See Sharon Gabel's lecture notes on potentiometric maps with an example and cross section. The potentiometric surface for the Madison limestone aquifer in central and eastern Montana is available from the Montana Bureau of Mines and Geology . The potentiometric surface follows the topography in central and eastern Montana but is more subdued. In the Little Belt Mountains the potentiometric surface is below the ground, but out on the prairie of eastern Montana it is above the ground. Radiocarbon dating is used to determine how long the groundwater has been underground.
Cosmic rays bombard the atmosphere converting some nitrogen into radioactive carbon.
This isotope has a half life of about 5730 years. The carbon reacts with oxygen in the atmosphere to make carbon dioxide. Carbon dioxide dissolves in rain water and surface water. The ratio of radioactive 6 C 14 to ordinary carbon 6 C 12 remains about the same as long as the water is in contact with the atmosphere. However, as soon as water goes underground the carbon 14 concentration decreases.