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Executive Summary, Main Report

Phase I, Groundwater Studies of Arsenic Contamination in Bangladesh

[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 available for purchase at cost; plus the Phase II reports, maps, data, and images.]

S1 Background to the Project

Arsenic in Drinking Water

S2.1  Scale of the problem
S2.2  Review of existing data

Historical Perspective and Previous Surveys
Ongoing Activities

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
Mobilization of the arsenic - redox processes
Transport of arsenic within the aquifers

S2.8  Future trends in groundwater arsenic concentrations

Influence of pumping and irrigation
Effects of floods

The mitigation strategy
Regional differences in the extent of contamination
Changes with time
Treatment options
Arsenic testing
Exploiting and protecting the deep aquifer
The future of groundwater use in Bangladesh

Map of arsenic-contaminated groundwater 

S1 Background to 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:

1. collate and review existing data for arsenic in Bangladesh groundwaters;
2. 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
3. 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 (NGO volume).

Arsenic in Drinking Water

S2 Phase l Findings

S2.1  Scale of the problem

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 testing.

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

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 (BAEC).

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.

Ongoing Activities

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

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

S2.5  Regional groundwater arsenic distribution and hydrogeochemical patterns

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:

• 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:

• 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 Value

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 density.

S2.6  Small-scale variability: the Special Study areas

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

Geological source of arsenic

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 arsenic.

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

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

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

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 investigation.

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

S3 Implications of the Present Study for the Arsenic Mitigation Strategy

The mitigation strategy

• 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

1. 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.;
2. 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;
3. 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.
4. 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

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.

Treatment options

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.

Arsenic testing

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

The future of groundwater use in Bangladesh