BY Dr. Thomas E Bridge, Professor Emeritus(Geology), Emporia State University,
Meer T. Husain, Environmental Geologist,Kansas Department of Health & Environment, Kansas
Bangladesh is faced with many potential natural disasters. One immediate threat is arsenic poisoned water used for daily personal needs. Water is essential to life and the availability of water determines the population density of an area. The quality of water is a major factor that determines the health of the population. Bangladesh is situated on a large delta and ground water is abundant and available almost everywhere as surface water and or from one or more buried aquifers. Water is present in sufficient quantities to supply all domestic and industrial demands if distributed properly and protected from pollution. Poisoning from groundwater obtained from tube-wells drilled in the past thirty years to prevent disease caused by using polluted surface water for personal needs has affected millions of people. The problem is wide spread with some areas affected more than others. The potential damage by the arsenic threat is so great that immediate action to provide potable water is needed in some areas. The severity of the poisoning for the short term is not believed to be as great a hazard to the people as the using of untreated surface water. However, long term affects may be even more hazardous to future generations because arsenic is both carcenogenic and mutagenic.
Groundwater arsenic contamination in Bangladesh was discovered about 5 years ago. It is unfortunate that peoples are still drinking and using poison groundwater for their daily requirements. We have had experience handling more than one hundred instances of contaminated or polluted water projects that were discovered in many different geological settings and environments. The successful investigation, remediation and monitoring of alternative safe water supplies for these projects leads us to believe that there is enough information to develop and distribute safe water to the affected areas in Bangladesh.
An excellent report was recently published by the British Geological survey and Mott MacDonald (UK).
Groundwater Studies of Arsenic Contamination in Bangladesh, Ministry of Local Government, Rural Development and Cooperatives, Department of Public Health Engineering, and Department for International Development (UK).
Excerpts and comments on parts of the report are as follows:
Several papers have been presented theories on the source of arsenic poisoning. The source of the arsenic is academic at this stage. However, the (UK) report states that
"Description of the problem is complicated by large variability at both local and regional scales. The arsenic is of geological origin and is probably only apparent now because it is only in the last 20-30 years that groundwater has been extensively used for drinking water in the rural areas. However, the arsenic has probably been present in the groundwater for thousands of years. It is difficult to say for sure whether it will get better or worse with time but the likelihood is that any changes are likely to be rather slow - seen over years or even longer".
The more important investigations are those that show locations and degree of contamination in tube-wells especially in areas where people show signs of arsenic poisoning. Investigations have shown that the problem is wide spread, is more concentrated in some areas than others and varies with depth, sediment age. and concentrations changes with time. Enough information is known to know that safe supplies exist in adequate quantities to supply the needs of the people and industries of Bangladesh.
(UK) "In many ways, the alluvial sediments of Bangladesh are ideal for groundwater development. The sediments are characterized by fining upward sequences of sand, silt and clay, with good aquifers in medium to fine sands. The unconsolidated sediments can be drilled by hand down to depths of 80 meters or more in a couple of days. The water table is high, typically less than 7 m below ground level, which means that ordinary suction hand pumps are able to extract the water in most places. In the drier areas, the hill tracts, and where intensive groundwater irrigation has increased the annual decline in the water table, positive displacement 'Tara' pumps must be used. The high rainfall ensures that the aquifers are fully recharged each year. This combination of circumstances has meant that the groundwater has been extensively exploited in recent years, a policy encouraged by government and other agencies".
The first step is to devise a plan for the distribution of good potable water to all of Bangladesh while simultaneously identifying multiple sources of good potable water.
(UK) "There are now about four million tube-wells in Bangladesh. The development of tube-wells has been responsible for the reduction of infant mortality from diarrheal diseases, and the achievement of food-grain self-sufficiency through groundwater irrigation. It is estimated that 95% or more of Bangladeshis now use groundwater for drinking water.
Much of Bangladesh is characterized by a two-aquifer system. A shallow aquifer typically extending from less than 10 meters to more than 100 meters below ground level, and a deeper aquifer below about 200 meters. A surface layer of silty clay forms a semi-confining layer and a lower clay layer sometimes separates the shallow and deep aquifers. In much of southern Bangladesh, the situation is more complex with a division of the shallow aquifer into two by a low permeability silt-clay layer.
The shallow (or main) aquifer has been most extensively exploited and is the source of the arsenic problem. Groundwater from depths of more than 150-200 m appears to be essentially arsenic-free. This confirms earlier findings. Indeed the extent contamination (1% of deep wells deeper that 200 m) observed in our survey was even less than in earlier surveys. This statement must be qualified by the fact that most of the deep wells sampled were from the coastal region where the deep wells have been sunk to avoid salinity in the shallow aquifer. However, some test deep boreholes sunk recently by DPHE in badly-affected regions farther north seem to confirm this but it is not yet established as a universal fact and needs further testing. The top of the shallow aquifer, at depths of less than 10 m, 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."
The sources of good potable water could range from purification of surface water from rivers, lakes, streams, building collecting galleries below or along side streams, lakes and rivers, drilling deep wells, collecting rain water, desalinization of sea water, removing arsenic and other chemical contaminants from water and etc. The final plan would probably include a number of the above mentioned sources. All water should be thoroughly tested before it is added to an integrated system and a plan for continued monitoring the quality of water at the source and distribution points established and maintained to identify and eliminate contamination in the future. The quantity and quality available from individual sources may vary from season and from year to year therefor the quality and quantity available at any distribution point may vary. A system of reservoirs should be established to store the water during low demand in order to supply adequate water as demand varied from one area to another. An inter connected system would provide adequate supplies to areas by careful regulation of all supplies. Sealed reservoirs to store the water as good water sources are developed. A sealed plastic pipe system should be established in local areas to distribute the water to where it is needed. There may be less expensive methods of water development and distribution but due to the many sources and kinds of contamination in highly populated regions this method would provide the greatest safe guards against contamination from all sources and be less expensive in the long run.
There are many sources of arsenic in the structurally active mountainous region from which the sediments come. Sulfides of arsenic, aresenopyrite (Fe As) S, orpiment As2 S3, realgar As S, in the presence of water and atmospheric oxygen, weather to hydrous arsenic oxides and oxides, iron oxides, and sulfuric acid. Arsenic is labile, therefore readily changes oxidation states or chemical form through chemical or biological reactions that are common in groundwater environments. The highly oxidized, arsenolite As2 O 3 and Fe2 O 3 are relatively insoluble in oxygen rich water. The reduced arsenous oxides (As O)aq+ , (As O)2aq-, and the dominate forms found in reducing environments in groundwater, arsenate H3AsO4aq, arsenite H3AsO3aq, are soluble in water. The arsenate/arsenite are non ionic species and the presence of one or the other is largely controlled by the Eh and pH of the groundwater. Clay, carbonaceous material, oxides, of iron, aluminum, and manganese, adsorb non ionic forms of arsenic decreasing its mobility in oxidizing environments near the surface. David B. Vance: Arsenic - Chemical Behavior And Treatment 1999; Internet Publication. States:
Ferric hydroxide under the right pH conditions has the capacity for cation or anion exchange and with an arsenic concentration of 50ug/L, ferric hydroxides can potentially adsorb 0.5 to 5 pounds of arsenic per cubic yard of aquifer matrix.’ Feric hydroxide is adsorbed by clay and forms concretionary growths at the groundwater table surface where the clay is alternately exposed to oxidizing environment and reducing environments as the water-table fluctuates up and down during seasonal changes. The iron oxidize to its insoluble form leaving a red ferric oxide or yellow hydrous iron oxide stain on the clay. If arsenic has been adsorbed and remains immobile in the oxidizing environment of the water-table the accumulation of these insoluble oxides would also form a concentrated source of arsenic for later release to the system in reducing environments. Sulfate minerals formed by the reaction of sulfuric acid with ions in solution are soluble and migrate out of the system. In an aggrading delta the iron and arsenic oxide layers are eventually buried and a reducing environment will cause these oxides to be reduced to their soluble form. The original arsenopyrites were concentrated along with other heavy minerals as placers and are not present in the sediments because they oxidized long ago. The soluble species are migrating away from their points of origin in response to ground water flow in the aquifers that contain them.
(UK) "The groundwater arsenic problem in Bangladesh arises because of an unfortunate combination of three factors: a source of arsenic (arsenic is present in the aquifer sediments), mobilization (arsenic is released from the sediments to the groundwater) and transport (arsenic is flushed away in the natural groundwater circulation). Previously a number of anthropogenic explanations had been for the occurrence of arsenic in groundwater. While it is possible that some may explain isolated cases of arsenic contamination, none of the anthropogenic explanations can account for the regional extent of groundwater contamination in Bangladesh and West Bengal. There is no doubt that the source of arsenic is of geological. The arsenic content of alluvial sediments in Bangladesh is usually in the range 2-10 mg/kg; only slightly greater than typical sediments (2-6 mg/kg). However, it appears that an unusually large proportion of the arsenic is present in a potentially soluble form. The high groundwater arsenic concentrations are associated with the gray sands rather than the brown sands. There is a good correlation between extractable iron and arsenic in the sediments and a relatively large proportion (often half or more) of the arsenic can be dissolved by acid ammonium oxalate, an extract that selectively dissolves hydrous ferric oxide and other poorly ordered oxides. It therefore appears likely that a high proportion of the arsenic in the sediments is present as adsorbed arsenic. This would not be true of arsenic present in primary minerals such as arsenic-rich pyrite.
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.
It is likely that the original sources of arsenic existed as both sulfide and oxide minerals. Oxidation of pyrite in the source areas and during sediment transport would have released soluble arsenic and sulfate. The sulfate would have been lost to the sea but the arsenic, as As(V), would subsequently have been sorbed by the secondary iron oxides formed. These oxides are present as colloidal-sized particles and tend to accumulate in the lower parts of the delta. Physical separation of the sediments during their transport and reworking in the delta region has resulted in a separation of the arsenic-rich minerals. The finer-grained sediments tend to be concentrated in the lower energy parts of the delta. This is likely to be responsible for the greater contamination in the south and east of Bangladesh. The map of arsenic-contaminated groundwater shows that highly contaminated areas are found in the catchments of the Ganges, Brahmaputra and Meghna rivers strongly suggesting that there were multiple source areas for the arsenic.
Mobilization of the arsenic - redox processes
Burial of the sediments, rich in organic matter, has led to the strongly reducing groundwater conditions observed. The process has been aided by the high water table and fine-grained surface layers which impede entry of air to the aquifer. Microbial oxidation of the organic carbon has depleted the dissolved oxygen in the groundwater. This is reflected by the high bicarbonate concentrations found in groundwater in recent sediments. There is a relationship between the degree of reduction of the groundwater’s and the arsenic concentration - the more reducing, the greater the arsenic concentration.
The highly reducing nature of the groundwater’s has led to the reduction of some of the arsenic to As(III) and possible desorption of arsenic since As(III) is normally less strongly sorbed by the iron oxides than As(V) under the near neutral pH groundwater conditions observed. Further reduction will lead to the partial dissolution of the poorly crystallized ferric oxide with consequent release of iron and additional arsenic. Other strongly sorbed ions, especially phosphate, will also be released by iron oxide dissolution. The relatively high phosphate concentrations present in the groundwater’s will compete with As(V) for sorption sites and is another factor that favors high groundwater arsenic concentrations. It may also make arsenic treatment more difficult.
The 'pyrite oxidation' hypothesis proposed by scientists from West Bengal is therefore unlikely to be a major process, and that the 'oxyhydroxide reduction' hypothesis (Nickson, R. et al. 1998 in Nature; v395:338) is probably the main cause of arsenic mobilization in groundwater. It is difficult to account for the low sulfate concentrations if arsenic had been released by oxidation of pyrite. Moreover, mineralogical examination suggests that the small amounts of pyrite present in the sediments have been precipitated since burial.
Present groundwater movement is very slow because of the extremely low hydraulic gradients found in the delta region. Except where modified by pumping, groundwater circulation is largely confined to the shallow layers affected, by local topographic features and the presence of rivers. Close to rivers, the enhanced groundwater flow may lead to a greater dispersion of arsenic along river banks. Annual fluctuations of the water table, typically about 5 m, will affect groundwater and arsenic movement in the shallow layers. There may have been some flushing of arsenic from the shallowest layers.
At greater depths, groundwater moves slowly in response to the low regional gradients. This is consistent with the old age of the waters. The lateral and vertical spread of contaminants is slow even without considering the retardation due to sorption. Modeling suggests that even in the most permeable layers, arsenic movement is likely to be limited to a few meters a year.
The permeability of the silty clay layers is low and in the case of a narrow horizon of silty clay, water will preferentially move through the adjacent more permeable sandy layers. This effectively protects the silty clay layers from strong leaching and possibly preserves arsenic-rich zones. This relative lack of water and arsenic movement and the strong stratification of the aquifer therefore both preserve the high concentrations of arsenic from leaching and lead to the great spatial variability observed. The conclusion from this is that in the absence of man's intervention significant short-term (less than a few decades) variations in arsenic concentrations are unlikely to occur at depth. "
The migratory nature of the arsenic and the uneven distribution of point sources produce a dynamic environment. The solution to the problem would be to initiate a dynamic system of water production and distribution that could respond to changes in water quality at production points and distribution points. A comprehensive plan should be worked out to provide a safe water supply for all of Bangladesh. The emergency measures to provide good potable water to severely affected areas should be the starting point for this overall plan. A closed tube system tied to closed reservoirs filled from a quality safe water source should be developed. Appropriate continued testing at various sources and dispersal points to guard against contamination is proposed.
UK) "There are no long-term water quality monitoring data to definitively establish how arsenic concentrations change over time. The few data that exist, extending over no more than two years, show that some wells have increased in concentration, but cannot yet be taken as proof of general or systematic changes. The Regional Survey showed a strong correlation between the year of construction and the proportion of wells that are contaminated above the Bangladesh Standard. On average, older wells are more likely to be contaminated than recently constructed ones. Only long-term monitoring will determine whether this actually corresponds to increasing concentrations at individual wells."
The plan should be thoroughly analyzed as to quantity of water needed at dispersal points now and in the future. As water sources and reservoirs are developed the requirements on the delivery system could be determined in accordance with point demands. Careful collection of drill samples as exploratory wells are drilled wells, samples from surface excavations, and soil samples should be collected during the establishment of safe water sources to aid in further research on origins and migration of arsenides and related ions. Vertical and horizontal environmental changes in aquifers should be monitored by careful core sample collection. Records kept of Eh-pH measurements in situe., elevation levels of well head surfaces , water table surface elevations, depth of stratigraphic changes and depths and locations of water samples collected from distinct aquifers and at different depths. An on going study to determine the origin, migration and changes in concentrations of arsenides and any other pollutants and their relation to changing environments in the aquifer such as the oxidation-reduction, pH, and other chemical changes resulting from aerobic or anaerobic bacterial action,. and mineralogy of sediments should be maintained through out the development of the project.
(UK) The types of sediment deposited in the delta region have been strongly influenced by global changes in sea level during the Pleistocene glaciations. For example, sea level was more than 100 m lower at the peak of the last lee Age around 18,000 years ago. At that time the major rivers cut deeply incised valleys into the soft sediments of the delta. All of the highly contaminated groundwater’s occur in sediments deposited since that time, while those sediments predating the low sea level stand contain little or no arsenic-contaminated groundwater."
There is a possibility that an influx of unweathered glacial derived sediments were delivered to the delta at the end of glaciation thus enriching the surface deposits with unweathered arsenic minerals that have weathered and concentrated arsenic in the upper layers. The difference may also be the time required to flush the buried aquifers after the arsenic has been mobilized by the reducing environment of deep burial.
When the research from the sample studies is completed the additional information may add insight to many of the questions posed by the (UK) report.