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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 groundwater.

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 earth.
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 either."

"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 environments.

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 water.

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 late.


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 Canada


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