Apports des techniques isotopiques à la connaissance des gisements d'eau minérale

Contributions of isotopic techniques to the understanding of mineral water resources

Université d'Avignon

Abstract

The first isotopes used were those of water itself, being an intrinsic part of the molecule and forming ideal tracers. Oxygen 18 and deuterium finally quashed the notion of juvenile water by demonstrating that mineral water had a meteoric origin. They also make it possible to locate the zone supplying the mineral water system by determining its mean depth on the basis of isotopic height gradients. Tritium from thermonuclear devices which was particularly abundant in precipitation during the decade 1963-1973 was used at an early stage to identify the presence of recent, post-nuclear water in mineral water springs.

Isotopes of dissolved elements, being exposed to chemical and biochemical fractionating, are a more delicate matter. They are used to determine the origin of the elements and to untangle the history of the water. Carbon 13 has been used to demonstrate the deep magmatic origin of the CO₂ in most mineral waters that contain it. It is used to investigate the interactions between the CO₂ gas, the bicarbonates and the limestone matrix, knowledge of which is essential for radiocarbon dating of ground water. The initial hopes that sulphur 34 would make it possible to distinguish sulphates dissolved in evaporites from those resulting from the oxidation of pyrite are now tempered by the extent and complexity of the fractionings resulting from the action of micro-organisms. Nitrogen 15 is still little used in the field of mineral waters, in which nitrate concentrations are usually very low.