SOS-arsenic.net
 

Arsenic Mobility and Groundwater Extraction in Bangladesh"

Detailed hydrochemical measurements, ý34SSO4 and 3H analyses were performed on 37 groundwater samples collected during February 1999, January and March 2000 from 6 locations in eastern and southeastern Bangladesh to examine redox processes that lead to As mobilization in groundwater. The study sites were chosen based on available nation-wide As surveys to span the entire spectrum of As concentrations in Bangladesh groundwater, and to represent 3 of 5 major geological units of the Ganges-Brahmaputra Delta: uplifted Pleistocene terrace, fluvial flood plain and delta plain. Arsenic was found to be mobilized under Fe-reducing conditions in shallow aquifers (<35 m depth), presumably of Holocene age. It remained mobile under SO4-reducing conditions, suggesting that authigenic sulfide precipitation does not constitute a significant sink for As in these groundwaters. The redox state of the water was characterized by a variety of parameters including dissolved O2, NO3-, Mn2+, Fe2+ concentrations, and SO42-/Cl- ratios. High dissolved [As] (> 50 g/l; or > 0.7 M ) were always accompanied by high dissolved [HCO3-] (> 4 mM), and were close to saturation with respect to calcite. Groundwater enriched in As (200800 g/l; or 2.710.7 M) and phosphate (30100 M) but relatively low in dissolved Fe (540 M) probably resulted from re-oxidation of reducing, As and Fe enriched water. This history was deduced from isotopic signatures of d34SSO4 and 3H2O (3H) to delineate the nature of redox changes for some of the reducing groundwaters. In contrast, As is not mobilized in presumed Pleistocene aquifers, both shallow (3060 m) and deep (150270 m), because conditions were not reducing enough due to lack of sufficient O2 demand (Volume 19, Issue 2, Pages 163-260 (February 2004) Arsenic in Groundwater of Sedimentary Aquifers, Edited by P. Bhattacharya, A. H. Welch, K. M. Ahmed, G. Jacks and R. Naidu, 2006).



(Book Review) Arsenic in Groundwater: Geochemistry and Occurrence, edited by Alan H. Welch and Kenneth G. Stollenwerk (2003), reviewed by Dr. Charles Harvey of MIT, groundwater-book review, vol. 47 No.2-groundwater-March-April 2004

In your above referenced review article you stated that "High arsenic concentrations in U.S. aquifers created by oxidation of arsenic-bearing sulfides (with little adsorption by metal oxides) contrasts with conditions in Bangladesh, where arsenic is primarily liberated from metal oxides as conditions become more reducing (with little sequestration by precipitation of reduced solid)."

I think your comment on the conditions of Bangladesh groundwater arsenic poisoning is not based on sound scientific data and evidence. I reviewed your work entitled "Arsenic Mobility and Groundwater Extraction in Bangladesh" and found that similar to Kinniburgh and others, you also have failed to examine the oxidation mechanism for releasing arsenic into groundwater of Bangladesh.

I do not see any hydrological, hydro-geological and geo-chemical data in your work that are essential to examine the oxidation mechanism. It appears that like David G. Kinniburgh et.al., you were also mistaken in establishing the "reduction" theory in Bangladesh mainly because of an important failure to understand the recent hydrological, hydro-geological and hydro-chemical history of Bangladesh.

Since 1998, the DPHE/BGS investigators(David G. Kinniburgh and others) have been suggesting that the dugwells in Bangladesh are free of arsenic contamination even in arsenic contaminated areas because they believe that the reduction mechanism is responsible for the mobilization of arsenic into groundwater. In 1999, Dr. Thomas Bridge and myself had raised questions and opposed the DPHE/BGS dug-wells contamination study. We found that their study was based on stagnant water samples and not on representative groundwater samples, and as a result DPHE/BGS investigators could not find high levels of arsenic contamination in the dug-wells. This is only one example of many regarding Kinniburgh et.al. arsenic study in Bangladesh?

On February 09, 2004 Dr. Dipankar Chakrabortti reported, "So far we have analyzed more than 700 dugwells from the arsenic affected areas of West Bengal and Bangladesh. We have found 90% dugwells are safe with respect to arsenic (less than 3 - 35 microgram per litre; Average 15 microgram per litre). There are some areas where we have found arsenic contamination above 50 microgram per litre (maximum 330 microgram per litre). One such area in Bangladesh is Samta village where many dugwells are arsenic contaminated and the dugwells are shallow (10- 20 ft ), recent and 2-3 ft diameter and waters have foul smell."

On February 09, 2004 Dr. Dipankar Chakrabortti reported, "So far we have analyzed more than 700 dugwells from the arsenic affected areas of West Bengal and Bangladesh. We have found 90% dugwells are safe with respect to arsenic (less than 3 - 35 microgram per litre; Average 15 microgram per litre). There are some areas where we have found arsenic contamination above 50 microgram per litre (maximum 330 microgram per litre). One such area in Bangladesh is Samta village where many dugwells are arsenic contaminated and the dugwells are shallow (10- 20 ft ), recent and 2-3 ft diameter and waters have foul smell

Dr. Chaakrabortti also failed to collect representative groundwater samples from the dug-wells. Even though he did not collect samples properly his data clearly support our prediction that the dug-wells are not free of arsenic contamination in arsenic contaminated areas in Bangladesh and West Bengal.

Dr. Harvey you are a hydrogeologist and as a hydrogeologist you know the conditions of how the contaminants migrate from up gradient to down gradient, cross gradient and side gradient etc. If the oxidation theory is correct, then obviously one should find the conditions for the generation and migration of contaminants from oxidation zone to the reducing zone(from up gradient to down gradient). Have you collected any data to examine these conditions? What are they?

If the reduction theory is a correct theory for the mobilization of arsenic into the groundwater of Bangladesh and West Bengal of India then one will not find the recent oxidation conditions for the generation of contaminants and the presence of arsenic in the oxidation zone. Can you tell us how the arsenic contaminants have migrated from reducing zone to the oxidation zone i.e., from down gradient to up gradient in Bangladesh and West Bengal of India?

Don't you think Dr. Chakraborti's dug well data disproves the reduction theory for the mobilization of arsenic into the groundwater of Bangladesh and West Bengal? Don't you think the presence of arsenic in dug-wells and the toxicological data of arsenic disproves the DPHE/BGS(Kinniburgh, D.G., et.al.) theory that the groundwater arsenic poisoning in Bangladesh has been present for thousands of years and which suggests that the arsenic disaster in Bangladesh and West Bengal of India is a recent environmental episode, and that if arsenic had been present in groundwater for thousands of years then the people of Bangladesh and West Bengal of India who used hand dug well water for thousands of years would have been poisoned by arsenic?

Wouldn't we witness people suffering from arsenic disease for generations? In West Bengal arsenic poisoned patients were first detected in mid 1980's, whereas in Bangladesh it was detected in the mid 1990's. According to Kinniburgh et.al. the lag time for the development of arsenic lesion is 10 years, according to Dr. Shah's study in West Bengal the lag time is about 2 to 5 years and according to other investigators it is about 8 to 14 years.

Research from David G. Kinniburgh and other's investigative study entitled "Arsenic contamination of groundwater in Bangladesh" has been incorporated into the recent publication "Arsenic in Groundwater: Geochemistry and Occurrence, edited by Alan H. Welch and Kenneth G. Stollenwerk(2003). Their work was not based on correct scientific data and evidence. The review of their work also revealed that DPHE/BGS investigators were biased in establishing the reduction theory for the mobilization of arsenic into the groundwater of Bangladesh.

Meer Husain, P.G. Environmental geologist Kansas Dept. of Health and Environment & Adjunct Faculty Cowley County Community College Kansas, U.S.A.July, 2004

As far as we know, BGS investigators have collected the most data of all the groups working in Bangladesh. However, they have failed to collect adequate and essential hydrological, hydrogeological and geochemical parameters and as a result they are biased in their support of the reduction mechanism as the principal cause for the mobilization of arsenic into groundwater.

Farraka dam in IndiaIn 1999, following their Phase-1 investigation we brought to their attention that in order to understand the time of poisoning and the role of surface water diversion/harvesting and groundwater abstraction in relation to the oxidation mechanism for releasing arsenic into groundwater, they should have collected/analyzed Post Farakka river water discharge data of the common rivers of Bangladesh and India. In their reports(Phase -1 and Final reports, page 2-2 and 17) they only presented pre-Farakka/pre 1975 river water discharge data of three major rivers of Bangladesh, whereas river water diversion started in 1975. They do not have any post Farakka/post 1975 river water discharge data and as a result they have failed to understand the cause of groundwater arsenic poisoning in Bangladesh.

In 2002 Meer Husain explained to Aggarwal et.al., BGS/DPHE and others why the age of arsenic poisoning as determined by isotopic study is not acceptable :

  • " If you collect arsenic poison water from shallow dug wells/tube wells and determine the age poisoning, you will find that the age poisoning varies from a few weeks to several months depending on the sampling periods/events.
  • " The age of the water and the time arsenic entered the water are distinct concepts. If arsenic was tied up in minerals that were stable below the water table when the sediments were first deposited and released when oxidation occurred as the groundwater table was lowered at a later date then the date of arsenic contamination relates to the groundwater lowering not the age of the water.
  • If prior to 1960 (before the onset of large-scale river diversions or irrigation) arsenic had been released by oxyhydroxide reduction mechanism and if arsenic poisoning had been present at shallow depth within the zone of fluctuation or less than 50 feet, then both tube well and dug well water users would have been poisoned by arsenic. We would surely witness people suffering for generations.
  • On February 09, 2004 Dr. Chakrabortti presented a large volume of dug well contamination data. He reported that "So far we have analyzed more than 700 dug wells from the arsenic affected areas of West Bengal and Bangladesh. We have found 90% dug wells are safe with respect to arsenic (3 - 35 microgram per litre; Average 15 microgram per litre). There are some areas where we have found arsenic contamination above 50 microgram per litre (maximum 330 microgram per litre). One such area in Bangladesh is Samta village where many dug wells are arsenic contaminated and the dugwells are shallow (10- 20 ft ), recent and 2-3 ft diameter and waters have foul smell."
  • If arsenic contamination had been present for thousands of years in the sediments and groundwater in Bangladesh and West Bengal of India as suggested by the proponents of reduction hypothesis, then arsenic would have been present in dug wells for thousands of years too. If groundwater arsenic poisoning/contamination had been present for thousands of years, then both shallow hand dug well and tube well water users would have been affected by arsenic poisoning prior to 1960/1975.
  • The area which was under water for thousands of years in Bangladesh is now dry land. India.s unilateral diversion of river water from more than 30 common rivers of Bangladesh and India, and over pumping of groundwater for the last 30-40 years are the root of the groundwater arsenic disaster in Bengal Basin. We do not understand why BGS and Aggarwal et.al. have been suggesting that river water diversion and over pumping of groundwater do not have any role for the creation of arsenic poisoning in Bangladesh and West Bengal of India without collecting pre and Post 1975 river water discharge data and adequate groundwater data.
  • If you collect arsenic contaminated water from shallow dug wells in Bangladesh and West Bengal of India, and determine the age of arsenic poisoning by isotopic methods, don't you think your isotopic analysis will reveal that the arsenic contamination in dug wells is a very recent phenomenon? Isotopic study can not determine the age of arsenic poisoning. In order to determine the age of poisoning scientists will have to analyze the historical groundwater use data, historical medical data, arsenic toxicological data, mineralogical, hydrological, hydrogeological and hydrochemical data.

    Bacteria responsible for breaking down arsenic in groundwater.

    Scientists have identified a special group of bacteria responsible for breaking down arsenic in groundwater. This was disclosed by a Bangladeshi scientist conducting a study in the UK on the cause of naturally occurring arsenic release into groundwater table. The discovery may provide a possible solution to groundwater arsenic contamination that now exposes an estimated eight million people in 61 districts in Bangladesh to serious health hazards.

    Millions of people, mostly in the rural areas, have developed various symptoms of poisoning from drinking arsenic-contaminated water from tubewells for decades. The study by Farhana Islam, supervised by other researchers, showed the special group of bacteria 'gains energy by respiring (breathing), using the metal iron and arsenic containing minerals in the earth sediments'. The young scientist said, "Our results show that these are the special anaerobic bacteria, as they don't need any oxygen to support their growth. They are known as metal-reducing bacteria. We are very interested in iron-reducing bacteria that use iron as their growth substrate, and can also use arsenic when the iron is used up." The bacteria cause changes in the mineral structure of the sediments, leading to release of arsenic into groundwater, the study says.

    Farhana, who studies in the Department of Earth Sciences and Williamson Research Centre for Molecular Environmental Science at the University of Manchester, told The Daily Star, "We are looking at how these processes of breaking down the mineral can be reversed so that the groundwater is safe to drink." She elaborated, "With our results, we found that maximum amount of arsenic was released from contaminated sediment into groundwater in the absence of oxygen." There were several hypotheses concerning the release of arsenic into the groundwater systems of West Bengal in India, where the researchers worked. Some suggested a role for aerobic bacteria (arsenopyrite oxidation), some suggested a role for metal-reducing bacteria while others considered the problem to be driven by geochemistry.

    The scientists from Manchester University conducted experiments in their laboratory with sediments collected directly from an area of West Bengal affected by arsenic. "We were the first group to combine geochemical, mineralogical and microbiological/molecular biology techniques to study this system, and have presented the first direct evidence to support a role for metal-reducing bacteria in arsenic release from the sediments. The organisms identified as playing a key role are iron reducing bacteria that can attack arsenic once they have exhausted iron as a growth element," Farhana explained. Studies showed that this type of bacteria is unable to use oxygen for growth, rather they use different metals to support their metabolism. Metal-reducing bacteria 'breathe' metals such as iron to get energy from their food, in the same way humans breathe oxygen to break down food.

    The iron-reducing bacteria use iron through the electron transport system in the anaerobic respiration and gain energy for their growth by reducing this iron. This process is known as dissimilatory iron-reduction. Explaining the process of arsenic contamination, Farhana said instead of using oxygen, the anaerobic bacteria gain their energy by respiring, (breathing) using iron-containing minerals in the sediments, a process called iron reduction. By doing this, the bacteria transfer electrons to iron oxide rust coating the sediments, causing changes in the characteristics of the minerals. And when the iron runs out, the bugs start to utilise other metals, such as arsenic, which occurs naturally. The chemistry of the arsenic is changed and the reduced arsenic is able to dissolve into groundwater, she said (Daily Star, August 14, 2004). Farhana Islam

  • Top of page
    Back to Source

    Last Modified: August 21, 2009

    Home