|The Daily Star||
Volume 2 Number 286 Wed. June 09, 1999
By Md Khalequzzaman
Arsenic contamination of groundwater in Bangladesh is now a well established fact of life. Preventive measures are imperative. However, before designing any successful arsenic mitigation plan it is necessary to determine the extent of the problem, as well as the geologic factors that control the source and mobility of arsenic in groundwater. Many studies are conducted and many others are underway to achieve this goal. In April 1997 a World Bank Fact Finding Mission visited Bangladesh to assess the situation and to initiate a mitigation programme. As a part of the "Rapid Investigation Programme" recommended by the World Bank, the UK Department for International Development (DFID) agreed to finance a project to evaluate the arsenic contamination. On behalf of the Government of Bangladesh (GOB), DFID appointed British Geological Survey (BGS) as overall consultants for the study. BGS in turn appointed Mott McDonald Ltd (MML) to carry out the bulk of the Phase I work. Recently, MML have submitted a report of Phase I.
I have read the "Executive Summary of the Main Report of Phase I, Groundwater Studies of Arsenic Contamination in Bangladesh" by BGS and MML for the GOB. This report is the most comprehensive compilation of pre-existing data and the results of new analyses on groundwater collected from Bangladesh. Both BGS and MML deserve a round of applause. They did a great job in summarizing the extent of the arsenic problem in Bangladesh. I understand that this is an executive summary of the first of two phase reports, and that a five-volume report is available for purchase. Although the full report probably has a detail explanation of all the findings, the executive summary stands alone and should be internally consistent with the elaborate report. It is based on this premise that I decided to make a few comments on the summary. I have organized my comments on various topics in the same order that the writers used in their table of content. However, I did not make comments on each topic listed on the table of content.
Review of existing data:Based on existing data it appears that arsenic contamination is maximum in the southern and northeastern Bangladesh at a depth of 10 to 100 meters. However, the authors mentioned the spatial variability and patchy nature of arsenic distribution. According to earlier reports by other authors and organizations (NIPSOM, DCH, etc.), arsenic contamination is very high in southwestern districts as well. According to the report, Arsenic seems to be of geologic origin. Sediments deposited on delta plain and coastal plain in Bangladesh originate in various parts of the Himalayas and are carried by rivers that flow through India. To understand the full extent of the spatial distribution of arsenic and the geochemical processes that control the transport and sink of arsenic, one will need to analyze both sediment types and arsenic concentrations in water for the entire Ganges-Brahmaputra-Meghna watersheds. If arsenic solely originates by adsorption-desorption of arsenic-rich iron oxyhydroxides then arsenic must be present in the aquifers of the entire watershed. Is it the case? We know that arsenic contamination is a problem in some parts of West Bengal. Did anyone look for arsenic in groundwater in, say, Uttar Prodesh, Bihar, Assam, Tripura. Geologic processes (such as flooding, arsenic contamination, etc.) are not bounded by political boundaries. We need to know the nature of distribution of arsenic at various reaches of the watersheds - from the place of origin (Himalayas) to the various depositional environments in India and Bangladesh. As the surface water and groundwater flow from outside Bangladesh, do the concentrations of arsenic increase or decrease? What geochemical processes are dominant along the flow path of groundwater?
Collection of existing data: It is mentioned in the summary that "the groundwaters in regional survey have characteristics typical of reduced groundwater: high dissolved iron, manganese, and low sulphate concentrations." Since the data were collected by various organizations, how could the author possibly know how reduced the groundwater was at the time of collection? Arsenic is a very redox sensitive element and its mobility and speciations are very much controlled by pH and Eh of the groundwater. This summary does not mention the pH and Eh range at the time of collection. These measurements should be done in the field, not in the lab, using flow-through cell, which are also used to measure other sensitive field parameters, such as pH, Temperature, dissolved oxygen, and total dissolved solid. The type of dissolved iron species are also pH-Eh sensitive and are important in arsenic mobility. It is not clear from this summary as to what methods or measures were taken to ensure the quality of these measurements.
Small-scale variability: the special study areas:Approximately 50 wells per thana (Nawabganj, Faridpur, and Laksmipur) were studied in greater detail to determine the small-scale variability in arsenic contamination. Lithologic logs were examined to determine the structure and continuity of aquifers. It is not clear if new wells were dug for this purpose. I have serious concern about pre-existing lithologic logs in Bangladesh. Firstly, to my knowledge (I worked for a relevant GOB organization) no detail lithologic logs are kept by DPHE. A lithologic log described by engineers or drillting technicians generally do not serve the purpose of understanding geologic nature of sediments and the depositional environments that various lithosomes represent. Lithologic logs described by geologists/hydrogeologists can only be used for this purpose. In addition, privately dug hand-pump owners do not keep any lithologic logs. They dig their wells based on experience about the water-bearing sand layer in a locality. Secondly, examination of pre-existing logs is not adequate to understand small-scale variability in mineral composition, because those logs were not meant to be used for such detail analyses. Thirdly, one log per 7 km2 is not adequate for small-scale variability or arsenic concentration. Several dozens of monitoring wells need to be placed around a hot spot (both up-gradient and down-gradient of the flow) of contamination - more like studying a plume from a landfill and leaking underground storage tank.
The summary also mentions that "not all of the wells in the hot spot are contaminated, but some are." The question is why? Obviously, there are geochemical variations in terms of mineral composition, extent of aquifer vs. aquiclude materials, organic content, pH-Eh, etc., which need to be determined.
The sedimentary depositions in a delta plain or coastal plain show a great deal of variability in terms of facies change. A simple layer-cake approach in analyzing spatial and temporal variability in the aquifer materials will lead to erroneous conclusions. This perspective seems to be missing from this summary report. To get a better understanding of the subsurface geology of Bangladesh, without which it will not be possible to understand the spatial and temporal variability of Arsenic contamination, a series of new wells will have to be dug and detail lithologic logs maintained by professional geologists. Once there are enough geologic control of the aquifers, then the pre-existing lithologic logs can be used to supplement the newly acquired data.
Geologic sources of arsenic: According to the summary, 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. Arsenic can be derived from both adsorbed arsenic (e.g. in pH >8 and Eh <-250 mv) and from arseno-pyrite by oxidation. Therefore, it cannot be concluded as to what is the major mechanism by which arsenic is being introduced in groundwater in Bangladesh. My understanding is that depending on Eh-pH conditions and other mineralogic parameters (e.g. amount of dissolved iron species, sulfate or phosphate present), arsenic mobility and chemistry varies from place to place and from surface water to aquifers.
The summary mentions about "physical separation of sediments during their transport and reworking in the delta region has resulted in a separation of arsenic-rich minerals in fine-grain sediments in lower part of the delta." According to the summary, "this is likely to be responsible for the greater contamination in the south and east of Bangladesh". This is probably true, however, the delta has been prograding over geologic time. As a result, the areas that are located inland at the present time used to be the lower reaches of delta in the past. Before any such generalization can be made, it will be necessary to determine the paleogeographic maps showing locations of various environments of the delta for various geologic times. To my knowledge, no such study has been done in great detail.
This summary also makes some comments about the sea-level fluctuations and its impact on arsenic concentration. According to the summary "all highly contaminated groundwater occurs in sediments deposited since last glacial period, i.e. 18,000 years ago.
This is a very naïve statement and lot more need to be understood (such as, ages of various layers and groundwater, paleogeographic reconstruction of the delta as mentioned earlier, occurrence of arsenic in incised valley-fill deposits, etc.). In addition, the location of lower part of the delta must have been beyond the "Swatch of No Ground" in the Bay of Bengal - not inland - during the low sea-level stand.
Influence of pumping and irrigation: According to the summary, "older wells are more likely to be contaminated than recently constructed ones." Why? No explanation was offered. Also, many older wells are constructed using metallic pipes. It need to be studied to see if corrosion of the pipes accelerate or enhance arsenic contamination. It is not known if the type of pipes (PVC vs. metallic) makes any difference in contamination.
Although it seems that shallow wells are more contaminated than the deeper ones, it cannot be concluded that all deep aquifers are safe, especially when reduced conditions (deeper wells must be more reduced than the shallower ones) control arsenic contamination and mobility.
Solutions?:Based on the extent of arsenic problem in Bangladesh and based on the complexity of arsenic mobility in groundwater, it appears to me that it is almost impossible to treat the aquifers in Bangladesh by any treatment procedure that is currently available. Our only option is to treat the pumped water at point of entry (POE) or point of use (POU). Ferric hydroxide, Fe(OH)3, apparently is the best sorbent of arsenic at low pH<8. Ferric hydroxide, which is very abundant in red soil of, say, Savar, Madhupur Garh, and the Barind Tract probably can be used in filtering mechanism. Since arsenic is a redox sensitive element and since our treatment will have to occur on surface water, any adsorption reaction that is feasible at high Eh (oxidizing environment) will be the only one that can be used. Ferric iron (as opposed to ferrous iron) is stable under high Eh.
Conclusions: While I think the BGS and Mott McDonalds' report is a very good compilation of existing data, it raises more questions pertaining to their interpretation of geologic nature and its relevance to arsenic contamination in Bangladesh than it answers. More study need to be carried out to better understand the spatial-temporal variability and the controlling geologic factors of arsenic contamination in Bangladesh. This report is a first step in the right direction. A holistic approach is necessary to solve the arsenic disaster in Bangladesh. Hopefully, the questions raised will be addressed or answered by the authority concerned before proceeding to the second phase of the arsenic study in Bangladesh.
The writer is Assistant Professor of Geology, Georgia Southwestern State University, Americus, GA 31709, USA.