Combating arsenic hazard |
The health problem due to arsenic contamination in groundwater was detected during mid 1990's in Bangladesh. These health problems arose from drinking water of mainly shallow rural supply wells. However, these health problems were scattered and did not occur in all arsenic contaminated areas. Meanwhile it was also observed that no cases of health hazard were found from some highly contaminated arsenic affected areas. Most urban water supplies are also from groundwater source, but arsenic health hazard had not been reported from urban areas as yet. People are known to have lived more than 90 years or so without any health problem due to arsenic in such areas by drinking tubewell water. Therefore the real cause of this health hazard needs to be determined carefully and properly.
Arsenic contamination in water supply wells is certainly not a crisis but a serious problem that arose due to human mistakes. This issue can be addressed by providing safe drinking water and medical care in the affected areas.
There are at present over six million private and government rural water supply wells. The Department of Public Health Engineering (DPHE) is the government agency in Bangladesh responsible for drilling these wells. The major donors have been the Danida, SDC and DFID through the UNICEF. Recent donors are IDB, JICA and Danida. Columbia University of the USA has also been doing a research project on arsenic problem in Bangladesh.
Most rural water supply wells, excepting the coastal belt, are shallow and called shallow tubewells (STW) as per DPHE definition with depth ranging from 30 to 250 ft -- the overall average could be 100 ft. In the coastal belt, shallow wells of depth 30 to 50 ft do exist in considerable numbers. Chloride content in most of these wells are generally between 1000 and 1500 mg/l, which is in excess of WHO desirable limit. As there is groundwater salinity in the upper aquifer system, which is within about 400 ft from the ground surface, wells are generally drilled deeper between 400 and 1200 ft (overall average is perhaps at 800 ft) in confined fresh water aquifers. The wells are drilled wherever the fresh aquifers are available in the coastal belt. These wells are known as Deep Tubewells (DTW). Perhaps 1200 ft is the maximum depth achievable by hand drilling method. A DTW generally costs 15 times more than an STW and therefore beyond the capacity of private ownership. It must be noted that the DTWs in the coastal belt generally have excellent water quality.
The water quality of these rural water supply wells is generally checked for iron and chloride only. Water containing excessive iron/chloride/hardness tastes bad. However, this is not true for arsenic contaminated water. Therefore the only way to know arsenic contamination in tubewell water is to test the water by a field kit or in the laboratory. During the very early 1990's the writer suggested to approve a programme to conduct full chemical analysis of tubewell water of at least three wells per Thana per year in order to ascertain the overall water quality of these wells, but the recommendation was not implemented. (It is worth mentioning here that full chemical analysis of water in all town water supply wells is done on completion of the production wells). Had this been done the arsenic contamination in rural water supply wells would have surfaced much earlier. A World Bank project is currently underway to determine arsenic content of every rural water supply well.
The presence of arsenic in aquifers is from hydrogeological times, in terms of millions of years. It could not be detected earlier because analysis for this element was not carried out for water supply wells as mentioned above. The writer has worked in several countries with alluvial deposits and it has been found that a good aquifer has better water quality than a poor aquifer. Good aquifer generally contains homogenous sand layer and, therefore, its permeability is high and allows water to flow through it at a much higher velocity than the poor aquifer. A poor aquifer is a heterogeneous mixture of sand and clay, such as clayey sand / sandy clay, whose permeability is relatively very low and, therefore, the groundwater flow through it is also very slow. Consequently, the flushing of the dissolved minerals from these formations has either not taken place or has been very slow. The writer's experience in well design in many water quality problem areas revealed that screening of poor aquifers/clayey aquifers was the cause for bad water quality, if any. The flow rates in these poor aquifers are low but sufficient for a hand tubewell. A hand tubewell in Bangladesh produces 3-6 gpm (gallons per minute), depending on the aquifer quality.
The upper aquifers in Bangladesh (except in the North West), particularly those within 100 ft from the surface, are generally clayey sand and/or sandy clay with low transmission capacity and, as such, the natural flow of groundwater has been unable to flush the aquifer. Consequently, the undesirable dissolved minerals, such as arsenic, still remain in these aquifers. To the contrary the upper aquifers in most North West Bangladesh (Bogra, Rangpur and Dinajpur) are generally sand with high permeability and the water quality of the STWs in these areas are also quite good with no arsenic hazard.
Bangladesh is a delta with thick alluvial deposits. Good quality aquifers are available in the depth zone 200 - 500 ft. Therefore if rural water supply wells are properly designed and drilled in these aquifers, good quality water is obtainable in most places. The urban water supply wells are drilled in this zone and there are few reports of excessive (more than the allowable limit) arsenic contamination in these wells.
The rural water supply hand tubewells use only 10 ft of strainer to produce 3-6 gpm. The use of short well intake offers the opportunity to design a better water quality tubewell by placing the screen in the most permeable aquifer. Also the small discharge generally causes a very insignificant drawdown, thus minimizing the risk of inducing leakage from the overlying/underlying any contaminant aquifers. The urban water wells are large capacity wells of discharges 15 - 60 l/s (liters per second) with well intake of (the strainer/screen) 60 - 100 ft. These wells were drilled well below the upper heterogeneous aquifers and have not yet exhibited any arsenic hazard in most cases. Although the relatively high drawdown can induce leakage from contaminant overlying/underlying layers, their contributions, due to very low vertical permeability, remain within the contaminant limit in most cases.
More than 600 large capacity Dhaka WASA wells plus many private water wells have been operating in Dhaka city for many years which already created an overdraft. These wells have been drilled in the permeable sand aquifers, available between 100 and 400 ft. Although this over development and continuous heavy pumping stressed the underlying aquifer system and caused the groundwater level to drop from 20 ft to over 150 ft, it has not yet caused any noticeable arsenic contamination from nearby arsenic hot spot, if any. The heavy pumping by the irrigation wells and the town water wells screening permeable sand aquifers between 60 and 250 ft in North West Bangladesh has yet to cause any noticeable arsenic contamination. Therefore, it is not true that heavy withdrawal of groundwater has caused any noticeable arsenic contamination as of yet. A model study has indicated that it might take many more decades to do so under these circumstances.
Therefore, water from arsenic contaminated wells, whose concentration are not too high, can be used for drinking purpose by using simple treatment such as three kolsi filtration method, etc. until a new safe water supply is established. These new techniques for providing safer water can be implemented either by drilling a new well in the available deeper safe aquifer or using treated surface water. In highly concentrated arsenic areas, new safe water supply source has to be established in the same way.
In Bangladesh, it is cheaper to develop safe groundwater for water supply in rural areas by hand tubewells as daily per capita water demand is very low as compared to urban areas. Treated surface water is the alternative where safe groundwater is not available. More than 80 per cent of people live in rural areas of Bangladesh.
The above has been prepared to demonstrate the real technical picture on the occurrence of arsenic hazard in many areas of Bangladesh. Groundwater is not only the most economic but also the safest and easily manageable source of water supply in Bangladesh as well as in most other countries of the world. What is important is to manage these groundwater resources properly and to design wells by positioning the screen/strainer in the right aquifer as described above. It is also essential to establish a network of tubewells to monitor groundwater quality in order to ascertain any changes over time.
Since the arsenic problem surfaced, many national and international seminars/workshops have taken place on this subject but things have not changed for the better technically. Do we really need so many seminars on this issue? What we need is to ascertain the real medical cause behind this arsenic health hazard and offer our rural people safe and potable drinking water in the affected areas. All agencies and donors need to be focused and strict during the project formulation and implementation phases in order to achieve the desired goal.
Luthfe Ali, now in New York, is a hydrogeology specialist and has worked extensively on international groundwater development projects in Bangladesh, Indonesia, Middle East, and the UK.