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Draft Development Strategy, National Water Management Plan
Arsenic in the Summary Of Options
3. Land and Water Resources
3.2 Arsenic
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 over large parts of Bangladesh. The first national survey was
completed by end-1998, but further surveys have extended the areas known to be
affected.
Arsenic is a major threat to human health. The GoB
currently adopts a standard of 0.05mg/litre (50ppb) as the limit of
acceptability of arsenic in water for human consumption, compared with the WHO
standard of 0.01mg/litre. Over the plan
period, it is likely that the GoB will adopt the present
WHO standard. By 2025, some 93 million people (51% of the population) will be
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. This is being studied but no results are expected to be available
before 2001.
Several possible sources of arsenic contamination have been
suggested. It is now generally accepted that the source is geological,
transported by rivers from sedimentary rocks in the Himalayas. Two mechanisms
for the release of arsenic, pyrite oxidation and oxyhydroloxide
reduction have been put forward, 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. If the first hypothesis was true, an embargo on tube well
irrigation may remedy the situation, although at huge cost to the economy.
Fortunately no such decisions appear to be required. Similarly suggestions that
phosphate in fertilizers or upstream abstraction of water from major rivers may
worsen the situation appear unfounded.
The majority tests to date have been carried out on shallow tube wells used
for drinking water. Significant number of tests have also been carried out on
deep tube wells down to 300m or more, used for drinking water, and other wells
(also referred to as deep tube wells) down to 100m used fro agriculture. The
tests show that at depths below 200m, the incidence of contamination falls off
and at 250 or more it is rare. Over 25 years, up to 50m horizontal movement may
be expected, but rather less vertical movement. This suggests that wells spaced
closer than 50m to an existing contaminated one will be at risk in the future.
In general, it appears that water drawn from depths greater than 250mis, and
will remain, arsenic free provided that irrigation wells do not start using the
same aquifer. Such wells usually have better quality in terms of iron and other
metals, and the same hardness as shallower wells. The 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.
Appendix - Issues of Special Interest
The Secretary, Ministry of Water Resources, in a letter to
the DG WARPO dated 1 August 2000, asked that the NWMPP identify
strategies/options to address six issues of particular concern. These are
discussed below:
1. Arsenic Contamination in Groundwater
Arsenic is a major concern for drinking water supplies, but there is no
confirmed evidence yet that it poses a threat to agriculture. Accordingly, NWMPP has identified
options for managing the former, and adopts a wait-and-see attitude for the
latter whilst further studies continue. Most drinking water is supplied by some
hand tube-wells (HTW), which are at threat not only from arsenic, but also from
seasonally low water tables due to irrigation abstraction. The latter is likely
to affect an increasing number of as farmers respond to the
Increasing demand for food by expanding and intensifying irrigation. Such
expansion by the private sector should be encouraged as, coupled with efforts to
increase crop yields, it is the most cost-effective and environmentally
sustainable way of feeding Bangladesh.
(i) Removal of Arsenic from Contaminated Sources
Arsenic can be removed from drinking and cooking water, and
various filters for use at household level with water from open wells and
hand-pump sources are being tested. These supply small quantities of water at
high unit cost, and are unlikely to he properly maintained.
(ii) Using Surface Water from Ponds
One alternative source for arsenic-free drinking water for
rural areas and small towns is ponded surface water, as relatively few towns and
villages are near perennial rivers. Ponded surface water is generally
contaminated and requires expensive treatment, and treatment systems are
difficult to rnaintain reliably.
(iii) Using Groundwater from 200m Depth
The aquifer at 200m depth is likely to remain uncontaminated provided
abstraction by irrigation tube-wells deeper than 150m is banned. Few if any such
wells exist at present in Bangladesh. Small deep tube-wells (DTW) costing
Tk50,000 can be fitted with hand-pumps, hut as hand-pumps serve only a few
people, per capita costs are high. If fitted with 1kW electric submersible pumps
with generators, they can serve 1000 people with 501/c-d through a piped water
supply system and become cost-effective and affordable. Such systems could be
installed and operated by the private sector, selling metered water to each
household on a sliding scale of charges so that even the poor could afford their
basic needs. The GoB would need to provide a 50% subsidy on capital costs
to attract private sector investment. The DTW systems appear to be
attractive as a forward-looking solution compatible with development needs, and
can be used in many coastal areas as well.
(iv) Using Surface Water from Perennial Rivers
Major towns will need to turn indreasingly to the perennial rivers.
Dhaka's continues expansion into the floodplains will expose it to greater risk
of arsenic, and the nearby rivers are unfit as a source of water. An 80km
long aqueduct from the Jamuna at Bangabandhu Bridge appears an obvious source,
as it is a secure, unlimited, and clean source, and the line of the aqueduct
would serve future growth areas. Chittagong has plentiful supplies on the
Karnafuli, provided Kaptai dam is operated for multipurpose use. Rajshahi
has secure supplies on the Ganges, but the flows to Khulna need to be
secured by ensuring that flows continue to enter the Gorai.
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