If it is city water I am guessing they are buffering the water. Iron water tends to be acidic, and common treatments are soda ash, calcium carbonate or magnesium oxide. Not sure if this is the case or not for you, but the water would tend to have a high(er) salt content. It may work in your benefit for a stout, but you'd be better informed if you got a copy of your city water report. Post it here if you get one, and the folks can help you.