The dissolution-exchange model for dating groundwater with 14C

In this model the dissolution of carbonate in the soil by the action of carbon dioxide generated by the decaying vegetation cover is considered to be responsible for the inorganic carbon content of groundwater: CaCO3+CO2+H2O→Ca2+2HCO3-. Both the 13C and the 14C content of the groundwater are thus primarily determined by this reaction. A further influence, however, is the process of isotopic exchange during infiltration between the resulting bicarbonate and carbon dioxide in the unsaturated zone. In the ideal model situation a relationship is derived that relates the 'initial' 14C content of the water to its chemical and stable carbon isotopic composition. This relationship is required for calculating absolute 14C ages of groundwater. The validity of other known correction procedures is discussed and compared with the DE model, using results from the literature and from unpublished field studies. (author)