Carbon isotope exchange rate of DIC in karst groundwater

The kinetics of isotopic exchange between dissolved inorganic carbon (DIC) of groundwater and calcite of the matrix of karst aquifers of Cyrenaica, Libya, can be deduced from 13C and 14C data. The aquifers are mostly confined, and the majority of the wells do not show any occurrence of modern recharge: in 1976–1980, in fact, the tritium content was below 1 tritium unit (TU) in most sites. ng that the isotopic exchange takes place through a first order reaction such as 14C radioactive decay, it can be shown that a linear correlation occurs between lnA and ln(δM−δ−εP), where A is the 14C activity, δM and δ are the 13C contents of matrix calcite and DIC, respectively, and εP is the 13C enrichment in CaCO3 precipitation. The slope of the correlation provides the half-life of the isotopic exchange process. For Cyrenaica karst groundwater, a half-life of about 11,000 years is obtained, i.e. about double that of 14C radioactive decay. otopic exchange kinetics also depends on the ratio between groundwater volume and the calcite surface exposed to the exchange process. Thus, other aquifers will show different exchange half-life values. The Cretaceous chalk aquifers of the Paris Basin, France and Lägerdorf, Germany give a half-life of about 4000 years, much shorter than that of Cyrenaica, which may be due to the high porosity, i.e. to the large surface available for the isotope exchange process. The Berkshire Chalk aquifer, UK, gives a half-life of about 10,000 years. Much higher half-lives, above 20,000 years, are obtained for two sandy aquifers in Flevoland, The Netherlands, and Texas, USA, which could be explained by the low CaCO3 content of the aquifer matrix. The highest half-life value, about 40,000 years, is obtained in an artesian limestone aquifer in Florida, USA.