Dating water can be as complicated as dating people

From July 29, 2004:

We just got back from Zion National Park. We passed some water coming out of the rock walls. The guide said the water was carbon dated to be about 4,000 years old. I thought for something to be carbon dated, it had to be organic. How can they carbon date water?

Simple, they just take a sample of water out to the garage, pour it into the Popeil Carbondate-O-Matic, push the start button and wait for the printout. Or something like that.

OK, so you’re sort of correct about the organic business. When they do carbon-14 testing on water, they’re usually lookingfor traces of organic chemicals dissolved in the water. The problem is they’re not really dating the water, they’re dating the trace chemicals. The ages of the two might not be the same.

Carbon-14 is naturally occurring radioactive stuff found in the atmosphere. When you or an animal or a plant take in air, your tissues soak up a bit of carbon-14. When you die, the carbon-14 starts to decay. Since we know carbon-14 has a half-life of 5,730 years, clever scientists can figure out how much carbon-14 is left in something and compare that with how much might be in it if it were still alive and thus figure out how old it is.

Carbon-14 dating is not the only way to find out how old water is. You could just ask someone. Like, “Say, how old is that water?” and if you got lucky the person just might happen to know.

That doesn’t seem very likely.

You could drill down into an aquifer and get a water sample and count the number of krypton-81 atoms in it. Krypton-81 is created by cosmic rays in the atmosphere and has a half-life of 229,000 years.

Or you could use a really complicated formula involving the ratio of the oxygen-16 isotopes in the water to oxygen-18 isotopes.

I’m told this is a crackerjack way to find the age of water, although I don’t know what the formula is, I don’t know how it works, and for what I get paid, I have no intention of figuring it out.