I've been led to believe that water filtered through limestone is some of the cleanest and purest forms of water you can get,I also know that a number of bottled water companies advertise that their water has been filtered through limestone so there must be something to it,and when it comes to bushcraft/survival websites we don't seem to see a lot of information on water even though it is one of the essential elements..so many people seem to concentrate on fire, food and shelter and take water for granted when we really should be examining this resource as carefully and meticulously as we can.
A place we use often has great limestone cliffs and a lot of little caves and fissures and regardless of the time of year or what the weather has been like there always seems to be drips coming down through the limestone..
and forming in little puddles and pools all over the place, now this is a very handy resource if the limestone has already purified the water,and the easy way to find out was to drink it..So I did, and it was delicious and after 3 days there were no after effects whatsoever.
Now I'm not saying that just because it's limestone filtered it's pure enough to drink as is, but I have drank from these pools on quite a few occasions with no adverse effects at all and in a survival situation I would be happy to continue to do this if I had no other means to purify the water available..I have also been told that limestone filtering can remove enough salt from sea water to make it potable for humans, now I cannot verify this but I do intend to find out one way or another if that is possible!!