A range of As compounds, both organic and inorganic, are introduced
into the environment through geological (geogenic) and anthropogenic
(human activities) sources (Fig. 1). Small amounts of As also enter the soil
and water through various biological sources (biogenic) that are rich in As
(Table I). Although the anthropogenic source of As contamination is increasingly
becoming important, it should be pointed out that the recent
episode of extensive As contamination of groundwaters in Bangladesh
and West Bengal is of geological origin, transported by rivers from sedimentary
rocks in the Himalayas over tens of thousands of years, rather than
anthropogenic.