Coupling between groundwater and surface water at Lake Willersinnweiher, a gravel pit lake in the Upper Rhine Graben without any surface in- or outflow, was investigated using both a groundwater model and the tracers 18O and SF6. Based on groundwater modeling, recharge and discharge areas around the lake as well as the residence time of the lake water were determined in a regional context. The uncertainty of the simulated flow field was assessed by sensitivity analysis. The tracers 18O and SF6 were measured in several observation wells and piezometers around the lake as well as in the lake's water column. They were used to verify groundwater flow directions found in the modeling. We found that the groundwater-lake interaction model had a large uncertainty even though relatively detailed information on input data was available. Independent information obtained from the environmental tracers allowed us to improve and verify the model.