If you haven't already done it, check this out: http://www.sandiego.gov/water/quality/
You can look for the 2011 or 2010 report for one of our regions.
I played around with the idea of doing something similar to what you were doing, but ended up taking the advice of a few local brewers which was to just use water from the tap (non garden hose). I have always ended up with a PH of 5.0 -5.3 by mid-mash. In fact, today's mash ended up being 5.1 at mid-mash.