Water samples from three quarry lakes and the surrounding fractured rock aquifer were investigated for δ18O and δ2H (H2O), δ15N and δ18O (), as well as anions and cations. Lake water and groundwater can be distinguished by their different chemical and isotopic composition. Because of evaporation processes, 18O and 2H are enriched in the lake water and can be used as natural tracers for the water dynamic of the lakes. The groundwater is characterised by high nitrate concentrations (up to 120 mg/l). Lake internal processes reduce the nitrate concentration in the quarry lakes. However, no enrichment of δ15N and δ18O in nitrate, typical for microbial nitrate degradation, is observed in the lake water. Because of the complex flow paths in the fractured rock aquifer and the intense chemical transformations at the interface between groundwater and lake water, isotopic and hydrochemical data of lake water and groundwater alone do not conclusively explain hydrological and hydrochemical processes of the investigated lake-groundwater system. †Revised version of a paper presented at the VIII IsotopeWorkshop of the European Society for Isotope Research (ESIR), 25–30 June 2005, Leipzig-Halle, Germany.
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