"Executive summary of the main report of Phase I, Groundwater Studies of Arsenic Contamination in Bangladesh, by British Geological Survey and Mott MacDonald (UK) for the Government of Bangladesh, Ministry of Local Government, Rural Development and Cooperatives, Department of Public Health Engineering, and Department for International Development (UK).

"arsenic, study, project, GOB, MLGRD, DPHE, DFID, Bangladesh, British, geological, survey, Mott, MacDonald, UK, hydrogeology, groundwater, water, contamination, pollution, sediment, redox"

Main report, Bangladesh GW Studies As Contamination

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). The Phase I reports (five volumes) may be available for purchase at cost.

Enquire to :Peter Ravenscroft <mottmac@bangla.net>

Mott MacDonald regarding availability and ordering information.

 

 

Table of Contents
Background to the Project
Arsenic in Drinking Water
S2 Phase l Findings
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
Mobilization 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
S3 Implications of the Present Study for the
Arsenic Mitigation Strategy
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

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.

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 - Modelling Studies

S4 - Hydrogeochemistry of the Special Study Areas

S5 - Arsenic Contamination of Groundwater in Bangladesh

(NGO volume).

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

 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 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.

 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 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.

 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

 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.

 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:

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.

 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.

 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 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:

 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.

 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 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 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.

Small-scale variability: the Special Study areas

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.

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 groundwater circulation).

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 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 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 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 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 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.

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 through flow 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

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 Strategy

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.

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 aquifer:

 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 emphasises the need for broad scale and statistically-based long-term monitoring programme for both the deep and shallow aquifers.

Treatment options

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 parts 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

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 the environment.
 

 
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