Arsenic is a metalloid element that, once lauded for its beneficial health effects, is now notorious for its toxicity and carcinogenicity. Arsenic is ubiquitous in the earth’s crust, although generally constituting less than 1% of most rocks, coals, and soils. Higher arsenic concentrations are associated with deposits of alluvial lacustrine in semi-arid zones, volcanic deposits, and geothermal systems. Arsenic is used extensively in manufacturing and agriculture. Arserous oxide (As4O6 ) is the starting point for the manufacture of other arsenic-containing compounds used in pesticides, cotton desiccants, and wood preservatives, in the manufacture of glass, ceramics, semi- conductors and dyestuffs, and in the processing of glass pigments, textiles, paper, metal adhesives, and ammunition. Arsenic can also found in coal and coal combustion by-products. Arsenic is usually found combined with one or more other elements such as oxygen, chlorine, and sulfur, with the highest mineral concentrations occurring as arsenides of gold, silver, copper, iron, and lead. Major arsenic-containing minerals are arsenopyrites (FeAsS), realgar (As4S4), orpiment arsenic trisulfide (As2S3). Arsenic combined with oxygen, chlorine or sulfur is referred to as inorganic arsenic, whereas arsenic combined with carbon-based molecules is referred to as organic arsenic. The form and the oxidation state in which arsenic is found are called its speciation. Because the different forms differ in aqueous solubility, the speciation of arsenic plays an important role in the mobilization and the transport of arsenic (Cullen and Reimer 1989). Arsenic in water commonly exists in two inorganic forms, trivalent arsenite, H2AsO3, and pentavalent arsenate, H2AsO4. Inorganic forms of arsenic dissolved in drinking water are the most significant forms of natural exposure. Organic forms of arsenic that may be present in food are much less toxic to humans. Common organic forms of arsenic are methyl arsonic acid, CH3AsO(OH)2, and dimethyl arsinic acid or cacodylic acid, (CH3)2As(OH).
Arsenite is more soluble and consequently more mobile than arsenate (Deuel and Swoboda 1972). In subsurface soil, arsenates are mostly absorbed to iron (III) hydroxides and aluminum hydroxides. These hydroxides can be discrete particles or thin layers around minerals such as clays. Arsenite absorbed by iron (III) sulfide (FeS2) minerals, which can include As in its mineral structure (Huerta and Morse 1992), can be absorbed directly on clay minerals (Ferguson and Gavis 1972), and on organic material (O’Neill 1990). There have been a number of documented mass exposures to excessive concentrations of As in water. For instance, in the 1960s, arsenic-contaminated wastes from mining operations leached into the spring waters of Antofagasta, Chile. Many people, especially a large number of children, were affected by drinking contaminated water (Borgono et al. 1977). In the Dzungaria Basin of Xinjiang Uighur Autonomous Region of China, As concentration in groundwater drawn from artesian wells ranged from 0.04 to 0.75 mg l–1 (Lianfang and Jianzhong 1994). Natural oxidation of sulfide-containing arsenopyrite minerals and wastewater discharged by mines resulted in stream water concentrations of up to 0.175 mg l–1 As in the Obuasi gold mining area of Ghana (Amasa 1975, Smedley et al. 1996).
High concentrations of As in surface soils were reported in south-west England resulting from mining and smelting activities (Colbourne et al. 1975, Abrahams and Thornton 1987). The main route of arsenic exposure, in this case, was accidental ingestion of contaminated soil and dust and eating vegetables grown in contaminated soil (Thornton 1995), causing melanoma of the skin, and peripheral neuropathy in the affected population (Heyman et al. 1956; Clough 1980). In the 1960s, concentrations of up to 0.6 mg l–1 As was discovered when some 65,000 artesian wells were sampled in Taiwan (Tseng et al. 1968). Speciation studies found the ratio of As(III): As(V) of 3.1. Many Taiwanese reportedly suffered from skin cancer and Blackfoot disease in the affected area. High levels of arsenic in ground water below the Ganges delta were first reported by Dr. K.C. Saha of the School of Tropical Medicine, Calcutta, in July 1983. Saha found arsenic above the WHO maximum permissible limit of 0.05 mg l–1 in tube wells in six districts of West Bengal bordering Bangladesh (Murshidabad, Maldah, Nadia, Bardawan, North 24 Parganas and South 24 Parganas (Quadiruzzaman, 1996). These six districts of West Bengal cover an area of 34,000 km2 and have a population of 30 million. A survey conducted between 1989 and 1996 by the School of Environmental Studies (SOES) in Jadavpur University estimated that more than a million people in 560 villages were drinking water containing an average As concentration of 0.20 mg l–1 (maximum concentration, 3.7 mg l–1 ) (Das et al. 1996). Of the 20,000 samples of tube well water analyzed by the SOES to date, 45% have been found to Arsenic contamination in Bangladesh groundwater 237 contain As above 0.05 mg l–1. Das et al. (1994a) reported that the majority of the samples of groundwater extracted from several thousand tube wells along the river Ganges in West Bengal had As concentrations exceeding 0.05 mg l–1 (maximum 0.4 mg l–1).
Most of the tube well water was found to contain equal amounts of both trivalent arsenite (H2AsO3) and pentavalent arsenate (H2AsO4). Neither MMAA (monomethyl arsonic acid) nor DMAA (dimethyl arsinic acid) could be detected (Das et al. 1994b). The source of As in the groundwater of West Bengal is geological. All affected districts are in the vicinity of the river Ganges, particularly in an alluvial formation along the banks of the river Bhagirathi. Chatterji (1998) associated As contamination of groundwater with arsenic-enriched pyrites in the sub-surface soils of several different geological strata in the lower Ganges basin. Several authors have suggested that the heavy extraction of groundwater for irrigation during the dry season causes a marked fluctuation of the water table. During such fluctuations, decomposition of the pyrite occurs as the ground water is aerated. The acid released due to this decomposition leaches the As from the pyrites (Das et al. 1996). Since the hydrological systems of the affected areas of West Bengal are similar to those of districts in neighboring Bangladesh (Khan 1991), a Bangladesh government project, initiated with financial assistance from the World Health Organisation (WHO), was undertaken to ascertain if similar contamination of groundwater had occurred in Bangladesh. Unfortunately, the limited scope of the project could not project the magnitude and urgency of the catastrophe awaiting the country. However, the project did lead to further investigations and by 1997 the human health hazard resulting from arsenic contamination of groundwater in Bangladesh had become a national issue.
1. M.G.M. ALAM, ALLINSON, STAGNITTI
School of Ecology and Environment, Deakin University, PO Box 423, Warrnambool, Victoria 3280, Australia.
Environmental Chemistry Division, National Institute for Environmental Studies, 16–2 Onogawa, Tsukuba, Ibaraki 305–0053, Japan.
School of Science, University of Ballarat, PO Box 663, Ballarat Victoria 3353, Australia.