Draft Development Strategy, National Water Management Plan
Arsenic in Annex C, Land & Water Resources
Section 6 - Water Quality
6.3. Arsenic in Groundwater
Arsenic in groundwater was identified in Bangladesh in
1993, and a summary of its occurrence and the background
issues are set out in Annex C Appendix 8. Previously there had been no policy
to test for arsenic, as no problem was suspected. The full extent has yet to
be identified but the problem is most strongly concentrated in the South West,
South East and North East regions as shown on Figure 6.1, which shows the
proportion of tube wells affected in each Thana. Some 20M people are now
considered at risk from arsenic in their drinking water.
6.3.1 Preliminary Findings
Preliminary findings indicate:
The problem mainly affects shallow
tube wells and although some deep tube wells ate
reported as contaminated, many reports ate based on erroneous
data (feet instead of meters, the use of term “deep”). Wells
over 250m deep are rarely contaminated.
- The distribution is far
from uniform. In some hot spots all tube wells are contaminated
while in other places both positive and negative results are
obtained for different tube wells in close proximity.
- Some tube wells, which
were not contaminated, are now affected.
The three main questions relating to the arsenic problem:
- Is the problem becoming worse and
if so, will it continue to worsen, or is it that more consistent and more
accurate testing continues to record an increased incidence of arsenic?
- Has the increased
development of groundwater for irrigation
arsenic to be mobilized and enter the groundwater?
- How acute is the risk that arsenic
in irrigation water will enter the food chain?
There are no firm answers to these questions at this time,
but it is evident that:
- Further testing increases the
known number of arsenic-infected wells, particularly since a new low cost
field test kit has been developed in Bangladesh. Few kits can
detect concentrations below 20ppb.
- Opinion was divided as to whether
increased abstraction of water for irrigation has caused an increase in the
concentration of arsenic in groundwater. There is no evidence to prove any
linkage apart from the fact that the recorded increase in the incidence in
arsenic in groundwater has coincided with the general increase in the use of
groundwater for irrigation. At the same time, the number of tube wells for
domestic water, and the number of people using this source of
water, has substantially increased over the same
period. The weight of opinion at present is that there is no connection.
- Recent research indicates that
arsenic does not accumulate in the
rice grains, but is concentrated in the roots. It has been shown in studies
that some green leafy vegetables accumulate arsenic.
The issue of arsenic
and the potable water supply is discussed further in Annex L. However, research
is urgently needed to improve understanding of the mechanisms involved and the
risks, particularly in the context of irrigated agriculture.
Farmers are likely to continue to use groundwater in
irrigation unless an equally reliable and cheaper alternative becomes available.
Should some crops be found to
accumulate arsenic then controls on their production will have to be considered.
However, this may be difficult to implement in a situation where the arsenic
contamination of the water is unevenly distributed.
Solutions for the arsenic problem are essential, and options
are discussed in Chapters 7 and
8 of the Main Report.
6.3.2.Variation of Quality with Depth
In general, there appears to be an improvement with depth of the borehole.
Results from the British Geological Survey of 253 samples showed the following
results [click on figure for larger image].
Figure 6.1 - Occurrence of Arsenic in Groundwater in Bangladesh [click on
figure for larger image]