Natural arsenic is mainly found in recent sedimentary contexts associated with large deltas ( Burkina Faso, Cambodia , India, Bangladesh, Nepal, Vietnam, Zimbabwe ).

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Other sources or arsenic are insecticides and chemical industries.

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Arsenic is cumulative poison ( causing skin cancers, liver cancers, leukaemia ).

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It exists in water as the reduced As ( III ) form, which is free and toxic,

or the oxidised As ( V ) form, which is less soluble and not toxic.

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The presence of As is mainly in groundwaters; in the Cambodian Mekong alluvial basin, the highest concentration in the aquifer are between 20 and 70 m of depth; the upper layers of the aquifer present less free arsenic ( oxidation ), as well ass the deep layers ( precipitation).

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This vertical variability depends on both geological and hydrological conditions, but must be checked to find alternative water resources for long-term projects.

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Arsenic concentrations may vary with seasonal events ( floods, dry season ) and also presents a huge geographical variability ( in the same village, some boreholes may be contaminated and others not ).

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The diseases caused by arsenic are generally linked to 10 to 15 years of water consumption.

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• WHO guideline value is 0.01 mg/l maximum.

• Some National Standards may differ a lot, e.g. Cambodian Std : 0.05 mg/l.

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In-situ analysis is recommended.

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