Radionuclides occur naturally in many rocks and minerals. Some radionuclides in rocks, like uranium, are present since the world formed. Others, like radon and radium, are the merchandise of the decay of these original radionuclides. As a result, these radionuclides frequently occur in groundwater. Uranium-238 may be a common chemical element in many rock types. Uranium is merely weakly carcinogenic, but is toxic and causes kidney damage at elevated concentrations when consumed in drinking water. Because uranium is very soluble in its oxidized forms but only slightly soluble in its reduced forms, its concentrations in groundwater depend upon redox conditions, also as on pH and on concentrations of bicarbonate.

Radon-222 may be a daughter product of the decay of uranium. Radon may be a dissolved gas that dissolves easily in water, and is present in most groundwater within the US. When water that contains radon is employed during a home, most of the radon is released from the water into the air and may be inhaled. Inhalation of radon poses a risk of carcinoma. Radium-226 and radium-228 are the 2 commonest isotopes of radium and both are daughter products of the decay of uranium. Both isotopes are carcinogenic. Radium dissolved in drinking water may be a human-health concern because it accumulates in bone and other tissues, increasing lifetime cancer risks. Other radionuclides are produced from human activities, like nuclear weapons testing, nuclear facility releases, and radioactive waste.