Isotopic methods and their hydrogeochemical context in the investigation of palaeowaters

Abstract

Abstract Isotope and geochemical techniques are the primary way in which the residence time, recharge conditions and subsequent evolution of palaeowaters can be determined. Isotopic species and noble gas concentrations are used as residence time and palaeoclimate indicators. Among the former, 14 C is pre-eminent in late Quaternary studies because of an age range which covers the Pleistocene-Holocene transition. However, its use is constrained by frequent difficulties in determining the dilution of dissolved 14 C due to water-rock interaction. A combination of 14 C data with 226 Ra and 4 He results may be useful for Holocene waters but they can also be used to validate the carbon systematics assumed for 14 C dating. For waters beyond the range of 14 C dating, 81 Kr, 36 Cl, 4 He and chemical tracers can be applied. Stable isotope ratios and noble gas concentrations primarily reflect climatic conditions at the time of recharge. While the noble gases provide absolute values for recharge temperatures, stable isotopes are only relative indicators that vary regionally. The PALAEAUX programme has examined these aspects in some detail by looking at the δ 18 O shift between Pleistocene and Holocene waters on the European scale, and by calculating δ 18 O/Δ T ratios from δ 18 O v. recharge temperature plots for aquifers at different distances from the Atlantic Ocean. Indications are that the more positive δ 18 O value of ocean water during the Pleistocene dominates in the more westerly European countries over the negative δ 18 O shift during cooler conditions. There are also indications that air-mass circulation during the Pleistocene was similar to the present day. The evolution of a palaeowater can best be studied by measuring chemical tracers; this is possible in freshwater aquifers, where a clear trend of geochemical reactions is observed, and in freshening marine aquifers. Chemical and isotopic tracers can also be used to study the movement of the front between palaeowater and younger components that must be identified in coastal aquifers to guarantee a sustainable water use.