San Francisco's municipal water is naturally soft, sourced largely from the Hetch Hetchy reservoir system in the Sierra Nevada — a genuinely clean, low-mineral starting point that gives a homebrewer or commercial brewer in the area a mostly-blank slate to build any target water profile from, rather than a strongly-flavored regional mineral character to work around the way Burton-on-Trent or Dublin brewers historically had to.
A soft baseline, not a defining regional mineral signature
The figures for San Francisco's water: calcium 24 ppm, magnesium 6 ppm, sodium 20 ppm, sulfate 12 ppm, chloride 15 ppm, bicarbonate 32 ppm. Every reading here sits well below the historic European brewing-city profiles this site also covers — nowhere near Burton's sulfate, nowhere near Dortmund's combined sulfate-and-chloride hardness, nowhere near Dublin's bicarbonate. That softness doesn't mean the water is characterless for brewing purposes; it means the water itself isn't dictating a single stylistic direction the way a strongly mineral-dominant historic water source did, leaving the actual style choice and salt additions to the brewer.
California's brewing history reflects that flexibility
San Francisco has real historical brewing significance independent of any single defining mineral, most notably as the home of California Common (Steam Beer), a hybrid lager-ale style developed there in the 19th century out of necessity — brewing lager-style beer without reliable ice or mechanical refrigeration, using open, shallow fermentation vessels to dissipate heat instead. That style's identity comes from fermentation technique and yeast choice far more than from any specific water-chemistry signature, which fits a city whose water itself doesn't push hard in one direction.
Using this profile
Because this water starts soft, it's a genuinely convenient base for building toward almost any target profile with brewing salts rather than needing dilution or reverse osmosis first the way a brewer working from a hard municipal supply would. A brewer chasing a hop-forward West Coast IPA or pale ale profile typically builds sulfate up with gypsum from this kind of soft starting point; PeriodicDeck's element reference is a useful background check on what calcium, sulfate, and the other ions in a water report actually are before making those additions. Note this is a current municipal reading rather than a historical reference figure the way most other profiles on this site are — actual tap water composition can still shift over time, so test your own water directly before brewing.