Executive Summary, Main Report
Phase I, Groundwater Studies of Arsenic Contamination in Bangladesh
by British Geological Survey and Mott MacDonald (UK) for Govt. Bangladesh, Min. Local Govt Rural Devel. Cooperatives,
Dept. Public Health Engrg., and for Dept. Intl. Devel. (DFID-UK).
This information was scanned from the printed copy
by ACIC - so please beware of any uncorrected OCR errors! Tables and figures
[Related information is available at the BGS website: another version
of this executive summary as part of their summary
of the Phase I reports; highlights
of the Phase I reports; the full hard copy Phase I reports (five volumes) may be
purchase at cost; plus the Phase
II reports, maps, data, and images.]
Table of Contents
S1 Background to the Project
S2 Phase l Findings
Arsenic in Drinking Water
S2.1 Scale of the problem
S3 Implications of the Present Study for the
Arsenic Mitigation Strategy
S2.2 Review of existing data
and Previous Surveys
S2.3 Laboratories and testing procedures
S2.4 Collation of existing data
S2.5 Regional groundwater arsenic
distribution and hydrogeochemical patterns
S2.6 Small-scale variability: the
Special Study areas
S2.7 Cause of the arsenic problem
Geological source of arsenic
S2.8 Future trends in groundwater arsenic
Mobilization of the arsenic -
Transport of arsenic within the
Influence of pumping and
Effects of floods
The mitigation strategy
Regional differences in the extent
Changes with time
Exploiting and protecting the deep
The future of groundwater use in Bangladesh
Map of arsenic-contaminated groundwater
S1 Background to the Project
Groundwater contamination by arsenic was first discovered in the west of
Bangladesh in late 1993 following reports of extensive contamination of
water supplies in the adjoining areas of India. Further testing in 1995
and 1996 showed that contamination extended across large parts of southern
and western Bangladesh. In April 1997 a World Bank Fact-Finding Mission
visited Bangladesh to assess the situation and to initiate a mitigation
programme. Part of their recommendations included a broad-ranging Rapid
Investigation Programme to collate the available data, fill in critical
gaps in knowledge and undertake surveys of the affected area. This eventually
lead to the project entitled 'Groundwater Studies for Arsenic Contamination
in Bangladesh' which was approved by the Government of Bangladesh in late
December 1997. The UK Department for International Development (DFID) agreed
to finance the project.
The Department of Public Health Engineering (DPHE), which is responsible
for water supply throughout the country other than in the cities of Dhaka
and Chittagong, is the executing agency for the project. The Bangladesh
Water Development Board (BWDB) and the Geological Survey of Bangladesh
(GSB) also provided counterparts for the study. On behalf of the Government
of Bangladesh, DFID appointed the British Geological Survey (BGS) as overall
consultants for the study. BGS appointed Mott MacDonald Ltd (MML) to carry
out the bulk of the Phase I work. A team of national experts were recruited
to assist with the work.
The project began in mid January 1998 and was structured to have a six
month Phase I and an eighteen month Phase II with about half the funding
allocated to Phase I. The principal aims of Phase I were to:
Phase I was due to be completed in July 1998 but was delayed due to the
need to reanalyse all of the 2000 regional survey samples in the UK. The
arsenic analyses were completed in October 1998, and the draft final report
was submitted for review in November. This delay did not prevent the Phase
II work beginning, and a number of Bangladesh sediments have now been analysed
for arsenic and other elements.
collate and review existing data for arsenic in Bangladesh groundwaters;
carry out a regional survey of arsenic in groundwaters in what was believed
to be the worst-affected parts of Bangladesh (approximately the southern
and eastern two-thirds of Bangladesh); and
to carry out a more detailed study of three small areas (thanas) to assess
the possible source, mobility and fate of arsenic in the aquifers.
The Final Report on the Rapid Investigation Phase is divided into a
Main Report and five Supplementary Volumes as follows:
Sl - Review of Existing Data
S2 - Regional Arsenic Survey
S3 - Modeling Studies
S4 - Hydrogeochemistry of the Special Study Areas
S5 - Arsenic Contamination of Groundwater in Bangladesh
Arsenic in Drinking Water
Arsenic is both toxic and carcinogenic. Inorganic forms of arsenic dissolved
in drinking water are the most significant forms of natural exposure. Organic
forms of arsenic that may be present in food are much less toxic to humans.
Clinical manifestations of arsenic poisoning begin with various forms of
skin disease, and proceed via damage to internal organs ultimately to cancer
and death. The symptoms of chronic arsenic poisoning may take between five
and fifteen years to reveal themselves. The principal treatment is to provide
the patient with arsenic free drinking water. The Bangladesh Standard for
arsenic in drinking water is 0.05 mg/l. This standard was based on World
Health Organisation (WHO) advice at the time when the regulations were
drafted. In 1993 WHO lowered their guideline value for arsenic to 0.01
mg/l. This value has not been adopted in either Bangladesh or India.
S2 Phase l Findings
S2.1 Scale of the problem
There is clearly a very serious problem of arsenic in groundwater in much
of southern and eastern Bangladesh. In terms of the population exposed
it is the most serious groundwater arsenic problem in the world. The contamination
occurs in groundwater from the alluvial and deltaic sediments that make
up much of the area. Description of the problem is complicated by large
variability at both local and regional scales. The arsenic is of geological
origin and is probably only apparent now because it is only in the last
20-30 years that groundwater has been extensively used for drinking water
in the rural areas. However, the arsenic has probably been present in the
groundwater for thousands of years. It is difficult to say for sure whether
it will get better or worse with time but the likelihood is that any changes
are likely to be rather slow - seen over years or even longer.
In many ways, the alluvial sediments of Bangladesh are ideal for groundwater
development. The sediments are characterised by fining upward sequences
of sand, silt and clay, with good aquifers in medium to fine sands. The
unconsolidated sediments can be drilled by hand down to depths of 80 metres
or more in a couple of days. The water table is high, typically less than
7 m below ground level, which means that ordinary suction hand pumps are
able to extract the water in most places. In the drier areas, the hill
tracts, and where intensive groundwater irrigation has increased the annual
decline in the water table, positive displacement 'Tara' pumps must be
used. The high rainfall ensures that the aquifers are fully recharged each
year. This combination of circumstances has meant that the groundwater
has been extensively exploited in recent years, a policy encouraged by
government and other agencies. There are now about four million tubewells
in Bangladesh. The development of tubewells has been responsible for the
reduction of infant mortality from diarrhoeal diseases, and the achievement
of food-grain self-sufficiency through groundwater irrigation. It is estimated
that 95% or more of Bangladeshis now use groundwater for drinking water.
Much of Bangladesh is characterised by a two-aquifer system. A shallow
aquifer typically extending from less than 10 metres to more than 100 metres
below ground level, and a deeper aquifer below about 200 metres. A surface
layer of silty clay forms a semi-confining layer and a lower clay layer
sometimes separates the shallow and deep aquifers. In much of southern
Bangladesh, the situation is more complex with a division of the shallow
aquifer into two by a low permeability silt-clay layer.
The shallow (or main) aquifer has been most extensively exploited and
is the source of the arsenic problem. Groundwater from depths of more than
150-200 m appears to be essentially arsenic-free. This confirms earlier
findings. Indeed the extent contamination (1% of deep wells deeper that
200 m) observed in our survey was even less than in earlier surveys. This
statement must be qualified by the fact that most of the deep wells sampled
were from the coastal region where the deep wells have been sunk to avoid
salinity in the shallow aquifer. However, some test deep boreholes sunk
recently by DPHE in badly-affected regions farther north seem to confirm
this but it is not yet established as a universal fact and needs further
The top of the shallow aquifer, at depths of less than 10 m, also appears
to be less contaminated than deeper down as indicated by the observation
that shallow hand-dug wells are usually uncontaminated even in areas of
high arsenic contamination. These wells, however, face the highest risk
of microbiological contamination.
S2.2 Review of existing data
Historical Perspective and Previous Surveys
Arsenic contamination of groundwaters was first detected in Bangladesh
in 1993 by the DPHE in Chapai Nawabganj in the far west of Bangladesh in
a region adjacent to an area of West Bengal which had been found to be
extensively contaminated in 1988. Extensive contamination in Bangladesh
was confirmed in 1995 when additional surveys showed contamination of shallow
and deep tubewells across much of southern and central Bangladesh. At the
same time, cases of chronic arsenicosis were being recognised by health
The oldest known analyses of arsenic in groundwater were for three municipal
tubewells in Dhaka City in 1990. All were below detection limits, and so
did not attract attention. Recent analyses have confirmed the absence of
arsenic contamination in Dhaka City.
Dr Dipankar Chakraborti of the School of Environmental Studies (SOES)
convened an international conference on arsenic in Calcutta in 1995 at
Jadavpur University in West Bengal. This first brought the scale of the
arsenic problem in West Bengal to a wider audience and it became evident
that there was an urgent need for more detailed studies of the similar
alluvial aquifers of Bangladesh. Early studies by the National Institute
of Preventative and Social Medicine (NIPSOM) highlighted the problem but
were not extensive enough to provide an overall picture.
With assistance from the WHO, two (and later all four) of the DPHE Zonal
laboratories were equipped to analyse for arsenic. Subsequently, several
thousand analyses have been carried out in these laboratories. Other early
data came from the Dutch-funded Eighteen District Towns project of DPHE.
The analyses were carried out in the Netherlands and confirmed the patchy
nature of the arsenic distribution. This project was also significant in
instituting regular monitoring of wells. Early arsenic data also came from
a survey by BWDB and analysed at the Bangladesh Atomic Energy Commission
Since 1995, data pointing to the extensive contamination of Bangladesh
groundwater have been collected by a large number of organisations. Extensive
arsenic surveys carried out by the Dhaka Community Hospital in association
with SOES in 1996 and 1997 were crucial in raising public awareness to
the extent of contamination. These involved the analysis of water samples
collected from the homes of arsenic-affected patients and confirmed the
seriousness of the arsenic problem. Classic symptoms of chronic arsenic
exposure were becoming increasingly apparent and Bangladeshi patients visited
West Bengal in order to seek a 'cure' for their illness.
The Asian Arsenic Network first visited Bangladesh in December 1996
following the publicity given to the West Bengal arsenic problem. They
made a detailed study of Samta village in Jessore and found that more than
90% of the tubewells were contaminated with arsenic.
A BGS survey of Chapai Nawabganj in early 1997 confirmed the extremely
high concentrations of arsenic - up to 2.4 mg/l - and low concentrations
of sulphate. University College London in collaboration with Dhaka University,
BWDB and MML carried out the first systematic geologically-based investigation
of the occurrence of arsenic in Bangladesh. A traverse from the ancient
terrace areas at Dhaka across the Brahmaputra and Ganges floodplains conclusively
demonstrated the geological control over the distribution of arsenic in
groundwater. Based on analysis of BWDB core samples, the study lead to
the publication of the main alternative explanation to the 'pyrite oxidation'
hypothesis for the origin of arsenic in groundwater. Other data collected
in 1997 included data collected by a DPHE Chemist studying in Austria.
This study also included high quality, multi-element data for a range of
Bangladesh groundwaters. Of 63 samples; 60% had arsenic concentrations
greater than 0.05 mg/l, the Bangladesh standard.
In early 1997, a randomised survey of wells in six districts in north-east
Bangladesh was undertaken by the Bangladesh University of Engineering &
Technology (BUET) for the North-East Minor Irrigation Project (NEMIP).
Some 1210 samples were tested for arsenic of which 61% were above 0.01
mg/l and 33% were above 0.05 mg/l. A further 751 samples were analysed
by the Bangladesh Council for Scientific Research (BCSIR) from the same
region and showed 42% of samples above 0.05 mg/l. These surveys indicated
extensive contamination of a region, well away from the area then believed
to be at the centre of the problem.
In 1997, there were an increasing number of studies of arsenic contamination
carried out by Government and University Departments, NGOs and other agencies.
These included patient surveys. A number of different field-test kits became
available and these were used by NGO Forum, BRAC, Grameen Bank and others
to test wells. The National Institute of Preventative Medicine (NIPSOM)
analysed nearly 3500 samples from various parts of the affected regions
of Bangladesh and found 28% with above 0.05 mg/l arsenic.
During 1997 two nation-wide surveys were conducted and gave the first
indication of the true extent of the problem. The first was carried by
NRECA and ICDDR,B who collected around 500 samples at 100 sites evenly
distributed across the country. The study analysed a variety of other parameters
in water, and collected soil samples at selected sites in order to investigate
(and subsequently reject) a highly publicised idea that arsenic contamination
was caused by leaching of wood preservatives from electricity pylons. A
more extensive survey of about 23,000 wells was carried out by DPHE with
assistance from UNICEF using simple field-test ('yes/no') kits. The lack
of precision of the test procedure was offset by the large number of samples.
For the first time, these surveys demonstrated that arsenic contamination
was a most serious in the southeast of Bangladesh. The seriousness of the
problem was brought home in 1998 when a field-kit survey by BRAC of all
12,000 wells in Hajiganj thana of Chandpur district showed that 94% of
the wells were contaminated. This survey also demonstrated the potential
for community involvement in testing programmes.
An international conference, organised by Dhaka Community Hospital and
the School of Environmental Studies, was held in Dhaka in February 1998.
This conference was the first major opportunity for the sharing of knowledge
and experiences of the arsenic problem in Bangladesh.
A number of detailed groundwater surveys have been, and are continuing
to be, undertaken at the municipality/village scale. There are some 63,000
mouzas (smallest administrative unit, containing one or more villages)
in Bangladesh so the task is formidable. A survey being undertaken by Dhaka
Community Hospital with UNDP funding is the largest of these surveys and
initially aimed to measure arsenic in every well within 200 villages in
the worst-affected regions of Bangladesh - this has recently been extended
to a further 300 villages. These tests will be carried out by field test
kits with some samples being checked in the DPHE laboratories.
All of these surveys have shown that while there is considerable variation
in arsenic contamination over distances of several tens of kilometres,
distinct 'high' and 'low' areas can be seen at a scale of a few kilometres
as in Chapai Nawabganj, Samta village and at Faridpur. There are therefore
both regional patterns and local patterns in the arsenic distribution.
S2.3 Laboratories and testing procedures
During 1997 and 1998, the laboratory facilities for arsenic testing within
DPHE were also being strengthened with help from WHO, UNICEF, DFID and
others. Nevertheless, the laboratory facilities available within Bangladesh
for testing arsenic on a large scale remain inadequate. During Phase I
of the project, the DPHE laboratory procedures were reviewed. New arsine
generators were purchased for the laboratories and supplies of good-quality
chemicals obtained, sometimes from overseas. The supply of 1-ephedrine
required for the arsenic analysis remained a problem throughout the survey.
However, many of the arsenic analyses were undertaken before any of the
improvements could be made and subsequent quality control checks showed
considerable variation between the DPHE and BGS laboratories with a general
tendency for the DPHE laboratories to under-report arsenic concentrations.
It was therefore decided to reanalyse all samples for arsenic in the UK.
Subsequent monitoring of the DPHE laboratories has shown an improvement
in the quality of results.
Compilation of recent evaluations and other information has produced
important information about the practicality of field-kit testing. Five
different kinds of field kit were tested, and while there were differences
between the kits, the results were sufficiently similar to be presented
in general. The general geographical distributions of arsenic contamination
indicated by field tests and laboratory tests are essentially the same.
However, there are problems in testing natural groundwaters with low levels
of arsenic contamination. Controlled field and laboratory testing in India
and Bangladesh showed that:
Field kits reliably identify highly contaminated water containing above
about 0.20 mg/l of arsenic.
Field kits do not falsely indicate the presence of arsenic in wells where
laboratory tests show the arsenic concentration is below 0.05 mg/l.
Field kits do not reliably identify the presence of arsenic in groundwater
containing between 0.05 and 0.20 mg/l of arsenic.
It should be noted that these tests were performed either by or under the
supervision of chemists. Therefore, actual results performed without supervision
may add additional uncertainty to the results.
S2.4 Collation of existing data
All of the available existing arsenic data sets have been collected, reviewed
and compiled into computer databases. All of the data have been geocoded
(i.e. codes that identify a location according to the district, thana and
union etc.) and, where possible, the data have been 'georeferenced' (i.e.
latitude and longitude were extracted or assigned). These data have been
incorporated into a Geographical Information System (GIS) system for analysis
and production of hazard maps. The combined computer database contains
the results of some 34,000 field and laboratory tests. This database has
been archived on CD-ROM and is available to field workers and researchers.
The disc also contains other water use and water quality information collected
under the project.
S2.5 Regional groundwater arsenic distribution
and hydrogeochemical patterns
The project undertook a new survey of 41 of the 64 districts of Bangladesh
between March and June 1998, covering what were believed to be worst-affected
parts of Bangladesh, namely most of southern Bangladesh (except the Chittagong
Hill Tracts) and the north-eastern districts. Altogether more than two
thousand samples were collected from 252 thanas (an average of 8 samples
per thana or 1 sample per 37 km2). The sampling strategy was designed to
give a uniform spatial coverage and a representative range of well types
and depths. The choice of wells sampled was not based on any prior information
about the possible arsenic concentration in the well water. Duplicate samples
were collected at each well. One sample was sent to the DPHE Zonal laboratory
and the other to the BGS laboratory in the UK. All of the samples were
analysed for arsenic and a subset was also analysed for iron and total
hardness. These analyses were undertaken in the four DPHE Zonal laboratories.
In light of the quality control checks, it was decided to analyse all of
the duplicate samples for arsenic in the BGS laboratories. In addition,
one sample from each thana was analysed in the UK for a wide range of other
solutes to provide information on the regional variation of groundwater
The results of the project's Regional Arsenic Survey broadly agree with
earlier survey data but provide better spatial resolution and probably
more reliable results at low concentrations. The median arsenic concentration
was 0.0108 mg/l, just above the WHO recommended drinking water limit. The
results of the 2022 samples analysed in the UK are summarised below:
About 20% of samples have arsenic concentrations of less than 0.003 mg/l
and may be considered essentially arsenic-free. In the survey area, there
were relatively few samples in the range 0.01-0.05 mg/l, though this is
not the case in all parts of the country. The minimum concentration was
below the lowest detection limit of all the methods used (0.0005 mg/l).
The maximum concentration found was 1.67 mg/l. Therefore the range of arsenic
concentration spans more than three orders of magnitude. Some 14% of the
samples were taken from wells deeper than 200 metres. Only about 1% of
the samples were contaminated above the Bangladesh standard. This compares
with 41% of contaminated wells in the shallower aquifers. Most of the shallow
wells are between 10 and 70 m depth with the water table usually in the
range 5-10 m below ground surface.
51% of the samples were above 0.010 mg/l (the WHO Guideline Value);
35% were above 0.050 mg/l (the Bangladesh Drinking Water Standard);
25% were above 0.10 mg/l;
8.4% were above 0.30 mg/l; and
0.1% were above 1.0 mg/l.
There is a distinct regional pattern in the arsenic-affected areas with
the most contaminated area to the south and east of Dhaka (Figure 1). This
reflects variations in the type of sediments and the spatial distribution
of deep and shallow wells. Groundwaters from the older aquifers beneath
the Barind and Madhupur tracts are not significantly contaminated with
arsenic. Also most groundwaters in the far south of Bangladesh (Barisal,
Barguna, Patuakhali and Bhola) were taken from the deep aquifer since the
shallow aquifer is saline. There were only two or three shallow wells sampled
in Barguna, Patuakhali and Bhola districts and hence these figures have
been omitted from the district-wise summary presented in Figure 1. The
shallow aquifer is most contaminated in Chandpur, Noakhali, Madaripur and
Lakshmipur districts. Table I gives a summary of the available arsenic
testing results in each district.
There is a strong correlation between the occurrence of arsenic and
the surface geology and geomorphology. The worst affected aquifers are
the alluvial deposits beneath the Recent floodplains. Older sediments beneath
the Barind and Madhupur Tracts and the eastern hills and their adjoining
piedmont plains are not significantly affected by arsenic. There are also
important differences with the floodplains. The floodplains of the Brahmaputra
and the Tista rivers in the north of the country show the lowest levels
of contamination. The most affected aquifers lie beneath the Meghna floodplains
of southeast Bangladesh. The Ganges floodplains, which have been the most
extensively sampled, show the greatest spatial variability.
The groundwaters in the Regional Survey area have characteristics typical
of reduced groundwaters: high dissolved iron (median 1.3 mg/l) and manganese
(median 0.3 mg/l) and low sulphate (median 0.7 mg/l) concentrations. The
groundwaters also had unusually high phosphate concentrations (median 0.6
mg/l). Data from the Special Study areas suggest that high ammonium and
boron and low nitrate concentrations are also typical of these reduced
waters. From the 253 detailed chemical analyses, the occurrence of the
following parameters are also of potential health significance was noted:
The maps derived from these data show regional hydrochemical patterns reflecting
the influence of geology, sedimentology and other geochemical factors.
Significantly, arsenic shows no strong, overall correlation with other
chemical parameters including dissolved iron. Therefore these other parameters
cannot be used to predict arsenic concentrations, at least on a regional
Boron exceeded the WHO Guideline Value at 13% of wells.
Manganese exceeded the WHO Guideline Value at 31% of wells.
Barium and chromium exceeded the WHO Guideline Value at three wells each.
In the Special Study areas, ammonium frequently exceeded the WHO Guideline
When the project and pre-existing survey data are combined with the
projected 1998 population densities, it is estimated that the probable
number of people exposed to arsenic concentrations above the Bangladesh
standard (0.05 mg/l) is about 21 million people. This number would be roughly
doubled if the WHO Guideline value of 0.01 mg/l were adopted as a standard.
The greatest density of exposed people is in the region of Chandpur, south-east
of Dhaka, where high arsenic concentrations coincide with a high population
S2.6 Small-scale variability: the Special Study
The three headquarter thanas of Nawabganj, Faridpur and Lakshmipur districts
were studied in greater detail than was possible in the Regional Survey.
Approximately 50 wells per thana were sampled (about one per 7 km2). A
wide range of chemical parameters was measured including dissolved oxygen,
redox status and arsenic speciation. Lithological logs were examined to
determine the structure and continuity of the aquifers. Groundwater use
and monitoring data were also compiled. This information was used to design
a three-dimensional groundwater flow and transport model for water and
arsenic transport in each thana.
In Chapai Nawabganj, concentrations of arsenic exceeded 2 mg/l. A large
proportion of the wells in and around Nawabganj town had high arsenic concentrations
(above 0.1 mg/l). Chapai Nawabganj represents an example of what have been
referred to as 'arsenic hot spots' - areas with highly localised extreme
concentrations within an area of regionally low arsenic concentrations.
The size of the Chapai Nawabganj hot spot is only a few kilometres across,
and is restricted to an area of a slightly older floodplain around the
town. Wells on the adjoining Barind Tract are not contaminated. Not all
of the wells in the hot spot are contaminated, but most are.
In Nawabganj 25% of the samples had arsenic concentrations greater than
0.05 mg/l. Arsenic is more uniformly distributed in Faridpur and Lakshmipur;
40% and 55% respectively of wells are contaminated. Groundwaters from depths
of more than 100 m in all the thanas typically have low arsenic concentrations.
Water from very shallow hand-dug wells also has low arsenic concentrations.
Speciation of the arsenic showed that the median percentage of As(III)
was close to 50% but there was a wide range of As(III) to As(V) ratios
and little relationship with other measured parameters. This confirms earlier
experience in West Bengal and Bangladesh. The more detailed chemical data
confirm that the waters are anoxic with high concentrations of dissolved
ammonium in Faridpur and Lakshmipur (but not Chapai Nawabganj), and low
concentrations of nitrate everywhere except where surface pollution was
suspected. In addition, carbon isotope studies support previous deductions
that micro-organisms play an important role in oxidising organic matter
and maintaining reducing conditions.
S2.7 Cause of the arsenic problem
The groundwater arsenic problem in Bangladesh arises because of an unfortunate
combination of three factors: a source of arsenic (arsenic is present in
the aquifer sediments), mobilisation (arsenic is released from the sediments
to the groundwater) and transport (arsenic is flushed away in the natural
Geological source of arsenic
Previously a number of anthropogenic explanations had been for the occurrence
of arsenic in groundwater. While it is possible that some may explain
isolated cases of arsenic contamination, none of the anthropogenic explanations
can account for the regional extent of groundwater contamination in Bangladesh
and West Bengal. There is no doubt that the source of arsenic is
of geological. The arsenic content of alluvial sediments in Bangladesh
is usually in the range 2-10 mg/kg; only slightly greater than typical
sediments (2-6 mg/kg). However, it appears that an unusually large proportion
of the arsenic is present in a potentially soluble form. The high groundwater
arsenic concentrations are associated with the grey sands rather than the
There is a good correlation between extractable iron and arsenic in
the sediments and a relatively large proportion (often half or more) of
the arsenic can be dissolved by acid ammonium oxalate, an extract that
selectively dissolves hydrous ferric oxide and other poorly ordered oxides.
It therefore appears likely that a high proportion of the arsenic in the
sediments is present as adsorbed arsenic. This would not be true of arsenic
present in primary minerals such as arsenic-rich pyrite.
The greatest arsenic concentrations are mainly found in the fine-grained
sediments especially the grey clays. A large number of other elements are
also enriched in the clays including iron, phosphorus and sulphur. In Nawabganj,
the clays near the surface are not enriched with arsenic to any greater
extent than the clays below 150 m - in other words, there is no evidence
for the weathering and deposition of a discrete set of arsenic-rich sediments
at some particular time in the past. It is not yet clear how important
these relatively arsenic-rich sediments are for providing arsenic to the
adjacent, more permeable sandy aquifer horizons. There is unlikely to be
a simple relationship between the arsenic content of the sediment and that
of the water passing through it.
It is likely that the original sources of arsenic existed as both sulphide
and oxide minerals. Oxidation of pyrite in the source areas and during
sediment transport would have released soluble arsenic and sulphate. The
sulphate would have been lost to the sea but the arsenic, as As(V), would
subsequently have been sorbed by the secondary iron oxides formed. These
oxides are present as colloidal-sized particles and tend to accumulate
in the lower parts of the delta. Physical separation of the sediments during
their transport and reworking in the delta region has resulted in a separation
of the arsenic-rich minerals. The finer-grained sediments tend to be concentrated
in the lower energy parts of the delta. This is likely to be responsible
for the greater contamination in the south and east of Bangladesh. The
map of arsenic-contaminated groundwater shows that highly contaminated
areas are found in the catchments of the Ganges, Brahmaputra and Meghna
rivers strongly suggesting that there were multiple source areas for the
The types of sediment deposited in the delta region have been strongly
influenced by global changes in sea level during the Pleistocene glaciations.
For example, sea level was more than 100 m lower at the peak of the last
lee Age around 18,000 years ago. At that time the major rivers cut deeply
incised valleys into the soft sediments of the delta. All of the highly
contaminated groundwaters occur in sediments deposited since that time,
while those sediments predating the low sea level stand contain little
or no arsenic-contaminated groundwater.
Mobilization of the arsenic - redox processes
Burial of the sediments, rich in organic matter, has led to the strongly
reducing groundwater conditions observed. The process has been aided by
the high water table and fine-grained surface layers which impede entry
of air to the aquifer. Microbial oxidation of the organic carbon has depleted
the dissolved oxygen in the groundwater. This is reflected by the high
bicarbonate concentrations found in groundwater in recent sediments. There
is a relationship between the degree of reduction of the groundwaters and
the arsenic concentration - the more reducing, the greater the arsenic
The highly reducing nature of the groundwaters has led to the reduction
of some of the arsenic to As(III) and possible desorption of arsenic since
As(III) is normally less strongly sorbed by the iron oxides than As(V)
under the near neutral pH groundwater conditions observed. Further reduction
will lead to the partial dissolution of the poorly crystallised ferric
oxide with consequent release of iron and additional arsenic. Other strongly
sorbed ions, especially phosphate, will also be released by iron oxide
dissolution. The relatively high phosphate concentrations present in the
groundwaters will compete with As(V) for sorption sites and is another
factor that favours high groundwater arsenic concentrations. It may also
make arsenic treatment more difficult.
The 'pyrite oxidation' hypothesis proposed by scientists from West Bengal
is therefore unlikely to be a major process, and that the 'oxyhydroxide
reduction' hypothesis (Nickson, R. et al. 1998 in Nature v395:338)
is probably the main cause of arsenic mobilisation in groundwater. It is
difficult to account for the low sulphate concentrations if arsenic had
been released by oxidation of pyrite. Moreover, mineralogical examination
suggests that the small amounts of pyrite present in the sediments have
been precipitated since burial.
Transport of arsenic within the aquifers
Present groundwater movement is very slow because of the extremely low
hydraulic gradients found in the delta region. Except where modified by
pumping, groundwater circulation is largely confined to the shallow layers
affected by local topographic features and the presence of rivers. Close
to rivers, the enhanced groundwater flow may lead to a greater dispersion
of arsenic along river banks. Annual fluctuations of the water table, typically
about 5 m, will affect groundwater and arsenic movement in the shallow
layers. There may have been some flushing of arsenic from the shallowest
At greater depths, groundwater moves slowly in response to the low regional
gradients. This is consistent with the old age of the waters. The lateral
and vertical spread of contaminants is slow even without considering the
retardation due to sorption. Modelling suggests that even in the most permeable
layers, arsenic movement is likely to be limited to a few metres a year.
The permeability of the silty clay layers is low and in the case of
a narrow horizon of silty clay, water will preferentially move through
the adjacent more permeable sandy layers. This effectively protects the
silty clay layers from strong leaching and possibly preserves arsenic-rich
zones. This relative lack of water and arsenic movement and the strong
stratification of the aquifer therefore both preserve the high concentrations
of arsenic from leaching and lead to the great spatial variability observed.
The conclusion from this is that in the absence of man's intervention significant
short-term (less than a few decades) variations in arsenic concentrations
are unlikely to occur at depth.
S2.8 Future trends in groundwater arsenic concentrations
Influence of pumping and irrigation
There are no long-term water quality monitoring data to definitively establish
how arsenic concentrations change over time. The few data that exist, extending
over no more than two years, show that some wells have increased in concentration,
but cannot yet be taken as proof of general or systematic changes. The
Regional Survey showed a strong correlation between the year of construction
and the proportion of wells that are contaminated above the Bangladesh
Standard. On average, older wells are more likely to be contaminated than
recently constructed ones. Only long-term monitoring will determine whether
this actually corresponds to increasing concentrations at individual wells.
The possible influence of pumping is a key policy issue for the water
sector. There is extensive withdrawal of groundwater for domestic use and
irrigation. Although the number of hand pumps is much greater than the
number of irrigation wells, they only account for about 10% of groundwater
abstraction by volume. The critical question is whether or not pumping
of groundwater for irrigation is either creating or exacerbating the problem
of arsenic in drinking water. The influence of pumping for irrigation could
be expressed as either the throughflow of groundwater through the aquifers
or by the lowering of the water table. To test these ideas, we looked for
a spatial correlation between the areas of most intense arsenic contamination
and the distribution of groundwater abstraction and also the deepest groundwater
levels. No correlation with either heavy abstraction or deep groundwater
levels could be found. In fact, the areas of greatest contamination never
coincide with either the deepest water levels or the most intensive abstraction.
Possible changes over time were also investigated through the use of
numerical groundwater flow and transport models. Modelling the impact of
a typical 0.5 cusec irrigation shallow tubewell (STW) with a 6 ha command
area indicates that even under conditions of relatively low arsenic sorption,
movement of the arsenic might be of the order of 50 m in 15 years. Therefore
while irrigation wells will enhance the movement and dispersion of arsenic,
this effect is likely to occur over the times scale of decades.
Although there is evidence that enhanced fluctuation of the water table
is not responsible for mobilising arsenic, this is not to say that irrigation
will have no influence on the arsenic problem. In particular, the widespread
cultivation of boro rice provides just the conditions that would minimise
air entry to the underlying aquifer and would therefore make any ongoing
reduction and arsenic release that much more effective. This process would
probably take a long time to have an effect, and cannot account for the
large-scale problem that currently exists. It nevertheless needs further
The effect of phosphate fertilisers also needs investigating. Phosphate
concentrations are abnormally high -frequently more than 0.5 mg/l (as phosphate-P)
and this could make the arsenic more soluble by competing with arsenic
for sorption sites on the iron oxides. However, we believe that most of
the phosphate is derived from natural geological sources.
The impact of using contaminated irrigation water from shallow tubewells
needs investigating from the point of view of possible entry of arsenic
into the human food chain, the animal food chain and any effect soil quality,
particularly its microbiological functioning.
Effects of floods
Floods are a normal occurrence in Bangladesh, and although the severe flooding
in the 1998 monsoon was exceptional, it is unlikely that floods have any
long-term effect on the arsenic problem. There may be some increased flow
in the uppermost part of the shallow aquifer but this will, if anything,
tend to flush out the arsenic that is found there.
S3 Implications of the Present Study for the Arsenic Mitigation
The mitigation strategy
Many national and international organisations are looking into how to overcome
the arsenic problem and the World Bank has recently announced that an initial
$44 million loan will be made available to the Government of Bangladesh
to begin to tackle the problem. The task ahead is enormous and it is clear
that there is not going to be a single, simple solution for all of Bangladesh.
There are many options inter alia including:
The challenge is partly technical - to design systems that work reliably
and that are both acceptable and affordable in rural Bangladesh. But the
problem also throws up many institutional challenges. The solution must
be organised by the rural communities themselves and this is going to require
a massive educational programme. Above all, the scale of the problem makes
implementing even a simple solution very demanding. There are almost certainly
more than half a million wells affected. The problem is clearly a long
term one but also demands immediate, emergency action.
use of surface water with treatment by pond sand fitter; sinking deep wells
into the arsenic-free aquifers;
rain water harvesting;
ring wells to tap the very shallow aquifer;
solar-assisted oxidation and sterilisation of existing groundwater; and
arsenic treatment at various scales.
It was not the purpose of this study to devise a mitigation strategy
- many others are already doing that - rather we hoped to inform those
devising such a strategy. Below we draw attention to some of the findings
of this study that may be helpful in this regard and particularly in selecting
priorities for the emergency action programme.
Regional differences in the extent of contamination
Figure 1 clearly shows large differences in the extent of contamination
of the shallow tubewells in different districts from 'hardly affected'
in the north-west to 'nearly all affected' in the south east. Four classes
of contamination and corresponding strategies can be defined for the shallow
low contamination (less than 10% of wells) areas where occasional 'hot
spots' are possible. The 'omission' of Chapai Nawabganj in the randomly
based Regional Survey highlights the difficulty of locating all of these.
Although these areas may receive lower priority for comprehensive testing,
a more efficient approach might be to conduct more intensive randomised
sampling across these areas, supplemented by local comprehensive surveys
as and when hot-spots are located.;
medium contamination (10-60%): extensive, preferably comprehensive, testing
will be required especially in the western region and in the north-east
where the pattern of contamination appears to be rather patchy;
high contamination (greater than 60%): two approaches are possible. Either
the areas can be considered so highly contaminated that the future effort
should concentrate on mitigation rather than further testing or use further
testing to locate the minority of safe wells and then use only these. Much
depends on what other options are available locally.
shallow saline groundwaters also contaminated by arsenic : there is no
problem with shallow wells in these areas because the water is too saline
to drink anyway. Deep wells are used in the south of Bangladesh to avoid
this problem and these have a very high probability of being safe. However,
all new wells should be tested before commissioning.
Changes with time
Limited water quality monitoring over the last few years show that the
arsenic concentration of some wells has increased slightly. The results
do not show, however, a universal tendency to increase. Survey data show
that higher proportions of older wells are contaminated by arsenic. This
trend is shown for wells ranging in age from their first year of operation
to about twenty years of age. This trend suggests, but does not prove,
that arsenic concentrations at wells increase over time. Certainly, the
wide range of concentrations observed strongly suggests that concentrations
do not increase at all wells, or if so, at such a slow rate as to be irrelevant
in human terms. In the context of an arsenic mitigation strategy it would
advisable to assume, until and unless proved wrong, that arsenic concentrations
are slowly increasing over a period of years. The implication is that a
well tested as safe, but in an affected area, cannot be presumed permanently
safe. Regular monitoring, at intervals of perhaps a few years, will be
required in the future.
Although this study has found no such evidence, there have been reports
from West Bengal and to a lesser extent from Bangladesh that once arsenic-free
deep wells have become contaminated over a matter of months or a few years.
Unfortunately, these are not well documented, which is not to say they
are not correct, but emphasizes the need for broad scale and statistically-based
long-term monitoring programme for both the deep and shallow aquifers.
The technology of arsenic removal is well known. This usually relies on
its very strong adsorption to iron and aluminium oxides, and if sufficient
of these are added, the arsenic concentration in the water can be reduced
to practically any desired concentration, certainly below the Bangladesh
drinking water standard. This essentially reverses the process that has
produced the arsenic in the first place. The challenge is to do this in
an acceptable long-term way and at an affordable price. This means the
minimum use of chemicals or filter media and a low capital cost especially
if it is to be implemented on an individual hand pump scale.
It has been traditional in Bangladesh to use alum to clarify drinking
water after times of flood. Alum is widely available in Bangladesh. This
will also probably remove some arsenic and if added in sufficient quantity
could be promoted as a possible arsenic treatment option. Then there is
the use of the naturally high concentration of dissolved ferrous iron in
many Bangladesh groundwaters. Oxidising this, and the arsenite present,
and allowing the floc to settle will remove some arsenic from the supematant.
This principle is being used successfully in some of DPHE's modified iron
treatment plants. Sunlight could be used to promote this oxidation.
The efficiency of this natural remediation will depend primarily on
the arsenic and iron concentrations in the groundwater and also to a lesser
extent on the concentration of other chemicals present such as phosphate.
It will not work very well everywhere because there is not enough iron
present everywhere. Our regional survey provides some indication of how
the concentration of the critical chemicals varies across the badly affected
areas. Allowing the drinking water to stand overnight to remove the iron
is already practised in pans of Bangladesh, and could be promoted more
widely to reduce the arsenic intake. Thus freshly drawn tubewell water
may pose a higher health risk from arsenic than stored water. On the other
hand, there is a risk of contamination if not stored property.
Experience shows that arsenic removal efficiencies using this approach
are typically 40-70% which may not be sufficient to reduce the arsenic
concentration to the desired level but it will always help, is simple,
costs little and could at least provide an emergency option to reduce the
intake of arsenic immediately. Overall removal efficiencies can be improved
by arranging for multiple separations using either a multi-stage system
or by using a column. This is the principle behind the various hand pump
filters being designed and tested in West Bengal and Bangladesh. Some kind
of alumina seems to be the most attractive media for such filters. Other
options could include any red or brown-coloured 'local' materials such
as the sand from Sylhet widely used for pond sand filters or even possibly
local crushed brick.
Other treatment options include the subsurface oxidation and precipitation
of iron and arsenic by injecting aerated water, or water with some additional
oxidising agent. This in situ technique has been tested successfully for
iron removal in the Netherlands but is untried in Bangladesh.
Both field and laboratory testing are required on a massive scale, or perhaps
a combination of the two, or perhaps mobile arsenic-testing laboratories.
Existing field test-kits have an essential role to play but ideally would
have better precision at the critical 0.04-0.06 mg/l level. New developments
are required to achieve this.
The existing laboratories, especially government laboratories, are not
equipped to cope with the scale of testing required, and do not have the
organisational infrastructure to run a modem laboratory efficiently. DPHE
needs a single person to oversee all water quality issues within the organisation
at a senior level. The private sector is beginning to take an interest
in the analytical possibilities presented. The challenge is to get the
price down to an acceptable level for a mass-screening programme. This
should be possible with modem automated instruments and round-the-clock
testing to counter the large capital costs.
Exploiting and protecting the deep aquifer
Available data shows that aquifers deeper than 150 - 200 m are essentially
arsenic-free over much of Bangladesh. However, the use of deep aquifers
is not a panacea - they are not always present. or may be difficult to
drill using current technology, or may be unsuitable for drinking because
of salinity in the extreme south of the coastal belt. Overlying silty-clay
layers should provide the necessary hydraulic protection to prevent any
shallow contamination from affecting the deeper aquifer provided that the
borehole annulus is properly sealed. However, the overlying silty-clay
layers may also be a potential source of arsenic albeit over a long time
scale. where practical, the screened interval should be some distance away
from these fine-grained layers. More protection may need to be given in
the north of the country where conditions tend not to be artesian. Design
and construction practices should be improved, and quality control procedures
applied, to protect water quality in deeper aquifers.
The future of groundwater use in Bangladesh
The discovery of severe arsenic contamination of groundwater in large parts
of Bangladesh came as a shock to all concerned. It affects about a third
of the wells in the Regional Survey area and perhaps a quarter of wells
in the country as whole. More than 20 million people are probably drinking
water that exceeds the Bangladesh standard. The Government takes the problem
extremely seriously, and donor agencies have pledged to assist. Understandably,
there has been something of a media backlash against the use of groundwater.
There have even been calls to abandon the use of groundwater completely.
Less radical proposals call on a moratorium on all new government- or donor-sponsored
drilling for a year or so until the situation is clearer. Amidst this debate,
it must not be forgotten that most wells are not contaminated and that
large parts of northern Bangladesh are hardly affected at all. In these
areas, there is no reason why the benefits that exploiting groundwater
has brought to Bangladesh should not continue. A rapid and widespread return
to the use of surface water would inevitably result in an increase in diarrhoeal
disease. The situation calls for a pragmatic combination of practical,
affordable and sustainable short, medium and long-term water supply programmes
aimed at minimising the combined risk to health of diarrhoeal disease,
arsenic and other natural and man-made chemicals that may be present in