Arsenic in groundwater of the Bengal Basin, Bangladesh:
Distribution, field relations, and hydrogeological setting
Peter Ravenscroft · William G. Burgess ·
Kazi Matin Ahmed · Melanie Burren · Jerome Perrin
Abstract Arsenic contaminates groundwater across much
of southern, central and eastern Bangladesh. Groundwater
from the Holocene alluvium of the Ganges, Brahmaputra
and Meghna Rivers locally exceeds 200 times the World
Health Organisation (WHO) guideline value for drinking
water of 10 g/l of arsenic. Approximately 25% of wells in
Bangladesh exceed the national standard of 50 g/l,
affecting at least 25 million people. Arsenic has entered
the groundwater by reductive dissolution of ferric oxyhydroxides, to which arsenic was adsorbed during fluvial
transport. Depth profiles of arsenic in pumped groundwater, porewater, and aquifer sediments show consistent
trends. Elevated concentrations are associated with finesands and organic-rich sediments. Concentrations are low
near the water table, rise to a maximum typically 20–40 m
below ground, and fall to very low levels between about
100 and 200 m. Arsenic occurs mainly in groundwater of
the valley-fill sequence deposited during the Holocene
marine transgression. Groundwater from Pleistocene and
older aquifers is largely free of arsenic. Arsenic concentrations in many shallow hand-tube wells are likely to
Received: 6 January 2003 / Accepted: 18 November 2003
Published online: 9 March 2004
Springer-Verlag 2004
P. Ravenscroft ())
Arcadis Geraghty and Miller International,
2 Craven Court, Newmarket, Suffolk, CB8 7FA, UK
e-mail: pravenscroft@arcadisgmi.com
Tel.: +44-1638-674786
Fax: +44-1638-668191
W. G. Burgess · M. Burren · J. Perrin
Department of Earth Sciences,
University College London,
Gower Street, London, WC1E 6BT, UK
K. M. Ahmed
Department of Geology, Dhaka University,
Dhaka, Bangladesh
Present address:
M. Burren, 39 Durrants Lane, Berkhamstead, Herts, HP4 3PL, UK
Present address:
J. Perrin, Centre of Hydrogeology,
Neuchatel University,
11 Rue E-Argand 2000, Neuchatel, Switzerland
Hydrogeology Journal (2005) 13:727–751
increase over a period of years, and regular monitoring
will be essential. Aquifers at more than 200 m below the
floodplains offer good prospects for long-term arsenic-free
water supplies, but may be limited by the threats of saline
intrusion and downward leakage of arsenic.
Rsum L’arsenic contamine les eaux souterraines dans
la plus grande partie du sud, du centre et de l’est du
Bangladesh. Les eaux des nappes alluviales holocnes du
Gange, du Brahmapoutre et de la Meghna dpassent
localement 200 fois la valeur guide donne par l’OMS
pour l’eau de boisson, fixe 10 g/l d’arsenic. Environ
25% des puits du Bangladesh dpassent la valeur standard
nationale de 50 g/l, affectant au moins 25 millions de
personnes. L’arsenic a t introduit dans les nappes par la
dissolution par rduction d’oxy-hydroxydes ferriques sur
lesquels l’arsenic tait adsorb au cours du transport
fluvial. Des profils verticaux d’arsenic dans l’eau souterraine pompe, dans l’eau porale et dans les sdiments des
aquifres montrent des tendances convergentes. Les
concentrations leves sont associes des sdiments sable fin et riches en matires organiques. Les concentrations sont faibles au voisinage de la surface de la
nappe, atteignent un maximum typiquement entre 20 et 40
m sous le sol, puis tombent des niveaux trs bas entre
100 et 200 m. L’arsenic est surtout prsent dans les eaux
souterraines de la squence de remplissage de valle
dpose au cours de la transgression marine holocne.
Les eaux souterraines des aquifres plistocnes et plus
anciens sont trs largement dpourvus d’arsenic. Les
concentrations en arsenic dans de nombreux puits creuss
la main doivent probablement augmenter au cours des
prochaines annes ; aussi un suivi rgulier est essentiel.
Les aquifres plus de 200 m sous les plaines alluviales
offrent de bonnes perspectives pour des alimentations en
eau sans arsenic long terme, mais ils peuvent Þtre
limits par les risques d’intrusion saline et la drainance
descendante de l’arsenic.
Resumen El arsnico ha contaminado gran parte de las
aguas subterrneas en el Sur, centro y Este de Bangla
Desh. Su concentracin en las aguas subterrneas del
aluvial Holoceno de los ros Ganges, Brahmaputra y
Meghna supera localmente en un factor 200 el valor gua
del arsnico en el agua potable, establecido por la
Organizacin Mundial de la Salud (OMS) en 10 g/L.
DOI 10.1007/s10040-003-0314-0
728
Aproximadamente, el 25% de los pozos de Bangla Desh
superan el estndar nacional de 50 g/L, afectando al
menos a 25 millones de personas. El arsnico ha llegado a
las aguas subterrneas por la disolucin reductora de
hidrxidos frricos a los que se adsorbe durante el transporte fluvial. Los perfiles del arsnico en las aguas subterrneas bombeadas, agua de poro y sedimentos del
acufero muestran tendencias coherentes. Las concentraciones elevadas estn asociadas a arenas finas y sedimentos ricos en materia orgnica. Las concentraciones de
arsnico son bajas cerca del nivel fretico, se incrementan
hasta un mximo que se localiza generalmente a entre 20
y 40 m bajo la cota del terreno, y disminuyen a valores
muy pequeos entre alrededor de 100 y 200 m. El arsnico se encuentra sobretodo en las aguas subterrneas
existentes en la secuencia de sedimentacin que tuvo lugar en el valle durante la transgresin marina del Holoceno. Las aguas subterrneas del Pleistoceno y acuferos
ms antiguos estn mayoritariamente libres de arsnico.
Es probable que las concentraciones de arsnico aumenten en los prximos aos en muchos pozos de tipo tubo
perforados manualmente, por lo que ser esencial efectuar
un muestreo regular. Los acuferos ubicados a ms de 200
m bajo las llanuras de inundacin ofrecen buenas perspectivas de abastecimiento a largo plazo sin problemas de
arsnico, pero pueden estar limitados por las amenazas de
la intrusin salina y de la precolacin de arsnico desde
niveles superiores.
Keywords Arsenic · Bangladesh · Contamination ·
General hydrogeology · Hydrochemistry
Introduction
Arsenic in groundwater in the alluvial and deltaic plains
of Bangladesh and West Bengal (India) has resulted in the
worst case of mass chemical poisoning in the world
(Smith et al. 2000). The occurrence of arsenic in the
Bengal Basin is unusual because most of the documented
cases of arsenic contamination of groundwater are from
mining and industrial regions or areas of recent volcanic
activity (e.g. Welch et al. 1988; Nriagu 1994). Further, the
Bengal Basin case is exceptional in the great areal extent
of arsenic occurrence and the number of people affected.
It is estimated that more than 25 million people in
Bangladesh drink water containing more than 50 g/l of
arsenic, and possibly an additional 25 million drink water
with 10–50 g/l arsenic (Department of Public Health
Engineering, Government of Bangladesh (DPHE) 1999).
In Bangladesh, more than 120 million people live in an
area of 144,000 km2. Land use is dominantly agricultural
but urbanisation and industrialisation are proceeding
rapidly. Until the 1970s, drinking water was drawn dominantly from surface-water sources, and water-borne diseases such as cholera and dysentery caused millions of
deaths. During the last three decades, at least 3–4 million
hand-pumped tubewells (HTWs) have been installed. The
typical HTW is manually drilled to between 20 and 70 m,
Hydrogeology Journal (2005) 13:727–751
installed with 3 m of 38-mm diameter slotted PVC casing,
and attached to a lever-action suction pump. Groundwater
provides over 90% of drinking water and a large majority
of irrigation supplies. Before the arsenic problem was
recognised, ready access to bacteriologically safe water
from HTWs was widely recognised as the principal factor
in the dramatic reduction in waterborne disease and infant
mortality in Bangladesh, and quoted as a development
success (UNICEF 1998).
Arsenic was first identified in the groundwater of West
Bengal in 1983, following a medical diagnosis of arsenic
poisoning (Saha 1984, 1995; Mazumder et al. 1988).
Investigations in India in the 1980s and early 1990s
progressively identified the extent of pollution there
(Public Health Engineering Department, Government of
West Bengal, (PHED) 1991; Das et al. 1994, 1996).
Unfortunately, this information was effectively unknown
in Bangladesh until the early 1990s. The earliest known
analyses of arsenic in groundwater in Bangladesh (reported by Dhaka Water and Sewerage Authority (DWASA) 1991) were from three municipal supply wells in
Dhaka City. All were below the analytical method
detection limit (10 g/l) and therefore attracted no
attention. Arsenic was first detected in groundwater in
Bangladesh by the DPHE in 1993. Between 1995 and
1998, a series of surveys revealed the extent of the
catastrophe (NRECA 1997; Jakariya et al. 1998 and
DPHE 1999).
Chronic exposure to arsenic in drinking water results
in skin ailments such as hyperpigmentation and keratosis,
and leads progressively to cancers of the skin, to damage
to internal organs, cancer and ultimately death (WHO
1993; National Academy Press 2001). Symptoms may
take five to fifteen years or longer to develop. The current
standard for arsenic in drinking water in both Bangladesh
and India is 50 g/l. In 1993 the WHO recommended a
provisional guideline level of 10 g/l, based on the
practical limit of detection at the time. In 2001 the
Environmental Protection Agency (EPA) in the United
States adopted a reduced standard in the USA of 10 g/l
for public water supplies. Even this lower limit is not
expected to be protective at the one excess cancer in 106
lifetime exposures (National Academy Press 2001). The
WHO guideline has not been adopted in either India or
Bangladesh. The treatment for arsenic poisoning requires
the removal of exposure to arsenic in drinking water.
Installing and maintaining safe water supplies in the
magnitude now required severely challenges the capacity
of the people and governments of Bangladesh and India.
The scale of the clinical and social effects of arsenic poisoning can be appreciated by reference to web
sites maintained by the West Bengal and Bangladesh
Arsenic Crisis Information Centre (http://www.bicn.com/
acic/) and Harvard University (http://phys4.harvard.edu/
%7Ewilson/ arsenic_project_main.html). The full human
dimension of the tragedy is still unclear and will depend
in part on the rate at which mitigation programmes can be
implemented.
DOI 10.1007/s10040-003-0314-0
729
Secondary impacts on health may result from agricultural activities whereby arsenic in soil or irrigation water
is taken up by crops, and thereby enters the human food
chain. Preliminary data on this subject are reviewed by
Huq et al. (2001) who conclude that it is a matter of
serious concern that requires immediate attention.
In this paper the principal observations of arsenic
occurrence at a regional scale (104–105 km2, DPHE 1999)
are combined with results from sub-regional scale studies
(103 km2, DPHE 1999) and localised studies (15 km2,
Burren 1998; Perrin 1998) to establish a hydrogeological
interpretation of arsenic in groundwater of the Bengal
Basin. In this paper the occurrence of arsenic in groundwater is considered in relation to the geological history of
the Bengal Basin, the groundwater chemical evolution,
and possible anthropogenic influences. A detailed description of mitigation options is beyond the scope of this
paper but, in conclusion, the aspects of mitigation that are
directly related to groundwater resources management are
discussed.
Geomorphology
Bangladesh has a tropical monsoonal climate. Mean
annual rainfall (Rashid 1991) is lowest in the west (e.g.
Rajshahi: 1435 mm) and increases both to the northeast
(Sylhet: 4177 mm) and the southeast (Chittagong:
2740 mm) (Fig. 1). Long-term average maximum and
minimum monthly temperatures at Dhaka range from
25.5 and 11.7 C in January to 35.1 and 23.4 C in April.
Despite the high rainfall, around 90% of river flows in
Bangladesh originate in India, Nepal and China. The
Bengal Basin (Morgan and McIntire 1959), which constitutes the major part of Bangladesh and the adjoining
state of West Bengal in India, is effectively the delta of
the Ganges – Brahmaputra – Meghna (GBM) River
system (Fig. 1). These rivers show a broad transition from
braided plains through meander belts to tidal and estuarine plains as they approach the sea, accompanied by a
general decrease in the median grain-size of the bed load.
They flood large parts of their alluvial plains each year
during the monsoon. The Holocene floodplains are
characterised by immature soil development over a thick
sequence of sedimentary deposits, and the formation of a
ploughpan beneath agricultural land (Brammer 1996).
The rivers converge to become the Lower Meghna River
to the south of Dhaka, and their alluvial plains combine to
form the largest delta in the world. The discharge of the
Lower Meghna (1.1106 Mm3/yr) makes it the third
largest river in the world, but in terms of sediment transfer
(c. 1109 t/yr) it is by far the largest (Friedman and
Sanders 1978). The active delta has advanced south by
about 100 km in the last 1000 years (Bakr 1977). The tidal
range in the Meghna Estuary is mostly between two and
four metres.
Hydrogeology Journal (2005) 13:727–751
Fig. 1 Location map, showing the names of places referred to in
the text. Solid shading indicates areas elevated above the surrounding alluvial floodplains; MT, Madhupur Tract, BT, Barind Tract,
and CHT, Chittagong Hill Tracts. The line ABC shows the line of
section in Fig. 2
Geology
Regional Geology
The Bengal Basin is bounded by the Himalayas and the
Shillong Plateau to the north, the Indian platform to the
west, and the Indo-Burman ranges to the east (Morgan
and McIntire 1959). The alluvial plains of the GBM river
system slope from north to south, smooth on a regional
scale but interrupted locally by ridges and basins. Pleistocene terraces – the Madhupur and Barind Tracts –
locally interrupt the flat topography of central Bangladesh, rising by up to 20 m above the adjacent floodplains.
These tracts have an incised dendritic drainage, with
channels filled by organic-rich muds of Holocene age
(Monsur 1995). It is convenient to consider the regional
geology in terms of these three major subdivisions – the
Tertiary hills, Pleistocene terraces and the Holocene
floodplains. Arsenic contamination in the Bengal Basin
occurs predominantly beneath the Holocene floodplains.
Stratigraphic correlation of the Bengal Basin has been
difficult (Brunnschweiler and Khan 1978), and the Quaternary (Monsur 1995) is particularly poorly defined
owing to the absence of well-exposed sections and the
difficulty of establishing absolute ages for the litholoDOI 10.1007/s10040-003-0314-0
730
Table 1 Simplified stratigraphic succession of Bangladesh
Age
Stratigraphic units
Lithology
Notes
Chandina Formation
Dhamrai Formation
Unclassified deposits
Lower Pleistocene Madhupur Clay
Barind Clay
Plio-Pleistocene
Dupi Tila Formation
Dihing Formation
Upward fining, grey micaceous, medium and
coarse sand to silt with organic mud and
peat.
Tough, red-brown to grey, silty-clay; residual
deposits; kaolinite and iron oxide.
Yellowish-brown to light grey, medium and
coarse sand to clay; very weakly consolidated; depleted in mica and organic matter.
Forms major aquifers beneath recent
floodplains. Probably <150 m thick.
Tertiary
Tipam Group
Mesozoic
Surma Series
Barail Series
Jaintia Group
Sylhet Traps
Yellowish-brown, weakly consolidated sandstone and mudstone.
Consolidated sandstone and shale
Consolidated sandstone and shale
Shale, limestone and sandstone
Basalt, shale and sandstone
Late Pleistocene–
Holocene
Often absent beneath Holocene floodplains.
Thickness 6 to 60 m.
Forms major aquifers beneath the terraces
and hills, and deeper aquifers beneath the
Holocene floodplains. Hundreds to thousands of metres thick
Minor aquifers in hills
No significant aquifers
After Alam et al. (1990), Khan (1991) and DPHE (1999)
gies represented. A simplified stratigraphic sequence for
Bangladesh is shown in Table 1. The nomenclature differs
from that used in West Bengal (Wadia 1975; PHED
1991), where arsenic occurs mostly beneath the Upper
Deltaic Plains, approximate time-equivalents of the Chandina and Dhamrai Formations.
Considerable thicknesses of Holocene sediment, including the Chandina and Dhamrai Formations, underlie
the floodplains of the Ganges, Brahmaputra and Meghna
rivers [Bangladesh Agricultural Development Corporation (BADC) 1992]. The Dupi Tila Formation and the
Barind and Madhupur Clays of the Pleistocene terraces
were deposited in the Lower Pleistocene or earlier
(Monsur 1995). Whitney et al. (1999) estimated that the
geomorphic surfaces of the Barind and Madhupur Tracts
dated from about 25,000 years BP and more than 110,000
years BP respectively. The underlying Tertiary strata are
alluvial or shallow marine clastic sediments and have
little direct influence on the exploited groundwater
resources (Ravenscroft 2003). Only in northern Bangladesh does the Holocene alluvium directly overlie the
Indian continental crust [Master Plan Organisation (MPO)
1987]. Due to the extreme incision of the GBM system,
there are few remnants of sedimentation from the Middle
or Upper Pleistocene, resulting in an age contrast of two
orders of magnitude between the two major alluvial
aquifer systems, comprising Holocene and Lower Pleistocene sediments.
Quaternary History of the Bengal Basin
Global climatic changes, uplift of the Himalayas and
subsidence in the Bengal Basin interacted to control
Quaternary alluvial sedimentation in Bangladesh (Umitsu
1993; Ravenscroft 2003). Himalayan glaciation suppressed monsoonal circulation, thus reducing rainfall
and river flows (Dawson 1992). For much of the last
120,000 years, global sea level stood about 50 m below its
present level, falling to a minimum of 130 m at 18 Ka BP.
During low-stands, the GBM system occupied deeply
Hydrogeology Journal (2005) 13:727–751
incised channels within a series of terraces now largely
buried beneath the Holocene floodplains. During the postglacial transgression, maximum sedimentation initially
occurred on the submarine fan between 12.8 and 9.7 Ka
BP (Kudrass et al. 1999) but shifted nearshore after about
11 Ka BP when sea level intercepted the coastal plain
about 50 m below the present surface (Goodbred and
Kuehl 2000). Kudrass et al. (1999) and Goodbred and
Kuehl (2000) concluded that during the mid-Holocene sea
level was slightly higher, the climate was warmer, and the
rivers discharged up to two and a half times more than in
present times. Goodbred and Kuehl (2000) estimate
subsidence rates of 1 to 4 mm a year during the Holocene,
the maximum being in the Sylhet Basin.
Hydrogeology
Highly productive aquifers occur beneath both the
Holocene floodplains and Pleistocene terraces, the shallowest generally encountered within 10 to 60 m of the
surface. Most water wells are less than 100 m deep.
Typically, 50–75% of the sequence can be successfully
developed to this depth (MPO 1987). Saline groundwater
is present in parts of the coastal area (Ravenscroft 2003)
and some inland areas (Hoque et al. 2003). In the coastal
region, fresh-water aquifers are encountered either within
the first 25 m or below about 150–200 m depth. An
equivalent deeper aquifer is also exploited at some towns
north of the coastal area (DPHE 1996), below an aquitard
which is present in places but appears not to be regionally
extensive.
Hydrostratigraphy
In the absence of reliable and extensive dating, the term
‘Holocene aquifers’ is used in this paper to describe the
young aquifers that underlie the alluvial, estuarine and
deltaic floodplains of Holocene age. The actual age of the
aquifer sediments is inferred generally to be younger than
DOI 10.1007/s10040-003-0314-0
731
about 11,000 years BP, coincident with a marine flooding
surface at about 50 m below modern sea level (Goodbred
and Kuehl 2000). Slightly older, but younger than the last
glacial maximum (LGM), sediments are inferred to be
present along the axes of major valleys such as the
Jamuna (Japanese International Cooperation Agency
(JICA) 1976).
The Holocene aquifers, which include the Chandina
and Dhamrai Formations, reach a maximum thickness of
about 100 m (JICA 1976; BADC 1992). Grain sizes fine
upwards, from coarse sands and gravels at the base, to
fine and very fine sands towards the top of the aquifer
(MPO 1987; Davies 1989; BADC 1992). Hydraulic conductivity values span at least four orders of magnitude
(Burgess et al. 2002), resulting in a highly transmissive
multi-layered aquifer (MPO 1987; Herbert et al. 1989).
Silts and clays predominate in the upper few metres,
forming a surficial aquitard, generally less than 10 m
thick, with typical specific yield values of 2–3%, and
vertical permeability values in the range 3–810-3 m/d
(BADC 1992). This aquitard is extensive, but may not be
continuous across active and recently abandoned riverbeds. The contact between the upper aquitard and the
exploited aquifers is gradational. The aquifers are mostly
medium-to-fine and medium-to-coarse sands, with permeabilities of 40–80 m/d. Short-term pumping tests on
the Holocene aquifers indicate a leaky response, but for
longer pumping periods the aquifer is best described as
regionally unconfined (MPO 1987).
The Holocene sands are grey, highly micaceous, often
containing abundant organic matter (Davies 1989, 1995),
and show relatively few signs of weathering, in contrast to
the thoroughly oxidised and weathered Dupi Tila sands of
Pleistocene age (BADC 1992). The principal mineralogical components of the Holocene sands are quartz, plagioclase feldspar, potassium feldspars, micas (muscovite,
biotite and chlorite), and clays (smectite, kaolinite, illite),
[Perrin 1998; Asian Arsenic Network (AAN) 1999].
Organic matter is present at up to 6% by weight and iron
oxyhydroxides occur as grain coatings and fine particulate
matter. Pyrite is rare; where observed it is framboidal and
apparently authigenic (Perrin 1998; Nickson et al. 2000).
The Pleistocene aquifer system is formed of Madhupur
or Barind Clay overlying Dupi Tila sands. The Pleistocene clays are thicker (up to 60 m) and more consolidated than the Holocene aquitards, with lower vertical
permeability and lower specific yield (BADC 1992). The
yellowish-brown Dupi Tila sand aquifer is tens of metres
to more than a hundred metres thick. The sands contain
less mica and less organic matter than the Holocene
sands. Permeabilities of the Dupi Tila sands are typically
20–30 m/d, about half that of Holocene sediments with
the same median grain-size, an effect attributed to the
presence of secondary clays and iron oxides which
partially clog pore throats (BADC 1982; Ahmed 1994).
Despite leaky or confined responses during pumping tests,
over periods of a few months the aquifer response is also
best characterised as regionally unconfined (MPO 1987;
BADC 1992).
Hydrogeology Journal (2005) 13:727–751
In the coastal regions, at Khulna, Barisal and Noakhali,
shallow fresh-water aquifers overlie saline groundwater at
depths of 20–30 m. Deeper sands, below about 150 m,
form productive fresh-water aquifers which are apparently protected from saline intrusion by intermediate clay
layers (e.g. Rus 1985). North of the coastal area, clayey
aquitards are present in some places and at varying
depths, e.g. at Meherpur, where there is an aquitard 30–
65 m thick at a depth of 160 m (Burgess et al. 2002), but
their lateral extent is only locally defined. Sands deeper
than about 150 m beneath the Holocene floodplains may
be equivalent to the Dupi Tila sands, but their identification is ambiguous because of the removal of the Barind
and Madhupur Clays at times of lower base levels and the
paucity of absolute dates. Where deep clayey aquitards
exist, the sands below are commonly referred to as the
‘deep aquifer’, although there is no generally agreed
definition. Where the aquitards are absent, the deeper
sands may be Pleistocene in age, but they effectively
constitute deeper regions of the same multi-layered aquifer that at shallower levels is formed of Holocene sands.
Across the Holocene floodplain in southern Bangladesh,
the deeper levels of the aquifer are exploited for potable
water supply to depths of up to 350 m at individual towns
(DPHE 1996).
Groundwater Circulation
In the north and centre of the country, the aquifer system
beneath the Holocene floodplains behaves essentially as a
single layer, but to the south and east layering becomes
increasingly important. Annual potential recharge is from
400 to >1000 mm (MPO 1987), but actual recharge is
much less because aquifer-full conditions develop during
the monsoon. In the absence of pumping, the water table
fluctuates seasonally by around 4–6 m (UNICEF 1994).
With the advent of pumping for irrigation, water table
fluctuations have increased. The effect is greatest in the
Dupi Tila aquifer beneath the Pleistocene terraces, where
seasonal depression of the water table to 15 m below
ground level (bgl) is common (BADC 1992; Hasan et al.
1998). Beneath the floodplains, the additional water table
lowering due to irrigation pumping is typically 2–3 m
(UNICEF 1994). Only at Dhaka City has continuous
pumping from the Dupi Tila aquifer for water supply
almost completely suppressed seasonal fluctuations and
caused long-term decline of the water table (Ahmed et al.
1999). Due to low topographic gradients on both the
Pleistocene terraces and the Holocene floodplains, hydraulic gradients are very small, commonly 0.0001 (e.g.
Burgess et al. 2002) and lateral groundwater flow in the
shallow aquifer is very slow, the Darcy velocity being
about 2 m per year. Three groundwater flow systems are
postulated to operate simultaneously on different scales:
– A local-scale flow system, between local topographic
features (floodplains, levees, flooded depressions,
minor rivers), to a depth of about 10 m over distances
of a few kilometres.
DOI 10.1007/s10040-003-0314-0
732
– An intermediate-scale flow system, between regionally
extensive topographic features (hills, terraces and the
major rivers), with flow paths up to tens of kilometres
long and residence times of hundreds to thousands of
years.
– A basinal-scale flow system, between the boundaries
of the basin and the Bay of Bengal, with flow paths
hundreds of kilometres in extent and residence times of
the order of tens of thousand years. Radiocarbon ages
for groundwater in deep coastal aquifers (e.g. Rus
1985; Aggarwal et al. 2000) relate to the closure of a
low-stand flow system of this scale, buried during the
Holocene transgression.
Where groundwater is pumped, the natural flow
systems are considerably disrupted and vertical components of flow dominate the groundwater flow system.
Water Quality
Before the discovery of arsenic contamination, the chemical quality of groundwater beneath the Holocene floodplains was thought to be generally good (MPO 1987;
Davies and Exley 1992), although the shallow groundwater is vulnerable to contamination by bacteria (Hoque
1998). Iron was known to be a widespread nuisance, and
salinity a constraint in the shallow aquifers of the coastal
area. Subsequently, in addition to arsenic, the DPHE
(1999) has identified manganese and boron as common,
naturally occurring constituents, present in places above
the WHO health-related guidelines for drinking water,
0.5 mg/l in both cases.
Groundwater beneath the Holocene floodplains is
mainly of the Ca-Mg-HCO3 type with relatively high
mineralization (EC 500–1000 S/cm), tending towards a
Na-Cl type water near the coast. This contrasts with
groundwater from the Dupi Tila sands aquifer beneath the
Pleistocene terraces, which is typically of Na-HCO3 type
and less mineralised, EC 200–300 S/cm (Davies and
Exley 1992; DPHE 1999). Groundwater beneath the
Holocene floodplains is characterised by high bicarbonate, with HCO3- commonly present at several hundred
mg/l. It is predominantly anoxic, and mostly strongly
reducing, locally to the extent of methanogenesis (Ahmed
et al. 1998; Hoque et al. 2003). Dissolved iron is typically
present at around 5–10 mg/l. Manganese commonly
exceeds 0.5 mg/l. Sulphate concentration is generally low
beneath the Holocene floodplains, mostly less than about
5 mg/l, although, as with nitrate, it is higher beneath areas
of settlement (e.g. Burgess et al. 2002).
These chemical characteristics reflect the conditions
under which groundwater beneath the Holocene floodplain has evolved. Groundwater gradients in the Holocene
sediments are likely to have been low since their deposition, and the aquifer would not have undergone the
flushing experienced by the Pleistocene and older sediments. The elevated bicarbonate concentrations, together
with the high dissolved iron and other indications of
reducing conditions, suggest that oxidation of organic
Hydrogeology Journal (2005) 13:727–751
matter (Lovley 1987), combined with hydrolysis of feldspar and weathering of mica (Breit 2001) are the dominant processes in the evolution of the groundwater
chemistry.
Hydrogeological Synthesis
The simplified hydrogeological section (after Ravenscroft
2003) through northeast Bangladesh in Fig. 2 generalises
and contrasts the aquifer conditions where elevated
arsenic concentrations occur with those where arsenic is
absent. The principal differences are between those
sediments that pre- and post-date the 18 Ka BP sea level
low-stand. In the central area, the thick Madhupur Clay
confines brown Dupi Tila sands with a relatively sharp
transition in grain size. The sands are weathered and
oxidised, and contain less mineralised, Na – HCO3 type,
water and lower concentrations of trace elements. To both
the east and west are the grey Holocene channel-fill
sediments that belong to the Dhamrai Formation in the
Jamuna valley and the Chandina Formation along the Old
Brahmaputra. The upper aquitards are thin, and they are
separated from the main aquifer horizons by thick fine
sands (marginal aquifers). The sands show few signs of
weathering, and the waters are more mineralised and
strongly reducing with high bicarbonate, iron and manganese contents. Brown clays at about 40 m depth in
the Jamuna valley are probably remnants of an Upper
Pleistocene terrace, although the underlying sands significantly post-date the adjacent Dupi Tila sands. The Old
Brahmaputra channel is apparently less deeply incised
than the Jamuna, and some wells may penetrate the Dupi
Tila. Thick clays with lenticular sand bodies characterise
the piedmont deposits at the foot of the Shillong Plateau.
Methods of Investigation and Sources of Data
The principal data sources for this paper are derived from
two survey programmes. The first is the Groundwater
Studies for Arsenic Contamination in Bangladesh (DPHE
1999), a project carried out by Mott MacDonald Ltd and
the British Geological Survey (BGS). The second is the
London-Dhaka Arsenic in Groundwater Programme conducted by University College London (UCL) and Dhaka
University (Nickson 1997; Burren 1998; Perrin 1998;
Burgess et al. 2001). The DPHE studies comprised the
compilation of existing data on arsenic in groundwater
and regional (104–105 km2 scale) and sub-regional
(103 km2 scale) surveys of arsenic and its hydrochemical
and hydrogeological context (DPHE 1999). The UCL and
Dhaka University studies focussed on detailed local scale
(10 km2) mapping of tubewell water quality, porewater
chemistry and hydrogeological controls on arsenic occurrence (Burgess et al. 2002; Cuthbert et al. 2002). Here
data are drawn principally from the DPHE (1999)
surveys, which covered two-thirds of the country (the
Regional Survey), mainly central and southern Bangladesh, the associated compilation of available data (the
DOI 10.1007/s10040-003-0314-0
733
Fig. 2 Simplified hydrogeological section through north-central
Bangladesh (after Ravenscroft 2003). The lithological section is
derived from several hundred individual logs reported by BADC
(1992) that have been averaged as the most probable lithology in
each 3-m depth slice within the local administrative unit (union)
through which the line of section passes. Lower case ‘g’ (grey) and
‘b’ (brown) denote the dominant sand colour. The bold dotted line
shows the inferred position of the land surface during the last
glacial maximum. Large arrows show the general direction of
regional groundwater flow. The depth of the section represents the
varying average depth of wells in each area, which is governed by
the greater thickness of clays and lower permeability of sands
beneath the Madhupur Tract. Permeability and EC data are also
from BADC (1992). Iron data are from Davies and Exley (1992)
Pre-existing Surveys), and the detailed survey of Meherpur town in western Bangladesh (the Meherpur Survey of
the London-Dhaka Arsenic in Groundwater Programme).
The Regional Survey was based on an average of
eight evenly-spaced groundwater samples in each of 250
upazilas (sub-district), stratified to take account of multilayered aquifers, but unbiased with respect to medical
reports, surface geology or the age of wells. The wells
sampled were predominantly hand-pumped tubewells.
Water samples were analysed for arsenic by hydride
generation – atomic absorption spectrometry (HG-AAS)
by the BGS in the UK, and spectrophotometry by DPHE
in Bangladesh. One sample per upazila was analysed by
the BGS for cations by inductively coupled plasma atomic
emission spectrometry (ICP-AES), and for anions by ion
chromatography (IC), to build up a geochemical baseline.
Quality control checks showed that the BGS results were
the more reliable, and these data are used in this paper.
Full analytical results are given in DPHE (1999).
The Pre-existing Surveys were evaluated according to
their sampling and analytical methodology, documentation, geographic referencing, and by comparison with the
unbiased sampling by others in the same region where
available (e.g. NRECA 1997). The medically-oriented
surveys aiming to confirm the cause of suspected arsenicosis were judged to overestimate the general levels of
contamination. Field-test methods (based on the mercuric
bromide stain method) tend to underestimate arsenic
concentrations, and only reliably indicate exceedance of
50 g/l when arsenic concentrations actually exceed
180 mg/l (DPHE 1999). Where statistical calculations are
performed on arsenic concentration data, results below
detection limits have been processed as half the detection
limit. The Regional Survey was considered to give the
most regionally representative view of arsenic concentrations, on account of the unbiased sampling strategy used
and the quality control of the analytical results.
The Meherpur Survey (Burren 1998; Perrin 1998) was
based on 150 groundwater samples from HTWs, irrigation
tubewells and deep public supply tubewells (DTWs) over
an area of 15 km2. Arsenic concentration was tested using
a field-test kit at 125 sites, from which 76 were sampled
for full hydrochemical analysis, selected to represent the
full range of arsenic concentration, borehole depth, and
pumping regime. Samples were filtered at the wellhead
using 0.45 m membrane filters (for anion analysis), or
filtered and acidified to pH 2 (for cation analysis). On-site
measurements were made of electrical conductivity, pH,
alkalinity, dissolved oxygen and temperature. Purging
prior to sampling was limited by time constraints to
approximately 40% of the HTWs volume. Shallow and
deep irrigation tubewells with motorised pumps were
pumped for at least 10 minutes to allow purging of at least
three well volumes. Anion analysis was by IC and cation
Hydrogeology Journal (2005) 13:727–751
DOI 10.1007/s10040-003-0314-0
734
analysis was by ICP-AES. Arsenic was determined by
hydride-generation AAS (detection limit 1 g/l). Full
analytical details are given in Burren (1998) and summarised in Burgess et al. (2002).
Sediment samples were preserved immediately following recovery on site by waxing the ends of the PVC
core sleeves. On extraction in the laboratory, approximately 100 g of sediment were mixed with distilled water,
stirred for five minutes and allowed to settle. The
porewater/distilled water mixture was then decanted,
and filtered or centrifuged prior to analysis. The results
must be interpreted in the light of the inevitable possibility of oxidation occurring during porewater extraction
prior to analysis. However, dissolved iron is generally
high in the porewaters, up to 30 mg/l, and has not apparently been oxidised and removed from solution. Also,
porewater calcium concentration is similar to that of local
groundwaters, which suggests that calcite precipitation
has not occurred to any great extent. Porewater composition is in general similar to pumped groundwater in the
region. Together these factors suggest that the integrity of
the porewater has been adequately preserved during
treatment, and the porewater hydrochemical profiles have
not been obscured.
Sediment samples were extracted from the wax-sealed
PVC core-sleeves, oven-dried, disaggregated and mixed
prior to analysis. One sub-sample from each core was
subjected to standard fusion with lithium metaborate;
another sub-sample was treated with hot 6 M HNO3 to
extract the readily soluble minerals (Perrin 1998). Analysis was by ICP-AES for cations, by IC for anions, and by
hydride-generation AAS for arsenic. Full analytical details are given in Perrin (1998) and summarised in
Burgess et al. (2002). It is emphasised that despite being
wax-sealed immediately on recovery at the drilling site,
and oven dried over a 24-hour period immediately on
extraction in the laboratory, some oxidation of the cored
sediments may have occurred. Breit (2001) demonstrated
the sensitivity of iron and arsenic to the redox environment by measuring oxidation of 50% of the extractable
iron and arsenic in grey Holocene sediment on exposure
to humid air for one week. Oxidation might therefore lead
to an increase in the reported iron oxide concentrations,
and could potentially result in oxidation of As(III) on the
sediments, but would not affect the total arsenic analysis.
Distribution of Arsenic in Groundwater
Regional Distribution
DPHE (1999) compiled the results of more than 30,000
arsenic tests from the Pre-existing Surveys, with results of
3,534 analyses for arsenic from the Regional Survey, and
the data were compiled into a GIS-database. The Preexisting Surveys, of which 72% of analyses were by fieldtest kit, covered all parts of the country except the
Chittagong Hill Tracts and some offshore islands. The
Regional Survey, conducted in two phases, covered the
whole country except for the Chittagong Hill Tracts. A
summary of the results, by region (administrative division), as the percentage of wells exceeding 50 g/l
arsenic, is shown in Table 2. The 25% exceedance as
indicated by the Regional Survey is the same as that by all
data sets combined. The Pre-existing Survey field testing
indicated a slightly lower percentage exceedance of the
50 g/l arsenic limit, at 21%. The Pre-existing Survey
laboratory test data indicated a higher percentage exceedance, at 34%. Inconsistencies between the data sets
might be expected due to differences in sampling strategy
and analytical methods. The field-testing had a relatively
unbiased geographical sampling frame but suffered from
unreliable detection of arsenic in the range of 50–180 g/
l. The laboratory results are analytically superior but in
many cases the Pre-existing Survey samples were selected
from known contaminated areas. The Regional Survey
gives the most realistic estimate of the frequency distribution of arsenic concentrations on account of the unbiased, consistent sampling strategy employed. The frequency distributions over seven concentration ranges are
given in Table 3.
Variations of arsenic concentrations with depth and
time (see below) must be considered when mapping its
distribution. Changes over time occur sufficiently slowly
to allow mapping of arsenic data collected within a few
years, even though the tubewell age is a significant factor
(DPHE 1999; Cuthbert et al. 2002). Variations of arsenic
with depth may be accounted for by mapping data from
Table 2 Summary of arsenic testing of water wells in Bangladesh
Number of tests
Percent of wells with more than 50 g/l of arsenic (%)
Division
Field kit
tests
Pre-existing
survey laboratory tests
DPHE
Regional
survey
All arsenic
tests
Field kit
tests
Pre-existing
survey laboratory tests
DPHE
Regional
survey
All arsenic
tests
Barisal
Chittagong
Dhaka
Khulna
Rajshahi
Sylhet
Bangladesh
1,396
3,232
6,175
4,819
5,891
1,264
22,777
403
1,094
2,189
2,036
1,712
1,440
8,874
295
445
988
474
1072
260
3,534
2,094
4,771
9,353
7,329
8,675
2,964
35,185
9
51
17
30
8
11
21
31
74
34
30
17
35
34
14
50
31
41
6
21
25
14
56
22
31
10
24
25
Source: DPHE (1999) and <exref type=”URL”>http://.www.bgs.ac.uk/arsenic/bangladesh</exref>
Hydrogeology Journal (2005) 13:727–751
DOI 10.1007/s10040-003-0314-0
735
Table 3 Frequency distribution of arsenic concentrations
Concentration range (g/l)
Classification
No. in class
Percent in class (%)
Percent exceeding lower
threshold (%)
<10
10–50
50–100
100–250
250–500
500–1,000
>1,000
Total
Below WHO guideline
Below Bangladesh standard
Above Bangladesh standard
2,041
606
315
324
178
61
3
3,534
58
17
8.9
9.2
5.0
1.7
0.09
42
25
16
6.9
1.8
0.09
Source: DPHE (1999) and <exref type=”URL”>http://.www.bgs.ac.uk/arsenic/bangladesh</exref>
Fig. 3 Percentage of wells with > 50mg/l arsenic. The map summarises >33,000 field and laboratory test data from drinking water
wells as compiled by DPHE (1999). The unit of aggregation is
the upazila, of which there are 490 in Bangladesh. A minimum
criterion of 10 results per upazila was applied. No depth classification was applied in selecting wells
wells with limited depth ranges, but the effect of vertical
flow within individual tubewell catchment areas must be
acknowledged (Burgess et al. 2002). Within these constraints, the distribution of arsenic can be mapped either
as concentration or as the frequency of exceedance of a
threshold concentration. Figure 3 shows the percentage of
wells exceeding the Bangladesh drinking water standard
(50 g/l) in each upazila with 10 or more tests, using data
Hydrogeology Journal (2005) 13:727–751
from the Pre-existing Survey and the Regional Survey
(DPHE 1999). Despite the coarse resolution and the
geologically arbitrary boundaries, chloropleth mapping
based on administrative unit allows incorporation of the
much larger number of non-georeferenced data and
qualitative field-kit results available from the Pre-existing
Surveys, and hence gives a fuller perspective on human
exposure. A geostatistical interpolation of the ‘most
probable’ arsenic concentration, interpolated from logtransformed arsenic concentrations at wells less than
150 m deep in Fig. 4, using data from the Regional
Survey, gives more insight into the geological controls.
The log-transformation is justified by the demonstration
of Johnston (1998) that arsenic concentrations approximate a lognormal distribution within discrete geological
units.
In whatever way the arsenic data are represented, the
broad regional patterns are the same. Arsenic concentrations exceeding 50 g/l are encountered in most parts of the
country, but most commonly in the southeast, southwest
and the Sylhet Basin (northeast) (Fig. 3). In many areas
adjacent to the Lower Meghna Estuary more than 80% of
wells exceed the 50 g/l concentration limit. For example,
Jakarya et al. (1998) have reported that 93% of 12,000
wells in Hajiganj upazila in southeast Bangladesh exceed
the limit. The probability of encountering extreme arsenic
concentrations, above 250 g/l, is also highest in the south
and southeast (Fig. 4). High concentrations of arsenic are
found in the lower catchments of all three major rivers of
the GBM system, indicating the existence of multiple
source areas and the likelihood of related mechanisms of
mobilisation across the entire Bengal Basin. Tongues of
high concentration of arsenic in groundwater extend upstream along the major river courses, expanding along the
course of the Meghna River into the subsiding Great Haor
Basin of Sylhet. The band of lower arsenic concentration
extending NNW-SSE along the Khulna-Jessore ridge is
also of note as it may indicate an accumulation of coarser
sediment along a Holocene course of the River Ganges.
Sub-regional Distribution
Arsenic concentrations vary systematically at a number
of smaller scales. Figure 5 shows the distribution of
arsenic in groundwater at Nawabganj upazila in northwest
DOI 10.1007/s10040-003-0314-0
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Table 4 Summary of arsenic determinations in Nawabganj Upazila
Surface
geological unit
No. of
wells
Arsenic concentration (g/l)
Mean1
Max.
>50
>10
Alluvial Sand
Alluvial Silt
Barind Clay /
Dupi Tila
21
22
15
7.0
4.9
<0.6
742
1,524
<0.6
24%
14%
0%
43%
45%
0%
1
Calculated as the mean of the logarithms of arsenic concentration.
All analyses by ICP method on acidified, filtered samples. Data
source DPHE (1999)
Fig. 4 Arsenic concentration in groundwater in drinking water
wells less than 150 m deep. The input data are laboratory analyses
from the Regional Survey (DPHE 1999 and http://bgs.ac.uk/
arsenic/Bangladesh). The areal distribution of arsenic concentrations was interpolated from logarithms of arsenic concentration at
3,500 evenly spaced points using the ArcView Spatial Analyst
software with the Inverse Distance Weighted method and including
the eight nearest neighbours. Areas with no shading have no or
insufficient data
Bangladesh, and a summary of arsenic concentrations is
given in Table 4. The survey of Nawabganj (DPHE 1999)
incorporated 58 groundwater samples over an area of
400 km2, approximately one sample per 7 km2. Nawabganj lies beyond the worst affected areas of south and
southeast Bangladesh (Fig. 4), but is an example of a
spatially restricted instance of excessive arsenic concentration, a “hot spot”, which was identified following investigations of high rates of arsenicosis in 1993. Extreme
arsenic concentrations, >250 g/l and even exceeding
1,000 g/l, are restricted to an area close to the urban
centre. A strong geological control is evident: the Dupi
Tila sands that underlie the Barind Clay on the Barind
Tract and beneath the River Mahananda floodplain are
unaffected by arsenic. Arsenic concentrations in the Holocene strata are spatially variable.
Although Table 4 lists a similar number of contaminated wells on the Alluvial Sand and the Alluvial Silt, the
map shows that most of the contaminated wells on the
Alluvial Sand are located near where it pinches out. These
Hydrogeology Journal (2005) 13:727–751
wells are probably screened in the underlying Alluvial Silt
unit (DPHE 1999). The Alluvial Sand corresponds to the
active floodplain of the River Ganges, while the older and
thinly bedded Alluvial Silt unit is believed to have been
deposited in the early Holocene when an estuary extended
deep along the Ganges valley (DPHE 1999). The contrast
between the Holocene units demonstrates how both
depositional environment and geological age are important factors in controlling arsenic mobilisation.
Arsenic concentrations have been mapped at a yet more
detailed scale at Faridpur by Safiullah (1998), Jessore by
AAN (1999), and Meherpur (Burren 1998), Chaumohani
part of the Noakhali urban area (Mather 1999), Magura
(Cobbing 2000) and Manikganj (McCarthy 2001), under
the London-Dhaka Arsenic in Groundwater Programme.
Arsenic concentration in groundwater from HTWs less
than 60 m deep at Meherpur, on the Ganges floodplain in
western Bangladesh, is shown in Fig. 6. The Meherpur
Survey, incorporating five samples per km2 over 15 km2,
illustrates arsenic variability at a scale nearly two orders of
magnitude more detailed than the survey at Nawabganj.
Arsenic concentration in groundwater around Meherpur
ranges from less than 1 g/l to nearly 900 g/l. Of the
boreholes sampled, 55% have arsenic concentrations
greater than 50 g/l and only 18% have arsenic concentrations of less than 10 g/l. The spatial distribution
indicates a belt of low arsenic concentration (<50 g/l),
500 m wide, with a north-south trend between Ujjalpur and
Meherpur. Within this belt the average arsenic concentration is 15 g/l; beyond it the average is 200 g/l (maximum
890 g/l). The pattern of this low-arsenic belt is suggestive
of a palaeochannel cutting across the present-day floodplain of the Bhairab River. The apparent “channel” is
defined by the extreme spatial variability along its margins.
For example to the northeast of Ujjalpur one borehole
beyond the “channel” pumps water with 250 g/l arsenic,
and lies just 105 m from a borehole within the “channel”
with arsenic at less than 1 g/l. Within the “high-arsenic”
regions on either side of the “channel”, occasional individual boreholes or small groups of boreholes have arsenic
concentrations of less than 50 g/l.
Safiullah (1998), AAN (1999), Mather (1999) and
McCarthy (2001) have also mapped lenticular bodies of
high and low arsenic concentration with widths measured
in hundreds of metres, that may correspond to palaeochannel and oxbow-lake deposits. In these cases, a
DOI 10.1007/s10040-003-0314-0
737
Fig. 5 Distribution of arsenic
concentrations in wells at
Nawabganj Upazila in relation
to surficial geology. Survey
data, all analysed at the BGS
laboratories, are taken from
DPHE (1999). The main channel of the River Ganges and the
Barind Tract (underlain by
Barind Clay over Dupi Tila
sands) are indicated by shading.
The Holocene floodplain units
are labelled as follows: asd,
Alluvial Sand, asl, Alluvial Silt,
asc, Alluvial Silt and Clay, ppc,
Marsh Clay and Peat. The geological boundaries were digitised from Alam et al. (1990). In
this area, the Alluvial Sand is
equivalent to the active Ganges
floodplain, and the Alluvial Silt
and Alluvial Silt and Clay are
equivalent to the Mahananda
floodplain. The map grid is the
Bangladesh Transverse Mercator projection
meander-belt sedimentary model could explain the rapid
lateral and vertical variations of arsenic occurrence in
groundwater, reflecting the contrast between relatively
oxic channel sands compared to more reducing overbank
muds. In general, these observations demonstrate the
possibility of linking spatial patterns of arsenic concentration in groundwater, and their variability, to sedimentological features that may have geomorphological manifestation, and hence the importance of such detailed
spatial surveys.
Table 5 Arsenic distributions in groundwater by well depth
Well depth
<10 m
10–30 m
30–100 m
100–200 m
>200 m
All
No. of
wells
Arsenic concentration (g/l)
Mean1
Max.
>50
>10
36
576
1,033
92
283
2,023
20
34
4.9
5.6
0.7
6.7
260
1,090
1,670
250
110
1,670
33%
52%
32%
20%
0.7%
35%
69%
78%
49%
54%
3%
51%
Source: GSACB Regional Survey, Phase I (DPHE 1999). 1Calculated from the mean of the logarithms of arsenic concentration
Depth Distribution
The majority of wells sampled in the Regional Survey and
the Meherpur Survey were hand-pumped wells with a
typical screen length of 3 m, and low discharge. In
aquifers hundreds of metres thick these wells approximate
point data, and hence it is reasonable to interpret the
concentration data in terms of the depth distribution of
arsenic in groundwater. The occurrence of arsenic concentrations exceeding 50 g/l is shown in relation to well
Hydrogeology Journal (2005) 13:727–751
depth in Fig. 7 and is summarised in Table 5. There is a
strong correlation between the occurrence of arsenic in
groundwater and the depth of wells, although the precise
pattern varies among regions. In general, the highest
concentrations, and also the greatest spatial variability,
occur a few tens of metres below the ground surface, and
decrease rapidly below about 100 m. In wells deeper than
200 m, arsenic concentration is generally negligible.
DOI 10.1007/s10040-003-0314-0
738
Fig. 6 Small-scale variation of
arsenic concentration near Meherpur Town (after Burren
1998). The location of sampled
wells less than 60 m deep are
shown with their arsenic concentrations divided into four
classes. Isopleths of arsenic
concentration show the alignment of a low-arsenic (<50 g/l)
band roughly parallel to the
present river channel. The map
grid is the Bangladesh Transverse Mercator projection
Wells shallower than 5 m, and especially dug wells, are
commonly uncontaminated. The absolute range of concentrations in most depth intervals spans several orders of
magnitude. Thus in Fig. 7 the data have been represented
as the proportion of wells in each 10-m interval (with
more than ten data points) that exceed 50 g/l. Figure 7
also shows profiles within three geomorphic sub-units
sampled in the Regional Survey. The profile from the Old
Meghna Estuarine Floodplain shows the simplest pattern
and the sharpest reduction with depth. Samples from the
Ganges floodplains suggest a second peak below 50 m
depth. The Sylhet Basin, where there is no overall
decrease of arsenic concentration to a depth of 150 m,
shows the greatest variation to this trend. It has been
proposed (Ravenscroft et al. 2001) that the depth peaks of
Hydrogeology Journal (2005) 13:727–751
arsenic concentration can be related to the diachronous
formation of paludal basins during the Holocene transgression, where degradation of peat provides the strong
redox driver required to account for the extreme arsenic
concentrations observed (McArthur et al. 2001).
The local arsenic depth profile at Meherpur is consistent with the regional picture (Burgess et al. 2002).
Groundwater with the highest arsenic concentrations (200
to 890 g/l) is pumped from depths shallower than 45 m.
Variability is highest in the shallow groundwater, where
the maximum range of arsenic concentrations, from
below the analytical method detection limit to 890 g/l,
is observed. Arsenic appears to occur at a lower and less
variable concentration, commonly around 100 g/l, in
groundwater pumped from depths greater than 45 m.
DOI 10.1007/s10040-003-0314-0
739
Fig. 7 Depth distribution of arsenic in groundwater. The traces
represent the average percentage of wells with total depths falling
in 10 m intervals and the arsenic concentrations exceeding 50 g/l.
The solid trace represents 1,786 samples from the Regional Survey
of DPHE (1999), while the ornamented traces are geologically
classified subsets from the survey, where triangles indicate wells on
the Ganges River Floodplain (n=772), squares the Old Meghna
Estuarine Floodplain (n=266), and circles the Sylhet Basin (n=97)
However, the data at these greater depths are too sparse to
draw firm conclusions on a local level.
At Meherpur, a more precise, site-specific view of the
depth profile of arsenic in the aquifer is illustrated by the
arsenic concentration of porewater eluates from a cored
borehole at Ujjalpur Village located away from HTW
pumping influences (Figs. 6 and 8). There is a single,
distinct peak concentration of arsenic in porewater between 18 and 21 m depth, where arsenic concentration
exceeds 300 g/l (range 50 to 500 g/l). In contrast, at
depths of less than 10 m there are elevated chloride
concentrations of between 80 and 90 mg/l. The chloride
profile probably reflects the limiting depth of active
groundwater circulation with anthropogenic influence on
chloride concentration, controlled partly by the subdued
topography and partly by the occurrence of a silty clay
layer just below 10 m depth.
Characteristic depth profiles of arsenic in groundwater
have been described at Lakshmipur and Chaumohani (part
of the Noakhali urban area), in the coastal region of
southeast Bangladesh (DPHE 1999; Mather 1999). Here
the shallow Holocene aquifer, <30 m thick, contains
groundwater with a high arsenic concentration, and a
deeper Pleistocene aquifer, >150 m deep, is almost free of
arsenic. In the Holocene aquifer at Lakshmipur, the
arsenic concentration in 87% of wells exceeds 10 g/l and
in 73% it exceeds 50 g/l. In 84% of wells in the
Holocene aquifer at Chaumohani it exceeds 50 g/l. In the
Pleistocene aquifer, the arsenic concentration exceeds
50 g/l in only one out of 17 samples at Lakshmipur, and
none of 10 exceed it at Chaumohani. In all other samples
from the Pleistocene aquifer in these areas the arsenic
concentration is below 10 g/l. Between 30 and 150 m
depth, there are few wells because the groundwater is
generally brackish.
Hydrogeology Journal (2005) 13:727–751
Temporal Trends
The Eighteen District Towns project (R. Dierx, pers.
comm. 1999), which covers towns in most of Bangladesh,
has monitored public water supply production wells for
arsenic since 1996. While some wells show no clear trend,
some wells do show an increase of arsenic over this short
period (DPHE 1999; Burgess et al. 2002). However, there
are no monitoring data of longer-duration on arsenic in
groundwater in Bangladesh. It is appropriate therefore to
use tubewell age as a surrogate time parameter. The
Regional Survey data are suitable for analysis because the
age of the sampled wells is reliably known and had not
been a factor in their selection. In addition, the wells were
sampled and analysed in a consistent fashion. The data
were transformed into the proportion of wells exceeding
50 g/l in each age group with 10 or more data points.
The results of this analysis (Fig. 9) suggest that in many
wells arsenic concentration increases to >50 g/l during
the first 5–10 years after installation, after which conDOI 10.1007/s10040-003-0314-0
740
Fig. 8 Arsenic concentration profiles in sediment and water at
Ujjalpur village in Meherpur (after Perrin 1998) are compared with
the lithological section at the site. Squares are porewater arsenic
concentrations (as ppb, equivalent to g/l); open diamonds represent the arsenic content of HNO3 extracts of the sediment; and
triangles represent the total arsenic content of the sediments (by
fusion, as ppb). The discrete bars represent the arsenic concentration in the vicinity of the cored borehole, and screened intervals of
hand tubewells
centrations level off. An independent analysis of the same
data using a semi-variogram approach (Richard Howarth,
Visiting Professor of Mathematical Geology, University
College London, pers. comm. 1998) reached the same
conclusion and identified a sill after about eight years and
significant increases (at the 99% level) in the scale of
exceedance of the 50, 100, 150 and 200 g/l thresholds.
At a regional scale the data are vulnerable to bias if
younger tubewells have been installed in areas of lower
arsenic concentration. The Meherpur Survey data, from
the same aquifer as investigated under the Regional
Survey, are illustrated in Fig. 8. Although the inter-annual
variations are unsurprisingly greater with a smaller data
set such as obtained in the Meherpur Survey, the same
overall trend of arsenic concentration increasing with
tubewell age is apparent at this local scale, which is
unaffected by spatial bias of the timing of tubewell
installation.
Increasing arsenic concentrations with time could be
attributed to lateral migration of arsenic in the aquifer,
leakage from adjacent or overlying aquitards, or a change
in redox conditions. There are no data to suggest redox
changes during pumping, though they may occur. Modelling of generalised groundwater flow scenarios by DPHE
(1999) suggests that arsenic is unlikely to move laterally
by more than a few metres to a few tens of metres a year
under the very low prevailing horizontal hydraulic gradients. Modelling of the specific conditions at Meherpur
(Cuthbert 1999; Cuthbert et al. 2002) suggests that for
tubewells screened below an arsenic-rich aquitard, arsenic
breakthrough would occur within 2–20 years of the onset
of pumping, depending on the sorption parameters specified. This timing of arsenic breakthrough is consistent
with the field data that indicate arsenic concentrations
increase with tubewell age, and supports the hypothesis
that vertical leakage is the principal cause of changing
arsenic concentration in tubewell discharge with time.
Hydrogeology Journal (2005) 13:727–751
DOI 10.1007/s10040-003-0314-0
741
Fig. 9 Temporal trends of arsenic in groundwater. The percentage of wells with arsenic
concentrations exceeding 50 g/
l in each year of construction is
shown by squares for wells in
the Regional Survey (after
DPHE 1999) and by crosses for
wells in the Meherpur survey
(Burren 1998). The figure also
shows the results of polynomial
regressions fitted to each data
set
Table 6 Occurrence of arsenic in groundwater related to surficial geology
Geological unit(s)1
Age
Geomorphic equivalent or
location
Arsenic concentration (g/l)
144
668
Alluvial Sand
Alluvial Silt and Clay
U. Holocene
Holocene
Deltaic Silt
Alluvial Silt
Holocene
Holocene
Chandina Formation
Old and Young Gravelly
sand
Dupi Tila and Dihing
Formations
Surma Series
L.-M. Holocene
U. Pleistocene–Holocene
Active floodplains
Brahmaputra and Meghna River
Floodplains and Sylhet Basin
Ganges Floodplain
Ganges and Brahmaputra River
Floodplains
Old Meghna Estuarine Floodplain
Tista Fan and floodplain
L. Pleistocene–Pliocene
Madhupur and Barind Tracts
Tertiary
Sylhet and Chittagong Hill Tracts
No. of
wells2
Mean3
Max.
>50
7
8
890
700
27%
32%
544
428
15
5
1,670
1,450
45%
26%
152
45
88
19
1,090
70
77%
16%
151
1
140
13%
26
2
130
4%
1
Data source: DPHE (1999). Geological units as per the map of Alam et al. (1990). The Dhamrai Fm underlies parts of the ‘Alluvial Silt’
and ‘Alluvial Silt and Clay’ units. Other Holocene units not referred to in Table 1 are stratigraphically ‘unclassified’. 2All wells less than
100 m deep. 3Calculated from the mean of the logarithms of arsenic concentration
Relation of Arsenic in Groundwater to the Geology
Stratigraphic Occurrence
To identify general relationships between surficial geology and arsenic in groundwater, all georeferenced well
locations were superimposed on a digitised version of the
Geological Map of Bangladesh (Alam et al. 1990) using
the ArcView GIS software. Detailed analyses are given in
DPHE (1999) and Ravenscroft (2001), and a summary is
given in Table 6. In Bangladesh, the surface geological
unit generally has a depositional association with the
aquifer that underlies it, to depths of about 100 m. The
Madhupur and Barind clays and Dupi Tila units have been
combined because they refer to underlying aquifers of a
similar age, depositional environment and diagenetic
history. Some errors may result from the coarse resolution
of the geological map, and also from situations where
wells are sunk in valleys filled by younger sediment
within the Tertiary hills and Pleistocene terraces, or
Hydrogeology Journal (2005) 13:727–751
penetrate oxidised aquifers beneath Holocene sediments
on the GBM floodplain.
Groundwater associated with the Holocene deposits is
most affected by arsenic. However, it is clear that
provenance and depositional environment are additional
controls on arsenic distribution. The map of Alam et al.
(1990) is based on lithology, and its units are not unique
to individual river systems. The same GIS overlay
technique was applied to a map of physiographic units
(FAO/UNDP 1988) and showed significant differences
between the various floodplains. Beneath the Ganges
floodplains, water in 35% of 1,747 wells had arsenic
concentrations exceeding 50 g/l, in contrast to 25% of
524 wells beneath the Brahmaputra (and Tista) floodplains, and 53% of 810 wells on the Meghna floodplains
(including the Sylhet Basin). It should, however, be noted
that provenance and grain size are related. In particular,
large parts of the Brahmaputra and Tista floodplains are
DOI 10.1007/s10040-003-0314-0
742
underlain by medium and coarse sand, while the sediments beneath the Meghna floodplains are finer grained.
Groundwaters in the Pleistocene and older aquifers are
largely free of arsenic. No evidence has been found of any
extensive or severe contamination in aquifers pre-dating
the LGM. It is yet to be established whether arsenic that
may originally have been present in groundwater has been
either immobilised in the solid phase or removed by
flushing.
Arsenic in Sediments
The occurrence of arsenic in alluvial sediments is not
unusual (Welch et al. 1988), and the arsenic content of the
GBM alluvial sediments is not particularly high, but it is
unusual for arsenic to be mobilised into groundwater so
extensively and at such high concentrations. The average
arsenic content of the Earth’s crust is 1.8 ppm and arsenic
is most abundant in shales (Mason 1966). Based on a data
compilation from West Bengal and Bangladesh, DPHE
(1999) report average total arsenic contents of fluviodeltaic sediments of 15.9 ppm for 134 onshore samples
and 10.3 ppm for 96 offshore samples. Although most of
the onshore samples were collected in areas of high
arsenic concentration in groundwater, this alone does not
account for the extensive and extreme contamination
encountered in the Bengal Basin. Based on a limited data
set, Datta and Subramanian (1994) report average arsenic
contents of riverbed samples to be 2.03 ppm in the
Ganges, 2.79 ppm in the Brahmaputra and 3.49 ppm in
the Meghna.
PHED (1991), Nickson et al. (1998) and Perrin (1998)
all note discrepancies between the arsenic content measured by selective extraction and by microprobe analysis
of individual mineral grains. A sedimentological study by
Imam et al. (1997) noted the ubiquitous presence of ironrich coatings on the sands, while analysis of grain coatings
from West Bengal found more than 2,000 ppm of arsenic
(PHED 1991). Nickson (1997), Perrin (1998) and AAN
(1999) all identified pyrite in the framboidal crystal
form indicating that it is of diagenetic origin and hence
primarily a sink rather than a source for arsenic under
present conditions. Furthermore, total arsenic in sediment
correlates strongly with iron but not with sulphur, sup-
porting the view that arsenic is primarily associated with
oxyhydroxides and not sulphides (Nickson et al. 1998 and
2000).
Nickson (1997), Perrin (1998), AAN (1999) and DPHE
(1999) all analysed sediments from cored boreholes on the
Gangetic Plains of Bangladesh, demonstrating the lithological and stratigraphical controls over the depth profiles
of arsenic in groundwater. The core analyses show that
arsenic content in sediments is greatest in fine-grained
strata, and usually also within the first few tens of metres
depth. At Meherpur, Perrin (1998) measured arsenic
content of the bulk sediment and of selective extractions,
in addition to porewater. Total arsenic content of sediments at Meherpur ranges from 1.4 to 35 ppm, spanning
the range of sediment analyses from Faridpur, 11 to
28 ppm (Nickson 1997), and Nawabganj, 2 to 11 ppm
(DPHE 1999). The depth profiles of arsenic in the aquifer
at Meherpur (Fig. 8) demonstrate a close agreement
between arsenic concentrations in porewater, the arsenic
content of selective extractions, and the arsenic concentration measured in nearby wells. The bulk sediment
arsenic content demonstrates the same trend but is much
higher in all cases.
Arsenic concentrations in the selective extractions
were positively correlated with iron, manganese and to a
lesser extent aluminium, and the bulk sediment analyses
indicated strong correlations of arsenic with both iron and
aluminium. This is possibly due to an association of both
iron oxyhydroxides and clay minerals within the finer
sediment fraction, but raises the possibility that the Fe-As
association could be due to dissolution of an aluminosilicate phase, as observed by Breit (2001).
Relation to Groundwater Chemistry
Groundwater in the Holocene aquifers beneath the GBM
floodplains characteristically contains negligible dissolved oxygen and registers low redox potentials on a
Pt-electrode under field conditions (NRECA 1997; also
see Table 7), with iron being extensively mobilised into
solution. High arsenic concentrations are restricted to
these strongly reducing conditions, though not all reducing waters contain arsenic (DPHE 1999). Figure 10 shows
Table 7 Groundwater chemistry in an arsenic-affected area: Meherpur, Western Bangladesh
Site
Depth
(m)
T
(
C)
O2
(%)
EC
(S/cm)
pH
As
(g/l)
Ca
Na
K
Mg
Fe
Mn
Cl
NO3
SO4
HCO3
S4
S3
S104
S107
S6
S1
S101
S205
S210
15
29
33
17
24
30
18
45
23
27.2
27.1
27.2
26.8
27.1
26.8
26.8
26.9
26.6
0
0
18
0
0
3
12
0
0
1,055
750
1,000
710
720
660
630
550
730
6.86
6.82
7.02
6.99
6.99
6.97
7.12
7.04
6.90
3
11
14
47
76
110
135
243
775
170
141
161
131
128
110
99
91
122
31.7
14.2
44.5
14.3
12.3
13.7
24.6
11.9
24.3
6.9
4.0
13.0
4.4
5.2
4.1
3.8
4.6
5.3
56.7
35.0
52.5
31.2
39.5
26.5
25.4
24.6
30.3
0.0
6.2
0.0
0.8
1.3
8.5
1.7
5.8
10.0
0.85
0.63
1.04
0.45
0.71
0.35
0.34
0.48
0.48
94.6
1.4
67.3
8.0
27.2
1.0
3.0
2.0
14.1
48.2
0.0
7.1
0.0
0.0
0.0
0.0
0.0
0.0
48.2
1.5
55.2
3.1
2.7
2.1
0.9
1.0
0.1
603
693
788
639
581
507
503
481
578
All units are mg/l except where stated otherwise. Source: Burren (1998)
Hydrogeology Journal (2005) 13:727–751
DOI 10.1007/s10040-003-0314-0
743
Fig. 10 Redox conditions and
arsenic concentration showing
survey data from NRECA
(1997) compared to stability
lines from Welch et al. (1988)
where the shadowed boxes indicate the thermodynamically
favoured form of arsenic in
water
the Pt electrode measurements, pH and arsenic in groundwater across Bangladesh.
The hydrochemical associations of arsenic are illustrated by reference to the Meherpur study of Burren (1998)
in Table 7 and Fig. 11. The tabulated data relate to wells
within an area of about 5 km2, centred on the location of
the cored borehole at Ujjalpur. Groundwater at Meherpur
is predominantly anoxic, and of Ca-(Mg)-HCO3 type. The
high background values of total dissolved solids (140–
1290 mg/l) are typical of groundwater in young, reactive,
alluvial sedimentary sequences (Hem 1985). Dissolved
oxygen is generally less than 6% saturation (Burren 1998),
suggesting that anoxic conditions are common, if not
pervasive, in the aquifer. Dissolved iron ranges up to
10 mg/l, reflecting the reducing conditions and the availability of iron in the sediments. Iron concentrations show
an approximately inverse relationship with nitrate and
chloride (Fig. 11). Nitrate is generally absent, except at
Meherpur town (NO3- 20 to 88 mg/l) and Ujjalpur village
(NO3- 20 mg/l), where it may result from on-site sanitation
in areas of dense human settlement (Burgess et al. 2002).
Chloride has a distribution similar to nitrate in the shallow
aquifer. Chloride reaches 150 mg/l at Meherpur and
55 mg/l at Ujjalpur, but is less than 50 mg/l outside the
main areas of settlement. Median bicarbonate values are
around 500 mg/l, with values greater than 700 mg/l
recorded beneath Meherpur and Ujjalpur.
At Meherpur, high arsenic concentrations in groundwater are associated with reducing conditions under
which oxygen is limited, nitrate is absent, and iron and
bicarbonate are at high concentrations (Fig. 11). However, in many cases groundwater with high iron content
contains negligible arsenic. Nitrate ranges from zero to
88 mg/l and has a strongly inverse relationship with
arsenic. The positive correlation between iron and arsenic
and the pervasively elevated bicarbonate concentrations
are similar to those recorded over broader geographical
areas (DPHE 1999; McArthur et al. 2001). The results
from Meherpur support the hypothesis (Nickson et al.
Hydrogeology Journal (2005) 13:727–751
2000) that desorption of arsenic has accompanied reductive dissolution of iron oxyhydroxides in the aquifer
sediments, and suggest this is the principal mechanism by
which arsenic is released to groundwater. Instances where
arsenic is present in groundwater despite the iron concentration being low (Nickson et al. 2000) may be related
to the precipitation of Fe carbonates (Mather 1999;
Nickson et al. 2000).
Relation to Irrigation Pumping
Das et al. (1994) and Mallick and Rajagopal (1996)
suggested that elevated arsenic concentrations in West
Bengal and Bangladesh are caused by extensive pumping
of groundwater for irrigation. In this scenario, pumping
lowers the water table, and arsenic-rich pyrite in shallow
sediments is oxidised, releasing iron, arsenic and sulphate
into solution. Certainly there is a temporal association
between the reports of arsenicosis and increases in
groundwater pumping for irrigation. However, there are
no arsenic analyses dating from before about 1983 in
India, or 1990 in Bangladesh, so such a hypothesis cannot
be directly tested. Statistical tests have been carried out
on an upazila-based compilation of data to identify any
spatial association between elevated arsenic concentration
in groundwater and the extent of groundwater pumping
for irrigation. The results are summarised in Fig. 12. The
percentage of arsenic concentrations exceeding 50 g/l in
each upazila was used as the measure of elevated arsenic
concentrations. The intensity, or impact, of groundwater
pumping was represented by two measures. First, since
irrigation accounts for more than 90% of groundwater
pumping in Bangladesh (UNICEF 1994), the spatial
impact of groundwater pumping was first described by
reference to the maximum recorded depth to the water
table over the period 1961–93. The second measure of
intensity was the percentage of the area of each upazila
irrigated by groundwater in 1996. The latter measure
DOI 10.1007/s10040-003-0314-0
744
Fig. 11 Hydrochemical associations of arsenic in groundwater
at Meherpur (after Burren 1998)
reflects the gross abstraction of groundwater per unit area.
Both measures are negatively correlated with arsenic
contamination. Although the proportion of variation explained by the regression equations is small, both relationships are significant at the 99% level and argue
strongly against irrigation pumping being a primary cause
of the elevated arsenic concentrations in groundwater.
Mobilisation of Arsenic into Groundwater
Mechanisms of Release of Arsenic to Groundwater
Both anthropogenic and geological sources have been
proposed to explain the elevated arsenic concentrations in
groundwater in the Bengal Basin. Suggestions for anthropogenic sources of arsenic have included mining wastes,
industrial pollution, burning of fossil fuels, agrochemicals,
Hydrogeology Journal (2005) 13:727–751
and wood preservatives in electric transmission pylons.
However, while some of the hypotheses may account for
isolated cases of pollution (e.g. Mazumder et al. 1992) none
can provide a general explanation (DPHE 1999). Only a
geological source can explain the extent and magnitude of
the observed arsenic occurrence, and the lithological and
sedimentological associations described above.
Two main explanations for the mobilisation of geological arsenic have been proposed. The first, ‘pyrite
oxidation – overabstraction’, considers that arsenic-rich
pyrite and arsenopyrite in the floodplain sediments are
oxidised due to water-table lowering caused by intensive
groundwater pumping (Das et al. 1996; Mallick and
Rajagopal 1996). The alternative ‘oxyhydroxide reduction’ hypothesis put forward by Bhattacharya et al. (1997,
2001) in India, and Nickson (1997) and Nickson et
al. (1998, 2000) in Bangladesh, proposes that adsorbed
DOI 10.1007/s10040-003-0314-0
745
– The spatial distribution of arsenic does not correlate
with either water-table depth or the intensity of
groundwater irrigation, but is associated with Holocene floodplains, and particularly with finer-grained
sediments;
– Maximum arsenic concentrations in groundwater are
found tens of metres below the depth of the deepest
water-table fluctuation, even in areas of little pumping;
– Pyrite is rather rare and where present occurs as an
authigenic rather than detrital mineral, more likely
acting as a sink for, rather than a source of, arsenic;
– There is a strong correlation between the iron and
arsenic content of the Holocene sediments, but no
correlation between iron and sulphur; and
– Sand grains in the Holocene sediments have pervasive
ferruginous coatings with appreciable arsenic content.
A third possible mineralogical source for arsenic,
which is not exclusive of an iron oxyhydroxide source and
may be consistent with the observed geochemical associations, is detrital biotite. Biotite is a common constituent of the Holocene sediments of the GBM floodplains, and is known to contain arsenic at sites in eastern
Bangladesh (Breit 2000). While evidence in support of
the ‘oxyhydroxide reduction’ hypothesis is strong, there
may also be a contribution from the weathering of biotite,
and the relative significance of the two processes may
vary with depth. Much remains to be done to identify the
mineralogical mechanisms of arsenic release in detail.
More detailed discussions of the alternative mobilisation
hypotheses and redox drivers are given by McArthur et al.
(2001) and Nordstrom (2000).
Natural Processes Controlling Arsenic Occurrence
Fig. 12a,b Relationship between elevated arsenic concentrations
and groundwater abstraction explored using two surrogate parameters to represent the intensity of abstraction. The first surrogate (a)
is the maximum depth to the water table, which under the extremely
flat conditions of Bangladesh is largely determined by irrigation
pumping. The second parameter (b) is the proportion of the total
area of each upazila that was irrigated by groundwater during 1996.
Graph (a) is based on 27,797 analyses in 309 upazilas with a
minimum of 25 tests per upazila. Graph (b) is based on 31,376
analyses in 340 upazilas with a minimum of 25 analyses in each
upazila. The data are from DPHE (1999), NMIDP (1997) and
UNICEF (1994)
arsenic is released by reductive dissolution of iron
oxyhydroxides as the floodplain sediments become buried
and reducing conditions develop. This latter explanation
emphasises the role of organic matter in generating
strongly reducing porewaters. The ‘oxyhydroxide reduction’ hypothesis is supported by the field evidence that:
– Arsenic-rich groundwaters are all strongly reducing;
– Arsenic-rich groundwaters generally have high iron
and bicarbonate concentrations but very little sulphate
or nitrate;
Hydrogeology Journal (2005) 13:727–751
The distribution of groundwater arsenic in Bangladesh
may be explained by a two-stage model that superimposes
the effects of Quaternary sea-level fluctuations upon a
continuum of fluvial-sedimentary processes, as summarised in Figs. 13 and 14. Arsenic enters the fluvial systems
in upland areas of India and Nepal by weathering of
sulphide and/or oxide bearing rocks. Arsenic released
during weathering is adsorbed by the iron, and possibly
also manganese and aluminium, oxyhydroxides. Breakdown of sulphides releases sulphate into solution. The
sediment load of the GBM system may be deposited and
resuspended many times before reaching the current
site of deposition. The upper reaches of the rivers
are characteristically braided; coarse-grained abandoned
channels may be preserved but little fine sediment
accumulates. In the lower reaches, the mud and organic
matter content of sediments increases, especially in
overbank deposits, allowing accumulation of colloidal
oxyhydroxides with their load of adsorbed arsenic.
Locally, and sometimes extensively, these are interbedded
with peat horizons (Brammer 1996). Each sedimentation
event provides an opportunity for fractionation of sulphate from iron and arsenic (DPHE 1999).
DOI 10.1007/s10040-003-0314-0
746
Fig. 13 Present day processes affecting the mobilisation, fate and transport of arsenic in the Bengal Basin. The figure shows an idealised
sequence of events that may occur between the upper catchment and the Bay of Bengal
Channel incision during the sea-level low-stands of the
late Quaternary divided the Bengal Basin into elongate
hills parallel to the main rivers, accounting for the sharp
subsurface discontinuities in age (though not necessarily
facies), aquifer hydraulic properties, and groundwater
quality in the transverse hydrogeological section portrayed in Fig. 2. During sea-level low-stands, transverse
groundwater flow was more important than at present,
driven by the large lateral hydraulic gradients. Suppressed
monsoonal circulation reduced rainfall and the regional
water table would have stood many tens of metres below
the surface of the Madhupur and Barind tracts. Recharge
would have percolated rapidly, promoting oxidative
weathering and flushing, leading to the removal of
organic matter, the development of circum-neutral, oxic
Hydrogeology Journal (2005) 13:727–751
conditions, and the recrystallization of amorphous oxyhydroxides as hematite or goethite. The combined effect
was to immobilise any arsenic that had not previously
been flushed from the aquifer system.
The Dupi Tila sands have experienced such conditions
over hundreds of thousands of years, allowing the almost
complete removal or immobilisation of arsenic. Buried,
Late Quaternary terraces in the Jamuna Valley and the
southeast coastal plain (with ages of >25 to 50 Ka BP)
experienced a briefer period of oxidative weathering
during the 18 Ka BP low-stand. It is anticipated that this
would have removed or immobilised much arsenic, by
adsorption onto residual Fe-oxide, but it is not certain that
these deposits are completely free of arsenic. These
sediments may account for the lower, but still significant,
DOI 10.1007/s10040-003-0314-0
747
Fig. 14 Palaeohydrological
processes controlling the accumulation, removal and immobilisation of arsenic in the
Bengal Basin. The ages (BP)
are indicative of the events that
may have occurred during and
after the last Glacial Maximum,
but in generalised form are
likely to have occurred many
times during the Quaternary
arsenic concentrations in groundwater at depths of between 50 and 120 m beneath the present-day floodplains
(Fig. 7).
Sea level rose rapidly from 18 Ka to 7 Ka BP, but a
major change in sedimentation occurred when sea level
intercepted the shallow coastal platform at about 11 Ka
BP (Goodbred and Kuehl 2000). The combination of a
broad, shallow shelf with higher rainfall, greater river
discharge and higher temperature provided ideal conditions for the formation of mangrove swamps and freshwater peat basins. Such waterlogged conditions provided
little possibility for the flushing of sediments by meteoric
waters. Fine sands deposited at the delta front and lower
fluvial regime are interbedded with organic-rich mud and
peat, the former providing the source of arsenic and the
latter driving the mechanism to generate strongly reducing groundwater conditions.
After deposition of the Holocene sediments, a sequence of chemical processes commences that may lead
to the mobilisation of arsenic in groundwater. Decomposition of organic matter progresses with the microbial
consumption of dissolved oxygen, followed by the reHydrogeology Journal (2005) 13:727–751
duction of any nitrate present, and eventually by the
reductive dissolution of solid phase ferric oxyhydroxides,
releasing adsorbed arsenic into solution in groundwater. If
reduction proceeds further and if sufficient sulphate is
available, iron and arsenic may ultimately be sequestered
in diagenetic pyrite, but this does not appear to have
occurred extensively. The key factors accounting for
widespread mobilisation of arsenic into groundwater in
the Bengal Basin appear to be (1) the efficiency of
separating sulphur (as sulphate) in river water from
arsenic and iron in sediment that eventually forms the
GBM floodplain deposits; (2) the abundance of organic
matter; and (3) the restricted groundwater flow due to the
low hydraulic gradients that have prevailed since deposition. Alternative geochemical pathways, due to variations in sediment composition, can lead to methanogenesis (Ahmed et al. 1998) or siderite formation, the latter
accounting for iron-deficient arsenic–rich groundwater
under conditions of siderite saturation (Mather 1999).
It is probable that the events described during, and
subsequent to, the terminal Pleistocene transgression
constitute a cyclical process that occurred many times
DOI 10.1007/s10040-003-0314-0
748
during the Quaternary, and would have affected many
large alluvial basins throughout the tropical world.
Recently, arsenic occurrence in groundwater at a similar
scale to that observed in the Bengal Basin, has been
described in the Red River Basin of Vietnam (Berg et al.
2001).
Human Influences on Arsenic Mobility
Human activities may modify the natural distribution of
arsenic in groundwater at a local scale, but none appear to
be regionally significant. Human waste (as sewage) might
be a source of nitrate and sulphate in groundwater beneath
areas of dense human settlement (Burren 1998); nitrate
may then oxidise ferrous iron to ferric oxyhydroxides
(Burren 1998) and partially remove arsenic from solution
by adsorption. Nitrate fertilisers may contribute to the
effect. Phosphatic fertilisers, on the other hand, may
compete with arsenate for adsorption sites and displace
arsenic into solution (Acharrya et al. 1999). In north
central and southeast Bangladesh, Davies and Exley
(1992) showed that phosphate in groundwater beneath the
Jamuna floodplain has concentrations in the range of 3–
8 mg/l. At a national scale, the spatial distribution of
phosphate is similar to that of arsenic (Ravenscroft et al.
2001), but NRECA (1997) and AAN (1999) show poor
(well by well) correlations between arsenic and phosphate. While high phosphate and high arsenic are both
restricted to the younger aquifers, it appears that phosphate and arsenic have a common origin rather than
phosphate playing a role in mobilising arsenic.
Pumping for water supply and irrigation has increased
aeration of the upper aquifer, possibly leading to the
precipitation of iron oxyhydroxides and immobilisation of
arsenic by re-sorption in the very shallow zone of waterlevel fluctuation. Arsenic in groundwater pumped for
irrigation is oxidised in the water distribution channels
and precipitated along with ferric iron in the fields
(BADC 1992). Where rice is irrigated from arsenicbearing aquifers, the transfer of arsenic to the soil zone
could be of the order of 1 Kg/ha/yr (assuming a gross
irrigation requirement of 1000 mm/yr for rice and an
input concentration of 100 g/l). The extent to which
arsenic added to the soil in this way might be leached to
groundwater or transferred to the atmosphere by
biomethylation is presently unknown.
Models of arsenic transport in groundwater flowing to
hand-pumped tubewells from an arsenic source zone at
20 m depth (Cuthbert et al. 2002) have demonstrated that
vertical leakage will tend to increase the arsenic concentration in the tubewell discharge with time, where the
tubewell screen is below the arsenic source zone. Where
the tubewell is located within the wider catchment area of
a deeper, more productive, water supply or irrigation
tubewell, arsenic will appear at the HTW more quickly,
and seasonal discharge from the irrigation tubewell could
lead to seasonal variations in the arsenic content of the
shallower HTW. These are, however, only secondary
influences on the occurrence of arsenic at HTWs.
Hydrogeology Journal (2005) 13:727–751
Mitigation and Resource Management Issues
Mitigation activities will involve extensive arsenic surveys, long-term groundwater quality monitoring, community awareness and mobilisation activities, and a range
of possible physical interventions including (1) treating
arsenic-rich groundwater at the source, (2) developing
alternative groundwater sources and (3) developing surface-water sources such as rivers, ponds or rainwater.
Existing uncontaminated shallow wells will continue to
be an important source of drinking water for many years.
However, with 25% of all wells containing >50 g/l,
critical questions relate to the sustainability of wells that
are presently arsenic-free despite being in affected areas,
especially given that arsenic concentrations appear to
increase over time (DPHE 1999; Cuthbert et al. 2002).
Detailed investigations of selected sites, and systematic
monitoring will be required to manage the risk to human
health.
Groundwater in deep aquifers, below about 200 m,
presently contains minimal arsenic, but faces a risk of
leakage from shallow aquifers in the long term. Preliminary modelling (DPHE 1999) based on a variety of
sorption scenarios suggests that cross-contamination would
take decades, probably longer. However, better definition
of vertical permeabilities and sorption characteristics under
changing redox conditions is required before predictions
can be made with confidence. Until these parameters are
better defined, the precautionary principle warrants planning on worst-case (e.g. low sorption) scenarios. The
sustainability of abstraction from deep aquifers may also be
constrained by the possibility of saline intrusion in the
coastal area and offshore islands. These aquifers are
confined downgradient by thick muddy sediments and are
not in continuity with the Bay of Bengal. Any negative
impacts are likely to take many years to develop. However,
the renewable yield of the aquifer is unknown, and a
quantitative resource assessment and monitoring network
are high priorities.
Conclusions
Groundwater in Holocene alluvial and deltaic aquifers
contains arsenic of geological origin at elevated concentrations over extensive areas of Bangladesh, threatening
the lives of more than twenty five million people. Arsenic
concentrations are highest in the upper 50 m of the
sedimentary sequence. Below 100 m, arsenic concentration reduces, and below 200 m the chances of drilling an
‘arsenic-safe’ well approach 99%. The arsenic is derived
from multiple source areas in the upper catchments of the
Ganges, Brahmaputra and Meghna Rivers and is thought
to have been transported through the river system adsorbed onto colloidal iron oxyhydroxides. Detrital arsenic-bearing phyllosilicates, such as biotite, may also
contribute to the arsenic content of the sediments and act
as an additional source to groundwater. Following deposition, degradation of organic matter has led to reductive
DOI 10.1007/s10040-003-0314-0
749
dissolution of iron oxyhydroxides, releasing adsorbed
arsenic to groundwater. There is, however, no evidence to
support a causal connection between the pumping of
groundwater for irrigation and widespread mobilisation
of arsenic in the aquifers. The sharp contrast between
arsenic-bearing groundwater in chemically reducing
Holocene aquifers and arsenic-free groundwater in the
oxidised Pleistocene aquifers is a result of the effects of
Quaternary sea-level fluctuations on the regional palaeohydrogeological evolution. Sediments that were not eroded during the last sea level low-stand were oxidised and
flushed by meteoric water, removing or immobilising
arsenic. The incision of the main river channels and
coastal plains created the space to accommodate younger,
Holocene deposits of organic-rich sediment and fine-tomedium sand that form the arsenic-affected aquifers
encountered today.
The occurrence of arsenic in groundwater in the
Bengal Basin provides a number of general lessons.
While the Bengal Basin may be exceptional, it is unlikely
to be unique. Groundwater in other open alluvial basins in
humid, and especially tropical, areas is likely to contain
arsenic at concentrations harmful to human health. A
more holistic and open-minded approach should be
adopted in the assessment of groundwater resources
where these conditions apply. Conventional scientific
wisdom did not recognise the possibility of widespread
and excessive arsenic occurrence in the alluvial aquifers
of Bangladesh. Only a rigorous application of the precautionary principle, whereby representative samples
from water supplies across the country were tested for
all naturally-occurring health-related constituents and
properties, would have identified the situation that is
causing the suffering seen today in Bangladesh.
Acknowledgements PR and KMA thank Mr Kazi Nasir Uddin
Ahmed, Additional Chief Engineer of the Department of Public
Health Engineering for his support in conducting the Groundwater
Studies for Arsenic Contamination project. We also wish to thank
the project staff and the staff of DPHE for their co-operation during
the project. We thank David Kinniburgh of the British Geological
Survey for planning and co-ordinating the analytical aspects of the
Regional Survey. The Groundwater Studies for Arsenic Contamination project was financed by the Department for International
Development (UK). The Natural Environment Research Council
provided an Advanced Course Studentship and fieldwork allowance
to Melanie Burren. Jerome Perrin was supported by a fieldwork
grant from the University College London Graduate School. We
thank Mizanur Rahman of the Bangladesh Water Development
Board for provision of core-samples from the Ujjalpur borehole.
Chemical analyses for the Meherpur study were carried out by the
Robens Institute for Public and Environmental Health at Surrey
University, the Environmental Mineralogy laboratory of the Natural
History Museum in London, and the Natural Environmental
Research Council ICP-AES facility at Royal Holloway College,
London. Grateful thanks for help with analyses are due to Andrew
Taylor, Chris Stanley, Vic Din, Nikki Paige and Tony Osborn
for assistance. Martin Gillham of Mott MacDonald Ltd, Mike
McCarthy of the Department for International Development and Dr
Babar Kabir of the World Bank are thanked for their support and
encouragement. The script has been much improved due to helpful
reviews by Kirk Nordstrom and Alan Welch of the USGS. Last, but
not least, we wish to extend our sympathies to those people in
Hydrogeology Journal (2005) 13:727–751
Bangladesh and West Bengal whose lives have been so tragically
affected by arsenic in groundwater.
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