First Water-Isotope-Map (δ18O, δ2H, 3H) of Austria: Applications, Extremes and Trends☆

Abstract The isotopic ratios of oxygen and hydrogen in water ( 2 H/ 1 H and 18 O/ 16 O) are important tools to characterise waters cycles. Tritium formed by natural cosmic radiation in the upper atmosphere and in the last century by tests of thermonuclear bombs, is an ideal age-marker during the last 60 years. To determine the origin and mean age of waters in many projects concerning water supply, engineering and scientific projects in the last 45 years on more than 1,350 sites more than 40,000 isotope measurements were performed in Austria. The median value of all sites of oxygen-18 is δ 18 O -10.7 ‰ and for hydrogen-2 δ 2 H -75 ‰. As the fractionation is mainly temperature dependent the lowest negative values are observed in winter precipitation (oxygen-18 as low as δ 18 O -23 ‰) and in springs in the mountain regions (δ 18 O -15.1 ‰). In contrast the highest values were observed in summer precipitation (up to δ 18 O – 0.5 ‰) and in shallow lakes in the Seewinkel close to the Hungarian border (up to δ 18 O + 5 ‰). The precipitation in the region of South of the main Alpine crest (East-Tyrol, Carinthia and South-East Styria) is approximately 1 ‰ higher in δ 18 O-values as sites at the same altitude in the northern part. This is most probably caused by the stronger influence of precipitation from the Mediterranean area. The median value of all 1,120 sampling sites of decay corrected (2015) tritium measurements is 6.2 tritium units (TU). This is somewhat smaller than the median value of all precipitation stations with 7.2 TU. The tritium concentration increases in the summer up to 10 – 11 TU and decreases in winter down to 3 – 4 TU. A mean tritium concentration in aquifers smaller than approximately 3.5 TU indicates large amounts of this water are older than 60 years. Waters containing approximately more than 12 TU containing still tritium from the 1960s and 1970s formed originally by thermonuclear bomb experiments. In Austria the highest Tritium values can be observed in the rivers Danube and March which show periodic or permanent tritium contamination up to 70 TU coming from nuclear power plants in the neighbouring countries.