How long does it take for waste-water to be filtered through natural filtration and not by water treatment?

I need a reliable source for source citation. I cannot find a source that says specifically how long it takes water, once used, to be filtered by nature back into a drinkable form. I know how long it takes to go through water treatment facilities, but not through the natural process.

I've always heard that once water travels through 12 feet of soil and subsoil it is considered "in speaking terms" to be filtered.
This in no way doesn't absolutely mean it couldn't contain a micro organism if there is a source of continual pollution upon entry into the soil.

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