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Draft Development Strategy, National Water Management Plan 

Arsenic in Annex C-8

C8.1 Arsenic

C8.1.1 History, Discovery and Causes

In the last few years it has been recognized that Bangladesh is affected by one of the worst cases groundwater contamination in the world. With the exception of diarrheal diseases and coastal salinity in rivers, until recently water quality was not perceived as a problem in Bangladesh, as may be recognized by reference to the earlier National Water Plan projects (MPO, 1986 and 1991). Medical evidence of arsenic poisoning was first discovered in West Bengal in 1978, although the connection with groundwater contamination was not recognized until some years later. Further surveys were carried out during the 1980s (eg Chakraborty et al, 1987; Mazumder et al, 1988), culminating a national report by PHED, India, in 1991. In 1993, DPHE, Bangladesh, tested for and identified arsenic at Chapai Nawabganj, near the border with West Bengal, but it was not until 1995 that the existence of arsenic in Bangladesh became widely known. It was not appreciated until early 1997 that arsenic contamination extended over large parts of the country. By the end of 1998, a national assessment of the extent of contamination across the country had been made (DPHE 1999). At around the same time, projects of both the Ministry of Heath and the DPHE as well as various NGO's began work on extensive testing, awareness raising and providing small-scale arsenic removal systems.

C8.1.2 Studies on the Distribution and Origin of Arsenic in Groundwater

The studies of arsenic occurrence in Bangladesh and West Bengal may be divided into two groups, those with a medical bias aimed at confirming diagnoses of arsenicosis and those with a hydrogeological bias aimed at estimating exposure and the causes of contamination. The following discussion concerns the theories about the origin of arsenic, which have important implications for water management policy. The principal competing explanations all point to a geological source. These ideas have been presented both in reports and scientific literature, and are given a detailed discussion below. Other, anthropogenic, sources have been proposed, but have generally been made in the popular press rather than in reports or in scientific journals.

C8.1.2.1Geological Source Hypotheses

The first major investigation of arsenic contamination in West Bengal by PHED (1991) contained much analytical and descriptive detail but did not suggest an origin for the contamination other than ruling out anthropogenic sources. In the absence of any formal explanation, a number of (non-hydogeological) studies (eg Das et al, 1994 and 1996; Chaterjee et al, 1995) went onto suggest that extensive seasonal pumping of groundwater for irrigation is responsible, although they did not put forward any direct evidence to support the idea. This idea came to be known as the ‘pyrite oxidation’ hypothesis and was formally described by Mallick and Rajagopal (1996), although again unsupported by field evidence. The idea is based on the assumption that arsenic is present in the sulphide minerals pyrite and arsenopyride. According to the theory, lowering of the water table due to pumping introduces oxygen, which causes the breakdown of pyrite and releases arsenic, iron and sulphate into the water. Recharge during the subsequent monsoon then flushes the arsenic into the underlying aquifers. This explanation, whereby the arsenic pollution is caused by man’s over-exploitation of groundwater, became well known and led to calls for the banning of tulle well irrigation.

In 1997, an alternative explanation was independently put forward by an Indo - Swedish group (Bhattacharya et al, 1997) and an Anglo - Bangladeshi group (Nickson 1997; Nickson et al, 1998 and 2000) and has become known as the 'oxyhydroxide reduction' hypothesis. Both these groups presented their ideas in international scientific journals where the origin of arsenic was the main subject of the papers. According to the theory, arsenic weathered from the hard rock areas of India is carried in the suspended load of the rivers adsorbed onto iron oxides or hydroxides. Following deposition of this fine sediment, decomposition of organic matter leads to strongly reducing groundwater conditions which causes dissolution of the iron oxyhydroxides and consequent release of arsenic into solution. This hypothesis also accounts for the pervasive occurrence of iron in wells beneath the floodplains.

The debate between these views hinges on a number of issues. First, the chemistry of the affected water points towards strongly reducing conditions and hence that release by oxidation is very unlikely. Evidence for reducing conditions includes direct measurement of redox potential, low-to-negligible concentrations 0£ dissolved oxygen nitrate and sulphate combined with high concentrations of iron, manganese and bicarbonate plus the occurrence of dissolved gases such as carbon dioxide and methane (Ahmed et al, l998). Only limited evidence has been presented for the occurrence of arsenopyrite or arsenin-rich pyrite, while Nickson et al (1998) showed that pryrite is present in a stable diagenetic (framboidal) form. This indicates that it has grown after deposition and is a sink rather than a source for arsenic. Imam et al (1998) identified the ubiquitous presence of iron-rich coatings on sands from the affected aquifers. Further, PHED (1991) showed very high arsenic concentrations (2000mg/kg) in the ferruginous coatings of aquifer sands. Chemical analyses of sediments from the arsenic affected aquifers show a good correlation between iron and arsenic but no correlation between sulphur and arsenic. In addition, arsenic concentrations are low at or immediately below the zone of water table fluctuation. The combination of reducing conditions, the problem of accounting for the absence of sulphur, and the near-neutral pH all point strongly towards the oxyhydroxide reduction hypothesis as being the predominant mechanism of arsenic release in the Bengal Basin. 

Subsequent field and laboratory investigations have all confirmed the ‘oxyhydroxide reduction’ hypothesis. These studies include investigations in Meherpur by Burren (1998) and Perrin (1998), in Jessore by AAN (1999) and in Noakhali by Mather (1999), as well as the major national investigation by DPHE (1999). In addition, there have been two published discussion on the 'oxyhydroxyde reduction' explanation in although again unsupported by field evidence. The idea is based on the assumption that arsenic is present in the sulphide minerals pyrite and arsenopyride. According to the theory, lowering of the water table due to pumping introduces oxygen, which causes the breakdown of pyrite and releases arsenic, iron and sulphate into the water. Recharge during the subsequent monsoon then flushes the arsenic into the underlying aquifers. This explanation, whereby the arsenic pollution is caused by man’s over-exploitation of groundwater, became well known and led to calls for the banning of tulle well irrigation. In 1997, an alternative explanation was independently put forward by an Indo - Swedish group (Bhattacharya et al, 1997) and an Anglo - Bangladeshi group (Nickson 1997; Nickson et al, 1998 and 2000) and has become known as the 'oxyhydroxide reduction' hypothesis. Both these groups presented their ideas in international scientific journals where the origin of arsenic was the main subject of the papers. According to the theory, arsenic weathered from the hard rock areas of India is carried in the suspended load of the rivers adsorbed onto iron oxides or hydroxides. Following deposition of this fine sediment, decomposition of organic matter leads to strongly reducing groundwater conditions which causes dissolution of the iron oxyhydroxides and consequent release of arsenic into solution. This hypothesis also accounts for the pervasive occurrence of iron in wells beneath the floodplains.

Other studies that contain relevant data but do not take a strong position regarding the origin of arsenic in water include Ghosh and De (1995), Nag et at (1996), NRECA (1997), Safiullah (1998), Talukder (1998) and Johnston (1998). Other hydrogeological and/or geochemical studies are underway by the US Geological Survey, the Lamont-Doherty Earth Observatory and the Massachusetts Institute of Technology (MIT). From correspondence with these groups, no major revision of the presently accepted (oxyhydroxide reduction) explanation is expected, although clarification of the mechanisms and rates of the processes can be expected.

Public interest in the origin of arsenic contamination has important practical implications if there is a relationship between tube well irrigation and arsenic. From the process perspective, the oxyhydroxide reduction hypothesis is totally inconsistent with the idea that arsenic pollution is caused by water table lowering due to irrigation pumping. In fact, that idea suggests that lowering the water table will have the effect of removing arsenic from shallow groundwater, an idea that is supported by evidence from dug wells (see below). DPHE (1999) carried out statistical tests for an abstraction between the intensity of arsenic contamination and both groundwater abstraction for irrigation and the seasonal depth to the water table. They found significant negative correlations with both parameters. This finding is consistent with the simple observation that arsenic is absent in the intensively - irrigated Bogra region and the pervasive contamination in the modestly - irrigated (by groundwater) Chandpur - Noakhali region. While it would be unjustified to suggest that pumping has no effect on arsenic, it can be stated confidently that there is no evidence pointing to a causal relationship between tube well irrigation and the occurrence of arsenic in groundwater.

As a minor modification to the above explanation, Acharyya et al (1999), while accepting oxyhydroxide reduction as the dominant mechanism, have suggested that phosphatic fertilizers might enhance mobilization of arsenic in the soil zone by displacing arsenic from adsorption sites. However, they have presented no evidence to support this, and it is clear that even if correct, the effect would be minute compared to the quantity of arsenic released by natural processes.

C8.1.2.2 Anthropogenic Hypotheses

Newspaper and magazine articles have suggested various manmade sources of arsenic such as fertilizers, pesticides, wood preservatives and even acid-mine drainage or interventions such as the construction of barrages on regional rivers. None of these ideas have been presented in reports or journals where a process of peer-review is possible, and none has gained support amongst the scientific community. Virtually all have been presented without verifiable supporting evidence and hence should be classified as unsubstantiated speculations. Nevertheless some have gained attention in the popular media and therefore cannot be ignored. The wood-preservative suggestion was formally investigated by NRECA (1997) who concluded that there was no connection.

As noted above, the chemistry of arsenic and phosphate are related. The geographical distribution of phosphate and arsenic are similar. However, it is more likely that phosphate and arsenic share a common or closely related origin than that one causes the other. The hypothesis of a causal relationship with fertilizer use is strongly opposed by fire the virtual absence of phosphate in the intensively irrigated Bogra-­Dinajpur and Gazipur-Mymensingh regions. The similar rare occurrence of nitrate in groundwater, especially in arsenic-affected areas, also argues against widespread fertilizer pollution.

Claims for connection between barrage building and arsenic contamination are highly tenuous, requiring pyrite oxidation as a mechanism, which as discussed above is rejected by most workers. It is claimed that arsenic poisoning began after the opening of the Farraka barrage. The absence of diagnosed patients is almost certainly due to the absence of testing. Further, with the first diagnosis in 1978 and a lag time of five to 15 years, this points to contamination being present at least in the 1960s. In addition, DPHE (1999) argue geologically that contamination is present mainly in aquifer sediments deposited in the last 18,000 years. Confirming this, the International Atomic Energy Authority have carried out radio-carbon dating on arsenic-contaminated groundwater which points to residence-times of a few hundred to a few thousands years. Moreover, the regional pattern of contamination is inconsistent with such a cause. For instance, whatever impacts Farraka may have had on the South - West region, it is inconceivable that it could have had a major impact on groundwater levels in the north of the Sylhet Basin.

All of these anthropogenic hypotheses face profound difficulties in explaining the observed geographical occurrence (and absence) of arsenic in groundwater, and all lack supporting evidence. These hypotheses are rejected as a basis for strategy development.

C.8.2 Safe Levels of Arsenic in Drinking Water

Arsenic in the environment may exist in a number of organic and inorganic forms. The most toxic form of arsenic is arsine gas, followed by arsenite and arsenate which are the forms of arsenic normally found in groundwater. Organic forms of arsenic such as found in many foodstuffs are much less toxic (Morton and Dunnette, 1994; FDA, 1993) than the inorganic forms in drinking water.

Arsenic is both toxic and carcinogenic (Morton and Dunnette, 1994). The clinical effects of chronic arsenic poisoning range from skin ailments (keratosis and melanosis), through damage to internal organs to gangrene and various forms of cancer. The clinical effects of arsenic in drinking water in Bangladesh have been extensively demonstrated by Dhaka Community Hospital and NIPSOM amongst others.

The present drinking water standard adopted in Bangladesh and India is 0.05mg/l (50ppb) of (total) arsenic, which followed WHO advice at the time the standards were set. In 1993, the WHO proposed a new provisional guideline value for arsenic of 0.01mg/l (10ppb). This limit is based on practical limits of detection of arsenic and not on the normal health risk criteria. Conventionally, limits for carcinogens are set at the level of one excess lifetime cancer per 10,000 of population exposed. The provisional value of 0.01mg/l corresponds to an estimated risk of six additional skin cancers per 10,000 exposed persons.

The European Union has adopted the guideline value of 0.01mg/l, while Heath Canada has adopted an interim maximum acceptable concentration of 0.025mg/l (25ppb). The USA still works to a standard 0.05mg/l, but this year (2000) the US EPA has proposed that the standard should be reduced to 0.005mg/l (5ppb). This is expected to become law within a year. The US EPA also proposes a Health Goal (the level of no known or anticipated health effects) of zero.

It is anticipated that there will be demands to lower the standard for acceptable concentrations of arsenic in drinking water m Bangladesh. However, in considering the implications of tbe arsenic drinking water standard, it is important that the intervention strategies consider the dose-response relationships. As indicated above, the number of cases of arsenicosis resulting from drinking water at the current standard is only a small proportion of the exposed population. However, published dose-response curves from China (Lianfang and Jianzhong, 1994) suggests that the prevalence rate of arsenicosis rises to 10% at about 0.4mg/l, and to the order of 40% of the exposed population at concentrations of around 0.6mg/1. As discussed below, such concentrations are not rare in Bangladesh.

In summary, epidemiological evidence from around the world indicates that the present Bangladesh Drinking Water Standard does not correspond to a safe level according to the standard criteria of WHO and others. On the other hand, the dose - response function makes it imperative that interventions are prioritized according to the concentrations to which people are exposed.

C8.3 Occurrence of Arsenic

C8.3.l Testing for Arsenic

The most reliable determination of arsenic in groundwater is carried out by analyzing filtered and acidified samples in a laboratory using an atomic adsorption spectrometer. However, the scale of the problem in Bengal calls for a field test kit. Field kits, however, have often been criticized for being unreliable. In addition, DPHE (1999) have demonstrated the unreliability and lack of quality control in many laboratories in Bangladesh. However, DPHE (1999) also demonstrated that any error consistently underestimates the true level of arsenic, and that field - kits can provide valid regional descriptions of arsenic occurrence. 

The practical detection limits of field - kits are close to the present drinking water standard of 0.05mg/l, and become unreliable for waters close to this value. Importantly, field kits do reliably identify highly contaminated wells. However, if a lower drinking water standard is adopted, the present field kits will not be able to differentiate between safe and unsafe water. The only field method applicable at these concentrations is a prototype field - kits known as the “Arsenator”.

The most comprehensive, systematic and reliable survey of arsenic in groundwater was carried out under DPHE’s Groundwater Studies for Arsenic Contamination in Bangladesh project in 1998 and 1999. The summary statistics from their national survey (excluding the Chittagong Hill Tracts) are given below in Table C8.3.l and they represent the least biased assessment of the groundwater affected by arsenic contamination. However, because the population density is not evenly distributed across the country, these percentages cannot be directly transformed into population affected.

Table C8.3.1: Frequency Distribution of Arsenic Concentrations in Groundwater

Concentration Class (mg/l)

No in Class

% of Wells in Class

Cumulative No. of Exceeding

Cumulative % Exceeding




































All Samples




Source: Groundwater Project for Arsenic Contamination in Bangladesh project and http://www.bgs.ac.uk/arsenic.

C8.3.2 Comparative Analyses

The DPHE study showed there are problems in measuring the concentration of arsenic with accuracy even in a laboratory, which need to be recognized when evaluating survey results. This shows up most clearly when the results of tests carried out in DPHE laboratories are compared with those carried out on the same samples in the UK, as is done in Table C8.3.2 below. This shows a statistically significant relationship that indicates the DPHE laboratory results overstate the concentration of As by 25% on average. This does not greatly change the findings about whether a particular well is harmful to health or not. However, differences can be large at higher concentrations.

Table [sic] C8.3.2: Comparison of DPHE and BGS Analyses (Click on the table for a larger view)

The differences are particularly important when reviewing the the occurrence of As in the deep wells, As in Table 8.3.3 below. Whereas DPHE results indicate a proportion of deep wells (>200m deep) exceed Bangladesh standards, this is not confirmed by BGS control tests.

C8.3.3.Comparison of Contamination with Depth, DPHE and BGS (Click inside for full view)

In the initial stages, it was suggested that testing had been concentrated in areas where arsenic was known to occur, a procedure that would lead to an over-estimation of the extent of the problem The plot in Table C8.3.4 below shows that this was not the case, as there was no meaningful relationship between the number of tests per Thana and the proportion found positive. 

Table C8.3.4: No of Tests V. Proportion Proving Positive

C8.3.3    Regional Distribution of Arsenic in Groundwater

The principal source of information on the distribution of arsenic comes from the surveys and data compilations of the Groundwater Studies for Arsenic Contamination project (DPHE, 1999). From the perspective of human exposure, the most useful method of representing the geographical distribution of arsenic is percentage of wells that exceed the drinking water standard. In this form it is possible to combine laboratory data with field kit results to access a much larger database. Combining this data on l8,000 tests in over 3,000 unions with the NRWD GIS map of unions, it has been possible to produce a probability Of arsenic occurrence across the country as shown in the four maps in Figure C8.3.1. The percentage of wells contaminated was assigned to the centre point of the union and then converted to a smooth surface using the inverse distance-weighted technique. 

The maps in Figure C8.3.l show the probability of exceeding the four threshold concentrations of 0.01lmg/l (the WHO guideline value), 0.05mg/l (the current drinking water standard), 0.20mg/l and 0.40mg/l. No depth separation of wells has been applied in this map, but, as described below, the deep aquifer system is almost completely uncontaminated. If a lower drinking water standard (e.g. 0.01mg/l) is adopted the affected area will expand considerably. On the other hand, the areas in which extremely high arsenic concentrations occur are much more restricted, pointing the way to prioritizing interventions.

Figure C8.3.1 shows that arsenic-contaminated groundwater may be found in most parts of the country but is strongly concentrated in the South West, South East and North East. In large parts of the South East, more than 70% of the wells are contaminated. In the case of Hajiganj upazila of Chandpur District, Jakarya et al (1998) tested all 12,000 wells and found that 93% were contaminated. There is currently very little data on the occurrence of arsenic in the Chittagong Hill Tracts, which are geologically very different from the areas tested.

The picture presented in Figure C8.3.2, which shows the occurrence of arsenic in wells deeper than 200m, is very different. Although far fewer deep tube wells have been tested, it is clear that the risk of contamination at this depth is much lower.

The regional patterns of arsenic distribution is closely related to its origin by reduction of iron oxy-hydro xides and has been described in detail by DPHE (999), and the key features are described only in summary here:

  • Arsenic occurs in the catchments of all major rivers of Bangladesh. This demonstrates the existence of multiple source areas.

  • Aquifers beneath the hills of greater Sylhet and terrace areas (Barind and the Madhupur Tracts) are virtually free of arsenic contamination. These aquifers are formed of older sediments from which arsenic has either flushrd out or has been immobilized.

  • Of the major rivers, the Brahmaputra and Teesta floodplains are least affected, while Meghna floodplains are worst affected.

  • Over large parts of North west and North Central Bangladesh and the Chittagong Coastal Plain the probability of wells being affected by arsenic is less than by 5%.

  • Arsenic is far less likely to occur in aquifers less than 200m.

Figure C8.3.1:Probability of Arsenic Exceeding Threshold Values (Click on the image for a better view)

Figure C8.3.2: Arsenic in Wells with Depth Greater than 200m (Click the inside for a larger view)

C8.3.4 Local Variation of Arsenic in Groundwater

The preview section described the systematic regional scale variation of arsenic. However, within file affected areas studies have shown striking and often systematic variations of arsenic concentrations on a scale of hundreds of metres to a few kilometres related to specific geological features. These variations take three main forms. First, at the boundaries of older units such as the Madhupur and Barind Tracts are sharp, predictable and sometimes extreme (DPHE, 1999). Sometimes within the floodplains the patterns of distinct sedimentary features such as buried channels or ox-bows can be identified in the arsenic maps (Safiullah, 1998; AAN, 1999). These features ate mappable and represent distinct contaminated and uncontaminated sections of aquifer, but they are not predictable from surface expressions. In other cases (eg Burren, 1998), the patterns could be related to mapped soil associations. At a larger scale, distinct higher or lower probabilities of contamination can be associated with different floodplain units (DPHE, 1999) and can be used for prioritizing surveys, but it is not clear how reliable the boundaries of these units are for predicting the occurrence of contaminated wells.

C8.3.5 Variations with Depth and Time

The probability of wells being contaminated is strongly related to the depth at which they are screened. The probability of exceeding the drinking water standard in the upper aquifer system is high, but as shown in Table C8.3.5, in wells screened at lower levels, the risk from arsenic is minimal. There has been some controversy (Chowdhury et al, 1999) as to whether arsenic increases or decreases with depth. However, as shown below and elsewhere, there is evidence that an increase in depth of screening is related to a sharp initial increase in the occurrence 0f contamination followed by a gradual decease.

Table C8.3.5 Arsenic Occurrence in Groundwater by Depth of Well

Depth Range

No of Wells

No >0.01mg/l (WHO Standard)

No >0.05mg/l (BWDS)

No >0.20mg/l

Maximum (mg/l)











































Source: Groundwater Studies for Arsenic Contamination in Bangladesh

It is clear from the table, in terms of both the probability of contamination and the maximum probable concentration, that well depth provides a clear basis for an arsenic avoidance strategy, at least in the short to medium term. Nevertheless, it is also important to note that in the DPHE survey 58% of the wells have concentrations below the WHO guideline and 75% of them conform to the current drinking water standard. Therefore there is no basis for a general abandonment of the shallow aquifers, again at least in the short to medium term. Figure C8.3.3 compares the depth-profile of the probability arsenic exceeding 0.05mg/l with the typical depth ranges of different types of wells, showing how different technologies are differentially exposed to arsenic.

Figure C8.3.3 also highlights a regional anomaly in Sunamganj where, due to the extreme rates of tectonic subsidence (4mm/yr), arsenic-affected aquifers occur deeper than elsewhere in the country.

A certain note of caution should be observed regarding the safety of deep aquifers. Wells deeper than 200m have only been extensively tested in the coastal area. The extension and safety of such deep aquifers in inland areas should be proven and should not be assumed. The question of sustainability of pumping from deep aquifers is addressed below.

Despite an obvious lack of long-term monitoring data DPHE (1999) have deduced that the proportion of wells that exceed the drinking standard plus concentrations of 0.10, 0.15 and O.20rng/l) increases with age up to an age of eight to ten years. In the long-term, concentrations of arsenic should decline as the stock of arsenic in the sediments is depleted. DPHE (1999) supported by modeling studies by Cuthbert (1999) attribute this to slow leakage from overlying aquitards. Their practical conclusion is that ensuring a distance of 10m or more between the well screen and an overlying aquitard massively reduces the risk of delayed contamination. Unfortunately, it appears to he common (and understandable) practice to screen at the top of the first available aquifer. The question of seasonal variations of arsenic concentration is uncertain. AAN (1999) have certainly reported differences between seasons, hut have not clearly proven seasonal fluctuations. Such fluctuations are possible, especially in shallower wells and where pumping is most intense. The Eighteen District Towns Water Supply Project (18DTP, 2000) periodically monitored selected wells from 1996 to 1999, but reported no consistent trend or pattern. Ahmed (2000) reports that nested monitoring wells installed in three Thanas under Phase II of the Groundwater Studies for Arsenic Contamination project showed no significant changes over seven months of monitoring except for a rise in the first few weeks after installation. The latter concentration has obvious implications for the testing and commissioning of new wells.

Figure C8.3.3 : Depth Distribution of Arsenic in Groundwater (Click on the Figure for a larger view)

C8.4 Arsenic in Water Sources

C8.4.1 Arsenic in surface Water

Arsenic contamination in Bangladesh is principally a problem in groundwater and it is often stated that surface water is safe from arsenic. However, concentrations up to 0.5mg/l (500ppb) have been reported from a partially dried up river in Meherpur (Burren, 1993). While this is believed to be an exceptional ease, it demonstrates that surface water cannot be assumed to be safe from arsenic. It is anticipated that arsenic contamination of surface water is most likely in ponds or in relatively stagnant water bodies that are fed by groundwater flow. This is likely to be a seasonal phenomenon, occurring in the dry season. The occurrence of arsenic in some surface waters also raises the possibility of arsenic uptake by fish.

C8.4.2 Arsenic in Dug Wells

It was noted earlier that the risk of arsenic contamination is low in very shallow groundwater. It has been widely reported that dug wells are safe from arsenic. Dr. Dipankar Chakraborty reports that analyzing more than 100 dug wells from West Bengal and Bangladesh and that none exceeded 0.05mg/l. The problem with dug wells is not arsenic, but the risk of diarrhoeal disease.

C.8.4.3 Arsenic in Tube wells in the Shallow or Main Aquifer

The majority of private water supplies are obtained from dried tube wells between 10 and 100 metres deep. There are no reliable figures but there may be between eight and eleven million such wells the country. These sources pose the principal threat to public health from arsenic. Nevertheless, the majority (>70%) of such wells have arsenic contamination levels of less than 0.05mg/l and they also pose a low risk of diarrhoeal disease. As noted above arsenic concentrations may increase over time (most probably in wells screened very close to aquitard layers). The continued safety of presently uncontaminated wells is an important water management issue.

C8.4.4 Arsenic in Deep Tube wells

Because of the differing nomenclature in irrigation and water supply circles, the term ‘deep tube wells’ regularly causes major confusion. Reports from BADC that arsenic occurs in deep tube wells can cause confusion because BADC refer to wells with depths in the range 150 to 350 feet (50 to 110cm). In addition, BADC set screens at depths of 50-80m, thus drawing water from the shallow aquifer, which is known to be contaminated.

Here, the term is used to refer to wells screened in aquifers deeper than 200m. However, it is strongly recommended that depths are always quoted when this subject is discussed. Most wells screened below 200m have arsenic concentrations below detection limits and less than 2% exceed the Drinking Water Standard, and even then only by a small amount. The distribution of arsenic in wells deeper than 200rn is shown in Figure C8.3.3, from which it is clear that the majority conforms to even the proposed new US standard.

C8.4.5 Changes over Time

The question is often asked if the arsenic map' will change in a few years' time. Of course, some change will occur as more data are collected and uncertainty is reduced. It is possible, but not at all certain, that there will be an increase in the percentage of wells contaminated in the badly affected areas, however, it seems unlikely that new areas of contamination will emerge. The greatest change that can be anticipated in the ‘arsenic map’ will come from a change in the regulations governing drinking water standards and not from any hydrological processes.

C8.5 Impact of Arsenic on Drinking Water Supplies

C8.5.1 Population Exposed

A simple assessment of the population exposed to hazardous levels of arsenic was made using the 1991 census and the National Arsenic Survey of the Groundwater Studies for Arsenic Contamination project carried out in 1998 and 1999. That survey is the only one that has a large, evenly spaced and reliable arsenic concentration data over the whole country (excepting the Chittagong Hill Tracts). The arsenic distributions shown earlier in' Figure C.8.3.2 have been combined with the population distribution to estimate the exposed populations at the 10, 50, 200 and 400ppb levels. The estimates are made for the year 2000 assuming a population growth rate of 2.2% per year and that 95% of the population draw their water from groundwater supplies. No explicit account has been taken of the difference in arsenic between the deep and shallow aquifers, however, the survey design was stratified to reflect the importance of deep and shallow aquifers in each upazila. The results of the assessment are given tnTahleC8.5.1 below.

Table C8.5.1. Estimated Population Exposed to Arsenic in Drinking Water

Arsenic Concentration

Exposed Population in 2000









Total Population


Notes: Table excludes the three CHT Districts due to lack of arsenic data.

The distribution of population exposed is somewhat different to the arsenic distribution itself because the most affected areas of the south - east of the country also correspond to the areas of highest population density.

C8.5.2 Options for Avoidance or Treatment

Preventing exposure to arsenic in drinking water by providing arsenic-free water supplies is the essential requirement for the improvement of health in the affected areas. There are three basic approaches:

The distribution of population exposed is somewhat different to the arsenic distribution itself because the most affected areas of the south - east of the country also correspond to the areas of highest population density.

C8.5.2 Options for Avoidance or Treatment

Preventing exposure to arsenic in drinking water by providing arsenic-free water supplies is the essential requirement for the improvement of health in the affected areas. There are three basic approaches:

The distribution of population exposed is somewhat different to the arsenic distribution itself because the most affected areas of the south - east of the country also correspond to the areas of highest population density.

C8.5.2 Options for Avoidance or Treatment

Preventing exposure to arsenic in drinking water by providing arsenic-free water supplies is the essential requirement for the improvement of health in the affected areas. There are three basic approaches:

  •  Treatment of arsenic contaminated groundwater;

  •  Develop arsenic-free groundwater sources; or

  •  Develop surface water sources such as rivers, ponds and rainwater. All three approaches have relevance in different settings, and all have some difficulties. Hydrological and climatic factors obviously constrain the technical selection of methods, while the scale (eg household, community, municipal) of the operation will determine their social and economic suitability. Detailed discussion of the selection of methods is given elsewhere in the DDS report. The choice of method must also be considered in relationship to different drinking water standards. It is known that common methods such as aeration and coagulation-filtration all effectively reduce arsenic to around 0.05mg/l but not to 0.01mg/l.

C8.6 Trends of Arsenic in Groundwater

Following the description of the occurrence of arsenic earlier, a number of sustainability questions arise which can be divided into those affecting the shallow or main aquifer (i.e. wells less than 100-120m deep) and those in the deep aquifers (wells more than 150-200m deep). Important questions affecting the use of the shallow aquifers are:

  • Will presently uncontaminated aquifers become contaminated in the future? This relates particularly to the extensive uncontaminated aquifers such as the Dupi Tila sands beneath the Madhupur and Barind Tracts.

  • Will uncontaminated wells in partially contaminated aquifers become contaminated in the future? There are many cases of sharp lateral variations of arsenic concentrations within villages, giving rise to obvious fears of lateral migration of arsenic.

The issues relating to the sustainability of abstraction from the deep aquifers are:

  •  Will wells in deep (>150-200m) aquifers that are overlain by contaminated aquifers become contaminated by downward leakage of arsenic? This is of greatest concern where increased pumping from the deep aquifer is accompanied by the abandonment of existing wells in the upper aquifer. This is usually a local problem, where the degree of risk depends on the intensity of abstraction and the properties of the intervening strata.

  •  Will increased pumping from deep aquifers induce saline intrusion in the coastal areas? Unlike the above concern, the risk results from the cumulative effects of deep well abstraction over a large region and will impact most on water supplies closest to the coast.

The above four questions may then be reformulated as three hydrogeochemical questions:

(a) Will conditions in uncontaminated aquifers change sufficiently to mobilize from sediments where arsenic is presently stable in the solid phase?

(b) At what rates will water, salinity (a conservative tracer) and arsenic migrate laterally and vertically through the aquifers?

(c) If arsenic is drawn into presently uncontaminated layers, will it remain in solution or be adsorbed on to the solid phase?

Regarding question (a), given that the main driving force for mobilization of arsenic is decay of organic matter in the sediment and this is a finite quantity, the answer is likely to be ‘no’. Only in the exceptional circumstances such as infiltration of landfill leachate is groundwater likely to become so reducing as to mobilize arsenic from the sediment.

With reference to question (b), tile migration of arsenic is constrained by the rates of water movement and by the effects of adsorption. Both have been studied by numerical modeling by DPHE (1999) arid Cuthbert (1999). There is currently significant uncertainty concerning the quantification of sorption parameters for arsenic. Both studies show even without including the effects of sorption that, under realistic scenarios, arsenic will not move more than a few meters a year laterally or vertically. With only modest sorption, the migration rates drop to less than a meter per year.

Uncertainty concerning sorption parameters prevents a precise answer to question (c). However, the direction of the process is clear and it will operate to some degree to slow down arsenic mitigation.

C8.6.1 Shallow Aquifers

Extensive areas presently free of arsenic are expected to remain so. The reasons are twofold. In the coarser aquifers (such as beneath the Brahmaputra and Teesta floodplains), little arsenic and organic matter accumulated at the time of deposition. In the older aquifers (such as the Madhupur and Barind Tracts and Dhaka City), arsenic has either been flushed out or is combined with more stable iron minerals (such as hematite or goethite) and the aquifers' redox conditions are apparently not sufficiently reducing to mobilize any arsenic that may he present in the sediment.

The future trends of arsenic in partially contaminated areas are more difficult to predict. The modeled migration rates and inferred increases over time strongly suggest that vertical leakage is the critical factor, leading to delayed contamination of tube wells. This may be expected to continue if well design practice is not changed. On the other hand, the risk of contamination due to lateral migration of more than a few tens of meters is small. For the 25-year planning horizon of the Plan, the probable upper limit of lateral migration is considered to be of the order of 50 meters, and vertical migration smaller still. Further research is needed to provide better estimates of migration rates for arsenic, particularly where inter-aquifer movement is involved. Sufficient is known to be confident of no very rapid movement, and some research is underway on this subject, but the results should be reviewed after a few years.

C8.6.2 Deep Aquifers

There is very little arsenic contamination in aquifers below 150-200m. The main and deep aquifers are usually separated by lower permeability strata. The vertical hydraulic resistance and anisotropy of the intervening strata suggest that the migration of arsenic from contaminated shallow aquifers would occur in a period of not less than decades and probably centuries. If the probable, but poorly quantified, effects of sorption are included, it is like~ that the deep aquifer system offers the prospect of permanent or at least very long term, sources of drinking water that is free of bot arsenic and bacterial contamination.

Use of the deep aquifer system will be of most importance in the coastal regions of Bangladesh. There the risk of arsenic must be assessed in parallel with that of salinisation. In this hydraulic situation, the critical issue is the source of water that replaces the water pumped from deep wells. While existing performance in the most heavily abstracted areas such as Khulna City is encouraging, it is impossible at present to say how much more abstraction can be supported. There is a need for a detailed and systematic assessment of the resources of the deep aquifer system to be implemented in parallel with the water supply development.

C8.7 Arsenic, Irrigation and Agriculture

The question as to whether there is a causal relationship between arsenic and tube well irrigation was addressed earlier, and it was concluded that here is no such causal link. However discussion of the inter-relation of arsenic and irrigated agriculture must he extended to ask whether arsenic in irrigation water affects the growth of crops, and whether arid how arsenic in irrigation water or soil might accumulate the food chain?

C8.7.1    Arsenic in Food grain

There are few analyses on arsenic in food grain in Bangladesh. However, no reports are available of analyses of rice or wheat containing arsenic at levels of health significance. Most analyses show arsenic below detection limits. 0f the food grains rice has the most potential to take up arsenic due to the anaerobic root zone (Chappell, 2000). No comprehensive study has been carried out, although a study is underway at CSIRO in Australia and results are expected in late 2000. The CSIRO study will make a survey of arsenic in vegetables and crops, plus their associated soils and water sources across Bangladesh and West Bengal. The long-term objective is to integrate the arsenic-crop-soil-water-medical relations to develop a landscape risk assessment model.

A case study of arsenic at two shallow tube wells in Faridpur by Hasan (1998) is described in Table 8.7.1 below. When considering the total arsenic concentrations in soil, it should be remembered that the arsenic may be present from the time of deposition of the sediment and has not necessarily been added from irrigation water. The high arsenic concentrations in soil at the new well with a low arsenic concentration in groundwater make this clear. Chappell (2000) reports that an arsenic content of 10mg/kg is typical for soils world-wide. Certainly the arsenic in the Devinagar soi1 profile could not have been accumulated from irrigation water in the single year of operation. This therefore makes it uncertain as to whether there has been significant accumulation of arsenic in the Shasa profile. The major uncertainty in the processes is the quantity of arsenic removed from the system (to the atmosphere) by biomethylation under the action of fungi, yeast and bacteria.

Table C.8.7.1 Arsenic Analyses from Irrigated Rice Production in Faridpur

Sample Type

Range of Arsenic Concentrations (mg/l or mg/kg)


Well-1 (Shasa Est. 1989)

Well-2 (Devinagar Est. 1998)

Tube well

0.246 (Fe - 8.8)

0.083 (Fe - 3.5)

Field Channel

0.198 (Fe - 4.5)

0.075 - 0.080 (Fe - 3.0)

Soil Samples

6.6 - 10.6

5.3 - 8.0

Rice Root



Rice Shoot



Rice Leaves



Rice (not paled)



Rice (paled)



Source: Hasan (1998)

At the two STW sites, no arsenic was found in the rice, but high arsenic concentrations were found in the root. This is consistent with the physiology of rice and other plants where the distribution of arsenic is in descending order from root to stem to leaf and edible parts (Huang, 1994). Also the results from the 'Devinagar' site suggest the possibility that arsenic in the root might be derived from the soil. There is also evidence that arsenic concentrations are being reduced by aeration along with iron between the well and the field.

The important conclusion from Hasan’s study is that irrigated rice plants may be exposed to sources of arsenic in soil and water without transferring the arsenic to the directly edible parts of the plant. However, the question as to whether the cultivation of irrigated rice is sustainable is fundamental to the formulation of the NWMP and any serious uncertainty in this area must be explicitly addressed.

C8.7.1.1 Arsenic in Vegetables and Fruits

Professor Imamul Huq of Dhaka University has shown that arsenic may accumulate in the edible parts of leafy vegetables. Although these are not specifically water management issues, they are extremely important and require urgent investigation. It is expected that the CSIRO will provide a major insight on these issues by the end of 2000.

C8.7.1.2 Other Pathways

In addition to direct consumption of arsenic in fruit, grains and vegetables, it is wise to consider whether there are indirect exposure pathways. It was noted earlier that there is slight accumulation in rice straw which is used as feedstuff for livestock; indeed any other fodder grown on arsenic contaminated soils are potential pathways of exposure. In particular it is important to check for mechanisms of bioaccumulation in different parts of the animals' anatomy. No data information has been identified from Bangladesh on this subject. However, the CSIRO study noted above includes animal studies of the bioavailability of arsenic from vegetables and it also quantifies ingestion of arsenic from processed foods such as cooked meat and milk.

Another potential exposure pathway is consumption of fish kept in groundwater fed ponds. The Noakhali Aquaculture Extension Project has analyzed tissue and liver from Common Carp, Gras Carp, Mrigal, Golda and Ruwi. Arsenic ranged up to a maximum of 2.0mg/kg of dry weight. The samples also contained between 0.11 and 0.66mg/kg (of dry weight) of mercury. A significant part off the arsenic found in fish is usually in less toxic organic forms, and the health significance of arsenic in fish is yet to evaluated.

C8.7.1.3 Yield Reduction in Rice

FAO Irrigation and Drainage Paper 29 (Water Quality for Agriculture) reports that arsenic is toxic to rice, and causes a reduction of yield, at the same levels as it is toxic to humans in drinking water (0.05mg/l). Based on studies in Japan, Huang (1994) notes that although there is a weak correlation with total arsenic content there is a strong correlation with soluble arsenic in the soils. The factors controlling crop yields are many and complex and some factors rarely dominate yields under normal conditions. The effect of arsenic on rice yield remains an area of uncertainty for water management, and it is believed (Chappell, 2000) that yield reduction in rice occurs at concentrations below those resulting in visible symptoms of photo-toxicity.

Arsenic is also photo - toxic to soybean, cowpeas and oats.

C8.7.2 Spatial Distribution of the Arsenic Hazard in Agriculture

Reference to Figure C8.3.2 shows that most of the areas of intensive ground water irrigation ( in the greater Districts of Dhaka, Mymensingh, Bogra and Rangpur) do not correspond to areas of severe arsenic contamination. The areas of potential risk are most in the Kushtia-Jessore and Chandpur-Brahmanbaria regions.

C8.7.3 Preliminary Assessment and Research Needs

Groundwater with potentially toxic levels of arsenic is being used as a source of irrigation water in significant areas of Bangladesh. Although no conclusion on the scope for yield reduction can be given at this time, some preliminary conclusions can be given on the scope for arsenic accumulation in food grains and vegetables.

  • Analyses for rice grains give little indication of bio - accumulation of arsenic.

  • Arsenic in food - stuffs is usually less toxic organic forms.

  • There is evidence that rice grown with contaminated irrigation water does not accumulate arsenic in the grain.

  • Arsenic in irrigation water may reduce the yield of rice.

  • Arsenic is accumulated by some vegetables.

  • There is shortage of relevant data.

Thus it might be concluded that there is no direct evidence to support a change in policy in the use of groundwater for irrigation from contaminated aquifers at this stage. However, there are reasons for caution in acting on this conclusion. First, the present data-base is too small to give a high degree of confidence. Second, the possible consequences of arsenic accumulation through long-term irrigated agriculture are simply too important to be left as a matter of scientific judgment. Intensive studies of the present situation and initiation of long-term monitoring are essential. Two lines of study are recommended below.

Long-term (ten to 20 years) monitoring of crop, soil and water monitoring should be established at say ten to 20 sites under differing agro-ecological conditions. The monitoring would concentrate on changes in the water and arsenic balances using the tube well catchments as a unit of measure. This is considered essential, but unfortunately will take many years to reach a definite conclusion, and a more rapid conclusion is needed..

DPHE (1999) used a cross-sectional sampling technique to infer with a high degree of confidence the temporal changes in arsenic concentration at wells. Beyond any reasonable doubt, arsenic contaminated irrigation water has been used in some areas for 20-30 years. If accumulation is a significant problem, it should already be apparent at older schemes. A stratified or randomized survey can therefore be designed to collect water, soil and crop samples at wells of different ages and with varying arsenic concentrations. Arsenic concentrations can therefore be tested against the cumulative loads of both arsenic and water applied. The survey may also be aggregated to identify the influence of soil type and climate. If the sample site is large enough, it should be possible to deduce the effect of arsenic concentration on crop yield. Other Groundwater Quality Issues

A summary of analyses of 3538 evenly spaced samples is given in Table C8.7.2 below, taken from the GSAC National Survey. In addition to arsenic the frequent exceedances of the health related parameters boron and manganese and occasional exceedances of barium and chromium. Although not-health related, iron exceeds that national standard at 4l% of wells.

Table C8.7.2 : Summary of Groundwater Quality Baseline Survey


Drinking Water Standard

Wells Exceeding

Maximum Concentration





Ratio to Standard







WHO guideline is 0.01 mg/l







New WHO guideline since 1998




























Not health related, but WHO aesthetics guideline is 1.0, and 0.3 preferred for public supply







Aesthetics guideline


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