Carbon-14 has been the tool of choice when dating old things, but it's not without its limits. For starters, it is considered accurate only out to 9 half-lives - or around 50,000 years. That'll catch most age-related things we're interested in, but certainly not all. For example, groundwater dating has some utility in ascertaining recharge rates, but some groundwaters can be very old indeed.
This is where krypton comes into play. 81Kr is a radioactive isotope that occurs when cosmic rays slam through the earth's upper atmosphere wreaking havoc with various particles in the process. These then fall to earth where they get stored in ice layers, oceans, groundwater and similar places. The great thing about 81Kr is that it has a half-life of about 230,000 years - allowing age dating on the scale of up to 1 million years old.
The actual process measures the capture rate of 81Kr and a close control isotope of 83Kr or 85Kr. The ratio of the 81Kr and the control isotope is then compared to an atmospheric sample of 81Kr which reveals the sample's age.
One problem has been capturing enough 81Kr. There is apparently a fairly low amount of the stuff around, and it is pretty soluble in water. Previous attempts using this method required huge amounts of sample material - like 16 tons of water to get enough Kr to date. But a new instrument - the Atom Trap Trace Analysis (ATTA) has changed the logistics. It now only requires two tons of groundwater to extract the required 81Kr.
Anyway, all this to tell you that the latest groundwater dating study on a well in western Egypt using the new ATTA equipment has dated that groundwater at 500,000 years old. That's pretty old water and indicative of zero recharge from the surface - at least in the last half a million years.
I might add that the Kansas Geological Survey has dated some of our groundwater (in southern Thomas County) and reports that it is relatively recent - from 3,900 to 4,700 years old. The KGS used multiple age determination methods, basically including Carbon-14 and an isotope of Tritium (3H). For a complete explanation of their age-dating process, see their 2012 annual index well report (Page 48 once you get there). I helped collect the samples and we only gathered up about a total of maybe 5 pounds of water for each of the 4 wells sampled. Had they used ATTA, I'd have given them one solid ATTA-BOY - but definitely would NOT have been involved in collecting the 8 tons of water required! Sure glad our groundwater was young enough to use Carbon-14!