Naturally arsenic-contaminated groundwaters do not necessarily coincide with high arsenic (As) concentrations in the ambient sediment. For example, in the highly polluted areas of the Bengal Delta Plain, the As content of the sandy aquifer material is mostly well below 10 mg/kg on average (Bhattacharyya et al., 2003). Over the years it became gradually evident that instead of the presence of a local, high-As source within the sediment, the enrichment of As in groundwater results rather from the coincidence and interplay of a few decisive factors. Among these, the local redox conditions and the mineral speciation of As, which primarily define its mobility, seems to be the main actuators (Nath et al., 2005). There is an increasing wealth of evidence that Fe- and Mn-oxyhydroxides in combination with an appropriate redox environment play a key role in the time constrained fixation and subsequent release of As into the groundwater. However, there is not often any correlation to be observed between Fe or Mn and As, either in the sediment or in the water, a matter of fact which sometimes is invoked to play down the role of these process in the occurrence of this natural calamity.
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