The radioactivity in the hydrosphere also has natural and artificial sources. The most important natural radioactive isotope in the hydrosphere is 40K, which, in the form of a potassium ion, is mostly found dissolved in water. Because of the lower salt concentration, the radioactivity of rivers is much less than that of seawater. For example, the mean radioactivity of the Danube River is 70—90 mBq/dm3, and the activity of seawater is about 10—15 Bq/dm3. Among the artificial radioactive pollu­tions, Sr, Cs, I, and I dissolve well in water, so the natural water mainly contains these artificial isotopes.

Of course, the radioactive isotope of hydrogen, tritium, is also present in natural waters as tritiated water. As mentioned in Section 7.3, tritium is formed in nuclear

power plants in 14N(n,3 4He)T and 14N(n, T)12C reactions. The emission of the nuclear power plants raised the tritium concentration by 1—2 orders of magnitude above the natural level. The tritium activity is generally expressed in tritium units (TUs). One tritium unit means that the ratio of the hydrogen (1H) and tritium (3H) atoms in 1018:1. The radioactivity of 1 TU is 0.1184 Bq/dm3.

The vegetation in water accumulates dissolved radioactive isotopes. The accu­mulation depends on the composition of water and the species present. The humus formed from the decomposition of the vegetation also uptakes the radioactive iso­topes, which in this way transfers to the lithosphere.