Seepage between a Regional Aquifer and a Flow-through Lake in a Melt Water Outwash Plain: Evidence of off-shore exchange

Groundwater discharge to lakes is often completely neglected or misrepresented in the lake water balance, mainly because of difficulties in quantifying the spatial distribution. Furthermore, knowledge of the spatial discharge pattern is important for tracing contaminant sources to the lake. Former studies have suggested that the primary discharge zone is near the shore line at shallow depths. However, discharge zones can also be linked with topographical breaks in slopes both at the shoreline and in the deeper parts of the lake. Numerical investigations corroborate these flow patterns for homogeneous systems. Lake Hampen in Western Denmark is a flow-through lake located in a geological homogeneous melt-water outwash plain. The lake consists of a deep (maximum depth of 13 m) and shallow basin (mean depth of 1.5 m). The complexity of the groundwater-lake water interaction was resolved by an integrated use of different methods performed at three different scales in order to determine direction and rates; 1) Near shore lines with water depths up to 1 m, 2) Off-shore in shallow to intermediate water depths (1-5 m), and 3) Off-shore in deeper parts of the lake with water depths of 3-13 m. Lake sedimentology was investigated using geophysical methods (Multi Electrode Profiling, seismic profiling and Ground Penetrating Radar) and these guided the choice of locations for measuring the spatial distribution of seepage. Direct and indirect methods were used to measure seepage; hydraulic gradients and seepage meter measurements and stable oxygen isotope profiles. The results show clear evidence of off-shore exchange between the regional aquifer and the lake. Discharge rates, ranging from 0.03-10.6 cm/day in the near shore zone, cannot account for all seepage to the lake. This suggests that a significant part of the discharge takes place further off-shore, which is also supported by observations of gradients in the shallow to intermediate zone and the deeper parts. Recharge is very low in the shallow near shore zone and found mainly to occur in a small area at the slope between the deep and shallow basins. Geophysical data show that this area is sandy and likely kept clear of gyttja by bottom currents. The sandy area enables a good contact with the regional aquifer where recharge can occur. This is supported by a δ18O profile down to 34 m below surface at the recharge end of the lake. The profile indicates a δ18O plume seeping out of the lake, not at the shore line, but rather near the transition from the deep to the shallow basin.