Assuming this is direct from the municipal supply, who's spec is it outside - theirs or yours?
For the former, the corrective action arguable rests with them as your supplier - you will want details of what they're doing about it. Is it significantly out of spec, and might it cause or indicate a problem? Can they provide monitoring data over a period of years so you can at least get a feel for how significant the shift is? Don't know where you are based and how things are set up, but we can relatively easily obtain full reports of annual chemical and micro testing for our municipal supply area.
In the first instance I'd try to get hold of someone in the analytical/technical department at the water authority with whom you can talk through the issue.