Groundwater can play an important role in lake water and chemical budgets. The aim of this study was to map and quantify groundwater discharge to Lake Steißlingen, a small lake in south-west Germany, using Rn as a natural groundwater tracer. ²²²Rn, nutrients, temperature and oxygen concentration were measured in depth profiles and in sediment-near water samples during April and June 2016. The spatial distribution of Rn activities (max. 944 Bq m⁻³) showed that groundwater discharges into the middle of the lake and along the northeastern shore from springs in the hypolimnion (11–14 m depth). Based on a Rn mass-balance groundwater accounts for ~70% of the total water input (GW_flux = 11 l s⁻¹) to the lake. There were significant positive correlations between methane and nitrite with Rn activity, suggesting the groundwater as a common source. Overall the inflow of groundwater causes a deterioration in lake water quality.