DannyD. wrote, on Wed, 09 Jul 2014 02:58:02 +0000:

the question is how long does it take for rainwater to percolate
down 500 feet (assuming it's all similar sandy stuff).

It's surprisingly hard to find how *old* the water is that we drink.
This document merely intimates that age by saying:
https://msnucleus.org/watersheds/Gen...er_history.htm
"The many naturally occurring wells came from the Niles Cone
ground water basin. This basin is filled with layers of clay
and gravel intermittent with water was caused by the many
previous floods and torrential rains."

We don't know how *many* years is "many" previous floods though ...

The Wikipedia on water in California also doesn't give us an age.
http://en.wikipedia.org/wiki/Water_in_California
It just says:
The largest groundwater reservoirs are found in the Central Valley.[3]
The majority of the supply there is in the form of runoff that seeps
into the aquifer. The freshwater is usually found in deposits of gravel,
silt, and sand. Below these deposits lies a layer of deep sediment, a
relic of the era when the Pacific Ocean covered the area."

So, all we really know is that the layer of "deep sediment" is millions
of years old; but we don't really know how old the water in that sediment
is.

One important point though, is that Wikipedia article implicitly implies
that we're drinking "old" water because we pull out far more than is
going in, according to that article.

The only way that can be sustained for decades is if the water has pooled
for a very long period of time, in order to build up that surplus.

So, "implicitly", we know the water must be old; but we still don't know
how old it is.