• Scale of the Problem
• Review of Existing Data
• Historical perspective and Previous Surveys
• Ongoing activities
• Laboratories and Testing Procedures
• Collation of Existing Data
• Regional Groundwater Arsenic Distribution and Hydrogeochemical Patterns
• Small-Scale Variability: The Special Study Areas
• Cause of the Arsenic Problem
• Geological source of arsenic
• Mobilisation of the arsenic - redox processes
• Transport of arsenic within the aquifers
• Future Trends in Groundwater Arsenic Concentrations
• Influence of pumping and irrigation
• Effects of floods

There is clearly a very serious problem of arsenic contamination 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 believed to be about ten 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 10 metres to 70 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 of 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. Some test deep boreholes sunk recently by DPHE in badly-affected regions further 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 and may account in part for the observation that shallow hand-dug wells are usually uncontaminated even in areas of otherwise high arsenic contamination. These wells, however, face the highest risk of microbiological contamination.

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 tubewells across much of southern and central Bangladesh. At the same time, cases of chronic arsenicosis were being recognised by health professionals.  

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. 

An international conference on arsenic was convened in Calcutta in 1995 by Dr Dipankar Chakraborti of the School of Environmental Studies (SOES), 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. 

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. Early arsenic data also came from a survey by BWDB and analysed at the Bangladesh Atomic Energy Commission (BAEC).  

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 led 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, NGO’s 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 with USAID support. They 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 most serious in the southeast of Bangladesh. The severity 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  

A number of detailed groundwater surveys have been undertaken, and are continuing, at the municipality/village scale. There are some 63,000 mouzas 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 400 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 or less as in Chapai Nawabganj, Samta village and at Faridpur. There are therefore both regional patterns and local patterns in the arsenic distribution.

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 although an increasing number of private laboratories are offering sophisticated arsenic testing facilities.

During Phase 1 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 l-ephedrine required for the arsenic analysis remained a problem throughout the survey. However, many of the arsenic analyses for this project were undertaken before all 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 contaminated 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. There is a substantial ongoing effort to improve these field test kits ready for a country-wide screening process.

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.

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 chemistry. 

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. 

About 20% of samples have arsenic concentrations of less than 0.003 mg/l and may be considered essentially 'arsenic-free'. 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 m and 70 m depth with the water table usually in the range 5-10 m below ground surface. 

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. A listing of the arsenic data can be downloaded in DBF format. 

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 253 detailed chemical analyses, the following exceedances were also noted: 

• Boron exceeded the WHO Guideline Value at 9% 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

Additional chemical analyses are being undertaken in Phase 2 to refine our understanding of the distribution of these and other trace metals in Bangladesh groundwaters. The maps so far 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 scale.  

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 approximately 20-30 million people. Another 10 million would be included 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 (Figure 2).

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 arsenic transport model for 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 thana, 25% of the samples had arsenic concentrations greater than 0.05 mg/l. Arsenic was more uniformly distributed in Faridpur and Lakshmipur; 40% and 55% respectively of wells were contaminated. Groundwaters from depths of more than 100 m in all the thanas typically had low arsenic concentrations. Water from very shallow hand-dug wells also had 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.

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 groundwater circulation).  

Geological source of arsenic 

Previously a number of anthropogenic explanations had been given 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. There have been insufficient analyses of the alluvial sediments to provide a regional picture but current data suggest that arsenic is usually in the range 2-20 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 brown sands.  

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 Ice Age around 18,000 years ago. At that time the major rivers incised deep 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. 

Mobilisation 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 concentration.  

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 arsenic 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 the ‘oxyhydroxide reduction’ hypothesis 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 layers. 

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.

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 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 timescales 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 suspect 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 on 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.

S1: Review of Existing Data 
S2: Regional Arsenic Survey 
S3: Modelling Studies 
S4: Hydrogeochemistry of the Special Study Areas 
S5: Arsenic Contamination of Groundwater in Bangladesh (NGO volume).