Arsenic in groundwater of the Bengal Basin, Bangladesh ... - UCL

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. 

RØsumØ 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 holoc›nes du 

Gange, du Brahmapoutre et de la Meghna dØpassent 

localement 200 fois la valeur guide donnØe par l’OMS 

pour l’eau de boisson, fixØe à 10 g/l d’arsenic. Environ 

25% des puits du Bangladesh dØpassent la valeur standard 

nationale de 50 g/l, affectant au moins 25 millions de 

personnes. L’arsenic a ØtØ introduit dans les nappes par la 

dissolution par rØduction d’oxy-hydroxydes ferriques sur 

lesquels l’arsenic Øtait adsorbØ au cours du transport 

fluvial. Des profils verticaux d’arsenic dans l’eau souterraine 

pompØe, dans l’eau porale et dans les sØdiments des 

aquif›res montrent des tendances convergentes. Les 

concentrations ØlevØes sont associØes à des sØdiments à 

sable fin et riches en mati›res 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 à des niveaux tr›s bas entre 

100 et 200 m. L’arsenic est surtout prØsent dans les eaux 

souterraines de la sØquence de remplissage de vallØe 

dØposØe au cours de la transgression marine holoc›ne. 

Les eaux souterraines des aquif›res plØistoc›nes et plus 

anciens sont tr›s largement dØpourvus d’arsenic. Les 

concentrations en arsenic dans de nombreux puits creusØs 

à la main doivent probablement augmenter au cours des 

prochaines annØes ; aussi un suivi rØgulier est essentiel. 

Les aquif›res à plus de 200 m sous les plaines alluviales 

offrent de bonnes perspectives pour des alimentations en 

eau sans arsenic à long terme, mais ils peuvent Þtre 

limitØs par les risques d’intrusion saline et la drainance 

descendante de l’arsenic. 

Resumen El arsØnico ha contaminado gran parte de las 

aguas subterrµneas en el Sur, centro y Este de Bangla 

Desh. Su concentración en las aguas subterrµneas del 

aluvial Holoceno de los ríos Ganges, Brahmaputra y 

Meghna supera localmente en un factor 200 el valor guía 

del arsØnico en el agua potable, establecido por la 

Organización 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 estµndar nacional de 50 g/L, afectando al 

menos a 25 millones de personas. El arsØnico ha llegado a 

las aguas subterrµneas por la disolución reductora de 

hidróxidos fØrricos a los que se adsorbe durante el transporte 

fluvial. Los perfiles del arsØnico en las aguas subterrµneas 

bombeadas, agua de poro y sedimentos del 

acuífero muestran tendencias coherentes. Las concentraciones 

elevadas estµn asociadas a arenas finas y sedimentos 

ricos en materia orgµnica. Las concentraciones de 

arsØnico son bajas cerca del nivel freµtico, se incrementan 

hasta un mµximo que se localiza generalmente a entre 20 

y 40 m bajo la cota del terreno, y disminuyen a valores 

muy pequeæos entre alrededor de 100 y 200 m. El arsØnico 

se encuentra sobretodo en las aguas subterrµneas 

existentes en la secuencia de sedimentación que tuvo lugar 

en el valle durante la transgresión marina del Holoceno. 

Las aguas subterrµneas del Pleistoceno y acuíferos 

mµs antiguos estµn mayoritariamente libres de arsØnico. 

Es probable que las concentraciones de arsØnico aumenten 

en los próximos aæos en muchos pozos de tipo tubo 

perforados manualmente, por lo que serµ esencial efectuar 

un muestreo regular. Los acuíferos ubicados a mµs de 200 

m bajo las llanuras de inundación ofrecen buenas perspectivas 

de abastecimiento a largo plazo sin problemas de 

arsØnico, pero pueden estar limitados por las amenazas de 

la intrusión salina y de la precolación de arsØnico 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 km 2 . 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, 


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 (DWA- 

SA) 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 10 6 

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

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 (10 4 –10 5 km 2 , DPHE 1999) 

are combined with results from sub-regional scale studies 

(10 3 km 2 , DPHE 1999) and localised studies (15 km 2 , 

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. 

729 

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 (1.1”10 6 Mm 3 /yr) makes it the third 

largest river in the world, but in terms of sediment transfer 

(c. 1”10 9 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. 


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 litholo- 

DOI 10.1007/s10040-003-0314-0

730 

Table 1 Simplified stratigraphic succession of Bangladesh 

Age Stratigraphic units Lithology Notes 

Late Pleistocene– Chandina Formation Upward fining, grey micaceous, medium and Forms major aquifers beneath recent 

Holocene Dhamrai Formation 

Unclassified deposits 

coarse sand to silt with organic mud and 

peat. 

floodplains. Probably

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 3–8”10 -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). 


731 

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-HCO 3 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-HCO 3 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 HCO 3 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 


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 – HCO 3 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 (10 4 –10 5 km 2 scale) and sub-regional 

(10 3 km 2 scale) surveys of arsenic and its hydrochemical 

and hydrogeological context (DPHE 1999). The UCL and 

Dhaka University studies focussed on detailed local scale 

(10 km 2 ) 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 km 2 . 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 


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 HNO 3 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 

All arsenic 

tests 

Field kit 

tests 

All arsenic 

tests 

Pre-existing 

survey laboratory 

tests 

DPHE 

Regional 

survey 

Pre-existing 

survey laboratory 

tests 

DPHE 

Regional 

survey 

Barisal 1,396 403 295 2,094 9 31 14 14 

Chittagong 3,232 1,094 445 4,771 51 74 50 56 

Dhaka 6,175 2,189 988 9,353 17 34 31 22 

Khulna 4,819 2,036 474 7,329 30 30 41 31 

Rajshahi 5,891 1,712 1072 8,675 8 17 6 10 

Sylhet 1,264 1,440 260 2,964 11 35 21 24 

Bangladesh 22,777 8,874 3,534 35,185 21 34 25 25 

Source: DPHE (1999) and http://.www.bgs.ac.uk/arsenic/bangladesh 


DOI 10.1007/s10040-003-0314-0

Table 3 Frequency distribution of arsenic concentrations 

Concentration range (g/l) Classification No. in class Percent in class (%) Percent exceeding lower 

threshold (%) 

1,000 3 0.09 0.09 

Total 3,534 

Source: DPHE (1999) and http://.www.bgs.ac.uk/arsenic/bangladesh 

735 

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 

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

736 

Table 4 Summary of arsenic determinations in Nawabganj Upazila 

Surface No. of Arsenic concentration (g/l) 

geological unit wells 

Mean 1 Max. >50 >10 

Alluvial Sand 21 7.0 742 24% 43% 

Alluvial Silt 22 4.9 1,524 14% 45% 

Barind Clay / 

Dupi Tila 

15

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. 

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 

Table 5 Arsenic distributions in groundwater by well depth 

Well depth 

No. of 

wells 

Arsenic concentration (g/l) 

Mean 1 Max. >50 >10 

200 m 283 0.7 110 0.7% 3% 

All 2,023 6.7 1,670 35% 51% 

Source: GSACB Regional Survey, Phase I (DPHE 1999). 1 Calculated 

from the mean of the logarithms of arsenic concentration 

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 (

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, 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. 

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 con- 


DOI 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 HNO 3 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. 

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) 

No. of Mean 3 Max. 

wells 2 >50 

Alluvial Sand U. Holocene Active floodplains 144 7 890 27% 

Alluvial Silt and Clay Holocene Brahmaputra and Meghna River 668 8 700 32% 

Floodplains and Sylhet Basin 

Deltaic Silt Holocene Ganges Floodplain 544 15 1,670 45% 

Alluvial Silt Holocene Ganges and Brahmaputra River 428 5 1,450 26% 

Floodplains 

Chandina Formation L.-M. Holocene Old Meghna Estuarine Floodplain 152 88 1,090 77% 

Old and Young Gravelly U. Pleistocene–Holocene Tista Fan and floodplain 45 19 70 16% 

sand 

Dupi Tila and Dihing L. Pleistocene–Pliocene Madhupur and Barind Tracts 151 1 140 13% 

Formations 

Surma Series Tertiary Sylhet and Chittagong Hill Tracts 26 2 130 4% 

Data source: DPHE (1999). 1 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’. 2 All wells less than 

100 m deep. 3 Calculated 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 


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, supporting 

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) 

O 2 

(%) 

EC 

(S/cm) 

pH 

As 

(g/l) 

Ca Na K Mg Fe Mn Cl NO 3 SO 4 HCO 3 

S4 15 27.2 0 1,055 6.86 3 170 31.7 6.9 56.7 0.0 0.85 94.6 48.2 48.2 603 

S3 29 27.1 0 750 6.82 11 141 14.2 4.0 35.0 6.2 0.63 1.4 0.0 1.5 693 

S104 33 27.2 18 1,000 7.02 14 161 44.5 13.0 52.5 0.0 1.04 67.3 7.1 55.2 788 

S107 17 26.8 0 710 6.99 47 131 14.3 4.4 31.2 0.8 0.45 8.0 0.0 3.1 639 

S6 24 27.1 0 720 6.99 76 128 12.3 5.2 39.5 1.3 0.71 27.2 0.0 2.7 581 

S1 30 26.8 3 660 6.97 110 110 13.7 4.1 26.5 8.5 0.35 1.0 0.0 2.1 507 

S101 18 26.8 12 630 7.12 135 99 24.6 3.8 25.4 1.7 0.34 3.0 0.0 0.9 503 

S205 45 26.9 0 550 7.04 243 91 11.9 4.6 24.6 5.8 0.48 2.0 0.0 1.0 481 

S210 23 26.6 0 730 6.90 775 122 24.3 5.3 30.3 10.0 0.48 14.1 0.0 0.1 578 

All units are mg/l except where stated otherwise. Source: Burren (1998) 


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 km 2 , centred on the location of 

the cored borehole at Ujjalpur. Groundwater at Meherpur 

is predominantly anoxic, and of Ca-(Mg)-HCO 3 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 (NO 3 - 20 to 88 mg/l) and Ujjalpur village 

(NO 3 - 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. 


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, 


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). 

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; 

Natural Processes Controlling Arsenic Occurrence 

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 

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 reduction 

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. 


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

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 

Bangladesh and West Bengal whose lives have been so tragically 

affected by arsenic in groundwater. 

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