Tritium and oxygen-18 in natural water samples from Switzerland

Tritium and 18O concentrations have been measured in precipitation samples at various stations in the Alps, in the foothills of the Alps and the Jura region. Mean 18O concentration and mean annual temperature follow a linear relationship at many, but not all, of the considered stations. For different regions 18O and altitude are differently related. The 18O and tritium content in precipitation is used as a basis for interpreting the T/H- and 18O/16O-ratios observed in rivers and underground waters. A spring in the Alps shows no seasonal variations in either tritium or ''18O. ''18O provides information about the mean altitude of its recharge area whereas tritium values, which are slowly decreasing, allow the computation of the mean residence time of the water in the ground. The isotopic behaviour of different inputs (precipitation, lake and river water) to the groundwater downstream from the lake of Thun is investigated. Measurements of both isotopes can prevent a misinterpretation, as demonstrated by the example of a karst spring. Distinct tritium variations occur with a sharp maximum in winter, which cannot, however, be due to delayed rain from the preceding summer, since ''18O is constant with time. The variations can be explained by assuming several well-mixed reservoirs of different ages with a mixing ratio varying with time. The isotopic compositions of the considered underground waters support the assumption that groundwater recharge occurs uniformly during all seasons. (author)