Contents
- 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
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:
- use of surface water with treatment by pond sand filter
- sinking deep wells into the arsenic-free aquifers
- rain water harvesting
- ring wells to tap the very shallow aquifer
- solar-assisted oxidation and sterilisation of existing groundwater and
- arsenic treatment at various scales.
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:
Low contamination (less than 10% of wells): areas where occasional ‘hot spots’ are possible. The ‘omission’ of Chapai Nawabganj in the randomly based Regional Survey highlights the difficulty of locating all of these. Although these areas may receive lower priority for comprehensive testing, a more efficient approach might be to conduct more intensive randomised sampling across these areas, supplemented by local comprehensive surveys as and when hot-spots are located;
Medium contamination (10-60%): extensive, preferably comprehensive, testing will be required especially in the western region and in the north-east where the pattern of contamination appears to be patchy;
High contamination (greater than 60%): two approaches are possible. Either the areas can be considered so highly contaminated that the future effort should concentrate more on mitigation than further testing or use further testing to locate the minority of safe wells and then use only these. Much depends on what other options are available locally. These should be high-priority areas for rapid screening.
Shallow saline groundwaters also contaminated by arsenic: the shallow wells in these areas are not used because the water is too saline to drink. Deep wells are used in the south of Bangladesh to avoid this problem and these have a very high probability of being safe. However, all new wells should be tested before commissioning.
Limited water quality monitoring over the last few years shows that the arsenic concentration of some wells has increased slightly. However the results do not show 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 may increase with 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. However in the context of an arsenic mitigation strategy, it would be advisable to adopt a precautionary approach and to assume that the arsenic concentration could increase slowly with time. 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 a broad scale and statistically-based long-term monitoring programme for both the deep and shallow aquifers.
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. There is also 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 supernatant. 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 properly.
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.
Both field and laboratory testing are required on a massive scale. Existing field test-kits have an essential role to play but ideally should have better accuracy and 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 modern 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 modern automated instruments and round-the-clock testing to counter the large capital costs.
Exploiting and protecting the deep aquifer
Available data show 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 for 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.
A high arsenic well that has been sealed in Chapai Nawabganj,
western Bangladesh.
