A branch of environmental geochemistry and isotope geology concerned with the occurrence of radioactive nuclides in sediment, water, air, biological tissues, and rocks. The nuclides have relatively short half-lives ranging from a few days to about 10⁶ years, and occur only because they are being produced by natural or anthropogenic nuclear reactions or because they are the intermediate unstable daughters of long-lived naturally occurring radioactive isotopes of uranium and thorium. The nuclear radiation, consisting of alpha particles, beta particles, and gamma rays, emitted by these nuclides constitutes a potential health hazard to humans. However, their presence also provides opportunities for measurements of the rates of natural processes in the atmosphere and on the surface of the Earth.

The unstable daughters of uranium and thorium consist of a group of 43 radioactive isotopes of 13 chemical elements, including all of the naturally occurring isotopes of the chemical elements radium, radon, polonium, and several others. A second group of radionuclides is produced by the interaction of cosmic rays with the chemical elements of the Earth's surface and atmosphere. This group includes hydrogen-3 (tritium), beryllium-10, carbon-14, aluminum-26, silicon-32, chlorine-36, iron-55, and others. A third group of radionuclides is produced artificially by the explosion of nuclear devices, by the operation of nuclear reactors, and by various particle accelerators used for research in nuclear physics. Some of the radionuclides produced in nuclear reactors decay sufficiently slowly to be useful for geochemical research, including strontium-90, cesium-137, iodine-129, and isotopes of plutonium. The explosion of nuclear devices in the atmosphere has also contributed to the abundances of certain radionuclides that are produced by cosmic rays such as tritium and carbon-14. Dating methods