Our water system is very old, and our water contains a lot of dissolved minerals; manganese, I'm told, is predominant and accounts for the reddish colour that can be seen in the standing water at the base of the hydrant.
When the city is flushing hydrants, don't wash your clothes, or they'll get that slightly red tinge, too. Necessitating another round of spring cleaning! By the way, our city water supply is artesian, very pure and quite tasty compared to some municipal water I've tasted.