Radioactive isotopes include both natural and man-made radioactive isotopes or elements that may be present in your drinking water. These unstable elements or their radioactive daughter products may be present in both private and public drinking water systems. These isotopes are regulated as part of the Primary Drinking Water Standards. As the radioactive isotope decays, it can emit radiation in a number of forms (depending on the particular isotope) and this radiation can be harmful to biological and genetic material. As the isotope decays, it releases energy (the radiation) and may create daughter isotopes that are also radioactive, posing another health risk.
There are many elements which have stable isotopes (such as potassium-39) but which also have unstable isotopes (such as potassium-40) which can be significant sources of radiation; such isotopes include: potassium-40, cesium-137, cobalt-60, iodine-131, and strontium-90. Then there are other elements which do not have any stable isotopes such as uranium, plutonium, radium, and radon; if you have any of those elements, they will be radioactive. Another way to assess the radioactives in water is to measure the amount of radiation produced by alpha or beta emitters without worrying about which particular isotopes are doing the emitting (gross alpha, beta emission).
Long-term exposure to radionuclides (unstable, radioactive isotopes) can result in cancer; exposure to uranium can result in kidney disorders, cancer, disruption of the reproductive system, organ malfunction; exposure can also have mutagenic effects. Note that some radioactive elements, such as plutonium, are not only highly radioactive, but also very chemically toxic.
Common Radioactive Isotopes below include Radium 226 and Radium 228, Uranium 238, and Radon 220 / 222. Tests of Radioactivity in drinking water below include Gross Alpha and Beta Particles.