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).

Well, I found out that the oldest water ever sipped by a modern
human is "between 1 and 2.6 billion years old":
http://www.treehugger.com/clean-wate...-terrible.html
http://grist.org/list/maybe-dont-dri...ear-old-water/

But that water was deeeeeep at 1.5 miles while we're only talking wells
that are probably 500 to 1,000 feet deep for the San Jose water supply.

This mine water in Minnesota has been dated to pre-cambrian times, which
dates to an ancient sea that existed before life on earth existed:
http://minnesota.cbslocal.com/2011/0...on-range-mine/

So, certainly it's *possible* that the water in San Jose's deep wells
is millions of years old ... but again ... I can't find anything that
says how old that water is.

Can you?