You CAN get into "water intoxication" because of depleting certain of your electrolytes. Sodium depletion is the most dangerous but the body has an EXTREMELY tight feedback mechanism that compensates after just a few days producing a nearly sodium free urine. UNFORTUNATELY, before the acclimitization occurs, sodium depletion and dilution can actually KILL.
an aside: [quote]In the current issue of Annals of Internal of Medicine, investigators reports on marathon runners who developed brain swelling as a result of "water intoxication." All had taken in too much water during their races, causing sodium levels in the blood to drop. From there, according to Dr. J. Carlos Ayus, excess water is absorbed into blood and fluid builds up in the brain. Eventually, fluid accumulates in the lungs, and athletes become breathless and nauseated.
Potassium is not quite so compensable because early man had no problems eating potassium excesses while sodium was RARE. Thus the body always tries very hard to rid itself of potassium and thus water intoxication can lower K+ (And many other ions) to dangerous levels.
I'll make a stab that more 2 gallons over several days, unless in a very hot climate and sweating profusely, is usually TOO much water.
Like all good things, even a good thing like water can be overdone.