administration (UK), Dhaka, 1992).
41. Michael, H. & Voss, C. Estimation of regional-scale groundwater flow
properties in the Bengal Basin of India and Bangladesh. Hydrogeol. J.
17, 1329–1346 (2009).
42. Williams, P. N. et al. Increase in rice grain arsenic for regions of Bangladesh
irrigating paddies with elevated arsenic in groundwaters. Environ. Sci. Technol.
40, 4903–4908 (2006).
43. Brammer, H. & Ravenscroft, P. Arsenic in groundwater: A threat to sustainable
agriculture in South and South-East Asia. Environ. Int. 35, 647–654 (2009).
44. Ravenscroft, P. in Groundwater Resources and Development in Bangladesh Background to the Arsenic Crisis, Agricultural Potential and the Environment
(eds Rahman, A. A. & Ravenscroft, P.) Ch. 3, 43–86 (Bangladesh Centre for
Advanced Studies, Univ. Press Ltd, 2003).
45. Breit, G. N. et al. in 11th International Symposium. Water-Rock Interaction
(eds Wanty, R. B. & Seal, R. R.) 1457–1461 (Balkema, 2004).
46. Dixit, S. & Hering, J. G. Comparison of arsenic(v) and arsenic(iii) sorption
onto iron oxide minerals: implications for arsenic mobility. Environ. Sci.
Technol. 37, 4182–4189 (2003).
47. Breit, G. N. et al. Compositional Data for Bengal Delta Sediment Collected from
Boreholes at Srirampur, Kachua, Bangladesh Report No. 2006–1222
(US Geological Survey, 2006).
48. The Study on Groundwater Development of Deep Aquifers for Safe Drinking
Water Supply to Arsenic Affected Areas in Western Bangladesh (Japan
International Cooperation Agency, Bangladesh, Dhaka, 2002).
49. Report on Deep Aquifer Characterization and Mapping Project, Phase I
(Kachua, Chandpur) (Ground Water Hydrology Division-I, Bangladesh Water
Development Board, Dhaka, 2005).
50. Goodbred, S. L. Jr & 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).
acknowledgements
We thank J. Davies for discussion of concepts informing the description of the Bengal
Aquifer System. M.A.H. is in receipt of a scholarship (BDCS 2006-37) from the
Commonwealth Scholarship Commission.
author contributions
All authors collaborated equally in the preparation of the manuscript.
additional information
The authors declare no competing financial interests. Supplementary information
accompanies this paper at www.nature.com/naturegeoscience.
nature geoscience | VOL 3 | FEBRUARY 2010 | www.nature.com/naturegeoscience
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manuscript NGS-2009-08-00993
SUPPLEMENTARY
INFORMATION
doi: 10.1038/ngeo750
Vulnerability of deep groundwater in the Bengal Aquifer
System to contamination by arsenic
W. G. Burgess1*, M. A. Hoque1, H. A. Michael2, C. I. Voss3, G. N. Breit4 & K. M. Ahmed5
1
Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT,
UK
2
Department of Geological Sciences, University of Delaware, Newark, DE 19716, USA
3
US Geological Survey, 431 National Center, Reston, VA 20192, USA
4
US Geological Survey, Box 25046 MS 964D, Denver, CO 80225, USA
5
Department of Geology, University of Dhaka, Dhaka 1000, Bangladesh
*
Correspondence: william.burgess@ucl.ac.uk nature geoscience | www.nature.com/naturegeoscience
1
supplementary information
doi: 10.1038/ngeo750
Fig. S1: Distribution of sediment lithology in southern Bangladesh. (a) A generalised W-E section of
lithological logs across southern Bangladesh. (b) Lithological cross-sections, Bangladesh. In (a) the
lithological logs within a 50 km W-E corridor are compiled into one section. In (b) 1573 lithological logs,
in a 2000x2000x3 m grid, were interpolated using the Rockwork mapping package (RockWare Inc.
Golden, CO, USA) applying the inverse distance algorithm. Driller’s logs are compiled from ref12, and
also from Bangladesh Water Development Board, University of Dhaka, Bangladesh University of
Engineering & Technology, Local Government Engineering Department, Bangladesh Agricultural
Development Corporation, and Dhaka Water Supply and Sewerage Authority (personal communications).
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nature geoscience | www.nature.com/naturegeoscience
doi: 10.1038/ngeo750
supplementary information
Fig. S2: Sediment colour distribution in southern Bangladesh. Lithological code prescript ‘b’
indicates yellowish-brown i.e. oxidized sediments; prescript ‘g’ indicates grey i.e. reduced sediments. ‘c’
= clay, ‘scs’ = silt-sandy clay, ‘vfs’ = very fine sand, ‘vffs’ = very fine to fine sand, ‘fms’ = fine to
medium sand, ‘ms’ = medium sand, ‘mcs’ = medium to coarse sand, ‘cs’ = coarse sand, ‘gs’ = sand with
gravel. Note that oxidized horizons occur at depth within predominantly reduced sequences in places (1,
4) and reduced horizons occur at depth within predominantly oxidized sequences of the Pleistocene inliers
(6). Lithological logs complied from Department of Public Health Engineering, Bangladesh Water
Development Board, University of Dhaka, Bangladesh University of Engineering & Technology, Local
Government Engineering Department, Bangladesh Agricultural Development Corporation, and Dhaka
Water Supply and Sewerage Authority (personal communications).
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