Millions of tube wells in Bangladesh draw from shallow aquifers where arsenic concentrations exceed safe limits. The method that grew into the Sediment Colour Tool works with what drillers can already see: the colour of the sediment they bring up tells them, reliably enough to act on, whether the water at that depth is likely to be safe.
Because it needs no laboratory and no instrument, it can be applied at the moment a well is being sunk — which is the only moment when the decision is cheap.
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