Drinking water supplies in Cambodia and Vietnam are dependent on groundwater resources (Berg et al., 2001, 2006; Feldman and Rosenboom, 2001; Fredericks, 2004). The Mekong and the Red River deltas are the most productive agricultural regions of South East Asia (see Fig. 1). Both deltas have young sedimentary deposits of Holocene and Pleistocene age. The groundwaters are usually strongly reducing with high concentrations of iron, manganese, and (in some areas) ammonium. TheMekong and the Red River deltas are currently exploited for drinking water supply using installations of various sizes. In the last 7–10 years a rapidly growing rural population has stopped using surface water or water from shallow dug wells because they are prone to contamination by harmful bacteria. Instead, it has become popular to pump groundwater using individual private tube-wells, which is relatively free of pathogens.The Vietnamese capital Hanoi is situated in the upper part of the 11,000 km2 Red River delta, which is inhabited by 11 million people and is one of the most populous areas in the world. The exploitation of groundwater in the city of Hanoi began more than 90 years ago and has since beenexpanded several times (Berg et al., 2001). Today, ten major well-fields are operated by water treatment facilities, which collectively process 650,000 m3/day. Due to naturally anoxic conditions in the aquifers, the groundwaters contain large amounts of iron and manganese thatare removed in the Hanoi drinking water plants by aeration and sand filtration (Duong et al., 2003). The urban water treatment plants exclusively exploit the lower aquifers in 30–70 m depth, whereas private tube-wells predominantly pump groundwater from the upper aquifers at 12–45 m (Hydrogeological Division II, 2000). Based on geological analogies to the Ganges delta,elevated arsenic concentrations in the aquifers of the Red River basin were expected (Berg et al., 2001). A first screening by us in 1998 confirmed this assumption and we studied the extent of arsenic contamination in a comprehensive survey from 1999 to 2000. The upper and lower Quaternary aquifers were investigated by analysing ground waters from small-scale tube-wells and pumped by the Hanoi drinking water plants. Groundwater arsenic contamination was identified in the Cambodian Mekong delta area in 2000 (Feldman and Rosenboom, 2001), and has since been investigated and addressed through close collaboration of local authorities and NGOs. The first international paper on arsenic groundwater contamination in Cambodia was published by Polya et al. (2005). In this paper, the arsenic levels in groundwater of the Mekong delta are presented including data for the Vietnamese delta part, which is reported for the first time. In addition to an overview of the magnitude of arsenic poisoning in this region, the limited information availablein the international literature on the geology and genesis of the Mekong and Red River delta is summarised.
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