Chlorine-36 dating of old groundwater. Chapter 6

Chlorine-36 has been widely used for dating old groundwater. The 36Cl used for this purpose is produced in the atmosphere through the interaction of cosmic rays with argon atoms. The 36Cl then mixes with ordinary atmospheric chloride (mostly derived from the ocean). The mixture is deposited on the land surface dissolved in rain or snow or as dry aerosols. Groundwater recharge carries the 36Cl into the subsurface where the radiometric 'clock' is set. Chloride is conservative in the subsurface and, thus, the 36Cl is rarely retarded with respect to the water velocity by adsorption or geochemical reactions. Groundwater age can be estimated using the radiometric decay equation and the decrease in 36Cl from the amount in the recharge water. The main complication is variations in the Cl concentration of groundwater. This can potentially be due to variable evapotranspiration during recharge or to the addition of Cl in the aquifer. If the cause of Cl concentration variations is understood, the age calculation can be corrected to account for the process. Chlorine-36 dating is generally applicable to water in the age range 100 ka–1 Ma.