Tomh is correct, it sounds like you have a leak in your water main that leads to the house. I had this happen to me a couple of years ago (plastic line failure) and it resulted in my front yard becomming a swamp before we realized there was a leak.
I would call you local water department. They will come out for no charge and determine if it is a problem or not. Unfortunately, if the leak is between the city shutoff to your house and your house, it will probably be your responsibility. If it is on the other side of the city shutoff, it should be their responsibility.
Good Luck!
Dodgeman