The Physics of Zam Zam Water

I flew via Jeddah recently, and Jeddah being a major destination for pilgrims going on Hajj or Umrah, the pilot told us when we flew over Mecca so that they could conduct their proper prayers. My customs form also informed me that I was able to bring up to 10 liters of water from the Zam Zam well, which makes me wonder how large that well really is. A well large enough to support a woman and her child, or even thousands of pilgrims in centuries past, may not be large enough to support the millions that come now. It might be, Mecca and the well are located at the lower end of a long geographic tilt which funnels what rain and underground streams fall in the region, but I still wonder, especially if each of the estimated two million people who come each year are allowed to take 10 liters with them. 20 million liters is a lot for a spring. Demand expands beyond those actually on a pilgrimage as well, as Zam Zam water is sold commercially outside Mecca (despite it being against Saudi law to do so). There is even counterfeit Zam Zam water, something to be avoided, as not only is not from the site that matters so much to the purchasers, at least one lot of it was found to have come from an area where arsenic tainted the groundwater, thereby poisoning the counterfeit water as well.