text: The presence of radioactive waste in the environment may cause long-term effects due to the activity and half-life of the radionuclides, leading their impact to grow with time. These particles exist in various oxidation states and are found as oxides, coprecipitates, or as organic or inorganic complexes, according to their origin and ways of liberation. Most commonly they are found in oxidized form, which makes them more soluble in water and thus more mobile. Unlike organic contaminants, however, they cannot be destroyed and must be converted into a stable form or extracted from the environment.
  The sources of radioactivity are not exclusive of human activity.  Natural radioactivity does not come from human sources: it covers up to three fourths of the total radioactivity in the world and has its origins in the interaction of terrestrial elements with high energy cosmic rays (cosmogenic radionuclides) or in the existing materials on Earth since its formation (primordial radionuclides). In this regard, there are differences in the levels of radioactivity throughout the Earth's crust. India and mountains like the Alps are among the areas with the highest level of natural radioactivity due to their composition of rocks and sand.
  The most frequent radionuclides in soils are naturally radium-226 (Ra), radon-222 (Rn), thorium-232 (Th), uranium-238 (U) and potassium-40 (K). Potassium-40 (up to 88% of total activity), carbon-14 (C), radium-226, uranium-238 and rubidium-87 (Rb) are found in ocean waters. Moreover, in groundwater abound radius radioisotopes such as radium-226 and radium-228 (Ra). They are also habitual in building materials radionuclides of uranium, thorium and potassium (the latter common to wood).
  At the same time, anthropogenic radionuclides (caused by humans) are due to thermonuclear reactions resulting from explosions and nuclear weapons tests, discharges from nuclear facilities, accidents deriving from the reprocessing of commercial fuel, waste storage from these processes and to a lesser extent, nuclear medicine. Some polluted sites by these radionuclides are the US DOE facilities (like Hanford Site), the Chernobyl and Fukushima exclusion zones and the affected area of Chelyabinsk Oblast due to the Kyshtym disaster.
  In ocean waters, the presence of tritium (H), cesium-137 (Cs), strontium-90 (Sr), plutonium-239 (Pu) and plutonium-240 (Pu) has significantly increased due to anthropogenic causes. In soils, technetium-99 (Tc), carbon-14, strontium-90, cobalt-60 (Co), iodine-129 (I), iodine-131 (I), americium-241 (Am), neptunium-237 (Np) and various forms of radioactive plutonium and uranium are the most common radionuclides.
  The classification of radioactive waste established by the International Atomic Energy Agency (IAEA) distinguishes six levels according to equivalent dose, specific activity, heat released and half-life of the radionuclides:
  *Exempt waste (EW): Waste that meets the criteria for exclusion from regulatory control for radiation protection purposes.
  *Very short lived waste (VSLW): Waste with very short half-lives (often used for research and medical purposes) that can be stored over a limited period of up to a few years and subsequently cleared from regulatory control.
  *Very low level waste (VLLW): Waste like soil and rubble (with low levels of activity concentration) that may also contain other hazardous waste.
  *Low level waste (LLW): Waste that is above clearance levels and requires robust isolation and containment for periods of up to a few hundred years and is suitable for disposal in engineered near surface facilities. LLW include short lived radionuclides at higher levels of activity concentration and also long lived radionuclides, but only at relatively low levels of activity concentration.
  *Intermediate level waste (ILW): Waste with long lived radionuclides that requires a greater degree of containment and isolation at greater depths.
  *High level waste (HLW): Waste with large amounts of long lived radionuclides that need to be stored in deep, stable geological formations usually several hundred metres or more below the surface.