However, this year the Namibian department of water affairs announced the discovery of a significant body of underground water - an aquifer dubbed Ohangwena II - that scientists say could supply the north of the country with enough water for centuries.

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But Mr Cheney said it was underground water that mattered most for long-term supply.

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Other solutions are more complex, but still cheap, such as injecting a sugary mixture into a water supply contaminated by hexavalent chromium to make it less toxic and less mobile underground.

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