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In the present study, the stable isotopes δ18O and δ2H were used for assessment of the water balance in a heterogeneously structured catchment area in the Lusatian Lignite Mining District, in particular, for estimation of the annual groundwater inflow and outflow (IGW and OGW) of Mining Lake Plessa 117. The application of stable isotopes was possible since the water exchange in the catchment area had reached steady-state conditions after the abandonment of mining activities in 1968 and the filling of the voids and aquifers by re-rising groundwater in the years thereafter. Diverging slopes of the Evaporation Line and the Global Meteoric Water Line manifested as evaporation from the lake catchment area. The calculated isotope water balance was compared with the commonly used surface water balance, which is unable to differentiate between IGW and OGW, and with a local groundwater model. The groundwater model calculated an IGW of about 811 000 m3 yr−1 and an OGW close to zero, whereas the isotope water balance showed fluxes of about 914 000 and 140 000 m3 yr−1, respectively. Considering the contribution of the groundwater inflow to the total annual input into the lake (ΔIT) and the mean residence time (τ), where the groundwater model and the isotope water balance calculated 42 and 47% for ΔIT and 4.3 and 3.9 years for τ, respectively, it was shown that both water balance calculation methods led to comparable results despite the differences in IGW and OGW.