Dear Thomas, thanks for answering. You correctly write "should be". Many cases show us that both rainwater and groundwater in particular, need to be treated because they are polluted. In the case of groundwater, it could be saline through intrusion of seawater, or the soil is very permeable so that is polluted by chemicals from the ground (the case of pacific islands). Rainwater also, it is not safe itself, but also the ways of storage rainwater could be obsolete and pollutant. Nowadays, I think, we can not take for granted that water from natural resources is potable.

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Dear Thomas, thanks for answering. You correctly write "should be". Many cases show us that both rainwater and groundwater in particular, need to be treated because they are polluted. In the case of groundwater, it could be saline through intrusion of seawater, or the soil is very permeable so that is polluted by chemicals from the ground (the case of pacific islands). Rainwater also, it is not safe itself, but also the ways of storage rainwater could be obsolete and pollutant. Nowadays, I think, we can not take for granted that water from natural resources is potable.