Quantifying groundwater dependence of a sub-polar lake cluster in Finland using an isotope mass balance approach

A stable isotope study of 67 kettle lakes and ponds situated on an esker aquifer (90 km²) in northern Finland was carried out to determine the role and extent of groundwater inflow in groundwater-dependent lakes. Distinct seasonal fluctuations in the δ¹⁸O and δ²H values of lakes are the result of seasonal ice cover prohibiting evaporation during the winter. An iterative isotope mass balance approach was used to calculate the inflow-to-evaporation ratios (<i>I</i>_TOT/<i>E</i>) of all 67 lakes during the summer of 2013 when the isotopic compositions of the lakes were approaching a steady-state. The balance calculations were carried out independently for ²H and ¹⁸O data. Since evaporation rates were derived independently of any mass balance considerations, it was possible to determine the total inflow (<i>I</i>_TOT) and mean turnover time (MTT) of the lakes. Furthermore, the groundwater seepage rates to all studied lakes were calculated. A quantitative measure was introduced for the dependence of a lake on groundwater (<i>G</i> index) that is defined as the percentage contribution of groundwater inflow to the total inflow of water to the given lake. The <i>G</i> index values of the lakes studied ranged from ca. 39 to 98%, revealing generally large groundwater dependency among the studied lakes. This study shows the effectiveness of applying an isotope mass balance approach to quantify the groundwater reliance of lakes situated in a relatively small area with similar climatic conditions