Those ranges are difficult to deal with. According to the information, two creeks supply the water. Interesting there is that much variation; I wonder if it is seasonal.
They do add something to maintain pH for corrosion control. It would be worth finding out what, exactly; I have soda ash added here in Greater Vancouver, which dissipates out fairly quickly so it is not a problem for fish. Could you see if there may be more info on this (and maybe the GH variation) on that "American Water" website they mention near the end?