DannyD. wrote, on Wed, 09 Jul 2014 03:09:17 +0000:

It's surprisingly hard to find how *old* the water is that we drink.

While the references quoted show our water "can" be billions of
years old, this document says that home well water is likely only
decades old, and, deeper wells (like those that San Jose Water Company
likely has) can easily be thousands of years old:
http://www.agwt.org/content/how-old-your-well-water
"Some deep confined ground water is thousands of years old and
yet is still slowly on the move."

Summarizing, depending on how deep the well is, the water can
be tens of years old to thousands of years to billions of years
old.

I still stick with the water under San Jose being millions of years
old, and a relic of the time when the area was flooded, but, we'd
need a reference to back up that hypothesis in order to be sure.