The GVRD adds sodium carbonate to raise the pH of our water to limit its corrosivity to piping. The pH can fluctuate depending on which reservoir they're drawing from, how much soda ash they're adding (or if the system is working at all) and what form of chorine they're using to disinfect.
The chemical analysis of Vancouver water can be found by digging around the GVRD website quality control reports
http://www.gvrd.bc.ca/water/quality-and-treatment.htm
or by phoning them.
SusanB