Triple-isotope mass balance of mid-latitude, groundwater controlled lake.

Abstract Lakes constitute an important part of the hydrological cycle on continents. Recent developments in high-resolution laser spectroscopy made feasible triple-isotope, high-precision analyses of water samples (δ2H, δ18O and δ17O). We conducted in-depth, comprehensive assessment of the potential and usefulness of the triple-isotope mass balance approach to quantify water budget of lakes and surface water bodies. A small, groundwater-controlled lake located in the vicinity of Krakow, southern Poland, was selected for this purpose. Approximately 30% higher incoming and outgoing groundwater fluxes derived from 2H-based budget, when compared to 18O- and 17O-based budgets, were obtained when isotopic composition of ambient moisture interacting with the lake was assumed to be in isotopic equilibrium with the local precipitation. Since the ambient moisture interacting with a given lake is in fact a mixture of the vapour released by the evaporating surface water body and the free-atmosphere water vapour, we proposed the procedure to calculate the isotopic composition of this mixture. This approach led to fully consistent triple-isotope mass balance of the studied lake. The underground water fluxes derived separately for the three isotope tracers in use (2H, 18O and 17O) turned out to be identical within the quoted uncertainties, thus supporting internal consistency of the obtained results. The magnitude of the calculated fluxes was in the order of 3200 mm year−1 for the incoming groundwater flux and 3100 mm year−1 for underground losses of lake water through seepage to the local aquifer. The calculated mean turnover time of water in the studied lake is only ca. 16 months, which guarantees a good quality of lake water, a critical parameter in the light of intensive exploitation of this lake as a popular recreational destination during summer.