f The water balance of the „Lange Lacke“ (Burgenland/Austria) – A Ggeophysical isotope hydrological, and hydrogeological study

A typical feature of the landscape of the Seewinkel area east of the Neusiedlersee in the province of Burgenland are the “Lacken” – shallow brackish little lakes – which are famous for their rich wildlife and specialised vegetation. Some of them dry up during the warm season. Up to now this behaviour was exclusively attributed to the influence of precipitation and evaporation. This study is focused on the area of the “Lange Lacke” with the goal to investigate the interaction between the groundwater and the water body of the “Lacke”. The hydrogeological conditions of the Seewinkel are characterised by low ground water recharge rates, little ground water movement, small depth of the subsoil water level, and considerable ground water table fluctuation during the year. In the last decades a subsidence of the groundwater level caused by drainage activities was observed. As a first step isotope hydrological investigations have been conducted in order to determine the origin of the water of the Lange Lacke and a possible interaction between ground water and lake water. 3H isotope data show that the origin of the water of the Lange Lacke can solely attributed to the precipitation. On the other hand oxygen isotope data of the surrounding ground water exhibit an enrichment in 18O in areas of ground water flow downstream of the Lange Lacke. This indicates that during times of low ground water table lake water drains off into the ground water.