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 C-13 and C-14 data. The aquifers are mostly confined, and the majority of the wells do not show any occurrence of modem recharge: in 1976-1980, in fact, the tritium content was below 1 tritium unit (TU) in most sites. Assuming that the isotopic exchange takes place through a first order reaction such as 14 C radioactive decay, it can be shown that a linear correlation occurs between lnA and ln(delta(M) - delta - epsilon(p)), where A is the C-14 activity, delta(M) and delta are the C-13 contents of matrix calcite and DIC, respectively, and epsilon(p) is the C-13 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 C-14 radioactive decay. The isotopic 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 Lagerdorf, 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. (C) 2002 Elsevier Science B.V. All rights reserved.