A rapid growth in research on arsenic occurrence and behaviour in the environment has occurred over the last decade or so. Today, environmental arsenic problems have been recognised and documented in numerous countries across the world, in a range of geological and climatic settings. Arsenic can be mobilised naturally in water and soils through weathering reactions and microbiological activity. Its mobilisation can also be initiated or exacerbated through anthropogenic activities such as metal mining and groundwater abstraction, and in some cases through the use of arsenical pesticides in agriculture and wood preservation. The relative roles of these processes vary from region to region although most workers would accept the dominant role played by natural biogeochemical processes in the mobilisation of As in waters from the areas of the world that have been worst affected.\ud Drinking water constitutes a major, and possibly dominant, pathway of exposure to As in humans. The WHO provisional guideline value for As in drinking water is 10 μg/L, having reduced from 50 μg/L in 1993. Most industrialised countries also take 10 μg/L as a statutory limit although most developing nations continue to use the pre-1993 WHO guideline value as a national standard because of difficulties with analytical detection and compliance.\ud Although the concentrations of As in drinking water are usually low, in some circumstances they can reach far in excess of these statutory drinking-water limits and thus cause a potentially severe threat to health. Groundwaters are generally more vulnerable to As contamination than surface waters because of the interaction of groundwater with aquifer minerals and the increased potential in aquifers for the generation of the physicochemical conditions favourable for As release. Indeed, the majority of the world’s recognised As-related health problems are linked with long-term use of groundwater for drinking. Some of the best-documented and most severe cases of As-contaminated groundwater occur in aquifers from Asia (e.g. parts of Bangladesh, China, India, Nepal) and South America (e.g. Argentina, Mexico).\ud A significant amount of the research carried out over the last decade has focused on characterising the occurrence and speciation of As in affected regions across the world and investigating the processes controlling As mobility in groundwaters and aquifers. This chapter describes the key features of high-As groundwater provinces with examples taken from a range of geological and climatic settings, and outlines some of the main biogeochemical processes involved
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