An off the wall question asked by my dear old sainted mother at dinner. How old is the water we drink?
The larger question is really – does water ever really go away? Or get created? I suspect that the same H2O molecules evaporate, condense, and fall…over and over again. Do any molecules fall apart, or leave the earth’s atmosphere? And is new water ever formed anywhere, anyhow?
So to answer Mom’s question – really, really old.
Water is created any time you burn some hydrogen. It’s destroyed every time some lightning zaps the ocean or something. Other than that, water mostly stays water.
Every time a water molecule is used by a plant for photosynthesis, it gets taken apart. Every time any (aerobic) living thing respires and breaks down the sugars created by photosynthesis, new water molecules are assembled.
What twoflower said, but even more so: breaking apart or building proteins and fats also create/use water molecules, and no doubt a whole bunch of other reactions common in living cells. Also, even in pure water, some of the hydrogen atoms naturally wander off from their water molecules; if the water is pure the hydrogen will eventually re-attach itself to a different water molecule, but with anything else in the water, the hydrogen might end up there.
So, bottom line, a water molecule might last a while (if you think swapping hydrogen atoms doesn’t really change it), but there are quite a few organic and some inorganic reactions that are constantly breaking and creating water.
Gasoline combustion produces water vapor too - the hydrogen in the hydrocarbons combines with the oxygen in the air, and presto - hydrogen oxide (H20 - water) and carbon dioxide (CO2) - among other things.
>the hydrogen in the hydrocarbons
which generally got there when a prehistoric plant put it there, right?
I knew a geologist who once dated the Calistoga aquifer. So it’s possible to take a segregated body of water and calculate how long it has been segregated. I don’t remember how that was tested for, although I remember that it wasn’t an intrinsic property of the water. It relied on the accepted geological history of the area and on rates of uptake and/or decay of chemical or radiological components.
If your water comes from rivers, lakes, or near water table wells, there are different things you can calculate. If by ‘how old’ she means how long since it last came down as rain, it may be possible to make a calculated guess.
I bet they made a lovely couple!
Lake Vostok in Antarctica is thought to have water that has been segregated for perhaps 20 million years. I bet it’s gone flat by now.
No body of water is truly segregated. The surface evaporates. The weather brings rain and snow. When you slug down a bottle of Aquafina, you are likely to get a molecule or more that was drunk and pissed by Plato. Yes! You are partly Plato piss! 
The most sincere religious bigot probably contains the water of Moses, Mohammed, and Jesus. :eek:
I always suspected we were ALL taking the piss.
Lake Vostok is under over 2 miles of ice, on a bed of granite. Pretty segregated, I’d say. It’s estimated residence time (the time the water stays in the reservoir) is over 1 million years. Compare that to Lake Ontario at 6 years, and your typical groundwater, at 10,000 years.
For a volume of water, you can look at all of the inflows and outflows and estimate a residence time. This represents the amount of time a given infinitesimally small drop of water spends in that volume before leaving again. It’s really an average figure, however, since some water may be cycling rapidly back and forth in and out of that body, and other parts will be relatively stagnant. It’s not precisely the “age” of that body, but it’s close enough to substitute in casual language.