Arsenic in the Main Report

Bangladesh is affected by one of the worst cases of groundwater contamination by arsenic in the world. Arsenic was first detected in West Bengal in 1978, but it was not until 1997 that it was recognized that arsenic extended aver large parts of Bangladesh.

The first national survey was completed by end-1998, but further surveys have extended the area known to be affected.

Arsenic can also affect human health by entering the food chain. The effects of arsenic in vegetable and crops, and the associated soils and water sources, are under study, but no results are expected to he available before end-2000. The typical concentration of arsenic in soils world-wide is 10mg/kg, while the few samples available so far in Bangladesh are in the range 5.3 to 10.6mg/kg. Arsenic has not yet been found in grain, although it is taken up in rice roots, which may be eaten by livestock. However, It may be taken up in leafy vegetables and also in fish, especially crustaceans. Arsenic in all these may be in the less toxic organic form, but the impact on health needs to he fully assessed. FAO reports that arsenic can reduce the yields of crops, even at concentrations below those at which crops show signs of phytotoxicity.

Several possible sources of arsenic Bangladesh have been put forward, as discussed in Annex C, but it is now accepted that the source is geological, transported by rivers from sedimentary rocks in the Himalayas over tens of thousands of years, rather than anthropogenic. Two mechanisms for the release of arsenic have been put forward, the “pyrite oxidation” and the “oxyhydroloxide reduction” hypotheses, but the weight of evidence now available supports the latter. The first associates the release with oxidation due to draw down of the water table, principally by irrigation abstraction, the second with reduction caused by decomposition of organic matter in the sediments.

The issue is important, as if the first hypothesis were true, an embargo on tube well irrigation might remedy the situation, although at a huge cost to the economy. Fortunately, no such decision appears to be required. Similarly, suggestions that phosphate in fertilizers or upstream abstraction of water from major rivers may worsen the situation appear unfolded. The probability of arsenic exceeding threshold values is illustrated in Figure 3.1(see annex C for full details).

The majority of tests to date have been carried out on shallow tube wells used for drinking water supply. Significant numbers of tests have also been carried out on deep tube wells used for drinking water, down to depths of 300m or more and other wells (also referred to as deep tube wells) down to 100m,used for agriculture. The tests show that at depths below 200m, the incidence of contamination falls off and at 250rn or more it is rare. Confusion between feet and meters has led to conflicting reports of contamination at depths greater than 250m in agricultural wells, but these have been followed up by NWMPP and shown to he incorrect.

In general, it appears that water drawn from depths greater than 250m is, and will remain, arsenic-free provided that irrigation wells do not start using the same aquifer. Such wells usually have better water quality in terns of iron and other metals and the same hardness as shallower wells. This deeper aquifer is likely to remain a potential source for drinking water in virtually all areas affected either by arsenic or areas of seasonally low water tables, using Systems discussed in Chapter 7.

Making arsenic-free water available to people in areas with badly contaminated water must be treated as a priority. Many of these people have invested in their own hand pumps and are reluctant to stop using them, particularly if safe sources are some distance away or their owners are reluctant to share their use. People who have invested in their own hand pumps, following GoB advice, are unwilling to pay for arsenic removal. A number of options for household treatment of arsenic contaminated water have been subjected to pilot testing, which is still ongoing. Treatments based on alum or potassium permanganate, which looks promising, are now considered unsafe due to potential health risks arising from excessive dosages, while the treatment alternatives may produce arsenic-rich residues which will require safe handling and disposal.

Arsenic-contaminated water is also found in some DTW-based urban supply systems. Tests by the 18DTP project showed that where iron was also present, an iron removal plant could reduce the arsenic content to below the current limit for acceptability. However the arsenic removal efficiency dropped as the iron content reduced. Ensuring adequate maintenance of the plants is also difficult. The cost of treating supplies into piped systems is effectively increased because all water is treated, not just the water needed for drinking and cooking. Investment in a removal plant has to be considered against the cost of a deeper borehole tapping arsenic-free water.

Unless safe, convenient and affordable arsenic treatment can be made available, treatment of water must be considered as a short-term measure, pending alternative arsenic-free supplies being made available. It is also likely that in the long term, the acceptable limit for arsenic will be lowered to the WHO standard, which will tend to make treatment more difficult and expensive in addition to being required in more areas than at present.

Current information indicates that the risk of arsenic occurring in groundwater drawn at a depth of 300rn is very low. The iron content also tends to be lower at this depth compared with shallower water, although the hardness may be higher. Where groundwater is available, even if it is deep, it is likely to be the most convenient and cost-effective source. Wells of the required depth will not be affordable to individual users but can form the basis of shared systems for relatively small groups. Currently it is difficult to drill large diameter DTWs to this depth in some areas. This can be solved by either using smaller wells or different drilling equipment.

7.7.4 Piped Distribution Systems

Piped systems offer the convenience of water delivered to the household. For users who have invested in their own hand pumps, a piped supply represents the next step on the path of progress. Piped distribution can be used to share the cost of developing a safe source of water, whether a deep tube-well or surface water source. The basic issues with piped distribution are the sharing of costs and the reliability quantity of supply. One impediment which prevents existing urban piped systems reaching all potential users in their service areas is the size of the connection charge. This may need to he subsidized for the poorer users, or provision made for payment by installment over one or two years.

Existing piped systems frequently have very high consumption per person connected. The causes of this are leaks and wastage, both at the system and

Large-scale piped systems also provide the potential to bring water into areas without adequate local sources. For example coastal regions can be supplied from fresh water sources further inland. However, the capital investments needed are substantial.

NWMPP has prepared a detailed proposal for a small piped system to serve peri-urban and rural areas based on a small DTW, which is included as Annex L Appendix 1. In summary, the system would use a well of about 1l/s capacity, which would serve up to 1000 people through small-bore pipes sized to deliver about 50l/c-d to storage tanks in the house or bari. This option offers considerable potential for cost sharing: GoB could fund the well as a means of providing a source of safe drinking water; the householders would par for their storage tanks; and a private operator would provide the distribution system.

7.7.5 Summary Discussion

The most immediate short-term requirement is the provision of safe drinking water in arsenic-affected areas. This can be accomplished either by treatment, which is unlikely to be a satisfactory long-term solution, or by development of alternative safe water sources as a matter of urgency, which can form part of the long-term investment in improved water supplies.

Consumers are reluctant to pay for arsenic removal or for less convenient alternative supplies than their own hand pumps, while GOB can ill afford the cost of providing treatment or alternative resources. The strategy must therefore be to move towards systems, which will attract investment by consumers because they represent an improvement in convenience of access, while also being sustainable in the long term. GoB funding for both capital and recurrent costs can then be reduced. The systems must be designed around consumer demand and actively involve women in their formulation and operation.

Substantial further investment is also required for water supplies in towns, some of which have existing systems although the coverage is often limited. The conventional systems based on large DTWs will be appropriate many areas, provided the wells are deep enough to provide safe water. Funds for expansion, sustainability and achieving higher coverage are the main issues. Government policies advocate involvement of the private sector, which needs to be given greater encouragement to respond directly to consumer needs, rather than government agencies acting as intermediaries who may opt to involve the private sector when it suits them.