All underground waters, except exclusions, are in some level radioactive. Their radioactivity has origin from radionuclide 238U, 235U and 232Th. In smaller level radioactivity originate from potassium -40K and rubidium -87Rb. Beside these, in underground water there are other radionuclide with a natural or artificial origin.
Content of radionuclide in underground waters is smaller then in rocks. The highest concentrations of radionuclide are in underground water uranium bearing.
Migration and concentration of radionuclide in underground waters, beside their character, depend on many factors, like
- geochemical conditions (pH - Eh)
- chemical composition of underground waters
- mineral and chemical composition of rocks,
- granulomethric composition of rocks,
- ions replacement etc.
Mineral particles of rocks don't absorb radionuclide iodine -131J and sulphur -35S, but weakly absorb other radionuclide like strontium -90Sr, ruthenium -106Ru, uranium -23 U and caesium -137Cs.
Concentration of radionuclide of atmospheric origin is smaller in underground waters then in surface waters. This radionuclide comes in underground water with precipitation. In deeper aquifers, concentrations of radionuclide with small sand are almost zero. So, tritium -3H we can find in shallow aquifers.
In underground waters from natural radionuclide the most frequent are uranium, radium and radon.
Uranium. As natural radionuclide, uranium occurs in a shape of several isotopes. 238U is the most frequent in underground waters. Uranium springs of underground waters are its minerals in acid igneous and in some sedimentary rocks. The biggest concentration of uranium occurs in underground waters of uranium bearing like in arid areas, where uranium concentration is very big because of evapotranspiration. Uranium migration is the biggest in acid mining waters, in zone of oxidation, and the smallest in zone of reduction with a presence of carbon hydrogen and organic matter.
Radium. From four isotopes of radium (223Ra, 224Ra, 226Ra and 228Ra), the isotope 228Ra is occurring in underground waters. In regard on uranium, concentration of radium is smaller and it is in correlation with a mineralization value. The smallest concentrations are in low-mineralized water.
Radon is the most important spring of natural radiation. Radon is gas without colour, taste and odour, and it is seven times harder then the air. Radon is occurring in a shape of three isotopes: 222Rn, 220Rn and 219Rn. The form of radon 222Rn prevails, and this isotope becomes from emanations of 226Ra. Radon is characteristic for many underground waters. Radon migration through fractures and faults is very good. The high concentrations of radon are occurring in zones with a fractures, as in underground waters of uranium bearing, phosphate bearing and in igneous rocks.