How does the groundwater influence the water balance of a lowland lake? A field study from Lake Stechlin, north-eastern Germany

The subsurface catchment of Lake Stechlin is quite complex. Water level measurements in groundwater wells with different screen depths show that groundwater in the lake's catchment may partly discharge to the lake and partly leak to deeper aquifers. Simple steady-state 2D vertical modelling of eight different scenarios (different leakage areas, different hydraulic conductivities) was conducted in order to explore the local and regional groundwater flow system around Lake Stechlin and the origin and possible magnitude of lacustrine groundwater discharge. A comparison of the different scenarios with in situ measurements of lacustrine groundwater discharge by temperature sticks and seepage meters revealed that only a part of the groundwater recharge in the catchment discharges to the lake while the rest leaks into deeper aquifers. This implies a differentiation of the subsurface catchment into a local and a regional flow system and impacts on travel times and chemical reaction time scales.