Bernard Blavoux

Apports du traçage isotopique naturel a la connaissance du fonctionnement d'un systeme karstique - teneurs en oxygene-18 de trois systemes des Pyrenees, France

Abstract Three karstic systems in the Pyrenees (France) have been subjected to a monthly systematic sampling in view of isotopic analysis of oxygen-18. In addition, a higher frequency sampling in overflow conditions has been made in one of the systems which is also continuously studied as regards hydrodynamics, geochemistry and biology. After analysing the sample representativity, an attempt has been made to find out the meaning of the variations of the contents in oxygen-18 in function of time. In the case of these systems of the Pyrenees, a relation between the altitude of the catchment area and the contents in oxygen-18 of their risings has been established which does not agree with Dansgaards relation. The variations of the isotopic contents in function of the hydrogeologic events confirm the heterogeneity of the submerged zone and the importance of the part played by the percolation, chiefly by considering the connection with the surface saturated zone, named “epikarstic aquifer”. These results, obtained with those of hydrodynamic, geochemical and biological studies of the karst, are not likely to be interpreted without the help of these disciplines.

Radiocarbon Dating of Groundwater of the Aquifer Confined in the Lower Triassic Sandstones of the Lorraine Region, France

Abstract Blavoux, B. and Olive, Ph., 1981. Radiocarbon dating of groundwater of the aquifer confined in the Lower Triassic sandstones of the Lorraine region, France. In: W. Back and R. Letolle (Guest-Editors), Symposium on Geochemistry of Groundwater — 26th International Geological Congress. J. Hydrol., 54: 167ndash;183. Stable-isotope and groundwater chemistry analyses on samples from about 40 boreholes in the Lorraine region (eastern France) are used to evaluate the paleohydrological regime of the Lower Triassic sandstone aquifer in the study of the mechanisms of the general water dynamics of the aquifer at present. In this sandstone aquifer, the estimation of the 14 C activity, A 14 , in the total dissolved carbon (TDC) to 100% of modern carbon is justified by the absence of carbonates in the reservoir and by the 13 C contents in the TDC. A significant relation could be determined between the 14 C activity in the TDC and 13 C: A 14 = —7.7δ 13 — 59.6%. This gradual enrichment of the TDC with 13 C results from the escape of the dissolved CO 2 . The meaning of the radiometric age is discussed as a function of piston and exponential-type flows. The study of the stable isotopes 18 O and 2 H revealed two water families. The most recent waters have δ 18 O-values higher by 1% 0 than the more than 10 4 yr. old waters. This 1 % 0 discontinuity corresponds to the improvement of weather conditions during the Holocene, which amounts to 2.5°;C. The isochrons of the aquifer reveal the existence of hydrogeological sections bounded by faults where the rates of circulation are different.