The spatial distribution of seepage at a flow-through lake in western Denmark was investigated at multiple scales with integrated use of a seepage meter, lake–groundwater gradients, stable isotope fractionation (d18O), chlorofl uorocarbon (CFC) apparent ages, land-based and off -shore geophysical surveys, and lake bed coring. Results were compared with a three-dimensional catchment-scale groundwater fl ow model using the MODFLOW and LAK3 codes for simulating lake–groundwater interaction. Seepage meter and model results of discharging groundwater to the lake compared well, if direct seepage measurements
from near shore were combined with measurements from deeper parts of the lake. Discharge rates up to 9.1 × 10-7 m s-1 were found. Ground-penetrting radar used to map the lake bed sediments proved very eff ective in recognizing low- and high-permeability areas but also in understanding the complex recharge patiern of the lake and relating these to the geologic history of the lake. Recharge of the surrounding aquifer by lake water occurs off shore in a narrow zone, as measured from lake–groundwater gradients. A 33-m-deep
d18O profi le at the recharge side shows a lake d18O plume at depths that corroborates the interpretation of lake water recharging off shore and moving down gradient. Inclusion of lake bed heterogeneity in the model improved the comparison of simulated and observed discharge to the lake. The apparent age of the discharging groundwater to the lake was determined by CFCs, resulting in ages between 3 and 36 yr with an average of 16 yr. The simulated average groundwater age was 13.2 yr.