Mapping and quantifying groundwater discharge to Lake Steiling and its influence on lake water chemistry

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 Steilingen, 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. 944Bqm-3) showed that groundwater discharges into the middle of the lake and along the northeastern shore from springs in the hypolimnion (11-14m depth). Based on a Rn mass-balance groundwater accounts for 70% of the total water input (GWflux= 11ls-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.