Naturally occurring krypton is made of five stable isotopes and one (78Kr) which is slightly radioactive with an extremely long half-life, plus traces of radioisotopes that are produced by cosmic rays in the atmosphere.
This is particularly challenging for dating pore water in deep clay aquitards with very low hydraulic conductivity.
An important goal of the Limited Nuclear Test Ban Treaty of 1963 was to eliminate the release of such radioisotopes into the atmosphere, and since 1963 much of that krypton-85 has had time to decay.
[13] The atmospheric concentration of krypton-85 around the North Pole is about 30 percent higher than that at the Amundsen–Scott South Pole Station because nearly all of the world's nuclear reactors and all of its major nuclear reprocessing plants are located in the northern hemisphere, and also well-north of the equator.
[14] To be more specific, those nuclear reprocessing plants with significant capacities are located in the United States, the United Kingdom, the French Republic, the Russian Federation, Mainland China (PRC), Japan, India, and Pakistan.