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This article describes the scale of the problem at hand, as well as it’s negative effects.
An estimate is given of around 13 million women and 3 million children being responsible for water collection needing to travel more than 30 minutes for this. The list of negative effects described in the paper is long with decreased hygiene being the most important. It also deals with the distribution of gender/age of who is gathering water, with women being disproportionately represented and far more children appearing than is acceptable.
Note that the study only considers 24 countries in subsaharan africa, so the problem will in reality be even larger than described here.