Stable isotope study of the Thingvallavatn area

The paper describes the stable isotopic composition of water within the catchment area of Thingvallavatn. It is therefore suggested that the spring water originates from Langjoekull ice cap but is slightly enriched in the heavier isotopes due to mixing with local precipitation on the 50 km way from the recharge area to the lake. At the southwestern shoreline of the lake several springs have been observed that are luke warm (7-11 deg. C) due to the geothermal activity at the Nesjavellir field. The oxygen isotopic composition of this water is slightly heavier than observed elsewhere within the catchment area due to contamination by the Nesjavellir thermal fluid, which has become considerably enriched in the heavy isotope 18O due to oxygen isotopic exchange with the bedrock. In the paper it is shown that the thermal water at Nesjavellir also originates from the Langjoekull ice cap. The results demonstrate the importance of studying simultaneously the oxygen and hydrogen isotopes and suggest that if the deuterium excess is <10 0/00, the water is isotopically different from its original isotopic composition due to evaporation. This is particularly important to bear in mind when the geographical distribution of delta values in precipitation is being studied and the δD values are used to trace the origin of a spring water. (BN)