Re: ZamZam
As I mentioned, according to the article on the Islamic Cultural Centre (which is funded by government of 19 Muslim states including Saudi Arabia, Iran, the UAE, Syria, Pakistan, and Bangladesh amongst others), the well has, on a limited number of occasions over the past 4000 years, fallen into a state of disuse due to either running dry or being blocked with sand.
The article does not state the time periods when that happened, so the population was probably significantly less than it is today.
During those period, I would guess that the city, historically being quite wealthy, purchased water from trading caravans in the area.