WATER DATING

To date groundwater, scientists determine how much of an isotope (such as carbon-14) is still present. An isotope is an element, such as carbon or chlorine, with one or more extra neutrons in its nucleus. By knowing the precise rate of decay of a radioactive isotope, scientists can then determine how long the water has been in the soil or an aquifer. Dating is also accomplished by analyzing the concentration of isotopes like carbon-14 and chlorine-36 in the groundwater, since such concentrations have varied over time due to changes in the earth's magnetic field. Scientists can also determine when a particular sample of groundwater fell as rainwater based upon the ratio of simple hydrogen (H1) to its isotope, deuterium (H2). The presence of deuterium is strongly influenced by the atmospheric temperature; during colder periods, for example, the last glacial period, rainfall contained less deuterium than today.