What Are Isotopes and What Can Be Learned From Them?  Elements
consist of atoms of different mass called isotopes. During the
evaporation and condensation of water, the concentration of
oxygen and hydrogen isotopes that make up the water molecule
undergoes small changes. Modern instruments can measure these
with great precision.  The history and pathway of water in
different parts of the hydrological cycle (click here for figure
2) can be followed by the abundance of the stable heavy isotopes
of hydrogen (2H) called deuterium, and oxygen (18O). In this
way, water in different environments develops isotopic
"fingerprints" with which it can be identified and its origins
traced.  Some isotopes are radioactive, which means that they
decay away with time. For example, those of tritium (3H) and
radiocarbon (14C), are naturally produced in tiny amounts in the
atmosphere. Large amounts of tritium and radiocarbon were also
injected into the atmosphere by nuclear bomb tests in the 1960s.
These isotopes are carried int