Plutonium 81 is a radioactive isotope that is produced in the atmosphere of cosmic rays. On the one hand, it is constantly generated, and on the other hand, the nuclear has been decaying. The two aspects have reached a balance. There are as many as 81 in American air, Chinese air, and Antarctic air. We tested it. The surface water exchanges with air, and the number of surface water 81 is the same, but once the water flows into the ground or when the water freezes, it is like the bottle cap is tightened and there is no fresh 81 added, then the isotope clock starts to go, 81 The number will be less and less over time. In different places, the water will measure the number of 81 in it, and you can know the age of the water body. The water itself has no age, but at what age the water body is and how long the water and air are separated, this information is very important. With this information, you can know how the groundwater flows, and you can figure it out.