Draft Development Strategy, National Water Management Plan
Arsenic in the Main Report
3 Country Setting
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 is a major threat to human health, as it is both
toxic and carcinogenic. Clinical effects include keratosis and melanosis. It is
ingested in drinking and cooking water, and the effects are not removed by
boiling the water. The GoB currently adopts a standard of 0.05mg/litre (50ppb)
as the limit of acceptability of arsenic in water for human consumption,
compared to the WHO standard of 0.05mg/litre. Elsewhere
in the world, notably USA and Germany, acceptable safe levels are being reduced
to half this figure. Over the plan period, it is likely that the GoB will adopt
the present WHO standard, and that WHO itself will revise safe levels downwards.
Priority will be given to areas where the GoB limit is exceeded, hut the NWMP
will need to address all areas where the WHO limit of 0.01mg/l
is exceeded. By 2025, some 93 million people (51% of the
population) will he living in towns and villages (both are affected) where
arsenic levels are already above this limit.
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
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).
Figure 3.3 : Probability of Arsenic Exceeding Threshold Values
(click on the image for a larger version)
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.
Arsenic from a point source within the aquifer can spread
horizontally and vertically, but the rate is controlled by adsorption of the
arsenic or clay. Over 25 years, up to 50m, horizontal movement may be expected,
but rather less vertical movement. Wells are often spaced much closer than 50m,
so wells found to be safe in places where others are contaminated
will be at risk in the future. There are also indications that arsenic
concentration may vary seasonally in shallow wells, which raises issues over the
efficacy of large-scale testing programs, and may rise in the first few weeks
after installation of wells.
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.
7 Towns & Rural Areas
7.7 Options for Water Supply
7.7.2 Treatment of Arsenic Contaminated Water
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.
area known to be safe or from a depth known to be safe.
Surface water will require treatment to remove suspended
solids and ensure that it is biologically pure. Simple filtration systems,
such as pond sand filters, can be effective, but require very frequent and
careful maintenance. However, the capacity of existing ponds is limited and
they are privately owned. New ponds require land. Consumers who have their own
pumps are reluctant to revert to using a shared source. The number of rivers
which have reliable dry season flaws are limited. During low flow conditions
these may contain relatively high concentrations of agricultural or industrial
chemicals, which are very expensive to remove. Intakes on the major rivers may
suffer from two problems: erosion during the flood
season; and being far from the low-flow channel
in the dry season.
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
household levels and undocumented connections. Measures are
required to control wastage. In some households the supply discharges into a
tank, which frequently overflows. Where water is charged by the size of the connection, there is no
incentive for the householder to save water. Such incentives
have to be created, either by metering, which requires some investment by
either GoB or the consumer and O&M expenditure, or through regulations
which empower the water utility to disconnect users who are wasting water.
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
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.