How much?

I have been wondering for a while about the fact that I am constantly drinking water.
I always have a water bottle with me and drink it all the time (and go to the bathroom every 30 minutes or so). I figure I drink about one gallon, while I am at work and maybe another quarter gallon once I am home.
Are there any health risks concerning too much water. Can I overdose on water? Note too, that I am NOT drinking the lead-contaminated DC water....