[2] Isotope hydrology applications are highly diverse, and used for informing water-use policy, mapping aquifers, conserving water supplies, assessing sources of water pollution, investigating surface-groundwater interaction, refining groundwater flow models, and increasingly are used in eco-hydrology to study human impacts on all dimensions of the hydrological cycle and ecosystem services.
Oxygen-18 (18O) occurs in approximately one oxygen atom in every five hundred and has a slightly higher mass than oxygen-16, as it has two extra neutrons.
Since precipitation in each rain or snowfall event has a specific isotopic signature, and subsurface water can be identified by well sampling, the composite signature in the stream is an indicator the proportion of the streamflow comes from overland flow and what portion comes from subsurface flow.
[6][7][8] The isotope hydrology program at the International Atomic Energy Agency works to aid developing states to create a detailed portrait of Earth's water resources.
The International Atomic Energy Agency maintains a publicly accessible global network and isotopic database for Earth's rainfall and rivers.