Arsenic contamination of groundwater in Bangladesh

A survey of well waters (n=3534) from throughout Bang- ' ladesh, excluding the Chitt;agong Hill Tracts, has shown that water from 27% of the 'shallow' tubewells, that is wells less than 150 m deep, exceeded the Bangladesh standard for arsenic in drinking water (50 flg L -I). 46% exceeded the WHO guideline value of 10 flg L-I. Figures for 'deep' wells (greater than 150 m deep) were 1% and 5%, respectively. Since it is believed that there are a total of some 6-11 million tubewells in Bangladesh, mostly exploiting the depth range 10-50 m, some 1.5-2.5 million wells are estimated to be contaminated with arsenic according to the Bangladesh standard. 35 million people are believed to be exposed to an arsenic concentration in drinking water exceeding 50 flg L-I and 57 million people exposed to a concentration exceeding 10 flg L -I. There is a distinct regional pattern of arsenic contamination with the greatest contamination in the south and south-east of the country and the least contamination in the north-west and in the uplifted areas of north-central Bangladesh. However, there are occasional arsenic 'hot spots' in the generally low-arsenic regions of northern _Bangladesh. In arsenic-contaminated areas, the large degree of well-to-well variation within a village means that it is diffic'ult to predict whether a given well will be contaminated from tests carried out on neighbouring wells. The young (Holocene) alluvial and deltaic deposits are __ m8-s_caffe_c.ted_whereas_the older alluvial_sediments in the ,I tion' hypothesis in which pyrite oxidation in the zone of water table fluctuation is assumed to release arsenic and ultimately to be responsible for the groundwater arsenic problem. There is no evidence to support the proposition that the groundwater arsenic problem is caused by the recent seasonal drawdown of the water table due to a recent increase in irrigation abstraction. Monitoring of groundwaters at two-weekly intervals at a number of sites, and at different depths, has shown some variation with time but there is as yet no convincing evidence for seasonal changes. Dramatic changes in contamination are not expected within such a short timescale. A monitoring programme should be undertaken at a range of sites to monitor possible long-term changes. In the three contaminated areas studied in most detail, the arsenic concentration increases most rapidly between 10-20 m below ground level. \Vhile arsenic is the single greatest problem in Bangladesh groundwaters, other elements of concern from a health point of view, are manganese, boron and uranium. Some 35% of the groundwaters sampled exceeded the WHO guideline value for manganese (0.5 mg L-I). The spatial pattern of the arsenic and manganese problem areas was significantly different and only 33% of shallow well waters complied with the WHO guideline values for both arsenic and manganese.