Arsenic Contamination: Too Formidable A Foe
by Sylvia Mortoza
The biggest mass-poisoning case the world has known is taking place
in Bangladesh. Arsenic-poisoning is not only a tragedy for the people but
also a test of the country's concern and integrity for arsenic-contamination
of ground water is seen by many as an opportunity to get rich. Now looked
upon by man as a God-sent gift or bonanza or at best a chance to prove
a theory or sell a ready-made "remedy," it is apparent that there
are a pitiful few who see it for what it really is, a disaster of such
potential it is not easy to overcome without a loss of a number of lives.
The recent euphoria over the award of this year's Nobel Prize for Peace
referred to the end of the thirty year old secular conflict that had cost
3,500 lives - the mass-poisoning case in Bangladesh has already cost us
over 2,500 lives and it will not take long to exceed their loss, if things
are allowed to continue unchecked. Some blame the World Bank for the delay
in coming to grips with the problem but as the first indication it received
was in 1993, some years after the first indication of trouble, the delay,
if there was any, was mainly in the cautious way the Bank operates. As
a result it took until after 1995 to confirm that there was indeed contamination,
but by that time, the high levels of arsenic in the millions of shallow
and deep wells that had been sunk in various parts of the country were
dispensing their own special brand of poison.
Now it was a public health matter and a real concern for a country where
health services were already under intense pressure for as contamination
spread the pressure increased for by that time a large number of people
had been affected by drinking water from the wells in more than half the
64 districts of Bangladesh, mainly in the south western, middle and north-eastern
parts of the country. Arsenic-contamination and its effect on the health
and well-being of the people of Bangladesh had now reached crisis proportion
because for most of the victims, there is no alternative to tube well water
unless it is to go back to drinking polluted surface water.
Faced with such a quandary, the World Bank took on the job of acting
as the co-ordinating body for the donor community and the Government of
Bangladesh. The only question is, does the World Bank understand its responsibility
to the people of this beleagured land? Hopefully they did for their first
response was to provide US$ 50 million for "investigations."
What investigations were actually carried out is hard to assess and it
can only be hoped the money was well spent. The Bank followed this with
a credit of US$ 32.4 million for the "arsenic mitigation programmes"
needed to stave off further illness and deaths caused by drinking arsenic-contaminated
Specifically, the project will provide alternative water supplies and
medical relief in the affected areas; establish the extent, nature and
causes of arsenic poisoning; and put in motion concrete action for long-term
solutions - including water treatment, public awareness and increasing
people's and government capacity to address similar crises.
Through on-going research, the Bank has been looking into the linkage between
the pumping of groundwater and arsenic-contamination and the impact of
arsenic-contaminated water on crops which has given rise to a report known
as the British Geological Survey (BGS) report.
The main point of this survey by the BGS is that the arsenic-contamination
of groundwater that has put millions of lives at risk is "natural
and not man-made" which may be true but does not tell us how to deal
with the many cases of arsenic- poisoning or help us to provide alternative
sources of pure drinking water. There is also some confusion for the earlier
belief that arsenic, already deposited in the soil, combined with the ground
water when the atmospheric oxygen invaded the aquifer in response to a
lowering of water level by extraction, is very strong.
According to the report, the BGS team tested more than 9000 samples
of groundwater collected from the shallow and deep tubewells drilled at
various depths across the country. It is now essential for these results
to be made public so that individual researchers around the world can follow
up on this information. As some of the laboratory tests were conducted
in Bangladesh and some in the UK, releasing this information to the public
is not difficult and individual analysis may come up with something new.
Although the arsenic-contamination of groundwater has been declared
a national disaster by the government, its seriousness is yet to be fully
comprehended, but as soil samples collected from the fresh shallow and
deep aquifers confirmed the presence of arsenic at various concentrations,
there is an immediate need to raise awareness to the problem through the
dissemination of information as laboratory analysis of the soil and water
samples have indicated that arsenic-contamination is "prominent"
in the shallow aquifers from which most people draw their supply of water.
As arsenic has been found in high concentration mostly at a depth of
between 30 and 100 feet, it is incumbent on us now to find alternative
sources of water as most of the very high levels, that is more than 0.25
mg/L arsenic, occur within a narrow range of 20 to 40 meters beneath the
Below 100 meter depth few water samples show concentration of arsenic at
levels of over 0.1 mg/L however, although arsenic-contamination of deep
tubewells sunk below 250 meters have not been reported, the possibility
of their contamination, according to the BGS, cannot be ruled out and they
say "there is every likelihood the deep aquifer is not free from arsenic
"As the arsenic-rich groundwater is mostly restricted to the alluvial
aquifers of the Ganges delta, the source of arsenic-rich oxyhydroxides
must therefore lie in the Ganges source upstream of Bangladesh," says
the report. If this is true, Bangladesh could now be in for further trouble
as the recent floods have deposited what may be "arsenic-rich silt"
across the country. This could mean that in the coming dry season, the
arsenic-contamination that now threatens more than half the country will
have spread to even those places where there was none. What a frightening
prospect but few appear to have given this much thought.
Although arsenic can be detected in traces everywhere - its abundance
in the earth's crust is 1.8 ppm i.e., making it roughly as abundant as
molybdenum or tin, arsenic concentration is considerably higher in soils
and shales than in the earth crust, because of its accumulation during
weathering and translocation in colloid fractions. The levels of arsenic
in the soil of various countries have been said to range from 0.1 to 40
ppm (mean 6 ppm). It is a major constituent of at least 245-320 different
minerals, but only a few occur in such quantities that they can be worked
economically. Arsenic in soil is highly mobile, resulting in possible groundwater
contamination. Any retention of arsenic by soils would occur by adsorption,
especially if the soil contains iron or aluminum oxides. This is especially
true in very acid pyritic soils, acid sulphate soils, high in iron oxide
in some areas of Bangladesh, have been reported to have many times more
arsenic than expected.
Arsenic concentrations in water samples from wells in a thin, alluvial
aquifer of the Madison River Valley, Montana, have been reported from 26
to 150 g/L. although the river itself originates in Yellowstone with an
arsenic concentration of 51g/L. This has been correlated with the intensity
of ditch irrigation in this semi-arid region. In the western USA, it was
found that arsenic is generally associated with one of four geochemical
The West Bengal Government's investigation revealed that a 450 km long
layer of arsenic rich silt clay is lying between the depths of 70 and 200
feet below the surface of the upper deltaic plain of river Bhagirathi.
All these zones are located between the Ganges-Bhagirathi river and the
western border of Bangladesh. The sediments on both sides of the border
have the same depositional history and geological environment. The area
is a part of the Ganga-Brahmaputra delta. The delta proper as well as the
flanking areas forming the so-called Bengal basin is divided into six macro-process
regions: laterite upland, Barind, upper delta plain of meander belt, valley
margin fan, marginal plain, lower delta plain and delta front. The aquifer
of the contaminated zone in West Bengal and Bangladesh are hydraulically
connected. The arsenic poisoning of the ground water in the lower Gangetic
delta (Bangladesh) has posed a serious threat. In spite of arsenic contamination
of the ground water no systematic study has been made so far. It is highly
desirable to form a study group with hydrogeologists, chemists, water supply
engineers, environmentalists and public health experts to carry out in-depth
investigation. Random sampling without considering the regional ground
water flow may lead to erroneous results.
Arsenic in sediment or water likes to move in adsorbed phase with iron,
which is available in plenty in the Himalayas. Combining with iron oxides,
about 100-300 mg arsenics/kg can be found in sediments under oxic conditions.
When these sediments were deposited in the tidal environment (Bengal basin
was under tidal condition), it came under anoxic condition. Sulfur reducing
bacteria combined the oxygen from sulfates (available in tidal basin) through
oxidation of organic carbon. As a result, organic carbon was lost as carbon
dioxide (CO2) or remained as bi-carbonate, and the sulfate was reduced
to hydrogen sulfide. Iron minerals and hydrogen sulfide rapidly tie together
to form iron sulfide containing arsenic because arsenic has been adsorbed
on the surface of the iron minerals. That gives either arsenic substituted
iron sulfide or arsenopyrite. These formations are stable unless they are
disturbed or exposed to oxygen.
These kinds of depositions are in 2 impervious layers under modern delta
formations of the Gangetic plain.
One within 15 to 30 meters depth and the other one below 100 meters depths.
These layers contained arsenic-pyrite, pyrite, iron sulfate, iron oxides
as revealed by x-ray studies conducted by the Indians, diffraction, electron
probe, micro analysis and laser microprobe mass analysis. The Himalayas
have pyrites and sedimentary formations as it is marine in nature origin.
Marine conditions are the ultimate resting place for metals or elements
or compounds. For example, the Indians got 150 kg arsenic/year from a single
tube well. So the arsenic is certainly a geologic source because no organic
arsenic compounds were found at high concentration.
Bangladesh and adjacent West Bengal has three aquifers: 1st one 2-15
meters, 2nd 40-80 meters and 3rd one below 100 meters. These aquifers are
also hydraulically connected to the major streams in Bangladesh, especially
the Ganges in the Northwestern region of Bangladesh. Ground water recharge
is low due to less rainfall and upstream diversion of Ganges water by India.
During the dry season, the water table falls to below more than 25-30 feet.
After the eighties the ground water fell drastically during the dry season
and a "drying zone" gradually developed. This caused a rapid
diffusion of oxygen within the pore spaces of the soil/sediments as well
as an increase in dissolved oxygen in the upper part of the ground water.
As this "oxic water" or oxygen came into contact with the 1st
impervious layer within 30-50 meters, the arsenic-laden pyrite became partially
oxidised and formed acid which became soluble and released the arsenic
(As), iron, (Fe) and sulfate plus hydrogen (acid). The oxygen is rapidly
consumed in forming sulfate, the Fe+2 acts as a catalyst to further decomposition
the as pyrites. So, these two-fold reactions released the arsenic in the
Shallow tube extracts water from the upper and intermediate aquifers.
The intermediate aquifer is just below the 1st impervious layer. As a result
arsenic is leaching from the 1st impervious layer and remains soluble in
the water of the intermediate aquifer. The oxidation theory also justified
the occurrence of acid sulfate soils in Jessore, Faridpur, Khulna. The
Ganges sediments are calcareous in nature, this calcium (Ca) neutralised
the acid formation. Otherwise we might have had lots of acid sulfate soils
like in Thailand and Vietnam.
Arsenic in certain solid phases within sediments, particularly iron
oxides, organic matter, and sulfides may be the primary source of arsenic
in groundwater. It is known that pyrite is the carrier of arsenic. Excessive
groundwater withdrawal may be one of the reasons for creating a zone of
aeration in the clay layers containing pyrite which in turn decomposes
to form iron sulphate and releases arsenic in the percolating subsurface
water. Mobilisation of arsenic in sedimentary aquifers may be, in part,
a result of changes in the geohydrochemical environment due to agricultural
irrigation. In the deeper subsurface, elevated arsenic concentration is
associated with compaction caused by groundwater withdrawal. Experts say
that an understanding of geochemical principles is essential for an explanation
of the mobilisation of arsenic by natural processes. Along with supergenic
mobilisation of elements arsenic is transferred from one phase to another
or within a heterogeneous phase such as soil by microbial mediated processes.
Aerobic bacteria and fungi often transform arsenic into coordination complexes
that are more mobile in soil water than the uncomplexed cation.
Environmental characteristics also strongly influence arsenic (As) movement
in soils. Movement is a strong function of speciation and soil type. For
a non-adsorbing soil (sand), the mobilisation of As (III) and As (V) in
groundwater are dependent on the dispersion coefficient and permeability
for solute transport. Soil pH also influences arsenic mobility. At a pH
of 5.8, As (v) is slightly more mobile than As (III). As pH changes from
acidic to neutral to basic, As (III) tends to become the more mobile species,
though the mobility of both increases with increasing pH. Arsenic is transported
at a slower rate in a strongly adsorbing soil as compared to the sandy
soils. Although there are two layers south of West Bengal and in Bangladesh,
these 2 layers combined together to form a thick layer in the northwestern
part, especially in the Malda district of West Bengal. As the aquifers
are hydraulically connected, people should get some arsenic. It depends
on the thickness of the layer and aquifers.
In the north-western part of Bangladesh, the rainfall is less than the
groundwater recharge. The aquifers in this region are also hydraulically
connected to the river Ganges. The reduced flow of the Ganges not only
caused reduction in ground water, it also extended the dry season from
March to May to December to May. During 1980's the flow of the Ganges was
at a record low. During these periods Bangladesh sometimes got only 6790
cusecs of water. (India Today - January 15, 1997 or April 30, 1997 by Rupen
Banerjee or Kamaluddin and Bailey).
When there is a reduced flow in the river, the riverbeds dry-up, groundwater
goes down, people use more groundwater through pumping deep down. During
1975, people used to use tubewells at a very shallow depth, over time they
have go down deeper and deeper to find water. Why? Because sufficient water
is not available at shallow depth. WASA had to change the well site often
in Dhaka. They used to say sufficient water is not coming. When there is
groundwater deep down, oxygen can easily go down because of partial pressure
increase of gases.
It is now very important to review all data and information that may
be available from any source and feed it into a central data base, after
which it can be properly analysed and hopefully, solutions found for the
people are not only getting arsenic from the groundwater, but also from
food such as rice, fish and vegetables. The urgency of coming to grips
with the problem cannot be over-emphasised for the "mining" of
water, which closely mirrors what happens in a metal mine, has certainly
exposed the arsenic-containing rocks to oxygen, thus releasing the arsenic
into the ground water.
But sifting fact from fiction or conjecture is not easy and whatever maybe
the truth about the cause/causes of the current arsenic-contamination of
Bangladesh, one thing is undisputed, it's effect on human health and human
relationships - and something has to be done about it before it is too
The author acknowledges the invaluable help of Mr. Khondker Rafiqul
Islam. Other acknowledgements are due to:
- The report of the British Geological Survey
- Dr. Delvin S. Fanning - Maryland University, USA
- Dr. Tom Lawand - The Brace Research Institute, McGill University, Quebec