The Increased Draw Down And Recharge in Groundwater Aquifers And Their Relationship to the Arsenic Problem in Bangladesh

Thomas E. Bridge, Professor of Geology(emeritus), Emporia State University, Kansas, USA & Meer T. Husain, Environmental Geologist, Kansas Department of Health And Environment, Kansas, USA.

ABSTRACT

Two explanations as to the origin of the arsenic contamination in the ground water in the Bengal Delta are presented. The Origin of the arsenic resulting from increased drawdown and oxidation of authigenic arsenic iron sulfides is discussed in some detail.

According to one explanation, arsenic poisoning of the ground water resulted from the reduction of arsenic laden iron hydroxides flushed from Pleistocene- Holocene sediments. The sediments were deposited in valleys eroded in the delta when stream base level was lowered with lowered sea stand during the last glacial advance. Valley filling occurred as sea level rose when the glaciers melted and sea level was restored. The organic mater deposited with the sediments reduced the arsenic bearing iron hydroxides and released the arsenic to the ground water. This theory seems to require that arsenic in groundwater has been present for thousands of years with out being flushed from the delta.

Another explanation is that the arsenic contamination is a recent phenomena. The arsenic contamination results from lowering the water table below deposits of organic matter containing authigenic arsenic pyrites concentrated in organic deposits. These arsenic pyrites oxidize in the vadose (draw down zone) releasing the arsenic as arsenic adsorbed on iron hydroxide. The arsenic adsorbed by the iron hydroxide is released and reduced to its soluble lethal forms when reducing conditions returnes with subsequent recharge during the rainy season.

The absence of reported poisoning prior to the nineteen eighties supports the theory that the poisoning of the groundwater is recent. Increased irrigation due to the reduction of surface water flow caused by a multitude of dams built in India on streams crossing the boarder into Bangladesh resulted in repeated lowering and recharge of the water table. Also the drilling of thousands of tube wells to supply domestic needs and prevent the use of polluted surface water resulted in repeated draw down and recharge around the well. The building of the dams and the drilling of tube wells occurred during the nineteen seventies and resulted in repeated wetting and drying of arsenic bearing sulfides in organic rich sediments.

Geochemical processes that are known to occur in regions containing arsenic laden sulfides are described and related to similar conditions in the Bengal Delta sediments.

Other environmentally destructive effects are mentioned that resulted from the construction of dams by India and the diversion of water from Bangladesh.

INTRODUCTION

Arsenic poisoning was not widely reported until in the 1980s and was attributed to arsenic contaminated ground water obtained from tubewells installed in the 1970s to prevent disease from polluted surface water. Unfortunately no groundwater analyses were made for arsenic when the tubewell were first installed. The exact source of the arsenic that is poisoning the tubewell water is not known but several possible sources have been suggested.

R.T. Nickson, , J.M. McArthur, et al, Attribute the arsenic to the reduction of arsenic in oxyhydroxides that were present in sediments washed into valleys cut by rivers when sea-level was lowered during the last glacial maximum (18 ka BP). During glacial maximum the rivers had base levels some 100 meters lower than in interglacial times. The original sediments had been deposited during Plestocene-Holocenes time and were oxidized and flushed during the low-stand of sea-level during this last glacial maximum. The sediments in-filling the valleys as the sea level rose during glacial melting were the characteristic weathered red brown Pleistocene-Holocene sediments. The authors attribute the release of arsenic to the groundwater reductive dissolution of arsenic rich hydroxide coatings on the sediments. The reduction is caused by microbial oxidation of sedimentary organic matter deposited with the sediments as the bacteria consume dissolved oxygen and NO3 from their surroundings. They further state:

"The As derives from reductive dissolution of Fe oxyhydroxide and release of its sorbed As. The Fe oxyhydroxide exists in the aquifer as dispersed phases, such as coatings on sedimentary grains. Recalculated to pure FeOOH, As concentrations in this phase reach 517 ppm. Reduction of the Fe is driven by microbial metabolism of sedimentary organic matter, which is present in concentrations as high as 6% C. Arsenic released by oxidation of pyrite, as water levels are drawn down and air enters the aquifer, contributes negligibly to the problem of As pollution."

Abstract,

Mechanism of arsenic release to groundwater, Bangladesh and West Bengal: R.T. Nickson, , J.M. McArthur,, P. Ravenscroft, W.G. Burgess,K.M. Ahmed Geological Sciences, University College London, Gower St., London, WC1E 6BT, UK Mott MacDonald International Ltd., 122 Gulshan Avenue, Dhaka -1212, Bangladesh Department of Geology, University of Dhaka, Dhaka -1000, Bangladesh. Received 4 January 1999; accepted 13 August 1999

Editorial handling by R. Fuge.

 

The process described by R.T. Nickson, , J.M. McArthur, et al, seems to require that the arsenic remains in solution for thousands of years and that movement of ground water through the delta sediments did not flush the arsenic from the system. They also state that the arsenic correlates with the dissolved iron in the groundwater. When the As and S are changed from Wt% to atomic proportions in the analyses of the water from contaminated wells, given in their paper, the oxydation of pyrite can not be ruled out.

The arsenic contamination with one or two exceptions is restricted to shallow aquifers (< 80 meters). The arsenic contamination is not uniform in distribution, some wells have high concentrations and others have low concentrations. Some wells that are relatively free of arsenic and were used for domestic use have become contaminated with arsenic. These observations suggest that environmental changes have on at recently and near the surface. This also suggests a non uniform distribution of source material. A non uniform distribution of organic mater in the sediments with arsenic pyrites would be the expected.

3Dipankar Das et al , conducted a geochemical survey in the six districts of west Bengal bordering the western part of Bangladesh. These districts are Mulda, Murshidabad, Bardhaman, Nadia, North 24-Pargana and South-24 pargana. Their subsurface investigation, some laboratory analysis, revealed the presence of arsenopyrite minerals in the sediments. They stated that the source of arsenic in groundwater and in the soil is from pyrite minerals containing arsenic. However they did not discuss how arsenic is released in groundwater from arsenopyrite. They cited the oxidation of pyrite process presented in the literature from the U.S. However, in their conclusions they state that: The way that arsenic enters the groundwater in these six districts is not well understood Our bore-hole analyses show arsenic-rich FeS2 in sediment layers. Since iron pyrite (FeS2 ) is not soluble in water, the question therefore arises as to how arsenic from pyrites enters the water."

Although pyrite is not soluble in water, it decomposes when exposed to air or in aerated water and proceeds rapidly in confined environments without the addition of oxygen from external sources as the acid level reaches a pH of 3.5 or lower.

A USA Government article on acid mine drainage (AMD) describes the process. 'The formation of acid drainage is a complex geochemical and microbially mediated process. The acid load ultimately generated is primarily a function of the following factors: Microbiological Controls; Depositional environment; Acid/base balance of the overburden; Lithology; Mineralogy; and Hydrologic conditions.

"Chemistry of Pyrite Weathering

A complex series of chemical weathering reactions are spontaneously initiated when surface mining activities expose spoil materials to an oxidizing environment. The mineral assemblages contained in thc spoil are not in equilibrium with the oxidizing environment and almost mediately begin weathering and mineral transformations. The reactions are analogous to "geologic weathering" which takes place over extended periods of time (i.e., hundreds to thousands o f years) but the rates of reaction are orders of magnitude greater than in "natural" weathering systems. The accelerated reaction rates can release damaging quantities of acidity, metals, and other soluble components into the environment. Thc pyrite oxidation process has been extensively studied and has been reviewed by Nordstrom (1979). For purposes of this chapter, the term "pyrite" is used to collectively refer to all iron disulfide minerals.

The following equations show the generally accepted sequence of pyrite reactions:

2 FcS2 + 7 02 + 2 H20 -> 2 Fc2+ + 4 SO4 + 4 H+ (Equation I)

4 Fe 2+ + 02 + 4 H+-> 4 Fe3+ + 2 H20 (Equation 2)

4 Fe3 + 12 H20-> 4 Fe(OH)3 { 12 H+ (Equation 3)

FeS2 + 14 Fe3 + + 8 H20 -> 15 Fe2+ +2 SO42- + 16 H+ (Equation 4)

In the initial step, pyrite reacts with oxygen and water to produce ferrous iron, sulfate and acidity. The second step involves thc conversion of ferrous iron to ferric iron. This second reaction has been termed thc "rate determining" step for the overall sequence.

The third step involves thc hydrolysis of ferric iron with water to form thc solid ferric hydroxide (ferrihydrite) and the release of additional acidity. This third reaction is pH dependent. Under very acid conditions of less than about pH 3.5, thc solid mincral docs not form and ferric iron remains in solution. At higher pH values, a precipitate forms, commonly referred to as "yellowboy."

The fourth step involves the oxidation of additional pyrite by ferric iron. The ferric iron is generated by the initial oxidation reactions in steps one and two. This cyclic propagation of acid generation by iron takes place very rapidly and continues until the supply of ferric iron or pyrite is exhausted. Oxygen is not required for thc fourth reaction to occur. The overall pyrite reaction series is among thc most acid-producing of all weathering processes in nature"

The article goes on to state that the raising and lowering of the water table (wetting and drying) in the reacting environment provides optimal conditions for the weathering of pyrite. The changes in the geochemical environment due to high withdrawal of groundwater resulted in the decomposition of pyrites and the release of arsenic.

3Dipankar Das et al mineralogical studies by XRD(X-ray defraction) shows the presence of FeSO4. (Welch et al., 1988) studied arsenic in the groundwater of the western USA and suggested that the "mobilization of arsenic in sedimentary aquifers may be, in part, a result of changes in the geochemical environment due to agricultural irrigation. In the deeper subsurface, elevated arsenic concentrations are associated with compaction caused by groundwater withdrawal."

If the time of arsenic contamination is after 1975 in Bangladesh, a probable explanation is that the changes in geochemical environment due to the high withdrawal of ground water resulted in the decomposition of arsenic bearing minerals that were stable in a reducing environment. These arsenic oxides if introduced to the reducing conditions below the water table are reduced to poisonous oxide forms. 3"The greatest arsenic concentrations are mainly found in the fine-grained sediments especially the gray clays. A large number of other elements are also enriched in the clays including iron, phosphorus and sulfur. In Nawabganj, the clays near the surface are not enriched with arsenic to any greater extent than the clays below 150 m , in other words, there is no evidence for the weathering and deposition of a discrete set of arsenic-rich sediments at some particular time in the past. It is not yet clear how important these relatively arsenic-rich sediments are for providing arsenic to the adjacent, more permeable sandy aquifer horizons. There is unlikely to be a simple relationship between the arsenic content of the sediment and that of the water passing through it."

The arsenic is associated with low energy sediments and organic matter would also tend to be associated with the lower energy environments also. Organic matter is present in the sediments below the water table in Bangladesh. Arsenic along with other trace elements, when present in the environment, is enriched in organic rich sediments. Sulfur from decay of organic matter combines with iron to form sulfides in reducing environments and these sulfides will incorporate arsenic if arsenic is present. When the groundwater table was lowered by increased irrigation during the dry season and the sediments were exposed to the oxygen from the atmosphere in a moist environment, arsenic rich sulfides associated with organic matter and other reduced arsenic bearing minerals would oxidize in this moist environment and release arsenic. Bacterial decay of the organic matter would produce H2CO3, HCO3- CO3-- , the kind of sulfates present are dependent on pH, and hydrogen sulfide, below an Eh of 3 or H2 SO4 above an Eh of 3. With the appropriate concentrations below an Eh of 3 and between a pH of 3 to 9 these carbonate and sulfur species would react with the ferrous iron in solution to produce siderite and pyrite.(Figure 1 after Robert M Garrels, Solutions, Minerals, and Equilibera)

FIG. 1. Stability relations of iron oxides, sulfides, and carbonate in water at 25 oC and I atmosphere total pressure. Total dissolved sulfur = l0 -6. Total dissolved carbonate = 10. Note elimination of FeS field by FeCO3 under strongly reducing conditions, and remarkable stability of pyrite in presence of small amount dissolved sulfur.

The beginning of poisoning does appear to have occurred after the drilling of the tubewells. Chemical analyses of many tubewell waters was made after arsenic poisoning was recognized and many tubewell waters do show levels of arsenic far above the levels considered safe for drinking. Not all tubewells have arsenic levels above safe limits. Some tubewells that had safe drinking water when first tested were found to be contaminated with arsenic when tested at a later time.

Many hand dug wells existed prior to the time the tubewells were drilled but no arsenic poisoning had been reported until after the nineteen seventies. The observation that tube wells that were once safe and then have become contaminated and the absence of poisoning prior to 1970 when people were drinking water from hand dug wells indicates that some environmental changes occurred after the tubewells were drilled.

What environmental changes occurred in the areas of contamination that could have released arsenic to the groundwater? The pumping of water from wells lowers the water table around the well forming a cone of depression or a cone shaped de-watered zone that expands outward as water is withdrawn. When pumping is stopped the cone of depression fills back up. The repeated draw down and recovery pulls oxygen into the dewatered area of the cone of depression and would in time cause oxidation of organic matter and the release CO2, oxidation of arsenic bearing marcasite or pyrite and the formation of hydroxides and sulfates of iron. Also during the 1970 time period when the tube wells were drilled major dams, such as the Farakka on the Ganges, were built in India on rivers flowing into Bangladesh. The diversion of water from Bangladesh resulted in increased irrigation in Bangladesh and a considerable lowering of the water table during the dry season along the flood plains of the rivers from which water had been diverted. The recharging of the water table during the rainy season would bring abundant oxygen to the sediments of the de-watered zone and have the same affect as mentioned for the cone of depression around tube wells.

An investigation should be made of the effects of the repeated draw down and recharge of the water table on the chemistry of the upper levels of the groundwater zone where most of the arsenic contamination is found. Also a determination should be made as to the possible connection between the changes brought about by the de-watering around tubewells and the appearance of arsenic in previously uncontaminated tubewells.

The UK/DFID report states that " the top of shallow aquifers, at depths of less than 10m, also appears to be less contaminated than deeper down as indicated by the observation that shallow hand dug wells are usually uncontaminated even in areas of high arsenic contamination. These wells, however, face the highest risk of microbiological contamination".

These shallow levels would be the levels most diluted by recent rains and sample when the wells were sampled with respect to recharge would be important. If the cone of depression around tube wells extend below the historical zone of water fluctuation that was established before increased irrigation or well pumping occurred then arsenic bearing minerals that were stable in reducing environments could become unstable by drawing in oxygen from the atmosphere or by the addition of oxygen saturated rain water. The depth of the cone of depression is a function of the permeability, rate and quantity of water withdrawn. The introduction of oxygen in open wells only affects the surface and diffusion is very slow and ineffective in oxidizing dissolved iron unless there is agitation of the water and continued introduction of oxygenated air into the well.

The climate of Bangladesh is conducive to the formation of laterite type soils from which most of the elements have been leached leaving behind only the insoluble oxides and hydroxides of aluminum and iron in a soil with a pH of from 5 to 9. Figure 2 (Brian Mason, Principles of Geochemistry, 167(1966). Compounds most stable in a laterite soil are aluminum hydroxide (gibsite) and ferric oxides and hydroxides.

Fig. 2. The stability of silica and alumna as a function of pH

The minerals present in saturated zone below the water table could be similar to minerals found in some marshes. Drainage of some tidal marshes or the exposure of acid-firming underclays results in acid sulfate soils (cat clays) that contain pyrite, jarosite, mackinawite, and alunite (Dost, 1973: Iverson and Hallberg,1976) Some of the minerals groups present include { Beudantite group (Sr, Be, Ca, Al, Pb) FeO3 (AsO4,SO4)(OH)6 Jarosite [K Fe3(SO4, AsO4)2(OH)6]} Alunite Group: AB3(XO4)(OH)6.

Arsenic concentrated in the organic mater in the sediments if converted to minerals such as beudantite during weathering and then released when the water table rose exposing this layer of accumulation to reduction of the hydroxides, arsenides and sulfides would release poisonous forms of arsenic oxides to the ground The arsenic bearing minerals brought in from the mountains would also be stable in the reducing environments below the water table. These minerals would also break down in a moist environment of the vadose zone if the water table were lowered.

An extensive sampling and mineral analysis of the zones at the interface of water table and zone of aeration may reveal the presence of arsenic bearing minerals. If these arsenic bearing iron hydroxides jarosite and the Alunite Group: AB3(XO4)(OH)6 are present, the reduction of these minerals could cause arsenic to be released to the groundwater. The subsequent rise of the water table could provide the reducing environments to reduce the hydroxides and form poisonous arsenic compounds that would migrate with the ground water flow through the sediments.

The high surface area of fine grained particles allows oxidation to occurs very rapidly. If the lowering of the water table began 23 years ago with the diversion of water by the Farakka barrage there is sufficient time to release and leach, absorbed arsenic if present in pyrites or iron hydroxides to the water table.

The crystalline iron sulfate occurs in several different morphological forms and the grain sizes ranging from invisible to several inches in size. The "framboidal" form is considered highly reactive and characterized by a small grain size and large surface area. Frequent lithofacies change, vertical and horizontal distribution of thickness are common in a delta. Water table elevations in the Bengal delta fluctuate in response to seasonal conditions forming a zone of cyclic wetting and areation. This provides optimal conditions for the oxidation and subsequent leaching of iron sulfides and associated weathering products.

Fig. 3. Sedimentary chemical end-member associations in their relations to environmental limitations imposed by selected Eh and pH values. Associations in brackets refer to hypersaline solutions. [From W. C. Krumbein and R. M. Garrels, Jour.Geol., 60, 1 (1952)]

 

Depleted Ganges threat to Sundarban wetlands.

WASHINGTON, Nov 30: More than 50 per cent of the world's major rivers are seriously depleted and polluted, poisoning surrounding ecosystems and threatening the health of tens of millions of people, says the World Commission on Water (WCW). A similar problem affects the Ganges, which serves 500 million people. It has become so depleted that during the dry season, the Sundarban wetlands in Bangladesh-one of the world's most unique ecosystems comes under serious threat. In Eurasia, the sickest rivers labeled "very unhealthy" by the commission-include in Central Asia, the Gages, which flows from the Himalayas to the Bay of Bengal.

During the summer months the high rain fall in Bangladesh may dilute the water in the upper part of the water table. On the other hand the oxygen brought into the sediments in the dewatered zone, created by irrigation during the dry season, may react with minerals previously protected from oxygen by the water table.

In order to determine the processes going on that are causing the arsenic problems the following kinds of information are needed: 1. The time of the year samples were taken, 2. records of precipitation, well inventory (depth to water depth of well, location, etc) 3. the depth at which water was sampled, 4. pumping history, 5. pH, and other geochemical data obtained at different depths in the well and at different times, 6. how the samples were collected and etc. Good sampling procedures need to be carried out over a wide area and over an extended period of time. Good records should be kept on rain fall and water table levels.

Conclusions:

To find a lasting solution to the problem of the arsenic poisoning that is plaguing millions of Bangladesh people the origin of the arsenic must be established and remedial measures taken to assure that further contamination of domestic water supplies does not occur. The different explanations for the arsenic source should be thoroughly researched and the source rigorously determined. A research proposal will be submitted to find the source(s) and cause(s) of arsenic contamination and establish a system of clean and healthy water management in Bangladesh.

R.T. Nickson, , J.M. McArthur,, P. Ravenscroft, W.G. Burgess,K.M. Ahmed, Mechanism of arsenic release to groundwater, Bangladesh and West Bengal:Geological Sciences, University College London, Gower St., London, WC1E 6BT, UK Mott MacDonald International Ltd., 122 Gulshan Avenue, Dhaka -1212, Bangladesh

Garrels, Robert M. and Christ Charles L., Solutions, Minerals, and Equiliberia. Freedman, Cooper & Company, 1736 Stockton Street, San Francisco , CA 941333, 450 pages.

Brian Mason, Principles of Geochemistry, second edition, John Wiley & Sons, Inc. New York, 1958, 310 pages.