Well...what I came to know (not sure how true it is) but apparently there is no water in ZAM ZAM.... they pour water in artificially these days and pull back through the pumps system. Although I am not sure about this issue but if someone knows good information with credible source then please do share...
Pumped water in my opinon is also TABARRAK....
so Zam Zam running out of water wont be a huge deal for me as it certainly is source of Tabarrak but not part of our faith...(unless I am unaware of something).