Thorium is one of the main radioactive elements in Earth crust. There are many minerals which consist thorium (about 120), but it is in small quantities. It is using in electric industry, as nuclear fuel and in production of electric bulbs.
Origin in water
Thorium can find in underground waters. Important minerals of thorium are thorite and thorianite.
In human metabolism, because the food or water has small thorium doses, thorium strives to distribute to skeleton. Thorium salts are chemical and biological hazards.
Long thorium loading cause malign neoplasm (it expects radioactive effects more then toxic).
Maximal allowable concentrations
International Commission for radiological protection give follow standards for thorium: 0,5 mg/1 solute thorium 232 and 0,5-9 mg/1 for indissoluble compounds.