Extreme Groundwater Discharge to a Eutrophic Seepage Lake and the Effect on Lake Restoration

Water residence time is an important variable in lake-ecosystem management but for lakes interacting with groundwater it can be difficult to estimate, due to the often unknown or badly determined inflow of groundwater or outflow of lake water into the groundwater aquifers. A study of Lake Væng, Denmark (16ha and max. 1.9m deep) with hydrogeological approaches demonstrates that the lake's water balance and probably also water quality (especially total phosphorus (TP)) to a large degree are driven by the surrounding aquifer. Results of the study confirm that the residence time in the lake is very short (20 days) and the δ18O signal of the lake confirms the high discharge of groundwater to the lake. The exchange of water between the aquifer and Lake Væng is analysed with a two-dimensional numerical flow model. It is demonstrated how the deeper groundwater is flowing in the eastern direction and is directed beneath the lake and enters at the sandy eastern shore area. Model results show that groundwater discharge to the western lake shore is substantial even with a small thickness (≤ 0.5m) of a conducting sand lens. In 2009 groundwater accounted for 66 % of the total water input to the lake (14.0 m/yr), surface water 30 % (6.4 m/yr), and precipitation 4 % (0.912 m/yr). In order to maintain a good ecological status, or clear water conditions in Lake Væng, lake TP concentrations should probably not exceed an ecological threshold for eutrophication of 30-50 µg/l of TP. In piezometer tubes placed systematically in the lake bank were measured phosphorus contents of between 20 and 220 µg/l of TP (median 90 µg/l of TP). We do not know the groundwater flux at each sampling points and thus we cannot estimate the water flux weighted concentration of phosphorus. A better quantitative estimate on the mass flux of phosphorus through the riparian zones and springs are planned in our future research, including dynamic water level changes in the lake and periodic flooding of the riparian zones. The naturally occurring phosphorus in the surrounding groundwater might be is so high that it is uncertain whether the positive effect of lake restoration with fish removals will have a continuous effect without periodic follow-up reduction in the population of roach and bream.